Low-Pressure Controls, Refrigerant Blends

May 1, 2006
/ Print / Reprints /
ShareMore
/ Text Size+
Low-pressure controls are used on many medium-temperature refrigeration systems to control case temperature. In order for this type of control to work properly, both its cut-in and differential pressure settings must be set correctly.

The control's cut in setting must be set to cycle the compressor back on at a pressure that will ensure adequate defrosting of the evaporator, and at a case temperature that is not too high for the product being refrigerated. For some applications, this may be at a saturation pressure corresponding to a 38°F saturation temperature.

Since refrigerant blends have a varying saturation temperature corresponding to a specific saturation pressure, a service technician must choose which saturation pressure/temperature relationship to use. When setting the cut-in pressure of a low-pressure control, the dew point temperature of a refrigerant blend should be used. This will cycle the compressor on at an appropriate coil/case temperature.

Once the cut-in pressure has been set, the control's differential setting must be set. The differential setting is used to cycle the compressor off at the lowest designed case temperature. The differential setting represents the difference between the cut-in and the cut-out pressure of the pressure control. For example, if the cut-in is set at 34 psig and you want the compressor to cycle off at 24 psig, the differential setting would need to be set at 10 psig (34 psig – 24 psig = 10 psig).

The appropriate cut-out pressure when working with a refrigerant blend must reflect the average saturation temperature within the evaporator minus any pressure drop through the suction line.

For example, if an average coil temperature of 25° is desired and the refrigerant blend enters the evaporator at a saturation temperature of 21° and is at 29° before the refrigerant becomes superheat, the cut-out pressure would then be set corresponding to the dew point temperature of the higher saturation temperature. This example would be 29° minus any pressure drop through the suction line.

If R-401A were the refrigerant used in our example and a pressure drop of 2 psig was assumed across the suction line, then the actual cut-out pressure would be 26 psig minus the 2 psig, which would equal a cut-out of 24 psig. If the cut-in value is set at 34 psig, then the differential would need to be set at 10 psig.

Calculating the average saturation temperature required and the pressure drop across a suction line may be difficult at times. Here's an alternate method that can be used to set the cut-in and differential pressure setting of a low-pressure control:

  • First set the cut-in value of the low-pressure control to the appropriate value for the refrigerant to be used.

  • Then initially set the differential of the low-pressure control to an extremely high value (a value the suction pressure should never reach under normal operating conditions; 30 psig should suffice).

  • Allow the system to run while monitoring the box temperature.

  • Once the box reaches its lowest desirable temperature, slowly adjust the differential counter-clockwise until the compressor cycles off.

    Using this procedure will ensure the differential settings have been properly set and the system will cycle properly.

    Joe Marchese is owner of Coldtronics of Pittsburgh. He can be reached at 412-734-4433, www.coldtronics.com, or joe@coldtronics.com.

    Publication date: 05/01/2006

  • Did you enjoy this article? Click here to subscribe to The NEWS Magazine

    You must login or register in order to post a comment.

    Multimedia

    Videos

    Image Galleries

    2014 ACI Home Performance Conference

    The 2014 ACI National Home Performance Conference & Trade Show was held in Detroit.

    Podcasts

    NEWSMakers: Doug Dougherty

    Doug Dougherty, president and CEO of the Geothermal Exchange Organization (GEO) joins the program to discuss the latest happenings with GEO, including the drive to get included in state and federal definitions of renewable energy, plus the future of geothermal.  Posted on July 25.

    More Podcasts

    ACHRNEWS

    NEWS 07-28-14 cover

    2014 July 28

    Check out the weekly edition of The NEWS today!

    Table Of Contents Subscribe

    R-22 PHASEOUT POLL

    Some in the industry are calling for the EPA to halt production of R-22 as of 2015. What do you think?
    View Results Poll Archive

    HVACR INDUSTRY STORE

    plumbing-hvac.gif
    2014 National Plumbing & HVAC Estimator

    Every plumbing and HVAC estimator can use the cost estimates in this practical manual!

    More Products

    Clear Seas Research

     

    Clear Seas ResearchWith access to over one million professionals and more than 60 industry-specific publications, Clear Seas Research offers relevant insights from those who know your industry best. Let us customize a market research solution that exceeds your marketing goals.

    DON'T MISS A THING

    Magazine image
     
    Register today for complete access to ACHRNews.com. Get full access to the latest features, Extra Edition, and more.

    STAY CONNECTED

    facebook icontwitter iconyoutube iconLinkedIn i con