Compression Losses in Cryocoolers

J.S. Reed, G. Davey, M.W. Dadd and P.B. Bailey

Department of Engineering Science, University of Oxford, Parks Road, Oxford, OX1 3PJ, UK.

Most of the loss mechanisms in Stirling and Pulse Tube cryocoolers are well documented and are relatively easy to analyse and estimate. One of these losses is related to the irreversible compression of gas in the cylinder, and the magnitude of this loss is such that it has a very significant impact on the overall system efficiency. Simple tests on a cryocooler give an estimate on the size of this loss, though the results of these tests include elements of other, known losses, such as clearance seal and pressure drop losses. The parametric variation of this 'lumped' loss suggests that it is not primarily due to these, or other known losses. Over a wide range of machines and conditions this 'lumped' loss varies with operating frequency, swept volume and pressure swing, suggesting that it is some kind of thermodynamic effect related to the area of the 'P-V' loop in the compression space. This loss appears to be quite independent of the refrigeration cycle, and takes place in the ambient temperature part of the system as a result of cyclic pressure changes in a typical Stirling cycle geometry. The loss does depend on geometry, and is typically higher on a 'split' Stirling cycle machine than on a more compact 'integral' one. This paper looks at the experimental data which is the basis of this loss, and compares this data with other studies carried out on compression losses in Stirling Cycle and other reciprocating machines.