Capacity Control of Centrifugal Compressors:
Centrifugal compressors need high tip speeds to create the essential pressure ratio. The high tip speed is attained by employing either a big diameter impeller or high rpm or both. Because of big u2, the velocities generally including the flow velocity C are high. Also, there ought to be a reasonable width of the shrouds to diminish friction and attain high efficiency. Therefore, due to the sufficiently large flow area (diameter D & width of shrouds b) needed and large flow velocity, the satisfactory volume that may be handled by a centrifugal compressor is approximate 30-60 cubic metres per minute. A single centrifugal compressor, hence, can be designed for a least capacity approximately of the order of 250 TR having R 11 & 150 TR having R 113 for the purpose of air conditioning.
One of the techniques to control the capacity of the compressors is by changing the compressor speed by a speed-reduction gear. The drop off in speed results in an operation on a lower head-flow characteristic giving a lower volume flow rate equivalent to the similar pressure ratio.
Capacity may be controlled by the use of variable inlet whirl vanes which are frequently employed along a constant speed drive. The capacity is varied by altering the angle at which the gas enters the impeller. The gas then enters along pre-rotation and it result in a decrease in flow.