Climatic test chamber with stable cascading direct expansion refrigeration system

Abstract

A climatic test system includes a test chamber adapted to hold a unit under test within the housing, and a refrigeration system that is adapted to cool air within the chamber. The refrigeration system includes a high stage, a low stage, and a cascade heat exchanger between said high and low stages. The low stage includes a refrigeration cycle having a low stage compressor, a low stage condenser, a low stage expansion device, and a low stage evaporator. The low stage evaporator is in heat exchange relationship with the chamber. The high stage includes a refrigeration cycle having a high stage compressor, a high stage condenser, and a cascade heat exchanger evaporator that is in heat exchange relationship with the low stage condenser, thereby defining the cascade heat exchanger. The high stage further has a first high stage expansion device supplying refrigerant from the high stage condenser to the cascade heat exchanger evaporator during cascade operation. The high stage further includes one or more additional high stage evaporator in heat exchange relationship with the test chamber and one or more additional high stage expansion device supplying refrigerant from the high stage condenser to the additional high stage evaporator(s) during single stage operation. A control controls operation of the refrigeration system in at least a single stage operation and a cascade operation. The additional high stage evaporator(s) is connected with the cascade heat exchanger evaporator wherein mass flowing through said at least one additional evaporator flows through the cascade heat exchanger evaporator.