Dear Mr mirmuddassira,

Your question remembers me of the time that I was a student and got those calculations as an exam.

First construct your compression diagram taking into account cooling, void space above the piston in top position.

In case of polytrophic compression use :

Power = n/(n-1) * (p1V1-p2V2) + n/(n-1) * (p2Void-p2V3) and use pV^n = constant.

In case of isothermal compression use :

Power = p1V1 * ln(V1/V2) + p2Void * ln(Void/V3) + and use pV^n = constant.

Also use your books about thermo dynamics.

In the compression diagram, you can locate the gas conditions for each crank position and calculated the forces.

The volume of pi/4 * Bore^2 * stroke * rpm is in the diagram between the top position and the bottom position of the piston. The void (or dead volume) is extra.

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Try this spreadsheet.

I think it works

success

Attachments

pistoncompr (ZIP)

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Hi Teus,

I have used your spreadsheet to calculate how much money we can save in a year by reducing the oulet pressure of our air compressors from 7.5barg to 6.5barg. As ours are screw compressors, less savings are expected. However I can still use the resulting figures for rough estimation. Do you have any clue how much higher is the efficiency of screw units when compared to piston ones?

Thanks and regards buddy,

Walter ■

Dear Walter,

A piston compressor is always compressing according the same thermo dynamic diagram.

Therefore the energetic efficiency is more or less independent of the delivery pressure.

A screw compressor has the highest efficiency, when working at the design pressure, due to the built-in compression ratio.

The choice of a compressor has to be made in relation to the characteristics of the installation that consumes the pressurized air (or gas).

Have a good day

teus ■

Hi Teus!,

You are right, I agree with you. I missed to mention my application is not for a particular case of pneumatic conveying.

Our compressed air is an utility for several plant purposes, such as pneumatic actuators (air cylinders), air rings where compressed air is blown to atmosphere, etc. etc. and dense phase pneu. conv. (all uses with individual pressure regulating valves for fine tunning).

It is common practice to set compressor's outlet pressure at 7barg. Thus, reviewing such by defect setting is worthwhile. If this is the optimal manufacturers design pressure, then my point is senseless.

But if not, and assuming V is kept constant then there is a chance to save energy when setting at a lower pressure (quick calculation using your simple excel file where polytropic and isothermal thermodynamics are considered). Let's explain it this way: we compress air up to 7barg (100psi) and then we reduce pressure to lower values in each application point. Does it make sense?

Something similar happens with steam boiler operating pressure where 10barg is like a standard pressure setting for general heating purposes.

If I were to dedicate a compressor unit for a particular use such as the moving fluid of a PneuConv facility, then of course the operating pressure should cope with the system pressure drop.

Hope I could now enlighten my concern a litlle better.

Take care!

Walter ■

Hi Walter,

Throttling air is an expansion process at constant enthalpy.

This means that the temperature stays constant and that there is no external energy involved.

Throttling is the pressure reducer process. and happens at nearly no losses.

(Joule Thomson effect, WIKIPEDIA)

Running a compressor system at 7 barg and throttling down to a lower pressure is OK.

Another advantage is that the compressor pressure can vary (due to the tankvolume and the momentary air demand), whereby the installation pressure(s) are kept constant.

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