Marine Refrigeration and Air Conditioning

Mohd. Hanif Dewan, Chief Engineer and
Maritime Lecturer & Trainer, Bangladesh.
MARINE REFRIGERATION
AND
AIR CONDITIONING

Construction and Working of Ships
Refrigeration plant:
The refrigeration plants on merchant vessels play a vital part in
carrying refrigerated cargo and provisions for the crew on board.
In reefer ships, the temperature of the perishable or temperature
sensitive cargo such as food, chemical, or liquefied gas, is
controlled by the refrigeration plant of the ship. The same plant or
a smaller unit can be used for maintaining the temperature of
different provision rooms carrying food stuffs for crew members.
The main purpose of ship’s refrigeration plant:
1. To avoid any damage to the cargo or perishable material so that
the cargo in transported in good and healthy condition.
2. Refrigeration prevents growth of micro-organisms, oxidation,
fermentation and drying out of cargo etc.

Main Components of Refrigeration
plants
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Main Components of Refrigeration plants

Any refrigeration unit works with different components inline to
each other in series. The main components are:
1. Compressor: Reciprocating single or two stage compressor
is commonly used for compressing and supplying the
refrigerant to the system.
2. Condenser: Shell and tube type condenser is used to cool
down the refrigerant in the system.
3. Receiver: The cooled refrigerant is supplied to the receiver,
which is also used to drain out the refrigerant from the system
for maintenance purpose.
4. Drier: The drier connected in the system consists of silica gel
to remove any moisture from the refrigerant

