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Thread: bohn cond. unit

  1. #1
    Join Date
    Jul 2001
    Posts
    159
    we are currenty replacing an old larken condensing unit with a blv2000... dont quote me on that model number i dont have the exact number at home. for superheat bohn/heatcraft recommend 30-40 degrees at the compressor while copeland wants nothing lower than 20 degrees according to bohn/heatcraft.
    what is coplands recomended superheat at the compressor for a discis compressor running r-22 +25 degree evap temp. they also dont mention what subcooling is spec. this unit has the dual low ambient valves and a subcooler coil next the condenser coil. the manual says 90% of reciever capacity should be proper charge but i cant find the reciever capacity written on the unit anywhere and the the reciever is heated and insulated so i cant see any tags

  2. #2
    ....and the question is?
    BTW what is a "dual low ambient valves"?
    Don't interrupt me while i'm talking to myself

  3. #3
    Join Date
    Jan 2004
    Location
    Earth
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    4,879
    Does it have a sight glass?
    A Diamond is just a piece of coal, that made good under pressure!

  4. #4
    Join Date
    Jul 2000
    Location
    Guayaquil EC
    Posts
    10,081
    Refer to your Heatcraft I&O manual pages 30-32.

    The 90% receiver figure is the maximum receiver capacity, not the required charge. The charging procedure is detailed quite well. If you have low ambient controls go to the section headed "Low Head Pressure Systems". It basically says to block the condenser airflow to bring the discharge pressure up to the equivalent of 105 Deg F and then clear the sightglass.

    The superheat is covered well also. Heatcraft recommends 30-45 Deg F compressor SH. If you have to compromise the evap SH to get there, do it.

    There is no mention of liquid subcooling in the book because this is commercial refrigeration and we don't care what the SC is.

  5. #5
    Originally posted by icemeister
    Refer to your Heatcraft I&O manual pages 30-32.


    There is no mention of liquid subcooling in the book because this is commercial refrigeration and we don't care what the SC is.
    sURE WE DO. nO SUBCOOLING EQUALS FLASH GAS IN THE LIQUID LINE ON LONG RUNS IN THE HOT OVERHEAD SPACE BETWEEN THE CEILING AND THE ROOF. WE JUST DON'T GET SUBCOOLING THE SAME WAY THAT PACKAGE UNIT DO. USUALLY HAVE TO DO IT WITH A HEAT EXCHANGER. EXCEPT THAT THIS PARTIICULAR CONDENSING UNIT DOES HAVE A SUBCOOLING PASS AFTER THE RECEIVER.

  6. #6
    Join Date
    Jul 2000
    Location
    Guayaquil EC
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    10,081

    refer dude..........

    I know where you're coming from with the flash gas in long runs , etc but this question referred to charging the system. With a typical commercial refrigeration condensing unit you're prime concern is to clear the sightglass, add enough additional to allow for cold weather operation and above all get your compressor suction superheat in line.

    With respect to liquid subcooling in a commercial refrig job, you get whatever the system is designed to give and that's it. There's no playing around with the charge to try to gain or lose subcooling as you would in a critically charged system.

    In commercial refrigeration we tend to deal with liquid subcooling only for unusual situations like long hot runs (as you mentioned), long vertical rise where you'd experience static pressure loss (and thus loss of subcooling), etc. Probably the most common cure to solving the flash gas at the TXV problem has always been the liquid/suction heat exchanger. This also increases your suction SH so be careful. Bigger systems like market racks often use mechanical subcooling with a plate heat exchanger primarily for efficiency boost. I used to use a 400 PSI rated D/B chiller barrel for mechanical subcooling back in the old days.

    The system in this thread has a subcooler loop after the receiver. This is a relatively new addition to the old standard unit designs that I'm used to. I think it's a big plus for promoting good stable system operation at a minimal initial cost. Good move for Mr Bohn.

    Anyway, my comment about us not caring about subcooling was intended as a jab to the A/C guys who always try to analyze refer systems' charge with the old SH/SC deal. It gets tiresome at times.

    ......and dude, turn off your caps lock. My ears hurt.

  7. #7
    Join Date
    Jul 2001
    Posts
    159
    i charged it up the way it says in the installation manual i was just curious as far as what normal subcooling would be on a say 80f day, and also what copelands recomended compressor superheat is, i know 20 is the minimum but it doesn't say what their range is. bohn has their own specs
    (30-45f)i just wanted to see if their specs were the same as copelands.


    thanks for the answers so far

  8. #8

    Re: refer dude..........

    Originally posted by icemeister

    Anyway, my comment about us not caring about subcooling was intended as a jab to the A/C guys who always try to analyze refer systems' charge with the old SH/SC deal. It gets tiresome at times.

    ......and dude, turn off your caps lock. My ears hurt.
    All of your points are well taken. I to have to bite my tongue every day with the HVAC guys. I worked 25 years in markets and industrial refrig. Now I am doing HVAC because it is easier on the old body, but I still prefer refrigeration. The A/C guys, with few exceptions, don't get too deep into the refrigeration side and as you note pay way too much attention to the number gathering with little or no real understanding of why they take these readings.

    Sorry about the cap locks. Was in quicken prior to loging in and I do that in cap locks.

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