Refridgiration Flashcards
What is the standard for Ozone Depletion Potential (ODP) judged against
R11
Symptoms of an under-charged refrigeration system?
. Low compressor suction and discharge pressures
. Large amount of bubbles in the liquid line sight glass
. Compressor running for unusually extended periods
. Reduced compressor load as indicated by ammeter
. Cold room temperatures rising
. High degree of superheat at compressor suction
Remedies for an under-charged refrigeration system
. Carry out a leak test and seal any leaks
. Top the system up with liquid on the high pressure side
. Do not over charge
Symptoms of an over-charged refrigeration system
High pressure side pressure will rise.
Remedies for an over-charged system
Remove excess refrigerant to an empty cylinder
Symptoms of air in the refrigeration system
. Air collects in the condenser and the pressure will be abnormally high but the liquid temperature leaving the condenser will be normal
. Small bubbles will be evident in the sight glass
Remedies of air in the refrigeration system
. with the cooling water on, pump down the refrigerant to the condenser.
. Isolate the compressor so that it cannot start
. Wait 15 minutes for the refrigerant to separate from the air
. The air can be purged through the purge valve on top of the condenser/receiver.
. Recharge the system with refrigerant
Symptoms of moisture in the system
. Moisture in the system tends to freeze through the TX valve, leading to a loss of cooling effect in the evaporator, resulting in a low suction pressure and a high condenser pressure
. TX valve may freeze open, leading to excessive refrigerant flow which may lead to liquid in the compressor and ice forming on the outside of the pipe
.
Remedies for moisture in the system
. Replace the dryer as required to remove the moisture
Symptoms of oil in the refrigeration system
. Loss of heat transfer in the evaporator and condenser
. Abnormally high condenser pressure
. loss of evaporator effectiveness
Remedies for oil in the refrigeration system
. Check oil type
. Check oil level in the sight glass
. Check oil separator operation
Effect of Sea Water Temperature:
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 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:
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 :
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.
