View Full Version : Deli fridge regular LP trip

19-09-2014, 07:36 AM
Hi all hope everyone is well

not really my thing so please bare with. Whilst working on air conditioning was asked to take a look at a deli fridge which wasn't chilling. On first inspection I reset the LP cutout and watched the unit run down to setpoint (5degC) without issue suction pressure was 3.5 bar R404A and sight glass was wet and clear. Stuck with it for a bit unit ran without issue for a good couple of hours controlling around 5degC. A week later got a phone call to say it's not working again ( looks like I've inherited this now) returned to site and sure enough tripped on LP. Check evap air flow all fans running, coil clear airflow good reset LP and unit runs fine again. Customers tells me the system is 13 months old and has been having this issue on and off a couple of months after install. The unit is cooling a deli well without doors or a cover but there isn't a clear pattern on trips so can't correspond it with high ambient temps. What we know is that when it works it works fine. Gas charge is ok (sight glass clear) could it be a prob with design or install. Could an over sized compressor be causing this. I have been back to site a hand full of times now over the last 6 months (same issue) but in that time I haven't seen any evidence of low pressure myself unit always runs fine when I'm around. Maybe it is a prob with the system but a usage problem ? Customer assures me they do not restrict the airflow around the well with salad containers etc. ???

Glenn Moore
19-09-2014, 12:50 PM
Hi Kirky
You say the sight glass is wet and clear, . If this is so it may simply be a case to replace the liquid line drier. What can happen with liquid line driers is that they can absorb moisture to a point that they can have a blocking effect due to the moisture contaminant, after some time of operation the drier becomes so choked with moisture that the system starts tripping on low pressure control. But once the system stops and the temperature rises this captive moisture in the drier can be released from the drier back into the system. By the time you get to the plant the blockage/restriction has disappeared and the system operates OK. Then over a period of operation the drier re absorbs the moisture until it causes the blockage again.
By fitting a new drier you will remove some of the moisture contamination in the system and most likely save the compressor from copper plating damage/failure at the same time..
I have seen this problem many times and often the service guy thinks there is no problem because by re setting the LP switch it all works ok until the next time.
The sight glass should change to reading DRY with the new drier fitted

19-09-2014, 03:16 PM
Stupid question maybe, but is there a solenoid on liquid line for pump down? Also i wouldn't fit a manual reset low pressure switch to this type of unit, auto reset will still protect compressor.

monkey spanners
19-09-2014, 06:57 PM
What pressure does the switch trip at? Also change the drier. You may need to change the sight glass too if it has been really wet as water will wash the colour change chemical out of the indicator paper.

19-09-2014, 08:39 PM
Gas charge is ok (sight glass clear)

This is not an indication of correct charge. If the unit is over charged you will still see a full sight glass.
A correct charge can only be determined by measuring superheat.

Glenn Moore
19-09-2014, 11:58 PM
Stick with the LP hand reset switch, as otherwise the compressor will short cycle and destroy itself with an auto reset switch. (Compressors cost a lot of money )
If the system works OK normally then it all points to the drier

20-09-2014, 01:19 PM
Are you sure it's not icing up,if it has a defrost then what are the settings? If it does not have an off cycle defrost I would fit one,it could maybe leading you up the wrong path because lp trips coil defrosts you reset it ice builds up over time and trips when you visit the ice has gone.

22-09-2014, 09:52 AM
Ok thanks for that guys. Next time I get a phone call I'll take a look at changing out the drier and sight glass. Give me chance to check charge and pressure test.

the LP switch is set for 1 bar. Set by installer
there doesn't seem to be a defrost customer tells me they didn't install one don't need it.
recently been told that the evap fans are from original install and we're not replaced
recently been told that they were having similar issues with a display fridge and the tech throttled open the expansion valve to raise suction pressure.

Lots to look at.

22-09-2014, 08:24 PM
the LP switch is set for 1 bar. Set by installer
there doesn't seem to be a defrost customer tells me they didn't install one don't need it.

Lots to look at.[/QUOTE]

Hmmm... I would imagine you will need a defrost of some kind. Probably off cycle. I would speak to the manufacturer and get their recommended parameters. Could be evaporator coil freezing up causing your LP trips.

Often cabinets are sent out with controllers factory set. Defrost settings etc should be programmed on commissioning to suit the cabinet. It's possible this has not been done correctly or at all.

23-09-2014, 12:00 AM
here here!

