Support
have been off and liquid refrigerant may have migrated to the
crankcase and dissolved in the lubrication oil. It is advisable for
the power to the heater to be on for at least an hour before you
attempt to start the compressor, or the oil may foam on start-up
and trip the compressor on oil pressure failure.
Inspect the wiring for blackened or discoloured wiring. This
may have been caused by loose terminals or current overload.
If the fault was recent there is usually the acrid smell of burnt
insulation.
A common fault is a failed hermetic or semi-hermetic
compressor motor. Check if any motor windings are burnt out
(open circuit) or earth fault. If so the burn-out may have caused
contamination of the refrigerant charge, remember to identify
the initial cause and follow burn out procedures.
CONTROLS
We find that controls fall into two categories, operational and
safety. Operational controls will switch the plant on and off to
meet the performance of the plant. Safety controls will switch the
plant or sections of it off to avoid damage to the equipment.
In all cases reference must be made to the plant wiring
diagram to establish the exact operation of the control devices
within the totality of the system.
Common areas you may look at are:
• Room thermostats which directly control a unit must be
set at the desired room temperature.
• Condenser water thermostats which cycle the fans on a
cooling tower or evaporative condenser.
• Freeze up thermostats on the water leaving side of
a chiller to stop the compression if the water leaving
temperature approaches freeze temperatures.
• A pressure switch which starts/stops the fan on an
air-cooled condenser to ensure that the condensing
temperature does not drop too low with low ambient air
conditions.
• A pressure switch which actuates a solenoid valve to load
or unload the compressor.
• A pressure switch which operates the ‘pump-down’ cycle
on a compressor. Sophisticated electronic or proportional
controllers may be fitted which will control multiple
steps of control on a cooling coil, loading/unloading of a
compressor or capacity of a water chiller.
If the plant is controlled by a management system, it may
be necessary for the technician to manually activate output
devices to switch on items of plant for testing and to reset to
automatic mode when testing is complete.
Some commercial refrigeration plants are fitted with pressure
regulators to regulate the evaporating temperature and hot gas
bypass regulators to regulate the discharge pressure.
A flow switch on the condenser water or chilled water line is
installed to prevent running of the refrigeration plant if water
flow is not established. These switches are sometimes wired
utilising a delayed timer.
A high-pressure safety switch is fitted to the refrigeration
compressor discharge to switch off the compressor if discharge
pressure rises too high. A low-pressure safety switch is fitted to
the compressor suction to switch it off if the suction pressure
drops too low. Typically, if the machine has lost its charge or
airflow restriction reduces airflow over the cooling coil.
An oil pressure differential switch is fitted to the compressor
oil pump discharge and the crankcase. If the oil pressure
generated by the pump is not high enough or liquid refrigerant
has mixed with the oil (causing it to foam) the switch will stop
the compressor. This pressure switch is usually fitted with a
manual reset button and a timer circuit to allow the pump time
to establish pressure. Once you have completed your work and
the plant is operational, complete the necessary documentation
including the plant logbook. It may be necessary to re-issue a
refrigeration Certificate of Conformance.
There are many fault finding guides available. The internet
may offer guidance or mislead you, but Gloria, when it
comes to fault-finding, your most important tool is in fact
your knowledge and understanding of air conditioning and
refrigeration systems. This is critical to you succeeding. RACA
Thank you for all your questions. Send your problems (and
sometimes your creative solutions) to [email protected]
with ‘Solutions Page’ in the subject line. You may include
pictures.
REFERENCES:
1. A CRA
2. SARACC A
BOOK
YOUR
STAND
TODAY
FRIGAIR 2021
9 - 11 June 2021
Contact Dale Macnamara | Tel: 082 929 9797 or +27 (0) 11 579 4940 | Email: [email protected]
www.hvacronline.co.za
RACA Journal I May 2020
61