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hot chiller

Something like this without the reciever under it ? note the 2 dual pressure controls. Has anyone actually checked the condensor capacity ? If they are both the same and only the header is different ....I'm no engineer but common sence says either the single is grossly oversized or the tandem is undersized.

If its been an ongoing problem god only knows what people have been doing to it to "fix things" Any idea if the old water cooled setup was a shell and tube ?

That system is about 40 tons on a dx coil and 4-5 years ago we changed out the evaporative condensor the only reason we didnt go straight air cooled was the size of the condensor coil.


  • blackoakbob
    blackoakbob Member Posts: 248
    Called to check a nagging reset.....

    on a McQuay chiller. There is a single 35hp cope on 1/2 of the barrel with it's own remote air cooled condenser and an uneven parallel 25 & 35 hp on the other side of the barrel with it's own remote air cooled condenser. When it's above 85F outside the dual will trip out on high pressure CO set at 350#. If one compressor of the dual is shut off on the breaker the other will keep running but will not cool enough for the load. This has been an on going problem. Here's the stats: single 35hp is running well, 10F SH on the TXV, good suction gas cooling, cond outlet pressure 240# and disc temp at cond inlet 151F and 85F at the oulet. the dual with both comp running suction 56#, 11F SH on valve, 260# high side press at the return from the condenser (king valve), discharge temp at the cond 194F and outlet of condenser 110F. Voltage ok, sight glasses clear and clean, and cond are very clean. I thought the dual was over charged so I pulled out 23# of R22 and found a 8# drop in the liquid line pressure and a lower temp at the cond in of 167F in and out 83F. No change in evap pressure or superheat. Another note both condenser are physically the same size with the same number of fans, they measure 123 inches long, 88 inches wide, 68 tubes at four rows thick and approx 12 fins per inch. The headers on each are different sizes 1 5/8 on the single 35 and 2 1/8 on the dual, Model No. APD045 on the former and APD055 on the later. Any ideas or tips would be greatly appreciated. Best Regards.
  • don_144
    don_144 Member Posts: 27
    it still

    sound like it overcharge to me.not sure where you live
    but here we have seen some very hot days,with a lots of coil
    loaded up,and running high back pressure and head.

    Being thats the case I would think your back pressure should be higher even with chill water.I would check flow thru the barrell and see what part of the building that has
    bad evaporator fans,dirty coils,or they are just turn off all together.
    Couple that with a tech that only understand pressure and you'll have a overcharge system.

    I'll leave you with a quote I herd on another site,when they started getting deep into vapor density.

    The evaporator is the boss and all other componants do what it says via its bully messenger,the vapor density.
  • blackoakbob
    blackoakbob Member Posts: 248
    I will ponder ....

    your comment on vapor density for awhile. Yes, you would think the low side pressure would be significantly higher due to the higher head pressure. But the TD on the condenser is really the factor that lowers the capacity and causes the shut down when the ambient gets higher. I'll be checking it again this weekend to follow up on the adjustment of the charge. Perhaps the best thing to do would be remove and reclaim all the refrigerant and check it for non-condensibles. Evacuate and recharge after a replacement of the filter/drier canisters. Best Regards.
  • don_120
    don_120 Member Posts: 15

    are correct and yes my post was properly out of context.
    I was just thinking out loud and maybe setting up an example of how I've seen scroll and vapor density cause trip safety.Something I was told about how it could spike where you would not even notice it on your guage.

    If I can help you from carrying that recovery machine,try this.

    Turn off your chiller for 15 mins.Read your high side guage.Take a look at the pt chart and see what the temps should be,then take the actual temp.If the temps dont match you have non condensibles.

    Hope this helps.
  • Marty
    Marty Member Posts: 109

    I would be looking long and hard at the condensor fan motors and how they are controlled.(assuming there has been no problem in the past) does the tandem side have a reciever ? Thats an odd setup all the dual circuit chillers I work on have the same capacity on both circuits.
  • blackoakbob
    blackoakbob Member Posts: 248
    Thank you for ......

    your input Don and my back thanks you about trying to test in-place, the refrigerant. I may try that Monday AM. Best Regards.
  • blackoakbob
    blackoakbob Member Posts: 248
    This has been.....

    an on-going problem and I''m not the first to try to solve the puzzle. Here's more info: With all the units running at the above readings there was a 6F drop across the chiller 46F in and 40F out. No complaints of comfort issues in any area of the building. Part of my original observations was to shut down the condenser fan disconnect to check rotation on each of four fans, you never know, it could have been that easy! The unit is a McQuay PWA-080-1 and was water cooled with a cooling tower as original equipment. Then retrofitted 10 years ago with the air cooled condensers. The story is a single, "huge" condenser was enroute to the job when the trucker got off the road by error and smashed it in to a low bridge. So these where rushed to replace it. (Everything has a story!) Niether of the condensers have receivers on them. The trouble shows up when it's extremely hot. Here in MN it's not the heat it's the humidity and we've had a good run of it so far this year. I thank you for your input, anothermarty. Best regards.
  • larry_9
    larry_9 Member Posts: 33


    living here in Minn I know what you mean about the humitity I was wondering if you did a compreesor effeincy test yet? Shut down your suction valve and see if it can hold a 17" vacum. Plus is the dual compressosr sent into one solonoid valve with one TXV? And are they piped in tandom with equalizing lines to both compressors? I have found one of my compressors leaking by with loose rings blowing by and pushing my oil into the second stage copressor causing higher headed pressures than normal (not real high) and causing a little higher than normal discharge tempatures But the oil sight glass gave it away, with a vee patern starting to develope on the glass.