5. Solenoids: Different solenoid valves are used to control the
flow of refrigerant into the hold or room. Master solenoid is
provided in the main line and other solenoid is present in all
individual cargo hold or rooms.
6. Expansion valve: An Expansion valve regulates the
refrigerants to maintain the correct hold or room temperature.
7. Evaporator unit: The evaporator unit act as a heat exchanger
to cool down the hold or room area by transferring heat to the
refrigerant.
8. Control unit: The control unit consist of different safety and
operating circuits for safe operation of the refer plant.
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Compressor safety devices
The compressor is protected by three safety switches;
1. The OP switch or Oil Differential Pressure switch compares
the measured lubricating oil pressure to the Suction (crankcase)
pressure. Should the differential pressure fall below a pre-set
minimum (about 1.2 bar) then the compressor will trip and
require a manual reset to restart. A time delay is built into the
circuit to allow sufficient time for the lubricating oil pressure to
build up when starting before arming the circuit.
2. The HP or High Pressure switch, is fitted to the outlet of the
compressor before the isolating valve. On over pressurisation
(dependent on the refrigerant, up to about 24bar bar for R22) the
switch will trip the compressor and a manual reset is required
before restart.
3. The LP or Low Pressure switch when activated ( at about 1
bar for R22) will trip the compressor and require a manual reset
before the compressor can be restarted.
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Compressor control devices
This normally takes the form of an LP cut out pressure
switch with automatic reset on pressure rise. The cut out
set point is just above the LP trip point say at about 1.4bar.
An adjustable differential is set to about 1.4bar to give a cut
in pressure of around 2.8 bar. The electrical circuit is so
arranged that even when the switch has reset, if no room
solenoid valves are open the compressor will not start. This
is to prevent the compressor cycling due to a leaky
solenoid valve.
In addition to this extra LP switches may be fitted which
operate between the extremes of the LP cut in and cut out
to operate compressor unloaders.
Some modern systems contain a rotary vane compressor
with variable speed (frequency changing) control
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Oil Separator
It is situated on the compressor
discharge line.
The purpose of the oil seperator:
- To return oil entrained in the gas,
back to the compressor sump.
The oil return may be float controlled
as shown, electric solenoid controlled
on a timer, or uncontrolled with a
small bore capillary tube allowing
continuous return.
With all of these methods a shut off
valve is fitted between separator and
compressor to allow for maintenance.
The oil gas mix enters the separator
where it is made to change direction,
the heavier oil droplets tend to fall to
the bottom.
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Filter Drier
Can be either a compacted solid cartridge or bags of
dessicant. The main purpose of this unit is to remove the
moisture from the refrigerant.
Moisture cause two main problems:
Firstly, it can freeze to ice in the evaporator and cause
blockage.
Secondly, it can form acids by reaction with the freon
refrigerants. This acid attacks the copper in the lines and
deposits its in other parts of the system. This can become
particularly troublesome when it is deposited on the
compressor mechanical seal faces leading to damage and
leakage.
Fine particles which could possible block the expansion
valve are removed.
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Thermostat and Solenoid Valve
These two elements form the main temperature control of the cold rooms.
The Thermostat is set to the desired temperature and given a 3 to 4 degree
differential to prevent cycling. When the temperature in the room reaches the
pre-set level the thermostat switch makes and the room solenoid is energised
allowing gas to the refrigerant liquid to the expansion valve.
A manual overide switch is fitted as well as a relay operated isolating contact
which shut the solenoid when the defrost system is in use.
System Operation
Assume that the rooms are all warm and the compressor is running with all the
solenoid valves open supplying refrigerant to the respective expansion valve
and evaporator.
Should one or two rooms be down to temperature the solenoids close thus
reducing the volume of gas returning to the compressor. The suction pressure
drops and the compressor unloads. If more rooms shut down then the suction
pressure will drop to cut out point and the compressor will stop. When the
rooms warm the solenoids open again, refrigerant passes back to the
compressor, the suction pressure rises and compressor starts. With more
rooms opening, the suction pressure increases and the compressor loads up
more cylinders.
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Thermostatic Expansion Valve (TEV):
Thermostatic expansion valve or TEV
is one of the most commonly used
throttling devices in the refrigerator
and air conditioning systems. The
thermostatic expansion valve is the
automatic valve that maintains proper
flow of the refrigerant in the
evaporator as per the load inside the
evaporator. If the load inside the
evaporator is higher it allows the
increase in flow of the refrigerant and
when the load reduces it allows the
reduction in the flow of the refrigerant.
This leads to highly efficient working
of the compressor and the whole
refrigeration and the air conditioning
plant.
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Functions of the Thermostatic Expansion Valve
The thermostatic expansion valve performs following functions:
1) Reduce the pressure of the refrigerant: The first and the foremost function of
the thermostatic expansion valve is to reduce the pressure of the refrigerant from
the condenser pressure to the evaporator pressure. In the condenser the
refrigerant is at very high pressure. The thermostatic expansion valve has an orifice
due to which the pressure of the refrigerant passing through it drops down
suddenly to the level of the evaporator pressure. Due this the temperature of the
refrigerant also drops down suddenly and it produces cooling effect inside the
evaporator.
2) Keep the evaporator active: The thermostatic expansion valve allows the flow
of the refrigerant as per the cooling load inside it. At higher load the flow of the
refrigerant is increased and at the lower loads the flow is reduced. It won’t happen
that the load on the evaporator is high and the flow of the refrigerant is low thereby
reducing the capacity of the evaporator. The thermostatic expansion valve allows
the evaporator to run as per the requirements and there won’t be any wastage of
the capacity of the evaporator. The TEV constantly modulates the flow to maintain
the superheat for which it has been adjusted.
3) Allow the flow of the refrigerant as per the requirements: This is another
