Thermo King MD-MT, Operator'S Manual

Page: 107 / 192

Refrigeration Maintenance
107
6. By the time the discharge pressure decreases
approximately 50 psig (345 kPa), the ball in
the receiver tank sight glass should begin to
move and the liquid level should decline.
a. When discharge pressure stabilizes, the
ball and liquid level (liquid level in liquid
line sight glass) will rise.
b. If the ball will not begin to move or if the
liquid level (liquid level in liquid line sight
glass) will not decline, the unit most likely
has an overcharge of refrigerant. The
refrigerant level should be adjusted.
Adjusting the refrigerant level
1. With the condensing unit operating in High
Speed Cool and the Zone 2 evaporator in
Cool, use a gauge manifold set to pump
refrigerant from the receiver tank into an
approved container.
2. When the liquid level begins to decline in the
receiver tank sight glass (or liquid line sight
glass), close the hand valve on the gauge
manifold immediately. Allow the new liquid
level in the sight glass to stabilize (1 to 2
minutes).
3. If the liquid level is less than a full sight glass,
add refrigerant vapor to increase the liquid
level. With the unit operating in high speed
Cool, add refrigerant vapor through the
suction service valve until the ball is near the
top of the sight glass. Allow the level to
stabilize (1 to 2 minutes).
NOTE: R-404A refrigerant must be added as
a liquid. Maintain a discharge pressure of
275 psig (1900 kPa) or more and control
refrigerant flow so that suction pressure
increases approximately 20 psig (140 kPa).
4. Repeat the overcharge test.
Moisture Indicating Sight
Glasses
The receiver tank is equipped with a moisture
indicating sight glass. The outer edge of the sight
glass has a colored ring approximately 0.1 in.
(2.5 mm) thick. The color of the ring indicates the
moisture content of the refrigerant, but it may not
be completely reliable.
Green = Dry
Chartreuse = Caution
Yellow = Wet
A system has to run for at least 15 minutes to
change the color of the indicator ring after the
moisture content of the system has been changed.
For example, evacuating a system to remove the
moisture will not change the color of the indicator
ring until the system has been recharged and then
operated for at least 15 minutes.
1. Floating Ball
2. Colored Ring
Figure 57: Moisture Indicating Sight Glasses
1
2
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