TENNEY ENVIRONMENTAL
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
PSlG 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. For 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,
pumpdown cycle, all power to the unit must be off
-
1
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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