The figure presents a simplified and partly schematic view of a two-stage rolling-piston compressor that is undergoing development for use as the prime mover in a vapor-compression heat pump. In comparison with equally rated compressors used heretofore in vapor-compression heat pumps, this compressor would perform better, would operate with higher efficiency, would be smaller, and would weigh less. Unlike prior compressors, this compressor could function in any orientation with respect to the gravitational field and/or acceleration, or in the absence of gravitation and acceleration.

Like most prior compressors, the present developmental compressor is designed to be part of a hermetic vapor/liquid-flow system wherein a refrigerant fluid and a lubricating oil are allowed to mix and flow together. In a typical prior compressor, a sump in the compressor housing is used to collect and hold most of the oil; this is necessary to prevent fouling of control valves and heat exchangers with excessive amounts of oil. The oil-retention function of the sump depends on the proper orientation of the sump with respect to acceleration and gravitation. In contrast, the present developmental compressor does not contain a sump; instead, as explained below, the compressor operates with flow-through lubrication, and oil is allowed to flow throughout the system. There being no sump and no need to concentrate oil at a single location in the system, the compressor and the rest of the system become insensitive to gravitation and acceleration.

The rolling-piston concept was chosen for development of this compressor for several reasons:

  • Rolling pistons can readily be designed to operate at speeds greater than those of reciprocating pistons; this makes it possible to reduce sizes and weights of moving components without reducing throughput.
  • The lubrication requirements of rolling-piston compressors appear to be minimal; a rolling-piston compressor can operate satisfactorily, without a sump, when lubricated by a small amount of oil (an oil mist) circulated with refrigerant.
  • The requirement for low oil circulation is consistent with the need to minimize the potential for fouling of heat exchangers and control valves with oil.
  • Rolling-piston compressors are quite tolerant of the entry of slugs of liquid - refrigerant or oil or both. Thus, in comparison with other compressors, rolling-piston compressors function with less dependence on precise control of liquid inventories.
This Two-Stage, Rolling-Piston Compressor offers potential advantages of better performance and less size and weight, relative to previously developed heat-pump compressors.

A typical preliminary design calls for the developmental compressor to circulate the refrigerant fluid between an evaporator at a temperature of 240 K and pressure of about 10 psi (≈69 kPa) and a condenser (radiator) at a temperature of 295 K and pressure of about 100 psi (≈690 kPa) while transferring heat from the evaporator to the condenser at a rate between 300 and 500 to 500 W. The refrigerant fluid would likely be R-134a (tetrafluoroethane). The lubricant would likely be a synthetic polyol-ester oil, the amount of oil being of the order of 10 to 20 mass percent of the refrigerant/lubricant mixture. The motor speed in typical operation would be of the order of 10,000 rpm.

Although the design calls for two stages to develop the pressure ratios needed for the required temperature lift of 55 K, both stages can be contained in a single housing, where they can be driven by one motor via one shaft. By suitable diversion of inlet and outlet flows, the two pistons can be operated in series (as two stages) to obtain a higher temperature lift at a smaller flow rate, or the two pistons can be operated in parallel (as a single stage) for a somewhat lower temperature lift at a greater flow rate. Depending on the required evaporator and condenser temperatures, a control system can select either the series or the parallel mode, whichever offers greater efficiency. The motor speed can be varied to control the capacity of the compressor in either mode.

This work was done by Daniel L. Fischbach, Russell Tetreault, Andrew C. Harvey, William Leary, Nathan Longo, and David H. Walker of Foster-Miller, Inc., for Johnson Space Center. MSC-22888

Motion Control Tech Briefs Magazine

This article first appeared in the October, 2000 issue of Motion Control Tech Briefs Magazine.

Read more articles from the archives here.