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  1. #1
    Join Date
    May 2010
    Location
    Brooklyn, NY
    Posts
    99

    what do set for low pressure control

    for these simple pump down systems, like walk in boxes, remote cooler/freezer, what are your typical settings for Cut In, and Cut Out, let say for a r134a, 1/3 HP, high temp compressor, with less than 20 feet lines.
    Let say you charge the system to the correct pressure/correct amount of refrigerant, in that case, does it really matter what you put for your Cut In pressure, since it will never come into play when there's a thermostat/LLSV controlling it already. Then the one thing that really matters the Cut Out. What do you guys think.

  2. #2
    Join Date
    Feb 2008
    Location
    California
    Posts
    2,064
    I think the Cut In pressure greatly matters...especially if you live in an area of low ambient seasons.

  3. #3
    Join Date
    Oct 2011
    Location
    Chicagoland Area
    Posts
    4,510
    If you are using the control for a pumpdown or safety, above 0psi is good for the cut out and cut in should be equal to or below the lowest ambient you will see.. Example. If -10* F is a temp the condensing unit will see during the winter, set the control to CI between -10* and -15* F/psi. If the condensing unit is in a ceiling that is unconditioned and the ambient is 25*F, then set the unit to CI between 20* - 25*/psi. If the condensing unit is in a mechanical room that is 100*, the CI should be equal to or below the box temp. 40* for a cooler and 0* for a freezer. A PT chart is your friend
    Officially, Down for the count

    YOU HAVE TO GET OFF YOUR ASS TO GET ON YOUR FEET

    I know enough to know, I don't know enough
    Liberalism-Ideas so good they mandate them

  4. #4
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    25,411
    http://www.heatcraftrpd.com/products...5CH-IM-64L.pdf


    Info is contained here.

    I typically set my cut in for the minimum expected ambient temperature to prevent a unit getting locked out on low pressure due to extreme low ambients.

  5. #5
    Join Date
    Jul 2007
    Location
    colorado
    Posts
    33
    Sorry to piggyback on to this post but i would like a little clarification rergarding very low ambients and outdoor condensers. If i am getting this right i need to forget what i learned back in CA and set my pressure controls to the existing conditions. If my typical lowest ambient is -20 and the refrigerant is 404-A, i should set my control to C/I at between 13#-17# and C/O at 0#-5#. Correct or not? And to really piggyback, When dealing with long stretches of -5 to -20 are there extra precautions or methods commonly used to protect the system?
    Thank you very much for any replies.

  6. #6
    Join Date
    Oct 2011
    Location
    Chicagoland Area
    Posts
    4,510
    The reason the pressure control is set the way it is in low ambient conditions is not to protect the system. Refrigerant likes cold. If the liquid in your receiver is at -20 degrees, your box is at 40 degrees, and the control is set to CI at lets say 75psi for 404a. The solenoid will open and the refrigerant is going to stay in the receiver. You can use heat tape wrapped around the receiver and insulated controlled by a temp control to keep the receiver warm
    Officially, Down for the count

    YOU HAVE TO GET OFF YOUR ASS TO GET ON YOUR FEET

    I know enough to know, I don't know enough
    Liberalism-Ideas so good they mandate them

  7. #7
    Join Date
    Feb 2008
    Location
    California
    Posts
    2,064
    There are systems out there that also incorporate a time delay in parallel with the low pressure control, to bypass it on start up. Allowing the system to buildup pressure and eliminate nuisance short cycles.

  8. #8
    Join Date
    Dec 2012
    Location
    columbus, OH
    Posts
    2,023
    Quote Originally Posted by Phase Loss View Post
    There are systems out there that also incorporate a time delay in parallel with the low pressure control, to bypass it on start up. Allowing the system to buildup pressure and eliminate nuisance short cycles.
    Would that be a beacon system?NO delay on break?

  9. #9
    Join Date
    Feb 2008
    Location
    California
    Posts
    2,064
    I don't know it Beacon does it, I've not worked on a Beacon system in years!

    I've seen it more recently on a HeatCraft condensing unit.

  10. #10
    Join Date
    Dec 2012
    Location
    columbus, OH
    Posts
    2,023
    I seen a beacon system today had a TDR in parallel with the LP. It was normally open, how do they usually work? Closes when energized and opens after set delay?

  11. #11
    Join Date
    Jul 2007
    Location
    colorado
    Posts
    33
    Really good stuff, thank you guys. I would think that with a heated and insulated receiver you could omit the time delay in series with the LP or perhaps both would compliment each other.
    One last thing......
    Now when "Summer" rolls around and ambient temps reach a scorching 78* for a month or two, do you need to make adjustments to the LP? Do they make self adjusting or regulating LP controls?
    Thanks again.

  12. #12
    Join Date
    Feb 2008
    Location
    California
    Posts
    2,064

  13. #13
    Join Date
    Dec 2012
    Location
    columbus, OH
    Posts
    2,023
    Quote Originally Posted by Phase Loss View Post
    Thanks for the link

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