air filtration system balancing manual

air filtration system balancing manual
800-500-9777
®
AIR FILTRATION
SYSTEM
BALANCING
MANUAL
312 SOUTH HWY. 73, PO BOX 398
FALLS CITY, NE 68355-0398
800-500-9777
Revision date 06/05/2008
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Index
Introduction ......................................................................................................... 3
System Components ........................................................................................... 4
System Terminology............................................................................................ 5
Basic Calculations ............................................................................................... 7
System Component Maintenance...................................................................... 10
Dust Collecting Hoods & Air Flow Control Gates ............................................ 10
Duct Work ...................................................................................................... 10
Fans ............................................................................................................... 11
Dust Collectors............................................................................................... 12
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Introduction
The information presented in this manual is intended to familiarize operating personnel with the
fundamentals of dust collecting systems, operation and proper balancing to assure maximum operating
efficiency.
A dust collecting system comprises a number of components, each of which must function in
accordance with the original design criteria to assure maximum efficiency. A malfunction in any one
part may cause the entire system to become inoperable.
The following instructions will detail these parts, describing their function in the system and the
procedure to use to ensure efficient operation. The original design data and drawings prepared when
the systems were installed should be available for reference. It is recommended that you study these to
become familiar with the arrangement of duct work, hoods and other equipment.
The system will have been balanced after the original installation was completed and a section of this
manual will describe “balancing”. It is imperative that you become familiar with this procedure and have
the tools on hand to rebalance should it become necessary. Read over all the instruction manuals to
become thoroughly familiar with each piece of equipment and keep recommended spare parts in stock.
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System Components
A typical system will normally consist of the following major items:
Dust Collecting Hoods & Air Flow Control Gates.
These are installed at dust producing areas and serve to confine the outflow of dust.
Duct Work.
The ductwork collects the dust from a number of hoods and is sized to ensure proper carrying
velocity.
System Fan.
The fan creates the vacuum necessary within the ductwork and hoods, which cause air to flow
inward through the hood, carrying dust particles with it.
Dust Collector.
This term generally refers to cloth or cartridge filters that separate the dust from the air stream,
allowing the clean air to dissipate to the atmosphere while confining the dust to a single
discharge point.
Rotary Valves.
These serve as airlocks under the dust collectors to discharge collected dust and to minimize air
flow either into or out of the collector depending on whether the system fan is on the dirty or
clean air side.
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System Terminology
Manometer. A Manometer is an instrument used to measure pressure. It consists of
a clear “U” tube with a graduated scale between the two legs, calibrated in inches.
The zero point on the scale is at the mid-point between the top and the bottom. In
use, the manometer is filled with water so that the level in each leg is at the zero
point. The pressure reading obtained is the difference between the two levels. The
difference is expressed as inches of water column, or WC. As an example, if the
water rises 1.5” in the leg connected to the hood, it will drop 1.5” from the zero point
on the leg that is open to the atmosphere, resulting in a total reading of 3” WC. Each
manometer is equipped with two rubber hoses, which can be of any length, usually
6” to 8” long each. However, in measuring static pressure, only one hose is normally
used. Today, electronic manometers are widely available.
3
2
1
0
1
2
3
3" Ps
3
2
1
0
1
2
3
SIMPLE MANOMETER
Velocity Pressure (Pv). Velocity Pressure is the difference between total pressure and static
pressure, and is used to calculate both FPM and CFM (refer to “Basic Calculations” on page 7).
Velocity pressure is measured through the use of a Pitot tube in conjunction with a manometer. A
velocity pressure of 1” WC to 1.5” WC is considered normal, equivalent to about 4,000 to 5,000 feet per
minute (FPM).
Static Pressure (Ps). Static pressure is measured in inches of water with a manometer, which is
described above.The word “pressure” is always used even
though in most cases we are talking about a dust control system
under a vacuum. This is the effect created by atmospheric
Pv
pressure due to movement of air created by a fan. Your vacuum
cleaner is a good example of this function. The fan does not
“suck” dust off the floor, it creates a vacuum inside the nozzle (or
hood), which causes outside
Pt
air to flow into the nozzle at
such a velocity that it picks up
Ps
Pt
and carries dust particles along
Ps
FLOW
with it.
