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Home Forums Energy Efficiency & Facility Management CAS 101 – Fooled by Published Compressor Nominal Power-Flow Data ?

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    Phun Lee Ming

    In my last 20 years experience, the most common complaints from the Facility Managers are “Overstated performance of Compressor by Manufacturers “!

    Let me explain.

    In Compressor industry, an air-compressor is rated by manufacturers at their nominal flow-power level in the published commercial leaflet. The nominal power only refers to the frame size of the electrical motor. The flow is measured according to the ISO1217 standard.

    For the Power, the actual running power for the electrical motor can be much higher than the stated nominal power. At full load, the running power is higher due to the usage of service factor. At partial load or unload condition, power is consumed during the transient period (when there is no compressed air is produced).

    For the Flow, the compressor performance is indicated at reference condition, which is the most favorable condition. In reality, almost no compressor can deliver the stated flow capacity in real life industrial environment.

    One more thing, most compressor manufacturer do not provide the Power Consumption level when the compressor is running at different Flow Rate. All compressor will have poorer Energy Efficiency when the compressor is running at lower flow rate. Without this information, how will the facility manager compare the Energy Efficiency Curve for different compressors or for different factory demand pattern ? 

    In short, it is time for Facility Manager, Energy Manager or Factory Management to start requesting the full performance data from the compressor manufacturer in order to make a right choice of the investment .



    Joshua Tan KM

    Recently I engage an M&V company to audit my compressors.

    1. My compressor GA145 actually consume about 168kW at full load and the flow measured at outlet port is actually 10% lower than original stated in catalogue

    2. The compressor is 7 years, and we did an overhaul about 2 years ago.

    3. Also, the measured result showed the efficiency of the compressor drop to 7.9 kW per m3/min over 2 days measuring period ; means the efficiency is 21% poorer than the original full load efficiency.

    I am planning to replace this unit, now that i have actual data in hand.


    Joshua Tan KM

    According to my calculation, the ROI for replacing this unit with a new one, will be around 2.4 years !


    Kelvein Peters

    Mr. Phun, I have a question.

    I have two units of compressors running at the factory. One unit 100hp and another unit 75hp. I noticed both compressors always change from loading to unloading frequently. Is this normal ? How will this affect efficiency ?

    Appreciate if you can advise.

    Phun Lee Ming


    First of all, if both of your compressors are running load & unload frequently at the same time, then there is something wrong with your compressor pressure setting. Most likely both compressors are having similar pressure setting, which is not advisable.

    During unloading, compressor still consume about 30% motor power. Also, the transient period from loading to unloading, the compressor consume more than 30% power. These are wasted energy, not producing any air.

    You should set the bigger compressor (the 100hp) to higher unloading point than the smaller compressor. This will let the bigger unit to run a full load, without any unloading. With this, the 100hp will be running at its better efficiency point (full load).

    Try this. You will see the energy saving by doing this simple adjustment.


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