LP switch pressure setting for Pump Down of Chiller

[venugopal]

Hi!!

I am facing a problem of rapid compressor cycling in the Air Cooled chiller package I am working with now. This is used for Air Conditioning application. The refrigerant used is R 22. Compressor used is Semihermetic reciprocating type. Lp switch setting at present is Cut-off 25 psig & Cut-in 35 psig. Chilled water outlet temperature is set at 45°F. Thermostat is controlling the liquid line solenoid vave. I feel that the compressor cycling is because of improper LP setting. What should be the ideal setting for this application? Pls share. Also pls suggest any other reason for this.

Thanks

[slingblade]

Lower it to say 5psi and widen the diff. to 30 psi.
Also check for an anticycle timer, 3 mins minimum.
Other reasons are faulty sol valve or compressor valves, try a pumpdown check on the liquid line and suction service valves to see if they hold.

hope this helps.

[fridg]

I would set the lp cut out at 2-5psig. just before the compressor runs into a vacuum.

Then the hp at your max ambient temp for the area
+10k for higher ambient +10k for safety.

[old gas bottle]

every time,just set the cut in to match the refrigerant,avoid short cycling,i allways set mine up with the gauges,as i,am sure you all do.?

[LRAC]

Check on the location of the sol valve is it close to the TEV or is it positioned after the reciever, if the first applies then set the L/P to cut out at 2-3 psi and the diff to the temp/pressure for R22 on the running pressure of the plant.If the secound point applies then a timer may have to be fitted as the compressor may bounce in and out a few times as the refrigerent is gradually being drawn through the TEV and evaporator to perform a pumpdown.

Regards
Lrac

[Andy]

Hi!!

I am facing a problem of rapid compressor cycling in the Air Cooled chiller package I am working with now. This is used for Air Conditioning application. The refrigerant used is R 22. Compressor used is Semihermetic reciprocating type. Lp switch setting at present is Cut-off 25 psig & Cut-in 35 psig. Chilled water outlet temperature is set at 45°F. Thermostat is controlling the liquid line solenoid vave. I feel that the compressor cycling is because of improper LP setting. What should be the ideal setting for this application? Pls share. Also pls suggest any other reason for this.

Thanks

Hi:)

the cut out pressure is set at 25 psi min to prevent the chiller from freezing and bursting, a kind of pressure/temp safety should the flow through the chiller stop.
I would not advise pumping down below this:)

there is cases were you can, but only with high water flow rates and a differential pressure switch fitted accross the chiller.

Better to bring the cut-in up and set the chiller controller for a selay of 10 minutes between off and on:)

Kind Regards Andy:)

[LRAC]

Thermostat is controlling the liquid line solenoid vave.

Hi Andy

Venugopal has stated in the above that the stat is controlling the pump down cycle when reaching temperature and as such no duty is required from the fridge plant, the refrigerent is just being recovered to allow the system to start unloaded.

However i do agree with the chance of icing up the evaporator is possible if the de-ice stat fails and the flow switch or Venugopal is confused about the solenoid position in the system i.e it may not be pump down but switches off when the compressor cycles on another control switch.

As always diagnosis is difficult if your not on site.

regards
Lrac

[gopalakrish]

Venugopal, there is recommended wiring diagram for pumpdown control given in Copeland OPI. But, they are not giving any specific set point for LP. So, as you experience compressor cycling there are only two ways:
1. Increase the differential. As you told, LP ct-in is alreaddy 35 psig, it is not wise to increase that. That will create complaint. So, the only way is to reduce cut-off. For this, around 10 pai seems to be better. Then 25 psi differential is possible.
2. Increase the anti recycling timer setting to 7 to 10 minutes.

A combination of both will be most useful.

[old gas bottle]

sliped up there,didnt take in the application, slap!!

[malik55]

Hi Venugopal,
Have you checked the water outlet temperature, there is possibility that chiller may cut off due to low load (satisfied thermostat),low water flow or a loose electical connection, check the electrical circuit and finds the actual cut off control,
Which make and model is of this chiller,
malik55