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in the oil. Copper flakes from copper planting processes, corrosion
products from system metals which have undergone corrosive
attack, and fine metal particles torn from bearing surfaces all form a
rough abrasive paste in the system.
Aside from increased wear, this can block screens and
strainers and reduce lubrication to the point where oil is not
getting to essential areas of the machine. This could be the cause
of total failure.
Normal cooling of the motor of a hermetic compressor is
by flow of cooler suction vapour across the windings. If the
windings are coated with fouling materials, this too can result in a
temperature increase.
Increased temperatures can cause carbonisation, in particular -
at the hottest part of the system - the compressor discharge valves.
A film of oil, being discharged with the hot gas continually coats
these valves as it moves through the system. Excessive heat will
break down the oil at the valves causing carbon deposits to form.
The carbon will build on the valves, preventing them from
seating properly. Hot gas will move back past the valves on the
down stroke of the piston reducing system capacity. Additionally,
the stressed discharge valves can break and lodge in the minimal
clearance space between the piston crown and cylinder head. This
will, of course, damage the piston and valve plate and result in
system failure.
All systems should have filter driers in their liquid lines. But never
depend upon the filter drier to remove moisture which you allowed
to enter into the system. That constitutes poor practice. View the
drier rather as a backup should some moisture enter the system.
Manifold charging line not secured,
loose on ground.
Refilwe, as you can see, moisture entering a refrigeration system is
to be avoided as it can result in total system failure. Most of the time
the entire situation can be avoided by following good practice.
Use a vacuum pump and follow all the correct evacuation
procedures:
When using manifold gauge sets, purge the lines to remove air
and moisture and when not in use do not allow the charging lines to
hang loose, dragging on the ground. Seal/secure the loose ends by
attaching them to the manifold blank off fittings and keep moisture
and contaminants out.
Seal/reseal tubing ends and components.
Manufacturers dehydrate refrigerant tubing, keep sealed and
reseal unused portions.
ACRA
Keep refrigerant oil containers sealed until needed, do not leave
open as the oil is hydroscopic and will attract moisture from the
ambient air.
Sealed tubing ends.
Refilwe, air in a refrigeration system is also very undesirable. Air may
have entered the system during installation or it could have been
introduced by poor service practice. Regardless, the air should have
been removed through proper evacuation procedures.
Dalton’s Law tells us that when gases share a space, each
behaves independently of the other, but their partial pressure are
summed to provide the total pressure of the system. That having
been said, it means that the condensing pressure will be further
increased for any combination of load and condenser water or air
temperature if air is present in the system.
As the air in the system causes increased discharge pressures
the compressor now has to continuously work against the higher
discharge pressure. This means that it will continuously draw more
electrical power, and its working temperature will also be higher.
The higher working temperature can be damaging to the
refrigerant and oil as it can cause carbonisation of oil and can start
chemical reactions and breakdowns between these fluids.
As air cannot be condensed in a normal refrigerant plant, air is
commonly termed a ‘non-condensable’.
In the system, the air moves into the condenser with the hot
gas, becoming trapped there. The result is that all of air which may
be present in the system is concentrated in the condenser. The
presence of the air reduces the capacity of the condenser which
further increases discharge temperatures.
Of course, we now know that this this may cause carbon build
up on the compressor discharge valves with all of the consequences
previously mentioned. As there is no such thing in nature as air
which is moisture free, any problem of air in a system will mean
there will also be a problem with moisture.
Refilwe, as you can see a little bit of moisture can in fact cause
quite a problem. I hope that this assists you with your compressor
failure rates. RACA
Thank you for all your questions. Send your problems (and
sometimes your creative solutions) to acra@netactive.co.za with
‘Solutions Page’ in the subject line. You may include pictures.
References: ACRA
www.hvacronline.co.za RACA Journal I September 2020 45