Total Pressure (Pt).
Total
Pressure is the combination of static and velocity pressures, and is
expressed in the same units. It is an important and useful concept to
use because it is easy to determine and, although velocity pressure is
not easy to measure directly, it can be determined easily by
subtracting static pressure from total pressure. This subtraction need
not be done mathematically. It can be done automatically in the
manometer.
Ps
AIR FLOW
Pt
Ps
PITOT TUBE
Pitot Tube. A Pitot tube is an instrument constructed as a tube within a tube. The inner tube is used to
measure total pressure, and openings in the outer tube allow measurement of static pressure.
Feet Per Minute (FPM). Term used to indicate the velocity of air in a duct. A velocity pressure of 1”
WC, as measured with a Pitot tube, results in a velocity of approximately 4,000 FPM, which is usually
sufficient to keep most dust in suspension in the duct.
Cubic Feet Per Minute (CFM). A term used to indicate the air volume from an individual hood or being
handled by the filter or fan. As an example, a fan may be selected to handle 10,000 cfm at 10” static
pressure. The 10,000 CFM is the total air to be handled by all hoods combined and the 10” static
pressure is that required to provide the desired airflow at the furthest hood.
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Anemometer. An instrument for measuring air velocity, used in system
balancing to measure face velocity for comparison to system design
specifications.
Face Velocity. Air velocity measured with an anemometer at the air inlet
or exhaust. Face velocity is expressed in feet per minute.
Air to Cloth Ratio. A term indicating the ratio of the total amount of air in
a filter divided by the total cloth area. 10,000 CFM in a filter having 1,000
square feet of cloth gives a ratio of 10 to 1.
Hot-wire and wind-vane
anemometers
Magnehelic Gauge. An instrument usually furnished with a filter that
provides a means of constantly monitoring the operating static pressure
drop, or differential pressure (DP) across the filter bags and indicates if the filter is operating normally.
A range of 0-10” WC is adequate on the dial. An increase above the desired pressure drop allowed for
in the system design would result in decreased airflow and a resultant decrease in the efficiency of the
entire system.
Cross Sectional Area. The area of the cross section of an air duct, usually expressed in square feet.
Cross sectional area is used in calculating CFM.
Traverse Readings. Traverse readings are taken in the interest of accuracy, since the velocity of the
air stream is not uniform across the cross section of a duct.
PITOT TUBE PLACEMENTS
AT CENTERS OF EQUAL
Friction slows the air moving close to the walls, so the velocity is
CONCENTRIC AREAS
greater in the center of the duct.
R
To obtain the average total velocity in ducts of 4” diameter or
larger, a series of velocity pressure readings must be taken at
points of equal area. It is recommended that at least 20 readings
be taken along two diameters In round ducts. In rectangular
ducts, a minimum of 16 and a maximum of 64 readings are taken
at centers of equal rectangular areas. The velocities are then
averaged.
These precautions should be
observed for best accuracy:
1. Duct diameter should be
at least 30 times the
.548 R
.707 R
diameter of the Pitot
.837 R
tube.
.949 R
2. Locate the Pitot tube in
TRAVERSE ON ROUND
a duct section providing
DUCT AREA
8½ or more duct
diameters upstream and
5 or more diameters down stream of the Pitot tube. This length
of duct should be free of elbows, size changes or obstructions.
3. Provide an egg-crate type of flow straightener 5 duct
diameters upstream of Pitot tube.
4. Make a complete, accurate traverse.
.316 R
In small ducts or where traverse operations are otherwise impossible,
a fairly good degree of accuracy can be achieved by placing the Pitot
tube in the center of the duct. Determine velocity from the reading and
multiply by 0.9 for an approximate average.
