Temperature and Altitude Effects on Fans - FE-1600

Temperature and Altitude Effects on Fans - FE-1600
FAN ENGINEERING
FE-1600
Information and Recommendations for the Engineer
Temperature & Altitude Effects
on Fans
Introduction
Temperature Effect
Fans are tested in laboratories with test setups that
simulate installations that are typical for that type of fan.
Usually they are tested and rated as one of four standard installation types as designated in AMCA Standard
210. These standard installation types are shown in
Figure 1.
Any temperature other than 70°F affects the air/gas
density. Fan pressure (P) and horsepower (H) vary
directly with the ratio of the air/gas density at the fan
inlet to the standard density; however, fan air volume
(CFM) is not affected by the air density. Fans are constant volume machines that, when operating at constant
speed will deliver the same CFM at 0.075 lb/ft3 density
air as they will with lower density air or higher density
air.
For example, Figure 2 illustrates the effect on the fan
performance of a density variation from the standard
value created by a change in fan inlet temperature.
Figure 1. Standard Fan Installation Types
Figure 2. Percent of Duct/Fan System Airflow – Q
Type A: Free Inlet, Free Outlet
DUCT SYSTEM A
@ 0.075 LB/FT3 DENSITY
AT FAN INLET
FAN PRESSURE CURVE
@ 0.075 LB/FT3
PERCENT OF DUCT SYSTEM
RESISTANCE AND FAN PRESSURE - P
Type B: Free Inlet, Ducted Outlet
Type C: Ducted Inlet, Free Outlet
100
60
P
40
Pc
PERCENT OF POWER - H
Products that are rated and certified by AMCA must
illustrate that they have been rated by one of the installation types shown above.
In addition to listing the test type, the ratings must
also be published at a standard air inlet density. The
fan industry has adopted a standard density of 0.075 lb/
ft3 at 70°F at sea level and at a barometric pressure of
29.92" Hg. All manufacturers’ ratings are made at, or
adjusted to, this standard. Whenever a fan is operated
in a system where any or all of these conditions vary,
corrections must be made in order to obtain accurate
results.
It’s not enough to make fan performance adjustments
based on density corrections. The designer must also
consider what effect the variables that are influencing
the fan air density might have on the structural components of the fan. Temperatures other than 70°F can
cause an alloy to become too pliable or brittle. Speed
adjustments can exceed the limits of the wheel, shaft
and bearings. Gases, other than air, that change the
inlet density may also be corrosive to vital structural
components. All these variables must be considered
when making fan inlet density adjustments.
DENSITY =
RATIO
ρc
=
ρ
0.0375 = 0.5
0.075
20
0
H
100
Type D: Ducted Inlet, Ducted Outlet
DUCT SYSTEM A
@ 0.0375 LB/FT3 DENSITY
AT FAN INLET
FAN PRESSURE CURVE
@ 0.0375 LB/FT3
80
H @ 0.075
LB/FT3
80
60
Hc
40
H @ 0.0375
LB/FT3
20
0
0
20
40
60
80
100
120
140
160
180
200
PERCENT OF DUCT SYSTEM VOLUME FLOW RATE Q
This density ratio must always be considered when
selecting a fan from a manufacturer’s catalogs or curves.
The dashed curve is representative of cataloged fan
performance at 70°F at sea level with a barometric pressure of 29.92" Hg. (standard air). The solid curve is
representative of the fan’s performance with an inlet
temperature of 600°F at the same altitude and barometric pressure.
The fan laws, with the size and speed remaining
constant, that apply here are as follows:
Qc = Q
Where:Q=CFM (cubic feet of air per minute)
Pc = P (ρc/ρ)
P=pressure (inches of water)
Hc = H (ρc/ρ)
H=fan brake horsepower
ρc/ρ
=
density
ratio
ρ=air density (pounds per cubic foot)
subscript c = converted value
So how do we determine the air density for tempera-
©2001 Twin City Fan Companies, Ltd.
tures other than 70°F? One way would be to calculate
it using absolute temperatures, absolute pressures and
barometric pressure, or we could simply refer to Table
1 where it’s been conveniently worked out for a range
of temperatures at sea level.
Table 1. Corrections for Temperature at Sea Level
Actually, these factors are used directly to determine
AIR
AIR
TEMPERATURE
FACTOR
TEMPERATURE
(°F)
(°F)
–50
0.77 275
–25
0.82 300
0
0.87 325
20
0.91 350
40
0.94 375
60
0.98 400
70
1.00 450
80
1.02 500
100
1.06 550
120
1.09 600
140
1.13 650
160
1.17 700
180
1.21 750
200
1.25 800
225
1.29 900
250
1.34
1000
FACTOR
3" SP x 1.53 = 4.59" SP
1.39
1.43
1.48
1.53
1.58
1.62
1.72
1.81
1.91
2.00
2.09
2.19
2.28
2.38
2.56
2.76
So for this example, if we select the same fan model,
our new requirements are for 15,000 CFM at 4.59" SP
at 70°F. The fan would operate at 1,742 RPM and
require 16.18 BHP. It then follows that the operating
conditions at 350°F would be as follows:
the corrected fan performance. The factor is equal to
the fan’s rating density (standard air) divided by the
actual air density at the fan inlet.
