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CAS 101 – Buy a Big Compressor to cater for future needs ?

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  • #6875

    Phun LM


    Once, one Facility Manager told me; he would like to buy a bigger screw compressor with the approved budget, to cater for future potential increase of air usage. What do you think of this idea as an Energy Manager ?

    As we know, bigger compressor will definitely has a better energy efficiency than a smaller one. In that case, does it mean the decision by above Facility Manager is correct ? Not really.

    Let me explain.

    If the choice of a bigger compressor is a Screw Compressor, we must aware of something call “Power consumed during Unloading duration”. In Screw Compressor operation, once the compressor hit the target pressure (Unload pressure level), the compressor will turn to Unloading phase.

    During this unloading duration, no additional compressed air is generated at outlet, BUT THE COMPRESSOR WILL CONSUME ABOUT 35% OF RATED POWER.

    Of course,  a bigger screw compressor will have a bigger flow capacity, therefore the loading period (when 100% rated power is consumed) will be shorter.

    Now, let’s combine both factors :

    • 37kW screw compressor at 60% load, 40% unload cycle will consume about 28kW
    • 75kW screw comperssor at 30% load, 70% unload cycle will consume about 41 kW

    It is clear from this simple illustration, it is not wise to get a MUCH bigger compressor than your requirement. It is important to understand the demand level and cycle before making the right choice.





    AS for the calculation

    I am right to say:-

    Example as above,

    For 37 kW screw compressor 60% load, 40% unload will consume

    37X 60% = 22.2kW

    and 40% X37 =14.8 kW X 35%(During unload condition) = 5.18 kW

    Total power consumed = 22.2 kW +5.18 kW = 27.38 kW

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