Flow rate of a compressor at different operating pressure 3

[ipocoyo]

Hi All,

I am trying to find out the flow rate that my air compressor has at different operating pressure. It has a maximum operating pressure of 10bar and a flow rate of 15.41m3/min at that pressure. If I were to operate 2 compressors at the same time to achieve a pressure of 16bar, how would I be able to estimate the flow rate? I would have assumed that the air would follow boyles law in a compressor system where P1V1=P2V2 but that does not seem to be the case. Thank you!

 

[Latexman]

It has a maximum operating pressure of 10bar and a flow rate of 15.41m3/min at that pressure.

I’m not so sure about the “at that pressure” part. Most compressor capacities are at suction conditions which is close to STP (standard temperature and pressure). I mean if it’s 15.41 m3/min at 11 bar-a, I would expect about 28.2 m3/min at 6 bar-a, suction flows being equal. But that’s way off the 18.54 m3/min of your chart. I think the flows are standard m3/min or essentially suction conditions. Check with the vendor tech support.

That may be why you haven’t figured it out yet.


Good luck,
Latexman

Engineers helping Engineers

 

[TiCl4]

Ipocoyo: The equation for a reversible adiabatic compression is P * V^gamma = C, where C is a constant and gamma = Cp/Cv (constant pressure heat capacity divided by constant volume heat capacity. Cp/Cv = (f+2)/f, where f is the number of degrees of freedom of the gas at the given conditions. For diatomics gases (air), gamma = 1.4. Boyles law does not apply here. That might be a better place to start.

It sounds like you would be placing the compressors in series to achieve 16 bar, since one alone is only rated to 10 bar for operation. Be very careful! If max operating pressure of the compressor is 10 bar, running up to 16 bar may exceed the MAWP of the compressor. Even if it does not, the excessive heat generated from the extra compression is likely to cause the metal of the compressor to expand beyond tolerances and wreck the pump. An intermediate air cooler may be required. I do not think running two of those compressors in parallel to 16 bar would work, as the single stage compression at each compressor would be above the vendor's operating limits (again leading to high temperatures and expanding metal).

Latexman is right - contact the vendor with your application.

 

[ipocoyo]

Thanks all. I will double check with the manufacturer in this case. Also, is it possible to say that the mass flow rate of the compressed air at "10bar" is 15.41x1.2(density of air at RTP)? The density of air changes as pressure changes as well. According to:

https://www.engineeringtoolbox.com/air-temperature-pressure-density-d_771.html

the density of air is 13.2 at 10barg of pressure. Would it be 15.41x13.2 for the mass flow rate at 10 bar instead?

Thank you!

 

[LittleInch]

Ipocoyo,

I don't know what type of compressor this is which might make a small difference but what you are proposing simply won't work.

Your data is given in FAD. FAD means Free Air Delivery ( or sometime Demand), i.e. the quantity of "free" or atmospheric pressure air entering the compressor.

https://www.engineeringtoolbox.com/rating-air-compressors-d_848.html

You won't be able to get any more air mass through this compressor at 10 bar.

However your then 6 bar compressor is expecting air at 0 barg, not 10 barg. The actual volume of the air at 10 barg will be about one tenth of the volume at 0 barg. Your 6 barg compressor, assuming it is a piston will simply let down the pressure back to about 0 barg as it has a big volume to fill which can't be met by the upstream compressor.

You would need 10 compressors feeding one compressor, but then the mass is x 10 and your little 6 barg compressor would explode.

If you want 16 barg, get a 16 barg compressor. Stop trying to make 10 + 6 = 16, because it won't.

"the density of air is 13.2 at 10barg of pressure. Would it be 15.41x13.2 for the mass flow rate at 10 bar instead?"
No because you would be creating mass from nothing. Even Einstein couldn't do that. See above for what FAD means.

The capacity really seems to die off after 9 bar for some reason....




Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ipocoyo]

Hi LittleInch,

Thanks for the reply! Sorry if some of my replies may seem silly, I am trying to learn as much as I can from my current internship.

Assuming a scenario where 3 compressors are connected to a pressure vessel with a MAWP of 16 bar(g). If each of the compressors have a maximum rating and cutoff pressure of 10 bar(g), am I correct to assume that overpressurisation within the pressure vessel is impossible due to overpressurisation of the compressors? It also seems to me that overpressurisation due to closed or blocked outlets is impossible as the pressure vessel is designed to be 6 bar(g) more than the maximum source pressure, as connecting the compressors in series will not increase the pressure beyond 10bar(g) and only increases the rate of flow when required.

Thank you!

 

[LittleInch]

ipocoyo.

You need to stop changing your question / design.

If you have three compressors in parallel and they all are rated to no more than 10 bar then your 16 bar vessel cannot overpressurise so long as your protection features / relief valves on the compressors work correctly.

you can't add one compressor to the inlet of another in series unless it is specifically designed for it as it then becomes a two stage compressor.

It sounds to me ( a drawing would help a lot) that you have three compressors in parallel and hence if two are running you will get more flow. but not more pressure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ipocoyo]

Hi LittleInch,

Thank you for the insight. In this case, does that mean a PSV is not necessary for the 16 bar(g) vessel as the maximum operating pressure of the 3 air compressors in parallel are 10 bar(g)? I've had a look at API 521 and there seems to be no credible scenarios of over pressurisation from happening. The most likely scenario is due to external fire since the air compressors are in a room beside the boilers. From my understanding, fitting a PSV for the event of an external fire is not going to be sufficient protection and only buys extra time. The compressor system will still be lost in the event of an over pressurisation due to external fire.

Is it recommended to still fit a PSV to the air receiver by using the maximum flow of the compressor of 18.54m3/min to determine the required effective orifice area? Or is it recommended to determine the required orifice area of the PSV when exposed to external fire?

Thanks!