3
Factor = 0.075 lb/ft
ρ
So if the dry air density corresponding to an air temperature other than 70°F is desired, it can be calculated
by simply dividing 0.075 by the factor.
Fan densities may vary from standard for reasons
other than temperature and altitude. Moisture, gas, or a
mixture of gases other than air are a few possibilities.
For these cases it will be necessary to obtain the
actual density of the inlet gas stream by some other
reference material. The factor can then be obtained by
substituting the new density for ρ.
Example 1: A fan is required to deliver 15,000 CFM
against 3" SP (static pressure). The fan is to operate at
350°F. This fan would be selected from a manufacturer’s
standard rating table or curve for 15,000 CFM at 3" SP
at 70°F and would operate at 1,621 RPM and require
12.25 BHP.
To determine the fan’s performance at 350°F, simply
divide the SP and BHP by the factor from Table 1. The
factor for 350°F is 1.53; therefore the operating static
pressure and brake horsepower would be as follows:
3" SP
= 1.96" SP
1.53
12.25 BHP
= 8.01 BHP
1.53
Although the fan RPM is within the speed range
specified in the performance tables, the impeller safe
speed needs to be verified for operation at the elevated
temperature. Most fan manufacturers will list safe speed
factors for operation at elevated temperatures in the fan
catalog and in their selection software.
Caution is required when selecting the motor. From
the BHP calculation it appears that either a 71⁄2 or a 10
HP motor could be used. But perhaps the motor selection should be based on a cold start of 12.25 BHP, to
allow the fan to start before the air warms up. In this
case the fan would require a 15 HP motor. An alternative to a larger motor, depending on the fan’s BHP
characteristics, could be a shutoff damper that would
not open until the air is up to temperature. For this
particular fan, the shutoff power requirement is 6 BHP
at standard conditions.
2
Example 2: Let us look at Example 1 another way.
Suppose the request is for a fan to deliver 15,000 CFM
against 3" SP at 350°F. In this case the designer is
asking for a fan to develop the 3" SP at 350°F inlet
temperature. In order to select the fan from the 70°F
standard performance tables, we must first convert the
static pressure at 350°F to 70°F. We accomplish this by
the factor established in Example 1.
4.59" SP = 3" SP and 16.18 BHP = 10.58 BHP
1.53
1.53
CFM and RPM would not change. And again, check the
maximum speed limitations of the impeller and proper
motor size for the cold starts.
Also, keep the following in mind when using temperature correction factors:
1.At temperatures higher than standard air (70°F) the
air density is less (lighter air); therefore both the pressure and brake horsepower will be less.
2.At temperatures lower than standard air the air density is greater (heavier air); therefore both the pressure
and brake horsepower will be more.
Altitude Effect
Fans operating at some altitude above sea level are
similar to fans operating above 70°F. The higher the
altitude the less dense (lighter) the air. Altitude correction
factors for 70°F air are listed in Table 2. Note that these
corrections correspond to average barometric pressure
at the stated altitude. Actual conditions will vary with the
weather.
Table 2. Corrections for Altitude at 70°F Air
ALTITUDE
0
500
1,000
1,500
2,000
2,500
3,000
3,500
4,000
4,500
FACTOR
ALTITUDE
1.00 5,000
1.02 5,500
1.04 6,000
1.06 6,500
1.08 7,000
1.10 7,500
1.12 8,000
1.14 8,500
1.16 9,000
1.18
10,000
FACTOR
1.20
1.22
1.25
1.27
1.30
1.32
1.35
1.37
1.40
1.45
Example 3: Select a fan to deliver 8,500 CFM at 21⁄2"
SP at 5,500 ft elevation. Since no temperature is given
it will be assumed to be 70°F. From Table 2, the factor
for 5,500 ft elevation is 1.22. Converting the static pressure to sea level to use the manufacturer’s performance
tables results in: SP = 1.22 x 21⁄2" SP = 3.05" SP at
sea level and 70°F. Selecting a fan for 8,500 CFM at
3.05" SP results in an RPM of 1,173 and 5.28 BHP at
sea level with 70°F entering air temperature. At the
operating conditions of 5,500 ft elevation the SP and
BHP would be corrected to:
3.05" SP
= 2.5" SP
1.22
5.28 BHP
= 4.33 BHP
1.22
CFM and RPM would not change. Confirm that the RPM
is within published speed limits. The motor horsepower
should be okay because the temperature does not vary
and the elevation cannot change.
Fan Engineering FE-1600
Temperature and Altitude Effect
When both temperature and elevation changes are present, the air density must be modified by a factor from
both Tables 1 and 2. An alternative to this would be to
use a single density ratio number such as can be found
in Figure 3.
