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murduc74
03-09-2006, 10:38 AM
hi guys

i would like to know, if i compress air at mass flow of say 3kg/s at STP, through a three stage centrifugal compressor. would my mass flow of 3kg/s change at inlet to each stage? and will my compression index (polytropic/adiabatic index) change at inlet to each stage assuming i am able to cool the exit air from each stage to the initial temperature?

pls help guys

US Iceman
03-09-2006, 09:35 PM
The mass flow should stay the same, unless you are adding mass in between the impeller stages.

As to your other question, I don't know. I guess I will have to think about that. My initial impression is that it might change slightly, but only if the pressure ratios are the same for each stage of compression. Bu thats not very scientific is it then?:o

TXiceman
04-09-2006, 06:54 AM
Your mass flow is constant unless you add mass flow between the stages. The ACFM (actual CFM) will reduce from stage to stage as you increase the pressure on a given mass flow.

For the 2nd part of the question, I will have to look in some old books at work on Tuesday (Monday is a holiday in the USA). Been a spell since I have had to work on this aspect of compression.

Ken

Peter_1
04-09-2006, 08:27 PM
This was once a specialised couse during my ATPL training and is in fact the same thing happening in a jet engine.

Mass flow will be the same, what comes in goes out.

But the compression index is determined by the construction of each stage.
The last stage will have the least compression index.

What I didn't understand in your question is that you're talking about an adiabatic compression ( no heat ineraction) but you cool it down each time to the initial temperature? So, an isentropic compression then?

An extract I translated from my school book (haven't re-opened it since 1999)

I just re-read my old school books about this topice and I have to admit that it has become almost Chinese for me. And I succeeded for my exams for this course.

Andy P
04-09-2006, 10:45 PM
Coolpack (much mentioned in dispatches recently) gives you a very quick answer - if your first stage suction was at 1 bar abs and 15C then the index of compression is 1.399 Assuming a pressure ratio of 5 then the inlet to the second stage is 5 bar abs and 15C so the second stage index would be 1.404. If the pressure ratio is 5 again then the 3rd stage inlet is 25 bar abs and 15C, so the index of compression is 1.436

Note 1: the index of compression is the ratio of specific heat at constant volume to specific heat at constant pressure, so it is just a property of the gas at given temperature and pressure conditions.
Note 2: Coolpack calculates the index of compression as a function of temperature and specific volume, so you need to calculate v from the T and P figures before you can calculate the index
Note 3: for air I would use 1.4 all the time, but for refrigerants nearly saturated (ie not much superheat) the index can vary a lot. However it will also vary during the refrigerant compression process - so really not worth worrying about. Pick a number and use it with confidence!!

Cheers
Andy P

US Iceman
05-09-2006, 02:31 AM
Coolpack (much mentioned in dispatches recently) gives you a very quick answer - if your first stage suction was at 1 bar abs and 15C then the index of compression is 1.399 Assuming a pressure ratio of 5 then the inlet to the second stage is 5 bar abs and 15C so the second stage index would be 1.404. If the pressure ratio is 5 again then the 3rd stage inlet is 25 bar abs and 15C, so the index of compression is 1.436


Pretty much what I expected. About a 2.5% deviation across the max. range.

In some of the older refrigerant tables these values are listed at one condition as a reference only. So be careful of using those values.



I just re-read my old school books about this topice and I have to admit that it has become almost Chinese for me. And I succeeded for my exams for this course.


Same problem for me too Peter. I think this falls into the category of "use it, or loose it".

murduc74
05-09-2006, 07:50 PM
thanks alot guys, you really helped me out.

one thing still bothers me though. Mass flow = Volume flow x density of the working fluid. the density of air depends on the temperature of the air. when compressing the air, temperature changes and thus changes the density. also the volume changes due to compression. is the mass flow/second not affected by these points?

US Iceman
05-09-2006, 09:56 PM
The suction mass flow is the same as discharge mass flow if NO mass flow has been added in between.

Of course the volume flow decreases with the compression process... that is how the pressure is developed.

The mass flow is constant unless the suction conditions change. The compressor has a fixed swept volume at a specific speed and pressure ratio.

Assuming the volumetric efficiency does not change, the mass flow can be changed by an increase in superheated specific volume. In other words, if the suction gas is superheated, the gas density decreases. This would lower the suction mass flow.

autt
07-09-2006, 12:49 PM
What I didn't understand in your question is that you're talking about an adiabatic compression ( no heat ineraction) but you cool it down each time to the initial temperature? So, an isentropic compression then?

Hi Peter,

I think adiabatic compression means in the process of each compress stage, not between stages, it's no compression there.

I remember that for multi-stage compression, ideal compress ratio for each is that they are equal, and cooling between stages make the total process near isothermal compression, which is best economical.

Regards