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Thread: Nominal HP

  1. #1
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    Nominal HP



    Can some one shed some light on the Nominal Horse Power rating of compressors and condensing units? I recall Prestcold codes being related to a nominal HP and still get asked for replacements for a unit of which the size is quoted in HP.

    Is there a rule of thumb or formula for working out the nominal HP from a units kW cooling capacity? Is there any difference between the figures for compressors and the figures for condensing units?



  2. #2
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    Just take the HP rating and multiply it by 0.7457 to give you the kW equivalent.
    There is no difference between the condensing units duty and the compressors duty other than they will be calculated on different conditions and voltages.

    Another old duty rating you may find out there is T(R) or Tons (refrigerant). This can be converted to kW by multiplying the T(R) rating by 3.157

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    Just take the HP rating and multiply it by 0.7457 to give you the kW equivalent.
    This is correct for horsepower defined as 550 ft-lbf /sec. Interestingly, "electric" horsepower is defined as 0.746 kW exactly.

    The Prof would think compressor manufacturers use the "electric" horsepower definition, not that it would make much difference...
    Prof Sporlan

  4. #4
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    It's probable that the compressor manufacturers calculated the duty using the electrical definition of HP as their was no real standards in those days. The HP rating is easily quantified in refrigeration terms due to its direct relationship to Joules/second.

    Still as always the rules keep changing lol.

  5. #5
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    For those who must know the details:

    1 hp (550 ft-lbf/sec) = 7.4569987 kW
    1 hp, electric = 7.46 kW (exact)
    1 hp, metric (75 m-kgf/sec) = 7.354988 kW
    1 hp, UK = 7.4570 kW
    1 hp, water = 7.46043 kW

    The original unit "horsepower" was defined by James Watt in the late 1700s. He determined a horse could lift 330 lbs of coal 100 ft up a mine shaft in one minute. Therefore: 330 * 100 / 60 = 550 ft-lbf/sec became defined as one horsepower.
    Prof Sporlan

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    Has the Prof got his decimal point in the wrong place? When I was taught 1hp = 746w.

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    Has the Prof got his decimal point in the wrong place?
    Arrggghhhh! The Prof isn't being particularly careful with his conversions.....

    1 hp (550 ft-lbf/sec) = 0.74569987 kW
    1 hp, electric = 0.746 kW (exact)
    1 hp, metric (75 m-kgf/sec) = 0.7354988 kW
    1 hp, UK = 0.74570 kW
    1 hp, water = 0.746043 kW
    Prof Sporlan

  8. #8
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    Quite right Frank!

    Originally posted by frank
    Has the Prof got his decimal point in the wrong place? When I was taught 1hp = 746w.

  9. #9
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    Never mind prof it don't matter unless your sizing the kit.......

  10. #10
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    Well thanks to all who have replied. Point is being missed I think, I wanted to know if you could relate the nominal HP to cooling duty. I am well aware of how to convert HP to kW etc! Perhaps some are trying to say that the Nominal HP was related to input power at a condition and by this relates to cooling capacity.

    Very busy at moment winding down for xmas break. I will post another reply with some data over xmas break so as to examine this theory.

  11. #11
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    Rule of Thumb:

    Low Temp App. (-10 deg F to -20 deg F coil range): 1 hp ~ 4000 btu/h.

    Med. Temp App. (20 to 25 deg F coil): 1 hp ~ 8000 btu/h.

    High Temp App. (45 deg F coil): 1 hp ~ 12000 btu/h.

    Those are just rules of thumb, you should always consult the compressor manufacturer.


    Example: A 3 hp medium temperature application compressor has an "approximate" capacity of 24000 btu/h... BUT the actual may only be 23200 btu/h...

    3 hp on a low temp application compressor would have an approximate capacity of 12000 btu/h... but again that is not actual... just a "rule of thumb" to get you close.

    Not sure about ultra-low etc....
    :D
    Dean
    Subzero*psia

    Extinction is simply proof of failure to adapt.

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