Application Note

2 Fluke Corporation Troubleshooting the HVAC/R refrigeration cycle for mechanical problems
To measure the pressures on
your DMM and pressure/vacuum
module without installing
your manifold gauges, take the
following steps:
•
Attach the module hose fitting
to any standard refrigerant
service hose.
•
Connect the refrigeration hose
to the service port on the
compressor.
•
Open up the service port and
read the refrigerant pressure
directly on the pressure/vac-
uum module.
You can read one pressure at a
time using a single pressure vac-
uum module, so you will need to
record the suction or discharge
pressure one at a time. Or if you
are using a DMM meter such as
a Fluke 179 with a min/max fea-
ture, you can record the suction
pressure as the min value and
then record the discharge pres-
sure as the max value.
To remove the pressure/vac-
uum module, simply reverse the
process you followed when
installing the unit. Caution: Be
careful when removing the high
side port of your gauges as it
can be under very high pres-
sure. To minimize refrigerant
loss, learn to practice safe refrig-
erant handling habits when
removing the module from the
high side of the system. This
includes shutting off the com-
pressor prior to gauge removal
and allowing the pressure to
equalize. When removing the
gauge from the low side of the
system it is not necessary to first
shut down the compressor.
Troubleshooting the
compressor discharge
line temperatures
Pressure and temperature are
fundamental tests that you can
perform to determine what is
happening inside the c
ompres
-
sor. To learn the proper methods
to check the refrigerant super-
heat and subc
ooling at the com-
pressor, refer to the Fluke
application note titled, “Trou-
bleshooting H
V
A
C
/R systems
using refrigerant superheat and
subcooling.”
T
o measure the temperature
of the c
ompressor, take the
following steps:
•
Use a pipe clamp accessory
with a DMM to measure the
discharge line temperature at
the discharge of the compres-
sor. High temperatures above
275-300 °F (135-148 °C) will
slowly destroy lubricant qual-
ities and performance of the
compressor. These high tem-
perature conditions can be
caused by high condensing
temperatures/ pressures,
insufficient refrigerant charge,
non-condensibles within the
system, high superheat from
the evaporator, restricted suc-
tion line
filters, or low suction pres-
sure. These conditions cause
the c
ompressor to ha
ve a
higher than normal c
ompres
-
sion ratio, work harder,
generate hotter inter
nal her
-
metic motor windings, and as
a result, cause compressor
wear, fatig
ue and failure.
A temperature survey is a critical
part of the service technician’s
job. A quick check of a system
’s
c
omponents not only helps to
diagnose troubles but also
allow
s you to antic
ipate failures
by regular monitoring of critical
temperatures. Use an infrared
thermometer to do a quick
survey of:
•
Compressor head tempera-
tures
•
Compressor oil sump temper
-
atures
•
Evaporator c
oil and suction
line temperatures
•
Discharge line temperatures
•
Condenser coil and liquid line
temperatures
•
Fan motor temperatures.
With an infrared thermometer
you can quickly survey a refrig-
eration system by scanning the
temperatures of various compo-
nents (see Figure 1). While
touching each of the compo-
nents often does this, a non-
contact infrared tool is faster. By
keeping careful records it is pos-
sible to detect trends that indi-
cate impending failure. This
allows you to keep the system in
top condition and avoid costly
failures.
Note: IR instruments read best
when measuring an object with
a dull (not shin
y) surface. If the
surface is shin
y, dull it with
either black markers, non-gloss
paint, masking tape, electrical
tape, etc. For more information
on taking temperature measure-
ments with an IR thermometer,
refer to the Fluke application
note titled “Non-contact temper-
atur
e measur
ements us
ing IR
thermometers.”
Recording a temperature
overnight
To check refrigeration system
performance, it is often useful to
record temperatures in the
refrigerated space. This allows
Figure 1