TENNEY ENVIRONMENTAL
Tenney Junior Test Chambers: Models TJR and TUJR, W942, or WF4
Page 40
INTRODUCTION
Note: Only qualified service personnel should ever be permitted to perform any service related
procedure on this chamber!
This information is written to help the refrigeration serviceman trouble-shoot and repair low temperature
cascade systems. It is assumed that the reader is familiar with standard refrigeration practice and is
interested in the special techniques applicable to cascade systems.
Important! Please remember that the following description may differ in some respects to the
refrigeration system equipped with your particular chamber.
History:
Prior to the development of low boiling point refrigerants such as R13 (-114 deg. F) and R503 (-127 deg. F),
reaching ultra low temperatures with mechanical refrigeration was difficult. R22 was used down to -80 deg.
F, but its system had serious drawbacks. Large and cumbersome, the machinery was subject to the many
troubles that afflict a compound system operating at suctions as low as 23 inches of vacuum. The modern
cascade system can reach as low as -120 deg. F with suction pressures of 0 PSIG or higher. Compact,
serviceable, and reliable, today’s cascade system is found on thousands of environmental test chambers.
How It Works:
Two types of popular cascade systems are expansion valve and capillary tube. The system described in this
manual is the capillary tube type.
Refrigerants with low boiling points have correspondingly high condensing pressures at normal ambients.
They cannot be liquefied by conventional air or water-cooled condensing units. Therefore, low temperature
refrigerants are condensed by a separate refrigeration system called “the high stage”. The main job of the
high stage in most cascade systems is to condense low stage refrigerant.
High Stage:
The high stage is a conventional single-stage system having a compressor, air or water cooled condenser,
expansion valve, and evaporator. The evaporator is the cascade condenser, serving the low stage. Modern
systems use R404a in the high stage, making -50 deg. F refrigerant temperature possible at 0 PSIG suction
pressure.
Low Stage:
The low stage is charged with refrigerant in vapor phase only to a specified gauge pressure. When the low-
stage is idle with all components stabilized at 70 deg. F, it will contain no liquid refrigerant. When the system
is activated, the low stage compressor will pump hot gas through the discharge line to the de-superheater
(on 1HP units only). The de-superheater (air or water-cooled) removes some heat from the refrigerant gas,
lightening the heat load on the cascade condenser. Leaving the de-superheater, the gas passes through an
oil separator and flows to the cascade condenser. Here it is liquefied by heat exchange with high stage
refrigerant and flows to the expansion valve.
21.0 SERVICING CASCADE REFRIGERATION SYSTEMS
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