Marine Refrigeration and Air Conditioning

Marine Refrigeration System

Vapour Compression System:

The vapour compression cycle takes place in a closed system, comprising a compressor, a condenser, a liquid receiver, an evaporator, and a flow control or expansion valve, interconnected by discharge, liquid and suction lines.

Principle of refrigeration:

1) Absorption of heat by the evaporation of a liquid refrigerant in the evaporator at a low pressure;
2) Raising the pressure ( to raise the condensing temperature ) of low pressure vapour coming from the evaporator by the compressor;
3) Removal of heat from the high pressure  vapour in the condenser so as to liquefy or condense the vapour;
4) By the use of the throttling device, reducing the pressure of the high pressure liquid ( from the condenser ) to the level of pressure needed in the evaporator.

Expansion valve ( flow control ):

The liquid refrigerant is stored at high pressure in the receiver. The liquid flows from the receiver through the liquid line to the Expansion valve, which regulates the rate of flow to the evaporator to suit the evaporation. As it passes through the Expansion valve, the pressure of the liquid is reduced to the evaporating pressure, so that the saturation temperature of the refrigerant entering the evaporator is below that required in the refrigerant space.

As it passes through the Expansion valve, some of the liquid boils off at the Expansion valve, taking latent heat from the remainder and causing its temperature to drop.

This vapour does not do any useful refrigerant work in the evaporator.
The presence of vapour in the liquid line will, also affects liquid carrying capacity of the liquid line and consequently the capacity of the system. Vapour also affect a lot of resistance to the liquid flow.

The Expansion valve throttles the liquid refrigerant and maintains the pressure difference between the condenser and evaporator, while supplying the evaporator at the correct rate.

It is thermostatically controlled in modern system.

Evaporator

The liquid-vapour refrigerant mixture then flows through the Evaporator, at a temperature lower than that of the surrounding secondary coolant ( air or brine ). It extracts heat ( receives latent heat ) from the refrigerated space  and changes to a dry saturated vapour at approximately the same temperature and pressure as that at which it leaves it left the Expansion valve.

Later the same heat is given up in the condenser, when the refrigerant is again compressed and liquefied.

In practice, the control system regulating the refrigerant flow is desired to ensure that the vapour leaving the Evaporator is slightly superheated, thus ensuring that only dry vapour is handled by the compressor.

In theory, superheating is non advantageous but in practice it is advantageous to make the vapour superheated at entry to the compressor, giving a fairly high superheat leaving the compressor.

As the liquid refrigerant flows through the evaporator, it absorbs heat and changes from liquid to saturated vapour. The vapour thus produced remains saturated as long as there is some liquid refrigerant present. Finally, all liquid is vapourised. Up to that point the vapour is in a saturated condition.

As the vapour continues to flow through the evaporator this vapour continues to absorb heat. But as there is no liquid left to boil off, the temperature of the vapour rises higher than the saturation temperature. This vapour now becomes superheated.

Actually, quantity of heat needed to superheat the vapour is very small as compared to the Latent heat needed to vapourise the liquid to vapour.

For example, less than 5 K.cal of heat is required to superheat 1 Kg of R-22 by 50C from the saturation point of 4.40C, where as 48.25 K.cal of Latent heat is needed to change 1 Kg of R-22 liquid to vapour at 4.40C.

Compressor 

In the compressor, the temperature and pressure of the vapour are raised by compression.
This causes its saturation temperature to rise, so that so that it is higher than that of the sea water or air, cooling the condenser.

The compressor also promotes circulation of the refrigerant by pumping it around the system.
Capacity of compressor must be such that it removes the vapour from the evaporator at the same rate as that as it is formed.

If the capacity is too small, (unable to vapourise refrigerant ) the excess vapour will accumalate in the evaporator, causing the pressure and saturation temperature to rise.
If the capacity is too large, it will remove the vapour from the evaporator too rapidly, causing the pressure and saturation temperature to fall.

To maintain a specified operating condition, a compressor must have a swept volume equal to the volume of vapour formed in the evaporator per unit time (m3/h ).

Condenser 

The vapour in the Condenser, first gives up its super heat as it is cooled from the discharge temperature to the saturation temperature corresponding to the condensing pressure, and then gives up its latent heat (originally from the evaporator) as it  condenses back to  a liquid.

Some undercooling, which is advantageous, will occur in the condenser. The greater the undercooling the less will be the flash off percentage. This represents a loss, as any vapor formed before the evaporation will not now extract its specific enthalpy of vapourisation from the brine, giving a resultant loss of refrigerating effect.

Receiver 

The liquid refrigerant, still at the pressure produced by the compressure, flows from the bottom of the condenser into the Receiver and then to the Expansion valve. Many of the circuits employ a liquid receiver after the condenser.

The capacity of the liquid receiver is usually sufficient to store temporarily of the whole system charge during repairs to an evaporator or condenser.

This completes the Refrigerating cycle.

For a small refrigerator the evaporator cools without the forced circulation of a secondary coolant.
In larger installations, the evaporotor cools air or brine which are circulated as secondary refrigerants.

A ) Refrigerating effect :-

The amount of heat absorbed by each unit of refrigerant as it flows through the evaporator is known as the refrigerating effect, and is equal to the difference between the enthalpy of the vapour leaving the evaporator and the enthalpy of the liquid at the flow control.

For example,  R-22 refrigerating system :-

Temperature of liquid at the inlet of expansion valve :           43.3 0C.

It is cooled to 4.4 0C ( this happens automatically, as the liquid refrigerant passes through the throttling device ) which is the temperature at which it enters the evaporator & boils.

