Trane PID Control User Manual
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.
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.
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.
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.
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
=