Optimization of Compression Chamber to Reduce the Viscous Friction in the Piston-Cylinder Clearance

摘要

The compression chamber of a hermetic reciprocating compressor is, normally, formed by a cylinder and a piston which present a constant clearance during the piston displacement inside the compression chamber. This configuration has the advantage of a good sealing, however, during the most part of compression, the pressure difference between compression chamber and internal compressor ambient is small enough to not provide any leakage from chamber to internal ambient. In this situation, during the most part of compression process, the sealing efficiency is not necessary, but the high viscous friction, due the small clearance between piston and cylinder, generates a considerable waste of energy by viscous friction. This article presents an analysis about a geometric proposal for the compression chamber that guarantees small clearance, necessary to the correct sealing of compressed gas, in a small region near the upper piston position. The second part of compression chamber is characterized by an increasing in the piston-cylinder clearance along the chamber length. The motivation for this configuration is to provide the adequate sealing only in the region where the leakage process is crucial to compressor performance, furthermore, the clearance growth provides a reduction on viscous friction between piston and cylinder, and both effects generate an expressive increase in the coefficient of performance. (number of transfer units), the thermal-hydraulic characteristics (friction factor and Colburn j-factor), and the operating conditions (heat transfer duty, core velocity, coil surface temperature, and fluid properties) is derived. It is shown that for heat exchanger with constant wall temperature (i.e., condensers and evaporators), there does exist a particular number of transfer units which minimizes the dimensionless rate of entropy generation. An algebraic expression for the optimum heat exchanger effectiveness, based on the working conditions, heat exchanger geometry and fluid properties, is also presented. The theoretical analysis led to the conclusion that a high effectiveness heat exchanger does not necessarily provide the best thermal-hydraulic design.