Recuperator Development Trends for Future High Temperature Gas

Recuperator Development Trends for Future High Temperature Gas
75-GT-50
`ice
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Recuperator Development Trends for
Future High Temperature Gas Turbines
C. F. McDONALD
La Jolla, Calif.
Mem. ASME
The current energy crisis and substantial increases in the costs of liquid and gaseous fuels.
combined with reduced pollutant emission requirements, make the higher efficiency
recuperative gas turbine cycle economically attractive for industrial and vehicular
application. For future low cost, high temperature, small gas turbines, with improved cycle
efficiencies, it is postulated that the complete hot section of the engine (combustor, ducts,
turbine nozzle and rotor) will be all ceramic and may include a ceramic heat exchanger. Few
of the answers are available today in the areas of ceramic recuperator performance, cost
and structural integrity and concentrated development efforts are required to demonstrate
the viability of a fixed boundary ceramic gas turbine heat exchanger. This paper briefly
outlines possible design and development trends in the areas of exchanger configuration,
surface geometry and materials, and it inc^udes specific sizes and economic aspects of
ceramic recuperators for future advanced low SFC gas turbines.
Contributed by the Gas 'Turbine Division of The American Society of Mechanical Engineers for
presentation at the Gas Turbine Conference & Products Show, Houston, Texas, March 2-6, 1975.
Manuscript received at ASME Headquarters December 2, 1974.
Copies will be available until December 1, 1975.
THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS, UNITED ENGINEERING CENTER, 345 EAST 47th STREET, NEW YORK, N.Y. 10017
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Recuperator Development Trends for
Future High Temperature Gas Turbines
C. F. McDONALD
INTRODUCTION
Current dramatically increasing fuel costs
ceramic stationary and rotating components curand reduced pollutant emission requirements make rently being developed for vehicular turbines,
the higher efficiency regenerative gas turbine there will be even more severe demands on the heat
cycle economically attractive for industrial,
exchanger structural integrity. To reduce engine
marine, and vehicular application. Substantial airflow (hence, pollutant volume) and reduce engine
gains in cycle efficiency can be realized in in- physical size (hence, initial power plant cost),
dustrial and vehicular gas turbines by increasing higher specific power cycles will be adopted which,
the turbine inlet temperature. A desirable flat by virtue of their higher pressure ratios, make
SFC-power curve can be achieved in the regenerative utilization of ceramic rotary regenerators quescycle by utilizing variable geometry in the turbo- tionable because of the increasing seal leakage
machinery to keep an essentially constant turbine and structural support problems. Material deinlet temperature over the operating spectrum.
velopments and forming techniques gained from reUnder these part-load conditions, the temperature generator programs over the last few years, in
into the heat exchanger increases substantially conjunction with recent ceramic technology advanceand, for vehicular applications in particular, a ments, should be directly applicable to a ceramic
high percentage of engine life may be spent at fixed boundary recuperator.
low power levels. Metallic recuperators capable For future low cost, high temperature engines
of withstanding these high temperatures will be with improved cycle efficiencies, it seems likely
prohibitively expensive for most applications, and that the complete hot section of the engine (comas even higher turbine inlet temperatures are made bustor, ducts, turbine nozzle, and rotors) will be
possible by using advanced blade cooling techniques all ceramic and may include a fixed boundary ceramfor industrial turbines and by utilizing uncooled is recuperator. Current development efforts are
NOMENCLATURE
AFR = exchanger frontal area, sq ft (m2 )
A x = conduction cross-sectional area, sq ft
( m2)
C = Flow stream capacity rate, Btu/hr-deg
F (w/C)
f = fanning friction factor
h = heat-transfer coefficient, Btu/sq fthr-deg F (w/m2 -deg C)
hp = horsepower
j = heat-transfer Colburn factor
K = material thermal conductivity, Btu/fthr-deg F (w/m-deg C)
kw = kilowatt
e = effective fin length, in. (mm)
L = heat exchanger flow length, in. (mm)
m = fin efficiency parameter
P = absolute pressure, lb/sq in. (Pa)
SFC = specific fuel consumption
Sphp = specific power hp/lb/sec (kw/kg/sec)
T = temperature, deg F (deg C)
It
V = heat exchanger core volume, cu ft (m3)
U = flow rate, lb/sec (kg/sec)
a = ratio of total heat-transfer area of
one side of exchanger to total volume
of exchanger, sq ft/cu ft (sq m/cu m)
/3 = ratio of total heat-transfer area of
one side of exchanger to volume between
plates on that side, sq ft/cu ft (sq
m/cu m)
s = material thickness, in. (mm)
A = denotes difference
F = heat exchanger effectiveness
'7f = fin efficiency
'7o = surface efficiency
X = axial conduction parameter
p = density, lb/cu ft (kg/m 3 )
if = ratio to free flow area to frontal area
(AP/P) = heat exchanger pressure loss, percent
= dollars
= cents
2
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aimed at demonstrating that attractive configurations can be realized using plain compact ceramic
surface geometries that can be readily formed by
simple low cost thin film techniques. Counterflow
configurations are necessary for high effectiveness levels, and integrally formed high pressure
air manifolds will provide a simple ceramic heat
exchanger assembly which, by virtue of its pro;iected low cost, can be regarded as a replaceable
element. Few of the answers are available today
in the areas of structural integrity, performance,
cost, and concentrated development efforts are required to demonstrate the viability of a fixed
boundary ceramic gas turbine heat exchanger.
This paper briefly reviews current metallic
recuperator developments, outlines design considerations in the areas of surface geometry, materials, and performance, and it includes specific
sizes and economic aspects of ceramic recuperators
for future advanced industrial and vehicular gas
turbines.
GAS TURBINE ADVANCEMENTS
The economics of using gas turbines become
more favorable as cycle efficiency and plant specific powez increases. In the case of the industrial gas turbine, higher cycle efficiencies
are necessary to combat the large cost increases
in liquid and natural gas fuels that have been
evidenced in the last year or so. For vehicular
gas turbines, high specific power is necessary to
minimize air throughflow and to reduce power
plant size. Improvements in cycle efficiency can
be realized by higher turbine inlet temperatures.
Increases in turbine inlet temperature have been
achieved by metallic material advancements and by
improved cooling methods. Material developments
are continuing and many new superalloys, which
must have high temperature strength and good corrosion resistance, are being evaluated. Although
it is impossible to predict with accuracy the
materials and associated creep strength properties
that will be available for future industrial and
vehicular gas turbines, it should be reasonable
to expect a continuation of at least 10 C/year
improvement in temperature capability.
In recent years, the ceramic industry has
been active in materials research and component
design for gas turbines. Utilization of ceramics
in the turbine section of industrial prime movers
will have two effects: (a) they will minimize
cooling air flow requirements and, hence, maximize
specific power, and (b) because of their superior
corrosion and erosion resistance will permit use
of lower cost residual fuels which tend to be high
in contaminants, such as vanadium and sodium. For
smaller gas turbines, the automotive power plant
in particular, the big incentive to use ceramic
components is to reduce engine initial cost.
Over 15 years ago, a British attempt to use
ceramics, notably silicon carbide, for the nozzle
vanes of an industrial gas turbine was reported
by Blakely (l),l although this work has not, perhaps, received the publicity it deserved. Much
more recently, progress in the development of
ceramics for vehicular gas turbines has been
reported by McClean (2). Since this publication,
substantial research work has been carried out on
determining the mechanical properties of representative ceramics, and many papers have been
presented on applications for vehicular and industrial gas turbines. Within the industry, development efforts are aimed at utilization of
ceramic turbine components with temperatures up
to about 2500 F (1370 C).
The heat-exchanged cycle is being utilized
in vehicular gas turbines to give a low sfc over
the full operating power range. In the case of
the industrial gas turbine, the rapidly increasing cost of liquid and natural gas fuels would
seem to favor the recuperative cycle. For cycles
with a high degree of recuperation, the optimum
pressure ratio is much lower than in aircraft
turbines (between )4 and 9, say, for heat exchanged
cycles). With the foregoing projected increases
in turbine inlet temperature and the relatively
low pressure ratios, the gas temperature into the
heat exchanger will be much higher than in existing engines and will almost certainly necessitate
use of a ceramic regenerator or recuperator. Because of intense development in the aircraft gas
turbine field over the last 30 years, the fixed
boundary recuperator has received much less development attention than the turbomachinery. To
match the projected increases in turbine inlet
temperature, concentrated development efforts are
necessary to demonstrate the practicality of a
fixed boundary ceramic recuperator.
RECUPERATOR-REGENERATOR COMPARISON
It is beyond the scope of this paper to make
detailed comparisons between rotary regenerators
and fixed boundary recuperators; it is felt, however, that experience gained to date on the various ceramic regenerator programs will be applicable
to recuperator developments. Most of the significant work on ceramic regenerators has been done by
Corning Glass and early work was reported by
Lanning (3). Application of the Corning Cercor
l Numbers in parentheses designate References
at end of paper.
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disks in vehicular gas turbines have been reported
in-depth by Penny (4, 5) and Ritchie (6). Current
vehicular development engines are still being endurance tested to demonstrate performance, emissions, and the life expectance of such critical
items as the rotary regenerator. The regenerator,
made from either metal or ceramic material, is
currently being used in fairly low pressure ratio
engine cycles (typically 5 :1 for a single-stage
radial compressor with good efficiency) for vehicular applications. Even after extensive development programs, the metallic-to-ceramic seal
problems of low leakage and long life have not
been fully resolved in these low pressure ratio
engines. For these low pressure cycles with
specific powers in the order of 100 hp/lb/sec
(165 kw/kg/sec), the current maximum disk diameter
of 28 in. (710 mm) limits the power to 400 to 500
Hp (300 to 375 kw) for the twin-disk variants with
effectiveness values on the order of 90 percent.
To realize the true potential of the gas
turbine, it is desirable to operate at higher compressor pressure ratios and turbine inlet temperatures. (However, initial plant costs must be kept
low for the small gas turbine to be competitive
with other prime movers.) These increased specific
power cycles introduce structural complexity and
seal wear and leakage problems that tend to offset
the potential low-cost advantage of the ceramic
regenerator, which is best suited for gas turbines
of relatively low power that are used where limited
life is acceptable. In heavy-duty, vehicular,
industrial, and marine gas turbine power plant,
the regenerator, whether made of ceramic or metal,
does not look as attractive as the fixed boundary
recuperator.
Aspects of recuperator technology for differing types of power plant have been described by
T-,cDonald (7). The metallic recuperator, because
of its high initial cost and temperature limitations, has not received the same development attention as the ceramic regenerator for vehicular
gas turbines. An obvious goal is to reduce current
engine size and lower the initial cost by utilizing
higher specific power cycles (i.e., increased compressor pressure ratios). Metallic recuperator
developments are being carried out for these new
cycles and these will be discussed in a later section. Clearly, there is no single answer to the
question of what type of heat exchanger is the
most cost-effective for a particular application.
