mech world: screw compressor-Fundamentals of operation

A screw compressor is best described as a positive displacement volume reduction device. Its action is analogous to a reciprocating compressor more than any of the other common compressor types. It is helpful to refer to the equivalent recip. Process to visualise how compression progresses in a screw. Gas is compressed by pure rotary motion of the two intermeshing helical rotors. Gas travels around the outside of the rotors, starting at the top and travelling to the bottom while it is transferred axially from the suction end to the discharge end of the rotor area.

SUCTION PROCESS :

Suction gas is drawn into the compressor to fill the void where the major rotor rotatesout of the female flute on the suction end of the compressor. Suction charge fills theentire volume of each screw thread as the unmeshing thread proceeds down the lengthof the rotor. This is analogous to the suction stroke in a reciprocating compressor asthe piston is drawn down the cylinder. See figure 1.

fig.1

The suction charge becomes trapped in two helically shaped cylinders formed by the screw threads and the housing as the threads rotate out of the suction port. The volume trapped in both screw threads over their entire length is defined as the volume at suction (Vs). In the recip analogy, the piston reaches the bottom of the stroke and the suction valve closes, trapping the suction volume (Vs). See figure 2

fig.2

The displacement per revolution of the recip. Is defined in terms of suction volume,by the bore times the stroke times the number of cylinders. The total displacement of the screw compressor is the volume at suction per thread times the number of lobes

COMPRESSION :

The male rotor lobe will begin to enter the trapped female flute on the bottom of the compressor at the suction end, forming the back edge of the trapped gas pocket. The two separate gas cylinders in each rotor are joined to form a “V” shaped wedge of gas with the point of the “V” at the intersection of the threads on the suction end. (See figure 3). Further rotation begins to reduce the trapped volume in the “V” and compress the trapped gas. The intersection point of the male lobe in the female flute is like the piston in the recip. That is starting up the cylinder and ompressing the gas ahead of it. See figure 4.

fig.3

fig.4

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 5. 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 6. 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 screw compressor to be able to run at much higher compression ratios than a recip..