Modern Refrigeration and Air Conditioning, 19th Edition Page 473 (495 of 1488)

Chapter 19 Installation and Troubleshooting of Domestic Refrigerators and Freezers
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side of a gauge manifold. Pinch the suction line as
shown in Figure 19-27. Next, run the unit to deter-
mine how much of a vacuum it will pull. It should
pull between 25″ Hg and 28″ Hg (7 kPa and 17 kPa) of
vacuum. Stop the compressor. If the pressure gradu-
ally rises to 20″ Hg (34 kPa) of vacuum, then toward
10″ Hg (68 kPa) of vacuum, the compressor is not hold-
ing vacuum. This indicates the exhaust valves of the
compressor are leaking. The motor compressor must
be replaced or overhauled. If the compressor holds a
vacuum, the refrigerant needs to be recovered, and
the part of the suction line that was pinched must be
replaced.
Another method of testing the compressor for
internal leaks is to install a piercing valve on the pro-
cess tube. Then, connect the quick-connect fitting on
the service hose to the valve adapter. Connect the
other end of that line to the compound gauge end of
the manifold. Then, pinch the suction line and start
the unit. The motor compressor should pull 16″ Hg of
vacuum (47 kPa) in about two minutes. Do not run the
compressor any longer. Otherwise, without a flow of
suction vapor to cool it, the motor will overheat. After
two minutes, stop the compressor and observe the
low-side pressure. If the low-side pressure increases,
the compressor valves are leaking.
Diagnosing Capillary Tube Problems
Capillary tubes must be correctly sized. Their
inside diameters (ID) and lengths must be correct for
the capacity of the system and the desired evaporator
temperatures.
Figure 19-28A shows a capillary system of correct
design. The undersized capillary tube in Figure 19-28B
creates too much resistance. It is either too long or it has
an undersized inside diameter. Note that the improp-
erly sized capillary tube causes a starved evaporator.
Pinched
suction
line
Compressor
Evaporator
Condenser
Capillary tube
Gauge manifold
Filter-drier
High-pressure vapor Low-pressure vapor
High-pressure liquid Low-pressure liquid
Vacuum
Goodheart-Willcox Publisher
Figure 19-27. A compressor’s capacity can be tested by pinching the suction line and then running the compressor. The compressor
must be allowed to run for only a very brief period of time, otherwise the motor will overheat. The compressor should pull 25″ to 28″ of
vacuum within a few seconds. The yellow area indicates the vacuum in the line.

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Extracted Text (may have errors)

Chapter 19 Installation and Troubleshooting of Domestic Refrigerators and Freezers
473
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
side of a gauge manifold. Pinch the suction line as
shown in Figure 19-27. Next, run the unit to deter-
mine how much of a vacuum it will pull. It should
pull between 25″ Hg and 28″ Hg (7 kPa and 17 kPa) of
vacuum. Stop the compressor. If the pressure gradu-
ally rises to 20″ Hg (34 kPa) of vacuum, then toward
10″ Hg (68 kPa) of vacuum, the compressor is not hold-
ing vacuum. This indicates the exhaust valves of the
compressor are leaking. The motor compressor must
be replaced or overhauled. If the compressor holds a
vacuum, the refrigerant needs to be recovered, and
the part of the suction line that was pinched must be
replaced.
Another method of testing the compressor for
internal leaks is to install a piercing valve on the pro-
cess tube. Then, connect the quick-connect fitting on
the service hose to the valve adapter. Connect the
other end of that line to the compound gauge end of
the manifold. Then, pinch the suction line and start
the unit. The motor compressor should pull 16″ Hg of
vacuum (47 kPa) in about two minutes. Do not run the
compressor any longer. Otherwise, without a flow of
suction vapor to cool it, the motor will overheat. After
two minutes, stop the compressor and observe the
low-side pressure. If the low-side pressure increases,
the compressor valves are leaking.
Diagnosing Capillary Tube Problems
Capillary tubes must be correctly sized. Their
inside diameters (ID) and lengths must be correct for
the capacity of the system and the desired evaporator
temperatures.
Figure 19-28A shows a capillary system of correct
design. The undersized capillary tube in Figure 19-28B
creates too much resistance. It is either too long or it has
an undersized inside diameter. Note that the improp-
erly sized capillary tube causes a starved evaporator.
Pinched
suction
line
Compressor
Evaporator
Condenser
Capillary tube
Gauge manifold
Filter-drier
High-pressure vapor Low-pressure vapor
High-pressure liquid Low-pressure liquid
Vacuum
Goodheart-Willcox Publisher
Figure 19-27. A compressor’s capacity can be tested by pinching the suction line and then running the compressor. The compressor
must be allowed to run for only a very brief period of time, otherwise the motor will overheat. The compressor should pull 25″ to 28″ of
vacuum within a few seconds. The yellow area indicates the vacuum in the line.

