TENNEY ENVIRONMENTAL
SERVICING CASCADE REFRIGERATION SYSTEMS
A
'
INTRODUCTION
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
(-1
1'4
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 modem cascade system can reach as low as
-120 deg.
F
with suction pressures of 0 PSlG or higher. Compact, sewiceable, 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 sole 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,
sewing the low stage. Modem systems use
R404a
in the high stage, making -50 deg.
F
refrigerant temperature possible at
0
PSlG 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
stagecompressor will pump hot gas through
the discharge line through an oil separator, and then to the cascade condenser where
it
is
liquefied by heat exchange with high stage refrigerant. It then flows to the capillary tube which
feeds the evaporator coil.
CASCADECONDENSER
The cascade condenser is the high stage system's evaporator and low stage system's
)
condenser. It can be either tube-in-tube with the low-stage refrigerant in the outside tube or
tube-in-shell with the low-stage refrigerant in the shell.
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