important function of the thermostatic expansion valve. It allows the flow of the
refrigerant to the evaporator as per the load on it. This prevents the flooding of the
liquid refrigerant to the compressor and efficient working of the evaporator and the
compressor and the whole refrigeration plant.
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Defrost system
Moisture freezes onto the evaporator eventually causing a
restriction and reducing the efficiency of the plant. This
must be periodically removed. For Veg and Flour rooms,
were not restricted to 0oC minimum by the back pressure
valve, this is carried out once per day. For the Meat and
Fish rooms this has to be carried out two or more times.
Due to the low temperature in the rooms it is necessary to
fit a drain heater.
When on defrost the solenoid valve is shut and the fan is
off. On some systems at end of defrost the solenoid valve
is opened momentarily before the fan is started. This allows
moisture to be snap frozen onto the surface of the element,
creating a rough increased surface area and thereby
increasing the heat transfer rate.
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Effects of under and over charge
The effects of overcharge are a full condenser/receiver
gauge glass. System pressures are not effected until highly
overcharged when a possibility of excessive HP pressure
exists. Undercharge causes failure to maintain cold room
temperatures and compressor cycling. Compressor cycling
is caused by there being insufficient gas to maintain the
compressor loaded even with all room solenoids open. In
extreme the compressor will cut in and out. Undercharge is
detected by low levels in the condenser/receiver gauge
glass/ bubbles in liquid sight glass, compressor cycling and
low suction pressures.
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Refrigeration Process:
Refrigeration is the heat removal process from objects and/or
spaces in order to maintain them at the temperature lower than
the ambient.
Refrigeration is carried out in two ways:
1. Vapor Absorption Refrigeration.
2. Vapor Compression Refrigeration.
Vapor Compression Refrigeration is being universally used for
almost all the applications of refrigeration. A Refrigerant gas is
generally used as the medium for heat transfer and, is
alternately condensed and evaporated at lower temperatures to
remove the heat from the spaces being cooled (refrigerated).
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© UNEP 2006
Refrigeration
Working cycle:
The refrigeration cycle
is shown in the Figure
and can be broken
down into the following
stages:
Cycle 1-2
Cycle 2-3
Cycle 3-4
Cycle 4-1
Condenser
Evaporator
High
Pressure
Side
Low
Pressur
e Side
Compressor
Expansion
Device
1
2
3
4
Vapor Compression Refrigeration
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1 – 2.
Low-pressure liquid
refrigerant in the
evaporator absorbs
heat from its
surroundings,
usually air, water or
some other process
liquid. During this
process it changes
its state from a liquid
to a gas, and at the
evaporator exit is
slightly superheated.
Low pressure
liquid refrigerant
in evaporator
absorbs heat and
changes to a gas
Condenser
Evaporator
High
Pressure
Side
Low
Pressur
e Side
Compressor
Expansion
Device
1 2
3
4
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The superheated
vapour enters the
compressor where
its pressure is
raised
Condenser
Evaporator
High
Pressure
Side
Low
Pressure
Side
Compressor
Expansion
Device
1 2
3
4
2 – 3.
The superheated
vapour enters the
compressor where its
pressure is raised.
The temperature will
also increase,
because a proportion
of the energy put into
the compression
process is transferred
to the refrigerant.
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The high pressure
superheated gas
is cooled in
several stages in
the condenser
Condenser
Evaporator
High
Pressure
Side
Low
Pressure
Side
Compressor
Expansion
Device
1 2
3
4
3 – 4.
The high pressure
superheated gas passes
from the compressor into
the condenser. The initial
part of the cooling process
(3-3a) de-superheats the
gas before it is then turned
back into liquid (3a-3b). The
cooling for this process is
usually achieved by using
air or water. A further
reduction in temperature
happens in the pipe work
and liquid receiver (3b - 4),
so that the refrigerant liquid
is sub-cooled as it enters
the expansion device.
Reeciver
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Liquid passes through
expansion device,
which reduces its
pressure and controls
the flow into the
evaporator
Condenser
Evaporator
High
Pressure
Side
Low
Pressure
Side
Compressor
Expansion
Device
1 2
3
4
4 - 1
The high-
pressure sub-
cooled liquid
passes through
the expansion
device, which
both reduces its
pressure and
controls the
flow into the
evaporator
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Vapour-compression theoretical graphs
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A-B, Isobaric Heat absorption in the evaporator
B-C, Isentropic compression in the compressor (frictionless
adiabatic compression in ideal cycle)
C-D, Isobaric Heat removal in condenser
D-A, Constant enthalpy expansion in expansion valve
Heat energy equivalent of work done = Heat energy
rejected- heat energy received
= Area ABCDA + Area under AD
Coefficient of performance = heat energy received/ Heat
energy equivalent of work done
The coefficient of performance for freon is about 4.7
It should be noted that undercooling increases the heat
received by moving point A to the left increasing the
refrigerant effect.
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The Critical Point:
The critical point is the point above which
1. The gas will not liquefy by the action of
pressure alone. This is an important temperature
for refrigeration systems which rely on the change
of state for heat transfer.
2. The gas will not liquefy by cooling alone
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Working of Ship’s Refrigeration Plant
- The compressor acting as a circulation pump for refrigerant has
two safety cut-outs- Low pressure (LP) and High Pressure (HP)
cut outs.
- When the pressure on the suction side drops below the set
valve, the control unit stops the compressor and when the
pressure on the discharge side shoots up, the compressor trips.
- LP or low pressure cut out is controlled automatically i.e. when
the suction pressure drops, the compressor stops and when the
suction pressure rises again, the control system starts the
compressor. HP or high pressure cut out is provided with
manually re-set
- The hot compressed liquid is passed to a receiver through a
condenser to cool it down. The receiver can be used to collect
the refrigerant when any major repair work has to be performed.
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- The master solenoid is fitted after the receiver, which is
controlled by the control unit. In case of sudden stoppage of
compressor, the master solenoid also closes, avoiding the
flooding of evaporator with refrigerant liquid.
