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TRG-TRC013-EN
User Manual
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Trane TRG-TRC013-EN User Manual

Air Conditioning Clinic
Air Conditioning Fans
One of the Equipment Series
TRG-TRC013-EN
Air Conditioning Fans
One of the Equipment Series
A publication of
The Trane Company— Worldwide Applied Systems Group
Preface
Air Conditioning Fans
A Trane Air Conditioning Clinic
Figure 1
The Trane Company believes that it is incumbent on manufacturers to serve the industry by regularly disseminating information gathered through laboratory research, testing programs, and field experience.
The Trane Air Conditioning Clinic series is one means of knowledge sharing. It is intended to acquaint a nontechnical audience with various fundamental aspects of heating, ventilating, and air conditioning.
We’ve taken special care to make the clinic as uncommercial and straightforward as possible. Illustrations of Trane products only appear in cases where they help convey the message contained in the accompanying text.
This particular clinic introduces the concept of air conditioning fans.
© 1999 American Standard Inc. All rights reserved
ii
TRG-TRC013-EN
Contents
Introduction ........................................................... 1
period one Fan Performance .................................................. 2
Fan Performance Curves ....................................... 11
System Resistance Curve ...................................... 17
Fan – System Interaction ....................................... 19
period two Fan Types .............................................................. 27
Forward Curved (FC) Fans ..................................... 28
Backward Inclined (BI) Fans ................................... 30
Airfoil (AF) Fans ..................................................... 33
Vaneaxial Fans ...................................................... 35
period three Fan Capacity Control ......................................... 39
“Riding the Fan Curve” ......................................... 40
Discharge Dampers ............................................... 44
Inlet Vanes ............................................................ 46
Fan-Speed Control ................................................. 48
Variable-Pitch Blade Control ................................... 49
period four Application Considerations ............................. 52
System Static-Pressure Control ............................. 53
System Effect ....................................................... 55
Acoustics .............................................................. 56
Effect of Actual (Nonstandard) Conditions .............. 58
Equipment Certification Standards ......................... 59
period five Review ................................................................... 60
Quiz ......................................................................... 64
Answers ................................................................ 66
Glossary ................................................................ 67
TRG-TRC013-EN iii
iv TRG-TRC013-EN
notes
Introduction
Air Conditioning Fans
axial
centrifugal
Efficient distribution of conditioned air needed to heat, cool, and ventilate a building requires the service of a properly selected and applied fan.
The types of fans commonly used in HVAC applications include centrifugal and axial designs. In a centrifugal fan the airflow follows a radial path through the fan wheel. In an axial fan the airflow passes straight through the fan, parallel to the shaft.
Figure 2
TRG-TRC013-EN 1
notes
period one
Fan Performance
Air Conditioning Fans
period one
Figure 3
Compared to compressors, the pressures generated by these air-moving devices within the ductwork of HVAC systems are relatively small. The measurement of these pressures is, however, essential to the determination of fan performance.
Measuring Pressure
atmospheric
duct
duct
pressure
pressure
One instrument that is available to measure these small pressures is a U-tube that contains a quantity of water. One end of the tube is open to the atmosphere (open leg), while the other end is connected to the ductwork (closed leg).
atmospheric
pressure
pressure
Figure 4
2 TRG-TRC013-EN
notes
period one
Fan Performance
Positive Duct Pressure
atmospheric
duct
duct
pressure
pressure
When the pressure within the ductwork is positive, that is, greater than atmospheric, the water column is forced downward in the closed leg and forced upward in the open leg. Conversely, a negative pressure within the ductwork causes the water column to drop in the open leg and to rise in the closed leg.
atmospheric
pressure
pressure
3 inches
3 inches
[76.2 mm]
[76.2 mm]
Figure 5
In this illustration, a positive pressure in the ductwork forces the water in the closed leg 3 in. [76.2 mm] lower than the water in the open leg. A pressure of 1 psi will support a 27.7-in. column of water. [A pressure of 1 kPa will support a 102-mm column of water.] Therefore, this length of water column is equivalent to a pressure of 0.11 psi [758 Pa].
However, to avoid having to convert units each time a reading is taken, inches (in.) or millimeters (mm) of water (H pressures generated by fans. Other common expressions of this measurement
are “water gage” (wg) and “water column” (wc).
