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use of crankcase heaters in these applications. It is
important, however, to never energize the crankcase
heater while the compressor is running because this may
overheat the compressor oil. (See Accessory Section 5.4
for applicable heaters.) Carlyle’s experience indicates
that many compressor failures occur during the first sev-
eral months of operation. Many of these failures
occurred during the initial start-up of the unit, and
inspection reports indicate flooding was the probable
cause of failure in many of these compressor returns. It
is believed that many of these failures could have been
avoided by using more care during the initial start-up of
the compressors. Refer to the recommended start-up
procedure in Carlyle’s Refrigeration Compressor Service
Guide (Lit. No. 020-611).
1.6 Proper Compressor Control and Protection
An important consideration to maintain good compres-
sor reliability is a proper control and protection strategy.
It is extremely important that the compressor controls
do not allow operation outside the compressor’s
approved application envelope. This may require the
addition of low-pressure switches to avoid too low of a
suction pressure and a high-pressure switch to limit the
allowable discharge pressure. In many refrigeration
applications an oil safety switch is required to avoid a
loss of lubrication.
It is also important to limit compressor start and stops.
Each time a compressor motor is energized a high
inrush current is reached, if only for a fraction of a sec-
ond. In addition the compressor bearing system is
stressed, and Carlyle has found a direct correlation
between excessive starts and higher failure rates. For
that reason Carlyle does not recommend more than 12
starts per hour for these semi-hermetic compressors.
In addition in refrigeration rack systems Carlyle has
found that compressors properly applied will typically be
limited to less than 75 cycles per day in low temperature
systems and 150 cycles per day in medium temperature
systems. Applications with much higher cycles per day
have typically been related to poorly adjusted controls.
Where possible it is recommended to add cycle counters
that will allow monitoring of the cycles per day.
1.7 Clean and Dry System
Clean and dry systems are essential for long compressor
and motor life, and satisfactory operation. This cannot
be over-emphasized. It is even more critical with the
introduction of new HFC refrigerants and POE lubricants.
The new POE lubricants are excellent cleaning agents
that will deposit system contaminants into the system or
compressor filters and screens, causing excessive pres-
sure drop or clogging, if the system is not kept clean.
The moisture levels of all systems must be kept within
acceptable limits. Excessive moisture when combined
with heat and refrigerant can form damaging acids. One
component that requires special attention is the com-
pressor lubricant. With compressors applied with miner-
al oil (MO) or alkaline-benzene (AB) lubricants, the
recommended limit for moisture is maintaining levels
less than 50 PPM. With Polyol-Ester lubricants (POE) the
allowable moisture level is higher at 100 PPM but it is
much more difficult to maintain because of the very high
affinity for moisture. Special care must be taken with
POE oils to avoid moisture getting into the oil.
Compressors with POE should only be opened for very
short periods. POE oil containers should be maintained
with a dry nitrogen holding charge to keep out moisture.
Liquid line refrigerant filter-driers maintain low moisture
content, and in the event of a motor burnout, prevent
contamination of the evaporator and other parts of the
refrigeration system. These filter-driers should be com-
patible with the new HFC refrigerants and POE oils if
used.
Liquid line moisture indicators are recommended in all
systems that use semi-hermetic compressors because
they provide a continuous check on the system’s mois-
ture content. Excessive moisture in combination with a
high operating temperature can lead to motor winding
breakdown and burnout. When moisture is indicated,
prompt corrective action, such as changing the filter-
drier core or dehydrating the system, can prevent seri-
ous compressor damage.
1.8 Prevent Excessive Discharge Temperatures
The actual discharge gas temperature at the compressor
discharge service valve must not exceed 275°F (135°C).
For HFC/POE applications the maximum recommended
discharge temperature is 250°F (121°C). For a given
refrigerant, this discharge temperature depends upon
the compression ratio as well as the temperature of the
superheated suction gas. Since an increase in either the
compression ratio or suction gas temperature causes the
discharge temperature to increase, both must be kept
within allowable limits. In low temperature R-22 applica-
tions, external desuperheating is required. See Bulletin
02T-3 for these application recommendations.
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