Help please

[dave lamuncha]

I have a cold food vending machine that is giving me fits. At first the evap was freezing up. I leak tested, replaced dryer, evacuated the system and recharged with 12 oz of R134 as per the mfg. The label says 12 0z R134 and the design pressures are 90 lbs low side and 230 high side. I checked all fans are working fine, I start the unit and the pressure on the high side slowly build to about 240 and then the compressor shuts down. It looked like a possible overcharge situation so I removed some of the charge. Unit runs longer, but when I put the enclosure back together compressor goes down again. I then remove more charge, still goes down. If I cut the charge way back the unit will run longer but it will not get cold enough, I checked the expansion valve and it appears to be working correctly. Any ideas on what I should do next?

Thanks.

[wambat]

Vending machines normally run about 36*f air off evaporator and 105*f condensing temp, so your low side pressure should be about 17# and hi side about 104#
This is a long way from the pressures you give for R-134. I think I would recover the charge pull a 500 micron vacuum hold for 15 min and weigh in another charge
I'm assuming this is not R-134a

[Lc_shi]

I wonder if there are LP and HP switch. Compressor is shut down by HP or LP or other ? Confirm it first. Why you guess overcharge? You checked the copressor current, is it normal ?
Would you pls collect the more data which is helpful to analyse?

rgds

[dave lamuncha]

Sorry about that it is R134A, and when first charged the low pressure side stays at around 45 lbs, the high pressure starts at around 175 and slowly creeps up to about 240 before the compressor cuts out. There is only a thermal overload on the compressor and I have changed it already. It does appear that the condensor and the compressor run hotter than I would normally expect, but the fan is moving a lot of air.

[coolman]

is the fan moving air through or just recirculating ?
Dit you completely clean out the condensor using water ?

Victor

[chemi-cool]

the high pressure starts at around 175 and slowly creeps up to about 240 before the compressor cuts out.

If its R-134a, then you are in trouble.
Even here at 40°C I don't get that kind of high pressure.
Check for blockage in the condenser,

Chemi

[chillin out]

The capillary is most likely blocked. Chop off the drier again and look into the end and you will see a coating on the inside which is restricting the flow.

[dave lamuncha]

My original issue was that the evaporator was freezing over. I thought I had a refrigerant leak but I could never find one. I did replace the dryer and ever since then I have had this problem. I changed the dryer again, same problem. The condensor isn't plugged, and the system doesn't have a capilarry tube it uses an expansion valve. I evacuated and weighed in the refrigerant. I notice that when it is running the condensor and compressor seem to run a lot hotter than they normally should. The condensor fan is pulling air through the condensor.

I agree there appears to be a restriction somewhere but how can I prove it? Could it be the expansion valve? Is that the reason why I had the freeze problem to begin with?

So what other trouble shooting information could I collect to help isolate wherre the problem might be? Or what are some things I could chek for or proactively do to try and isolate the problem.

Any trouble shooting help would be appreciated.

[wambat]

you could have a loose TXV bulb, non-condensibles,high evap air flow, or dirty condenser, all with hi amp draw
I lean toward non-condensibles; did you pull a hi vacuum on both sides of system?

[Lc_shi]

From your data,the evap temp is around 10C and condensing temp is around 60C it's higher than normal (50C).
pls check :
# evap frosting may be due to tube blockage;
# displace the sensor bulb of TEX to confirm if it's OK
others has been mentioned by above posts.

rgds
LC

[Gary]

So what other trouble shooting information could I collect to help isolate wherre the problem might be? Or what are some things I could chek for or proactively do to try and isolate the problem.

Low side:

Evap air in temp
Evap air out temp
SST (saturated suction temp)
Suction line temp near compressor

High side:

Cond air in temp
Cond air out temp
SCT (saturated condensing temp)
Liquid line temp near condenser

[dill]

just a thought, if you are sure there are no restrictions as looks likely, are you sure the refrigerant you are using is correct and of virgin quality. i have had problems in the past were r22 was put in a bottle marked 134a wich would give you simler problems

[Jase]

Check that the condenser fan motor is the correct size for the condensing unit as it may have been replaced at some point.

regards

Jase

[chilly]

I agree with above posts, Only extra I can offer is, a) My instict tells me the expansion valve is the most likely cause.
b) Is there information we dont have, Ie ask the end user when was the last time it was worked on, when did it fail, did it fail slowly or sudenly, do they have a copy of the last engineers time sheet, had it just been cleaned before the failure? How old is the system?

If all else fails I would be looking for of the beaten track problems, for instance has someone else changed the tev or orifice for the wrong type or size.

Also when you pull a vac, I would hold a vacume and monitor with a tor guage, or better still a micron guage.

Best of Luck