TENNEY ENVIRONMENTAL
Tenney Junior Test Chambers: Models TJR and TUJR, W942, or WF4
Page 42
Frosted Lines Are Typical:
A low-stage characteristic is frosted liquid and suction lines. In a normal cascade system, the liquid line is
always below +32 deg. F. The suction line, returning from a -100 deg. F evaporator, assuming 15 degree
superheat, will also be far below freezing.
Leak Testing:
Loss of refrigerant is the most common cause of refrigeration failure. Because of temperature extremes
experienced by its metal parts, the cascade system is particularly susceptible to leaks.
Check the entire system with an electronic leak detector. If the system is empty or at low pressure, boost
pressure to 200 PSIG with inert gas (not oxygen) diluted with a percentage of high stage refrigerant. Test
again.
A leak check while the system is at low temperature, -80 deg. F or colder, is a necessity. Expansion valve
flanges, superheat adjustment caps, and other mechanical joints should be tightened and checked for leaks
while at low temperature.
You may use a Halide torch to locate large leaks, but make your final test with the more sensitive electronic
leak detector. This is especially important on the low stage. The low stage is gas charged with a relatively
small quantity of refrigerant. Because of this, small leaks can quickly incapacitate the system.
Testing by Static Charge:
One advantage of a gas charged system is that its tightness can be checked by periodic observation of static
or standby pressure. You must read the pressure with all parts of the system at ambient temperature. This is
important. The unit must be shut down at least 24
hours before a static pressure reading is taken. To eliminate the possibility of cooling the cascade condenser
with the high-stage, pump-down cycle, all power to the unit must be off during the shut down period.
When reading static pressure, consider ambient temperature. Most static charge data are for a 10 deg. F
decrease in temperature. Due to a large system’s considerable thermal mass, several days may be required
for all components to completely stabilize at a particular ambient.
Evacuation:
Granted, refrigerants R23 and R404a are expensive, but there are times when charges must be recovered. A
contaminated system must be cleaned and evacuated regardless of refrigerant expense.
If there is a possibility that moisture, non-condensibles, or the wrong refrigerant contaminated a system,
recover the charge and evacuate.
Select a two-stage pump capable of pumping the system down below 200 microns, and connect an
appropriate gauge to ready system pressure. The ordinary compound refrigeration gauge is inadequate,
however a thermocouple gauge is ideal. Evacuating a leaky system is an exercise in futility. Therefore, make
sure the system is absolutely tight before beginning evacuation.
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