23-09-2014, 10:39 AM
Stupid question maybe, but is there a solenoid on liquid line for pump down? Also i wouldn't fit a manual reset low pressure switch to this type of unit, auto reset will still protect compressor.
I see where you are going with this AL:cool:

Auto reset LP with the restart at around 3 degrees C, using R404a on the PT chart.

If the drier, refrigerant or TX is twitchy, at least one can see the drama in real time rather then showing up on site with the kit shut down on a manual reset....then a restart shows bugger all drama's.

I've been there and bought the "T" shirt.:) Captain Morgan says: Cheers.

13-02-2015, 09:58 AM
The saga continues.

some time ago I was lumbered with a faulty deli well fridge which was misbehaving. Basically the fridge would randomly trip on LP. On arrival the fridge manual LP switch would be reset and the unit would run without issue. It could run for months, weeks or days then trip again randomly not following any sort of pattern. Each time I returned to site the dam thing would run fine without issue. I even sat watching it all day one time and nothing happened so I left and sure enough 10 days later had another call out same issue. Anyway I checked the system charge which was low due to a leak on the LP cutout flared connection. Repaired it pressure test it all ok so recharged with gas quantity detailed on the commissioning report Great I thought cracked it. 6 weeks later same issue bugger. Luckily for me I then went on pat leave and left it to somebody else. 5 weeks later another call same issue. On arrival noticed the expansion valve was hunting like mad valve had been throttled almost fully open, LP switch had been changed for auto reset and extra evap fans had been installed. Decided to decant the refrigerant and recharge to ensure the system charge was ok. The system took another 800g gas on top of stated install charge sight glass clear. I left the unit happily controlling around 5degC setpoint. Subcooling 15deg and superheat 10 deg. Pressure seemed ok 5 bar suction 20 bar liquid all looked well. Lasted 6 weeks and went again. Luckily I was already on site working on other stuff. Fridge had stopped system was at 25 degC on stat. Carried out a technical reset ( turned off and on) compressor tried to start but wouldn't. Didn't have a permit so told them I would return Monday to check out the comp. months of running with the LP problem had finally killed it or so I thought. Returned Monday checked comp all ok turned fridge on everything ran fine except superheat was on the low side at 6 deg. Unit must have been out on LP but customer told me unit had been down since day before. What am I missing ? I've been in this game now a good 10 years and should have cracked this by now. We usually work on air conditioning so this sort of thing isn't the norm any help most welcome The fridge is a deli well without a cover or door. The setpoint is 5 degC. R404a refrigerant there is no defrost fitted on the system they turn the unit off at night. The TEV bulb has a good connection I have never seen any evidence of freezing on the evap. Could this be a design issue undersized evap coil etc ????

13-02-2015, 01:53 PM
You can set the LP to 0.5 bar without any worries.
I believe the condenser fan works all the time and there is no head pressure control.
It can certainly be low ambient at night when there is no heating and low pressure will drop under the set point.

As others have mentioned, air defrost at 4 hours intervals for 20 minutes will help the evaporator stay free of ice.

14-02-2015, 01:10 AM
Think I would definitely set the lp lower,setting an auto reset to certain parameters can work as a defrost system

monkey spanners
14-02-2015, 03:48 PM
Wonder if evap fan relay is intermittent, would drop the fans out and then the tev would shut and lower the suction pressure. Or are they EC fans? Think they will stop if there is a dip in mains power, need turning off to rest.

14-02-2015, 06:39 PM
How does it control on thermostat..pump down?

15-02-2015, 01:06 AM
If it had a pump down would it not have an auto reset lp?

15-02-2015, 03:42 PM
Hi guys sorry for the late reply

the fridge set point is 5 degC it will run at 5 degC for around 30seconds then kill the compressor. Evap fans stay in. Once it reaches 7 degC the comp is restarted. There is no pump down, no sign of a solenoid valve. The fridge is switched on at 7am each morning and runs until 3pm when it is switched off overnight. We are not allowed to work in the area during food service so I haven't actually seen the fridge being used to keep food chilled. I have tried to simulate it to see if when the food pots are put in it effects the fridge but the fridge ran fine no issues. As for room temp the site is 24/7 so room temp stays between 21-23 degC. The LP cutout switch is set at 0.5 bar. EC fans ? What's that please elaborate thanks for your time on this guys

15-02-2015, 03:58 PM
Some info about AC fans:

EC stands for Electronically Commutated, and it combines AC and DC voltages bringing the best of both worlds. The motor runs on a DC voltage, but with a single phase 230VAC or three phase 400VAC supply. The motor incorporates voltage transformation within the motor. The non-rotating part of the motor (stator) is extended to make room for an electronic PCB board which includes power transformation AC to DC, as well as the controls.

