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superheat even when there is proper subcooling. In some
cases, it’s not possible to achieve the required subcooling
without having a superheat of zero degrees. If there is zero
degrees superheat with a TEV, then the TEV is defective and will
need to be replaced.
CHARGING BY MASS
I would be remiss not to mention the method of charging
according to mass. This method, known as the weigh in method,
can be very accurate if the exact length of the refrigerant lines is
known. The outdoor unit usually comes pre-charged with enough
refrigerant for the outdoor unit, a standard indoor unit, and 3 – 6
metres of line set. Refrigerant must be added for any line length
over what is specified by the manufacturer. Refrigerant may also
have to be added or removed depending on what indoor unit
or indoor coil is used. When charging by weigh in, it’s still good
practice to check the charge using the subcooling or superheat
methods to ensure correct operation.
DIAGNOSIS
Data from superheat and subcooling measurements can be used
to determine various conditions within the system, including
checking the refrigerant charge and verifying the operating condition
of the metering device. These measurements can also be used
to determine the efficiency of the condenser, evaporator, and
compressor.
Before drawing conclusions from the measured data, it is
important to check external conditions that influence system
performance. In particular, inspection and verification of proper
airflow across coil surfaces.
Remember to look for obvious problems at the coil surfaces, such
as dirty air filters inside the unit, or leaves and debris restricting airflow
on the condenser. The superheat value can indicate various system
problems, including a clogged filter-drier, refrigerant undercharge,
overcharge, faulty metering device, restricted airflow, or improper fan
motor or blower direction. Suction line superheat is a good place to
start diagnosis; a low reading suggests that liquid refrigerant may be
reaching the compressor.
In normal operation on traditional systems, which use mechanical
metering devices such as TEVs or capillary tubes, refrigerant entering
the compressor is sufficiently superheated above the evaporator
boiling temperature to ensure that the compressor draws only
vapour and no liquid refrigerant. On systems that use electronic
expansion valves and solid-state controllers, it is possible to see the
superheat setting as low as three degrees.
A low or zero superheat reading indicates that the refrigerant did
not pick up enough heat in the evaporator to completely boil into a
vapour. Liquid refrigerant drawn into the compressor typically causes
slugging, which can damage the compressor valves and/or internal
mechanical components. Additionally, liquid refrigerant in the
compressor, when mixed with oil, reduces lubrication and increases
wear, which leads to premature failure.
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System showing subcooling.
On the other hand, if the superheat reading is excessive, it indicates
that the refrigerant has picked up more heat than normal, or that the
evaporator is being starved of refrigerant. Possible causes include a
metering device that is underfeeding, improperly adjusted, or faulty.
Any additional problems with high superheat could indicate a
system undercharge, refrigerant restriction, moisture in the system,
blocked filter-drier, or excessive evaporator heat loads.
USING SUBCOOLING FOR TROUBLESHOOTING
Refrigerant is usually subcooled towards the outlet of the condenser.
An improper subcooling value can indicate various system
problems, including overcharge, undercharge, liquid line restriction,
or insufficient condenser airflow (or water flow when using water-
cooled condensers).
For example, a very low reading indicates that the refrigerant
did not lose the normal amount of heat in its travels through the
condenser.
Possible causes for this condition include insufficient airflow over
the condenser where the problem may be simply that the condenser
coil surface needs to be cleaned thoroughly to eliminate airflow
restriction. This shows how measurement of the subcooling in the
system will indicate possible issues in the system.
Excessive subcooling (over condensing) means the refrigerant has
been cooled more than normal. A possible explanation is faulty head
pressure control during low ambient conditions.
Karel, I hope that this assists you with a better understanding of
subcooling in the refrigeration system which is critical knowledge we
should all have.
REFERENCES:
ACRA
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.
RACA Journal I January 2020
69