material and may not be acceptable on material handling fans.
Two types of dampers are available:
• Outlet Dampers mount on the fan outlet to add resistance to the system when partially closed. These are available with both parallel and opposed blades. Selection depends on the degree of control required (opposed blade dampers will control the flow more evenly throughout the entire range from wide open to closed).
• inlet Dampers mount on the fan inlet to pre-spin air into the impeller. This reduces fan output and lowers operating horsepower. Because of the power savings, inlet dampers should be considered when the fan will operate for long periods at reduced capacities.
Variable pitch blades are available with some axial-type fans. The fan impellers are designed to allow manual or automatic changes to the blade pitch. "Adjustable" impellers have a blade pitch that can be manually changed when the fan is not running. "Variable" impellers include devices to allow the blade pitch to be changed pneumatically or hydraulically while the fan is operating.
A Variable Frequency Drive (VFD) may also be used to control flow. A VFD will control the fan speed, rather than varying the fan inlet flow conditions or the outlet area to change the fan's point of operation. This type of control varies both the flow rate and the fan static pressure.
The VFD control unit is connected in-line between the electric power source and the fan motor. It is used to vary the voltage and frequency of the power input to the motor. The motor speed will vary linearly with the line frequency. Most VFD applications use a direct drive arrangement; however, belt drives are occasionally used.
For a typical system with fixed physical characteristics, the attainable points of operation will fall on the system curve.
Forexample, Figure 6-10 shows points A I and A2 on a system curve. These two points of operation can be attained with a VFD by adjusting it for speeds of RPM] or RPM2. This will result in fan curve PQ] or PQ2, respectively.
VFDs do have disadvantages. They may have a low speed limitation. Most AC motors are designed to operate at their nameplate speeds. If a VFD is used to run a motor well below its nominal speed, the motor's efficiency will be reduced and losses will increase. This can increase motor heating and may cause damage.
The VFD can cause harmonic distortion in the electrical input lines from the power source. This may affect other electrical equipment on the same power system. Such distor- tion can be reduced with the addition of isolation transformers or line inductors.
To properly apply a VFD, the equipment supplier needs to know about its intended usage, about the building's power supply and about other electrical equipment in use. In general,
for applications where the minimum system air flow is 80%
or more of the maximum system air flow, the VFD's losses and higher initial cost may make use of the inlet damper a better choice for flow control.
An advantage of the VFD orthe Variable Pitch Blade over the dampers is often a dramatic power and noise reduction.
However, these accessories usually require additional con- trolling equipment. An advantage of dampers is their rela- tively simple installation and use and their lower initial costs.
6.3.2 Rating Tables: Fan size and operating RPM and Power usually are obtained from a rating table based on required air flow and pressure. Tables are based on FTP or FSP:
Fan TP = (SPoutlet + VPoutJet) - (SPinlet + VPinled [6.2]
Fan SP = SPoutlet - SPinlet - VPinlet [6.3]
Fan Rating Tables are based on requirements for air at stand- ard conditions (0.075 Ibm/ft3). If other than standard condi- tions exist, the actual pressure must be converted to standard conditions. See Section 6.3 .8, "Selection at Air Densities Other Than Standard."
The most common form of table is a "multi-rating table"
(see Table 6-1) which shows a range of capacities for a particular fan size. For a given pressure, the highest mechani- cal efficiency usually will be in the middle third of the "CFM"
column. Some manufacturers show the rating of maximum efficiency for each pressure by underscoring or similar indi- cator. In the absence of such a guide, the design engineer must calculate the efficiency from the efficiency equation
where:
QxFTP CFxPWR
Q x (FSP + VPoutlet) CFxPWR
11
=
Mechanical efficiency Q=
Volumetric flow rate, cfm FTP=
Fan total pressure,"wg FSP=
Fan Static Pressure,"wg PWR=
Power requirement, hpCF
=
Conversion Coefficient, 6362[6.4]
Even with a multi-rating table, it is usually necessary to interpolate in order to select fan RPM and BHP for the exact conditions desired. In many cases a double interpolation will be necessary. Straight line interpolations throughout the multi-rating table will introduce negligible errors.
Certain types of fans may be offered in various Air Move- ment and Control Association(63) performance classes identi- fied as I through IV. A fan designated as meeting the requirements of a particular class must be physically capable of operating at any point within the performance limits for that class. Performance limits for each class are established in terms of outlet velocity and static pressure. Multi-rating tables
TABLE 6-1. Example of Multi·Rating Table
Inlet diameter: 13" 0.0. Wheel diameter: 22%"
* Outlet area: .930 sq. ft. inside I Wheel circumference: 5.92 ft.
