DESIGNS

Positive displacement compressors

-    Piston
-    Rotary
-    Helical (screw)
-    Kinetic
-    Centrifugal

Positive displacement compressors are named after on account of the displacement of the cylinders whose size and rate determines the capacities of the compressors. In general, the speed is known (for typical hermetic compressors, it is 1750 rpm or 3500 rpm), the volume or weight of the gas pumped has in a mathematical relation with the rate of cylinders in a unit time.

The kinetic compressors, also called as turbo-compressors (centrifugal) is considered as one of the member of the turbomachinery family, encompassing the fans, impellers and turbines where pumping forces are obtained by angular moment between the speed of rotating impeller and the flowing fluid (the refrigerant). The volumetric capacities of the turbomachineries where the flows are continuous are bigger than that of the positive displacement equipment of the same size. Nevertheless, the design and cost of such compressors are only cost effective for small capacities (50 tph or less). On the other hand, the centrifugal machines may have the capacities start from 80 to 100 tph and go up to 8,000 tph. Among the positive displacement compressors, piston compressors with the capacity range 100 to 150 tph with small horsepower rating is widely accepted. This is the breaking point to shift to the centrifugal ones.

The rotary compressors are widely used in the cooling systems with the fractional horsepower. The rotary compressors are not preferred in the commercial markets. This might be due to their lack in efficiencies in pumping against high discharge pressures while operating at low suction pressures.

The helical (screw) compressor is a design of a positive displacement and satisfactory in wide range of condensing temperatures. Since 1950s, the screw compressors are used in cooling service in United States of America. Its original design was developed and patented in 1930 in Sweden.

The compressing cycle of a screw compressor is as follows:

The refrigerant is sucked in to fill inside the gaps facing one another. As the rotors turn, the gap between the screws passes the suction port that isolates the gaps between the screws. Continuous rotation decreases the space occupied by the gas and finally ends up with compression. When the gap faces the discharge port, the gas will discharge.

The capacity control is ensured by the circulation of gas. By this way, up to 10% of the design capacity can smoothly be regulated. The screw compressors with the capacities in the range between 100 tph and 700 tph are in conformity with ARI nominal conditions for the cooling water systems. The screw compressors cannot be used within the small-size cooling or acclimatization purposes like centrifugal compressors. The piston compressors are used in 90% of such applications. Their capacities range between the fractions of the horsepower up to 100 tph. They are therefore the starting point for learning the compressors. Following sections are now reserved for the design considerations.