From the table of R-22 :-

Enthalpy @  43.3 0C    :             23.58 K.cal / Kg
Enthalpy @  4.4   0C    :             11.90 K.cal / Kg

Excess enthalpy           :              11.68 K.cal / Kg
(to be removed )

At the outlet of the throttling device, on reduction in pressure, a portion of the liquid vapourise, taking the latent heat of vapourision itself.

Latent heat of vapourision @ 4.4 0C  :    48.18 K.cal / Kg

Out of this, 11.68 K.cal / Kg,  i.e., 11.68 / 48.18 = 0.24 Kg ( 24 % of the full latent heat @ 4.4   0C ) will have to be used for cooling the liquid from 43.3 0C   to  4.4   0C .

This leaves only 0.76 Kg ( 76 % ) of the liquid   for boiling in the evaporator.

This 0.76 Kg takes ( 48.18 – 11.68 ) = 36.5 K.cal as latent heat capacity of the refrigerant liquid is available for useful refrigeration.

This is called “ net refrigeration effect ” =  36.5 K. cal


B ) Effect of subcooling :-

In the above case, we assume that there was no subcooling of the liquid.

For example,  if R-22 is subcooled from the condensing temperature 43.3 0C  to 35.5 0C, the liquid needs to be cooled only  35.5 0C to 4.4 0C on entering the evaporator instead of 43.3 0C to 4.4 0C as in the previous case.

From the table,

Enthalpy @  35.5 0C    :             21.16 K.cal / Kg
Enthalpy @  4.4   0C    :             11.90 K.cal / Kg

Excess enthalpy           :              09.24 K.cal / Kg
 ( to be removed )

So,   9.24 / 48.18 = 0.19 Kg  ( 19 % ) ) will have to be used for cooling the liquid from 35.50C   to  4.40C

This 0.81 Kg takes ( 48.18 – 9.24 ) = 38.95 Kcal  as latent heat capacity of the refrigerant liquid is available for useful refrigeration

So,  “ net refrigeration effect ” =  48.18 – 9.23 = 38.95 K cal / Kg of Refrigerant.

Hence we are able to utilize 81% of the latent heat capacity of the refrigerant as against 76% in case there is no liquid subcooling i.e., gain 5%.

C ) Refrigerating Capacity

The rate at which a system will absorb heat from the refrigerated space or substance is known as the refrigerating capacity.

Refrigerating Capacity = mass flow of refrigerant through the evaporator (kg / s ) X refrigeration effect ( KJ / s ).

D ) Effect on machine size.

Each Kg of CO2 entering totally as liquid and leaving as dry saturated vapour would absorb 275 KJ, being the specific enthalpy of vapourisation of CO2. (for NH3, this figure is 1314 KJ ).

5 Kg of CO2 would extract 1375 KJ has a volume of 0.085 m3 whereas 1 Kg of  NH3 has a volume of 0.51 m3,

   i.e., for same refrigeration effect a high mass flow rate for CO2 but a very high volume (capacity) rate for NH3 , which effect machine size.

For F-12,  8.5 Kg extracts 1350 KJ/Kg and has a volume of 0.79 m3 i.e., the specific volume is 0.093 m3/Kg and the specific enthalpy of evaporation 158.7 KJ/Kg.

5 Kg of CO2     -  volume of 0.085 m3                                  would extract 1375 KJ
1  Kg of  NH3  -  volume of 0.51 m3                                   would extract 1314 KJ
8.5 Kg of F12  - volume of 0.79 m3                    would extracts (158.7x8.5) 1350 KJ

E ) Need to increase the refrigerant vapour pressure by Compressor

If the refrigerant vapour coming from the evaporator at 4.40 deg. C ( for example ) is to be condensed at same temperature. Water or air at a temperature lower than 4.40 deg. C will be needed.

As the temperature of the available water or air is always is much higher than the temperature of the refrigerant vapour coming from the evaporator the refrigerating vapour cannot be condensed unless its condensing temperature is raised by some means.

The condensing temperature of vapour can be raised by increasing the pressure of vapour.

Compressor is used to raise the vapour pressure and sending it to the condenser.

F ) Coefficient of performance 

The ratio of refrigerating effect to the heat of compression is known as the coefficient of performance ( COP ).

G )  Measuring Superheat

The steps to measure the superheat are :

1. Take the temperature of the suction line at the place where the valve feeler bulb is mounted.
2. Take the suction pressure and find the corresponding temperature from the pressure - temperature chart.
Add 1.1 deg.C to this temperature reading to care of the suction line pressure.

For example, saturated pressure at compressor = 2.46 Kg/cm2,
                              corresponding temperature  = 3.33 deg.C
                                                               add      = 1.1 deg.C
say, temperature at the suction line                   =  10 deg.C
so,  Superheat of valve ( 10 deg.C – 4.4 deg.C)=  5.6 deg.C

System Components

Filter:

Compressors are protected by metal gauze filters from dirt entering with the suction gas. When first running a newly erected plant it is usual to fit felt socks inside these gauzes to catch the initial dirt in the pipe systems.

Fine liquid filters are fitted to protect the small orifices in expansion valves. Any clogging of one of these reveals itself by a cooling of the liquid pipe downstream of the filter. The choked filter causes a pressure drop in the liquid so that it is acting as an expansion valve, causing a loss in pressure and reduction in temperature.

Drier:

Purpose of drier is to keep all traces of water out of refrigeration systems to prevent ice formation at expansion valve. Moisture in Freon systems also causes further troubles, viz, corrosion of steel parts, transference of copper from copper pipes on to steel parts of compressors causing a copper film and forming sludge in the lubricating oil.