2"SP 4"SP 6"SP 8"SP 10"SP 12"SP 14"SP 16"SP IB"SP 20"SP 22"SP CFM OV
RPM BHP RPM BHP RPM BHP RPM BHP RPM BHP RPM BHP RPM BHP RPM BHP RPM BHP RPM BHP RPM BHP 930 1000 843 0.57 1176 1.21 1434 1.93 1653 275 1846 3.64 2021 4.59 2184 5.62 2333 6.68 2475 7.81 2610 9.01 2738 10.2 1116 1200 853 0.67 1183 135 1439 2.12 1656 2.98 1848 3.90 2022 4.89 2182 5.95 2333 7.07 2473 8.23 2606 9.45 2733 10.7 1302 1400 866 077 1191 1.51 1445 2.33 1660 3.22 1852 4.20 2025 5.23 2183 6.31 2333 7.47 2474 8.68 2606 9.95 2731 11.2 1488 1600 882 0.89 1201 1.69 1453 2.56 1668 3.50 1857 4.51 2030 5.59 2188 6.72 2337 7.92 2474 9.13 2606 104 2734 11.8 1674 1800 899 1.01 1213 1.88 1463 2.81 1676 3.81 1863 4.86 2035 5.98 2194 7.16 2340 8.38 2479 967 2610 11.0 2735 12.4 1860 2000 917 114 1227 2.09 1474 3.09 1685 4.13 1872 5.24 2040 6.39 2199 7.62 2344 8.89 2484 10.2 2613 11.6 2735 130 2046 2200 937 1.29 1242 2.32 1484 3.37 1694 4.48 1879 5.63 2048 6.84 2206 8.13 2351 9A3 2487 10.8 2618 12.2 2741 13.6 2232 2400 961 1.45 1257 2.56 1497 3.68 1704 4.85 1889 6.07 2056 7.33 2212 8.64 2357 10.0 2493 11.4 2622 12.8 2745 14.3 2418 2600 984 1.62 1275 2.81 1513 4.02 1717 5.25 1900 6.53 2065 7.84 1212 9.22 2364 10.6 2501 12.1 2631 13.6 2750 15.1 2790 3000 1038 2.02 1313 3.36 1543 4.73 1744 6.11 1924 7.52 2088 8.96 2241 10.4 2383 12.0 2517 13.5 2644 15.1 2766 16.7 3162 3400 1099 2.50 1358 3.99 1580 5.52 1775 7.05 1952 8.60 2115 10.2 2265 11.8 2405 13A 2538 15.1 1665 16.8 2783 18.5 3534 3800 1164 3.07 1407 4.69 1620 6.37 1812 809 1984 9.79 2144 11.5 2290 13.3 2428 15.0 2562 16.8 2684 18.6 2803 10.5 3906 4200 1232 3.75 1462 5A8 1665 7.31 1851 9.19 2018 11.0 2174 12.9 2320 14.8 2458 16.8 2587 18.7 2708 20.6 2825 22.5 4278 4600 1306 4.56 1520 6.39 1717 8.38 1894 lOA 2058 12.4 2209 145 2355 16.5 2489 18.6 2614 20.6 2736 227 2852 24.8 4650 5000 1380 5.49 1582 7.41 1770 9.53 1941 11.7 2100 13.9 2247 16.1 2390 18.3 2521 20.5 2645 227 2766 25.0 2883 27.3 5022 5400 1457 6.56 1647 8.57 1827 10.8 1990 13.1 2146 15.5 2291 17.8 2428 20.2 2558 22.6 2681 25.0 2798 27.3
5394 5800 1535 7.79 1719 9.93 1885 12.2 2045 14.7 2194 17.2 2334 19.7 2469 22.2 2594 24.7 2717 27.3 2830 29.8 Performance shown is for fans with outlet ducts and with inlet ducts. BHP shown does not include belt drive losses.
usually will be shaded to indicate the selection zones for various classes or will state the maximum operating RPM.
This can be useful in selecting equipment, but class definition is only based on performance and will not indicate quality of construction.
Capacity tables which attempt to show the ratings for a whole series of homologous fans on one sheet cannot be used accurately unless the desired rating happens to be listed on the chart. Interpolation is practically impossible since usually only one point ofthe fan curve for a given speed is defined in such a table.
(='WR)
/
0::
:~
Ld 0 [y
u) IX:
Ifl L']
Lj.l
b
ct:
0 0
FLOW f\ATE (0)
FIGURE 6-7. Typical fan performance curve
Today, most fan manufacturers have "electronic catalogs"
available. These catalogs are computer programs which can be used to calculate the correct fan speed and horsepower based on input data such as desired flow rate and fan static pressure or fan total pressure. Some electronic catalogs in- clude estimates ofthe affects of various fan accessories such as dampers and inlet boxes.
