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Setting up low pressure contol to control temp

Discussion in 'Refrigeration' started by kena, Jan 28, 2016.

  1. kena

    kena New Member

    Messages:
    783
    Can anyone explain the easiest way to do this? I have been allot of work lately for customers who were using another company prior to calling me. The guy liked to use low pressure controls vs thermostats to control temp and low pressure. I've had to replace a few, and what I have been doing is setting them up like they were which is set the cutout at the pressure I want the system to come on, and setting the differential to when I want it to shut off. The easiest way I have found is to wait until the desired temp for the unit to come on (say 38 degrees), I set the cutout to start the system. Then when the desired low temp is reached, set the differential to shut it off. This is how they were setup prior.
    The problem is I end up getting 3-4 consistent cycles... then the unit will stray. Instead of 33-38 it will run from 35-37 which starts to short cycle a bit. I like thermostats and a low pressure switch... but the customers just want me to replace the control. Any help on best practice on how to set these up would be appreciated.
     
  2. pecmsg

    pecmsg Junior my ASS

    Messages:
    22,377
    Location:
    Eastern LI
    For med temp
    Depending on TD and air flow
    R-134A CI-35 CO-13
    R-404A CI- 85 CO-45

    Adjust pressures accordingly
     
  3. kena

    kena New Member

    Messages:
    783
    This system is R22 and I just don't get why it will run well for several cycles and then the pressures will change enough to effect the temps the way it is. There's no leaks in the system and
    I just installed a new condenser fan.
     
  4. kena

    kena New Member

    Messages:
    783
    Looks like the system should come on around 68 psi and go off around 61 psi running on R22 for a cooler. Agree, disagree?

    However.... while running, the low side pressure in the system is around 45 psi. That's what weirds me out about setting these.
     
  5. Texas-Tech

    Texas-Tech Official Geezer*

    Messages:
    18,167
    Location:
    duhh
    Low side has to be running at a pressure lower than what your setpoint. At 45psi your evap is 21° which is well below freezing so maybe your evap is frosting up.
     
  6. kena

    kena New Member

    Messages:
    783
    I'll check it today. So I should add refrigerant to bring the pressure up to above 61?

    Don't want to screw around with expansion valve... customer doesn't wanna pay for the labor to check the system for proper superheat.

    That's a whole different issue. I'd love to find just one customer who is willing to pay what it takes to setup their system to run efficiently. Do they exist?
     
  7. Texas-Tech

    Texas-Tech Official Geezer*

    Messages:
    18,167
    Location:
    duhh
    No, your evap must be at least 10°below the temp your trying to maintain. The unit has a defrost on it or are they using the off cycle with the fans runing?
     
  8. pecmsg

    pecmsg Junior my ASS

    Messages:
    22,377
    Location:
    Eastern LI
    Whats going on with the head pressure?

    R-22 CI 68 CO-35
     
  9. kena

    kena New Member

    Messages:
    783
    The defrost is air defrost (fans on)

    When you say temp I want to maintain.... do you mean max, min, or average?

    At lowest temp... pressure would be about 47-48 psi (low pressure) if evap is 10 degrees less than min temp I want to control.
     
    Last edited: Jan 28, 2016
  10. kena

    kena New Member

    Messages:
    783
    Head pressure of about 140

    Thanks
     
  11. Texas-Tech

    Texas-Tech Official Geezer*

    Messages:
    18,167
    Location:
    duhh
    When you say temp I want to maintain.... do you mean max, min, or average?

    I'd say average, but how critical is the temp in the box? What's in it? Remember even though the air temp may rise a few degrees the product inside doesn't react that fast.
    I used to maintain a milk cooler for a friend of mine, 3 evaps in a box about 30 ft long. The temp would fluctuate a lot but the milk pretty much stayed around the same temp
     
  12. kena

    kena New Member

    Messages:
    783
    They want it to stay under 40. I like them to run around 34 - 39. So if I said I wanted it to run around 36 I would make sure the low side pressure is around 52 psi using the 10 degree difference?

    Right now it's running in the mid 30's.... but running on and off with about a 2 degree difference. So the compressor is running more often.
     
  13. flange

    flange Act like you care and do SOMETHING!!

    Messages:
    13,082
    Location:
    IMT--in my truck
  14. kena

    kena New Member

    Messages:
    783
  15. coldsnap

    coldsnap -20 on the money

    Messages:
    4,637
    Location:
    Wisconsin
    The last r22 remote I did was 66ci 34co. You have to tweak it to your specific conditions.
     
  16. kena

    kena New Member

    Messages:
    783
    ok.... i feel like a dumbass trying to figure this out. I set the cut in at 66 psi which made the unit come on like I figured it would (around 38 degrees). I was calculating for a 55 psi cutout
    which would be in the low 30's. So that came out to 11 psi with differential. However.... I had to set it at 21 psi to shut off in the low 30's. Was this about the 10 degree colder coil? That would make sense....
     
    Last edited: Jan 28, 2016
  17. IRONMAN

    IRONMAN New Member

    Messages:
    9
    I don't like to control temperature by sensing pressure. If controlling temp sense temp. an a19 and a solenoid valve will solve your problem. Do it even if you don't make money on the first one the customer will be soo pleased you will get the job to change them all. also a pressure switch out of the box isn't all that accurate. set it to cut in by the scale on the control then check it before installing by using your gauges and nitrogen you will be surprised just how much they are off







    i
     
  18. kena

    kena New Member

    Messages:
    783
    A simple thermostat would work.
     
  19. mike1020

    mike1020 entitled millennial

    Messages:
    18,887
    Location:
    on the roof of target
    Yes, you have to factor in the temperature differential of the coil.

    So if you wanted the run 32 to 38 degrees, you would have to set the pressure control to 38 degree sst cut-in, and 22 degree sst cut-out (assuming 10 degree td)
     
  20. heavyevans

    heavyevans FNG

    Messages:
    1,326
    Location:
    Indy
    I would get a handle on this before trying to control box temp with a pressure control.
     
  21. crackertech

    crackertech assholiness *

    Messages:
    92,837
    Location:
    Fl/IN/Ga/Ms/TX/NJ/VA/NC/TN
    X2 140 is low
     
    22blood likes this.