3 replies to this topic

[ayan_dg]

I read engineers use polytropic head and ploytropic efficiency when describing Centrifugal Compressors but adiabatic head and adiabatic efficiency when describing positive displacement compressors.

Why this is so ?

Why we cannot use adiabatic efficiency and adiabatic head for centrifugal compressor even though it can be easily referenced to thermodynamic properties and easily understood.

[PaoloPemi]

you may find answers in several old threads and many books,

as introduction I recommend the post from art here,

http://www.cheresour...as-compression/

[ankur2061]

ayan,

 

Before we move on to the why's of the compression process or compression efficiency type (adiabatic or polytropic) it is essential that the difference between the two be understood. 

 

1. Adiabatic Compression: The terminology used by some compressor specialists is "Near-Adiabatic (Isentropic) Compression". Practically it is impossible to have a perfect adiabatic compression which by the theoretical definition of "adiabatic" means that during the compression process no heat enters or leave the system when the system is represented as a perfectly insulated box from its surroundings. Positive displacement compressors come closest to adiabatic compression or near-adiabatic compression. It is important to note that even for dynamic compressors (centrifugal / axial) the compression process and the compression efficiency was defined as adiabatic more than 30 years ago

 

2. Polytropic Compression: The practical understanding of the polytropic process and its usage in defining the compression process and efficiency is a development of the last three decades. In a polytropic compression process infinitesmal changes occur in terms of heat absorbed or heat removed during the entire compression process. If you plot a P-V curve for the compression process and represent the curves for all three compression processes viz. adiabatic, isothermal and polytropic, you will find the polytropic compression curve lies in-between the adiabatic and isothermal compression process. The polytropic process represents the real world compression process. Compressor manufacturers realised that the centrifugal compression process mimics the polytropic process more closely than the adiabtic process. There is a reason for it. In centrifugal compressors gas slippage occuring across the impeller(s) in the compressor casing is much more pronounced than the gas slippage encountered in a reciprocating (PD) compressor volume chamber. This gas slippage leads to mixing of compressed and slip gas which are having different temperatures. This pronounced slippage in centrifugal machines mimics the polytropic compression process which as earlier mentioned is the infinitesmal change in heat gain or heat loss during the entire compression process. That is the reason  that the compression process for centrifugal compressors is defined in terms of polytropic compression and polytropic efficiency.

 

To conclude, the performance and efficiency of a centrifugal compressor can be defined in terms of an adiabatic compression process but the polytropic compression process provides a more realistic performance of a centrifugal compressor compared to an adibatic compression process.

 

Regards,

Ankur.

Edited by ankur2061, 26 March 2013 - 06:02 AM.

[ayan_dg]

Thanks Ankur for your explanation.
I read that for a polytropic process the polytropic efficiency is independent of pressure ratio whereas for an adiabatic compression process the adiabatic efficiency is dependent on pressure ratio.
Is this correct ?