View Full Version : mcquay high pressures
09-12-2006, 01:35 AM
mcquay (alpc0045 ??)air cooled, 20 horse copeland 4re compressor, air conditioner. they just replaced the compressor two weeks ago and now after getting it going again, the pressures are way too high, and sight glass is not clearing. pulled out 32 #'s, weighed in to 20 and the suction and discharge shot up. I arrive and start checking things. Suction at 76 psig and 71 deg 6" off compressor, discharge at 364 psig and 230 deg 6" off compressor, liquid line pressure at 350 psig and 91 deg. air entering condensor at 84 deg, leaving air on condensor 107 deg, cold deck at 54 deg, space temp at 74.8, set point at 74, both compressors running fully loaded (manually commanded). I need more time to go throught the computer controls, I am questioning the unloaders being staged on by the thermostat on the computer whether they are N.C. or N.O., but that is of little relevance to this right now. the customer had blown out condensor with nitrogen and you could see light through the fins, it isn't that dirty or old. After talking with tech, I think his manifold set failed on the high side so he wasn't evacuating the high side and now he has got non-condensables in the system. I ran out of time to check it any further, but hope to make it back tomorrow. any other ideas or opinions?
thanks in advanced
09-12-2006, 01:53 AM
What's the compressor model? We need to know the refrigerant to check the superheat.
[Edited by The Saint on 09-12-2006 at 01:57 AM]
09-12-2006, 05:50 AM
Sounds to me like you have air in the system.
09-12-2006, 09:17 AM
refrigerant is r-22, I don't recall the exact model number of the compressor I think it started with 4re, I wrote it down, but I left my notes there. I'm hoping to make it back today and recover all the refrigerant, evacuate and check microns and recharge system with virgin. The sight glass is running about 50% liquid vapor and it acts like the condensor is stacked up and overcharged, yet the glass isn't clear and my superheat is high. I don't know why the suction pressure is so high with such a low load, it does't seem right.
I would think next step is to recover, replace drier(s), deep evac with micron gage is possible, and recharge
09-12-2006, 12:32 PM
that's my plan, I am hoping to make it back there today. The on-site guy is supposed to be recovering right now, so when I get there I can throw my micron gauge on the system, see what sort of vacuum he can pull and we'll go from there. This is a problem unit they said they do a compressor every other year, and I see a lot of little problems like txv sensing bulb weren't mounted in right locations, nor were they tight. I've got my work cut out for me to go through the system thoroughly. Thank you all for your replies, it's good to hear I'm on the right track.
09-12-2006, 04:35 PM
I see a lot of little problems like txv sensing bulb weren't mounted in right locations, nor were they tight
This alone will kill compressors . Your superheat will always be high and you will flood the compressors due to it !! It sounds like you are on the right track with recovering and putting virgin refrigerant back in . Be sure that you have no air traps in the system (between LLSV and TXV) .
09-12-2006, 07:03 PM
recovered all refrigerant and pulled good vacuum, got down to 500 microns, recharged system, head pressure now looks in line, but suction and superheat is way out. running 255 psi head @ 195deg, 80 psi suction @71deg 6" from comp. indoor air at 74, outdoor at 85. liquid line 255psig @ 113deg, txv outlet 71 deg, evap outlet 74. i am thinking txv is bad, you can adjust all you want and it doesn't make any difference on the temperatures. unit is mcquay air handler-lml128eh, condensor-ald045c, compressor copeland 4de3r18me-tsk-800. i never thought non-condensables would effect the head pressure like they did, it was a good lesson. any ideas on the txv, or did I miss something. thanks in advanced
09-12-2006, 07:31 PM
I would agree on replacing the TXV . What probobally happened was that whoever has been changing out the compressors has not been finding out why they went bad in the first place . I would also bet that someone has played with the TXV to an extent where it is no longer any good . Your LL is kind of warm also . Did you replace the drier ? What is the pressure drop across it ??