O) are often used to measure the
2
TRG-TRC013-EN 3
notes
period one
Fan Performance
Inclined Manometer
atmospheric
atmospheric
pressure
duct
duct
pressure
pressure
reservoir
reservoir
Since some of the pressures observed in air conditioning systems are very small, the U-tube has been modified to improve the ability to read such small differences in water levels. The modification replaces one leg of the tube with a liquid reservoir and the other leg with an inclined tube. This instrument is called an inclined manometer.
pressure
Figure 6
Knowing the slope of the tube to be 10:1, a pressure applied to the reservoir causes the liquid to travel ten times further up the inclined tube to achieve the liquid level difference between the tube and the reservoir. This allows the pressure difference to be read with greater accuracy.
For example, a 0.5 in. [12.7 mm] H the device causes the water to travel 5 in. [127 mm] up the inclined leg.
Other instruments commonly used to measure pressures related to fans include the electronic manometer and mechanical gages.
O pressure applied to the reservoir end of
2
4 TRG-TRC013-EN
notes
period one
Fan Performance
Total Pressure
fan
fan
static
static
pressure
pressure
velocity
velocity
pressure
pressure
total pressure (P
total pressure (P
The total amount of pressure generated by a fan has two components: velocity pressure and static pressure. The velocity pressure is due to the momentum of the air as it moves axially through the duct, while the static pressure is due
to the perpendicular outward “push” of the air against the duct walls.
The total pressure is the sum of the velocity pressure and the static pressure.
) = static pressure (P
) = static pressure (P
t
t
) + velocity pressure (PP
) + velocity pressure (
s
s
)
)
v
v
Figure 7
Velocity Pressure vs. Static Pressure
vane
fan
fan
vane
damper
damper
Figure 8
For example, assume a fan is attached to a straight piece of duct that has a damper at the open end. To observe air movement, a hinged vane is suspended from the top of the duct.
TRG-TRC013-EN 5
notes
period one
Fan Performance
Velocity Pressure vs. Static Pressure
damper
damper
fully open
fully open
Figure 9
With the fan operating and the damper fully open, air moves through the duct unimpeded. The impact of the moving air causes the vane to swing in the direction of airflow. The pressure exerted on the vane is due to the velocity of the air moving through the duct, not the static pressure exerted on the walls of the duct.
At this point the outward, or static, pressure exerted on the duct walls is negligible. Nearly all of the usable fan energy is being converted to velocity pressure.
Velocity Pressure vs. Static Pressure
damper
damper
partially open
partially open
Figure 10
Partially closing the damper increases resistance to airflow. The fan generates enough pressure to overcome this resistance (static pressure loss), but this
occurs at the expense of velocity pressure. Part of the fan’s usable energy is now being devoted to generating enough static pressure to overcome the resistance of the damper.
6 TRG-TRC013-EN
period one
Fan Performance
notes
This build-up of static pressure results in reduced air velocity (velocity pressure) and therefore a reduction in the airflow delivered by the fan.
Notice that the hinged vane has moved toward a more vertical position. The reduced velocity pressure on the face of the vane causes it to move to a more neutral position.
Velocity Pressure vs. Static Pressure
damper
damper
fully closed
fully closed
Figure 11
Finally, when the damper is closed fully, airflow stops and no velocity pressure exists in the ductwork. All of the usable fan energy is now being converted to static pressure. The pressure on the back side of the vane equals the pressure on the face of the vane and it assumes the neutral (vertical) position.
Measuring Static Pressure
inclined
inclined
manometer
manometer
Figure 12
Both velocity and static pressures can be determined using the inclined manometer. Static pressure is measured directly by inserting a probe through the duct wall with its open end perpendicular to air movement. In this position only the outward, or static, pressure within the duct is sensed.
TRG-TRC013-EN 7
notes
period one
Fan Performance
Measuring Total Pressure
inclined
inclined
manometer
manometer
Figure 13
Another probe can be placed in the duct with its open end facing into the air
stream. This probe senses total pressure—the combination of velocity pressure plus static pressure.
Therefore, static pressure can be read directly, while velocity pressure is derived by subtracting the static pressure from the total pressure.
An alternate method would be to attach the open end of this manometer to the duct system, using it to measure static pressure. With one end measuring total pressure and the other end measuring static pressure, the difference read on the manometer scale would be equal to the velocity pressure.
8 TRG-TRC013-EN
notes
period one
Fan Performance
Fan Performance Test
throttling
throttling
device
fan
fan
manometer
air
straightener
air
straightener
manometer
dynamometer
dynamometer
The characteristics of a fan’s performance under various duct pressure conditions is tested by an apparatus similar to the one shown here.
