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Modern Refrigeration and Air Conditioning
A starved evaporator could also be caused by a par-
tially clogged filter-drier or capillary tube.
The amount of refrigerant in a capillary tube sys-
tem is critical. Refer to Figure 19-29. Notice the change
in head pressure as the charge of refrigerant changes.
If the system is undercharged, as in C, the evaporator
will not receive enough refrigerant. The system may
run all the time. If the system is overcharged, as in B,
the liquid refrigerant may flow down the suction line
and cause oil-slugging in the compressor. The suction
line will sweat and even frost up all the way to the
compressor.
Checking for a Clogged Capillary
Tube or Filter-Drier
To check the capillary tube, run the system for a
few minutes. Stop the unit and listen where the capil-
lary tube enters the evaporator. If there is no hissing
sound, the capillary tube is clogged.
Heat the evaporator end of the capillary tube
with a rag and warm water. (Do not use a flame.) If the
clogging is from ice, there will be a hissing sound as
it melts. A clogged filter-drier or capillary tube will
cause refrigerant to back up in the condenser. The com-
pressor may stop or it may overload during start-up.
Condenser
A B
Condenser
Evaporator Evaporator
High-pressure vapor
Low-pressure liquid
High-pressure liquid
Low-pressure vapor
Goodheart-Willcox Publisher
Figure 19-28. The effects of properly sized and improperly sized capillary tubes. A—A capillary tube that is properly selected for
capacity prevents excess liquid from backing up into the condenser and keeps the evaporator properly charged. B—A capillary tube
that does not have enough capacity results in too much resistance in the tube. Liquid backs up into the condenser and the evaporator
is “starved.” The discharge pressure may be abnormally high and the suction pressure may be abnormally low. The bottom of the
condenser is subcooled.
Run
Cut-in
A B C
Cut-in Cut-in
Cut-out Cut-out Cut-out
Low-side
Low-side
Low-side
Pressure
Condenser Condenser Condenser
Idle Run Idle
Time
Idle Run
Normal cycle
Overcharge
Undercharge
Goodheart-Willcox Publisher
Figure 19-29. Pressure-time diagram showing the refrigeration cycles of capillary tube systems charged with different amounts of
refrigerant. A—Proper charge. B—Overcharge. Overcharge will usually cause a frosted or sweating suction line. C—Undercharge.