If the drier located in the liquid line it should be arranged so that the liquid enters at the bottom and leaves at the top. This is to ensure that there is uniform contact between the liquid refrigerant and the drying agent and that any entrained oil globules will be flooded out without fouling the particles of the drying agent.

If located in the suction line, the gas should enter at the top and leave at the bottom so that any oil can pass straight through and out.

The common drying agents are silica gel and activated alumina, both of which can be reactivated by heating to 1400 deg.C for a number of hours. The drying agents are supported on a stiff-gauge disc.

In most installations the driers have by-passes so that they can be isolated without the running of the plant and interfering with the running of the plant and drying agent renewed or activated.

For modern installation, the strainer/drier pack is replaced completely.

Liquid Indicator ( liquid sight glass ):

A sight glass ( circular glasses ), consisting of a small observation “window” in the pipe, is often provided to allow a visual check on the liquid flowing from the condenser.

This is provided to ascertain whether or not the system is fully charged with refrigerant. If undercharged, vapour bubbles will appear in the sight glass.

To be most effective indicators should be installed in the liquid line as close to the liquid receiver as possible. A sight glass arranged some distance from the condenser is unsatisfactory as there may be a pressure loss in the pipe between condenser and glass ( or a pick up a heat if the pipe is running through a warm engine room ) causing some gassing to be visible in the glass even when the charge is correct.

Solenoid Valve:

The solenoid valve is a servo controlled electromagnetic valve which provides automatic opening and closing of liquid and gas lines.
When current flows the solenoid attracts up the valve and holds it open. It closes when the coil is de-energized and the valve drops.

Thermostats:

Thermostats are temperature controlled electric switches, which can be used for both safety & control functions. When fitted to compressor discharge lines, they are set to stop the compressor if the temperature is too high.

Thermostats are also used to control the temperature in a refrigerated space by cycling the compressor ‘on & off’, or by ‘opening or closing’ a solenoid valve in the liquid line.

Three types of element are used to sense & relay temperature changes to the electrical contacts.

1. A fluid filled bulb connected through a capillary to a bellows
2. A Thermistor
 3.   A bi-metal element

Liquid Receiver:

It is usual to provide a welded steel pressure vessel into which liquid refrigerant from the condenser can flow. The receiver is of sufficient capacity for temporary storage of the whole system charge during repairs to an evaporator or condenser.

The receiver may be incorporated permanently into the circuit, or so arranged that it be only used during overhaul.

A level glass is fitted permanently in the circuit, but in normal operation the level should be low enough in the sight glass to show the complete refrigerant charge when necessary. As the receiver is subjected to high pressures, a safety valve or bursting disc must be fitted.

  Safety Controls and Devices

Most of the controls are electric switches, actuated by bellows movement via amplifying leverage. The bellows move in response to pressure changes transmitted from the sensing point via a connecting tube.

High pressure safety cut outs:

This is used to protect against too high a discharge pressure, which will overload a compressor and may damage components. The control is usually set to cut out and stop the compressor motor at a pressure of about 90% of the maximum working pressure of the system. Some controls restarts the compressor automatically on drop in pressure; others have a mutual reset mechanism.

Low pressure safety cut outs:

This is used to protect against too low a suction pressure, which usually indicates a blockage or loss of refrigerant. The control is normally set to stop the compressor at a pressure corresponding saturation temperature 50 deg.C below the lowest evaporating temperature. In some small plants, it is also used as a temperature control, stopping and starting the compressor to maintain the desired pressure and hence temperature.

Oil pressure safety cut outs:

An oil pressure failure control stops the operation of pressure-lubricated refrigeration equipment when the oil pressure falls below a safety limit for a specified time. This control serves as a combination differential pressure control and a time-delay relay.

The differential pressure control measures the useful oil pressure. The time-delay prevents the the compressor from cutting out when it should be operating.

The useful oil pressure in a pressure lubricated compressor is the difference between the oil pump discharge pressure and the suction pressure of oil pump ( suction side of the oil pump is connected with the crankcase pressure). The oil safety switch therefore has two metallic bellows, one connected to the delivery side of the oil pump and the other to the crankcase of the compressor.
As there is no oil pressure when the compressor has just started, a time delay mechanism is incorporated in the oil safety switch. The time-delay relay permits the compressor to operate for about one or two minutes to establish the correct oil pressure differential. If the pressure differential does not come up to the cut-in point within this predetermined time, the compressor motor shuts off.

Some times oil pressure switch of a manual reset type. In that case compressor can not be restarted until –

1. The heater and bi-metal have cooled, and
2. The timer switch contacts on the control are reset manually by the button.

Electrical overload:

All motor starters are equipped with overload elements to stop the motor when the current drawn by the motor exceeds its rated level.

Primary & Secondary Refrigerant – Desirable properties &

Montreal Protocol

Primary refrigerants: Primary refrigerants are the working fluids used in vapour compression systems. 

The desirable properties are :

1) The refrigerant must be non-toxic, non-explosive, non-flammable; 

2) It should not contaminate foods and damage the environment in the event of a leak.

3) The refrigerant must be chemically inert and non-corrosive to any material used in the structure of the plant;

4) The refrigerant should have no action with lubricating oil. ( the fact that most  refrigerants are miscible may be advantageous, i.e., removal of oil films, lowering pour point, etc., provided separators are fitted.);

5) The refrigerant must have high coefficient of performance (COP), i.e., low compressor per input power unit of refrigerating capacity;

6) The volume of vapour that has to be pumped around the circuit for a given cooling effect must be low;

7) The working pressure on the high pressure side must be low enough to keep down the mechanical strength required in the compressor, pipe work and condenser;

8) The working pressure on the low pressure side must not be too low  as pressure below atmospheric result in air being drawn into a system through any minute leaks. This air carries water into the system which freezes out and causes chokes;

9) High specific enthalpy of vapourisation ( to reduce the quantity of refrigerant in circulation and lower machine speeds, sizes, etc. );

10) Low specific volume in vapour state which reduces size and increases efficiency;

11) High critical temperature ( temperature above which vapour cannot be condensed by isothermal compression ); 

12) The refrigerant must have a satisfactory heat transfer characteristics;

13) Easy leak detection;

14) The refrigerant must be cheap, easily stored, and obtained through out the world;

15) The current concern of depletion of ozone layer has resulted in new requirement that refrigerant should be environment friendly. 

