Refrigeration system faults, causes and remedy ?

Effect of Sea Water Temperature:

              For good heat transference, a temperature differential of 8oC between cooling water inlet
and the condensing temperature is used. Under cooling, which is advantageous, under
standard condition, will be 5 oC. Therefore any changes of the sea water temperature will
affect the degree of under cooling.

When sea water temperature increases :

• The degree of sub cooling is reduced.
•  Increases the flash off percentage, efficiency of plant drops.
• The condensing pressure will be higher.
•  Superheat increases discharge temperature may be too high, compressor.
• overheated

When sea water temperature decreases :

• Very low or no super-heat, liquid may be drawn into the compressor, causing damage to it
• Less flashing off of the gas at the expansion valve

Air in the system:

                Air is admitted in the refrigerant system during repairs, or can leak into the low pressure
side, when the plant is running with evaporator pressure below atmospheric. This air
is pumped around the system as far as the condenser, where it is accumulated, as it
cannot be condensed.
The symptom, which indicates air in the system, is a steadily increasing reading on
the condenser pressure gauge the accumulation of air reduces the effective area of
condenser available for condensing refrigerant. The efficiency of the plant is reduced.

Checks:
               If air leakage is suspected, it can be checked by stopping the compressor, closing the
condenser outlet liquid valve with the condenser sea water still circulating. After a few
hours, equilibrium is reached in the condenser and the condenser thermometer should
read exactly the same on sea water temperature, if no air is present. If the thermometer
reading is higher, then air is present.

Action:
                Air can be released from the system, by crack opening the purge valve on top of the
condenser, or if no valve is fitted, by slacking a pipe connection on top of the condenser.
Dirty condenser or insufficient cooling water
Dirty condenser or insufficient cooling water is indicated by compressor high discharge
pressure and the effect of the poor cooling will be that the refrigerant will not be efficiently
liquefied and the condenser ends and pipes will feel hot.
The cooling water can be restricted due to:

• Choked condenser tubes
• Choked cooling water pump strainers
• Choking of the cooling water system
• The cooling water pump fault

Moisture in the system:

                   Water circulating with the refrigerant, tends to freeze on the thermostatic expansion
valve, causing a build up of pressure on the condenser side and drop in pressure on the
evaporator side due to the blockage. The plant tends to be stopped by the high pressure
safety cut-out.

Remedy:
                    Driers are used to remove moisture, and icing of the expansion valve would indicate
that the chemical is no longer effectively removing the moisture. The chemical, either
activated alumina or silica gel is renewed and the compressor restarted after the ice has
melted. Usually the ice in the expansion valve will melt due to the ambient temperature.

Undercharge :
                    Symptoms of undercharge are a low compressor discharge pressure, lack of frost on
suction pipe, lengthy running times and large bubbles in the liquid line sight glass.
The compressor will tend to become very hot. The result of undercharge is that the
performance falls off.

Check:
                     A leak test should then be carried out over the system to determine the fault and enable
its rectification.
The quickest, but least efficient, test is to examine for traces of oil; wherever oil is
weeping, refrigerant is also being lost. Attention should be concentrated on compressor
glands, flanges and pipe joints, particularly the flared ends of copper pipes.

Following are some of the methods used to detect leak:

•  Leak detector lamp
•  Soap solution
•  Electronic beeper

                    A leak detector lamp for Freon refrigerants may be of the methylated spirit type, but
more commonly uses gas (butane / propane).
The lamp has a pale blue or colorless flame which turns to green, when Freon is drawn
into it by a sampling tube. The open end of the tube is held close to joints and other
potential leakage points around the pipe-work.
The limitations of this type is that a large leak will cause the flame to burn violet and it is
sometimes necessary to ventilate the space to clear excess gas before the leak can be
pinpointed. This lamp can not be used for Freon- free refrigerants.
A soap solution is also used to detect the escaping refrigerant by brushing soapy water
over the joints and flanges. The leak is magnified in the form a soap bubble.
Electronic beeper is used to detect leaks in a Freon- free refrigerant system. This
instrument will sound a beep, in the presence of Freon- free refrigerant.