Credit to the www.

Another thought that comes to mind, assuming evaporator is clean and all of its fans run in the right direction.
WHen people from the shop arrange the food in the fridge, they must leave a space at the back to allow proper air flow. look at that please.

And my last advice to you, add some refrigerant to raise the low pressure and you will not hear from them for a long time.

15-02-2015, 05:42 PM
You said that when you attended for same issue after pat leave that an auto reset LP switch was fitted? So what was suction pressure then? Was it short cycling on the auto switch or had the comp gone out on clixon with no suction cooling? It was mentioned above that you should have some form of defrost on the system. Although you have not seen any frost try giving it a couple of off cycle defrosts, just 20 mins a couple of times during its operational hours.

15-02-2015, 07:51 PM
Thanks for the info

following PAT leave I was on site already went in for a brew and the restaurant manager collared me. Took a quick look and the thermostat was reading 27 degC unfortunately at the time I didn't have a permit to work or any tools plus it was mid service. All I did was turn it off and on checked the evap fan were running and the coil wasn't frozen. Everything as you would expect was very warm like it had been off a while. The compressor tried to start but didn't. I told them I would return the following day to check out the comp. when I returned the unit had been turned off. I turned it on the compressor started and the unit ran without issue getting from 24 degC to 5degC in no time. Suction 5 bar liquid 22 bar. The unit then ran fine until last Friday (1 week) before I had another phone call. I have advised the restaurant staff not to block the airflow leaving decent gap they say they do and from what I've seen they are. As I've said I haven't yet been stood over it when it trips always plays nice for me. I will defo take a look at the evap fans and program in a defrost or 2 cheers

23-02-2015, 12:56 PM
Well turns out that it is LP that's tripping that deli fridge anymore. During a recent visit it was reported that the fridge had stopped working following a good week without issue. On arrival turned unit on a thermostat read 31 degC? Anyway the compressor started and the fridge ran down to 12 degC when the compressor cutout at 5 bar suction 21.bar liquid. The compressor then proceeded to cut in and out a few times before restarting and running down to setpoint. The compressor was not hot infact it was cool to the touch. The suction pressure did not drop below 5 bar so it wasn't the LP cutout causing this and the head pressure stayed at 22 bar which is high but not high enough to stop it on high head pressure. Ideas ?

Glenn Moore
23-02-2015, 04:24 PM
What is the make and model of the condensing unit and compressor. What controller are you using?.
The condensing pressure is ridiculously high at 22 bar, and it may well be you are tripping on the compressors internal overload. Check that the condenser is clean and with a good air flow through it, if it isnt get a chemical spray to clean it, as the discharge pressure should be closer to 13 or 14 Bar. I know you AC boys like to see a high head guage but us fridge guys like to save our customers money by keeping the head guage down as low as possible and making our compressors life easier .
You say the compressor stopped and started a few times before restarting and running down to temp, could you hear any noise from the compressor starter as if something was tripping and resetting??? Kr Glenn

monkey spanners
23-02-2015, 08:12 PM
Does the fan stop when the compressor stops? If its the compressors internal protection stopping it the fan will keep running.

24-02-2015, 07:02 AM
pull the charge and refill it to the correct weight.then see whats happening

31-03-2015, 01:49 PM
The compressor was starting and stoping rapidly so it looked to me that the condenser fan stayed in but to be honest the comp switched that fast it wouldn't have time to stop. There is no sign of a clicking or any other device resetting, checked the klixon and it's not that. The wiring on this thing is terrible spent a good hour the other day ensuring all connections are good. The only info I can lift from the unit is that it's a Tecumseh CAE945OZ 12G10191808 4B88230202 which means nothing to me. LAE SDU12T0RE controller the compressor itself has no ID badge. I've watched this thing go from 34 degC to 12 DegC without issue the compressor then cuts in and out a few times before running down to setpoint 5degC. It then controls around setpoint for a while before the compressor starts to stop and start again same as at 12 degC. At no point does the compressor stop peminantly it only does that when I'm not their resulting in yet another call out because the well is at 20 odd degs. When I arrive turn it on the same thing happens again as above. I think I'm going to replace the controller starting from scratch setting up the parameters hopefully it's the controller playing silly buggers. Condenser is clean with good airflow. It's not head pressure stopping it either. Just to prove at what pressure it trips on HP I obstructed the condenser the head pressure rose to 25 bar without stopping before I bottled it and removed the cardboard. Unit continued to run and did not trip throughout test. So it's not LP or HP or the klixon. I will post following controller replacement.