PITOT TUBE PLACEMENTS AT
CENTERS OF EQUAL
RECTANGULAR AREAS
TRAVERSE ON RECTANGULAR
DUCT AREA
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Basic Calculations
Velocity Pressure (Pv) = Total Pressure (Pt) - Static Pressure (Ps)
Pv = Pt - Ps
Feet Per Minute (FPM) = The square root of the Velocity Pressure (Pv) x 4004.4
FPM = 4004.4
Pv
Sq. Ft. of Cross Sectional Area (CSA) =
CSA =
(3.14) x Duct Radius (R) ÷ 144 *
R²/144
Cubic Feet per Minute (CFM) = Cross Sectional Area x Feet Per Minute (FPM)
CFM = CSA x FPM
*Divide by 144 when duct radius is measured in inches.
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FIELD TEST SHEET
SKETCH OF INSTALLATION
SHOWING POINTS OF READING
FAN OWNER
FAN MFR.
FAN NAMEPLATE DATA
DATE
BY
FAN RPM
APPX BHP
MOTOR NAMEPLATE DATA
SP
(OUTLET)
READINGS
SP
(OUTLET)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
TOTAL
VP
VP
L
L/8
L/4
L/4
L/4
L/8
W/8
W/4
W/4
W
W/4
W/8
PROBE POINTS IN A RECTANGULAR DUCT
XXXXX
AVERAGE
Density = .075
=
"k" =
"k" times
Barometric Pres.
29.92
530
460 + °F
lbs. per cu. ft.
R
.075
=
Density
SP =
VP =
BHP =
" SP
" VP
" BHP
average duct velocity = 4000
at st'd. density
VP
= 4000
=
.316R
.548R
.707R
.837R
.961R
fpm.
CFM = velocity x duct area
=
x
=
Add to the
to to
duct
friction
Add
the SP
SPany
anyloss
lossdue
due
duct
friction
between pints
of readings
and fan
Between
points
of readings
anddue
fantodue to
poor inlet
inlet and
poor
andoutlet
outletconnections.
connections.
FAX 402-245-5196
PROBE POINTS IN A ROUND DUCT
CFM
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System Balancing
System balancing is very important and every operator should be aware of how it is done and be
familiar with the terminology used.
Fan speed is selected to provide the required static pressure at the furthest hood; therefore the hoods
nearest the fan will have a higher available pressure unless the air flow control gates are adjusted so
that only the calculated static pressure is provided.
See “Industrial Ventilation, A Manual Of Recommended Practice For Design” by ACGIH for a
comprehensive discussion of exhaust system design procedure.
Balancing is the procedure of directing airflow to the pickup points through
the use of blast gates. This is accomplished with the aid of a manometer
(either clear tube or electronic) or a 0-3” magnehelic gauge.
2
Start at the hood closest to the dust collector. Drill a small hole in the duct
MAGNEHELIC
between the hood and the blast gate. Usually, a 1/8” hole will suffice,
unless it is desired to install a permanent tap. Place the free end of the
hose attached to the vacuum side of the manometer. Adjust the gate until
the manometer shows 1” WG. If an anemometer is available, check the
face velocity of the hood, and compare to the design specifications.
Proceed to the next hood on this branch and repeat this procedure.
Continue this process with each hood until the hood farthest from the collector has been adjusted. Start
over at the first hood and check each one in order again. Some adjustments will probably be required.
By adjusting the gate, the desired conditions can be established at which point the gate should either
be locked or at least marked for future reference.
1
0
ADJUSTABLE BLAST GATE
INCHES OF WATER
(TYP 1")
1/8" HOLE
3
When determining the total differential pressure
(DP) across the filter from the air inlet to the air
outlet side, the same technique can be used. Drill a
1/8” hole in the inlet and outlet ducts and measure
the pressure at each point. The static pressure
reading on the clean air side will be higher than on
the dirty air side if the filter is on vacuum. The
pressure drop is the difference between these two
readings.