Charging refrigerant :
                    When charging refrigerant, the refrigerant storage bottle is connected to the charging
valve on the regulator outlet, loosely. The bottle valve is crack open to clear air from
the connecting pipe and the nut is tightened on the nipple of the charging valve. The
charging connection is made to the liquid stop valve or suction stop valve if there is no
valve after the regulator. The bottle must be kept upright to prevent entry of liquid when
the connection is made to the suction side of the system.
On some refrigerant storage bottle, separate valves are provided for liquid and vapour.
The charging valve is opened to one turn off the back seat and with the compressor
running; the bottle valve is fully opened. Charging is continued until the bubbles appear
from the sight glass or the suction and discharge pressures of compressor are normal.
Charging will correct the pressure gauge readings (i.e. condenser gauge about 7oC
below the evaporator on the equivalent saturation temperatures for the pressures).

Overcharge:
                   Overcharge only happens when the system has been charged. It is indicated by the
compressor high discharge pressure and condenser sight glass indicating full liquid.
There are other faults which will result in compressor high discharge pressures. These
include poor cooling, having air in the system and icing of the thermostatic expansion
valve. Such faults occur spontaneously.

Remedy:
                   The charge is pumped to the condenser and the excess refrigerant is released to
atmosphere through a pipe connected up for this purpose.
Oil on cooling coils
Heat transfer would be reduced by oil deposits in condensers and evaporators.
Oil carry over is cut down by the oil separators fitted on some compressor discharge
side. Oil deposit is removed by chemical cleaning as described in the heat exchanger.

Frost:
                   Frost on the evaporator coils reduces the efficiency of the plant by acting as an insulator
between the evaporator and the air in a direct expansion system. The air flow is also
restricted by the blockage, by the formation of ice. Automatic defrosting is carried out by electrical heating, keeps the coils free of ice but
failure of the defrost arrangement allows excessive icing.
Ice on the evaporator can be manually removed by washing it off with a hot water.
Choked expansion valve/moisture in system:
                    A blockage in the system may be caused by moisture forming ice or dirt in the
thermostatic expansion valve, but it can be due to blocked strainers, closed valves or
solenoids which have failed to open.
This is manifested in suction pressure drop and a rise in discharge pressure of the
compressor.

Remedy:
                  The drier should be examined and the drying chemicals will probably need replacement.
Strainers and solenoids needs to be inspected.

Short cycling:
                  “Short cycling” is the term used to describe a compressor unit repeatedly running for
a few seconds and then cutting-out. This is the result of operation of the low pressure
controller. The control is arranged to operate when suction pressure drops, to stop the
machine. It restarts it when the suction pressure rises. Thus any condition which varies
suction pressure over this range will cause the compressor to cut-in and cut-out.

Cause:
                   If a solenoid is opened in the normal way by high cold room temperature, the refrigerant
in passing through will build up suction pressure and the compressor will be started. If
the supply is restricted and insufficient for compressor demand, the suction pressure will
drop and the low pressure controller will stop the machine. The thermostatic valve may
be the restricting device, due to low superheat of the gas leaving the evaporator (from
the insulating effect of ice on the coils) it will be closed in to reduce the refrigerant flow.
Electrical faults
                 These are responsible for a large number of refrigerator problems. Ship vibration, in
turn, is the reason for many of the electrical faults. Loose connections and broken wires
and earths, resulting from chafed insulation, are examples.

Safety Procedures:
Plants are protected by:

• High pressure cut outs
• Low pressure cut outs
• Oil failure cut outs
• Safety valves or bursting discs.

High pressure cut outs are usually set to the pressure corresponding to 49oC i.e. about
5oC above highest expected condenser gauge.
Low pressure cut outs should be set about 5oC below the lowest expected evaporation
gauge reading.
Oil failure cut outs are differential pressure devices as crankcase pressure is not
atmospheric and oil pressure is determined relative to crankcase pressure.
Safety valves are used to protect the condensers against excessive pressures.