Low Side Compressor R-502 Icing Up

[DTBtech]

Hey everyone, Looking for some feedback as to why this is happening...Had a couple thoughts as to why but cant figure it out.I work for a testing facility with a rel chamber. Its a cascade system. R-13 & R-502, stand by pressures are 125 & 150, oil pressures are both about 50psi running with a suction pressure of about 8 and 10psi for both. Compressors run fine till the bypass selonoid kicks in on the 502 for a satisfied temp of the chamber but the already iced up low side begins to ice up more and the oil pressure obviously begins to drop and trip on low oil pressure.. checked with a gage to make sure it wasnt the oil pressure switch tripping out. I seemed to have fixed that. Now the 13 side, first stage is tripping out on high suction temp... any suggestions?

[Gary]

Normally, when the chamber temp is satisfied the low stage compressor shuts off.

[DTBtech]

the both stages are on a timer, once the jc shows its satisfied it runs for 2 minutes then shuts off until its called for again... the 502 is still icing up before that is even close... but like i said that side is operating ok now but still icing which i have seen in other chambers here, but the 13 side is trippin on high suction temp cant figure that one out

[Gary]

Are you saying that the hot gas bypass is open during the 2 minute shut down period and then both compressors shut down?

Or does the low stage compressor shut down immediately and then the high stage compressor runs for an additional 2 minutes?

[DTBtech]

runnig the chamber at room temp 73f there is first stage r 13 and second stage r-502 both are in series to the 2 minute pump down. they both turn off after the 2 mins together as well as coming on. Like i said earlier though this really does not make much difference because it is happening even if the jc is calling constant for cooling.. it begins to ice up rite at start up on the 502 second stage even after on standby for 1 -3 days...
my question with that is, isnt that usualy a sign of liquid refrigerant coming back in the suction side?

secondly, what could cause the high suction temp to trip? meanwhile the 13 is only at about 8 to 10 psi on the suction

[Gary]

runnig the chamber at room temp 73f there is first stage r 13 and second stage r-502 both are in series to the 2 minute pump down. they both turn off after the 2 mins together as well as coming on. Like i said earlier though this really does not make much difference because it is happening even if the jc is calling constant for cooling.. it begins to ice up rite at start up on the 502 second stage even after on standby for 1 -3 days...
my question with that is, isnt that usualy a sign of liquid refrigerant coming back in the suction side?

I have never seen a cascade where both compressors start at the same time.

What normally happens is that the high stage (R502) compressor starts and must reach a predertermined temperature/pressure before the low stage (R13) compressor is allowed to run. During this time, the high stage suction line will frost. This is normal and does not necessarily indicate floodback.

Also, the low stage compressor normally shuts off first, during which time the high stage suction line once again frosts. Again, this is normal.

The high stage suction line frosts because the low stage compressor is not running.

On some cascades the high stage compressor runs non-stop.

I have never seen a hot gas bypass on a cascade system, so possibly you have something unusual there.

[Gary]

secondly, what could cause the high suction temp to trip?

If the low stage compressor is allowed to start before the high stage is cold enough, this could cause the low stage to trip on high suction temp.

[Gary]

I seemed to have fixed that. Now the 13 side, first stage is tripping out on high suction temp... any suggestions?

I would suggest that you unfix whatever you fixed and start over.

[lowcool]

as you have mentioned a pump down im inclined to think that half your problem maybe liquid line solenoid valve is not seating properly

[DTBtech]

cool...thanx for the input.. i just like to get other peoples ideas as well.... im the only hvac guy in the building so i dont have many people to talk to about this. i will take into consideration what you said and apply it... i will get back to you within the week to let you know how i made out

[DTBtech]

hey gary just thought id let you know it was icing up because i had only 30 lbs of 502 in it with the site glass bubbling... that particular chamber took 39 lbs of 502....issue has gone away...thanks for the chat sry i took so long to reply