# What is the maximum compression ratio

## Main Question or Discussion Point

what is the maximum "compression ratio"

http://www.starrotor.com/Engines [Broken]
what is the maximum "compression ratio", so to speak, achievable by compressors? i don't think it is very high.

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Single stage compression ratios are limited to around 1.4. Beyond that, flow separation can damage the engine. I don't know if there is a limit to the total compression ratio.

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There is no real "mechanical" limitation on the compression ratio achievable by compressors, except for the relationship effects with temperature (strength reduction, oxidation, friction ...). Compressors are more limited by efficiencies, where an isothermal compression process is ideal, but a perfect isothermal process is impossible to achieve, because gases heat up when you compress them and you would need perfect heat transer - so, for a compromise, multiple stages are often used with intercoolers, and because of this phenomena, compression ratios are kept low. 1:4 normally, sometimes as high as 1:7 for some positive displacements (reciprocating, scroll, screw, ...) Note that the compression stages in newer higher performance turbines are running upwards of 1:25.

@brianc. what do you mean by flow separation could damage the engine.
@canadieng. what is the compression for a centrifugal compressor?

Most centrifugal compressors have a compression ratio of around 1:2.5 to 1:4, however, special units can be upwards of 1:10 with a single stage.

That's a bit of a how long is a piece of string question. For a single stage device yes the value is quiet low, if you have a multistage machine you start to run into temperature problems after 3 or 4 stages, if you have a multi stage machine with inter-stage coolers there shouldn't be any limit.

i have a design where i use a compressor to compress air fuel mixture to a c.r. of about 8 and then cause an ignition, pretty much similar to the star rotor mentioned in the first post. i only have a doubt as to how efficient the compressor would be for a single stage.

@brianc. what do you mean by flow separation could damage the engine.
I'm taking a propulsion course, and was told that compressors can literally explode if too much flow separation occurs. I assume that the rotors will be torn apart from the drag forces.

Keep in mind that the mole weight of gas being compressed plays a very significant role in the compression ratio that can be achieved in dynamic compressors. Light gases like hydrogen need more stages to get the same pressure than heavier hydrocarbons or chlorine for example.

hey i need help solving this problem .
3 KJ/s enters the low pressure cylinder of 2 stage compressor, the overall pressure ratio is 8:5. the air at inlet to the compressor is 100 KPaat 30 degree . the index of compression in each cylinder is 1.3. find the intercooler pressure of perfect intercooling and also find the minimum power for compression and % of power saved over a single stage compressor.
i want to know where to substitute what in the formula

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