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  1. #1
    Join Date
    Jan 2006
    Location
    DFW Metroplex
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    4,910

    Compressor Over Sized - Need Suggestions (Long Read)

    Alright, I need some suggestions based on the information below (it’s a long read – sorry)

    Reason For Call:
    Medium temp compressor short-cycling and all circuit temps +50F or warmer

    Background:
    Old Mom and Pop type grocery store with 1 compressor for all low temp and 1 for all medium temp.

    MT (R401A) is a 6RA4-200A-TSK with 2 unloaders – #1 set at 20/12 and #2 set at 25/20. Suction control set at 12/2. There are 7 circuits on this compressor and all but 1 is controlled by t-stat/sol. (solenoids mounted in comp room). 24’ service meat is controlled by suction stop epr.

    Outside ambient -9F. Roof condenser is a circuited – med/low temp. Header end fan set to run constant – 1st & 2nd stage controlled by ambient temp.

    This is a new account and my first time on a call here. Log book in mechanical room indicated MT body had been changed 4 times in the last 7 years – the most recent being November of last year.

    Repairs and Observations:
    Receiver level 0%. Disconnected heat reclaim temporarily and opened disconnect on condenser. Added 40lbs R401A - pressures came up to 24/225 briefly and settled out at 20/125 after condenser disconnect was closed. I adjusted hold back to 110 psig and receiver leveled out at 15-20%.

    Even at +50F the circuit loads weren’t sufficient to load up all 3 heads. Within 15 minutes run time, the compressor actually started to short cycle again as circuits were becoming satisfied.

    I pulled info off circuits/compressor - came up with roughly 131,000 Btuh capacity/108,000 Btuh load.

    At one point, there was only 1 circuit (4 door ZZ cooler) calling at 8,000 Btuh.

    It doesn’t take a rocket scientist to figure out why the compressor is trashing itself every 1 ˝ years or so.

    Possible solutions (or band-aids):
    My first thought is to change the compressor to one that is suitable for the load but I just don’t feel right walking into a new account and telling them that $xx,xxx.xx is needed to fix the problem especially after they just spent God knows how much on repairing what they have.

    Second thought is to change all t-stat/sol. to epr’s to help balance out the load. The compressor will still be over sized and return gas temp will higher than recommended but I’m thinking slow death by running with low load rather than fast death from short cycling.

    Adding extra load doesn’t seem feasible.

    More than anything, I'm trying to think of a cost effective means of extending the existing compressor life but I am open for ANY thoughts or suggestions on this one.

  2. #2
    EPR's is probably the way to go here. If you're concerned about temp of return gas is it possible to install head cooling fan and/or demand cooling? How is your comp ratio?

  3. #3
    Join Date
    Jul 2000
    Location
    Guayaquil EC
    Posts
    10,392
    That's a tough one to walk into for sure.

    Adding EPR's will certainly help even out the load on this system and reduce the frequency of the compressor cycling off on low pressure, but it won't solve the problem entirely.

    I suggest going with a HGBP with a quench or desuperheating valve as the first move. It'll give you a solid baseline load to keep that pump pumping. If you set it up with a HGBP valve capable of bypassing 5-6 tons you should be able to set and hold around 10# (or even higher if you tighten up on the unloaders) and keep the compressor happy and cool. This would likely be the least expensive option as well. Mom and Pop will like that aspect.

    You could go for EPR's after that, but I don't think you'll need them if the HGBP is doing its thing.

  4. #4
    Join Date
    Mar 2004
    Location
    Las Vegas, Nevada
    Posts
    1,647
    Dude -
    I don't know the term for this, I've seen systems with somewhat similar problems that were solved by bleeding Discharge Gas into Suction Header. Controlled by a Pressure Sensing Reg Valve. Wish I had more terminology....sorry man. Anyway, it'll keep your load up without "adding load". I'm sure the gurus here know what I'm trying to say. Someone help me out here....
    The views and opinions posted here are my own. They do not reflect the corporate policies of my employer and will most likely get me fired at some point.

  5. #5
    Join Date
    Oct 2006
    Location
    Emerald Coast, FL 30.1N 85.8W
    Posts
    681

    from Tran Recip Refrigeration manual

    Maybe adding these solutions may add to the thread:


    Cause: Inoperative compressor loading system. Solution: Repair or replace faulty contol

    Cause: Low pressure control differential set too close. Solution: Reset differential




    Other than that, I can see why you think you have an oversized compressor.

    Hope this adds to the thread; if not, oh well

  6. #6
    Join Date
    Jul 2001
    Location
    Gold Coast of Connecticut
    Posts
    4,593
    Quote Originally Posted by icemeister View Post
    That's a tough one to walk into for sure.

    Adding EPR's will certainly help even out the load on this system and reduce the frequency of the compressor cycling off on low pressure, but it won't solve the problem entirely.

    I suggest going with a HGBP with a quench or desuperheating valve as the first move. It'll give you a solid baseline load to keep that pump pumping. If you set it up with a HGBP valve capable of bypassing 5-6 tons you should be able to set and hold around 10# (or even higher if you tighten up on the unloaders) and keep the compressor happy and cool. This would likely be the least expensive option as well. Mom and Pop will like that aspect.

    You could go for EPR's after that, but I don't think you'll need them if the HGBP is doing its thing.
    This is exactly where I would go with this. The compressor has to keep running, most likely the short cycling kills the compressors.

  7. #7
    Join Date
    Jun 2001
    Location
    Michigan
    Posts
    12,077
    Pat, your fellow cheesehead will come home and help on this one. I work for free Leinenkugels.

    This is a great call. This is what gives me a hard on about this trade.

    Dude. Simple simple simple.

    Total candidate for my specialty. Hot Gas Bypass, with desuperheating valve. Make sure you got a headmaster. No fan cycling. NO FAN CYCLING.

    EPR every Load. Run the pump at lowest circuit.

    There my freinds is a rack on a single. Your done.

    Or. Slap in a new Glacier Scroll that can modulate to 5%.

    You have very little load at the moment in the ambients your dealing with right now. I bet that pig is right at close to capacity on a design load day.

  8. #8
    Join Date
    Dec 2006
    Location
    Vancouver Island, British Columbia Canada
    Posts
    87
    I've experienced a similar problem with a r-22 based curling rink. Problem was solved using a hotgas by pass regulator and de-superheating txv. That I believe will be the easiest and most cost effective, and If the customer really wants to reap some benefits of running that compressor all of the time, you could put eprs on it to maintain a more constant temp in the cases.IMHO
    A Man Built It,A Man Can Fix It

  9. #9
    Join Date
    Jan 2006
    Location
    DFW Metroplex
    Posts
    4,910
    I appreciate all the responses. This morning I printed out literature from the Sporlan website and others. Since my experience in sizing, installing, setting up and operating HGB and desuperheating is somewhat limited, I will be spending some time studying.

    That being said, from the brief perusal of information this AM, it doesn't look terribly difficult.

    Thanks for the help everyone.

    ..................Hey Dow, I'll stock up on Leinies, when ya commin? (should I just order a whole truck? )

  10. #10
    Join Date
    Apr 2002
    Location
    Dallas, TX
    Posts
    2,987
    I agree, hot gas bypass and a desuperheating TEV is the easiest and least expensive solution. Save the redesign for another day.

    Any questions or concerns about the hot gas and desuperheating sizing, post the particulars.

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