I inherited a every other year compressor unit. in fact it is a mcquay. i would not allow it to go online untill i went through it completly. mine was wired incorrectly. If the controller was calling for second stage, the only safety that would stop the compressor was the oil pressure safety. pressure swithes and solid state mpdule was bypassed.
This was after i cleaned the system up. with multiple compressor failures you would expect the system to contaimnated as it sounds. good luck dont put it online till it is right or someone else is willing to take the heat.
09-13-2006, 12:41 PM
I have left the system down, they are supposed to change the txv and call when they have that done and are ready for startup. they tell me that the first year after this unit was installed that mcquay came to the site and removed the accumulators, but i think they might mean receivers, no one knows for sure why they removed them. i think it had to do with the system charge over 50#'s and the epa reporting. the diagram on the unit shows a 4 stage thermostat with 1 and 2 being n.o. for compressors and 3 and 4 n.c. for the unloaders. they have the system hooked up to bas, i am not sure what brand or anything, no one knows much about it and i need more time to go through it and verify operation. thanks again for all the replies. i am impressed with everyones knowledge and willingness to share it.
09-13-2006, 07:37 PM
you mentioned the liquid line @ 255 psig and 113F...that only equals 5F subcooling...that is not much...where is this measurement taken? hows the liquid line sight glass?...what happened to the old compressors? if they mechanically failed, you probbably have a lot of debris out in the system. if liquid line cores were not installed (or not installed properly), you may have some debris in the txv. i know that you are going to change it anyway but it would give the customer a lot of confidence in you if you could find out what happened first...chicken or egg.
09-13-2006, 08:03 PM
Next time the comp gets changed blow all the oil out of the system w/ nitro. Is this a package or a split? If you put the new txv in, don't start playing around with the sh adjustments. Check your supply and return temps, if your doing 20 degrees across, what more do you want it to do? When these compressors go bad..are they the valves going bad or is it a burn out?
I am not a fan of controlling unloaders with temp controller. The temp may be higher than set point but the evap or chiller barrel may not be suppling enough heat load to need the compressor loaded.
I perfer a reverse acting pressure switch to load and unload the compressor. You can add it to the circuit at the compressor with out changing the control network.
Bottom line the temp controller must be calling for a loaded compressor AND the suction pressure must be above 78 PSI. This way the compressor starts unlaoded and if a call for loaded compressor is activated the compressor will load and unload like a mechinical unloader.
You mentioned THE TXV was being changed, meaning it has a single TXV?
The compressor may flood back when you get it cleaned up and running.
09-14-2006, 12:23 AM
thanks for all the replies, i don't know any of the history on the old compressor failures as they are not on site anymore and i get different answers from the techs. with the system the way it was, i believe poor service practice is one factor and i don't think the system was ever started up and setup correctly. i too like the reverse acting unloader control, and i will be looking into their controls as soon as they let me. driers are being changed and have been changed when the system is opened. the liquid line pressure was taken off the drier canister lid and the temp was taken at the txv inlet. it has one txv per circuit- split system with 2 circuits, one 20horse, one 25 horse. the sight glass will not clear, one thing that concerns me was they told me mcquay removed the accumulators from the unit, but I think they mean receivers and I would think a system of this size and application should have them. this air handler is servicing an office building that is zoned and is a hot deck, cold deck, the cold deck temp was 58 deg when all these readings were taken, that's why the 80# suction and such high line temps concerned me. hopefully they can get the txv changed and i can see what is going on then. thanks again for all the help.
09-14-2006, 12:53 AM
need to take the temp and press at the same place...pressure is at liquid line filter cap (inlet) but your temp is after the filter...maybe way after the filter...could be plugged filter- drier...where is sight glass, before or after the filter. if it is after the filter-drier, then you need to recheck your pressure drop. discharge superheat at 77F which is not terrible. a 4RE3-2000-TSK unloads down to 50%. suction super heat is at 24F...14F temp drop across coil. about how much air is going across cold deck (50%, 75% 100%)? was the other compressor online when these values were taken? if so, how well is it running?
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