Figure 3. Air Density Ratios at Various Altitudes
and Temperatures
600
E IN
ITUD
ALT
550
500
400
TEMPERATURE (F)
300
250
50
0
-50
80 70 60 50 40 30 20 10
00 00 00 00 00 00 00 00 0
0.4
0.5
0.6
0.7
0.8 0.9 1.0 1.1
DENSITY RATIO
1.2
1.3
1.4
1.5
Example 4: Select a fan to deliver 8500 CFM at 21⁄2"
SP at 5,500 ft elevation at 250°F. From Table 1 the
factor for 250°F is 1.34 and from Table 2 the factor for
5,500 ft elevation is 1.22. The overall factor is obtained
by multiplying these factors: 1.34 x 1.22 = 1.63. To use
a fan manufacturer’s performance tables, convert the SP
to standard air:
2.5" SP x 1.63 factor = 4.08" SP
The fan will be selected for 8,500 CFM at 4.08" SP and
will operate at 1,287 RPM, 6.96 BHP. Converting to
operating conditions results in:
6.96 BHP
4.08" SP
= 2.5" SP
= 4.27 BHP
1.63
1.63
And again, CFM and RPM will not change. Also, if the
fan is to start cold, it will still be at 5,500 ft elevation.
Therefore, to obtain the “cold” horsepower, divide the
standard air horsepower by the altitude factor only.
6.96 BHP
= 5.70 BHP
1.22
Identical results can also be achieved by using Figure
3. Locate the temperature on the left-hand scale and
proceed horizontally to the intersect of the altitude curve,
and then follow it vertically down to the density ratio at
the bottom of the graph. For a temperature of 250°F
and an elevation of 5,500 ft, we read a density ratio of
0.613. The density ratio is simply the reciprocal of the
factor.
1
1.63 factor
= 0.613 density ratio (DR)
ACFM vs SCFM
These two terms are commonly used in design work,
and they should not be confused as this greatly influences the fan selection.
3
0.0375 lb/ft3
0.075 lb/ft3
= 5,000 SCFM
10,000 SCFM
x
0.075 lb/ft3
0.0375 lb/ft3
= 20,000 CFM
Inlet Suction Effect
100
x
Select the fan for 20,000 CFM.
200
150
10,000 CFM
Selecting a fan when SCFM is specified requires us to
calculate the ACFM. If the fan was specified for 10,000
SCFM at 600°F, then an equivalent weight rate of flow
is desired at 600°F.
350
-100
0.3
SCFM (standard cubic feet per minute) — Air volume
corrected to standard density conditions. This term is
commonly used when a given weight rate of flow is
required. For example, to determine the SCFM of a fan
delivering 10,000 CFM at 600°F, we would multiply the
CFM by the density ratio or divide it by the factor.
T
FEE
450
ACFM (actual cubic feet per minute) — Represents the
volume of gas flowing anywhere in the system independent of its density. ACFM or CFM is the value that is
used when selecting a fan.
A common influence on density, especially on exhaust
systems, is suction. When system resistance is placed
on a fan’s inlet, the suction creates a partial vacuum at
the inlet. This negative inlet pressure (partial vacuum)
lowers the barometric pressure at the inlet and therefore
the inlet density. This correction is rarely accounted for
unless the suction pressure exceeds 10" SP. In any
event, this negative inlet pressure effect can be accounted for in the following manner:
Inlet density (lb/ft3) =
3
Gas density (lb/ft ) x
Atm. Press. (IWG) + Inlet SP (IWG)
Atm. Press. (IWG)
— or —
DR (inlet) = DR (gas) x
Atm. Press. (IWG) + Inlet SP (IWG)
Atm. Press. (IWG)
Where:Atm. Press. = atmospheric pressure = 407" w.g.
(at other than sea level divide 407 by the altitude factor to get the atmospheric pressure)
Density of standard air = 0.075 lb/ft3
Density ratio (DR) of standard air = 1.00
Inlet SP is normally a negative number.
Example 5: A fan is to deliver 10,750 ACFM at 22" SP.
20" of this pressure is at the fan inlet.
DR (inlet) = 1.00 x
407 + (–20)
= 0.951
407
SP = 22" ÷ 0.951 = 23.1" SP at 70°F
Therefore, a fan is selected for 10,750 CFM at 23.1" SP
which results in an RPM of 1,873 and a BHP of 63.81.
The corrected BHP would then equal 63.81 x 0.951 or
60.68 BHP.
Example 6: If conditions were at 200°F instead of standard air, then:
407 + (–20)
DR (inlet) = 0.80 x
= 0.761
407
SP = 22" ÷ 0.761 = 28.9" SP at 200°F
The fan would now be selected for 10,750 CFM at
28.9" SP resulting in a speed of 2,073 RPM and a BHP
of 79.64. The corrected BHP would then equal 79.64 x
0.761 or 60.68 BHP.
Both selections could be operated with a 60 HP
motor; however, if the 200°F fan were to be subjected
to cold starts without a shutoff damper, then a 100 HP
motor would be required.
Fan Engineering FE-1600
Twin city fan & blower | www.tcf.com
5959 Trenton Lane N | Minneapolis, MN 55442 | Phone: 763-551-7600 | Fax: 763-551-7601
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