Page 186 - Boiler_Operators_Handbook,_Second_Edition

Refrigeration & AC 171

that builds up pressure after the compressor starts just ically differential pressure switches that measure the
like your automobile. In that case provision includes a pressure at the inlet and outlet of the heat exchanger and
contact that bypasses the low oil pressure switch for a open contacts to shut down the compressor whenever
few seconds during startup. If you have a compressor the water flow is so low that there is a minimal pres-
that starts then stops almost immediately check the oil sure drop. Their operation should be tested annually by
level again. Do not repeatedly try to start a compressor closing down on the outlet valves of the condenser and/
that will not continue to run because bearing damage or evaporator water circuits and attempting to start the
can occur. Since refrigerant flow cools the motor and in- compressor. An established number of turns open for
rush current heats up the windings during starting you each valve (degrees, percent, or notches on a butterfly
should give it a chance to cool off. During annual inspec- valve) should be determined and included in your SOPs
tion simply lifting a wire from the bypass circuit should as starting points. You should be able to use a test light
prove to prevent the compressor operating. If the com- at the control panel terminals to detect switch operation
pressor is fitted with an oil pressure gauge make it a point when the equipment has both switches. You can then
to observe the switch operation and record the pressure gradually open the outlet valve and note when the first
at which the contacts close. Note also that oil pressure switch in the circuit closes, record the inlet and outlet
switches are actually differential pressure switches that water pressures then do the same thing for the other.
compare the oil pressure to the suction pressure (typical- Using a test light is preferable to testing that requires
ly in the compressor crankcase) because the flow of oil is frequent starts of a large compressor but you should still
determined by that differential. confirm that the compressor can’t start when one or both
Large compressors can also have high and low of the switches are actuated by low flow.
oil temperature switches. The oil temperature switches A low temperature switch is normally provided
will prevent compressor operation if the oil tempera- in any evaporator used to chill water to prevent freez-
ture is too high or low. Don’t confuse an oil temperature ing of the water. Even though water flow is maintained
switch that controls the heater with a high oil tempera- through the evaporator, a flow imbalance created by
ture switch, especially if you decide to set the switch by foreign substances or associated with the piping con-
markings on the switch. nections to the evaporator can reduce flow sufficiently
You may be wondering if the compressor could that ice could form in one or more tubes. The result is
continue to operate completely unloaded. What you’ll similar to what is shown in Figure 5-30. The thermowell
discover is that unloading is never 100% complete. Ei- for the switch is typically inserted into the refrigerant
ther the compressor is set up with one or two of the cyl- space to sense the refrigerant temperature which, being
inders always in operation or throttling devices like the at saturation, should be consistent throughout the evap-
adjustable plug of a screw compressor doesn’t unload orator. During each annual inspection of an evaporator
everything and the inlet vanes of a centrifugal compres- containing water the operation of the low temperature
sor simply leak. Remember that most motors are cooled switch should be confirmed by removing the sensing el-
by refrigerant flow; therefore every compressor should ement (normally a bulb in a thermowell at the bottom)
be protected by high and low pressure switches and a and inserting that bulb in a glass of ice water. After a few
flow switch wouldn’t hurt. minutes in that condition you should not be able to start
Testing of high and low pressure switches should
be performed on an annual basis. On individual com-
pressors throttling the suction and discharge valves
while observing suction and discharge pressures on
your gauge set should serve to confirm operation. As
with other operating limits you should record actual
values whenever possible in the log where you describe
the testing. Be certain you’ve restored normal valve po-
sitions and any other alterations suggested for testing
by the instruction manual before leaving the equipment
even if the switches are shown not to work.
Water flow switches are used to prevent operation
of the compressor whenever water flow through the
condenser or evaporator is not proven. These are typ-
Figure 5-30. Chiller tube after freezing