EPR frosting

[B_roche]

From time to time I see an EPR on a rack thats frozen on it's outlet. The tech's im with generally pay it no mind, except maybe double checking the fans and gaskets in the case.

I have some confusion here, even if there are multiple TXV's there should be superheated vapor at those EPR's. It looks to me like metered liquid.

Im just wondering if this is a sign to anyone that this case should be checked out more, maybe pull em and look for a flooded evap? maybe my EPR setting is causing too much run time for the setpoint? I'm assuming a superheat check before the EPR would give me my answer?

Also can this be an issue with regards to compressor flooding, I've seen some foamy oil sight glasses which seem to also be ignored.

I start doing my own mantainences Monday and these are a few things I should probably find clarification on before I start writing "all ok" in the log book

[hvaccop]

Just because you see frost doesnt neccessarily mean you have liquid present. Keep that in mind especially with a low temp case. If you had a case with a -30 evap at the compressor you may have a 0 degree line that is frosted but liquid isnt present

[B_roche]

yeah I get that, my issue is with a drastic change across the EPR that goes away at the suction header. I remember seing a Copeland video where the compressor was totally iced and no liquid was present.

[hvaccop]

What is the temp diff across valve

[B_roche]

not sure, I'm generalizing here. I'll have the oppertunity to look deeper into these Monday, for now I'm speaking of several systems I've seen.

[B_roche]

I took a pic of this one today. I'm not concerned about the frost on the header or the compressors, but I do believe that there is something about this particular circuit that is off

[gerryboy00]

Condensation in the pilot line

[B_roche]

No kiddin? I never woulda considered that.

[gerryboy00]

it is

[B_roche]

Ok I brought it up in class but couldnt really get any more info....could someone elaborate on this scenerio for me?

[jaysongoff]

What I've usually seen is when you have a circuit that has the EPR set several pounds higher than the suction header, the valve modulates constantly and uses discharge gas throught the pilot line to force this modulation. The relief port exits to the suction line through the equlization line. This acts like a mini evaporator and causes the ice build-up on the EPR. During defrost, some of the ice may melt and drop down causing the cascading iceberg like in the picture.

[gerryboy00]

The relief port exits to the suction line through the equlization line. This acts like a mini evaporator and causes the ice build-up on the EPR..

exactly, but to build ice, we need:


liquid refrigerant or mixture of vapor/liquid(in pilot line)
pressure drop (orifice in the (s)orit)
low pressure side (suction header)




During defrost, some of the ice may melt and drop down causing the cascading iceberg like in the picture.

during defrost (from any other circuit on the same header) hot gas demand from that particular circuit swept the hot gas pipe from any condensed refrigerant causing ice to melt until defrost end and condensation reoccur.

[B_roche]

ok I get it, thanks.

[gerryboy00]

- a good way to verify...


close the circuit with the iced up EPR
close the hot gas supply line to the pilot for that particular EPR
make sure that no hot gas defrost is activate on any circuit while testing



the symptom will deviate to another EPR close to the one you just shut