- The room or hold solenoid and thermostatic valve regulate the
flow of the refrigerant in to the room to maintain the temperature
of the room.
- For this, the expansion valve is controlled by a diaphragm
movement due to the pressure variation which is operated by the
bulb sensor filled with expandable fluid fitted at the evaporator
outlet.
- The thermostatic expansion valve supplies the correct amount
of refrigerants to evaporators where the refrigerants takes up the
heat from the room and boils off into vapours resulting in
temperature drop for that room.
- This is how temperature is maintained in the refrigeration plant
of the ship.
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A safety system includes alarm, cut offs, and trips which safeguards the
machinery and its parts from getting damage.
The main safeties adopted for refrigeration plants are
Low Pressure or LP cut off:
This is a compressor safety which cut off the compressor in the event of
pressure drop in the suction line. The pressure of the suction line is
continuously sensed by the control unit and when it goes below the set
value, which means the room is properly cooled, the LP cut out will
auto trip the compressor. When the pressure rises, indicating there is
flow of refrigerant in the line due to increase in room temperature, the
LP switch will start the compressor.
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High pressure or HP cut out:
As the name suggests, the high pressure cut out activates and trips
the compressor when the discharge side pressure increases above
the limit value. The HP cut out is not auto reset and has to be done
manually. The reason behind it is to manually attend the fault which
is leading to rise in pressure, else this situation can lead to
overloading of compressor parts and may damage the same
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Oil differential cut out:
This safety is again for compressor as it is the only machinery in
the circuit having rotational parts which requires continuous
lubrication. In the event of low supply or no supply of lube oil to the
bearing, the differential pressure will increase and activates a trip
signal to safeguard the bearing and crankshaft.
Relief valves:
Relief valves are fitted in discharge side of compressor and will lift
and safeguard the compressor in the event of over pressure. One
relief valve is also fitted in the condenser refrigerant line to avoid
damage to the condenser if there is high pressure in the discharge
line.
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Solenoid valves:
Master solenoid valve is fitted in the common or main line
after the condenser discharge. It closes when compressor
stops or trips to avoid over flow of refrigerant in to
evaporator. All holds or rooms are fitted with individual
solenoid valve which control the flow of refrigerant to that
room.
Oil heater:
Oil heater is provided for the compressor crank case oil
and prevents compressor from getting excessively cold
which may effect the lubrication of the parts.
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Effect of less Refrigerant gas in the Ref. System:
While in operation, the refrigerant used in the refer plant
gets consumed or is reduced in quantity because of
leakage in the system. Reduction in quantity of refrigerant
may lead to troubles in the plant such as-
- Short Cycling of Compressor
- Too low suction pressure
- Difficult to maintain temperature of rooms and holds
- Reduction in the efficiency of the plant
When the above mentioned problems occur, it indicates
less refrigerants in the system and the plant has to be
charged with the refrigerant.
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Charging of Refrigeration Plant:
There are two methods for charging reefer plants:
Liquid charging and Gas charging.
Now a day’s gas charging is preferred over liquid charging
because it is more safe and simple.
Gas Charging of Refrigeration Plant:
For gas charging, a special T piece valve block with
mounted pressure gauge is provided to combine three
connectors inter-connecting:
-Vacuum pump
-Charging Cylinder
-Charging Point
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Following steps are to be
taken for charging gas into
the reefer plant:
1. Connect gas bottle or
charging cylinder, vacuum
pump and charging point in
the reefer system to the
valve block.
2. The discharge of the
vacuum pump is to be
connected in the empty
recovery bottle
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3. First open the valve between vacuum pump and charging
bottle located in the valve block without opening the main valve
of the charging cylinder. This will remove all the air inside the
pipe. Once vacuum is reached, close the valve of charge
cylinder in the valve block
4. Now open the valve of the charging point pipe in the valve
block and run the vacuum pump until the vacuum is reached.
This will remove the trapped air from this pipe. Then shut the
valve in the valve block
5. Now keep the system idle for 5 minutes to check there is no
pressure drop. This will ensure there are no leakages in the
system
6. Now open charging bottle pipe valve and the charging point
pipe valve located in the valve block. This will set the line for
charging. Ensure that the vacuum pump valve is shut
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7. Now open the main valves in the charging cylinder and
charging point of the reefer system
8. Do not overfill the system. Make sure the receiver has 5
% space for expansion
Ensure that no refrigerant is leaked out in the environment
as these effects the ozone layer in the atmosphere.
Gas bottle is kept on weighing scale for measuring the
amount of charged supplied to the system.
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What is a Refrigerant?
• Refrigerants are used as working substances in a
Refrigeration systems.
• Fluids suitable for refrigeration purposes can be classified into
primary and secondary refrigerants.
• Primary refrigerants are those fluids, which are used directly
as working fluids, for example in vapour compression and
vapour absorption refrigeration systems.
• These fluids provide refrigeration by undergoing a phase
change process in the evaporator.
• Secondary refrigerants are those liquids, which are used for
transporting thermal energy from one location to other.
Secondary refrigerants are also known under the name brines
or antifreezes
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Desirable properties of a refrigerant
1. Low boiling point (otherwise operation at high vacua becomes a
necessity)
2. Low condensing pressure (to avoid heavy machine plant scantling
and reduce the leakage risk)
3. High specific enthalpy of vaporisation ( to reduce the quatity of
refrigerants in circulation and lower machine speeds, sizes etc.)
4. Low specific volume in vapour state (reduces size and increases
efficiency)
5. High critical temperature (temperature above which vapour cannot
be condensed by isothermal compression)
6. Non corrosive and non solvent (pure and mixed)
7. Stable under working conditions
