What is the correct low side pressure

[MESHACK2]

I have a VMC33 coke machine that does not cool good. I think it is low on freon.It has R12 installed in it now. The tag shows that it holds 1lb. I have some R12 and would like to add to what is already in the unit.My question is What is the low side pressure suppose to be.
Thanking you in advance for yur help.

[johnieG]

It really depends on the application & system size, but in general between 18-26 psi for R12 in medium temp applications like soda machines.

[Larry]

Check to make sure that the condenser is clean and the fan motors are operating.  Either one of these will cause high head pressure and affect or is it effect the cooling. 

Also, check for oil on the tubing or in the cabinet, that might indicate a leak.  The sytems are sealed.  If you see a port on the lines, odds are there was a leak.  The valve stem, if there, could leak through the schrader valve (bike tire valve).

Make sure you tubing isn't rubbing on any metal or other tubing.  The vibration, in time, can wear away the copper or aluminum tubing, and cause a leak.

One last thing to check is to touch the two pipes coming out of the compressor.  The discharge side will be hot, be careful.  The suction side should be cold and preferably sweaty.  Be careful not to burn yourself, catch your hand in a moving fan blade, and watch out for wires. 

The pressures on a fridge for R-12 at 72F should be around 32 - 36 PSIG.

[johnieG]

Hmm, we can agree to disagree, but I'd never like to see sweat back on the suction line at the compressor, to me that would indicate an overcharge, or a flooded evap coil, plus with the coil to air transfer of heat (cold) you'd never get the box air temp down past 40-degrees at best with the pressure at 36 psi, but I prefer to measure the low side up at the evap itself ( as close as possible to the cap' tube,if room allows for a service valve) and again, I set my pressures a little lower than most, as most customers prefer a lower temp when serving adult beverages from the machines. I guess I'd stick with 28psi for soda. ( and I use hot-shot as opposed to R12)

[MESHACK2]

I want to thank  you very much for your help.
First let me say that I tapped the sucking line down below on the front of unit.I charged it to 20 plus psi and got ice on line back at the comp. I raised the psi to 30 which melted the ice and had sweat all the way back to the comp. The  low line was cold and the high side was hot. The temp inside box never got below 45.I have now removed some freon and the psi is 25. I have frost at comp and sweat at evapor. Do not know yet what the temp will pull down to. will check the first thing in the morning.
I pulled the shroud off the evapor to make sure it was clean and noticed the stat tube was not attached to anything at the outer end. It is the stat with the long tube . It has two or three circles and then looks like a coil spring but the end is not attached to anything.
My question is where does this tube attach. is there someplace in the evapor that you stick the end of tube or do you attach it to something.
again thanks for all your help.

[globalcompressors]

If you're running the "sweat" line back to the compressor, you really
are "overcharging" the system. Sweatback, on a medium temperature refrigeration unit, with an accumulator, at 72 degrees,
means you have a fully saturated evaporator coil as well as the accumulator. This does not allow room for expansion of the refrigerant as it trys to "evaporate" as it travels through the coil. If the refrigerant
cannot "flash" from a liquid to a vapor, you run a risk of
"liquid migration", washing the bearings, and locking up the compressor as well as never reaching temperature.

When I'm charging, on the bench, temperature 80 degrees,
I'll run it to about 18-22 psi or when the frost, on the cap tube at the entrance of the evaporator coil, just breaks free. 1/2" of sweat, up above the cap tube, you're done.

Eric

[Larry]

I stand corrected.  It's been at least fifteen years since I looked at a R-12 system.  I remember, or at least I thought I did, a pressure in the low to  mid 30's.  As for the cold and sweaty pipe near the compressor, that  works for a R-22 system.  I was wrong.  Well at least mt other tips should be correct.

I dug around to find an old pt chart and I still think, according to the chart, that a psig of 29 with a temp of 31F, would be the lowest allowed before the coil would freeze.  My guess of 36 was too high, and globals 18 seems awfully low bringing the evaporator temp to 15 F.  Wouldn't the coil eventually freeze?

I have two Coke machines and they both work properly.  There isn't any add on valves.  If I remember correctly, and that seems to be a problem lately, the CS-96's evaporator inlet line is frosted slightly and then sometimes its sweaty.  I only noticed this when I was reloading up the beers at a party.  I don't know if it was calling or not.  I would like to investigate, but my machines are burried right now.  I wonder if the pressures are in the teens.  Hmmmm.

[MESHACK2]

Well we are getting close! I let the unit run all night with the stat calling for low temp psi at 25. The temp inside box was 26 this morning but the low line had ice from the compressor out about 14".
I took out some freon a little bit at a time. Went from ice to sweating to frost. I am now at 19 or 20 psig and the cut off temp inside box is 34. The unit sounds good and it is cooling good. I have frost on the low side line from the comp out about 14". Which way should I go??
#1 Should I take out some more freon? Would this hender the cooling?Since I am at the frost stage if I took out some more would this bring on the ice like too low on freon?  As I said above it is cooling really good at this level. #2 Should I add a little freon to clear up the frost? This would start it back to a sweat. #3 Should I leave well enough alone?
Thank all of you very much for all your help!

[Yarochrehc]

Have to agree with Eric, your still flooding the evaporator coil. Let a little more refrigerant out to get rid of the frost.