Compressor Reverse Flow Regulation
A method of stepless control suitable for reciprocating compressors is reverse flow regulation, by which the intake flow is caused to vary according to requirements, by delaying the closing of the suction valves. This delayed closing is achieved by means of special unloaders. These are actuated by the variable pressure of a piston built into the valve cover (usually a diaphragm piston in a servo-cylinder). A number of unloader springs are tensioned by means of an unloader plate, which in turn keeps the valve plate of the regulated suction valve open against the reverse flow of gas during the compression stroke of the cylinder. Part of this gas flows back into the suction duct and reduces the output capacity by an equal amount. During the compression stroke, the velocity of the gas flowing back rises steadily with increasing piston speeds as do the flow forces acting on the unloader and valve plate in a closing action. These forces vary with piston position and eventually exceed the hold-open force which has been adjusted to a certain value. This causes the valve to close (after a delay) and the reverse flow is terminated. The remaining gas in the piston is compressed and discharged in the usual manner. This permits adjustment to any desired capacity between 100% and 40% of full load (see Figure 6) and in certain cases stepless reductions down to 15% or less may be achieved.
A further method of controlling the output is by throttling the intake, but this requires more power at part load conditions than the methods described above.