Chapter hvac engineering Fundamentals: Part 1 Introduction

Design Procedures: Part 6
249
Figure 8.32
b
Three-way diverting valve.
two-way valve, the valve operator must be designed to provide tight
closure at the shutoff pressure differential expected, which may be as
high as the ‘‘shutoff ’’ head of the pump or the static pressure of the
system.
The appropriate valve size is determined by means of the valve con-
stant
C
V
.
This number is furnished by the manufacturer and is used
to relate capacity to pressure drop or head:
Q
⫽
C
兹
H
V
where
Q
⫽
flow rate, gal / min for water, lb / h for steam
H
⫽
head loss (pressure drop), usually in lb / in
2
C
V
⫽
determined with valve wide open and decreases as valve
modulates closed
Selection of the control valve is made by determining the flow rate
needed, assuming a head loss, determining a
C
V
, and selecting a valve
with that
C
V
from the manufacturer’s catalog. Because the calculated
C
V
seldom equals a catalog
C
V
rating, most valves are chosen to be
somewhat oversized. Shutoff pressure, plug type, and valve materials
are also part of the selection criteria.
Design Procedures: Part 6
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