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Chapter 4 Applications
38 CNT-APG002-EN
The PCL program in Table 15 stages two cooling-tower fans. Figure 28
shows the same program in TGP. The behavior of the stages programmed
in this program is illustrated in Figure 30 on page 41.
Figure 28: TGP program for staging cooling tower fans
Table 15: PCL program for staging cooling-tower fans
Line Result 1st Arg Operator 2nd Arg Description of Statement
---- -------- --------- -------- -------- --------------------------------------
1 FAN_CALC = CWST DDC:1 CW_SETP DDC loop compares water temp to setpt
2 *IFF = CDWP1ST OR CDWP2ST If both chilled-water pumps are off
3 FAN_CALC = *0.0 set output to 0 to turn off all fans
4 *IFT = CWST FAIL If sensor has failed
5 FAN_CALC = *100.0 set output to 100 to turn on all fans
6 *IFT = FAN_CALC GT *63.0 If PID result > 63
7 CT1SS = CONTROL ON then turn on stage 1
8 *IFT = FAN_CALC LT *10.0 If PID result < 10
9 CT1SS = CONTROL OFF then turn off stage 1
10 *IFT = FAN_CALC GT *90.0 If PID result > 90
11 CT2SS = CONTROL ON then turn on stage 2
12 *IFT = FAN_CALC LT *36.0 If PID result < 36
13 CT2SS = CONTROL OFF then turn off stage 2
14 *END
Deadband blocks set
on/off points for each fan
Integral and derivative gains need
values even though the PID block
is set to proportional only