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  1. #1
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    R134a operating pressures


    Hi there ,
    I am hoping that someone will be able to assist me here.
    I have a cold store that I am trying to get working.
    The client called me to site as the evaporator was completely blocked with ice.

    In short I checked the condensate drain, and pump
    checked for pressure drop across the filter drier in the liquid line - it was ok

    Suction pressure was just about 10 psi and discharge at about 80 -100.

    checked the subcooling which was normal ( i doubt this now having said that!)

    and as such deduced that there muct be a restriction in the flo of refrigerant. with no other area to check ( i checked the operation of the solenoid valve in the liquid line also by the way)

    I deciided to change the orifice in the expansion valve.

    I started up the system and it happily came to temperature.
    I was then called back to site with the same problem.
    I then noticed that the fans were not running and noticed that the evaporator sensor was reading very high and had a sensor fault.

    I replaced the sensor and started the system again which soon came to temperature.

    I was then called back AGAIN .. .guess what ...same problem... this time the controller was playing up and the manufacturer reset it ovver the phone and the sensors were reading correctly again .

    Back again this time with the same problem.
    I am now thinking that the system is short of gas, but i dont want to overcharge the system.
    The cold store is supposed to be running at 2-4 deg C and i seem only to be able to get about 12 psi onn the suction side.

    Could someone please tell me what sort of pressures I would be expecting on this type of r134a system.

    I charged the sstem to the sight glass which is free from bubbles. I am just worried that I may over charge it if I am not careful.

    Failing all of the things above the only other thing i can think of is the TEV bellows not working correctly.
    I have checked the phial which seems ok and is placed correctly.

    As I left the system the evaporator was frosting about half way down.

    But i fear I may ave over charged it as the condenser had a great TD between the top and the bottom of the condenser.

    Really frustrated with it and so is my client.
    Any ideas / help would be appreciated.

    Chilly Matt



  2. #2
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    Re: R134a operating pressures

    What is the superheat reading Matt ?

    If you have sensor problems why does that make you think that the gas charge is incorrect?

    It does sound like there is a problem with the controller which is giving you trouble.

    Are the parameters correct for fan operation etc.?
    Brian - Torquay, Devon, UK
    I have to stop saying "how stupid can you be?" to my co-workers.
    They're starting to take it as a challenge...

    BASIC MAINTENANCE. If it doesn't move and it should then use WD40. If it moves but it shouldn't then use Duct Tape.

  3. #3
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    Re: R134a operating pressures

    Chilly Matt.
    Lots of questions here.
    But simply put if the op temps are from +2 to -4 then 10 - 12psi is way to low, That equates to -12 to -14c.

    If the evap is iceing up often increase the defrost times.
    But firstly you are correct to consider the expansion valve.
    Given that the operating frost line on the evap is half way down.
    I suspect that the txv could be opened a little.
    As Brian suggests take a superheat reading and adjust to get roughly 6c s/h below chamber temp at time of calculating.
    Once you have done that provided that's where the issue is.
    You can then fine tune the s/h and or defrost settings.
    If that does not resolve the issue then at least you can discount it.
    Oh also you need to confirm the evaporator is clear and has a good airflow through it.
    If this is blocked in anyway, say by a slab of ice within the centre.
    All you will get is a repeat call back every time.
    A blocked or blocking evap equates to low or lowering Suction Pressure / temp.

    Grizzly

  4. #4
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    Re: R134a operating pressures

    Hi again,
    The superheat I originally got was around 10 which is obviously too high. However the subcooling was about 6 which didnt freak me out too much but led me to believe that the problem was with the TEV. To be honest my temperature probes are well.... not the best..
    If you had a system that was plodding along nicely what sort of suction and discharge would you expect ?
    I was really hoping not to have to fiddle with the TEV as I have heard horror stories from doing so.

    The reason I think I am having gas charge problems is because i have replaced the sensor with a new one and reset the controller and I am still having problems. I am just weary of over charging. The reciever holds 2.35 l and there is a pipe run of about 5 meters.
    I estimated a charge of 1.8 kg based on this.
    Matt

    Matt

  5. #5
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    Re: R134a operating pressures

    Go on be brave!

    Just open it slightly to drop the s/h.
    then see what suction pressure you have.
    If you still can't adjust anything, try adding a little more refrigerant.
    Grizzly

  6. #6
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    Re: R134a operating pressures

    You reseted the controller, what controller is this?
    I suspect the TEV lost it's charge partially.
    If it ran OK in teh past with a normal SH (+/- 7K)and you'r sure you have enough gas, then your TEV injects not enough to feed the evaporator.
    Or water in your system and the TEV freezes up. Sight glass green??
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

  7. #7
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    Re: R134a operating pressures

    grab onto the phile and see if the valve opens up and floods the evap, if not the charge has probably been lost.

    Again when in doubt audit the system! start from point one and move systematicaly.

    Condnser clean and air free flowing? Evap the same? TXV bulb secure making good thermal contact? EQ Line properly attached? Screen clear on the TXV?

    And so on. usualy by just going over the whole system the problem jumps out and bits you!

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