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  1. #14
    Join Date
    May 2011
    Location
    Spring City, Pennsylvania, United States
    Posts
    190
    Quote Originally Posted by timebuilder View Post
    Are you missing the point on purpose?

    Making a gas hotter does not increase its density. Making it colder, OR raising its pressure do that.

    In this situation, the gas is more dense because of the increased pressure, not because of the associated increased temperature. We can have an increase in temperature AND an increase in pressure because of the nature of the system which is containing the gas.

    A lower temp evap has less capacity because of the low pressure at which it operates, and the less dense gas means there are fewer gas molecules per cubic volume that are present to carry away the heat being introduced into the evaporator.

    I just don't want folks to think of an increase in density being associated with an increase in temperature, because if the pressure remains the same, they are inversely, and not directly, proportional.
    Mike, The answer is the second to last paragraph

  2. #15
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    18,259
    Quote Originally Posted by mikeweber3 View Post
    Timebuilder,

    I agree with everything you have said, however my point was .................... If you look at the -30 column (or any column for that mater) it shows a differrent BTU capacity for LBP than MBP. With the same refrigerant and assuming the same condensing temp, and same evap temp, why would the BTUs be different?

    Thanks
    Mike.

    P.S. I like your signature
    I'm not a compressor engineer, but from what I know of engines, I would think that the compressor has a fixed volumetric efficiency potential, and in the chart, we are looking at data for two types of applications that would be using the same compressor. In one application, the compressor design is capable of a little more capacity than in the other application.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

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  3. #16
    Join Date
    Dec 2000
    Location
    Flat Rock, NC
    Posts
    463
    I think you are referring to the high low med back pressure ratings for starting torque on compressor motors. Low generally has time to equalize pressure from high to low before it will start. Med and high generally have extra starting caps and or relays to increase torque to allow starting a compressor immediately. This rating has nothing to do with gas density just the pressure difference exerted on the discharge valve between high and low.

  4. #17
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    18,259
    Quote Originally Posted by codgy View Post
    I think you are referring to the high low med back pressure ratings for starting torque on compressor motors. Low generally has time to equalize pressure from high to low before it will start. Med and high generally have extra starting caps and or relays to increase torque to allow starting a compressor immediately. This rating has nothing to do with gas density just the pressure difference exerted on the discharge valve between high and low.
    It's just one compressor.....

    ...and it has different BTU ratings based on evap temp.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

    AOP Forum Rules:







  5. #18
    Join Date
    Jan 2010
    Posts
    6,209
    OK...here's a good rule of thumb. A 1HP compressor is good for 12,000 btuh's per ton for high temp.application (a/c)
    A 1HP compressor is good for 8,000 btuh's per ton for medium temp.applications.
    A 1HP compressor is good for 4,000 btuh's per ton for low temp. applications.
    So you can see where the same 1HP compressor, has different capacities, depending on the evaporator temperature.

  6. #19
    Join Date
    Feb 2008
    Location
    California
    Posts
    2,065
    Quote Originally Posted by VTP99 View Post
    depending on the evaporator temperature.
    exactly

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