Primary Refrigerants include Carbon dioxide (CO2), Ammonia (NH3), R12, R22, R404a, R502, R134a.

No refrigerant has all the desirable properties, each one having various advantages and disadvantages.

Environmental effect & Montreal Protocol:

An ozone layer, surrounding the earth’s stratosphere ( above 11 km above the earth surface ) substantially arrest the ultraviolet rays from the sun from reaching the earth. If there is a depletion of the ozone layer in the stratospheric, the ultraviolet radiations reaching the earth will cause – 

1. Global warming

2. Increased potential for skin cancer

3. Melting of polar ice caps

4. Interfere with immune system, harm aquatic system

Ozone depletion is caused by chlorine, which reacts with ozone and converts it into oxygen. Most of the refrigerants in use are halogenated hydrocarbons or chlorofluoroCarbons ( CFCs ). Though many of the CFCs are heavier than air, some eventually migrate to stratosphere, once the release to atmosphere. Time taken to reach stratosphere may be considerably long ( may be a decade or two or more ). 

Chlorine reacts with ozone converting it into ordinary oxygen and thus depleting the protective ozone layer. 

Refrigerant                      ODP                  GWP

R11 (CFC)                          1.0                    1.0

R12 (CFC)                          0.98                  3.05

R 502 ( part CFC )              0.23                  5.1

R 22 (HCFC)                      0.05                  0.365

Ammonia                            0.00                  0.00

R134a                                 0.00                  0.81

R123                                   0.02

R125                                   0.00

Montreal Protocol:

To protect the global environment, an international agreement, the Montreal Protocol, signed in 1987, controls the use and production of CFC refrigerants and other ozone depleting substances throughout most of the world. 

CFC are characterised under the Montreal Protocol according to the extend to which they damage the ozone layer. 

The most damaging CFCs are given an ‘ozone depletion potential’ (ODP) of 1, and all other CFCs are then assigned an ODP between 0 and 1, according to their destructive potential relative to the most damaging CFCs.

Likewise, CFCs are assigned a ‘global warming potential’ (GWP) compared to base line R11. The ODP and GWP values of the refrigerants used in marine refrigerating plants are listed in Table . 

Secondary refrigerant

A secondary refrigerant is one which is used as a heat transfer medium, with a change of temperature but no change of state. 

The secondary refrigerants used in marine plants today are brine and trichloroethylene.

Brine: Brine is solution of a salt in water. By mixing soluble salts in water, the freezing point of the solution becomes lower than that of the water.

Brine in common use are of Sodium chloride, Calcium chloride, Glycols, such as Ethylene glycol, Propylene glycol, etc. Brine is normally used for temperatures down to –340C, below which it is extremely viscous resulting in unacceptable pumping losses.

Brine is bitter in taste and will contaminate food stuffs. 

Calcium chloride and Sodium chloride brine: These are corrosive, particularly when too much air gets mixed up with the brines. So the brine tanks should be kept closed as far as possible.

These brines can attack copper, steel parts. This may lead to pipe failure, chiller tube failure, etc.

To prevent corrosion, inhibitors are used. Sodium chromates and dichromates are used as inhibitor for both Calcium chloride and Sodium chloride brines to maintain slight alkaline condition ( pH value 8.5 to 9.5 ). In addition, an alkaline such as caustic soda (NaOH ) would be used at a concentration of about 1 % of solution.

Ethylene glycol and Propylene glycol brine: These are less corrosive, but they can be corrosive when air gets mixed. Therefore inhibited glycols should be used. Sodium chromates and dichromates should not be used with glycols ( as inhibitors ) as oxydation of glycol can occur, making the solution more corrosive. 

Ethylene glycol is more toxic than Propylene glycol. 

Trichloroethylene:

1. Trichloroethylene is used for temperatures down to  -730C. 

2. The gas, which is both toxic and heavier than air, has a maximum permissible concentration in air of 200 ppm,

3. Trichloroethylene acts as a solvent to most synthetic rubbers and jointing materials.

4. It is non-flammable and non-toxic.

5. The liquid is heavier than, and immiscible with, water so any water in the system will freeze at temperature below 00C.

Advantages of Secondary refrigerant system  compared to direct expansion (DX) system are :

1) the primary refrigerant is only present in machinery space, and the risk of loss by leakage more readily controlled.

2) Difficulty of oil recovery from evaporators far away (half a ship’s length) from the compressors are avoided.

3) The brine system is easily controlled to give accurate temperature control. Greater flexibility in simultaneously carrying cargoes at different temperatures in spaces is possible.

4) DX system are considered to be cheaper to install and run than brine systems.

5) The weight of refrigerant in the DX system is however, several times greater than in a brine system, which is, on the other hand, more bulky.

Operation: 

Refrigeration is a process of heat removal from a substance or space and rejects it else where. The primary purpose of refrigeration is to lower the temperature of the substance, say perishable goods or a space, like accommodation.

The purpose of refrigeration is to prolong the storage life of perishable goods, by lowering its temperature such that metabolic deterioration is prevented.

