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Rotary Screw Compressors - Discussion And Calculations

Rotary Screw Compressors - Discussion And Calculations Dear All,
Rotary Screw compressors are fast replacing piston type reciprocating compressors all over the world. With advancement in technology of precision machining the helical screws required in screw compressors, they are becoming the first choice for selection where otherwise a piston reciprocating compressor would have been used in earlier days.
I had provided a chart for compressor selection in one of my posts on “Cheresources” at the following link:

While this chart is quite handy for compressor selection, latest advancements in technology of screw compressors allow far higher capacities and higher pressure ratios (discharge pressure / suction pressure) compared to what is shown in the chart.
Some general advantages of screw compressors in the category of positive displacement compressors are listed below point wise:
1. Less pulsations or surging of flow compared to piston type compressors since the gas compression process with a rotary screw is a continuous sweeping process.
2. Low mechanical vibrations similar to centrifugal machines compared to piston reciprocating compressors.
3. The inlet or suction volume flow and power consumption increase linearly with the compressor speed at constant discharge pressure.
4. Compared to a centrifugal compressor the inlet or suction volume is nearly constant for variation of pressure ratio or gas molecular weight with no surging limit.
5. The achievable pressure ratio per compressor stage is not limited by the gas molecular weight but limited only by the allowable discharge temperature and mechanical limits of the compressor due to high temperatures.
6. Very high pressure ratios up to 10 per compressor stage can be achieved by liquid injection for cooling.
The pioneers of Screw compressor technology were the Swedish company “Svenska Rotor Maskiner” abbreviated as SRM.
The history of the development of the screw compressor can be found at the following link:
SRM main web page can be found at the link provided below:
With this blog entry I am also attaching an excel workbook which provides some design equations for calculating the shaft power of oil-free or dry screw compressors. The reference for the design equations is the book “Compressors – Selection and Sizing” by Royce N. Brown which I personally consider amongst the best books for compressors. The book provides equations in USC units which I have converted to SI units. The reference for the rotor diameters and L/d ratios is taken from the reference provided in the link below:

Other diameters and L/d ratios used by various manufacturers may be utilized in the excel workbook.
I welcome comments on my blog entry from the knowledgeable members of the “Cheresources” community.
Quick note from the admin: You can download the MS Excel workbook that accompanies this blog entry in the File Library.

Great job as always Ankur!

If i am using the Rotary Screw air Compressor for Natural gas purpose.Whether there is any Hazards form.

How to Reduce compressor pressure of 7Bar to 2Bar  

If i am using the Rotary Screw air Compressor for Natural gas purpose.Whether there is any Hazards form.

How to Reduce compressor pressure of 7Bar to 2Bar  

Consult the rotary screw compressor manufacturer for change in compressed gas. If sealing system is designed for air (air leakage is not a hazard) then the compressor requires modifications to cater to natural gas which is hazardous.


Install a Pressure Control valve to reduce pressure.




Thank you for reply,

1)How to calculate the Discharge pressure for pump and compressor.

2)How to calculate the Vessel size based on Flow-rate and pressure.

    Fluids is a Natural Gas 





which is the best compressor for flare gas recovery unit, and how can be design please

Interesting but the OES History didn't seem to give much on the history of the screw compressor. Not the pages I found. Wiki doesn't report the history either.

If I had to guess I would suggest it evolved from the Swedish IMO 1934(?) patented birotor (Twin helix) flowmeter..... "the most accurate water meter ever manufactured".

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