SCREW COMPRESSORS(Types,Operation, Compression ratio calculations)

SCREW COMPRESSORS

As explained in introduction, there are broadly two types of screw compressors “Oil free” or “Dry” type and “Oil injected” or “Wet” type. There have been developments over a period of time. Different designs available now can be classified as below:

 DRY OIL FREE COMRPESSORS:

In this, there are only two moving components, the rotors themselves. Timing gears are also installed on the rotors and are primarily used for synchronisation. The gears synchronise the rotation of rotors and prevent contact of rotors between themselves and with the casing.This type of compressors are used for air and many process gas applications which do not allow contamination with oil. Suitable sealing arrangement is designed for each service.

 LIQUID INJECTED COMPRESSORS:

Compression is accompanied by volume reduction and increase in temperature This increase in temperature puts limitation on compressor design. Temperature increase in certain gas mixtures poses problems like ‘polymerisation’, ‘gumming’ etc. Hence suitable liquids like water, benzol, other solvents etc are injected in the process gas which control the temperature rise by absorbing heat of compression.

OIL INJECTED SCREW COMPRESSORS:

The fundamental difference between the oil free and oil injected screw compressors is that in oil injected, lubricant is added to the gas being compressed and removed again after the compression is complete.

It can be stated that the power absorbed in compressing a gas all appears as heat in the system. Normally the vast majority of this heat appears in the gas itself as increase in temperature, the remainder being absorbed in the compressor and its cooling systems. However, in an oil injected compressor, a large part of the mass flow going through the compressor is made up of the injected oil and hence this absorbs the heat. The mass of oil is relatively large compared to the gas mass flow because the oil is in the liquid phase, but the volume of the oil relative to the gas is normally less than 1% and therefore, the effect of oil volume on the gas throughput of the compressor is negligible.

The addition of the appropriate quantity of oil to the compressor related to the absorbed power therefore, controls the compressor discharge temperature regardless of the pressure ratio over which it is operating. As long as the cooling for the oil is designed to remove the heat absorbed by it, the system remains under accurate control with great flexibility. A typical oil injected screw compressor is
shown on the next page.

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 istransferred 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 rotates out of the female flute on the suction end of the compressor. Suction charge fills the entire volume of each screw thread as the unmeshing thread proceeds down the length of the rotor. This is analogous to the suction stroke in a reciprocating compressor as the piston is drawn down the cylinder. See figure.

 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 27

 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 on the driving rotor.

 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 28). 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 compressing the gas ahead of it. See figure 29.

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.

 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.

 VOLUME RATIO:
In a reciprocating compressor, the discharge valves open when the pressure in the cylinder exceeds the pressure in the discharge manifold. Because a screw compressor does not have valves, the location of the discharge ports determine the maximum discharge pressure level that will be achieved in the screw threads before the compressed gas is pushed into the discharge pipe.

Volume ratio is a fundamental design characteristic of all screw compressors. The compressor is a volume reduction device. The comparison of the volume of the trapped gas at suction Vs, to the volume of the trapped gas remaining in the compression chamber when it opens to discharge Vd, defines the internal 

volume reduction ratio of the compressor. This volume index or Vi determines the internal pressure ratio of the compressor and the relationship between them can be approximated as follows:

Vi = Vs / Vd

where, Vi: Volume ratio or index
Vs: Volume at suction
Vd: Volume at discharge

Pi = Vi k

where, Pi: Internal pressure ratio
k: specific heat ratio of the as being compressed.

Only the suction pressure and the internal volume ratio determine the internal  pressure level in the trapped pocket before opening to the discharge port. However, in all refrigeration systems, the condensing temperature determines the discharge pressure in the system, and the evapourating temperature determines the suction pressure.

If the internal volume ratio of the compressor is too high for a given set of operating conditions, the discharge gas will be kept trapped too long and be raised above the discharge pressure in the piping. This is called as overcompression and is represented in the pressure-volume curve in Figure 33.

In this case, the gas is compressed above discharge pressure and when the port opening occurs, the higher pressure gas in the screw thread expands out of the compressor into the discharge line. This takes more energy than if the compression had been stopped sooner, when the internal pressure was equal to the system discharge pressure.

 When the compressor volume ratio is too low for the system operating pressure, this is called as undercompression and is represented in Figure 34. In this case, the discharge port opening occurs before the internal pressure in the compressor trapped pocket has reached the system discharge pressure level. The higher pressure gas outside the compressor flows back into the lower pressure pocket, raising the thread pressure immediately to the discharge pressure level. The compressor then has to pump against this higher pressure level, rather than pump against a gradual build up to discharge pressure level if the volume ratio had been higher, keeping the trapped pocket closed longer.

 In both cases the compressor will still function, and the same volume of gas will be moved, but more power will be required than if the discharge ports are correctly located to match the compressor volume ratio to what the system needs. Variable volume ratio compressor designs are used in order to optimise discharge port location and minimise compressor power.