Refrigerating machine plant provides “refrigeration cooling effect” for the preservation
foods and other stuff. The plant is a refrigeration system design and is typically used
for the sole purpose of providing cooling effect in a different fixed provision
compartments on board. Various equipments are incorporated in the system and the
structure is based on the system requirements.
7.8.1. The Pressure-Enthalpy Diagram
Fig.7.8.1-1 Pressure-Enthalpy Diagram below shows the typical refrigerant work
cycle in a vapor-compression system. Two phase zone is identified in the shaded
part (liquid-vapor zone). The cycle is indicated by the numbers. Isotherms in the
vapor region (right side of the two phase zone) are shown by arcs. In the two phase
zone, isotherms and isobars coincide, and in the saturated liquid region the
isotherms are vertical.
7.8.2. Refrigerant
Refrigerant is a substance that provides/removes heat from the compartment in the
refrigeration system. It is simply called as “coolant” as its purpose to provide cooling
effect in a compartment.
There are different types of refrigerants that are being used, but the desirable
properties are; be non-toxic, non-flammable, operate at modest positive pressures
(to minimize pipe and component weights (for strength) and avoid air leakage into
the system), have a high vapour density (keeps the compressor capacity to a
minimum and pipe diameters relatively small), be easily transportable, be
environmentally friendly, be easily re-cycleable, relatively inexpensive to produce,
and compatible with the materials of the refrigeration system (non-corrosive, miscible
with oil and chemically benign) One example is R12 (chloroflourocarbons-CFC) which is ideally good refrigerant but can deplete the ozone layer in the atmosphere. With the discovery of the damaging
effect of CFCs and HCFCs refrigerants to the ozone layer, the HFC family of
refrigerant has been widely used as their replacement. R134a is also known as
Tetrafluoroethane (CF3CH2F) from the family of (hydroflourocarbon) HFC refrigerant
is one of being widely used as a replacement.
7.8.3 Outline of Refrigeration Units
A refrigerating machine plant onboard are normally two sets of refrigerating
condensing units and each are consisting of refrigerating compressor, condenser, oil
separator, liquid separator and the control equipments such as pressure gauges and
temperature gauges. Valve panel is provided for thermo expansion valves, magnet
valves, thermostats and temperature gauges for each compartment.
Each unit has enough capacity to cool down the required temperature in each
compartment, and it is possible to operate both units for quick cooling down when
new cargoes (provisions) have been loaded. Even if two units are in operation,
the refrigeration capacity is maintain to 100% capacity as the piping are designed
based in one unit in operation.
a. Components of Refrigerating Plant
i. Compressor
The compressor is the device responsible for removing refrigerant and reducing
the pressure in the evaporator and; compressing the refrigerant to raise its
pressure above its saturation temperature so that condensing process is more
easily. There are various types of compressor that is being used in any refrigerant
system and the most common is the “semi-hermetic” type. Semi-hermetic
compressors are partially sealed unit, serviceable and connected to a separate
driving unit which is using an electric motor.
ii. Condenser
Condenser is a heat exchanging device that condensed the high pressure
refrigerant coming from the compressor into a high pressure liquid. Heat taken
from the evaporator is dissipated to the cooling sea water by the refrigerant.
iii. Expansion Valve
The most common used expansion valve for the refrigerating unit onboard is the
“thermostatic” type. The purpose of the thermostatic expansion valve is to
regulate the flow of the refrigerant and reduce its pressure. The high pressure
liquid refrigerant from the condenser is converted to low pressure liquid
refrigerant into the expansion valve so that it can be easy to absorb heat from the
evaporator.
iv. Evaporator
Evaporator is the opposite of the condenser. The low pressure liquid refrigerant
from the expansion valve is evaporated in the evaporator into low pressure gas
refrigerant. When the evaporation takes place, the heat from the surrounding is
being absorbed thus, cooling effect takes in place.
v. Oil Separator
The purpose of the oil separator is to separate oil from the refrigerant and to
ensure that the oil is to maintain in the compressor crankcase. Oil is mixing with
the refrigerant when the system is in service and can cause component trouble
as oil has no heat transfer capability as with the refrigerant.
vi. Liquid Separator
The purpose of liquid separator is that no liquid refrigerant will enter the
compressor. Liquid cannot be compresses and can cause trouble with the
compressor.
vii. Filter Drier
The filter drier is the prime protection of the expansion valve and other
components in the system by trapping dirt and other substances. This is to
maintain that the refrigerant is free of water and dirt after the condenser.
