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Thread: pt chart usage

  1. #1
    Join Date
    Sep 2010
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    miami,fl.
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    605

    pt chart usage

    i was curious to know more about the pt chart, i feel like im missing some thing i see alot of people ask what should be the correct operating pressures for X refrigerant. the way i determine the pressures for example r22 cooler with a low pressure control as a temp control. cut in is about 72 psig = 42 degrees then to determine the cut out temp would be your desired box temp ex: 35 degrees = 61 psig then minus the 15 degrees td would be 45 psig cut out = 21 degrees evap coil. (this will get you close, but many variables will affect the cut out/ cut in pressures.

    if i work on something im not familiar with, the best is to call tech support and ask the operating pressures. if you can thats always nice. but sometimes they never return your call. so basically ill go off the box temp minus 15 delta t will get me close then check my superheat and comp. amperage.
    but some evap coils are designed with 10 degrees td. Can you give me more pointers or more ways i can use the pt chart as far as figuring pressures.

  2. #2
    Join Date
    Dec 2009
    Posts
    147
    Quote Originally Posted by anthonyac1 View Post
    i was curious to know more about the pt chart, i feel like im missing some thing i see alot of people ask what should be the correct operating pressures for X refrigerant. the way i determine the pressures for example r22 cooler with a low pressure control as a temp control. cut in is about 72 psig = 42 degrees then to determine the cut out temp would be your desired box temp ex: 35 degrees = 61 psig then minus the 15 degrees td would be 45 psig cut out = 21 degrees evap coil. (this will get you close, but many variables will affect the cut out/ cut in pressures.

    if i work on something im not familiar with, the best is to call tech support and ask the operating pressures. if you can thats always nice. but sometimes they never return your call. so basically ill go off the box temp minus 15 delta t will get me close then check my superheat and comp. amperage.
    but some evap coils are designed with 10 degrees td. Can you give me more pointers or more ways i can use the pt chart as far as figuring pressures.
    I always go with 10F TD.
    It sounds like you have a pretty good handle on the PT chart.
    Whenever possible I NEVER use low pressure to control temperature when the unit contains R22. I find that trying to adjust the pressure control with R22 can be almost impossible. Best to use LP control as a safety and wire in a thermostat in series with a solenoid coil.

  3. #3
    Join Date
    Sep 2010
    Location
    miami,fl.
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    605
    I see techs on here ask what should be my normal operating pressures then I see people reply go look at your pt chart! But if they don't know to minus the 10-15 degree td then the pt chart won't really do them any good or am I wrong????? I'm trying to see how they will find the answer vs how I find the answer

  4. #4
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    Oct 2011
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    Someone asking for operating pressures is different than what you are asking/stating. There are too many variables to give a definite answer. Outdoor ambient, the load on the space, etc to make a blanket statement of what they should be. In your case, setting a lpc to control temp will also vary depending on type of coil. Static, gravity, or one with fans. After setting a control, I make it a priority to return in a day or two after the box has had a chance to stabilize, and go thru it's "normal" workday and tweak if necessary. I don't know if I have any R22 units on pressure controls. They are mostly, if not all, some variation of 12, or low temp 404a
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  5. #5
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    Sep 2010
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    Quote Originally Posted by 2sac View Post
    Someone asking for operating pressures is different than what you are asking/stating. There are too many variables to give a definite answer. Outdoor ambient, the load on the space, etc to make a blanket statement of what they should be. In your case, setting a lpc to control temp will also vary depending on type of coil. Static, gravity, or one with fans. After setting a control, I make it a priority to return in a day or two after the box has had a chance to stabilize, and go thru it's "normal" workday and tweak if necessary. I don't know if I have any R22 units on pressure controls. They are mostly, if not all, some variation of 12, or low temp 404a
    your right about operating pressures differ upon many different variables, i agree, i was just asking because i felt like i was missing something. thanks for all the info.

  6. #6
    Join Date
    Sep 2008
    Location
    Western PA
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    As 2sac is saying, it is pretty difficult to come up with "blanket" type rules.

    TD depends on design criteria that is selected by the engineer according to the product needs of the unit.

    If manufacturer's data isn't available then you've kind of got to "wing" it by using your best guess.

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