Discus Compressors

Question: I have several questions. First, what is a discus compressor? What are its salient features and how does it differ from the reciprocating compressor?

Also, what is a floating head pressure system? How does it differ from the conventional fixed head pressure system? What are the advantages? What equipment is required to operate such a system? Are there any special refrigerant lines to be run or alterations to be made? Due to the advantages, is it recommended that the conventional systems are changed to this system?

­ -R. Sheshadri, Madras, India

Answer: The Copelametic “Discus” compressor is a semi-hermetic reciprocation compressor that was designed to meet the changing market needs for higher efficiency compressors for commercial refrigeration and air conditioning applications. The major difference between the “Discus” compressor and the conventional “reed” compressor is in the valve plate assembly. But you cannot interchange the valve plates because the cylinder decks, suction reeds, pistons, and cylinder heads are different.

Compressors using the conventional reed valve design have a re-expansion volume built into the valve plate because the discharge reeds are on top of the discharge ports. There is a small amount of re-expansion due to the clearance volume between the piston at the top of its stroke and the bottom of the valve plate. The high-pressure refrigerant trapped in these spaces has to re-expand during the suction stroke pressure before the suction reeds will let refrigerant into the cylinders. This re-expansion results in less refrigerant being pumped with each stroke of the piston and reduces the volumetric efficiency of the compressor.

Volumetric efficiency is defined as the actual volume of refrigerant that the compressor pumps divided by the volume displaced by the pistons.

The Discus valve design developed by Copeland has a discharge valve that is conical in shape and is made of materials that are lightweight, wear resistant, and capable of withstanding high temperatures.

The Discus valve plate is hollow, allowing the suction vapor to enter the cylinder through a large annular suction port. A ring-type suction reed is used to close this intake port and allow a dome on top of the piston to fit up in the center of the suction reed to reduce the clearance volume between the piston and the bottom of the valve plate to a minimum.

The Discus valve fits into the discharge port such that it seats at the bottom of the valve plate. This design eliminates the re-expansion volume of the discharge port, allows the suction reed to open sooner, and increases the pumping capacity of the compressor.

In some models of compressors, the Discus valve design can increase the compressor's capacity (Btuh) as much as 25%. This could allow you to use a smaller displacement compressor and increase the compressor efficiency as much as 16%. This increased efficiency means lower operating costs compared to conventional reed compressors with the same system capacity.

-Glen Hannegan, Copeland