  • blackoakbob
    blackoakbob Member Posts: 248
    Compressor test.....

    I did close the suction on each loop and pumped down each compressor and they each held with no rise in pressure. Yes, the uneven compressors are on the same loop, they are copeland semi-hermetics and are joined at the rear. No oil problems have been noted on the dual system with the exception when either one is shut off, to keep the system from shutting down on high pressure, the off one gains oil level which is normal. There is no receiver on either system which is common with these McQuay chillers. I have worked on a number of them and they are alike. The fans used to be cycled on ambient temp but are set to be on continuously now. I'll change that if I find a solution or the temperature drops, which ever comes first. Thanks for your reply and insight. Enjoy the rest of the summer, it passes so quickly here. Best Regards.
  • michael terry
    michael terry Member Posts: 30

    wonder what the actual liquid line pressure is instead of discharge pressure. even though the single is giving no trouble it sounds like there is a lot of subcooling going on there.also it might sound strange but i have heard of excessive hydraulic pressures caused by oil at the instant of start up to cause nusance trips. a friend of mine moved a pressure switch to the liquid line instead of its original location near the compressor discharge.
    food for thought

  • rick_28
    rick_28 Member Posts: 4

  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Definitely a Problem Bob

    Sorry it took so long for me to get back to you, but it looks like this is the situation:

    You have an air cooled condenser on which you checked the rotation and operation of all the fans and also verified that the condenser coil is clean.

    You mentioned that the TXV is maintaining approximnately 10 degrees of superheat in the chiller barrel and that the water entes the barrel at 46 degrees and leaves at 40 degrees.

    There is typically a 7-degree approach temperature between the evaporator saturation temperature and the temperature of the water leaving the barrel. In this case, we have an evaporator saturation temperature of 33 degrees, which corresponds to a low-side pressure of 59 psig. This is close to the number you provided.

    Here's the deal.

    Since the water is leaving the chiller barrel at a temperature that is about 5 degrees below design, the amount of heat that is absorbed by the refrigerant is reduced, causing an increase in the superheat. Since the TXV is only responsible for maintaining constant superheat, the TXV will close to bring the superheat back to its desired level.

    So far so good.

    Well, if the TXV is closing, the refrigerant will back up in the receiver. BUT YOU SAID THERE ARE NO RECEIVERS!
    The receiver stores refrigerant until needed by the metering device without causing the head pressure to rise too high. The refrigerant is therefore backing up into the condenser, reducing the effective surface area of the coil, causing the head pressure to rise.

    Basically, what is happening is that you are working on a TXV system that is piped in as a critically-charged system. In this situation, all of the refrigerant is moving through the system all the time (or at least trying to do so).

    Be a hero... install receivers!

    Keep us posted.
  • blackoakbob
    blackoakbob Member Posts: 248
    That's it ,,,,,

    the tandem is like the picture you posted. I believe the old water cooled unit was shell and tube and mounted above the compressors because there is no cover over the compressors. I had a thought over the weekend of adding some kind of a subcooler to these units. I had a foggy recollection of a supermarket job I was on, decades ago, that had an undersized condenser and we routed the liquid line thru a heat exchanger on a separate system before it went out to the liquid distribution header. I don't believe the budget of this place will stand a couple of new condensers but maybe a couple flat plate heat exchangers and a single phase condensing unit or just a water valve and dump line to a drain would work during the peak demand. It's better than a garden hose on the condenser and the damage that can do long term. Thanks for your replies. Best Regards.
  • blackoakbob
    blackoakbob Member Posts: 248
    The high side .....

    pressure reading I am giving are from the King valve or return line from the condenser ( you say either, I say either). But the high pressure manual reset is coming off of the dischage port of the compressors. That's an idea, moving it to the former which would stretch the cutoff point a little longer. A set point of 350#, on the HP cutout could be raised or changed to 400#, too. We've had break in temps today and I am going to check with the Maint Chief to see if the removal of the refrigerant has caused any difficulty.Thanks for your input, michael terry. Best Regards.
  • blackoakbob
    blackoakbob Member Posts: 248
    This spring .....

    the staff pulled the fans and seriously cleaned the condensers, straightened fins and used some type of wash out chemical. The air flow off each fan seems like new. I know what you mean when people think just a little rinse off the bottom means it's clean. No unloaders are set up on this unit just the staging of the compressors.Best Regards, Rick.
  • blackoakbob
    blackoakbob Member Posts: 248
    That does lead to......

    what anothermarty refered to when he asked about a shell and tube water cooled condenser. Which would have some space in it used as a receiver. Although, I know of a similar unit,actually larger in horsepower, that runs without a receiver. This is a simple solution, anyway. Now, should I put them inside or out? It's never run in the winter, so outside will do and probably a more precise fan staging control will help maintain head pressure. Thanks, Professor and thanks to all who posted. I'll read everything over again and make a proposal. Best Regards to all!
  • michael terry
    michael terry Member Posts: 30
    bob could not

    resist one more comment on this situation.I think I remember you saying something about the oil level increasing in the compressor that was idle,was wondering if when this compressor started that due to the increased oil level and maybe even a defective crankcase heater,the hydraulic surge thing was popping that HPS.
    any how thank you for the thought provoking problem.
    I have enjoyed pondering it,

    mike terry
This discussion has been closed.