The content of filter drier is either activated alumina or silica gel.
7.8.4 Procedures for Operation
a. Starting
i. Fully open the inlet and outlet valve of the sea water to condenser and supply
cooling sea water and purge with air. Start ventilation fan to the evaporator unit.
ii. Open the following valves:
a. suction valve (crack open) and delivery valve of the compressor (valve name, n
umber etc.)
b. refrigerant inlet and outlet valve in the condenser
c. liquid outlet valve
d. stop valve before and after back pressure regulating valve
e. stop valve (suction side) in the compartments.
iii. Start the compressor and fully open the suction side gradually while checking the
suction pressure to avoid liquid back to the suction side of the compressor.
iv. Check the temperature of oil return piping between oil separator and compressor
and it should be slightly hot than the ambient temperature in normal operation.
b. Stopping
i. Close the liquid outlet valve of the condenser and wait until five times stopping of
the compressor by the pressure switch.
ii. Push stop button of the compressor in the control panel and close the suction and
delivery valve.
iii. Close the inlet and outlet valve for filter drier.
Precautions:
i.) In case that the ambient temperature is below 0oC, drain off cooling sea water
into the condenser to avoid freezing.
ii.) In case that the refrigeration plant is to stop for a long period of time such as
during drydocking, etc., pump down the refrigerant in the condenser and never
leave any refrigerant into the piping system.
c. Procedures for Pumping down of Refrigerant
“Pump down” means collecting all the refrigerants temporarily into the condenser.
i. Supply cooling water to the condenser.
ii. Open the compressor suction and delivery valve.
iii. Open the condenser refrigerant inlet valve and close the outlet valve.
iv. Open the liquid outlet solenoid valve manually or electrically.
v. Start the compressor manually until it sucks the refrigerant in the system and stop
when the suction pressure becomes 0 MPa. Leave for awhile and wait for the
suction pressure to increase and restart again. Repeat this procedure at least 3 times and confirm that the suction pressure is not increasing and maintains to 0MPa.
vi. Stop the compressor close the compressor suction valve and condenser inlet
valve as soon as possible.
vii. Stop the cooling water into the condenser and close all the valves.
d. Procedure for Charging Liquid Refrigerant
i. Pump down the refrigerant into the condenser and ascertain the level of the
refrigerant.
ii. Weight in the cylinder bottle to ascertain the content and the amount of refrigerant
to be charged. Connect the cylinder bottle charging line into the refrigerating unit
charging line into. Maintain the connection cap slightly loose in the refrigerating
unit and purge the air by crack opening the outlet valve of the refrigerant cylinder.
iii. Fully tighten the connection cap when air has been purge and open the charging
valve.
iv. Crack open the liquid outlet stop valve in the condenser and start charging.
v. Charge the correct level of the refrigerant (about 4/5 in the sight glass) into the
condenser.
e. Procedure for Replenishing and Extracting Refrigeration Oil
Replenishing Oil
i. Pump down the refrigerant into the condenser and close the valves of the
compressor.
ii. Take off the plug for the refrigeration oil in the compressor crankcase.
iii. When charging oil, start the compressor motor momentarily (abt.5~6 seconds) to
speed up the charging process.
iv. Ascertain the oil level to correct level and tighten up the oil plug in the
compressor crankcase.
v. Bleed off air in the compressor casing by slightly opening the suction valve and
loosening connecting caps from the high pressure gauge (In case connection
fittings for vacuum pump are provided, use the vacuum pump to extract the air
from the compressor crankcase).
Extracting Oil
i. Pump down the refrigerant into the compressor only. Ensure that refrigerant
pressure is higher than the atmosphere.
ii. Loosen the oil drain plug in the compressor crankcase and extract the amount oil.
Precautions:
i.) Care to be taken when charging oil so that no air will be trapped in the
compressor crankcase.
ii.) Ensure to bleed off air every after charging and extracting refrigeration oil.
iii.) Air that is trapped into the system will cause fluctuating pressure to the delivery of
the compressor (air is non-condensable) and may become moisture when not
properly purge that can cause clogging to the expansion valve due to icing.
f. Purging of Non-Condensable Gas
If the refrigerant pressure is abnormally high even sufficient amount of cooling
water is supplied into the condenser, the possibility of non-condensable gas is
intermixed in the refrigerant piping.
i. Pump down the refrigerant into the condenser and close all the valves.
ii. Increase the cooling water flow in the condenser until the inlet and outlet
temperature of the cooling water is almost equivalent. Wait for about 5 minutes
until the gas refrigerant was fully condensed.
iii. Slightly open the purging valve in the condenser and purge the air mixed with the
refrigerant.