6.3.3 Point of Operation: Fans are usually selected for operation at some fixed condition or single "Point of Opera- tion." Both the fan and the system have variable performance characteristics which can be represented graphically as curves depicting an array of operating points. The actual "point of operation" will be the one single point at the intersection of the fan curve and the system curve.
Fan Performance Curves: Certain fan performance vari- ables are usually related to volumetric flow rate in graphic form to represent a fan performance curve. Figure 6-7 is a typical representation where Pressure (P) and power require- ment (PWR) are plotted against flow rate (Q). Other variables also may be included and more detailed curves representing various fan designs are provided in Figure 6-4. Pressure can be either FSP or FTP. This depends on the manufacturer's method of rating.
It should be noted that a fan performance curve is always specific to a fan of given size operating at a single rotation rate (RPM). Even with size and rotation rate fixed, it should be obvious that pressure and power requirements vary over a range of flow rates.
System Requirement Curves: The duct system pressure also varies with volumetric flow rate. Figure 6-8 illustrates the variation of pressure (P) with flow rate (Q) for three different situations. The turbulent flow condition is representative of
ld Ct:
~)
iUi~8ULE.N T FLOW 61" = C02
i LUVV F\AI[~ (0) I AMII\jAf;; Ff.CJW
6 p CO
~ f--- .. - ... -.- ... .
l_d CL Q.
rL.OW f~AT[
CONSTANT HEAD 6 P
=
C FIGURE 6-8. System requirement curvesduct losses and is most common. In this case, the pressure loss varies as the square of the flow rate. The laminar flow condi- tion is representative of the flow through low velocity filter media. Some wet collector designs operate at or close to a constant loss situation.
The overall system curve results from the combined effects of the individual components.
6.3.4 Matching Fan Performance and System
Requirement: A desired point of operation results from the process of designing a duct system and selecting a fan. Con-
sidering the system requirement or fan performance curves individually, this desired point of operation has no special status relative to any other point of operation on the individual curve. Figure 6-9 depicts the four general conditions which can result from the system design fan selection process.
There are a number of reasons why the system design, fan selection, fabrication, and installation process can result in operation at some point other than design. When this occurs, itmay become necessary to alter the system physically which will change the system requirement curve and/or cause a change in the fan performance curve. Because the fan per- formance curve is not only peculiar to a given fan but specific to a given rotation rate (RPM), a change of rotation rate can be relatively simple if a belt drive arrangement has been used.
The "Fan Laws" are useful when changes of fan performance are required.
6.3.5 Fan Laws: Fan laws relate the performance vari- ables for any homologous series offans. A homologous series represents a range of sizes where all dimensional variables between sizes are proportional. The performance variables involved are fan size (SIZE), rotation rate (RPM), gas density (p), flow rate (Q), pressure (P), power requirement (PWR), and efficiency (11). Pressure (P) may be represented by total pressure (TP), static pressure (SP), velocity pressure (VP), fan static pressure (FSP), or fan total pressure (FTP).
At the same relative point of operation on any two perform- ance curves in this homologous series, the efficiencies will be equal. The fan laws are mathematical expressions of these facts and establish the inter-relationship of the other variables.
They predict the effect of changing size, speed, or gas density on capacity, pressure, and power requirement as follows:
Q = Q (SIZE2 )3(RPM2)
2 , SIZE
,
RPM,
[6.5]P2 = P1(SIZE2 J2(RPM2 )2
(E.?.)
SIZE1 RPM1 P1
[6.6]
PWR2
=
PWR1(SIZE2 )5(RPM2 J3(E.?.) SIZE, RPM1 P1[6.7]
As these expressions involve ratios of the variables, any convenient units may be employed so long as they are consis- tent. Size may be represented by any linear dimension since all must be proportional in homologous series. However, impeller diameter is the most commonly used dimension.
6.3.6 The Effect of Changing Rotation Rate or Gas Density: In practice, these principles are normally applied to determine the effect of changing only one variable. Most often the fan laws are applied to a given fan size and may be expressed in the simplified versions which follow:
• For changes of rotation rate:
FLOW RATE (0) A '-/\N ANU SYSITM MATCH!::!)
l,d IX ::::J ()l (f) I~J I): - It
fAN '-~~
UESIR[I)
-~ ,
ACTUAL
i.OW I,A 11:_ (0)
AMEF-(l(' i\J CONF'E:EENCE:
o Ti' C () VF:P,NMENTAL,
INDUSTH,1 AL HYGIENIS'rS
DATEw 0::
::::J U]
U) Lu
CJ.:::
;l
FAN
ACTUAl
Fl_OW f~ATE (0) 8. WiWNC F/\N
FLOW F,AfT (0)