6.1.3.9
Fan System Effect Factors. Fans are tested and rated
based upon a certain standard ductwork arrangement. If installed
ductwork creates adverse flow conditions at the fan inlet or fan
outlet, loss of fan performance is defined as a system effect
factor. The system effect factor can be caused by obstructions
or configurations near the fan inlet and outlet. For example,
failure to recognize the affect on performance of swirl at the
fan inlet will have an adverse effect on system performance. See
Figure 4 for methods to minimize fan system effect factors.
Refer to Air Movement and Control Association (AMCA) 201, Fans
and Systems for additional information on fans and system
effects.
6.1.4
Ductwork Details
6.1.4.1
Branches.
See Figure 5 and Figure 6.
6.1.4.2
Elbows.
See Figure 7.
6.1.4.3
Offsets and Transitions.
See Figure 8.
6.1.5
Testing and Balancing. Ensure duct design includes
adequate provision for testing and balancing, including straight
sections of duct with ports for velocity measurement. Air
straighteners may be required if sufficient lengths of straight
duct are not available.
6.2
Fans for HVAC Systems
6.2.1
Fan Selection
6.2.1.1
Major Types of HVAC Fans.
See Table 7.
6.2.1.2
Size. In most applications, the fan capacity required
is a function of heating and cooling loads, except where there is
a minimum prescribed air movement, such as an operating suite in
a hospital.
For the total room sensible heat load, calculate the
minimum supply air quantity to satisfy the sensible heat load as
follows:
EQUATION:
CFM = Qs/(Tr - Ts) * 1.10
(12)
where:
CFM = supply air quantity (cubic feet per minute),
Tr =
room design temperature (degrees F dry bulb), and
Ts =
supply air temperature (degrees F dry bulb)
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