The fan is connected to a long piece of straight duct with a throttling device at the end. The throttling device is used to change the air resistance of the duct. The fan is operated at a single speed and the power applied to the fan shaft is measured by a device called a dynamometer. As discussed on the previous slide, a single manometer is used to measure the velocity pressure—the difference between the total and static pressures.
device
Figure 14
The test is first conducted with the throttling device removed. This is called wide-open airflow. With no resistance to airflow, the pressure generated by the fan is velocity pressure only—the static pressure is negligible.
The throttling device is then put in place and progressively moved toward the closed position. The pressures are recorded at each throttling device position. When the throttling device is fully closed, only static pressure is being generated by the fan because there is no airflow. This point is called the blocked-tight static pressure.
TRG-TRC013-EN 9
notes
period one
Fan Performance
Determining Fan Airflow
Velocity Pressure (Pv) = Pt – P
Velocity (V) = Constant ×
√
s
P
v
ρ
Airflow = Velocity × Fan Outlet Area
Figure 15
Next, the measured velocity pressure is used to calculate the airflow delivered by the fan. The manometer measures the velocity pressure (P static pressure (P calculated by dividing velocity pressure (P square root of the quotient, and multiplying by a constant. Finally, the fan airflow is determined by multiplying the air velocity (V) by the outlet area (A) of the fan.
For example, assume the test readings for a specific throttling-device position are as follows:
■ Total pressure (P
■ Static pressure (P
■ Velocity pressure (P
Pa]
■ Fan outlet area (A) = 1.28 ft
■ Fan speed = 1,100 rpm (revolutions per minute)
■ Air density (ρ) at standard air conditions = 0.0749 lb/ft
) from total pressure (Pt). Next, the air velocity (V) can be
s
) = 2.45 in. H2O [62.2 mm H2O or 610 Pa]
t
) = 2.0 in. H2O [50.8 mm H2O or 491 Pa]
s
) = Pt – Ps = 2.45 – 2.0 = 0.45 in. H2O [11.4 mm H2O or 119
v
2
[0.12 m2]
) by the air density (ρ), taking the
v
) by subtracting
v
3
[1.204 kg/m3]
Proceeding with the calculations,
P
V1,096
Airflow V A× 2,686 1.28× 3,438 cfm (ft
V1.414
Airflow V A× 14.06 0.12× 1.69 m
It is determined that at this point, the fan, operating at 1,100 rpm, is delivering 3,438 cfm [1.69 m
10 TRG-TRC013-EN
3
v
------ 1,096
ρ
P
v
------ 1.414
ρ
/s] against 2 in. H2O [491 Pa] of static pressure.
0.45
----------------- - 2,686 fpm (ft/min)== =
0.0749
119
-------------- - 14.06 m/s== =
1.204
3
/s== =
3
/min)== =
notes
period one
Fan Performance
Plotting Fan Performance Points
2.0 in. H
O
2.0 in. H
O
2
2
[491 Pa]
[491 Pa]
static pressure
static pressure
3,438 cfm
3,438 cfm
3
3
[1.69 m
/s]
[1.69 m
/s]
airflow
airflow
Fan Performance Curves
This point can then be plotted on a chart that has static pressure on the vertical axis and airflow on the horizontal axis.
Figure 16
Plotting Fan Performance Points
static pressure
static pressure
airflow
airflow
Additional data from the fan tests establish other static pressure and corresponding airflow performance points for a given rotational speed (revolutions per minute or rpm).
Figure 17
TRG-TRC013-EN 11
notes
period one
Fan Performance
Fan Performance Curve
blocked-tight
blocked-tight
static pressure
static pressure
1
1
,
,
1
1
0
0
0
0
r
r
p
p
m
static pressure
static pressure
airflow
airflow
When a series of points is plotted, a curve can be drawn. The resulting curve graphically illustrates the performance of this fan when it is operated at a constant speed.
m
wide-open
wide-open
airflow
airflow
Figure 18
Notice that the curve extends from blocked-tight static pressure, with a corresponding zero airflow, to wide-open airflow, with a corresponding zero static pressure.
12 TRG-TRC013-EN
notes
period one
Fan Performance
Fan Speed
f
f
a
a
n
n
s
s
p
p
e
e
e
e
d
d
1
static pressure
static pressure
7
00 r
00
7
r
p
pm
m
5
5
0
0
0
0
r
r
p
p
airflow
airflow
9
9
m
m
Next, the fan laws are used to calculate the performance characteristics of this same fan at other rotational speeds. The subscript performance conditions; the subscript conditions.
