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  1. #1
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    CPR improving hotgas defrost? any thoughts



    I have been told to install a CPR onto a R404A system.

    The fridge has a badly designed drainage/defrost system. Hotgas Defrost.

    The freezer sits on approx -18c to -25c all day, and the evap just turns to one huge block of ice.
    The theory behind installing the CPR is they think while 'Hotgas Defrost' is in operation, the CPR will help raise the temperature in the evaporator, because the hotgas coming through will be restricted at the CPR and raise pressures in the evaporator, helping the defrost operation.

    Although i like the idea of having a CPR in a system with hotgas defrost, to compressor does not recieve hot suction vapour. I do not believe it would help raise the evaporator pressure/temp.

    Anyone have thoughts on this???



  2. #2
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    Re: CPR improving hotgas defrost? any thoughts

    Hi young gun
    Yes the cpr will help increase the temp/pressure in the evap during hgd. Also will protect the compressor on startup if the accumulator is on the small side. this will aid the defrost but if the drainage is poor - the weakest link in the chain, iceups are likely to still occur

  3. #3
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    Re: CPR improving hotgas defrost? any thoughts

    If you do install a CPR, then perhaps you'd want to increase the flow through the HGP, to correct the defrost issue?
    Engineering Specialist - Cuprobraze, Nocolok, CD Technology
    Rarefied Technologies ( SE Asia )

  4. #4
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    Re: CPR improving hotgas defrost? any thoughts

    young gun ,

    Does this system have multiple air coolers which defrost individually , or only one ?
    Give some more details how it is piped up ?

  5. #5
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    Re: CPR improving hotgas defrost? any thoughts

    hey young gun , yes hgp is ok , but if its not set up serfectly, you could end up cooking the comp, you may even need liquid injection to cool the comp,< just to warn you <

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    Re: CPR improving hotgas defrost? any thoughts

    young gun ,

    Unless hot gas pressure is controlled above 0 deg C saturated nothing will happen anyway .
    Its not the sensible heat of the gas that defrosts evaporator , its latent heat .
    If we don't know how its piped up I think everyone is guessing , myself included .

  7. #7
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    Re: CPR improving hotgas defrost? any thoughts

    How many defrosts a day?

  8. #8
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    Re: CPR improving hotgas defrost? any thoughts

    It is a freezer especially designed for transport refrigeration, installed on a truck. The compressor is a off engine drive (sanden 24v DC) reciprocating compressor. The type of fridge is a new style Korean designed fridge (a cheap one i think) lol.

    There is also a totally seperate freezer also installed that runs off mains power over night, this system has hotgas defrost, and very good sufficient drain heaters, this system has no defrost issues.

    The problem is the mains power freezer runs perfectly all night, and cycles at approx -25c.
    The off engine drive system has poor inadequate drain heaters, but they are apparintly the best ones you can get for 24v DC. The system has a poor controller where parametres cannot be changed to run more often, and long hotgas defrosts. The evaporator cover is run very level, so any water dripped down does not run down to the drains lol. The drains are tiny 1/2 holes. And the hotgas pipe only runs through the evaporator coil, it does not run through the drain pan first.

    I have put in thicker drains, modified the evaporator cover so that is slopes down, so easy flow into the drains, i have modified the 1/2 hotgas line to run through the drip tray before entering the evaporator coil. And have also installed a CPR.

    The system is a basic set up.... Compressor - oil seperator - condenser - liquid reciever - drier .. liquid line/suction line heat exchanger... liquid line solenoid... tx valve... evaporator... CPR.... and back into the compressor....
    Hotgas being tee'd in just before the condenser, through a solenoid, through bottom of evaporator cover, then tee'd in just after the tx valve
    It does not have a suction accumulator.

    The issue is now fixed, but i believe more so because of the modified hotgas pipe run through the evap cover (the way it prob should have been designed for starters). I would have personally preferred to be able to fit the hotgas solenoid just before the Tee after the txvalve, so that there is hot discharge pressure there all the time.

    Still not sure that the CPR made an impact on the hotgas defrost performance though, although of course it is much safer to have it there for compressors sake.

    lol i do no not call the shots at work, i just do as told though.
    haha long enough explenation?

  9. #9
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    Re: CPR improving hotgas defrost? any thoughts

    If young gun ran the hg over the drainage holes then bent the pipes into a long circut through the condensor (over the length it may loose enough heat & pressure) to be tee'ed in to the super-heated vapour returning to the compressor.

    I wouldnt know how to measure this though!

  10. #10
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    Re: CPR improving hotgas defrost? any thoughts

    >.> oops! that was a dopey comment -.-

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