6 replies to this topic

[SadiqChemy]

How can I calculate the polytrophic and isentropic efficiency of compressor If I only know the suction condition and the pressure ratio .

 

The compressor type is axial flow .

[PingPong]

With only that you can't.

 

You need not only suction temperature but also discharge temperature, and gas composition to calculate back what efficiency is.

 

Efficiency is a property of the compressor, so if you have vendor data for this particular compressor you should be able to find the efficiency in there.

 

Why do you need to have the efficiency?

[SadiqChemy]

I want to calculate the actual discharge temperature

[PingPong]

For that you need to have the efficiency.

 

You can not calculate both the discharge temperature and the efficiency, one of the two has to be known to calculate the other.

 

Is this an existing compressor? If so, why can't you simply measure the discharge temperature?

 

If not existing, then what is the objective of any calculation?

 

If you have the volumetric suction capacity (at P₁ and T₁) of the compressor, it may be possible to do an educated guess about its efficiency.

[SadiqChemy]

Yes it is an existing compressor and it is part of the gas turbine cycle .I want to study the effect of the ambient temperature on gas turbine performance. The isentropic efficiency is unknown .Hence, I cannot determine the actual discharge temperature at different ambient temperature ( suction temperature ).

[PingPong]

If you know the vendor and type of the GT you could calculate them back from the specific fuel consumption and the outlet temperature of the GT.

 

Or you could ask the GT vendor about the compressor and turbine efficiencies.

 

I expect both to be in the range of 80 to 90 %, depending on the size of the GT, so I would say: in the absence of better data simply assume 85 %.

 

To determine the impact of ambient temperature on GT performance it does not matter much what the exact efficiency is that you assume.

Edited by PingPong, 03 October 2014 - 03:21 PM.

[Padmakar Katre]

How can I calculate the polytrophic and isentropic efficiency of compressor If I only know the suction condition and the pressure ratio .

 

The compressor type is axial flow .

 

Hi,

From your post it seems that this is an existing system, I will suggest you to check the vendor curves and impose your current operating conditions to know the efficiency. Otherwise you can always refer to fluid mechanics handbook to understand compressor theory.