Howden screw compressor curve

[reach1e]

1. May I know how does the compressor power requirement varies with the system pressure ratio?

2. Based on the attached curve, does it mean that I can reduce my power requirement if I lower my system discharge pressure? I assume tha pressure ratio is the system pressure ratio as the compressor is with fixed Vi.

Any idea?

[Lodiev]

1. May I know how does the compressor power requirement varies with the system pressure ratio?

2. Based on the attached curve, does it mean that I can reduce my power requirement if I lower my system discharge pressure? I assume tha pressure ratio is the system pressure ratio as the compressor is with fixed Vi.

Any idea?

This means exactly that. The same principle as if u bring down the suction pressure the demand comes down at the motor side. Just always be careful that if the plant should find itself in a fault condition may it be air in the system or hi suction pressure that the motor can handle this condition, or that your controls are of such that it can ramp up.

[sterl]

Pressure ratio is the system pressures....and a high Vi machine operating at low pressure ratios overcompresses; which means the internal pressure of the machine is higher than the discharge port (which would be a little higher than the system condensing pressure). A machine that odes not have enough Vi compared to the system's pressure requriements will undercompress...so the compression continues even after the "thread" has reached the leading edge of the discharge port. For practical purposes: The air compressor curve you are looking showed the result of undercompressing with the pressure ratio just over 8; and the result of overcompressing with the pressure ration under 7. (the peak of the total efficiency curve...) So this machine "likes" pretty high pressure ratios (its a higher Vi machine).

With refrigeration screws alone, not paying attention to the effects of low discharge pressure on the rest of the system: The Oil Separator velocity goes higher so the oil carryover may actually increase; and the superheat tends to decrease so the machine actually requires lower oil recirculation to the rotors and housing. And the shaft torque as well as the horsepower decrease...All providing the machine is not overcompressing drastically....

[reach1e]

Thanks for the explanation. I understand that if the pressure ratio is too high, it might damage the thrust bearings cos of the high thrust load.

Is there any requirement for a minimum suction pressure on these twin screw compressors?