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LOw Pressure Controls

Morning!It is a loss of charge safety device , it is a pump down control, it can also be reverse acting( like in ice machines),but mostly it is used as a temperature control. My question is, how do most of you mechanics set the cut-in and cut-out(differantial).Just courious!Enjoy your day!


  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Great Question

    In order to determine the settings for a low pressure control that is being used for temperature control, we must first determine the range of temperatures that we want to see in the box.

    For example, the owner of an R-134a medium temperature walk-in box may want the box to get no colder than 35 degrees and no warmer than 45 degrees. This means that the system will be operating when the box is just above 35 degrees and be off when the box is just below 45 degrees.

    So, 35 degrees is the lowest box temperature. Given that the evaporator saturation temperature is often 10 to 20 degrees cooler than the box temperature, we can start odd by assuming that the evaporator coil temperature will be about 20 dgrees when the system cycles off (35 degrees - 15 degrees, splitting the difference between the 10 and 20 degree delta-t). A 20 degree evaporator saturation temperature corresponds to an evaporator saturation temperature of 18.4 psig for R-134a. Therefore 18.4 psig is the cut-out setting for the control.

    Now, the highest temperature we want in the box is 45 degrees, as per our example. When the box reaches 45 degrees, the compressor is still off, so the pressure that corresponds to 45 degrees is 40.1 psig for R-134a (P/T chart).

    So, for this example, the cut-in pressure is 40.1 psig and the cut-out pressure is 18.4 psig.

    The differential is the difference between the cut-in and cut-out pressures, 40.1 - 18.4, or 21.7 psig.

    How do you do it, Terry? (better?)
  • Techman
    Techman Member Posts: 2,144

    Hi Professor!O.K. O.K. I'll let you know how I set LoPress controls . First, I set the cut-in (hi event)EXACTLY as you said. So, assuming, a box temp of 45*(cut-in )the LP setting would be 40 psig(rounded off)for R-134a.And as you said ,this info is directly off of the fabulous P/T chart for saturated refrigerant's .NOW ,comes the tricky part of setting the cut-out(differential)(low event). I don't know of any maker of any refrigator,freezer ,R.I. or W.I. Delicatessen show cases ,salad cases,meat cases, or veggie cases that gives the"evaporator temperature difference" of their system. Just like the Proff. said, the evap t/d has to be figured in ,and, known to set the cut-out properly.With all of the hundreds of possible combinations of the customers ACTUAL PRODUCT COOLING REQUIREMENTS(humidity and tempature) vs. evap coil size vs. cond unit size vs. which repair co. changed out either the evap or comp. or cond unit over the years. Thats a lot of unknowns for us mechanics to think about.Especially when this is a new customer with an old problem, the "system" or piece of
    equipment ,new to me," DOES"T WORK RIGHT!??!!??!"

    So, I do it the easy way out.It takes time ,AND , the new customer will be happy with the results ,and happy customers call again !

    After setting the cut-in properly I turn the cut-out way down and let the unit run,and run(sometimes hours sometimes days) until the CUSTOMERS PRODUCT is at the RIGHT REFRIGERATED REQUIREMENTS. Then I "fingerprint"the running conditions ,adjust the cut-out to turn off the comp. and that system is now set and any fine tuning is easily done . Then re-fingerprint!

    This way, even any minor piping screw ups (like short ell's instead of the proper long ell's)can be compensated for because the LP control is far away at the cond unit ,20'-50' away . On closer coupled systems the piping loss is less
    So ,out of 10 W-I Refrig's ,I'll have 6-7-8- different LP settings
    Work's for me! Enjoy your day!
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380

    I guess that nobody else is setting low pressure controls?

This discussion has been closed.