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umeshuv
13-05-2008, 10:05 AM
Hi all,

I have a question regarding use of R134a in a freezer. The compressor manufacturer's specs indicate a lowest evap temp (-) 35°C. At this temp, the suction pressure will be at something like 9 psia or about (-) 6 psig. Is there any problem with a system that operates at such low pressures in the long run ?? Can somebody throw light on this ??

old gas bottle
13-05-2008, 05:47 PM
obviously R404A would be better but R134A is similar to the old R12 and there were enough running away happily on that,most domestic freezers were on it too,low pressure should not be a problem unless it drops into a vacum then theres oil trouble.:eek:

750 Valve
17-05-2008, 12:40 PM
You must be talking of a domestic system, in a commercial applicaion there are much more favourable refrigerants, like r404a or r507.

I have serviced freezers on r12 and its replacements and found that you pretty much need to run a vacuum to acxhieve the desired SST (-35 as you have stated). I'm not sure that oil is a problem in a vacuum... that is the first I have heard of that, a vacuum - as in 10 inches of mercury is merely a form of reading pressure, yes it is below atmospheric pressure but as the system is not open to atmospheric pressure everything will operate fine provided there are no leaks - then air will be sucked in. There are plenty of systems that run vacuums to achieve desired conditions especially in low temp industrial on NH3 and centrif liquid chillers, most contain auto air purgers incase of air ingress though.

Billy Ray
17-05-2008, 08:01 PM
i've seen a few refeer units on R13A at low temp (-20 deg C).

Not sure why this is? or why it should not be?

Apart from the obviuos vacuum issue ! Any thoughts anyone?

Billy ray

powell
17-05-2008, 08:47 PM
Is there any problem with a system that operates at such low pressures in the long run ?? Can somebody throw light on this ??

umeshuv,

If this compressor you have is listed as a LT R134a unit then it's ok to run it in the operating range listed by the manufacturer. Just watch the capacity because it drops quite a bit as the SST drops.

If it's a R404A compressor and you just want to use R134a then it wouldn't be advisable.

Many old low temp R-12 freezer systems ran in a vacuum with no problem.

old gas bottle
18-05-2008, 09:16 AM
most of the old systems on R12 low temp used to run just above 0 psi,if they were set up propely and running correctly not in a vac,were talking old stuff here and in a lot of cases old semi hermetics ,i found anyway that if they had run short of gas or running in a vac for other reasons that after a time a good percentage of the compressor oil had been laying elsewere in the system,once it was running right again it would start returning to the compressor. how many times have you been to a job where the system has lost its gas and theres no oil in the sight glass ! running in a vac is just that,yes you might get the system to balance but its not good practice. thats how i see it anyway.:p

EmCoastHVACR
14-07-2008, 03:42 AM
I don't like the idea that the system would operate in a vacuum on the low side.

I heard some brazing solders are prone to leakage in R134 (Sil Phos? -- don't remember).

paul_h
18-09-2008, 08:00 PM
Anyone else here have the priviledge of refitting old r12 open drive systems for freezers to r134a?
Bloody nightmare, had to babysit that thing due to it running in a vacuum, checking the oil to make sure it all doesn't disappear.
Not to mention the fact it was open drive and is in a vacuum you never know what's going to happen with a crankshaft seal on r134a (we had a few failures on higher temp open drives after charging with r134a).
Domestics seem to run OK for some reason, most don't run in a vacuum anyway, maybe oversized evap, shorter capillary?
But putting r134a and TXV into a freezer design for r12 meant it would run in a vacuum most of the time. We had to make sure it never got below -16C in the freezer if we didn't want to babysit it or have any problems, it broke down every month otherwise, usually on a friday afternoon.

Harrison
21-09-2008, 01:42 AM
obviously R404A would be better but R134A is similar to the old R12 and there were enough running away happily on that,most domestic freezers were on it too,low pressure should not be a problem unless it drops into a vacum then theres oil trouble.:eek:
what are these oil troubles by the way...