On board the ship you have two refrigerant compressors, condensers, liquid receivers, expansion valves. The refrigerant after passing through expansion valve flows in to the evaporator coils which are common for both the circuit. The refrigeration system is in continuous operation to maintain the temperature of domestic stores or provision rooms.

The circuit shown above contains compressor, condenser, expansion valve and evaporator and controls for automatic operation.

Before starting

1. Check the refrigerant oil level in crank case of the compressor.
2. Check the power supply is available to the compressor motor which is to be put in use.
3. Check the belt tension from the electric motor to the compressor.
4. Check the cooling water inlet and outlet to the condenser are open.
5. Check the following valves for the refrigeration circuit which is to be put in to operation are shut. They are:-
• Compressor inlet and outlet valves.
• Condenser inlet valve and receiver outlet valve.
• Filter drier bypass, inlet and outlet valves.

During start

1. Open the following valves in the refrigeration circuit. They are

• Compressor outlet valves.
• Condenser inlet valve and receiver outlet valve.
• Filter drier inlet and outlet valves.
• Cross check the aforesaid refrigeration system valves and cooling water inlet and outlet to the condenser are open.

2. Switch on the breaker for the compressor.
3. Open the compressor inlet valve by half a turn and switch on the compressor. The compressor will start working.
4. Open the compressor inlet valve slowly. Let it be open fully.

Automatic operation of the Freon system

The compressor supplies number of room cold compartments through thermostatically controlled solenoids. In response to the pressure at the compressor is started and stopped by LP (low pressure) controller. Subsequent temperature rise of cold room temperature will cause the solenoids to re-open by individual room thermostatic switches.Pressure rise will cause the LP controller to start the compressor.

Each cold room has a TEV through which liquid refrigerant passes through. Master solenoid is fitted in the system such that in case if the compressor stops due a fault, then the maser solenoid will close. Thereby flooding by liquid refrigerant is prevented.

Fan blows air through evaporator coils. Here air is the secondary refrigerant. Regular defrosting is carried by electrical elements provided which are timer operated.

Checks during running
• Check the condensing pressure, evaporating pressure, oil pressure drop.
• Inspect the compressor unit and check for any abnormality and vibration
• Check the discharge and suction temperatures
• Check the oil level and check the oil is returned to the crankcase by oil separator, and oil return line is warmer than the crankcase.
• Check the shaft seal for oil tightness

Stopping

As the temperature of each room is brought down, its solenoid will close off the liquid refrigerant to that space.
When all compartment solenoids are shut, the pressure drop in the compressor suction will cause the compressor to be stopped through LP controller.

Maintenance

The planned maintenance schedule is developed based on manufacturer’s recommendations and class requirements. However, it is the responsibility of the ship owner and staff to periodically review the schedule considering the operational conditions.
The following maintenance schedule gives an overview of refrigeration system maintenance; every engineer should follow planned maintenance to ensure trouble free operation of the system. The following table is a general guide and precise instructions can be obtained from individual makers.

Daily

• Check the condensing pressure, evaporating pressure, oil pressure drop.
• Inspect the compressor unit and check for any abnormality and vibration
• Check the discharge and suction temperatures
• Check the oil level and check the oil is returned to the crankcase by oil separator, and oil return line is warmer than the crankcase.
• Check the shaft seal for oil tightness

Every three months

• Grease the bearings
• Check the safety cutouts
• Check the belt tension

Every six months

• Renew the compressor oil and driers
• Inspect and clean the oil filter and suction gas strainers
• Check and clean the condenser tubes. Inspect the sacrificial anodes.
• Check the compressor valves, by timing the rate of pressure equalization on stopping the machine and shut the suction stop valve

Every twelve months

• Examine all float operated valves
• Examine external piping’s and connections
• Examine air cooler fans
• Recharge the refrigerant driers
• Every twenty four months Examine the tube plates of the “shell and tube” condensers
• Inspection and overhaul the system components in entirety

Maintenance - For reciprocating compressors

Periodic maintenance or running hour based maintenance depends on the operation time and is different for different types and makes of the compressor.

The following table is a general guide and precise instructions can be obtained from individual makers.

5000 hours

• Check the operating valves and replace the worn out parts
• Change the oil, clean the crankcase, and clean the oil level sight glass and oil strainer.
• Check the unloading mechanisms and replace the o-rings
• Inspect the cylinders for scratches and seizures
• Inspect the pistons for any damages.
• Check the function of monitoring devices and safety cut-outs

10,000 hours

• Replace the operating valves
• Inspect the bearing surfaces
• Check the piston ring gap.

Maintenance - For screw compressors


1000 hours - Check the function of monitoring devices and safety cut-outs

2500 hours - Check the alignment of compressor and electric motor
- Clean all the system filters
  - Lubricate the electric motor bearing

5000  hours - Check the bearings of the oil pump

10000 hours - Change the oil
- Replace the coupling between electric motor and compressor

40000 hours -  Disassemble for total overhaul and replace the axial bearings


Trouble shooting

Faults may occur, while the refrigeration system is in operation, despite periodical maintenance. As a watch-keeping engineer, you should be able to identify reason and rectify the fault. You can identify the faults by carefully observing the parameters and taking necessary action to prevent failure of machinery and down time.

1.Short cycling:Compressor starts and stops too frequently.

