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Thread: oversized??

  1. #1
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    oversized??



    Hello everyone, it seems every few months or so I come across
    the most unique applications.
    I had to change out a failed copeland compressor.
    Compressor rated at 3400 btu's @ 35F evaporator
    Evaporator rated @ 1200 btu's @ 45F
    System has a reciever and a sporlan 1/4 ton valve.
    This is a 134a system, box temp set to control 60-62F
    I called the manufactuer and he stated this was a common practice.
    Any thoughts??
    Bernie



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    You're unlikely to run the evaporator at a 15F TD with this matchup, nor at 45F (40 psig suction for R-134a). Depending up your design load, however, things will balance out somewhere. Question will then become: are the operating pressures and temperatures appropriate for the application?

    Here's a wild guess: suction pressures are running well below 40 psig...
    Prof Sporlan

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    60 deg F? I thought even wine fridges controlled at a lower temperature than that.

    I would guess that you have a single or two-door reach-in refrigerator. If my guess is correct, then I would hope you have a CPR valve to protect the compressor from high suction pressures.

    With all my assumptions in place, I don't see why this shouldn't work. Sounds like a custom built box for extremely humble duty. Is humidity a concern at all?

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    I'm thinking this is similar to the discussion in another thread "Freezer is now a cooler" in the commercial refrigeration section.

    The oversized coil results in the TD matching the TXV superheat, with the coil partially active.

  5. #5
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    The oversized coil results in the TD matching the TXV superheat, with the coil partially active.
    The relationship of superheat, room temperature and coil temperature is one that I have never seen observed until Gary tied it together. Pure Techmethod stuff.

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    This case is all hardwood. It is an open display for fine chocolates.
    The professor was right in his wild guess, suction was at 25 psi.
    Super heat at compressor was about 15F
    liquid temp was @ 83-85F
    Humidity is a concern but the airconditioning is set to maintain
    68F with the help of a hotgas bypass to keep pressures year round.
    The temp speced for the case is 60-62F,
    Discharge air temp is speced @ 50-52F
    The case does not employ a cpr for protection, but what I cant understand is why Im not experiencing high head pressure troubles.
    Seems to me the condensor is probably large enough to compensate for the small load. Is this a case were we can undercharge the system and achieve the same desired results??
    Bernie

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    The case does not employ a cpr for protection, but what I cant understand is why Im not experiencing high head pressure troubles.
    The condenser must handle the heat load picked up in the evaporator. You would only have head pressure problems if the evaporator were experiencing excessive load.

    During pulldown from a warm box, which would be the heaviest load, the TXV is running wide open, at which point the heat load is limited by the size of the orifice. If the orifice is properly matched to the compressor, there will be no overload.

    In a stable closed loop system, the refrigerant mass flow is equal at all points, and is normally regulated by the metering device. Under normal operation, the metering device dictates the rate of refrigerant flow, and the rest of the system adjusts itself to match that mass flow rate.

    Is this a case were we can undercharge the system and achieve the same desired results??
    Undercharging the system would limit the the mass flow by underfeeding the TXV. However this would de-stabilize TXV operation. It would be FAR better to match the TXV orifice to the compressor to limit mass flow.

    The temp speced for the case is 60-62F,
    Discharge air temp is speced @ 50-52F
    Assuming the evap leaving air temp is indeed maintained at 50-52F, this should give you RH of about 70% at 60-62F box temp.
    Last edited by Gary; 05-04-2003 at 07:46 PM.

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    Conventional wisdom,(which is always wrong in the refrigeration field) tells me if I have 10 gallons of water, how is it possible to fit all 10 gallons in a 5 pound bucket???
    This compressor has no means to unload and the evaporator is clearly mismatched.
    It seems on the surface the compressor would certainly suffer some type of operational diffuculties, weather it be high amps
    or head pressure .
    Now taking a logical approach to this tells me mass flow is decreased in this application and the compressor dosent have to work very hard at all. This would explain why their isn't any
    negative effects to the compressor operation.

    Can it be said that compressor efficiency is directly realated to the amont or mass flow thru its suction valve???
    If this this is true, this compressor cant possibly have the total mass flow back to it required for a long service life.
    The compressor that failed was installed in 1998.

    Its great to be thinking again. I changed jobs about 9 months ago and have been out of the refrigeration end of it for awhile.
    Its nice to be back.
    Thanks
    Bernie

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    Gary is correct in noting head pressure is a function of your evaporator load.... the higher the load, the higher your head pressure. The high head pressure problems are not likely related to having an undersized evaporator, if the Prof is permitted to call the evap 'undersized' for the moment. The undersized coil is causing it to operate at a very high TD, let's figure it at 62F - 29F (25 psig R-134a) = 33F, which is apparently the intent of the manufacturer.

    The Prof suspects Bernie is speculating the undersized evap is reducing the internal volume of the refrigeration system, causing refrigerant in the system to be backed up in the condenser and the high head pressure problems.

    The DX variety evaporator, however, doesn't hold much liquid refrigerant. The receiver (if present), and condenser can be expected to hold a greater amount. The liquid line, if it has any length, will also contain more refrigerant liquid than the evaporator.

    The head pressure problems are unlikely related to the undersized evaporator. More likely the case is a high load condition, excess refrigerant in the system, and/or an undersized condenser coil.
    Prof Sporlan

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    The prof was correct again, I almost think he's a mind reader.
    I was concerned refrigerant would back up, but his logic , and
    all who have responded have set me on the right path again.
    Bernie

  11. #11
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    If you will provide the basic list of measurements, we can pinpoint any system problems, and probably figure out why the compressor failed.

    Low side:

    evap air in temp
    evap air out temp
    SST (saturated suction temp)
    suction line temp near evap coil
    suction line temp near compressor

    High side:

    cond air in temp
    cond air out temp
    SCT (saturated condensing temp)
    liquid line temp at outdoor unit
    liquid line temp at metering device inlet

  12. #12
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    Hi Bernie,
    I have read the various posts, some are inclined to beleive that there is a high head pressure problem with this system. I can not see this in you posts, but you have said
    Im not experiencing high head pressure troubles
    You have also stated that
    Humidity is a concern but the airconditioning is set to maintain
    If the compressor is oversized, hot gas operatin will be activated quite often, this will lower the head pressure by virtue of sending the most of the compressor displacement straight back to the evaporator.
    Prolonged hot gassing will increase the suction superhead, even if the hot gassing point is just after the TEV.
    High suction superheats are extemely harmful to suction cooled compressors.
    To me this is a possible reason for you compressor failure, in fact considering the system design I think the compressor did quite wel to last as long as it did.
    Regards. Andy

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    Hi,
    sorry the second quote should has mentioned the hot gass bypassing.
    Regards. Andy

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    Re: oversized??

    Originally posted by bernie
    Compressor rated at 3400 btu's @ 35F evaporator
    .... but your evaporator temperature is NOT 35F. It is 25F. At 25F the compressor Capacity will decrease as a result in the increase of the compression ratio.

    If I reference a compressor with about the same capacity (at 35F) as you describe, then reference it's capacity at 25F I see about 2400btuh. With an evaporator rated at 1200buth (@10degF TD), or more accurately, 120btuh per (1)degF TD, the result is a design TD of 20degF, which is proper math for candy storage.



    PS: In such candy cases (as described), I find that these are most commonly utilized in Mall kiosks, and department stores that are usually somewhat dusty conditions. The most common cause of any call for service is a dusty/dirty condenser coil, and I would presume a most likely cause of compressor failure, but certainly NOT the evap/compressor match-up.
    Last edited by herefishy; 07-04-2003 at 03:36 PM.

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