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Slide inlet valve principle

Slide inlet valve principle for rotary screw compressors, Working principle: The sliding inlet valve offers a continuous capacity control by varying the inlet air trapped at the inlet of the element. The sliding valve opens a variable section just after the inlet of the element causing a recirculation of a part of the trapped air back to the inlet system. The amount of recirculation is a function of the valve position, this way the valve controls the compressed air volume and thus the delivered air flow.
A serious disadvantage of the sliding inlet valve is its bad performance at decreased flow. With a nearly fixed outlet volume this way of controlling causes the volume ratio to drop significantly, thus also dropping the built-in pressure ratio of the element more than linear. Dropping the built-in pressure ratio of the element results in a maximum isothermal compression equal to the built-in pressure, from that point on the compressor will start its highly inefficient pumping process in order to deliver against the net pressure. The p-v diagram below shows the losses in comparison with isothermal compression.
The figure below shows a cross-section of this sliding valve mechanism. The valve is controlled hydraulically by means of an external actuator. Operation and service is very complex. Operation: The mechanical combination of actuator coupled with a sliding valve has an inertia which can cause problems for strongly fluctuating demand. There is no control on the valve position. In case the valve fails in the open position (full recirculation to inlet) , the compressor will keep on working in its worst energy mode, very low built-in pressure ratio, very high losses.An operator will not detect this, since the compressor is still delivering a part of the nominal flow (by pumping) at the operating pressure. Service: The hydraulic valve is a complicated component creating an excessive amount of functional parts submitted to wear and service. Due to the complexity and the integration of the valve, a failing will lead to serious downtimes for repair. These extra components (actuator, valve parts...) will need periodical service adding to the downtime of the machine and service cost.