• Low pressure control has interrupted - Check and adjust
• Low pressure control difference too small- Check and adjust
• Wrongly adjusted capacity regulator- Check and adjust
• Restriction of refrigerant supply- Get rid of restriction
• Refrigerant charge too small - Pump down and then charge the refrigerant
• Leaky refrigerant plant - Identify and rectify the leak
• Air in the condenser - Purge out the air
• Expansion valve partially blocked with ice or dirt - Get rid of restriction
• Expansion valve bulb wrongly placed - Refer manual and place the bulb correctly
• Filters in the suction line clogged - Inspect and clean
• Leaky discharge valves -  Inspect and replace

2.Compressor runs continuously:

• Welded contacts in the motor - Inspect and replace
• Refrigerant charge too small - Pump down and then charge
• Leaky refrigerant plant - Identify and rectify the leak
• Too much of loading of the plant -Reduce the load partially
• Defective or leaky suction or discharge valves - Inspect and replace
• Too low compressor capacity - Check and rectify 

3.Compressor abnormally noisy:

• Oil pressure too low- Adjust oil pressure regulating valve
• Oil is foaming in the crankcase - Drain little oil
• Liquid in the suction line - Identify and rectify
• Improper adjustment of coupling or slack coupling- Check and replace
• Defective oil pump - Check and replace
• Worn out or defective bearings - Check and replace
• leaky discharge valves - Check and replace 

4.Pressure:

Condenser pressure too high

• High pressure control has interrupted -  Check and adjust
• Overcharge of refrigerant - Purge out the system
• Insufficient cooling water - Give sufficient water
• Air to condenser - Purge out air from the system
• Temperature of cooling water high - Adjust accordingly
• Non-condensable gases in condenser - Purge out non condensable gases
• Fouling of condenser - Clean the condenser
• Water valve does not function -  Make it functional

5.Temperature:

Discharge pipe temperature too high

• Low pressure control difference too small - Check and adjust
• Restriction of refrigerant supply - Identify and clear the restriction
• Refrigerant charge too small - Charge the system
• Insufficient cooling water to the condenser - Give sufficient water

6.Oil:

Oil temperature too high

• Worn out or defective bearings - Identify and replace
• Leaky discharge valves- Identify and replace

Oil in the crankcase disappears

• Fouled oil in the evaporator -  Get the oil back to the system.
• Defective piston rings - Identify and replace

Oil in the crankcase foams

• Compressor capacity too high during start - Check and adjust
• Expansion valve gives too small a superheat - Replace the valve with correct configuration

Oil pressure too low

• Oil pressure control has interrupted - Check and adjust
• Oil charge too small - Charge some amount of oil
• Defective oil pump - Check and replace
• Choked oil strainer - Identify and clean 

7.Crankcase sweating or crank case frosting Liquid in suction line: Avoid liquid in the suction line

• Expansion valve bulb wrongly placed - Get the correct placement, refer to makers manual
• Expansion valve gives too small a superheat - Replace the valve with correct configuration
• To high a compressor capacity during start - Adjust the same.

8. Reduced cooling capacity: The cooling capacity of the plant has reduced and it is not being able to maintain the provision room or cargo hold temperature

• Inadequate refrigerant - Charge refrigerant in the circuit
• Insufficient or damaged insulation in the room - Check and renew the insulation
• Room or hold is over packed - Ensure that the room is not filled above its capacity
• Malfunctioning solenoid or Thermostatic Expansion Valve (TEV) - Check the functioning of these valves and renew if not functioning properly
• Poor thermostat location that senses cold temperatures - Place the sensor of the TEV in proper location i.e discharge of the evaporator
• Room door is kept open Ensure to close the door while exiting the provision room

Indirect system

Indirect system is usually fitted for large installations. Here the system uses “secondary refrigerant”. The secondary refrigerant is a liquid which is cooled in the refrigeration machinery room by the primary refrigerant and pumped around the ship to the cargo space.

Advantages of indirect system

• Primary refrigerant is only present in the machinery space, and the risk of loss by leakage is minimized and controlled.
• Difficulties of oil recovery from evaporators, which are located almost half the ship’s length, away from the compressor is avoided.
• The brine system is more easily controlled to give accurate temperature control.
• Greater flexibility in simultaneously carrying cargoes at different spaces at different temperatures is possible.

Air Conditioning System

The human body’s comfort is influenced by
• Surrounding air temperature
• Surrounding air humidity
• Surrounding air velocity
• Radiant heat loss or heat gain from the space boundaries.

In marine air conditioning the first two points are of great importance.

Air conditioning is a process by which the temperature, humidity and freshness of air within the space by means of heating or cooling.

Winter conditioning relates to increasing the temperature and humidity.

Summer conditioning relates to decreasing the temperature and humidity.

Comfort is a subjective matter and varies from person to person. Comfort zone is defined by the temperature and humidity of a place where “most people” feel comfortable. By “most people” is meant a very large proportion of people.

Types:

Air conditioning system can be classified into two main classes:

Individual unit systems
Individual unit systems are also called as self contained unit type.
It is an air conditioning system installed in the space it is to serve. Each room or space contains its own small self contained unit.

Central systems
It is an air conditioning where larger refrigeration machinery units are installed and their output is distributed throughout the ship by use of air ducts and blowers.

Central systems may further be subdivided in to
• Zone control system
• Double duct system
• Reheat system


Principle

Before venturing in to air conditioning you know the basic principles and related terms in air conditioning.

Air conditioning is a process by which condition of air is modified and maintained for human comfort. The condition of air such as temperature, humidity, and freshness of air is conditioned and maintained by either cooling or heating, with necessary humidification or de-humidification.

When a liquid evaporates a cooling effect is produced. Common examples of liquids providing this effect are “after shave lotion” or “eau-de-cologne” poured on to the palm of your hand. As the liquid evaporates rapidly it gives a cold sensation. This is because as the liquid evaporates, it removes the heat from the skin. Similarly a refrigerant is evaporated to achieve cooling.

