Chapter hvac engineering Fundamentals: Part 1 Introduction

376
Figur
e
11.8
Traditional
arrangement
for
double-duct
AHU
.
Equipment: Part 3
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377
Figur
e
11.9
Two-fan
double-duct
AHU
.
Equipment: Part 3
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378
Chapter Eleven
Figure 11.10
Volume damper for duct pressure control.
CFM. A reduction to 50 percent of the design CFM could result in a
theoretical reduction to one-eighth of the design fan work. In practice,
the method used to reduce the fan CFM determines the energy sav-
ings, and the full theoretical savings is never realized, due to mini-
mum system pressure requirements, to mechanical friction, and to air
turbulence. Three methods are used to reduce fan CFM.
1.
Damper in duct, either upstream or downstream of the fan
(Fig.
11.10).
This forces the fan to ‘‘ride up the curve’’ (Fig. 11.11), i.e.,
to increase the fan pressure at the lower CFM. Little or no energy
is saved.
2.
Inlet vane damper.
The inlet vane damper alters the fan perform-
ance, and a portion of the theoretical saving is realized. For actual
savings, consult the fan manufacturer. See the discussion in Sec.
5.2.5.
3.
Fan speed control.
Fan speed control allows most of the theoretical
savings to be realized—except for mechanical and motor efficiency
losses. Mechanical belt and variable-pitch pulley systems change
the fan speed while the motor speed remains constant. These sys-
tems are satisfactory for small motors and are usually limited to
residential and small commercial applications. Variable-speed
clutch drives—hydraulic and magnetic types—allow constant mo-
tor speed. Some of these systems are satisfactory for large motors,
but they have been largely superseded by variable-speed motor
drives. Variable-speed motor drives of the variable-frequency type
are the preferred method today (see the discussion in Sec. 8.3.3.3).
Equipment: Part 3
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