This paper outlines possible development trends in
the area of fixed boundary recuperators only.
RECUPERATIVE GAS TURBINE OPERATING REGIMES
The approximate operating regimes for re-
cuperative gas turbines are shown in Fig. 1, In
the high horsepower class, conservatively designed
recuperators embodying low compactness plain surface geometries have been utilized for industrial
and marine gas turbines. These recuperators have
been manufactured using mild steel with brazed and
welded types of construction. Most of these units
have been used in conservatively designed large
industrial plants where the gas temperatures have
been below 1000 F (540 C) and oxidation has not
been a problem. These units have accumulated
millions of operating hours and have demonstrated
a high degree of reliability. Mild steel has outstanding attributes of low cost, good fabricability, good weld and brazeability, but is temperature
limited to-about 1000 F (540 C) because of the
poor oxidation resistance. Over a hundred of these
units are in service on large industrial gas turbines. Increasing utilization of this type of
recuperator for utility and marine gas turbines
is being evidenced, since large engine operating
economics are sensitive to fuel costs, which, in
the last year, have increased dramatically.
At the lower power levels, stainless-steel
recuperators embodying compact plate-fin surfaces
have been constructed using methods inherited from
the aircraft heat exchanger industry. In this
type of design, solid sealing bars were used and
low compactness end sections were necessary to
channel the hot and cold fluids to and from the
counterflow section. This type of construction
is characterized by a large number of components,
and with the resultant high labor effort required,
the costs of these units in small volume production have been high. While these units have demonstrated their performance and structural integrity
for limited vehicular and industrial gas turbine
application, their initial costs of $20 to 40/hp
(t27 to 54/kw) were not acceptable to the prime
mover operator. Utilizing compact heat-transfer
surfaces, it can be seen that in the 4000-hp
(3000-kw) class, this type of recuperator is an
order of magnitude lighter (and in volume) than
the heavy industrial type.
For vehicular application, the evolution of
a satisfactory metallic recuperator design is a
complex function of the total environment of the
gas turbine. Basically, the problem is not the
design of a device that will transfer the heat
within the limitations of the installation envelope. There are several heat-transfer surfaces
or elements that will do this. The problem is to
incorporate in a recuperator the correct combinations of materials and surfaces that will satisfy
the goals of durability and low cost. The predominant mode of failure in existing plate fin
recuperators has been low cycle fatigue resulting
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+OG. 0001 151,000
• HEAVY DUTY INDUSTRIAL AND MARINE GAS TURBINES
• CARBON STEEL RECUPERATORS (TEMP. LIMITED TO 1000 ° F)( 1 53E ` C)
IOC 000
• MATERIAL COST 154/LB
• TYPICAL RECUPERATOR COST $1.2/LB (APPROXIMATELY a12/HP)
APPROXIMATE OPERATING
RANGE OF CURRENT
CERAMIC DISC ROTARY
REGENERATORS
• EXISTING STAINLESS STEEL COMPACT PLATE-FIN RECUPERATORS
WITH CONSTRUCTION METHODS INHERITED FROM AIRCRAFT
HEAT EXCHANGERS
• LOW VOLUME PRODUCTION, HIGH LABOR COST DESIGNS
• RECUPERATOR COST $20-40/HP
000
'-
h
L 6
10.0 HF (110 KW):
AUTOMOTIVE
ENGINE
IOL
10
AUTOMOTIVE
GAS TURBINE—►-^
A
µO00 HP
(3000 KW)
INDUSTRIAL
GAS TURBINE
600 HP
(450 KW)
TRUCK
7 I ENGINE
j-.
II 75 00
100^
1000
• ADVANCED STAINLESS STEEL FORMED PLATE (AND FIN) TYPE
CURRENTLY BEING DEVELOPED FOR VEHICULAR GAS TURBINES
• TEMPERATURE LIMITED TO APPROXIMATELY 13C0 ° F (70-. ° C)
• TYPICAL MATERIAL
MATERIAL COST $1/LB
• RECUPERATOR COST $1-2/HP IN LARGE VOLUME PRODUCTION
•
•
•
•
(
KW
10,000
7 5, 000
PROJECTED CERAMIC RECUPERATOR REGIME
HIGH TEMPERATURE CAPABILITY
MATERIAL COST 20-40C/LB
IN LARGE VOLUME PRODUCTION FOR AUTOMOTIVE GAS TURBINES
RECUPERATOR COST APPROXIMATELY 30-SOC/HP
100,000
HORSEPOWER
TRUCK
GAS TURBINE
NOTE: SOLID SYMBOLS REPRESENT EXISTING UNITS AND UNITS
CURRENTLY BEING DEVELOPED. OTHER SYMBOLS ARE UNITS
PROJECTED FOR FUTURE GAS TURBINES
RECUPERATED INDUSTRIAL GAS TURBINES
Fig, 1 Approximate operating regimes for recuperative gas turbines
from high thermal stresses in the core during
transient operation. Where thin-to-thick sections
exist in the core, the resultant temperature
gradient during engine transient operation results
in stresses often greater than the yield strength
of the parent material. Advanced metallic plate
fin recuperators, of both prime and secondary
surface types, are under development for vehicular
and industrial application and are shown in Fig.
1. These designs embodying features to minimize
thermal stress problems also require a minimum of
manual labor. By eliminating thermal capacitance
incompatibility in the core components the fatigue
problem is substantially reduced and thinner gage
materials can be used; hence, the core weight and
material cost are reduced. In these designs, the
seals are formed by the tube plate itself, and the
high pressure air manifolds are made integral with
the tube plate and thus eliminate the costly operation of having to weld on separate ducts after
the brazing operation. Perhaps the biggest challenge for the metallic heat-exchanger manufacturer
is the vehicular gas turbine field which includes
automotive, truck, and off-highway equipment. As
shown in Fig. 1, the main development efforts for
advanced metallic recuperators are in the 150-hp
(112-kw) and 600-hp (450-kw) power classes for
automotive and truck gas turbines, respectively.
These designs utilize stainless steel which, in
the thin foil form, costs about 11/lb ($2,2/k_g)
will probably cost in the order of $1-12/hp (^1.35w2,1/kw) in large volume production.
The projected regime for ceramic recuperators
is also shown in Fig. 1, and again the vehicular
gas turbine, particularly for automotive application, is where concentrated development efforts
will be made. Low cost, high temperature capability and elimination of dynamic seals and drive
mechanisms are a few of the features that favor
utilization of a ceramic fixed boundary recuperator. At this stage, it is not obvious which is
the most attractive type of surface geometry for
ceramic recuperators (i,e., plate-fin, primesurface, or tubular). By using extremely compact
surfaces both the weight and volume of the ceramic
unit will be less than the advanced metallic variants. With raw material costs in the range of 20
to 40¢/lb (44 to 88t/kg), very low cost recuperators are projected for materials like silicon
nitride and silicon carbide.
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OPERATING REGIME OF ADVANCED HIGH
SPECIFIC POWER AUTOMOTIVE GAS
TURBINES WITH TEMP. LEVELS
NECESSITATING USE OF CERAMIC
orRECUPERATOR OR ROTARY REGENERATOR
00
o f
IL,
I-
180
oC
1100
100
"TURBINE
INLET
TEMPERATURE
I-
z
L
170
f
I-
1w
J
900
o
oC
F 1400
2500
160
150
N
PART LOAD TEMP. LINES FOR
REGENERATIVE G.T. WITH
VARIABLE TURBINE GEOMETRY
TO GIVE FLAT SFC OVER WIDE
POWER RANGE
240 1300
230
800
Of
Lu
PROJECTED INCREASES IN
TURBINE INLET TEMP. BY
USING ADVANCED BLADE
COOLING TECHNIQUES IN
LARGE ENGINES. AND BY
UTILIZING UNCDOLED
CERAMIC COMPONENTS
(COMBUSTOR, SCROLLS.
NOZZLES AND ROTORS) IN
SMALL GAS TURBINES
220 1200
210
x
w
200 1100
I-
1900
1800 1000
1000I
0
10
J
I
20
AUTOMOTIVE GAS TURBINE OPERATES FOR APPROX.
80 PERCENT OF ITS TIME BETWEEN IDLE AND 25
PERCENT POWER
I
'
'
1
1
40
50
60
70
30
ENGINE POWER, PERCENT III'
80
90
100
L
►
1970 1975 1980 1985
YEAR
TRUCK GAS TURBINE OPERATES
FOR APPROX. 80 PERCENT OF
ITS LIFE IN THIS REGIME
Fig. 2 Projected temperature trends for gas turbines utilizing ceramic components
The current operating range of engines using
ceramic disk rotary regenerators is also shown in
Fig. 1. Most of the truck gas turbines in the
300 to 4-00-hp (225- to 300-kw) class currently
under development utilize rotary regenerators.
As mentioned previously, at power levels above
about 500 hp (375 kw), the regenerator disks exceed the current 28 -in, (710 -mm) maximum, and the
structural support and sealing become a problem
for the higher specific power cycles. At the low
horsepower end of the scale, for automotive gas
turbines, it is not obvious that the rotary regenerator offers the lowest cost heat exchanger
solution since seals and driving mechanism costs
do not scale down directly with power. It is believed that the ceramic fixed boundary recuperator
will find initial acceptance in the small power
automotive gas turbine.
RECUPERATOR TEMPERATURE--MATERIAL CONSIDERATIONS
For industrial and vehicular units, turbine
inlet temperatures will certainly continue to in-
crease with material advances, more sophisticated
cooling techniques, and utilization of ceramic
components. A general trend in advancements is
shown in Fig. 2, with a turbine inlet temperature
of 1800 F (985 C) being assumed for the current
generation of vehicular and more advanced industrial gas turbines. Since recuperative gas turbines optimize out at fairly modest pressure
ratios, the projected turbine inlet temperature
increase results in extremely high gas temperatures
into the heat exchanger.
For vehicular gas turbines a flat SFC curve
over a wide power operating range is necessary if
the gas turbine is to match the performance of the
diesel engine. Part-load SFC characteristic in the
heat exchanged cycle is a strong function of turbine inlet temperature; further, as recuperator
effectiveness increases SFC becomes a weak function
of pressure ratio, even at low pressure ratios.