In the following example, the procedure is outlined with the hoods numbered in the order in which they
should be balanced.
8
2
9
13
3
7
FREE AIR IF
REQUIRED
1
10
11
14
4
5
6
12
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System Component Maintenance
Dust Collecting Hoods & Air Flow Control Gates
Dust is generated each time a granular product is disturbed, such as being dropped out of a bin onto a
moving belt, from one belt to another, from a truck or rail car into a hopper, or from an elevator leg into
a bin, etc. Properly designed hoods are installed at each of these points and equipped with an airflow
control gate (normally called a blast gate), which is adjusted to assure that each hood is handling the
designed CFM (cubic feet per minute) of air.
Hoods positioned over a belt or a bin normally require little attention other than occasionally checking to
make sure the blast gate is functional and set at the desired point to assure proper dust control as
determined by the initial balancing procedure.
Hoods installed under a belt pulley, for example at the discharge end, can be more of a problem since
they have to handle floating dust and carry-over that clings to the belt. These hoods should be checked
more frequently to make sure that an excessive amount of dust has not accumulated which would
restrict the air flow and, if allowed to continue, would cause the hood to fill up and eventually spill over.
An accumulation of dust in the bottom of one of these hoods might be caused by trash such as paper,
cloth, etc., or an obstruction in the branch line from this hood to the main duct. Occasionally a belt will
stop while still carrying product, which would cause an abnormal flow of product into the hood. Since a
dust control system is not to be considered a pneumatic conveying system, the hood will have to be
cleaned out before restarting the belt. It is also possible that the air control gate may have been
accidentally closed off.
Other types of hoods may be required to suit specific dust producing areas. They all serve essentially
the same purpose. As long as they are kept clean and free of an accumulation of trash, and as long as
unauthorized personnel are not allowed to re-adjust the blast gates, they should not cause any
problem. If they are damaged in any way or if the blast gates do become inoperable, it is essential that
they are repaired immediately.
Duct Work
When it is necessary to salvage the collected dust because of its value, only one type of dust can be
handled per system at one time. The ductwork may cover a fairly wide area as a result. The main duct
gradually becomes larger to accommodate the increased airflow and hold the air velocity to a
reasonable level as each hood is added. High velocity results in increased wear on the duct work and
increased fan HP. Low velocity results in settling out of larger particles which may eventually plug the
line completely. Duct velocities in the range of 3,500 to 4,500 FPM are typical.
Duct cleanouts are recommended. If cleanouts are not used, it may be necessary to open up the longer
horizontal runs for periodic inspection. Ductwork installed outdoors may leak and accumulate moisture,
which will hasten the plugging problem in the duct as well as in the filter.
System ductwork is designed for use with a specific number of hoods. The branch line should be
capped if a hood is removed. If it is unlikely that another hood will be installed in the near future in the
same general location, then it may be desirable to remove the branch line all the way to the tee in the
main duct and then put a metal cap over the tee. Install a slide gate in the cap to bleed in sufficient air
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to maintain a minimum conveying velocity. A velocity pressure reading should be taken to determine
this.
If additional hoods are required at some other point in the system, a check of your ductwork drawing
will determine whether this is feasible. A change in ductwork might be necessary if the hood is to be
installed near the end of the system furthest from the fan. If it is to be installed near the fan, it is not so
critical as long as an airflow control gate is provided and proper balancing is utilized.
A more common problem occurs when additional hoods are added with no change in duct sizes, fan
RPM or motor HP. The fan will normally handle a fixed amount of air at designed total static pressure;
therefore, adding hoods merely robs other nearby hoods. This may not be serious if the resultant dust
control is still adequate. However, in most cases you end up with only partial dust control at an
increased number of points rather than adequate dust control on the original hoods.
The primary things to watch for are worn areas in elbows, leaking flanges, dented sections which
restrict air flow and hoods no longer in use but still connected to the system. An accumulation of small
leaks is the same as adding another hood with the results as outlined above. In any event, the worst
possible thing to do when ductwork becomes plugged is to use a hammer to clean it out. This only
aggravates the problem.
