7 replies to this topic

[Attiq Ahmad]

Hello EveryOne,
Can anybody define POLYTROPIC HEAD OF CENTRIFUGAL COMPRESSOR? I've used this term for calculation of power requirement of centrifugal compressor but I don't know its defination/concept.
Thank you
Attiq Ahmad

[Art Montemayor]

Attiq Ahmad:

First, let’s get the basic pre-requisite definitions down before going into your query:

Polytropic Process
A Polytropic Process is one in which changes in gas characteristics during compression are considered. This is considering that an Adiabatic Process is one during which there is no heat added to or removed from the system (Q = 0), and Isentropic Process is one wherein the entropy remains constant (Delta S = 0), and an Isothermal Process is one in which there is no change in temperature (Delta T = 0). A Polytropic Compression Process is typical of a dynamic-type compressor – such as a centrifugal compressor. A reciprocating compressor typically follows an Adiabatic Compression Process very closely.

A Polytropic Process is defined in Thermodynamics as an internally reversible process, which conforms to the relation PVⁿ = Constant.

If you plot the compression curves for Isothermal, Adiabatic, and Polytropic compressions on a Pressure versus Volume graph, you will find that the Polytropic curve falls between the other two, but closer to the Adiabatic curve.


Polytropic Head
The Polytropic Head is an expression used for dynamic compressors to denote the foot-pounds of work required per pound of gas.

That’s all that the Polytropic Head represents. Its definition/concept is that straight forward. I strongly recommend that you seek out more information on these subjects through the Google Search engine. You will find a wealth of information on the Polytropic Process and other related processes in Thermodynamics.

P.S.: I deleted your duplicate post in the Free For All Forum because duplicate posting is very confusing and gains no advantage for either the poster or the respondees. If you want to secure good responses that you can track then you should only post one thread on the subject and follow it through to its completion.

[Attiq Ahmad]

Hello Art Montemayor,
Thanx for this nice explaination and SORRY for duplicate post. I think it posted due to my fault.
Anyway its always nice to hear some valueable advice from your side.
Attiq Ahmad

[Rohit R]

HI Art,
Really appriciate ur explanatory anws given.
Thanks..
I will be happy if u can provide me the calculations or steps for the same.

[Zauberberg]

Hello Attiq,

Just to add one good technical article related to this topic, apart from Mr Montemayor's reply. Take a look at:

http://www.revamps.c...cuments/166.pdf

Regards,

[Porch]

Hi Art,

I just came across your post, which is from a long time ago. If you're still here, can you explain how a compressor can be Isothermal? I mean in a centrifugal compressor, typically the suction temperature (and pressure for that matter) are lower than that of the discharge temperature (and pressure).

Thanks!
Porch

[Art Montemayor]

Porch:

I like your query. It challenges my explanation and allows me to digress further on the subject of gas compression – on a practical level, and not on a theoretical basis. I didn't explain the frailties of Isothermal Compression in my prior post because it didn't enter into the basic question. I hate it when professors in thermodynamics start throwing thermo processes at you, expecting a student to be able to know inherently what is fiction and what is realistic – and not pointing out the fact that some thermodynamic processes presented in the class room are simply not feasible from a practical point of view. Isothermal Compression is one of those, thank you.

I defy any human being reading this to devise a machine that can compress a real gas without raising its temperature – as measured in the discharge port of such a machine. If you can imagine, you would need an infinite number of finite, differential stages – each with an intercooler – compressing the gas. The greater the number of stages, the less the noticeable discharge temperature. However, this is totally impractical and impossible to build due to mechanical and economic limitations. It would prove nothing from a practical aspect and only serve to reinforce what we already know theoretically.

ALL gas compressors – whether centrifugal, reciprocating, screw, vane, lobe, or whatever other type can be built – will deliver a compressed gas with an increased discharge temperature. You are absolutely practical and engineering-like in asserting this fact. Keep up your practical thermo outlook and insist on a thorough and detailed explanation of what is occurring thermodynamically and why. This type of probing thought can only make you a better engineer.

[djack77494]

Just a minor addition to what Art has said-
Even a perfectly reversible/100% efficient compressor is not isothermal. Due to the manner in which pressure and temperature are interrelated in real gases, you can expect temperature to rise when pressure is increased. This is just the opposite of gas expansion where temperature drops as pressure is reduced. I believe that there are some exceptions to this, however, and wonder if the wildly impractical theoretical isothermal compression might be possible for such gases? But, if we constrain ourselves to the real world (as engineers should do), then we can forget about isothermal compression outside the classroom.