Controlling 2-8 degree C Walk-In Refrigerator

[Elanhank]

I am interesting in controlling a 2-8 degree C Walk-In Refrigerator using a PLC. I have two compressors that are configured for master/ slave. Compressor comes on when temperature in Walk-In is Out of Spec (2-8). Also, every 3.5 days the master/ slave (lead/ lag) configuration changes. We currently have a Honeywell 4500 Chart Recorder Controller with external relays that's having problems. Any suggestions?

 

[itsmoked]

Here's a time porpotioning thermostat: (2nd from bottom item)

http://www.flaminsystems.com/page/page/2456297.htm

If you want to do that with the PLC I would ditch the 3.5 day thingy. Run two totalizers, one for each compressor. Whenever cooling is called for you turn on the compressor with the lowest totallizer time. The two compressors then play leap-frog from then on, always tending towards equal run times. Works great.

 

[OldBean]

What size are the compressors? When you say master and slave are they both feeding the same evaporators? Is there a common oil recovery system. How many stages of loading unloading are there? Depending on the answers to above there are many options open to you. Control from Suction pressure, Loading unloading control off room temp. Head pressure control and condenser control.

 

[Ashereng]

Elanhank,

What specific problem(s) are you having?