DISCHARGE PROCESS
In the recip. Compressor, the discharge process starts when the discharge valve first opens. As the pressure in the cylinder exceeds the pressure above the valve, the valve lifts, allowing the compressed gas to be pushed into the discharge manifold. The screw compressor has no valves to determine when
compression is over. The location of the discharge ports determine when compression is over. See figure 30. The volume of the gas remaining in the “V” shaped pocket at discharge port opening is defined as the volume at discharge, Vd.
compression is over. The location of the discharge ports determine when compression is over. See figure 30. The volume of the gas remaining in the “V” shaped pocket at discharge port opening is defined as the volume at discharge, Vd.
A radial discharge port is used on the outlet end of the slide valve and an axial port is used on the discharge end wall. These two ports provide relief of the internal compressed gas and allow it to be pushed into the discharge housing. Positioning of the discharge ports is very important as this controls the amount of the internal compression.
In the recip., the discharge process is complete when the piston reaches the top of the compression stroke and the discharge valve closes. The end of the discharge process in the screw occurs as the trapped pocket is filled by the male lobe at the outlet end wall of the compressor. See figure 31. The recip. Always has a small amount of gas (clearance volume), that is left at the top of the stroke to expand on the next suction stroke, taking up space that could have been used to draw in more suction charge. At the end of the discharge process in the screw, no clearance volume remains. All compressed gas is pushed out the discharge ports. This is a significant factor that helps screwcompressor to be able to run at much higher compression ratios than a reciprocating one.
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