The flow range limitation of aerodynamic compressors, even with practical variable geometry features, means that for reduced power, there must
be a reduction in pressure ratio or turbine inlet
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temperature, or both. However, since with high
effectiveness recuperation, SFC is more affected
by turbine inlet temperature than by pressure
ratio, modulating power primarily by changing
pressure ratio and mass flow, while maintaining
the turbine inlet temperature results in a relatively flat SFC curve. In typical two-shaft
vehicular gas turbines, there are two methods of
maintaining a high turbine inlet temperature at
part-load operating conditions. In a fixed geometry engine, a means can be incorporated of
drawing off varying amounts of compressor turbine
power and adding it to the power turbine output
shaft. Thus, by suitably varying the power division between gas generator and power turbines a
high top temperature can be maintained at any
total power, and hence a good part-load SFC can
be achieved. In contrast with variable turbine
geometry controlled to reduce the proportion of
pressure ratio across the compressor turbine, a
high turbine inlet temperature can also be maintained as power is reduced. The temperature can
be arranged to vary in any desired manner within
the limits of the variable turbine geometry and
compressor flow range. The latter method of improving part-load SFC by means of variable stagger
power turbine nozzle vanes seems to be favored for
vehicular gas turbines. Variable vanes have the
added advantages of improving the acceleration if
swung open, accommodating variation in accessory
loads on the compressor turbine, and also permitting braking of the power turbine.
The aforementioned methods of minimizing
SFC at part load are included because they have
a significant effect on the recuperator, Maintaining the turbine inlet temperature constant as
the pressure ratio drops, of course, implies that
the recuperator gas inlet temperature will increase
substantially at part-load conditions. Typical
part power temperatures are shown in Fig. 2 which
is meant to illustrate trends rather than specific
values since the curves are influenced by pressure
ratio and component efficiencies. At today!s level
of turbine inlet temperature, the recuperator experiences gas inlet temperatures from 1100 to
1200 F (595 to 650 C) at 100 percent power, to
values in excess 6f 1400 F (760 C) at 25 percent
power. As turbine inlet temperatures will increase
over the years, it can be seen that temperatures
into the heat exchanger get prohibitively high for
existing types of metallic recuperators. The low
power levels are particularly significant since
automotive gas turbines operate for a high percentage of their life in this regime. Although
the internal pressure differential in the recuperator is less at part power, the very high temperatures make this operating regime the most critical
from the structural integrity standpoint. This
situation could be improved by scheduling a constant turbine exhaust temperature (recuperator
inlet temperature) and letting top temperatures
fall somewhat with reducing load. This mode of
operation does not give as good an SFC as constant
top temperature, but the curve is still much flatter than a high pressure ratio simple-cycle gas
turbine.
The factors of primary importance in the
selection of recuperator materials are mechanical
properties, including the coefficient of thermal
expansion, hot-corrosion resistance, fabricability,
stability and cost. A recuperator material must
have adequate mechanical properties to withstand
the stresses due to thermal transients, and to
both fluctuating and steady-state pressure differentials. The material (in its brazed or welded
form) must be capable of resisting simultaneously
the effects of high temperature corrosion and high
stresses, particularly in the presence of contaminants. Cost is a factor of prime importance
in all recuperators. For the recuperator to find
a wide range of acceptance in gas turbine applications, the cost must be reduced by utilizing
lower cost materials and types of construction to
minimize the manufacturing labor content.
In order to attain an operational life of
say 10,000 hr, the structural design of the recuperator must be very carefully evaluated, and
the detail design optimized so as to reduce the
stresses in the components to levels commensurate
with the strength of the selected materials and
the required life. The steady-state thermal and
pressure stresses in the core matrix must be kept
below the 1 percent in 10,000 hr (or the specified
design life) creep stress or the yield stress,
whichever is less. The hotter sections of the
core will, therefore, be designed by creep criteria, whereas the cooler side design will probably
be dictated by the specified short-term proof and
burst pressure requirements. The steady-state
thermal stresses are inherently reduced to a minimum by selection of the counterflow core configuration. To ensure long core life, it is usually
preferable to keep the levels of the transient
thermal stresses below the material yield strength
at the appropriate metal temperature. However,
this is not always possible in metallic units and
local yielding can be allowed to occur and still
maintain a unit with adequate service life. Transient thermal stresses occur as a result of thicker
elements in the matrix responding slower than relatively thin ones to a thermal transient input (engine start or shutdown). If the time constant of
the thicker components is longer than the duration
of the imposed transient, some transient thermal
7
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CRYOGENIC
EXISTING GAS
TURBINE HEAT
AU TO MOTIVE, INDUSTRIAL, AND AIRCRAFT
•
HEAT EXCHANGER REGIME
EXCHANGER
REGIME
AIRCRAFT LIQUIDTO-LIQUID AND LIQUIDTO-AIR HEAT EXCHANGERS
TITANIUM HEAT
EXCHANGERS USED
IN CHEMICAL
PROCESS PLANT
ALUMINUM
AIRCRAFT
AIR-TO-AIR
UN ITS
AUTOMOTIVE
RADIATOR
I
-
CD
z
IN
u
SPACE
POWEROSYSTEMENICKEL
AND COBALT BASE
ALLOY HEAT EXCHANGER
U
a
z
CURRENT ST. ST. GAS
TURBINE RECUPERATOR
TITANIUM
ADVANCED GAS TURBINES
UTILIZING CERAMIC ROTARY
REGENERATOR AND CERAMIC
FIXED BOUNDARY RECUPERATOR
HOT PRESSED DENSE
SILICON NITRIDE AND
SILICON CARBIDE
dw
ADVANCED GAS TURBINE
Wz
TOR
(HIGH L COST E UN PITS)
\/
,
SUPER
ALLOYS
ALUMINUM
xl \
STAINLESS
STEELS
ALUMINA AT
LOW DENSITY
CERAMICS
COPPER
0
0
100
200
0
202 30
400
f
5006 00
400
600
800
1000
,700
1200
0
1400
900
1000°C
1800
'
2000 OF
TEMPERATURE
Fig. 3 Approximate operating boundaries for
various heat exchanger materials
stress will exist. For long life, it is obviously
mandatory to achieve thermal response compatibility
between the various elements of the core and parts
attached to the core.
Approximate operating boundaries for various
heat-exchanger materials are shown in Fig. 3. The
data are presented to give the heat exchanger designer an appreciation for the specific strength
regime of various materials. The operating islands
shown are very approximate, and, of course, for a
particular application the actual properties of the
candidate material must be examined in detail over
the operating temperature range. For future recuperator applications with gas inlet temperatures
in excess of 1400 F (760 C) (particularly at part
power where an automotive gas turbine will operate
for a high percentage of its life), it can be seen
that stainless steel is no longer applicable.
Nickel and cobalt base alloys do have sufficient
strength, but are prohibitively expensive for industrial and vehicular gas turbine application,
Clearly, at these high temperatures, only ceramics
offer a combination of high strength and low cost
for fixed boundary recuperators or rotary regenerators. The bands of data for ceramics are approximate only and cover a wide range of materials.
RECUPERATOR PERFORMANCE CONSIDERATIONS
Surface Geometry
The main theme in this paper is the projected
utilization of ceramic fixed boundary recuperators
for future high temperature gas turbines. In initial development units, it is postulated that experience gained from ceramic regenerator cores
will be directly applicable and that plate-fin
type of core construction will be used. In this
type of construction, heat transfer takes place
between parallel flat plates or channels which
incorporate finned secondary surfaces. Considerable flexibility is afforded with this type of
geometry in that the air and gas side passage
heights and fin form can be varied independently
to give an optimized unit, the actual choice being
8
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SURFACE AEFF'NCE
EXISTING ELATE-FIN RECUPERATOR
OFFSET SURFACES FOR VEHICULAR
AND INDUSTRIAL GAS TURBINES
OI II TA 0550!-. C=RAMIC SURFACE GLOMS TRIES CAMP(.CTNC Si
2500
8200
2000
.60
1500
-T.
-.
l; . .
.
FT ([ .TA FROM REF 5•0
.. FT)
OFF T r l;. M1i^^LLIC
U UPE
i
A
MINUTE EEL Me" FOR CURRENT
CERVIX
CERAMIC
SURFACES USED IN RG TART
REGENERATORS FOR VEHICULAR
GAS TURBINES
F
.. FT
'_D IN CUR3ENT P[ATL-FIN
I. FT (DATA FEOM REF. 8,
a
FR GJ 'COD REGIME FOR CERAMIC
RE
IND USTR I AL RS SOT R DI EU LAP AND
INDUSTRIAL GAS TURBINE APPLICATION
r^
/
1.
4
ADVANCED METALLIC RECUPERATOR
REGIME UTILIZING COMPACT
s RFC US DEVELOPED FUR
AEROSPACE HEAT UX HANGERS
Li
y20 (0.38)
8)
I
50
80"
S 0. G ,)
35 (
is
30) 1.18)
I:
O. 3)
`1 3)
0
0
7U
IS
X00 lilt
0.075
[1
'
iI
(0"5)/FI N S/INCH
7 i
,3
38)
25)1.0)
600
Sou
LOU
40 l l 57)
0.025
1000 328 1
I
FINS,MM)
N0.10
tN1
^1 ^)
°' 1 j
13 " A)SURFACES
i00 Sjb
150
FAsSAGE
HEIGHT,
NS
tn+ntl
O.IL
(51
I
CONVE NTI ONAL PEA
v^
0.30
0 40
07,6) )1
5 00.2 c)
) o. sS
,11
^1 5 D2)0
o.0
USED
IN
EARLY RECUPERATORS
GS LAR GE
I INDUSTRIAL
GAS TURBE INES
-
_ «. _}
U.G UTYPICAL DE
N
}
tl1'I
,.
_ t,
fir• .__ _
I
-
REG I ME FJ.,
75
9 0)
z 0.
COMPACT PLLIN
'
G CERAMIC JIFR26
ar
j
Fig, 4 Heat exchanger surface compactness spectrum for plate-fin recuperator geometries
-
^--[
_ _ _{ l Ef ICS+L k IME F03 ^^
FF-,=T M THL_IC
A
p
p lot of the range
g of surface compactness
p
I
°
loo
'v
+
I
i - CUPCR:,TOR OUT GEOMETRIES
FIN SURF"(
dictated by the compactness one can achieve within
established design and cost limitations. --
,_
r^
.+1
00
^a^YNOLDS NuMeFR
for plate-fin surface is shown in Fig. 4. Early Fig, 5 Comparison of heat transfer and friction
plate-fin units had 9 values around 300 ft 2 /ft 3characteristics for representative ceramic and
(985 m 2 /m 3 ) and this compares with typical auto- metallic plate-fin surface geometries
mobile radiator surface geometries. Existing industrial recuperators have /3 values in the 500 to
direction of flow. This provides periodic inter600 ft 2 /ft 3 (1640 to 1970 m2 /m 3 ) range, and in
ruption
of the boundary layers and thereby in
advanced metallic units this will probably increases
the heat-transfer coefficients. With this
2
The
crease to around 800 ft /ft 3(2630 m2 /m3 ),
essentially
strip-fin configuration, it is feasible
approximate regime for current ceramic surfaces
to have very short flow length fins and thus very
used in rotary regenerators, with /3 values aphigh heat-transfer conductances. Pasic data for
proaching 2000 ft 2 /ft 3(6560 m2 /m3 ), is also
representative recuperator offset fin surfaces
shown in Fig. 5. Attractive recuperator core
from Kays and London (8) are shown in Fig. 5.