8. Non flammable and non explosive
9. No action with oil ( the fact that most refrigerants are miscible may
be advantageous e.e. the removal of oil films, lowering pour points etc,
provided separators are fitted
10. Easy leak detect
11. Non toxic
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Environmental and safety properties of Refrigerants
At present the environment friendliness of the refrigerant is
a major factor in deciding the usefulness of a particular
refrigerant. The important environmental and safety
properties
are:
• a) Ozone Depletion Potential (ODP): According to the
Montreal protocol, the ODP of refrigerants should be
zero, i.e., they should be non-ozone depleting
substances.
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• b) Global Warming Potential (GWP): Refrigerants should
have as low a GWP value as possible to minimize the
problem of global warming. Refrigerants with zero ODP
but a high value of GWP (e.g. R134a) are likely to be
regulated in future.
• c) Total Equivalent Warming Index (TEWI): The factor
TEWI considers both direct (due to release into
atmosphere) and indirect (through energy consumption)
contributions of refrigerants to global warming. Naturally,
refrigerants with as a low a value of TEWI are preferable
from global warming point of view.
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• d) Toxicity: Ideally, refrigerants used in a refrigeration
system should be non-toxic. Toxicity is a relative term,
which becomes meaningful only when the degree of
concentration and time of exposure required to produce
harmful effects are specified.
In general the degree of hazard depends on:
- Amount of refrigerant used vs total space
- Type of occupancy
- Presence of open flames
- Odor of refrigerant, and
- Maintenance condition
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• e) Flammability: The refrigerants should preferably be non-
flammable and non-explosive. For flammable refrigerants
special precautions should be taken to avoid accidents.
• f) Chemical stability: The refrigerants should be chemically
stable as long as they are inside the refrigeration system.
• g) Compatibility with common materials of construction (both
metals and non-metals)
• h) Miscibility with lubricating oils: Oil separators have to be
used if the refrigerant is not miscible with lubricating oil (e.g.
ammonia). Refrigerants that are completely miscible with oils
are easier to handle(R12).
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ECO-FRIENDLY REFRIGERANTS
HCFC
R22,R124
HFC
R134a,R152a
NATURAL REFRIGERANT
NH3, HC'S
CFC
ALTERNATIVES.
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Halocarbon Refrigerants
• Halocarbon Refrigerant are all synthetically
produced and were developed as the Freon
family of refrigerants.
Examples :
– CFC’s : R11, R12, R113, R114, R115
– HCFC’s : R22, R123
– HFC’s : R134a, R404a, R407C, R410a
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F Gas Stakeholder Group, 14th October 2009 Slide 53
HFCs
• Remain a popular choice
– especially for R22 phase out
• Good efforts at improving leakage performance
– e.g. Real Zero project
• Interest in R407A to replace R404A
– 50% reduction in GWP
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HCFC
• Transitional compounds with low ODP
• Partially halogenated compounds of
hydrocarbon
• Remaining hydrogen atom allows Hydrolysis
and can be absorbed.
• R22, R123
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Inorganic Refrigerants
• Carbon Dioxide
• Water
• Ammonia
• Air
• Sulphur dioxide
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AIR CONDITIONING
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The basic principals of air conditioning
Air conditioning is the control of humidity, temperature,
cleanliness and air motion. Winter conditioning relates to
increasing temperature and humidity whilst summer
conditioning relates to decreasing temperature and
increasing humidity.
The components of the system such as the Compressor,
Evaporator, oil seperator, filter drier, condenser and air
handling unit .
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Air Handling Unit
One or more is fitted. In the diagram above a single unit
contains two individual evaporators which are
independently supplied by a compressor. A belt driven fan
delivers air to the evaporators via a fine mesh air filter. This
filter is removed on a regular basis and washed in a soapy
solution containing disinfectant.
The air passes over the evaporator where it is cooled and
releases water vapour. The water condenses and is fed
away via a drip tray and pipework.
A perforated pipe is fitted after the evaporator allowing low
quality steam to be fed into the air improving its humidity
when too dry.
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HEALTH HAZARDS:
1. Contamination of ships air conditioning systems
by legionnella bacteria
2. Legionnaires disease is caused by bacteria
which flourishes in stagnant water or sludge . It
can also be found in wet matrix filters, which may
be found in the ships filtration system for the air
conditioning plant.
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Dew point:
When a mixture of dry air and
water vapour has a saturation
temperature corresponding to the
partial pressure of the water
vapour it is said to be saturated.
Any further reduction of
temperature (at constant
pressure) will result in some
vapour condensing. This
temperature is called the dew
point, air at dew point contains all
the moisture it can hold at that
temperature, as the amount of
water vapour varies in air then
the partial pressure varies, so the
dew point varies.
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Psychrometric chart
This chart is used for
finding the relative
humidity of air which has
been measured using a
'wet and dry bulb'
thermometer. This is a
pair of thermometers, one
of which has its bulb
wrapped in a damp cloth.
The drier the air,the
greater the evaporation of
water off the cloth and
therefore the lower the
reading on the 'wet bulb'
thermometer.
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Troubleshoot Faults in Shipboard Refrigeration Systems
Undercharging of Refrigeration System
Indication:
• Compressor is running hot and performance of the compressor falls off due to high
superheat temperature at the suction side of compressor.
• Suction and discharge pressure of the compressor is low.
• Large vapor bubbles in the liquid sight glass.
• Low gauge readings in the condenser.
• Ammeter reading for the compressor motor is lower than normal.
• Rise in room temperature which is to be cooled.
• Compressor is running for extended period of time.
Causes:
• Leakage of refrigerant at the shaft seal, flange couplings, valve gland etc.
• Expansion valve may be blocked at the strainer.
• Partial blockage of refrigerant at the filter or drier or evaporator may cause
undercharging.
Action:
• Identify and rectify the leakage of refrigerant from the system.
• Clean the filter and drier.
• Charge the system with fresh refrigerant as required.

