Ahmed Sidi-Yekhlef

Design, Development, and Testing of a Cryogenic Reciprocating Expansion Device

A lightweight, easily maintained helium expander system has been developed to provide reliable, low cost refrigeration in the 20 K to 40 K temperature range. The expander consists of a 2 inch diameter piston controlled by a combined hydraulic/pneumatic system. During the expansion process some of the energy released from the helium gas is stored in the hydraulic system. The stored energy is used to return the piston to its lower position. The expander assembly is built as a self- sealed removable pod that can be replaced with a new pod without warming or contaminating the main system. The expansion engine is controlled by a processor, which reads an LVDT signal and opens and closes valves in a timed sequence. The LVDT rod moves up and down with the piston giving a signal to the processor which controls the valve timing logic. This design is unique in the sense that the pressure of the gas after expansion can be easily adjusted by the valve timing. The hydraulic nature of the drive mechanism also allows us to compensate for the high back-pressure during the resetting of the piston. A typical reverse Brayton cycle operates with a compressor discharge of 1.7 MPa and a suction pressure of.1 to.138 MPa. This system is designed with the option of a higher suction pressure, allowing for higher throughput at similar power input. An adiabatic efficiency of 60% was demonstrated during testing.