The Effect of Mean Pressure on Large Pulse Tube Cryocoolers

Abstract

We have been developing large scale, Stirling-type pulse tube cryocoolers. These cryocoolers are operated at or near 60 Hz and are directly powered by a dual-opposed linear motor pressure wave generator (PWG). The PWG is non-lubricated, requiring no oil removal from the helium working fluid. The PWG drives the unit directly, with no need for a rotating valve. The power input to the PWG and cryocooler can be scaled according to the refrigeration need, as opposed traditional refrigeration where capacity control is achieved by on-off operation.

These cryocoolers are hermetically sealed. As ambient conditions change, or cooling water temperature varies, the mean pressure in the cryocooler can vary over time. If the power input to the cryocooler changes, this will change the amount of heat produced by the motor and the motor temperature, which will also change the mean pressure.

Seemingly small changes in the mean pressure can affect cryocooler efficiency, capacity, and overall performance.

This paper presents simulation results showing how variation in the mean pressure impacts large cryocooler performance, in particular, the amount of refrigeration produced by the cryocooler. The paper also discusses various strategies for compensating for performance losses caused by mean pressure fluctuations.