dimensions cannot be realized using surfaces this
The operating range shown is fairly typical of
dense, and the projected regime for ceramic rethe Reynolds number range of interest for many
cuperators is initially felt to be with plain
gas-to-gas heat-exchanger applications. It extriangular surface geometries with /3 values around
tends from well down into the laminar region out
1400 ft 2 /ft 3 (4600 m2 /m3 ). High heat-transfer
to the transition region. Very compact heat-excoefficients are not necessarily synonymous with
changer surfaces are rarely used at high Reynolds
high surface compactness, and this is discussed
number; in fact, as the surface is made more comin the following,
pact by employing smaller hydraulic diameter passages, the tendency is to push the operation
Surface Characteristics
range to lower values of Reynolds number,
In most metallic compact gas-to-gas heat
One of the characteristics of very compact
exchangers offset type of finned surfaces are
heat exchanger surfaces with high blockage factors
utilized for minimum volume and' weight (not necesis that the core design tends toward short flowsarily minimum cost, however). With this type of
lengths and large frontal areas, These designs
surface, the fins are systematically pierced in
the direction of flow and then offset normal to the are difficult to integrate with the turbomachinery,
9
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often have flow maldistribution effects, and have
adverse part—load effectiveness characteristics
because of axial conduction effects, and these
will be discussed in the following section. From
Fig. 5, it can be seen that at the very low Reynolds numbers, i.e., well in the laminar flow
region, the interrupted offset type of fin surfaces
tend to lose their heat transfer to friction power
advantage over the smooth passage plain type of
geometry. Heat transfer and friction data for
plain triangular ceramic compact geometries as
established by Howard (9) are shown for comparison in Fig. 5. At the lower optimum Reynolds
numbers for these very compact surfaces, it can
be seen that reasonable values of heat-transfer
modulus (j) can be realized. In the fully developed laminar flow regime, the heat-transfer
coefficient (h) becomes constant and independent
of velocity; thus, for extremely compact surfaces,
there is no gain in attempting to provide boundarylayer interruptions in the form of waves or offsets. The smooth plain passages are also easier
from the manufacturing standpoint, and will probably be used in early ceramic recuperator cores
which may be injection molded, extruded, or roll
formed. In this paper, emphasis is placed on the
plate-fin type of geometry since material forming
experience gained in extensive rotary regenerator
development programs should be directly applicable
for early ceramic recuperator configurations.
NOTE; CURVES DRAWN FOR DESIGN POINT EFFECTIVENESS OF 0.85
AND CORE PRESSURE LOSS (AP/P) . 6.0 PERCENT
1.00
0.91
0.91
0.91
0.9
0.9 1
0.81
0.8,
0.8
0.8
0.8
0.T
0.7
0.7
0.7
ME■■■■■■■■■ ■N■
■■■■■
0.70 0
10 20
30
40
■
50 60
■SOME■■
70 80 90 100
RECUPERATOR AIRFLOW, PERCENT
Fig. 6 Typical curves of recuperator part-load
effectiveness showing influence of axial conduction for compact surface geometries
-
Performance
As mentioned earlier, a flat SFC-power curve
is essential for economic operation of vehicular
gas turbines, At part-load, the aerodynamic components operate at reduced efficiency values so a
high.level of heat exchanger effectiveness at part
power is essential. In many heat exchangers, the
assumption that heat conduction in the separating
walls is only in the direction normal to the flow,
and can be neglected in the direction parallel to
the flow, is reasonably valid. In counterflow gas
turbine recuperators of high effectiveness, however, the temperature gradient that exists in the
matrix is at a maximum value, as the hot end of
the core is essentially at maximum fluid temperature, and the cold end is close to the minimum
fluid temperature. The flow of heat through the
material in this type of situation results in a
loss of heat from the hot end and an addition of
heat to the cold end, both of which have adverse
effects on heat-exchanger performance. Under
these conditions, there will be continuous heat
conduction in the axial direction, and this will
inevitably result in a decrease in the effectiveness of the recuperator. The problem gets more
severe for units of very high effectiveness level,
and a comprehensive study was carried out by
Bahnke and Howard (10) to determine the effects
of axial conduction on heat exchanger performance.
A non-dimensional conduction parameter, %, may be
developed which can be used to describe this axial
conduction behavior in heat exchangers. The parameter is defined as follows:
X=
KA
x
CL
min
and for preliminary design purposes the loss of
effectiveness can be approximated as A e /e = T.
This simple relationship is only a crude approximation of the actual effect of axial conduction,
and for final recuperator designs, the data given
in the foregoing reference should be used. Utilization of very compact surfaces results in units
with a large frontal area and a small flow length,
both of which have an adverse effect on performance
because of axial conduction. Since the conduction
parameter is inversely proportional to flow, it is
to be expected that performance degradation will
be worse at part-flow conditions, and this can be
seen in Fig. 6 for a recuperator with a design
point effectiveness of 0.85. For the metallic
units, it can be seen that the part-load effectiveness falls off significantly as the surface compactness is increased. Projected reduction in
core volume, by utili:ing very compact plate-fin
surface geometries, is, therefore, limited in the
10
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case of metallic units for automotive gas turbines
because of the poor part power effectiveness.
Lowering the value of X by reducing the metal
cross section area is limited because the plate
and fin thicknesses are essentially dictated by
structural criteria. The other alternative is
selecting a material with low thermal conductivity,
although this represents a procedure quite alien
to most heat-exchanger design ground rules. The
effect of this is clearly seen in Fig. 6 for a
representative compact ceramic core design that
exhibits an attractive off-design effectiveness
characteristic,
METALLIC RECUPERATOR DEVELOPMENTS
Existing plate-fin recuperators, with construction methods essentially inherited from aircraft heat exchanger practice, have performed
satisfactorily and remained structurally sound in
limited operation on industrial and vehicular
types of gas turbine. It was apparent, however,
that their costs, even in high volume production,
were far too high for the projected vehicular and
industrial gas turbine markets of the late 1970 1 s.
Development activities seem to be in progress in
three areas, (a) further developments of the large
recuperators for heavy industrial turbines, (b)
continuing development of the aircraft heat exchanger type embodying efficient offset fin
surfaces, and (c) simple formed plate primarysurface designs for very low cost. These three
areas of development are briefly outlined as
follows.
Recuperator for Large Industrial Gas Turbines
Utilization of mild steel for gas turbine
recuperators has to date been limited to the large
industrial type, as described by Wolfe (11), where
many years of satisfactory operating experience
have been accumulated with gas inlet temperatures
up to about 1000 F. The main problem with mild
steel, of course, has been the poor oxidation
resistance at elevated temperature. Increased
temperature capability for this type of unit can
be achieved by utilizing chromized mild steel.
It would seem that use of stainless steel for
these heavy units negates their main advantage of
low initial cost. To make the size (and weight)
of these recuperators more compatible with the
compact nature of modern industrial gas turbines,
the use of more compact heat-transfer surfaces is
necessary.
Compact Seconda ry-Surface Plate-_Fin_ Recuperators
The predominant mode of failure in existing
compact plate-fin recuperators has been low cycle
fatigue resulting frorrr high thermal stresses in
the core during transient operation. In these
units, where there is a considerable difference
in the thermal inertia between the solid sealing
bars and the thin foil plates, the resultant temperature difference could cause stresses as high
as 90,000 psi (622 kpa) for a typical 5 -sec engine
start. To alleviate the stresses in existing
units, the tube plate thickness has been increased
to as much as 0.016 in. (0.40 mm); however, for
high volume production, this would be unacceptable
because of the very high material cost.
In new low cost designs, the need for solid
sealing bars has been eliminated and the high
pressure end seals are integrally formed from the
tube plates themselves. This alleviates the high
stress situation during transient operation and
results in the adoption of reduced thickness fins
and plates, hence, lower material cost. Other
low cost features, such as integrally forming the
high pressure manifolds from the parent tube-plate
and eliminating the cost of welding on separate
ducts are also being incorporated in these new
designs. For this type of unit to find acceptance,
two high cost areas that must receive attention
are the methods of forming and brazing the heattransfer elements.
The surface geometry compactness in these
new units will probably be essentially the same as
in existing recuperators so that the volume of the
core will be of the same order. As mentioned
previously, the offset type of interrupted fin
surface exhibits good heat transfer and friction
characteristics and is being utilized in advanced
recuperators. The performance of these fins,
especially the friction data, is affected by the
thickness and character of the fin leading edge.
These fins are currently formed by a machine cutting die punch press that inevitably leaves a
slightly bent and scarfed fin at the leading and
trailing edge. The degree of scarfing depends
upon the fin material and the sharpness of the
cutting tool. A few ten-thousandths of an inch
of scarfing can have a considerable adverse effect
on the characteristic of the fin. This effect becomes more noticeable as the compactness is increased, and very much higher friction factors are
observed with usually not much effect on heat
transfer, The associated higher (f/j) values
imply cores with larger frontal areas and smaller
flow lengths, both of which have an adverse effect
on part power effectiveness because of axial conduction effects. The current die punch forming
operation is a slow process (inches per minute)
and would not be economically acceptable for high
volume, low cost recuperator manufacture. Corrugation gear rolling techniques as used in the
11
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automotive radiator industry have the potential which was designed and developed in England. In
for generating fins at a rate of several hundred this design, the basic element is a single plate
feet per minute. Early indications are that for with a rhomboidal area of longitudinal triangular
gear-rolled compact offset fins, in stainless
depressions in the counterflow portion. The basic
steel, the scarfing is a problem and that tool life unit consists of two of these plates, which when
is limited. An alternative to offset fins are the laid face to face form a simple tube functioning
wavy or herringbone type which are used extensively element. These basic elements are stacked up to
in heat exchangers in both this country and in form the core with welded or mechanical configuraEurope. The heat-transfer augmentation, by bounda- tions at the periphery to form the high pressure
ry disturbance, is slightly less efficient than an seals, More advanced designs with compact wavy
offset surface and will result in a slight heat form depressions in the counterflow section have
exchanger weight penalty. The wavy fins can be performance characteristics (heat-transfer rate
gear rolled at very high feed rates and are thus per unit of volume) very similar to plate-fin decheaper to form than offset fins made by the con-
signs embodying offset fin surfaces. The main
ventional die punch machine. The strength of the problem with the prime-surface type to date has
wavy fin is much greater, and this can be used to been the structural integrity during thermal
advantage in future compact metallic recuperators. cycling.
