MEMS Refrigerant Expansion Valve

MicroElectronic Mechanical Systems (MEMS) are an offspring of the silicon processing industry and have been under development as a separate entity since the mid 1970's.  Although they bear considerable resemblances to integrated circuits, they are significantly different in that they incorporate miniature mechanical devices such as diaphragms, cantilevers, gears, etc. This industry has matured over the past 30 years to the point of MEMS being incorporated into our everyday lives.  Some examples include MEMS accelerometers which act as automotive air bag triggers, microfluidic mixers and pumps are commonly explored in the medical fields for drug delivery, air pressure sensors for tires, and gyroscopes for air and space applications.  However, to date no such devices are used in heat pumping/air conditioning machines. 

Greater efficiency in a vapor-compression cycle may be realized by controlling the refrigerant distribution in an evaporator. Laboratory measurements have shown that the penalty for mal-distributed refrigerant flow may be as much as 41% in the most extreme cases. By finely controlling the refrigerant distribution, this penalty can be avoided. Controlling refrigerant distribution requires separate refrigerant valves located at each circuit inlet. Currently available macro-scale valves could be used for this purpose, but their high price renders their application uneconomical. However, revolutionary advances in MEMS make it feasible to manufacture low-priced refrigerant valves that would be able to control refrigerant distribution in a heat exchanger.  The goal of this project is to design and demonstrate through testing the practicality of such a MEMS valve for applications with a low refrigerant mass flux. 

The refrigerant expansion device under study involves a small flow obstruction that is able to move into and out of a capillary tube, thereby creating a partial blockage.  In this situation, the capillary tube acts as the main refrigerant expansion device, and the obstruction adds additional constriction to the capillary tube upon demand.  To perform this task, the flow obstruction is positioned on top of a thin membrane, which seals an oil filled cavity.  When a small amount of heat is added to the cavity through resistive heating, the oil expands and pushes the obstruction upwards into the flow path of the refrigerant.

David Yashar
HVAC&R Equipment Performance Group
(301) 975-5868
david.yashar@nist.gov