Mechnical

This makes about as much sense as your last post, non what so ever.

1. the output power of a 4.2MW genset is 4.2MW

2. The centrifugal compressor (or any machine efficiency) = Power Out/ Power in

In compressor power out - calculate in terms of the output head of the fluid and the flow rate.

Power In - from the prime mover

Re: Can any one told me what is mean of output power of DG set if it 4.2 MW?

You're asking what is the meaning of 4.2 MW as the output power of a DG set?

MW is a unit of measurement for power, it is megawatts, typically a measurement of electric power. It can be converted to various other units of power, for example, Hp (horsepower). (and others, I'm just drawing a blank atm (at the moment))

Re: How to i calculate my centrifugal compressor effiency by using its operation paraments?

Is it driven by an electric motor? Determine the input electric power (by measuring the input volts, amps, and power factor) and the output power in the compressed gas (by measuring the output flow and pressure), and make a ratio:

efficiency = output power / input power

That's actually the efficiency of the combination of the motor and the compressor. Offhand, to get just the efficiency of the compressor, find out the efficiency of the motor and correct the input power in the calculation above by the efficiency of the motor.

You can make similar calculations for a compressor driven by something other than an electric motor, I don't know how to calculate the input power to, for example, a diesel motor--maybe at a steady speed measure the rate of fuel consumption and get the BTU value of the fuel.