Short Cycling blues & low pressure control switch
Hello All,
I'm dealing with a condenser (404a),where the compresser short cycles before beginning a cycle, and after the compresser stops. I'm not sure, if the liquid line solenoid is leaking; if the pressure control has a leak in the diaphragm; or if the pressure line from the compressor is actually increasing in pressure and causing the pressure control contacts to close. I have been experimenting with different pressure control settings but have not found a solution. Help, anybody that can tell me how to test pressure contols switches, solenoids, pressure leaks through compressor valves, etc.
Re: Short Cycling blues & low pressure control switch
To test a pressure switch you really need it disconnected from the system so that you can use a pressure pump, or nitrogen, to check its range and setting accuracy.
As far as checking for leakage around the various components goes it needs a bit of monitoring to see what is happening around the system. Do you have sufficient pressure access ports to see what is changing?
Are there any service valves that can be closed to restrict any gas flow so that leakage monitoring can be done?
If you are saying that the compressor restarts after a planned stop then it can be that the pump down setting is too high leaving vapour in the suction line which will expand after shutdown. This sometimes happens also on long pipe runs.
If however the compressor stops shortly after a planned restart it sounds as though the low pressure switch is operating before the system has stabilised. If so then perhaps the LP setting is too high, as in scenario above. Also, fitting a delay timer across the LP switch would allow for stabilisation.
Re: Short Cycling blues & low pressure control switch
You can check the solenoid valve during the pump down cycle.
After the thermostat de-energizes the solenoid valve, the liquid line will start to get cold downstream of the solenoid. The liquid line may produce some condensation or frost. After the compressor shuts off, the frost on liquid line should disappear if the liquid has all been removed.
It may be common for the compressor to run several times in an effort to remove all of the liquid from the line.
If the compressor cycles on the low pressure switch after pump down, this is called a recycling pump down. The compressor continues to cycle on and off to keep the pressure down.
Another version of this is called a non-recycling pump down. In this case, the compressor only pumps down once. Once it is off, an auxiliary contact locks the compressor out. When the thermostat again energizes the solenoid valve the compressor can operate as normal.
The non-recycling pump down saves the wear and tear on the starter and compressor. However, you can still have refrigerant condensation back in the suction line.
Pros and cons to each.
If the low pressure switch is not set low enough the compressor may short cycle on the pressure switch. This is a setting you have to determine for the system.
The easiest way to check the compressor valves is to jump out the low pressure switch temporarily. Close the suction service valve and let the compressor pump down.
It should only take less than a minute for the compressor to pull a decent vacuum. Shut the compressor off. If it takes longer than a minute, the suction valves are leaking or the compressor is in a bad way.
If the crankcase pressure does not rise, the suction valves are OK. If the crankcase pressure rises rapidly (after you shut the compressor off) the discharge valves are leaking or broken.
That's it.
Check the compressor first as it is the easiest to check.
Re: Short Cycling blues & low pressure control switch
Quote:
I'm dealing with a condenser (404a),where the compresser short cycles before beginning a cycle
Fit a delay (on start) timer...job done.
Quote:
and after the compresser stops
Adjust the LP switch lower but not in a vac.
Has the system got an oil seporator?
If so make sure the heater is working.
Chillin:) :)
Re: Short Cycling blues & low pressure control switch
Thank you all for the recommendations. The system does have a long lineset, about 125ft. I have had the pressure settings at 5 psi cut out and cut in at 30. I also noted that the compressor starts as though someone just bumped the contactor briefly, then less then a minute heard the solenoid click, and the compressor started a new cycle.
Re: Short Cycling blues & low pressure control switch
I have had the diafram on the D**foss solenoid valves ripple on the edges passing liquid.It's normally on R404 freezer aplications when the valve is inside the case or room.It may be moisture seperating the diafram or the r404.not sure.Also I have had chatter from D**foss LP switches they seem to lose dif settings.Especialy if mounted were vibration.Check these
Re: Short Cycling blues & low pressure control switch
Is this a new system?
Has the system got more than one case running on it with seperate contols?Each with it's own solenoid?You may have one case cutting in and the comp is oversized for the one case but is ok when the other is calling.If so you may have to fit off loaders controled on a pressure switch.
Re: Short Cycling blues & low pressure control switch
Quote:
Originally Posted by Andy T
You may have one case cutting in and the comp is oversized for the one case but is OK when the other is calling.If so you may have to fit off loaders controlled on a pressure switch.
I good point to make.
Just to be safe, I recommend you also check the control wiring to verify nothing strange has been done.
When you say the compressor ran briefly does this mean one or two seconds, or less than 60 seconds?