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474
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.

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472
Modern Refrigeration and Air Conditioning
Locating Compressor Faults
The most expensive service item for a domestic
refrigerator is the replacement of the compressor. The
technician should carefully check for all other electri-
cal and mechanical failures before determining that
the compressor needs replacement. When a compres-
sor is in good condition, the most common reason for
replacement is an electrical fault.
Other electrical problems often mislead a ser-
vice technician into thinking the motor compressor
is at fault. To check a motor compressor, first discon-
nect power to the unit and then clean the outside of
the compressor dome. Then, remove the cover over the
electrical connections. Disconnect the system wiring
from the compressor: relay, capacitors, overload cutout,
and all. Use an ohmmeter to check motor windings for
continuity, shorts, and grounds. Refer to Chapter 14,
Servicing Electric Motors and Controls. If the unit checks
out correctly, connect a starting circuit to the motor
compressor. Use the correct size of capacitors and over-
load cutout. Connect the starting circuit as shown in
Figure 19-26.
If the system starts and operates correctly with
these manual-start electrical connections, the problem
is in the external system. It may be in the wiring, ther-
mostat, relay, or overload cutout. If the internal electri-
cal motor is faulty, the compressor must be replaced. If
the electrical system operates correctly, the compressor
may not be pumping.
The best check of the compressor is its volt-ampere
(watt) reading at normal low-side and high-side pres-
sures. If the volt-ampere reading is below the motor’s
rating, the pump may be worn out.
If the compressor is operating correctly but not
building up pressure, check for internal problems
with the compressor. To check the compressor’s pump-
ing ability, install a piercing valve on the suction line
and attach a compound gauge or the compound gauge
Overload
protection
C
R
S
W
G
B
Starting
switch
Starting
capacitor
High-pressure vapor Low-pressure vapor
High-pressure liquid Low-pressure liquid
Goodheart-Willcox Publisher
Figure 19-26. To test a capacitor-start motor compressor, remove all electrical leads and then connect a test cord as shown. If the
motor compressor operates properly when manually started, the problem is external.

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Page 3Section 1 Refrigeration System Fundamentals click to expand contents

Page 43Section 2 Service Basics click to expand contents

Page 109Section 3 Refrigerants click to expand contents

Page 199Section 4 Basic Electricity, Magnetism, and Electronics click to expand contents

Page 251Section 5 Motors and Electric Control Systems click to expand contents

Page 337Section 6 Compressors, Valves, and Metering Devices click to expand contents

Page 411Section 7 Domestic Refrigerators and Freezers click to expand contents

Page 499Section 8 Indoor Air Fundamentals click to expand contents

Page 603Section 9 Air-Conditioning Systems click to expand contents

Page 769Section 10 Heating Systems click to expand contents

Page 957Section 11 Energy Management and Conservation click to expand contents

Page 1007Section 12 Commercial Refrigeration Systems click to expand contents

Page 1177Section 13 Designing Commercial Refrigeration Systems click to expand contents

Page 1227Section 14 Installing and Servicing Commercial Systems click to expand contents

Page 1343Section 15 Professional Development click to expand contents

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