PID control

PID control• P: Measures the difference between setpoint and actual

value (error value)

• The bigger the error value, the more the controller reacts

• I: Measures the duration of the error value

• The longer the error takes, the more the controller reacts

• D: Fast reaction on error variatons (for instance because of a sudden load change)

PID control• Example: setpoint = 7 & LWT = 16

P: 16 – 7 = 9 big difference: all compressors to 100%

P: Measures the difference between setpoint and actual value (error value)

The bigger the error value, the more the controller reacts

The bigger the error value, the smaller the pulse interval

PID control

• Example: setpoint = 7 & LWT = 8

P: 8 – 7 = 1 small difference: compressors at 25% (example)

(not at startup >9,7)

LWT stays at 8 for 2 minutes (example)

I: long duration of error value compressors load up (>25%)

I: Measures the duration of the error value

The longer the error takes, the more the controller reacts

The longer the error takes, the smaller the pulse interval

PID control

• Example: setpoint = 7 & LWT = 18

P: 18 – 7 = 11 big difference: all compressors to 100%

after 1 min: LWT = 10

D: fast error variation (18 10 in 1 min) (example)

D reduces compressors to 50% (example) to avoid LWT to drop below setpoint of 7