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CNT-APG002-EN 29
®
Chapter 4
Applications
This chapter describes several HVAC applications that use PID control. It
includes specific settings and recommendations for each application.
Discharge-air temperature control
When controlling hot/chilled-water valves in discharge-air applications, a
PID loop controls the position of a valve to increase or decrease the flow of
hot or chilled water. This section focuses on control of hot-water valves,
but control of chilled-water valves is almost identical. Seasonal
changeover control may be required in these applications, but is not dis-
cussed here.
In this application, one hot-water valve and one chilled-water valve con-
trol the discharge-air temperature serving a large space. The hot-water
valve and chilled-water valve each require a PID loop. Since the two
valves should not be open simultaneously, the hot and chilled-water valve
programs share valve position data.
Table 9 shows a PCL program for controlling a hot-water valve. Note that
the variable CWVALVE provides the position of the chilled-water valve.
Table 9: PCL program to control a hot-water valve
Line Result 1st Arg Operator 2nd Arg Description of Statement
---- -------- --------- -------- -------- --------------------------------------
1 HEATCALC = DISCHTMP DDC:4 HEATSP HEATCALC is an analog variable that
holds result of PID calculation
2 *L0 = DISCHTMP FAIL Has discharge-air sensor failed?
3 *L1 = CWVALVE GT *0.0 Is chilled-water valve open?
4 *L2 = *L0 OR *L1
5 = FANOFF
6 *IFT = *L2 If sensor has failed, chilled-water
valve is open, or fan is off
7 HEATCALC = *-10.0 then close hot-water valve
8 *IFT = FANOFF AND HEATOPEN If fan is off and hot-water-valve-open
request (override) is true
9 HEATCALC = *100.0 then fully open hot-water valve
10 *END =
11 HWVALVE = CONTROL HEATCALC Control hot-water valve to calculated
position
12 *END =