Overcharge of Refrigeration System
Indication:
• The liquid level in the condenser is too high (high condenser gauge
reading). This will reduce the available condensing surface, with
corresponding increase in the saturation temperature and pressure.
• High pressure switch of the refrigerant compressor activates and
stops the compressor.
• The suction and the discharge pressures are high.
Causes:
• It may be due to the reason that excessive refrigerant has been
charged in the system.
• Air in the system may also cause over charging indication.
• It may also be due to the formation office on the regulator.
Action:
• Remove the refrigerant from the system. This is done by connecting
a cylinder to the liquid line charging valve, starting the compressor, and
then operating the charging valve.
• Purge the air from the system and maintain effective cooling.
• Remove ice from the regulator by using any of the defrosting
methods.
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Moisture in the System
This normally comes with the ingress of air in the system. Moisture may
freeze at the expansion valve, giving some of the indication of under
charging. It will contribute to the corrosion in the system. It may cause
lubrication problems and breakdown of the lubricating oil in the refrigerant
compressor.
Action:
- Renew silica gel in case of minor moisture.
- collect refrigenant and remove all air and moisture by vacuum pump if
the amount is huge.
Air in the System
Indication:
• This may cause the refrigeration compressor to overheat, with a high
discharge pressure and normal condensing temperature.
• There are possibilities of small air bubbles in the liquid sight glass of the
condenser.
• Condensing pressure of the refrigerant in the condenser may be high.
• If there is excessive air, it may reduce the cooling capacity of the system,
making the compressor to run for the extended period of time.
• It may cause the gauge pointer of the condenser to jump indefinitely.
7/17/2014 66

