Capacity control

[Josip]

Hi, Sparrow

Hello I was asked a question on a mock exam paper,and I am unsure of the correct answer.

The question was Capacity control of a screw compressor is generally accomplished by:

(a) Hot gas bypass

(b) throttling the suction vapour

(c) Cylinder unloading

(d) sliding valve

I did not get this correct,and i am unsure still.

I think it might be (a) though.

sparrow

During start we can utilize (a) if it's installed, to reduce starting torque - easier start....also sometimes (after long shut down) we have to start the screw compressor with throttled suction stop valve thus (b) too,

...as we do not have any pistons and cylinders on screw compressors we cannot use (c) at all,

....normal operation capacity control of screw compressors is obtained by sliding valve (d) controlling opening or closing suction port,

...on new compressors we have also Vi slide valve to control the opening of discharge port, mainly, but having some affect also to suction port...according to relation between suction and discharge pressure...some of them with automatic some with manual positioning...

Best regards, Josip