The final joining of the matrix elements can A unique prime-surface recuperator developed
be done using brazing or welding techniques. All for a high-pressure gas turbine has been reported
current plate-fin cores are designed for brazing,
by Engel (12). This annular unit consists of an
which can be carried out either in a vacuum or in assembly of circular hydroformed plates which are
a hydrogen atmosphere to remove oxides and provide held together with axial tie bolts to form the
an uncontaminated environment for braze flow. Con- recuperator structure. The simple hydroformed
ventional vacuum furnace techniques of brazing are plates have embossed dimples and depressions to
not attractive for large recuperator cores because improve heat-transfer performance. The unit is
of the long cycle time which ties up capital equip- made self-manifolding by means of welding between
the plates locally to form a simple headering arment and adds to the cost of the heat exchanger,
rangement. This compact recuperator integrates
A forced convection hydrogen brazing process has well with the turbomachinery to give an engine of
been developed which substantially reduces the braze cycle time, Currently used braze alloys extremely high specific power density (kw/m 3 ).
If all the structural problems can be reare expensive ( 3, fl/lb — ^6.6 to 18.8/kg), and
solved,
prime-surface designs should offer a low
in some recuperator designs, they can represent cost
solution
since they do not require expensive
up to 10 percent of the total material cost. In
fins
and
braze
alloy and may be designed to elimihigh volume production, techniques must be de- nate
the
costly
and time-consuming furnace brazing
veloped to minimize the amount of braze alloy re- operation.
quired, and this probably necessitates applying
the alloy to the fin corrugation nodes rather than
FUTURE TRENDS FOR METALLIC RECUPERATORS
to the tube plates.
At this stage of recuperator development,
While the main emphasis in this paper has
it is not obvious that the plate-fin type of
secondary surface design offers the lowest cost
been on the ultimate utilization of ceramic resolution for high volume gas turbine application,
cuperators for vehicular and industrial gas turbines, it is realized that a considerable developand alternate configurations are outlined as fol- lows.
ment period is necessary to demonstrate their
viability, since the many problems associated
Prime Surface Recuperator Designs with ceramic rotary regenerators have not yet
From the standpoint of minimum cost, a re- been fully resolved. There will not be a sudden
cuperator design should consist of a stack of
change to ceramic recuperators, but rather a
mechanically bonded heat-transfer plates, with a gradual transition from metallic to ceramic units
counterflow fluid path, embodying integral air as the technology becomes available. However,
manifolds and thus obviating the need for complex
turbomachinery advancements will require a heat
secondary surfaces and expensive brazing and weld- exchanger capable of withstanding much higher gas
ing operations. Prime-surface geometries have
temperatures than those experienced in current
been used for several gas turbine recuperators
recuperators (approximately 1200 F, 650 C).
over the last 20 years. Perhaps the unit which
For automotive gas turbines in particular,
has had the most publicity, without much direct
many thousands of hours running must be carried
gas turbine application, is the Nicholson type, out to demonstrate engine performance, emissions,
12
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CURVES BASED ON:
• METALLIC PLATE-FIN RECUPERATOR
• CORE (CP/P) = 6 PERCENT
• COUNTERFLOW CONFIGURATION
•COMPRESSOR PRESSURE RATIO • 7.0 1900'F(I038°C)
• TURBINE INLET TEMP
W
SECONDARY
AIR
AIR
GG
P
COMBUSTOR
LOW PRESSURE RATIO FAN FOR
SECONDARY AIR INJECTION
TO GIVE REDUCED GAS TEMP.
INTO HEAT EXCHANGER
H%
TOAD MIXED CONDITIONS
GASD AT RECUPERATOR INLET
T
00
0.95
EFFECTIVENESS CURVE WITH AIR
INJECTION TO GIVE CONSTANT
1100°F (593 ° C) MIXED GAS
TEMPERATURE INTO RECUPERATOR
0.9E
0.9 4
0.9C
I
0.9€
t'iiiIIi
0.8£
w
^
NORMAL PART LOAD
EFFECTIVENESS CURVE
0.8E
0.84
x53
=LLB
^t.40
1.20
00
800
EGINEL PART POWER T EI MPERAT UI RE FOR
EGENERATIVE
IRE CYCLE W TH VAR ABLE
R ECENET
POWER TURBINE G[OMETRY TO GIVE FLAT
SFG CURVE OVER WIDE PO WE0. SPECTRUM
900
1600
800
°F
00
N
S€CONDARY AIR INJECTIO(S93°C)
TO GIVE C*STRNT 1100 F
MIXED GAS TEMPERATURE
INTO RECUPERATOR
700
7200
i
-
600«-_-__ __ _
000
0
10
20
30
40
_
-- - - -
50
60
70
80
90 100
ENGINE POWER, PERCENT
Fig. 7 Effect of secondary air injection at turbine exit to give reduced part power gas temperatures for metallic recuperators
and reliability. Over the development period of
several years, many hundreds of engines will be
built, all of which will require heat. exchangers.
There is obviously an incentive for the metalforming industry to develop a cost-effective recuperator capable of withstanding higher temperatures, The three areas that require attention
for a future high temperature metallic recuperator
would appear to be materials, construction methods,
and cycle control, and these are briefly discussed
as follows.
Materials
The limitations of mild steel have been
discussed in a previous section; however, with
its low cost of 15 to 20 cents/lb (33 to 44 cents/
kg), it should not be completely discounted. Improved oxidation resistance can be achieved by
various surface treatments, such as chromizing
and aluminizing. Above 1000 F (540 C), there is
not much strength left in mild steel, and this
necessitates thicker fin and plate section sections, This aggravates the part power axial conduction problem, and integration with the turbo-
machinery becomes more complex because of the
heavier recuperator core. Use of mild steel in
the lower temperature portion of the core may be
a solution to the high cost problem, and this will
be discussed as follows.
Type 347 stainless steel is an excellent
recuperator material and has been used widely in
these applications. It has good creep strength
and hot corrosion resistance for long-time service
up to temperatures of about 1300 F (700 C). In
the thin-foil form, the cost of this material is
1 to $1,5/lb ($2.2 to $3,3/kg). For a compact
plate-fin recuperator embodying offset fin surfaces, the weight of the matrix is in the order
of 70 lb/lb/sec for an effectiveness of 0.85.
For a 150-hp (112-kw) automotive gas turbine, the
metallic recuperative core will weigh in the
order of 100 lb (46 kg) and even at the low end
of the scale, the material cost (excluding braze
alloy, forming and fabrication, etc.) will be
$100. For an automotive type power plant, a recuperator cost goal in the order of half this
value is postulated.
The superalloys, such as Inconel 625,
Incolo, and Hastelloy X, have excellent
properties at elevated temperature and would
probably be suitable for heat exchanger gas inlet
temperatures 'slightly over 1500 F (815 C). At
costs of $5/lb ($11/kg) in the thin-foil form,
these materials are just too expensive for commercial recuperator application, Possible combinations of the foregoing materials in a single
recuperator matrix have been considered, and this
aspect of heat exchanger construction is outlined
as follows,
Construction
To establish the most cost-effective recuperator for a given gas inlet temperature and
to find the most economic way of providing temperature uprating potential, (for a given life
requirement of 10,000 hr say) thought has been
given to bi-metallic and tri-metallic constructions. With a counterflow configuration the
axial temperature gradient is in the flow direction so that fin and tube plate materials can be
varied through the core in the flow direction.
In this manner, a core with mild steel, stainlesssteel 347, and Inconel 625 could be considered.
For some applications, bi-metallic designs (i.e.,
mild steel and stainless steel) would be satisfactory, but for high temperature application,
the hot end of the unit could be made from Inconel
625. Utilizing different fin materials in the
core is simple, but the problem of tube plate
material transition has to be resolved. One possibility involves butt welding the materials at a
13
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Fig. 9 Typical ceramic disk and drum regenerator
cores (by Courtesy Corning Glass)
Fig. 8 Simple cross-flow plate-fin ceramic heat
exchanger module (by Courtesy American Lava Corporation)
thickness of 0,012 to 0.016 in. (0.03 to 0.04 mm)
and then rolling down to the required thickness
to ensure good metal flow in the transition section. The actual materials selection will be
determined after a detailed structural analysis
to ensure that stress levels associated with the
differing expansion coefficients are acceptable.
An interesting side note here regarding bi-metallic components is with reference to commercially
available hacksaw blades. These are manufactured
in high volume production with the actual saw
teeth of high quality steel welded to a strip of
low quality steel which forms the main body of the
blade. The strip welding operation is done in an
automated process with a linear speed in excess
of 20 fpm (0.10 m/sec). The bi-metallic on trimetallic type of construction can be utilized in
both plate-fin and prime surface designs to lower
the overall cost of the unit while at the same
time giving the metallic recuperator a higher
temperature capability.
Another feature that can be built into a
recuperator to give high temperature capability
during transient operation is a thermal buffer at
the gas inlet face. This device, essentially a
thermal capacitor, absorbs heat during overtemperature periods such as engine starts, and by
taking the "spikes" out of the temperature profile, protects the recuperator matrix. Ceramic
cellular structures with their high specific heat,
high temperature capability, and low flow resistance, are well suited as thermal capacitors for
metallic recuperator cores.
Engine Cycle Considerations
In several of the two-shaft vehicular gas
turbines under development a variable geometry
power turbine nozzle is used to keep the turbine
inlet temperature constant and thus give a flat
SFC curve over a wide power range. As discussed
previously this mode of operation results in a
very high heat exchanger gas inlet temperature.
Controlling the turbine exhaust temperature to an
acceptable value at low power levels (particularly
for automotive gas turbines) to protect the metallic recuperator results in a SFC penalty. As
turbine inlet temperatures increase over the years
with the utilization of ceramic turbomachinery
components the higher exhaust temperatures will
necessitate a ceramic recuperator or regenerator.
Again, to protect the metallic recuperator
a control mode to keep turbine exit temperature
constant over the power spectrum could be established, but this also would result in a SFC
penalty which may not be acceptable from the
standpoints of emissions and vehicle operating
economy, With a constant turbine inlet control
mode secondary air could be injected into the
gas stream before the heat exchanger gas inlet
face as shown in Fig. 7. With this arrangement
a constant recuperator gas inlet temperature can
14
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be maintained, and this would allow use of the
lower grade alloys in the heat exchanger. By
changing the capacity rate ratio in the recuperator,
a more attractive part-load effectiveness curve can
be realized as shown. However, the air temperature
into the combustor is lower because of the reduced
recuperator extreme temperature difference, and
this has an adverse effect on SFC. The secondary
air pressure is only slightly above atmospheric
pressure, but the pumping power and associated
mixing loss, together with the increased gas side
pressure loss in the recuperator at part power,
must also be factored into the cycle analysis to
see if this means of protecting the recuperator
is viable. For military vehicles this system may
have merit because the lower gas leaving temperature will result in a substantially lower infrared
signature.
Fig. 10 Ceramic plate-fin counterflow recuperator
module (by Courtesy Advanced Materials Engineering
CERAMIC RECUPERATOR DEVELOPMENTS
Ltd.)
Very little work directly related to application of fixed boundary ceramic recuperators for
gas turbines has been reported in the open literature. Some development efforts in related high
temperature fields have been carried out, and an
example of a simple cross-flow plate-fin ceramic
heat exchanger core is shown in Fig. 8. For gas
turbine application, the high effectiveness requirement (for vehicular units in particular)
virtually necessitates a counterflow arrangement,
and this will be discussed later.