Air in the System
Causes:
• During charging, air may enter in to the system.
• If Freon-12 is used air may leaks in to the suction line because
the working pressure of the Freon-12 refrigerant is less than the
atmospheric pressure.
Action:
• Air in the system can be removed by collecting the system gas in
the condenser, leaving the condenser cooling water on and venting
out the air from the top of the condenser because air will not be
condensed in the condenser but remains on top of the condenser
above the liquid refrigerant.
• Connect the collecting cylinder to the purging line of the
condenser, open the valve, and collect air in the cylinder.
• After purging the air from the system don’t forget to shut the
purging valve.
• Check the level of the refrigerant in the system. If required, charge
the system with fresh refrigerant.
• Restart the compressor with all safety precautions.
7/17/2014 67

Oil in the Refrigeration System
Indication:
• Temperature is not dropping in the cold rooms as normal, due
to fact that oil act as insulation in the evaporator.
• It may cause excessive frost on the suction line.
• Refrigerant compressor runs for the extended period of time.
• Lubricating oil level in the compressor will drop.
• Refrigerant level will fall if oil has caused blockage.
Causes:
• This may happen if the oil separator is not working properly.
• Oil may carry over from the compressor and may not come
back to the compressor due to blockage in the system.
• Defective piston rings or worn out liner of the compressor may
cause the oil to carry over along with the refrigerant.
• Compressor may take high capacity current during starting.
7/17/2014 68