Fans
Fans are usually driven by an electric motor through a V-belt drive. Normal maintenance consists of
proper use of the right bearing lubricant at the correct intervals. Be careful not to over-lubricate. Refer
to the manufacturer’s maintenance manual.
Fans are subject to wear when they are installed on the dirty
air side of a collector. Excessive wear on the blades will
cause an unbalanced condition which will be noticeable in
excessive vibration and reduced efficiency. Worn bearings
will result in overheating and a change in the fan’s sound.
Once an operator becomes familiar with the normal sound of
a properly functioning fan, any abnormal sound will become
apparent and should be checked immediately. Spare
bearings and a spare wheel are good insurance against an
extended down period.
Be sure the fan is operating in the correct rotation after
performing any work that requires removing or disconnecting
the motor. A switch in lead wires is easy to overlook.
V-belts should be periodically checked for correct tension.
Belts that are too loose will result in belt slippage, causing overheating. Belts that are too tight will
cause the bearings to overheat. Never replace only one belt. Always replace the entire set. Correct belt
tension can typically be determined by placing a straight edge over the complete set of belts and
pushing downward so that the same pressure is exerted on each belt. A deflection of approximately 1”
at the midpoint between sheaves is considered normal. Refer to your belt manufacturer’s guidelines for
correct tensioning procedure.
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Dust Collectors
Collectors today are usually assumed to be felt cloth or cartridge filters as opposed to the older cyclonic
type of collector. Filters operate at roughly 99.99% efficiency or better as compared to 80% to 90% for
most cyclones. Efficiency in this case means the percentage of dust removed from the air stream in the
collector. Therefore, 80% efficiency would mean that 20% of the dust is being emitted to the
atmosphere.
Cyclones normally operate at a fixed pressure drop, but with varying efficiency that is dependent upon
a number of factors including:
the size and weight of the dust particles
velocity of the air inside the cyclone
atmospheric conditions
A cloth filter, on the other hand, operates at a fixed efficiency and a fairly constant pressure drop under
normal operating conditions and when properly maintained. If any malfunction occurs in the filter that
would cause added resistance to the air flow, less air will be handled by the individual hoods thus
causing more internal dusting.
It is apparent, therefore, that a properly operating, self-cleaning filter is the heart of any dust collecting
system, and the filter is usually the first piece of equipment to check when a change is noticed in
internal dust control. Selection of a filter with the proper cloth area is based upon the air volume to be
handled, the type of dust and atmospheric conditions. The filter pressure drop, or differential pressure,
will level off after the initial startup period. Differential pressure readings will fall within a relatively small
range unless trouble occurs. Filters are normally equipped with a magnehelic gauge, which indicates
the pressure drop across the filter bags. 2” to 5” is considered normal.
When a change in internal dust control is apparent by the appearance of more float dust in the
atmosphere, the mechanical operation of the filter should be checked immediately by referring to the
instruction manual. If there is no mechanical malfunction, then check the appearance of the filter tubes
themselves. Filter bags normally operate with a coating of dust. If the dust becomes caked due to
moisture or for any other reason, the automatic cleaning mechanism will not be able to clean down the
tubes as effectively. Caking will cause a rise in the differential pressure, which in turn causes a drop in
the total air flow with a resultant increase in float dust inside the plant.
Excessively caked bags should be removed and replaced with new bags. The operating manual will
give you instructions on the proper method to use for replacing the bags. Filter bags are made of a
variety of materials, with synthetics most commonly used now. It is important to rebalance the system
after a complete set of filter bags is replaced. Check the magnehelic gauge after bag replacement. The
initial differential pressure should be quite low, perhaps under 1” of water. Never replace only one bag.
Always replace the entire set of bags.
If you have any questions regarding the material in this manual, please contact your AIRLANCO
representative at 800-500-9777 or www.airlanco.com.
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