8.25
Fan Controls B. G. LIPTÁK
Fan
(1995, 2005)
Flow sheet symbol
INTRODUCTION The transportation of gases and vapors is an important unit operation in all plants, particularly in the unit operations that involve the moving of large volumes of air, such as dryers, boilers, cooling towers, and HVAC systems. The reduction in the cost of air or gas transportation contributes to plant optimization. This section describes the automatic control of fans and provides some advice on how to operate them at minimum cost. Fans transport large volumes (up to 1 million CFM) of gases at low pressures, usually at discharge pressures of a few inches of water to up to a couple of PSIG. For a detailed discussion of fan controls for HVAC, boiler, and cooling tower applications, refer to Sections 8.2, 8.6, 8.16, and 8.17. The emphasis of this section will be on the industrial application of fans.
(in. H2O), with the pressure-flow characteristics as shown in Figure 8.25a. If the operating point is to the left of maximum pressure on the fan curve, and particularly if the fan pressure exceeds 10 in. H2O, it is likely that pulsation and unstable operation will occur. This maximum point on the fan curve is referred to as the surge point or pumping limit. All fans must always operate to the right of the surge point. Under low load conditions, the fan can be kept out of surge by artificially increasing the load by venting the gas that is not required by the process. The other possible way of eliminating surging is to substitute the use of dampers on the fan discharge as final control elements with blade pitch, speed, or vane control.
FAN CONTROLS Fan Throttling
FAN TYPES Fan designs are classified into axial flow and radial (also called centrifugal) flow types, because of the difference between the nature of the flow through the blade passages. They move large volumes of air at relatively low pressures
Figure 8.25b shows both fan curves and system curves. The system curves, the relationship between flow (velocity) and fan discharge pressure, in mostly friction transportation systems are parabolic. The operating point for the process is where the fan curve crosses the system curve (point A on Figure 8.25b). Axial fans Tube axial
Propeller
150
Centrifugal fans
100
50
100 150 Percent of rated flow
e urv
0
Fo rw Ra ard c urv di al e tip
c ard
flow
0
ckw
50
Ba
Axial
Percent of rated static pressure
Vane axial
200
230
FIG. 8.25a 1 Different fan types exhibit different pressure-flow characteristics.
1946 © 2006 by Béla Lipták
Forwardcurved
Radial
Backwardinclined Backwarecurved
Airfoil
8.25 Fan Controls
1947
Blade motion
Static pressure (fan curve) Operating lever Gears
Sta tic effi cie P nc sys arab y tem ol cu ic r v Zo es ne of op tim um pe rfo rm
an ce
Blade
Drive shaft Center
Outlet velocity
Surge line +16° +12°
89%
Fan curve
B C
Horsepower (kilowatts)
E
Power
D
C
+4° 0°
11 −4° −8°
12
B
+8°
86% 80% 75% ies 70% enc i c 60% Effi
Sy ste m
D E
10
88%
cu rv e
A
Pressure
Total pressure
System curve
A −36° −32°
−28°
−24 °
−20°
−16°
−12°
Pitch angles
Capacity
Volume
FIG. 8.25b Upper curve shows optimum operating zone for fans; lower curves 1 describe the effect of suction vane throttling on the fan curve.
When the flow and pressure requirements of the process correspond to a point that is below the curve of the constant speed fan (points B to E in Figure 8.25b), one way to bring them together is to introduce an artificial source of pressure drop. In this figure, that source of pressure drop is a suction damper (a vane), which, as it is throttled, modifies the fan curve (from point A to E). While the introduction of this artificial pressure drop does result in the wasting of transportation energy, its advantage is that this throttling shifts the surge point to the left and thereby, at lower flows, allows for stable operation. Tube-axial fans are provided with adjustable pitch blades that permit the balancing of the fan to match the varying process load either automatically or by infrequent manual adjustments. Vane-axial fans are also available with controllable pitch blades (that is, pitch that can be varied while the fan is in operation) for use when frequent or continuous flow adjustment is needed. Throttling by varying the pitch angle retains high efficiencies over a wide range of conditions. Figure 8.25c illustrates both the variable-blade pitch design and its performance curves. The efficiency is near maximum at zero blade pitch angle, and it drops off as the pitch angle is increased.
