Minimum Operating Pressure for 7Kg/cm² Screw Compressor

The manufacturer will give an operating envelope for the compressor. At 7kg/cm² and no flow, the unit will consume minimal energy. It may be that below 4.5kg/cm² the unit is consuming the maximum energy, producing the maximum flow, and may be close to its motor's peak current. Therefore it is undersized for the job if it is operating here for any length of time, and a larger one would be needed.

What you write is 100% valid for centrifugal compressors.

The "screw compressor" is as far as I know a volumetric pump type (at least it is what appears after a short check with google help to make sure that I am on the right track).

So that if load pressure is low the flow will not increase if the rpm are not modified. At maximal pressure the resistant torque will be maximal and if speed stays constant then maximal power will be asked from the motor.

Or am I wrong?

May be there are other reasons, what do you think?

I would appreciate a comment.

I suspect there may also be issues with the oil flooding system with the screw and possibly an oil foaming problem with prolonged operation.

When in doubt, follow the manufacturer's recommendations.

You are right. Screw compressor being a positive displacement will require maximum motor power when the output is choked and minimum when the output is free. Hence I too am unable to understand what is the likely problem if the compressor works at low pressure.

The only thing I can think of is the turbulence at the oullet stage due to the low pressure and as I remember reading some paper this excess velocity may cause some problem.

However we do work with screw pumps in oure lube units and very often these pumps do work at extremely low outlet pressures. Just for example, 12 bar pumps are usually run at about 5-6 bars (the other portion is kept as a factor of safety to account for drops, filter choke etc)

I really do not find any reason for problem in low pressure operation except that, as I mentioned, due to excess velocity at the exit stage of the screw and the likely pressure turbulance.

I know I am right but in fact I expected a reaction from the author of #1 comment and am disappointed it did not come.

Now considering the low pressure work. In fact the pressure is build up by the net resistance or by the pressure limiting valve of the reservoir. In modern installations to avoid power losses the rotational speed is regulated according to user needs via pressure measurements and the valves are only used as safety devices.

The delivered specific volume can be higher since the air is less compressed. This can be the reason for a flow speed increase but this will not increase the power consumption, it will reduce it. A flow increase could have some effects on the lubricating oil carrying capability. As far as I know there are often oil injections for cooling and if the flow speed is more important the mass of oil coming into the circuit could be higher but, again as far as I know, in such systems there are outlet filters to recover the oil so that... I shall have a look to see if this guide line for low pressure is function of the compressor type or general for screw compressors which ever type they are. I will come back when I will have consistent results.

Hello PWSlack,

GA to you for your explanation Sir.

Take care.

Guest in reply #4 got it right.

Hi PWSlack,

When i operate the plant air at 4.5kg/cm² instead of 7kg/cm² (switch off one or more unit of 26Nm³/min compressor) and i noticed that compressor running current is lower,which is meaning that loading is lower. eg. 160Kw (Atlas Copco GA160W) loading current at 7kg/cm² is 312Ampere at 415V 3ph 50Hz. and loading current at 4.5kg/cm² is 260Ampere.

Our production plants needed 550Nm³/min and pressure at 4.5kg/cm² of course running with many units of srew and centrifugal air compressors to pump in to air net system with total power of approx. 2650Kwh.

If i maintain the plant air pressure at 7kg/cm² the total power needed is about 3400Kwh

So may i say when i operate at lower pressure i will save more electricity ?

Correct me if i am wrong please

Thank you and Best Regards

Yes, if can can operate at a lower pressure you will definetly save electricty. For every 0.141kg/cm² or 2 psig you reduce pressure you will reduce your horsepower or kw by 1% which will reduce your electricy needs.

But, keep in mind that you don't want to reduce below the minimum needed to operate your plant and I as said earlier you need a minimum pressure at the screw compressors to keep the oil flowing inside your screw compressors to lubricate them.

You will need to put some gauges at the furthest point from the compressors and monitor the pressure there as you adjust the pressure at the compressors. The reason is that you will have pressure drops from the compressors through any air dryers, filters, and all your piping to the users at the end of the lines.

Probably 4.5kg/cm^2 pressure is advised for an oil flooded machine. As oil is supposed to be circulated within the system for efficient cooling of the compressor.

In short as per the tests 4.5kg pressure was found suitable to cool the system nothing else..!!