Motor KW Rating for Reciprocating Two-Stage Compressor

The current required is the difference in enthalpy for the compressible fluid between its compressed state and its uncompressed state, multplied by the mass flowrate, divided by the system efficiency, divided by the power factor, divided by the supply voltage. The starting conditions can, of course, be looked up in the manufacturer's Operation & Maintenance Manual and on the commissioning sheets that the Commissioning Engineer filled in while running-up the equipment after initial delivery and installation.

If the current is too high while not achieving pressure, it may indicate a mechanical fault inside the equipment that is causing the compressible fluid to leak back from the high pressure side to the low pressure side within the equipment. It may indicate a faulty bearing or a faulty seal also, or even a clash between rotating parts and non-rotating parts, though this would be accompanied by noise and vibration, which aren't mentioned in the original post. Obtain copies of the maintenance records, and call the equipment manufacturer in for advice with those records to hand.

There are other vital information I think u have failed to tell us. Is the mottor runing hot? What starting method did u adopt for the motor? Is the motor perment Delta, star or Star/Delta? How long have u been using the motor? Has the motor at one time or the other been rewound? Is the machine situated in a properly ventilated place? Is the cooling fan and fan cover of the motor in place? Has the body of the motor been properly cleaned from dirts to allow for free flow of air?

Find a megger and let me know the insullation resistance of the various phases with all links removed. Also get the between pole test.

I need all the above because the no load current of the motor is too high.

Dickson

Dear Friends,

The motor has Star/Delta connection. Bearings temperature does not exceed 40 Deg Celsius and motor as well remains cool. The suction filter is clean and there is no abnormal sound from the compressor.

Thanks

We do not know the detailed design of the compressor, so these figures are purely theoretical. Assuming adiabatic compression from 1 bara to 8.5 bara, k=1.4, compressibility Z=0.99 and mol mass of 28.96 you would require 118 kW at FAD conditions. (n is difficult to determine so from a practical point of view you can use k. It is usually close enough for practical purposes).

Giving a 10% margin on power gives 130 kW. Pretty tight but should just do the job with 132 kW motor if everything is in order.

To see if valves etc are a problem calculate outlet temperatures and then measure them. I can not do it for you because I do not know the pressure ratios of the two stages, nor the inter-cooled temperature to the second stage. Check that the inlet to the second stage is at pretty much the same temperature as the inlet to the first stage. Poor inter-cooling could be part of your problem.

By the way I can not (on the back of a cigarette box) make sense of the dimensions given and the FAD if the "RPM - 750 KW"

To calculate power

Dear Mr. kaisan,

Thank you very much for your precious help. I accept my mistake for writing kW for the RPM but it did not hinder your willingness to help.

Regards.

RS Sahni

LP Bore - 385mm, HP Bore - 235 mm, Stroke - 150 mm, FAD 21.27 Cu M/Minute, Max delivery Pressure - 7.5 Kg/cm sq, RPM - 750 KW. The compressor is oil lubricated and water cooled. The description of motor is 415 V, 226 Amps, 132 KW, 1485 RPM, 95.7% efficiency and S1 duty.

When phe pressure is raised beyond 6 kg/cm Sq, the current of motor shoots up up to 250 Amps at 7.5 Kg/ cm Sq. pressure.

Please tell me all factors (with formula) for calculating the required KW of motor for compressor. The current consumed without load (V belts removed) is 90 Amps. In full load all three phases were checked and current found was R=256 Amps, B= 254 Amps and Y = 256 Amps (Max.)