Work has been reported by Miwa (13) on the
use of a tubular ceramic air heater in a novel
equi-pressure cycle gas turbine designed for burning residual fuels. In other high temperature
fields work has been reported by Browning (14) on
the development of a tubular ceramic heat exchanger
constructed from alumina. Ceramic heat exchangers
are employed in industrial plant, such as small
glass melting tanks, and soaking pits in steel
rolling plants for preheating air up to about
2000 F. In the field of MI-ID power generation,
considerable development efforts have been made
in large ceramic heat exchangers and these have
been reported in depth by Heywood (15), Recent
studies on the use of High Temperature Gas Cooled
Reactors (HTGR) for process heat applications have
shown the need for extremely high-temperature heat
exchangers, and the possibilities of ceramic designs have been reported in depth by Hyrniszak
(16).
In the gas turbine field, most of the ceramic heat exchanger development work, in the form
of rotary regenerator disks, has been done by
Corning Glass Works. Significant regenerator advancements have been made in the last few years
for small vehicular gas turbines. Early regenerator configurations utilized a drum matrix, but
it was found that disks could be better integrated
with the turbomachinery, had simpler seal arrangements and drive mechanisms, and were superior from
the structural standpoint. A view of a drum matrix
and two compact ceramic regenerator disks is shown
in Fig. 9.
The most encouraging work, reported in the
open literature on ceramic recuperator developments, has been done by Advanced Materials Engineering in England as described by Stoddart (17).
Fig. 10 (taken from this reference) shows a compact ceramic counterflow recuperator module. Made
from reaction bonded silicon nitride using flexible
thin film forming techniques, the construction of
this small module represents a significant step
toward the ultimate goal of utilizing a full-size
ceramic recuperator on a high temperature gas
turbine.
Various aspects of ceramic recuperator material, surface geometry, and performance requirements are briefly discussed in the following
sections. The actual material specification can
only be defined after in-depth chemical and
structural analyses. The material must be nonporous, be readily formable into thin wall structures, have good thermal shock capability, and
have sufficient strength at elevated temperature
to provide pressure containment. To be acceptable
for high volume automotive gas turbines, the basic
material must be available in large quantities at
low cost. In this paper only plate-fin types
of surface geometry are discussed. It is felt
that experience gained from rotary regenerator
15
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CURVES BASED ON
EFFECTIVENESS = 0.85
MATRIX (AP/P) = 6 PERCENT
FIN AND PLATE THICKNESS = 0.005 IN.(0.127 MM)
CERAMIC K= 1.0 BTU/FT. HR . F (1.73 W/M C)
PLAIN TRIANGULAR FINS
COUNTERFLOW CONFIGURATION
°
70
°
PLATE-FIN GEOMETRY
m
3
0 3
u
PASSAGE HEIGHT
0.04 IN.
(1 MM)
30 FINS/IN.
(1.18 FINS/MM)
3
10
5 250-
(0.39)
Li
,; 4 200-
6c
CURVES BASED ON
EFFECTIVENESS = 0.85
MATRIX (0P/P) - 6 PERCENT
CERAMIC K
1.0 BTU/FT.HR F
(1.73 W/M C)
COUNTERFLOW CONFIGURATION
3 150
FINS/INCH
(FINS/MM)
5(
c,
LI
30
(1.18)
4C
X
MZ
0
U
a
1 o.z
0
2
( .5)
2C
3 0.6 KG/SEC
0 2 0.4
U0
.1
z
1 25
J
50
(2.0)
0
°
3d 4 0.8
40
( 1 .57)
3 3(
°
--
2 50-
20
79) -
0.08
(2.0)
0.06
0.2
0.3
.004
.00B
.012
MATERIAL THICKNESS, 6 INS.
0.4 MM
.o16
Fig, 11 Effect of ceramic surface geometry on
plate-fin recuperator weight
(15) 0.04
(1.0)
PASSAGE
HEIGHT,
INCHES
(MM)
Nj
0.02
(0. 5)
30 fins/in. (1.2 fins/mm). To examine other
variables, these geometry values were assumed,
and they correspond to a compactness (f3) in the
order of 1400 ft 2 /ft 3 (4600 m 2 /m 3 ). In compariFig. 12 Effect of ceramic material thickness on
son, current ceramic regenerator surfaces have
counterflow recuperator size
passage height around 0.020 in, (0,50 mm) and
surface compactness values approaching 2000 ft 2 /
ft3 (6560 m 2 /m 3 ). Since the actual material
development programs will be applicable in the
areas of material forming, assembly, and firing.
thickness will be determined by structural and
Most of the ceramic regenerator disks in producforming considerations for the selected ceramic,
tion'utilize plain triangular plate-fin geomthe influence of plate and fin thickness on core
etries.
size, for the arbitrarily selected geometry, is
shown in Fig. 12,
Because of - the uncertainty of the final
ceramic material specification for fixed-boundary
One of the early problems postulated for
heat exchanger application some simplifying assump- ceramic secondary surface heat exchangers was the
tions were made in the preliminary thermal analyexcessive volume because of the low material
sis. A material density of 140 lb/ft 3 (2243 kg/
thermal conductivity. If existing metal surface
m 3 ) and a thermal conductivity of 1.0 Btu/ft-hr F
geometries were used, this would be the case be(1.73 w/m/c) were assumed. For most vehicular gas
cause of the low fin efficiency. For the very
turbine applications, a maximum recuperator efsmall fin lengths associated with compact ceramic
fectiveness value of 0.85 has been assumed. This
surfaces, thermal conductivity effects on the
value corresponds to an effectiveness of about
specific size of the recuperator are insignificant.
0.88 for regenerative variants, taking into acA comparison of typical surface geometries for
count seal leakage and carryover losses in the
metallic and ceramic plate-fin recuperators is
cycle calculations. For an assumed material
shown in Table 1. For an assumed ceramic thermal
thickness of 0,005 in. (0.125 mm), the effect of
conductivity of unity (regarded as a very low
fin pitch and passage height on counterflow core
value for representative ceramics, but used to
weight is shown in Fig. 11. From the thermal
emphasize the point), it can be seen that a fin
analysis, it was noticed that excessive core
efficiency in excess of 70 percent can be refrontal areas resulted for passage heights below
alized and with the very small effective fin
abour 0.040 in. (1 mm) and for fin counts above
lengths, the overall surface efficiency is es16
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Table 1 Counterfiow Recuperator Plate-Fin Surface
Geometry Comparisons
1 Core Material
Metallic
Surface Type
Fluid stream
Ceramic
Retangular
Triangular
Offset Fins
Plain Fins
Air
Gas
1
Air and Gas
Passage height, in. (mm)
0.10 (2.54)
0.15 (3.81)
0.040 (1.0)
Fins per inch (per mm)
20 (0. 79)
16 (0.63)
30 (1.18)
Fin thickness,
in. (mm)
0.004 (0. 10)
0. 004 (0.10)
0. 005 (0.127)
Plate thickness, in. (mm)
0.006 (0.15)
0.006 (0.15)
0. 005 (0.127)
0. 0052 (1. 58)
0.007 (2. 13)
0.0022 (0. 67)
Hydraulic diameter, ft. (mm)
1
Compactness,$ ft 2 /ft 3 (m2 /m3)
680 (2231)
508 (1667)
1400 (4593)
Compactness, cx ft 2 /ft 3 (m2/m3)
260 (853)
290 (951)
620 (2034)
Total compactness,( Tft 2 /ft 3 (m2/m3)
550 (1805)
0. 34
0. 51
0. 34
Fin area/ Total area
0. 676
0.70
0. 648
11.0 (19.0)
Material p, lb/ft 3 (kg/m3)
Fin effective length
e'
in. (mm)
Typical heat transfer
coefficient
i
h, Btu/ft hr. F (w/m
C)
Fin
n pparameter m = Fin efficiency
500 (8009)
i
2h
r, f
Surface efficiency
^10
f
sentially the same as for representative metal
fin surfaces,
Specific sizes and weights of compact
ceramic plate-fin recuperators for vehicular and
industrial gas turbines are shown in Fig. 13,
This type of curve array, particularly useful
for engine preliminary design studies, clearly
illustrates what influence effectiveness and
pressure loss have on the core size and weight.
It should be restated here that the data given
in Figs. 11 through 13 are of a very preliminary
nature and are included more to show trends than
0. 05 (1.27)
0. 075 (1. 90)
50 (284)
50 (284)
I
1240 (4068)
Free flow area/frontal area
Material K, Btu/ft hr °F(w/m°C)
1
1.0 (1.73)
140 (2243)
0. 026 (0. 66)
50 (284)
165
165
490
0.865
0.750
0.740
0.910
0.825
0.832
1
specific values. Once material evaluations have
been carried out and the formability aspects of
various geometries have been resolved, more realistic performance estimates can be performed
for actual engine operating conditions.
The structural design of the plate-fin core
matrix must consider combined stresses due to
thermal stresses incurred by steady-state and
transient temperature differentials and stresses
due to internal pressure loadings. Materials,
such as silicon nitride and silicon carbide, are
brittle materials and application of these materi-
17
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CORE WEIGHT INCLUDES END SECTIONS
AND INTEGRALLY FORMED AIR MANIFOLDS
BUT DOES NOT INCLUDE DUCTS AND
SUPPORTING STRUCTURE
• COUNTERFLOW CONFIGURATION
GAS
O UT4
•CERAMIC PLATE-FIN SURFACES
30 FPI (1.18 FPM), 0.04 IN. (1 MM)
•COMPRESSOR PRESSURE RATIO = 7.0
AIR
IN
°
TURBINE INLET TEMP. = 1900°F (1038 C)
u
SECTON FLOW
SECTION
I)
4C
1
AIR
M
U1
34
32
2(
21
21
nor i►^ ^
ti ^►
sU
AIR
OUTLETS
TYPE A
AIR
IN
/
i
GAS
OUT
♦
Q
//J
\
)
INTEGRALLY
FORMED AIR
MANIFOLDS
—
GAS
IN
TYPE q
CERAMIC OR
METALLIC
MANIFOLDS
AIR
t OUT
21
21
11
if
11
.1•
'Lipid
• TYPICAL DESIGN POINT FOR
AUTOMOTIVE AND INDUSTRIAL
GAS TURBINE RECUPERATOR
Fig. 13 Specific sizes and weights of compact
ceramic plate-fin recuperator
als to gas turbine recuperators will require special design criteria to minimize joining stresses
and impact problems. Ceramic materials are much
more prone to failure by tensile stresses than compressive stresses, and thus thermal gradients that
give rise to tensile stresses are of greatest
concern. Although the thermal expansion of ceramics are much less than stainless steels, careful
attention must be given to the core design to
ensure thermal inertia compatibility of the fins,
plates, and integral manifolds, and thus avoid
high stresses during rapid thermal cycling. In
recuperator matrices, the forces affecting their
structural integrity are mainly static, which can
be interpreted as an advantage for brittle materials. For ceramic recuperators, the material
thicknesses in the core are based on the appropriate strength to rupture, and extensive materials
development efforts are in progress to establish
allowable working stress levels that can be
factored into future heat exchanger designs. It
is important to remember that knowledge of the
basic material properties must be supplemented
by actual data taken from representative module
geometries subjected to realistic engine simulated tests, to verify their suitability in a
AIR
PASSAGES
PLATE-FIN
CO NS T UCTION
GAS
PASSAGE
GAS
IN
A
CERAMIC
TRANSITION FROM \
W
CORE
CERAMIC CORE TO
METALLIC DUCTS
SHOULD BE IN
SIMPLE CIRCULAR
SECTIONS Q OR Q VIEW SHOWING POSSIBLE MANIFOLDING
ARRANGEMENT FOR COUNTERFLOW
CERAMIC RECUPERATOR
WY
TYPICAL
CERAMIC
PRIMARY
TYPE OF
A
A
G
A
G
A
A
WY
WY
b. PRIME
SURFACE
CONSTRUCTION
SECTION X-N THROUGH
CORE SHOWING EITHER
OR SECONDARY SURFACE
CONSTRUCTION
Fig. 14 Ceramic recuperator core configurations
gas turbine environment. For vehicular application, a simple gas-side ducting arrangement is
necessary to keep the cost of the recuperator
installation low. A simple "hot-box" housing,
perhpas formed in the main engine casting, could
be used to support the removable ceramic core
element. Careful design of the ceramic core
support with this arrangement is necessary to
ensure that vibratory excitations from the turbomachinery and vehicle do not cause high stress
levels.