1
,
,
1
1
0
0
0
0
0
0
0
r
r
0
r
r
p
p
m
p
p
m
m
2
m
refers to the tested
1
refers to the calculated performance
Figure 19
Airflow
-----------------------
Airflow
Static Pressure
-------------------------------------------- -
Static Pressure
Input Power
------------------------------------ -
Input Power
Fan Speed
2
--------------------------------
=
Fan Speed
1
2
1
2
1
Fan Speed
2

--------------------------------
=

Fan Speed
1
Fan Speed

--------------------------------
=

Fan Speed
2
2
1
3
2
1
The result is a family of curves that represents the specific fan’s airflow capacity at various speeds and static pressures.
TRG-TRC013-EN 13
notes
period one
Fan Performance
Input Power
0.5 hp
0.5 hp
[0.37 kW]
[0.37 kW]
static pressure
static pressure
1 hp
1 hp
[0.75 kW]
[0.75 kW]
Finally, using the measurements from the dynamometer and the fan laws,
curves can be calculated and plotted to represent the fan’s power consumption at each operating condition.
2 hp
2 hp
[1.5 kW]
[1.5 kW]
airflow
airflow
3 hp
3 hp
[2.2 kW]
[2.2 kW]
input power
input power
Figure 20
Fan Surge
high
high
pressure
fan
fan
low
low
pressure
pressure
When most fans approach the blocked-tight static-pressure condition, instability is encountered. This condition is known as surge.
Surge occurs when the quantity of air being moved by the fan falls below the amount necessary to sustain the existing static-pressure difference between the inlet and outlet sides of the fan. When this occurs, the pressurized air flows backward through the fan wheel, instantaneously reducing the pressure at the fan outlet. This surge of air enables the fan to re-establish the proper direction of airflow. The resulting fluctuation in airflow and static pressure within the fan and ductwork can result in excessive noise, vibration, and possibly damage to the fan.
pressure
Figure 21
14 TRG-TRC013-EN
notes
period one
Fan Performance
Fan Surge Line
surge
surge
surge
line
line
line
static pressure
static pressure
airflow
airflow
A surge line is established during the fan test procedure to indicate the area on
a fan performance curve where surge occurs. As long as the fan’s operating point falls to the right of this line, the fan will operate in a stable manner. If the fan is operated at a point that falls to the left of this line, the fan will surge.
Figure 22
Percent of Wide-Open Airflow
surge
surge
surge
static pressure
static pressure
Finally, to serve as a guide for selection, curves are established to indicate the percentage of wide-open airflow being delivered by the fan at various operating points.
This completes the typical fan performance curve. It shows the relationship between pressure and airflow, and can be used to graphically represent the fan’s interaction with the system.
line
line
line
50%
50%
airflow
airflow
60%
60%
% wide-open
% wide-open
% wide-open
airflow
airflow
airflow
70%
70%
80%
80%
90%
90%
100%
100%
Figure 23
TRG-TRC013-EN 15
notes
period one
Fan Performance
Tabular Performance Data
Figure 24
Fan manufacturers may present their performance data in graphical and/or tabular form. Similar to using the fan curve, by knowing the desired airflow and pressure-producing capability of the fan, the table can be used to determine the
fan’s speed and input power requirement.
For example, assume an application requires 6,800 cfm [3,210 L/s] at a static pressure of 2.5 in. H requires a rotational speed of 821 rpm and 4.25 hp [3.17 kW] of power to meet the requirements.
O [622.75 Pa]. Using this sample table, this particular fan
2
16 TRG-TRC013-EN
notes
period one
Fan Performance
System Resistance
return duct
return duct
supply duct
fan
fan
damper
damper
cooling
cooling
coil
coil
System Resistance Curve
Now that a typical fan performance curve has been developed, let’s see how the fan will perform within a system.
supply duct
return air grille
return air grille
supply
supply
diffuser
diffuser
Figure 25
With each airflow, an air distribution system imposes a certain resistance to the passage of air. The resistance is the sum of all of the pressure losses experienced as air passes through the ductwork, supply air diffusers, return air grilles, dampers, filters, coils, etc. This is the resistance, or static-pressure loss, that the fan must overcome to move a given quantity of air through the system.
System Resistance
2.0 in. H
O
2.0 in. H
O
2
2
[491 Pa]
[491 Pa]
static pressure
static pressure
3,500 cfm
3,500 cfm
3
3
[1.65 m
/s]
[1.65 m
/s]
airflow
airflow
Assume that a system is designed to deliver 3,500 cfm [1.65 m overcome the system pressure losses, the fan must generate 2.0 in. H [491 Pa] of static pressure.
Figure 26
3
/s], and that to
O
2
To illustrate how a system resistance curve is developed, this point is plotted on the same chart used to develop the fan curve.
TRG-TRC013-EN 17
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