Texv influencing suction pressure or lazy compressor?

[Drew]

Another strange one........

Had a job yesterday where on a cold room normally running at -5c sst on 134a, for a 2c room we had a suction running at 0c with an acceptable superheat of 7c. The room was at 4c so suction seemed high. The sight glass was also showing short and hp was down but it was cold ambient with no hp fan control. I assumed the compressor was not pumping correctly so pumped it into a vacuum and watched it hold a 20 inch vac for half an hour . All ok. I then noticed oil on suction rotalock , tightened and topped up 134a. Suction still at 0c with similar superheat, clear sight glass. I then opened texv a turn which brought the suction up to 5c and noticed system flood back? Then shut down valve which lowered suction to -5c? Super heat up to 10c. During this all I thought I had heard a 'tick' coming from the condensing unit and so am not sure whether me shutting the valve or the tick noise lowered the suction. Could I have freed up a stuck suction valve which would have shown my effeciency test to be ok by still holding a vacuum, but still be inefficient? Or could my valve be playing up? An expansion valve shouldn't influence my evap pressure unless it's flooding back or starving and in this case I seemed to have two different evap pressures with acceptable super heat. One pressure however to high and the other correct. I'm confused with the fact that a texv shouldn't influence my evap pressure or a compressor that had all the signs of not pumping properly , but an effeciency test proved it to be ok and then it suddenly started pumping and suction dropped?
what a way to end the week.
any ideas!

[RANGER1]

Drew,
How are you measuring superheat exactly?

You say you have a room temp at 4c sst 0c & 7c superheat?

You need to check readings over a time period so readings are real & system is settled.
Maybe you are taking a snapshot of readings that are not true

[Drew]

Yes I was quite rushed and need to see how the system reacts over time. This unusually high suction with ok superheat was noticed on a call out by a colleague the day before after a 'room to warm ' call came in?The condensing unit is on the roof of cool room so I'm measuring the superheat off the compressor suction valve with thermometer on suction ( insulated) maybe 300mm from comp. there are no heat exchangers in the lines. It's a very basic little room , no bypasses , oil seps etc....maybe 3hp. To see my suction up , then after an effeciency test , a little bit of gas and playing with the texv all seems ok. ( for now)

[Drew]

The evap super heat I assume to be 5c due to 0c suction and allowing 2c for suction pipe?
therefore I'm measuring more compressor super heat than evap super heat but it's a short insulated run.

[Drew]

Conclusion: established that system was designed to run on 404a, but was charged with 134a with a 134a texv. This kept the suction high, but still maintained an average superheat. Took the client a while to notice as he has 2units in the room so the other one was carrying the load. This mismatch gave all the indications of an inefficient compressor or an oversized evap. I changed texv and recharged with 404a and all is good.

[RANGER1]

Conclusion: established that system was designed to run on 404a, but was charged with 134a with a 134a texv. This kept the suction high, but still maintained an average superheat. Took the client a while to notice as he has 2units in the room so the other one was carrying the load. This mismatch gave all the indications of an inefficient compressor or an oversized evap. I changed texv and recharged with 404a and all is good.

The good old days R502, R22 & R12! oh yeah & ammonia of course.

[paul 717]

Am having high head pressure on 3 stage system

[Drew]

The good old days R502, R22 & R12! oh yeah & ammonia of course.

Yes, a little easier then with stats that went ' click' . Don't know much about that last smelly gas though.