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

7/17/2014 3 Mohd. Hanif Dewan, Chief Engineer and

Maritime Lecturer & Trainer, Bangladesh.



















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.

7/17/2014 Mohd. Hanif Dewan, Chief Engineer and

Maritime Lecturer & Trainer, Bangladesh. 14

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. 7/17/2014 15

Mohd. Hanif Dewan, Chief Engineer and


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



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. 7/17/2014 17



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.

7/17/2014 18



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.



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.



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.



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. 7/17/2014 22



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.





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).



26 © 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 Pressure Side

Compressor Expansion Device

1 2

3

4

Vapor Compression Refrigeration



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 Pressure Side


1 2

3

4



28

The superheated vapour enters the compressor where its pressure is raised

Condenser

Evaporator

High Pressure

Side

Low Pressure

Side


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.



29

The high pressure superheated gas is cooled in several stages in the condenser

Condenser

Evaporator

High Pressure

Side

Low Pressure

Side


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



30

Liquid passes through expansion device, which reduces its pressure and controls the flow into the evaporator

Condenser

Evaporator

High Pressure

Side

Low Pressure

Side


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



Vapour-compression theoretical graphs



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.



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



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.



- 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.



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.



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



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.



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.



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.



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



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

7/17/2014 42



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



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.



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



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 12. cheap, easily stored and obtained 7/17/2014 46



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.




• 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.




• 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 7/17/2014 49




• 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).



ECO-FRIENDLY REFRIGERANTS

HCFCR22,R124

HFCR134a,R152a

NATURAL REFRIGERANTNH3, HC'S

CFCALTERNATIVES.



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



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



HCFC

• Transitional compounds with low ODP

• Partially halogenated compounds of

hydrocarbon

• Remaining hydrogen atom allows Hydrolysis

and can be absorbed.

• R22, R123



Inorganic Refrigerants

• Carbon Dioxide

• Water

• Ammonia

• Air

• Sulphur dioxide



AIR CONDITIONING



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 .

7/17/2014 57







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.



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.



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.



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.

7/17/2014 63



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. 7/17/2014 65



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.



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.



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.



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!



Documents

Marine Refrigeration and Air Conditioning