The liquid refrigerant is evaporated in the evaporator in a vapor compression refrigeration cycle. Since the refrigerants are too expensive to be blown away in to atmosphere, after cooling job in the evaporator the refrigerant is collected for re-liquefaction. Air is the secondary refrigerant which flows over the evaporator coils. An electrically operated fan is used for this purpose.

This is achieved by using a compressor which sucks the low pressure refrigerant from the evaporator and delivers it as hot compressed high pressure gas to the condenser.

In the condenser the high pressure refrigerant is liquefied by circulating sea water or fresh water. The latent heat removed from the evaporator is transferred to the cooling medium.

To complete the circuit the liquid refrigerant flows from the condenser to an expansion valve. In the expansion valve the refrigerant flows from high pressure to low pressure.

The pressure drop causes the saturation temperature to drop due to which the liquid refrigerant to boils at low temperature. As a result it takes up the latent heat from the remaining liquid refrigerant causing its temperature to drop.

The refrigerant from the expansion valve enters the evaporator at a temperature lower than the secondary coolant, say air or brine, receives latent heat and evaporates. The cycle continues.

Specific humidity - is the ratio of the mass of water vapour to the mass of dry air in a given volume of mixture.

Per cent relative humidity - is the mass of water vapour per m3 of air compared to the mass of water vapour per m3 of saturated air at the same temperature. This also equals the ratio of the partial pressure of actual air compared to the partial pressure of the air if it was saturates at the same temperature. i.e.
m/mg = p/pg  ;where g refers to the saturated condition.

Partial pressure, Dalton's Laws
Barometer pressure = partial pressure of N2 + p.p.O2 + p.p.H2O,
From Dalton's Law we know that
• Pressure exerted by, and the quantity of, the vapour required to saturate a given space, at any given temperature, are the same whether that space is filled by a gas or is a vacuum.
• The pressure exerted by a mixture of a gas and a vapour, of two vapours, or of two gasses, or a number of same, is the sum of the pressure which each would exert if it occupied the same space alone, assuming no interaction of constituents.

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 vapor varies in air then the partial pressure varies, so the dew point varies.

It can be seen that cooling a superheated vapour at constant pressure will bring it to the saturated vapor line, or Dew point. It can also be seen that cooling at constant temperature raises the partial pressure until the dew point is reached.

Therefore from the above equation for determining the relative humidity,
%R.H. = m/mg x 100 = p/pg x 100
= pdew/pg point x 100
where g refers to the saturated condition. This means dry air contains the maximum moisture content (100% R.H.) at the saturation conditions.

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 greater the evaporation of water off the cloth if air is dry. The 'wet bulb' thermometer reading will be lower.

Central systems- Operation

Air conditioning incorporates heating with humidification and cooling with de-humidification. Comfortable conditions depend on the temperature and humidity, but are also sensitive to air movement, air freshness and purity.

On board the ship you have two AC compressors, condensers, liquid receivers, expansion valves. The refrigerant after passing through expansion valve flows in to the evaporator coils which are common for both the circuit. The refrigeration system is in continuous operation to maintain the temperature of accommodation.

The circuit shown above contains compressor, condenser, expansion valve and evaporator and controls for automatic operation.

Before starting


1. Check the refrigerant oil level in crank case of the compressor.
2. Check the power supply is available to the compressor motor which is to be put in use.
3. Check the coupling from the electric motor to the compressor.
4. Check the cooling water inlet and outlet to the condenser are open.
5. Check the following valves for the refrigeration circuit which is to be put in to operation are shut. They are:

•      Compressor inlet and outlet valves.
•      Condenser inlet valve and receiver outlet valve
•      Filter drier bypass, inlet and outlet valves.

During start

6. Open the following valves in the refrigeration circuit. They are:

• Compressor outlet valves.
• Condenser inlet valve and receiver outlet valve.
• Filter drier inlet and outlet valves.
• Cross check all the system valves and cooling water inlet and outlet to the condenser are open.

7. Switch on the breaker for the compressor. Let the compressor working.
8. Crack open the compressor inlet valve by half a turn and switch on the compressor. The compressor will start working.
9. Open the compressor inlet valve slowly. Let it be open fully.

Automatic operation

The circuit is very similar to refrigeration except for difference in TEV and evaporator.

The AC compressor circulates the refrigerant through the circuit. The refrigerant temperature drops after its expansion in TEV.

Fan blows air through evaporator coils. Here air is the secondary refrigerant.
Air gets cooled and is taken to the accommodation spaces.

Checks during running
• Check the condensing pressure, evaporating pressure, oil pressure drop.
• Inspect the compressor unit and check for any abnormality and vibration
• Check the discharge and suction temperatures
• Check the oil level and check the oil is returned to the crankcase by oil separator, and oil return line is warmer than the crankcase.
• Check the shaft seal for oil tightness

Capacity control
The capacity control of the compressor is adjusted manually or automatically to take care of cooling. The un-loader system may be used for capacity control by successively cutting in or cutting out cylinders. The unloading gear adjusts the total output of the compressor.

Zone control system-with single air duct
This is the most popular of the central system type because of its basic simplicity. The accommodation is divided in to zones, having different heating and cooling requirements.
Double duct system

This system is also called as twin duct system. This system has two separate ducts running from central unit to each of the air terminals. Two air streams are carried at the terminals either in summer or winter, for individual mixing.

Reheat system
In winter, the air is preheated at the central unit. The temperature is automatically controlled. The air terminals are equipped with hot water elements.

Maintenance
The planned maintenance schedule is developed based on manufacturer’s recommendations and class requirements. However, it is the responsibility of the ship owner and staff to periodically review the schedule considering the operational conditions.

The following maintenance schedule gives an overview of air conditioning system; which is normally found on a motor vessel. This is laid out to suit a particular machinery room layout. It may be modified to suit different installations.

