Chapter hvac engineering Fundamentals: Part 1 Introduction

Design Procedures: Part 6
247
Figure 8.30
Flow versus plug lift in a control valve.
Figure 8.31
Coil heating capacity versus hot water flow rate.
Design Procedures: Part 6
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248
Chapter Eight
Figure 8.32
a
Three-way mixing valve.
the flow rate of hot water through the coil. At lower flow rates, the
water temperature drop increases, with a resultant increase in heat-
ing capacity per pound. Thus, a flow rate decrease of 80 percent may
result in an output decrease of only 20 to 30 percent. The shape of the
curve is determined by the temperature difference between the enter-
ing water and the entering air; as the difference decreases, the curve
becomes flatter. A similar effect, although not so pronounced, is ob-
tained in a cooling coil.
In Fig. 8.28 the fluid flow direction is indicated as tending to open
the valve, i.e., to raise the plug off the seat. With flow in the opposite
(wrong) direction, as the valve plug approaches the nearly closed po-
sition, the velocity pressure of the fluid will tend to completely close
it—there is sufficient hysteresis in the operator linkage to allow this.
When the valve closes and fluid flow stops, the velocity pressure goes
to zero. This allows the valve to crack open, flow is restored, and the
process is repeated. The result is called
chatter,
which results in
water
hammer,
which can sometimes destroy a valve or piping system. Each
control valve has an arrow cast into the outside of the body to show
the flow direction. Three-way valves are made for mixing or diverting
service (Fig. 8.32
a
and
b
) and must be used only in the designated
arrangement. Because the fluid pressure differential tends to open the
Design Procedures: Part 6
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