Chapter hvac engineering Fundamentals: Part 1 Introduction

8.3.3
Controlled devices
In HVAC work, controlled devices are usually valves, dampers, or mo-
tors. As previously noted, these devices may be controlled in two-
position or modulating modes.
Design Procedures: Part 6
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Design Procedures: Part 6
245
Figure 8.28
Straight-through (two-way) control valve.
Figure 8.29
a
Quick-opening (flat seat) valve.
8.3.3.1
Control valves.
A control valve (Fig. 8.28) includes a body,
within which are passages for fluid flow; a seat; and a plug. The plug
is connected to a stem, which in turn is connected to an operator.
Where it penetrates the body, the stem is provided with some kind of
seal or packing to prevent loss of fluid. Many different materials are
used for the various elements to suit the requirements of temperature,
pressure, and fluid characteristics.
The three principal plug types encountered are shown in Fig. 8.29
a,
b,
and
c. Plug lift
refers to opening the valve by lifting the plug off
the seat. The flat or quick-opening plug is suitable only for two-
position control, as shown in Fig. 8.30; a very small lift results in a
large change in the design flow rate. The linear plug is notched or
tapered in a straight line; this results in a linear or near-linear re-
Design Procedures: Part 6
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