Precautions:
i.) When purging, keep sufficient flow of cooling water in the condenser to keep the
refrigerant in the liquid state.
ii.) Since refrigerant pressure in the condenser is high, never position yourself near
the outlet of the purging valve to avoid injury.
iii.) Air or non-condensable gas should be properly removed in the refrigerant piping
system as this will turn into moisture and will clog the expansion valve; which will
cause erratic movement of the control elements and can damage the expansion
valve itself.
g. Refrigerant Leak Check
Every after maintenance procedure of refrigeration system especially with the
piping, refrigerant leak check must be always considered ensuring that refrigerant
will not be wasted to the atmosphere. There are various methods for refrigerant
leak detection, below are the most common and economical methods applied
onboard.
i. Leak Detection Using Halide Torch
Refrigerant leak detection by halide torch is applied when very small amount of
leakage is observed and is useless when big amount of leak and the refrigerants
are scattered in the surrounding atmosphere.
When carrying out leak check, make sure to vent the surrounding thoroughly so
that refrigerant leakage can be found easily by pointing the exploring hose of the
torch while burning in the suspected area of the refrigeration system, ie., joints
and fittings, etc. When the color of the torch flame changed from blue to light
green color this indicates the refrigerant leakage. Rectify the leakage, flush the
surrounding atmosphere of fresh air and carry out again leak checking until
satisfy.
ii. Leak Detection Using Soap Suds
Another method of leakage detection is using the soap suds. Apply soap suds
directly to the suspected area of leakage, ie., joint and fittings, etc. When bubbles formed and increase this indicates the leak points. Soap suds is very efficient when there is big amount of leakage is observed.
h. Defrosting of the Evaporator Coil
To maintain the performance efficiency of the evaporator in the refrigerated
compartment, defrosting should be conducted periodically, i.e. every 12 hours, etc.
Over the time, moisture content of the air circulating in the compartment will
become ice and cling into the evaporator coil. The ice adherence will build up and
inhibit heat transfer in the refrigerated compartment thus, many times that the
required temperature in the compartment is not being met.
Defrosting is the procedure of removing the ice or clearing the evaporator coils
from any adherence of ice. Defrosting methods can be as follows:
i. Water Spray
The refrigeration system and air circulating fans are stopped and water is
sprayed over the coils. It may be necessary to heat the water supply in the event
normal water temperatures are below 4.4ºC. Water supply and drain lines should
be built to assure rapid drainage of refrigerated space when defrosting is
completed.
ii. Hot Gas
Compressed vapor from the compressor is utilized to apply heat directly to the
evaporator and in some systems to the drain pan. Most systems use the latent
heat of condensation of the compressed vapor as the heat source, but some use
only sensible heat of highly super heated vapor. Most hot gas systems introduce
the hot gas at the suction connection and bypass the expansion valve through a
relief valve into the suction line downstream from a suction solenoid valve which
closes during defrost. The defrost time control will, in this case, operate the
compressor during the defrost cycle and shut off the circulating fans. At the same
time, it will energize the hot gas solenoid valve and allow the hot gas to enter the
evaporator coil and warm it, thus removing the buildup of frost.
In reefer provision plant onboard, hot gas defrosting is commonly done in manual
operation procedure, where a bypass valve in the delivery line of the compressor
should be operated to enable the hot gas to flow in the evaporator.
iii. Electric Defrost
Usually the heat is applied externally to the evaporator as opposed to the internal
application of heat in the hot gas method. Such systems, therefore, require a
longer defrost period than hot gas methods, usually 1-1/2 times as long. Heating
elements used in supplying the defrost heat may be in direct contact with the
evaporator or may be located between the evaporator fans and the evaporator. In
each instance, the temperature limiting device should be used on or near the
evaporator to prevent excessive temperature rise if any controlling device fails to
operate. The defrost time control in this method will, therefore, turn the
compressor off during the defrost cycle and turn on the heaters for the time
required to defrost the coil.
Precautions:
Never use air and blow torch when defrosting, as this may cause rupture of the
evaporator tubes.
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