Every engineer should follow to ensure trouble free operation of the system. The following table is a general guide and precise instructions can be obtained from individual makers.

However maintenance schedule given in the direct expansion system remains the same for primary refrigeration circuit.

Time Period Checks and routines:

Daily

*Temperature indicators and recorders:- Check all accommodation space temperatures are within limits

*Compressors:-

• Check no failure alarms
• Check for satisfactory functioning, and Check ammeter reading. Should be satisfactory
• Check the main compressor for oil level gauge readings and super-heat.

 *Fans:-

• Check no failure alarms
•  Check for satisfactory functioning, and Check ammeter reading. Should be satisfactory

*Condenser  Pumps:-

• Check the pump gland for leakages
• Check no failure alarms
• Check for satisfactory functioning, and Check ammeter reading. Should be satisfactory

*Condenser:- 

• Check liquid level in the sight glass
• Check the cooling water flow is sufficient
• Check the temperature drop of the cooling medium is correct.

*Evaporator:-

• Check for satisfactory functioning, and
• Check the temperature drop of the refrigerant medium is correct
• Three monthly Cleaning or renewal of filters

*Others:-

• Regular inspection and using 50 ppm super-chlorinated solution at possible locations of Legionella bacteria colonies such as air inlet area, below cooler, in air filters, in humidifiers
• Six monthly Lubrication of bearings
• Adjustment of belt tension
• Yearly Examination of insulation of compressor motor

Trouble shooting:

Faults may occur, while the air conditioning system is in operation, despite periodical maintenance. As a watch-keeping engineer, you should be able to identify reason and rectify the fault. You can identify the faults by carefully observing the parameters and taking necessary action to prevent failure of machinery and down time.

The following is a general guide for air handling system and precise instructions can be obtained from individual makers.

1. Bubbles in liquid sight glass

• Compressor capacity too high during starting - Check and adjust
• Refrigerant charge too small - Charge the system
• Filters in the liquid line clogged - Inspect and clean the same

2. Low refrigerant level in the receiver

• Refrigerant charge too small - Charge the system
• Leakages in the plant - Identify and rectify

3. Capacity regulator hunting

• Wrongly adjusted capacity regulator - Check and adjust the same

4. Frosting or icing on the refrigerant line

• Choke in the line preventing the flow of refrigerant - Check and clear the choke

Secondary circuit
1. Low air circulation

•  Choked air inlet filters - Renew or clean the same
• Defective fans - Replace
• Fans tripped due to overload trips - Check and reset

2. Overloading of air handling plant

• Air leakage at the access doors Identify air leakage and replace the gaskets and fasteners
• Atmospheric air mixing in large quantities Decrease fresh air mixing

Charging refrigerant in the system

• Put the drier in the system by opening the inlet and outlet of the drier valve and shut the by pass valve.
• Weigh the Freon gas cylinder before charging and after charging to ascertain the quantity of gas charged in the system.
• Collect the gas by shutting the receiver outlet in the refrigerant system. The compressor will cut off on LP trip. Check the liquid level in the sight glass.
• Connect the charging pipe to the gas cylinder.
• Connect the charging pipe to the liquid side of the system and crack open the cylinder valve. This will purge out any entrapped air. Tighten the charging connection.
• Open the charging valve and liquid valve in the cylinder. The liquid refrigerant will start flowing in to the system.
• Start the compressor and continue to charge the system. Observe the liquid level in the sight glass.
• Close the gas cylinder liquid valve and charging valve.
• Close the drier inlet and outlet valve and open the bypass valve.
• Start the system by opening the receiver outlet valve and observe its efficiency for 20 minutes. Check the liquid level in the receiver.
• If some more gas charging is required, repeat the procedure.

Purging out air from the system

• The symptom which indicates air in the system is a steady increase in condenser gauge reading.
• The accumulation of air reduces the effective area of condenser available for condensing refrigerant.
• Close the condenser outlet liquid valve and condenser will trip on LP cut off. The condenser sea water is left circulating. Note the condenser gauge reading.
• Allow few hours for equilibrium.
• If no air is present in the condenser, the condenser gauge will read exactly the same when the compressor was stopped.
• If air is present then condenser gauge reading will be higher than the previous value.
• The refrigerant will condense and collect in the receiver but air stratifies and collects on top of the refrigeration liquid. Now crack open the vent cock to purge out air from the system.

Adding oil to the system

• Oil should be replenished when oil level is below half the sight glass .
• Oil prescribed by the manufacturer should be used.
• Clean oil from sealed containers should only be used.
• Close the liquid valve at the receiver outlet and collect the refrigerant.
• The compressor will cut off on LP trip.
• Close the suction and discharge valves of the compressor
• Remove the oil filling plug slowly allowing the remaining pressure in the crankcase to be released.
• Add oil to the crankcase by piston arrangement or siphon arrangement.
• Fit the plug. Open the suction and delivery valves. Open the liquid receiver outlet valve. Start the compressor.

Testing compressor discharge valves

• Close the liquid valve in the receiver outlet and collect the refrigerant.
• The compressor will stop on LP trip.
• Shut the suction and discharge valves quickly.
• Observe the suction and discharge pressure gauges. If the discharge pressure falls roughly by 1 bar and above in five minutes and simultaneously if suction pressure rises, then discharge valve is leaking.

Testing of compressor suction valve

• Run the compressor under manual control.
• Close the suction valve slowly to prevent foaming of lubricating oil in the crankcase.
• With the suction valves shut, the compressor should develop a vacuum of 0.4 bar or more.
• This indicates the suction valves are ‘holding’ and functioning correctly.