Formability aspects of ceramic structures
for gas turbine recuperators have yet to be resolved, particularly low cost methods which are
essential for vehicular applications. Recent
developments in silicon nitride fabrication techniques have shown that flexible sheets in the
green silicon form can be made. These plasticized sheets offer new possibilities in cold or
hot forming by extrusion, corrugating, rolling,
and moulding. Shaping of the part takes place
in a plastic state allowing for a high forming
accuracy which can be maintained during the
process of removing the organic binder and the
following nitridation. An important consideration
for recuperators is that silicon nitride can be
made sufficiently impermeable. With the ceramic
18
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in a plastic form high volume production methods
such as extrusion and injection moulding are possible.
The final form of the recuperator will be
determined only after an extensive development
period, but several suggested configurations are
shown in Fig. 14. These arrangements reflect
metallic recuperator forms but include features
desirable in a ceramic design, i.e., counterflow
arrangement, integral air manifolds, etc. Since
it is not obvious yet which is the best surface
geometry type in the ceramic form, the configurations could embody either prime or secondary
surfaces.
For the candidate material detailed stress
analyses must be carried out in the manifold areas
since this appears to be a critical area. As in
most recuperators, there is a conflicting design
requirement in that the air manifolds must have
sufficient thickness to withstand the internal
pressure loading, and yet have thermal inertia
compatibility with the thin-walled core structure
to avoid high thermal stresses during engine
transients. Providing smooth transitions from
one section to another to minimize adverse
temperature gradients is an important design
criteria. Careful consideration must also be
given to the transition from the ceramic core to
the metallic engine ducts since metals and ceramics
exhibit different behavior as far as conduction of
heat and thermal distortion is concerned. Preferably, this transition (perhaps a mechanical bond)
should be in simple circular duct section. If
leaks occur in ceramic cores, there is a possibility that these could be simply repaired by
flushing the matrix with a ceramic slurry and
refiring the core.
As outlined by McDonald (18, 19) fouling
and fire problems have occurred in compact metallic plate-fin recuperators. With increasing
emphasis on cleaner combustion for low pollutant
emission, the problem of recuperator fouling, even
in very compact surfaces, will be substantially
reduced. In future recuperators, surface temperatures will be much higher, and this combined
with the fact that the coefficients of expansion
of fouling products and the ceramic core are
substantially different, implies that deposits
will not accumulate in the matrix during operating
periods of poor combustion efficiency. Most of
the fires reported in current recuperators (with
gas inlet temperatures around 1200 F (650 C)]
were due to either inadequacies in the engine
design, which allowed excess fuel to become
trapped in the ducts and heat exchanger matrix,
or to control malfunction. Limited test data
shows that as the temperature level increases
[1400 to 1500 F (760 to 815 C)J, the danger of a
matrix metal fire is increased. Ceramic heat exchangers will not burn or ignite in the presence
of excess fuel since they are already oxides, but
engine over-temperature excursions have shown that
glazing of the ceramic surfaces occurs.
Increasing use of ceramic materials is expected in low emission (and low cost) gas turbine
combustion systems. These may be in the form of
porous plate combustors using surface combustion
techniques. Looking farther into the future combined ceramic recuperator-combustor systems of the
type described by Topouzian (20) may be utilized
in very high temperature, low cost, gas turbines.
In this ceramic system, fuel is injected into the
compressed air upstream of the heat exchanger and
heat added to the fuel-air stream should assist
in vaporizing the fuel. An igniter located in
the vicinity of the downstream edge of the recuperator ignites the fuel-air mixture as it
leaves the exchanger. The flame front could be
anchored on or removed slightly from the surface
of the heat exchanger. Improved efficiency may
result from the homogeneous fuel-air mixture produced by the relatively long mixing time. Fuel
vaporization in the ceramic heat exchanger would
increase heat transfer and reduce the wall temperature. Again, looking to the future, it may
be possible to impregnate the ceramic core with
a suitable catalyst to improve the emissions from
small recuperative gas turbines. With literally
thousands of square feet of surface area in the
recuperator, a substantial amount of catalyst
could be exposed to the gas, but with the small
dwell times in the heat exchanger the effectiveness of such a system may be low; nevertheless,
it warrants at least a preliminary chemical investigation.
RECUPERATOR COST CONSIDERATIONS
Gas turbine recuperator cost data are usually of a proprietary nature and are dependent on
the engine application and the production quantities. Cost is a factor of prime importance in
all recuperators. It is clear that for the recuperator to find a wide range of acceptance in
gas turbine applications, the cost must be reduced by utilizing lower cost materials and types
of construction to minimize the manufacturing
labor content. For high volume vehicular production, it is projected that close to 80 percent of
the heat exchanger cost will be basic material.
Because of uncertainty in the cost of manufacturing ceramic cores, the data given in this paper
is for material costs only. Because ceramic developments are ,still in their infancy, cost data
19
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Table 2 Recuperative Comparisons for Vehicular
Gas Turbines
Application
Highway Trucks
Power, hp (kw)
Specific Cost Goal
600 (448)
150 (112)
$10/hp to be competitive
with diesel engine
Production engine cost, $
Recuperator Type*
Automobiles
6000
450
Counterflow Plate-Fin
Effectiveness
Counterflow Plate Fin
0.85
Recuperator life goal, hrs
0.85
10,000
3,500**
Recuperator cost goal
10-15% of engine cost ($750)
Core Material
Metal
Surface, S ft 2 /ft 3 (m2 /m3 )
600
Core frontal area, ft 2 (m2 )
Counterflow length, ins (mm)
5.70 (0.53)
9.00 (229)
Core volume, ft 3 (m3 )
4.28
Core weight, lb (kg)
Core material cost, $/lb ($/kg)
Recuperator material cost ***$
Material considerations
$3/hp to be competitive
with gasoline engine
Ceramic
(1970) 1 1400 (4593)
9.60 (0.89)
(0.121)
400.0 (181)
1.0
(2.2)
400.0
3.40 (0.86)
2.72
(0.077)
140.0 (64)
0.40
(0.88)
56.0
10-15% of engine cost ($56)
Metal
Ceramic
600
(1970)
1400 (4593)
1.42 (0.13)
2.42 (0.22)
9.00 (229)
3.40 (86)
1.07 (0.030) 0.69 (0.019),
100.0 (45.4)
35.0 (15.9)
1.0
(2.2)
0.40 (0.88)
100.0
1
14.0
Stainless steel economically
acceptable
Stainless steel too
expensive
• Development engines to
demonstrate performance,
emissions, reliability
Stainless steel recuperator
Bi-metallic recuperator
(superalloy-stainless
steel combination)
• Production engines
Stainless steel recuperator
changing to ceramic as
engine temperature increases
Ceramic recuperator
Notes ;
*
o Metal recuperators of brazed formed plate and fin elements
• Ceramic units of extruded, injection molded, or formed plate and fin
elements, with integral air manifolds
**
Assumed life of 7 years at 500 hrs/year (approximately 105,000 miles)
***
Approximate values for metal-ceramic comparisons. Manufacturing and
labor costs not included.
have not been completely defined, but for the
purpose of cost comparisons, it has been assumed
that most non-metallic materials are under 50//lb
(,1.10/kg). Stainless steel used in current
plate-fin recuperators (Type 347) costs in the
order of /1 -1.5/lb (/2.2-,3.3,g) in the thin
foil form. Representative nickel and cobalt alloys (Inconel 625, Incoloy 800) in the thin foil
form are in the range of ,3 ,'6/lb (/6.6 /13.2/kg).
Recuperator comparisons for vehicular gas
turbines are given in Table 2. The cost figures
given do not correspond to any one particular gas
turbine manufacturers values, but are appropriate
values selected by the author from the many gas
turbine economic studies that have appeared in the
open literature over the last few years. While
the actual magnitude of the figures may vary within the industry, the general trend of recuperator
costs is felt to be representative for compact
types of plate-fin surface geometries. For both
truck and automotive gas turbines a recuperate:'
cost goal of 10-15 percent of the engine cost has
been assumed.
For truck gas turbines [projected power of
600 hp (450 kw) for future highway trucks], it
can be seen that a metallic core material cost,
assuming stainless steel at %l/lb (/2.2/kg) is
economically acceptable. This material cost does
20
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U
not include braze alloy, which at /3 /4/lb (/6.6,8.8/kg) could have a significant affect on the
overall cost of the recuperator. Manufacturing
and labor costs are not included in Table 2, and
considerable ingenuity on the part of the metal
forming industry will be required to keep these
costs to a minimum.
For the much more challenging automotive
market it can be seen that utilization of a stainless steel recuperator is not economically acceptable. It is also questionable whether bimetallic or tri-metallic variants could satisfy
the economic goals. For low cost and high temperature capabilityj, use of a ceramic material
such as silicon nitride at an assumed cost of
40¢/lb (88X/kg) satisfies the heat exchanger cost
goals.
In the future, it is hoped that as metallic
and ceramic recuperators are developed for vehicular gas turbines, respective industries will
publish projected manufacturing cost data so that
more realistic cost comparisons than shown in
Table 2 can be carried out.