Oil in the Refrigeration System
Action:
• Check the oil separator for proper functioning.
• Check the drier for proper cleaning and if its require cleaning
clean it
• Evaporator coil should be drained to remove any trace of oil.
• If there is oil in the cooling coils, increase the condenser and
evaporator temperature differentials and remove excess frost on
the suction pipe.
• Heat pipes with blow torch.
Flooding of Refrigerant in the System
This is seen as liquid getting back to the suction of the
refrigerant compressor. It may be due to a faulty or incorrectly
adjusted expansion valve and also due to solenoid valve
leakage. It may also result from overcharging of the refrigeration
system. Flooding may lead to an iced up evaporator.
7/17/2014 69

Evaporator Coil Icing:
Icing of the evaporation coils which may happen due to:
1. Cause:Too low temperature setting
Action: Increase the coil temperature by adjusting TEV or
it’s sensor.
2. Cause: The coil capacity is less
Action: Install large capacity evaporator coils
3. Cause: Defrost is not operational
Action: Check if the defrost system is functioning at regular
intervals.
7/17/2014 70

Compressor Start and Stops Frequently:
If while maintaining the correct temperature of the ship’s provision room or
reefer cargo, the reefer compressor is frequently cutting-in and out, then such
problem needs to be sorted out immediately. The most normal causes for such
operation are:
1. Cause: Wrong Setting of Cutouts: It may be because the high pressure (HP)
cutout is set too high or LP cutout is set too low
Action: Check and change the setting to advisable limit
2. Cause: Differential Setting Span is Small: The low pressure (LP) cut out is
provided with starting and stopping pressure setting. If the setting span is too
small, it will lead to frequent cut-in and cut-out of the compressor
Action: Change the setting and increase the span between starting and
stopping compressor pressures.
3. Cause: Defective Valves: If the compressor discharge valve is leaky or the
line solenoid valve is not closing properly, this will lead to variation in sensor
pressure and result in frequent cut-in and cut-out of compressor
Action: Replace all the defective valves
4. Cause: Clogged Suction Filters: Compressor is provided with a filter in the
suction line. If that is clogged, it will lead to frequent LP cut out
Action: clean the filter.
7/17/2014 71

Compressor Starts But Stops immediately
When the compressor in the reefer circuit starts and suddenly stops, it
can be because of the following reasons:
1. Cause: Low pressure cut out gets activated
Action: Ensure that all the suction line valves are in open condition, the
refrigeration is properly charged and the low pressure cut out is not
defective.
2. Cause: Defective oil pressure cut out
Action: Check for proper functioning of oil pressure cutout and replace
the defective cutout.
3. Cause: Defrosting timer is getting activated frequently
Action: If the defrost timer is getting activated frequently, leading to
cutout of compressor, check and repair defrost timer.
4. Cause: The lube oil level is below required level
Action: This can be because of leakage of lube oil from seal or carry
over of oil. Rectify the leakage and refill the oil level.
5. Cause: Foaming of oil leading to reduced oil pressure
Action: Ensure no foaming takes place, renew the oil if required.
6. Cause: Motor overload cutouts are activating
Action: Ensure that electrical motor trips are working properly.
7/17/2014 72

ANY QUESTION?
THANK YOU!
7/17/2014
73

Marine Refrigeration and Air Conditioning

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