Chapter hvac engineering Fundamentals: Part 1 Introduction

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Chapter Eleven
Figure 11.12
Runaround bypass for VAV supply.
Figure 11.13
VAV return-air fan volume control.
difference between the supply and return CFM is needed, to match
the fixed exhaust CFM in the building. Because the two fans will al-
ways have different operating characteristics, it is not sufficient to
simply track speed. Either flows or pressures must be measured. Var-
ious methods have been proposed for doing this, some involving com-
plex and expensive control systems. The general rule is to avoid using
return-air fans unless the return-air system has a high-pressure loss.
Then a flow-sensing system such as shown that in Fig. 11.13 can be
used. In this system, the return-air fan volume controller is reset by
the supply airflow. Single-point flow sensing can be used, but greater
Equipment: Part 3
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