The compressor could have just started on the low pressure switch, then pumped the suction line back down. Shortly after this the liquid line could open. This might be what you are seeing.
If the compressor did start up, then run for just a little bit of time, then shut off, then the liquid opened and the compressor started again this could prove to be normal.
Re: Short Cycling blues & low pressure control switch
Hi Mc cool:)
If the fans were stopped during pump down this would happen.
Timers are ok but something more to go wrong, try opening the LP diff up to stop cycling, the old school way:D
A pump down check on the compressor would be a good idea;)
Also if any other more than one cabinets are on the compressor this will also happen:)
Kind Regards. Andy:)
Re: Short Cycling blues & low pressure control switch
Thanks for all the many insightful perspectives. I had to make another call to this location today (4/8/06) for another system problem. The dairy case with the short cycling problem, was still at it.
I checked, and found the crankcase heater out. When the system went off on a planned shutdown, I checked to be sure the thermostat had opened, which it did. It being tied to the solenoid, I had both out of the control circuit, but the compressor started for 3 or 4 seconds then shut off again.
So, it appears the pressure is coming from the compressor to close the solenoid contacts and restart the compressor.
I reset the cut in at 60psi, high enough that the case stat will cycle sooner than the pressure setting does. However, I am worried about the ice build-up on the compressor suction valve!
Any additional ideas on what might be going on here will be appreciated.
The next step I believe is a pumpdown, to check the source of the pressure build up through the compressor suction valve as a recycling pumpdown, previously mentioned by US Iceman.
Pressure settings have been set today at 60psi- cut in, and 10 psi cut out. This seems to have ended the compressor recycling, prior to the thermostat cutting in at a 30 degree setting, ( there are a total of 4 cases, with a Copeland Discus 2DC3-050E-TFC-200/ Chandler- 41,290 Btu condensing unit.
Input appreciated.
Re: Short Cycling blues & low pressure control switch
Quote:
Originally Posted by US Iceman
The easiest way to check the compressor valves is to jump out the low pressure switch temporarily. Close the suction service valve and let the compressor pump down.
It should only take less than a minute for the compressor to pull a decent vacuum. Shut the compressor off. If it takes longer than a minute, the suction valves are leaking or the compressor is in a bad way.
If the crankcase pressure does not rise, the suction valves are OK. If the crankcase pressure rises rapidly (after you shut the compressor off) the discharge valves are leaking or broken.
Check the compressor first as it is the easiest to check.
Do as USIceman suggests. Check the compressor valves first.
Re: Short Cycling blues & low pressure control switch
Just a thought, but have you connected gauges to the suction and watched what happens when the comp cycles on/off?
I've had this previously where the sol valve opens as the comp kicks in and the suction keeps dipping below cut out until 3 or 4 stop/starts when pressure start to come up. Especially as you have long pipe runs.
We ended up putting in a delay on timer to allow pressures to settle before the compressor kicked in. As mentioned earlier.
Re: Short Cycling blues & low pressure control switch
Quote:
Originally Posted by Frank
I've had this previously where the sol valve opens as the comp kicks in and the suction keeps dipping below cut out until 3 or 4 stop/starts when pressure start to come up. Especially as you have long pipe runs.
We ended up putting in a delay on timer to allow pressures to settle before the compressor kicked in. As mentioned earlier.
Hi Frank.
Was the solenoid wired in parallel with the compressor contactor so they both started at the same time?
I have also seen something suspiciously similar.
Re: Short Cycling blues & low pressure control switch
Quote:
Originally Posted by US Iceman
Hi Frank.
Was the solenoid wired in parallel with the compressor contactor so they both started at the same time?
I have also seen something suspiciously similar.
yes it was. It was for a high risk food prep area, 10C indoor temp, semi hermetic comp, pump down control.
Everytime the stat called for cooling the sol valve would energise, the comp started and then a cycle of stop/start until the system stabilised. We watched the gauges and could see that the comp was too powerful on suction and caused several LP trips until pressures equalised.
Fitting the "delay on" timer allowed the liquid pressure to increase before the compressor started. Problem solved.
System's been running fault free now for 5 years. :)
Re: Short Cycling blues & low pressure control switch
Quote:
Originally Posted by Frank
Everytime the stat called for cooling the sol valve would energise, the comp started and then a cycle of stop/start until the system stabilised. We watched the gauges and could see that the comp was too powerful on suction and caused several LP trips until pressures equalised
Sounds a little like the TXV's were not being supplied with sufficient liquid pressure so that the evaporators could not maintain the suction pressure.
I seem to remember Dunham-Bush using a similar type of switch on their chillers some time ago.