RECUPERATOR APPLICATIONS
Vehicular Gas Turbines
The biggest challenge for the heat exchanger manufacturer is the vehicular gas turbine
field which includes truck, automotive, and offhighway equipment. Utilization of low cost materials and high volume manufacturing techniques,
as described in other sections of this paper, are
necessary to keep the initial cost of the recuperator or regenerator to an acceptable value.
Development engines in the 300- to 400-hp (225-to
300-kw) range are currently being endurance run
in highway trucks and buses. Current engines in
this power class are regenerative and utilize both
metallic and ceramic disk regenerators. Encouraging progress has been made in the ceramic regenerator field for these low pressure ratio engines
with specific powers in the order of 100 hp/lb/
sec (165 kw/kg/sec). Because of the large amounts
of money invested in these engine development
programs, departure from the regenerative configuration to a recuperative variant is extremely
unlikely. Looking to the future it would seem
that the trend will be toward higher specific
power cycles and related aerospace technology,
say in the area of advanced two-stage radial compressors of high efficiency, will be used. At
these higher pressure ratios, the regenerator
will lose some of its advantages because of the
more severe sealing and support problems. In
Table 2, it has been shown (based on some rather
arbitrary assumptions) that in the 600 hp (450
kw) class, a stainless-steel plate-fin recuperator
can satisfy the economic goals at today's level of
engine temperatures. In stainless steel, however,
the recuperator probably has very limited temperature uprating potential, and almost certainly
turbine inlet temperatures will increase as stationary ceramic components (nozzles) and cooled
blades are utilized. With the resultant higher
heat exchanger inlet temperatures (particularly
at part load) stainless steel will no longer be
suitable for the recuperator. Complete units
made from super-alloys are out of the question
because of the excessive cost, and the intermediate solution would seem to be stainless
steel-superalloy bimetallic configurations. The
fabricating problems and the associated cost of
this type of unit are unknowns at this time. The
justification for a large ceramic recuperator
development program aimed at a power plant of
this size will depend very much on the projected
market. A more lucrative market for the development of a ceramic recuperator is the projected
high volume automotive gas turbine (approximately
150 hp, 112 kw) and if this materializes, then
the larger truck engines could take advantage of
the ceramic technology. Motivation for gas turbines in passenger automobiles is rapidly increasing as new ceramic technologies are applied.
In this paper, it has been shown that for
a 150-hp (112-kw) automotive gas turbine, a recuperator cost goal in the order of /50 cannot
be satisfied with a metallic heat exchanger capable of operating at the high temperatures associated with part power operation. The choice is
then between a ceramic rotary regenerator or a
ceramic fixed boundary recuperator, the final
selection being on the basis of cost. At this
stage, it is not obvious that the rotary regenerator offers the lowest cost solution, since the
cost does not scale down (from current truck engine size) directly with power because of the
complex sealing system and drive mechanism. The
cost of a fixed boundary ceramic recuperator,
however, is equally hard to estimate since little
development in this area has been reported to
date. If the ceramic recuperator is to find acceptance in the gas turbine industry, it will be
initially for small automotive type power plants,
and as advancements are made they will be applied
to larger vehicular and industrial gas turbines.
Industrial Gas Turbine
For the industrial gas turbine, the utilization of a heat-recovery device is optional and is
very dependent on the application, the load profile, and fuel cost, etc. One of the main problems in many early recuperators was that of pro21
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longed structural integrity in the cyclic environment associated with gas turbine operation. Many
designs experienced fatigue failures, resulting
from the high transient thermal stresses associated with the thermal inertia incompatibility of
the core components. In current recuperators,
careful attention to the core design has eliminated the high thermal stress problem and has
resulted in configurations that satisfy the necessary long life goals required of the heat exchanger. Changing market conditions and economic
considerations have brought renewed interest in
the recuperative cycle to minimize SFC and to
keep turbines competitive for the future, and
help the users of industrial gas turbines to meet
changing economic conditions as fuel prices continue to increase. Blade cooling techniques developed for aircraft gas turbines are now being
utilized in industrial gas turbines. As higher
cycle efficiencies can be achieved by turbine inlet temperature increases, development programs
are underway to utilize stationary ceramic components in industrial gas turbines. With their
high temperature capability and good oxidation
resistance, uncooled ceramic nozzles combined with
air-cooled blades will improve cycle efficiency.
For continuous duty turbines, the operating
economics can be substantially improved by utilizing a recuperator. The incorporation of a
heavy-duty recuperator on a simple-cycle industrial gas turbine to improve fuel economy (by
approximately 20 percent) and maintain engine
competitiveness as gas and liquid fuel prices
increase to significantly higher levels has been
reported in Reference (21).
With large increases in both liquid and
natural gas fuel costs in the last year, the recuperative cycle should find more acceptance in
the industrial gas turbine market. The added
cost of the heat exchanger can often be paid for
in fuel savings after only a few thousand hours
of operation. Similarly, the extra weight and
volume of the recuperator can be recovered fairly
quickly, and this is important for military and
marine applications where fuel logistics may
dictate the choice of prime-mover.
Because of the relatively small market, it
is unlikely that concentrated ceramic recuperator
development efforts will be made for industrial
gas turbines. However, the technology gained
from extensive vehicular programs will be directly
applicable and will result in smaller recuperators, of lower cost and higher temperature capability for future industrial gas turbines.
SUMMARY
Increasing fuel costs and low pollutant
emission requirements make the higher efficiency
recuperative cycle gas turbine economically attractive for many industrial and vehicular applications. Heat-exchanger technology has not
progressed as rapidly as turbomachinery advancements in recent years, and if a low cost recuperator was available today with dimensions compatible
with the compact nature of the turbomachinery, it
would find acceptance for many industrial applications. Over the next few years, turbine inlet
temperatures are expected to increase substantially in industrial gas turbines as technology gained
from aircraft engines is factored into new and uprated engine designs. In the larger industrial
gas turbines, more sophisticated turbine blade
cooling methods, combined with increasing use of
ceramics in stationary parts, will enable higher
cycle efficiencies to be realized by virtue of
the higher turbine inlet temperatures. In the
smaller automotive power class, the goal of low
cost virtually dictates the use of ceramics for
the complete hot section of the engine.
For future gas turbines, it is clear that
if the recuperator or regenerator is to find acceptance, it must have high temperature capability
and low cost. To reduce engine airflow and engine
physical size, higher specific power cycles will
probably be adopted in the future, which, by
virtue of their higher pressure ratios make utilization of ceramic rotary regenerators questionable because of the increasing seal leakage and
structural support problems. The emphasis in
this paper has been on the utilization of fixed
boundary recuperators for vehicular and industrial
gas turbine applications. The most challenging
application is the automotive gas turbine, and
there can be no doubt that turbine power plants
are now serious contenders for the car of the
future.
In the short-term, there is a need for a
low cost metallic recuperator capable of withstanding high temperatures at part load in existing types of gas turbines with fairly modest turbine inlet temperatures. Use of stainless steel,
while probably initially acceptable, does not
allow for much engine temperature uprating during
the extended development period. Use of superalloys in the recuperator allows much higher gas
temperatures, but the cost of the heat exchanger
would be prohibitively expensive and would be
acceptable only for a few engines to demonstrate
performance, emissions, and reliability of rotating components. A solution to the high cost may
be the use of bi-metallic or tri-metallic recuperator constructions for automotive, truck and
industrial gas turbine applications. The ultimate
low cost (and high temperature) solution is the
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use of ceramics in the heat exchanger, and all of
the metallic variants will merely provide a transition until the ceramic technology is available.
The time frame for the introduction of ceramic
recuperators is very dependent on the development
efforts carried in support of the automotive gas
turbine. Truck and industrial gas turbines will
benefit from the automotive ceramic heat exchanger
development program. In the latter field, increasing use of ceramics with their excellent
oxidation resistance will enable low cost residual
fuels to be used and with a high temperature
ceramic heat exchanger, the exhaust heated cycle
may be considered for some applications.
The preliminary thoughts presented in this
paper were aimed at studying the feasibility of
utilizing ceramic cellular structures for counterflow gas turbine recuperators. Plate-fin surfaces
were considered mainly because the experience
gained in current ceramic regenerators could be
directly applied in the areas of forming, fabricating and firing, etc. To reduce recuperator
volume and establis`i a counterflow configuration
that can be well integrated with the turbomachinery, it is necessary to use extremely compact
surfaces. For fixed boundary recuperators, however, there is a limit to compactness that will
give reasonable core frontal areas and flow
lengths. In this study, a compactness limit of
around 1400 ft 2 /ft 3 (4600 m2 /m3 ) was established,
compared with values of over 2000 ft 2 /ft 3 (6560
m2 /m3 ) being evaluated in advanced rotary regenerators. For recuperator application, it was
found that at the low Reynolds number operating
range associated with very compact surface designs, satisfactory heat-transfer and friction
performance could be realized with plain triangular passages, thus obviating the need for
complex and expensive wavy and offset type surfaces. For recuperator passage heights in the
order of 0.04 in. (1 mm), it has been shown that
good surface efficiencies can be realized using
the low thermal conductivity associated with many
ceramics. In fact, in the case of very compact
geometries, it has been shown that a low value of
thermal conductivity is desirable to minimize
axial conduction effects and give a high value of
recuperator effectiveness at engine part-load
conditions. While plate-fin surfaces appear to
yield satisfactory core sizes, they should not be
regarded as the optimum type at this stage since
ceramic forming techniques (bearing in mind the
requirement for a counterflow configuration with
integral headers) may show prime-surface geometries
to be more applicable.
This paper is not intended to imply that the
technology of ceramic heat exchangers has already
been established; in fact, few of the answers are
available today in the areas of structural integrity, performance and cost, and concentrated development efforts are required to demonstrate the
viability of a. fixed boundary ceramic gas turbine
recuperator. The projected automotive gas turbine,
while probably many years away from production,
will provide the stimulus for a ceramic recuperator development program. As the ceramic technology becomes available, ceramic components (including the heat exchanger) will be utilized for
many vehicular and industrial gas turbine applications. The goal in all of the development efforts will be to produce very low cost components.
Although heat exchanger costs are sensitive to
the production quantities and, in most cases, the
economic ground rules are regarded as proprietary
by the manufacturer, it is apparent that for
future recuperator applications where a high percentage of the final cost is in the basic material,
the trend toward higher temperatures indicates
that only ceramic materials are capable of satisfying the demanding economic goals.
ACKNOWLEDGMENTS
The author wishes to thank John Egenolf of
Advanced Materials Engineering Limited, John
Lanning of Corning Glass Works, and Dr. Paul
Davis of American Lava Corporation for the photographs showing different types of ceramic heat
exchangers, The views expressed in this paper
are those of the author, and do not necessarily
represent any company affiliation.
REFERENCES
1 Blakely, T. H., and Darling, R. F., "The
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3 Lanning, J. G., and Wardale, D. J. S.,
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8 Kays, W., and London, A. L., Compact
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of A Ceramic Air Heater for Burning Residual Fuels
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24
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