© 2006 by Béla Lipták
FIG. 8.25c The design and characteristics of axial-flow fans with variable pitch control are described here. (Lower part is from Reference 2.)
From the standpoint of power consumption, the most desirable method of control is to vary the fan speed to match reduced process loads. If the load does not change too frequently, belt-driven fan drives can be considered. The speed in such designs is adjusted by changing the pulley on the drive motor of the fan. When the process load varies often or when continuous fan flow modulation is desired, electrical or hydraulic variable-speed motors are required. Figure 8.25d illustrates both the fan curves and the power consumption of a variety of load controls on fans at partial loads. From the standpoint of noise, variable-speed is preferred to the variable-blade pitch design. On the other hand, both the variable-speed and the variable-pitch throttling designs are much quieter and more efficient than the discharge damper or suction vane throttling-type systems. Safety Interlocks All fans should be provided with safety interlocks, such as the ones illustrated in Figure 8.25e. In the illustrated design, interlock #1 will stop the fan if either excessively high pressure develops on its discharge side (PSH-02) or excessively high vacuums are detected on its suction side (PSL-03). Both of these pressure switches are protecting the ductwork from
1948
Control and Optimization of Unit Operations
Discharge damper
Inlet vane
0 − 30' TD 05
Variable speed
3
2
2
5
Pressure
4
Pressure
Pressure
1
SS 1 01 Off-Auto
2
7
o −10" XCD c − 60" 06
R 04
1
M
Fan
8
FC
9
6
1
PSL 03
PSH 02
1
1
2
1
4 5 6
8 9
Power consumption-% includes motor and VAV system losses
Capacity
Capacity
Capacity
SS-01 Off
Power consumption 120
Damper control
100
Variable inlet vane (VAV)
Actions
PSH-02
PSL-03
R-04 (Reset)
—
—
—
High
140
Auto-On
— Low
XCD-06 TD-05 Fan (Damper) (Note #1) Stop
Close
—
Stop
Close
Time
Stop
Close
Time
Stop
Close
Time
Run
Open
Reset to start
Not high Not low Not reset Reset
— = Any state or condition (don’t care) Note #1: Shut-down delay used in interlock 2
60 40
AC adjustable frequency fan drive
20 Fan curve 20
40
Time
—
80
0
60
80
100
Rated CFM (RPM)-%
FIG. 8.25d Turndown efficiency and energy conservation are directly related. (Upper part from Reference 2.)
bursting (PSH) or from collapsing (PSL) due to the extreme pressure conditions. Both of these conditions can be caused by accidental blockage of the airflow. Once the fan is shut down, it cannot be restarted until the reset button (R-04) is pressed. This gives the operator an opportunity to eliminate the cause of the abnormal pressure condition before restarting the fan. Whenever the fan is stopped, its discharge damper (XCD-06) is automatically closed to protect it from flow reversal, which can occur if several fans are connected in parallel. This discharge damper is designed for fast opening and slow closure, to make sure that it is always open when the fan is running. The time delay (TD-05) guarantees that once the fan is started, it will run for a preset period, unless the safety interlocks turn it off. This protects the motor from overheating as a result of excessive on/off cycling. The fan cycling interlock (#2) is described below, in the paragraph titled Optimizing Multiple Fans. Fan Controls in HVAC Applications Air-handler controls are discussed in detail in Section 8.2. Therefore, here only the highlights will be mentioned of a
© 2006 by Béla Lipták
Conditions
7
Power
3
Power
Power
Interlock 1
FIG. 8.25e Illustration of the functioning of safety interlocks, which evaluate the state of suction and discharge pressures plus reset button status and, based on them, determine the fan, damper, and time delay status.
fan configuration where supply fan(s) is/are transporting heat or cooling by sending conditioning air into a number of spaces (users), while return air fan(s) bring most of that air back for reheating or recooling. Figure 8.25f shows the fan controls of a variable-volume HVAC system. EA
FC
FE 14
RF
PIC 11
RA FC
XP 17
PIC 20 D/A
RA Stops fan
FSL 14 SP = 90% FFIC 14
SF
FC XP 18
PSH 20
