Air Conditioning and Refrigeration

Construction and working of refrigeration system

Before going to the system, let us take a quick look at some associated terms.
(a) Sensible heat is heat exchanged by a body or thermodynamic system in which the exchange of heat changes the temperature of the body or system.
(b) Latent heat is defined as heat energy that is absorbed or released during the transition phase of a substance.
(c) The saturation temperature is the temperature for a corresponding saturation pressure at which a liquid boils into its vapour phase.

(d) The critical temperature of a substance is the temperature at and above which vapour of the substance cannot be liquefied.
(e) Subcooling temperature is the temperature below the condensing temperature.
(f) Degree of superheat is the difference between the temperature of the refrigerant vapour leaving the evaporator (measured at the expansion valve bulb) and the temperature of the saturated vapour at the evaporator pressure. saturated vapour pressure can be read on the suction pressure gauge.
(g) Tons of refrigeration is the cooling effect of one ton or 2000 lbs. of ice melting in 24 hrs.
(h) BTU or British thermal unit is the quantity of heat energy required to raise 1lb of water up to 1 ℉ under atmospheric conditions. The 1-ton machine can remove 288000 BTU/24hrs.
(i) Refrigeration effect is the amount of heat absorbed by each unit mass of refrigerant as it flows through an evaporator is known as the refrigerating effect.
(j) Refrigerating capacity is the rate at which a system will absorb heat from the refrigerated space or substance.

There are two main refrigeration systems in commercial use, the Absorption system and the Vapor compression system. Most marine refrigeration systems are of the vapour compression type. The basic components of the Vapor compression cycle are compressor, condenser, expansion valve and evaporator.
Most refrigerants have similar characteristics and properties to steam except they have a much lower boiling point. The latent heat is much more than the sensible heat and this can be seen in the Ice-water-Steam-Phase changes diagram, explained in the article at a later stage.

Refer to the system flow diagram and pressure enthalpy diagram for understanding the working principle of the system.
The compressor raising the pressure of the vaporized refrigerant, causes its saturation temperature to rise so that it is higher than that of the seawater or the air cooling the condenser. The compressor also promotes the circulation of the refrigerant by pumping it around the system.
In the condenser, the refrigerant is liquified by being subcooled below the saturation temperature relating to the compressor delivery pressure, by the circulating seawater or air. It removes both heats of compression and heat of space to be cooled.
The expansion valve controls the flow of the refrigerant from the H.P to the L.P side. So saturation temperature to fall. Some of the liquid boils off at the expansion valve, taking latent heat from the remainder and causing its temperature to drop.
The refrigerant entering the evaporator coil, at a temperature lower than that of the surrounding secondary coolant(air or brine) receives latent heat and evaporated, changes to dry saturated vapor.

1-2 Compression: Vapor is compressed which raised the temperature and pressure. The condition of the refrigerant at entry should be either 'dry' or superheated so that liquid does not enter the compressor and damage the internal components. Compressor discharge is always superheated.
2-3 Condensing: The condenser cools and condenses the vapour so that liquid is produced at the outlet. The condenser normally undercools the liquid. Note that the discharge pressure of the compressor is dictated by the coolant temperature, so that if the temperature rises, so does the compressor discharge temperature.
3-4 Expansion: Expansion reduces the pressure of the liquid, and thus cools the liquid. The flow through the expansion valve is regulated by the temperature of the refrigerant leaving the evaporator. Thus as the temperature rises then the valve will allow more cold refrigerant to flow into the evaporator.
4-1 Evaporation: Heat transfer takes place between the cold room and the cold refrigerant. Thus the temperature of the room will fall, and the refrigerant will absorb latent heat, which will change its state into a gas.

Working Diagram:

Starting from the compressor the gas refrigerant is allowed to pass through the oil separator. An HP cutout switch is provided at the compressor outlet. From the Oil separator, the gas gets condensed in the condenser and collected in the receiver. The liquid refrigerant the pass-through filter and drier. Stop valve(5) which allow the liquid to pass when cooling is required in the respective room. Then the liquid passes through TEV(2) and starts changing its phase. Superheated gas is obtained at the evaporator outlet. This gas which is having a low pressure enters the compressor. LP cutout switch is provided at the inlet of the compressor. A back pressure valve(10) is fitted at the evaporator outlet of the veg room. The charging connection is at the receiver outlet.

Refrigerant circuit components

1. Filters (Scale traps):
Compressors are protected by fine metal gauge filters from dirt entering with the suction gas. When first running a newly erected plant. It is usual to fit felt socks inside these gauges to catch the inertial dirt in the pipe system. If steel pipes have been welded in situ for the gas system some internal scale is unavoidable.
Fine liquid filters are fitted to protect the small orifices in expansion valves. Any clogging of one of these reveals itself by cooling of 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.

2. Oil separator:
A refrigerant plant is fitted with one or more oil separators to prevent the capacity from being reduced by oil which has been carried over with the refrigerant. The separator is fitted between the compressor and condenser. The oil separator consists of a closed cylinder fitted with a refrigerant inlet and outlet, an oil drain, stop valve an automatic oil return valve with float and a deflector plate. The high-pressure gas enters the separator and impinges on the deflector plate, causing the heavier oil droplets to be deposited on it.

The gas diffuses outwards in a radial direction, but it is prevented from rising directly upwards by a baffle plate. The remainder of the oil still presents in the gas is deposited upon the separator wall by the centrifugal effect of gas flow. The separated oil is get collected at bottom of the separator where a float controlled return valve is fitted. When the oil level exceeds the preset level, the valve opens and the oil is returned under pressure to the crankcase. The valve automatically shuts off when the oil level has fallen sufficiently, The hand-operated oil drain is also fitted to drain the surplus oil during a running-in period of a new compressor. The heavily polluted oil must not return to the crankcase.

3. Driers/Dehydrators:
It is important to keep the traces of moisture out of the refrigerant system otherwise it will form ice at the expansion valve. The reaction between the water and freon will produce acids that attack the system causes corrosion of steel parts, the products of which will also block the expansion valve. A transference of copper from copper pipes onto steel parts of compressors causing a copper plate film and forming sludge in lubricating oil. Driers are used in the liquid line from the condenser to remove any water accidentally admitted in the system. In large plants, the drier is always in the circuit, a bypass being provided to allow the plant to continue running when the drier is being recharged/renewed.

In small plants, driers are fitted temporarily after any repair have been made likely to have allowed moisture into the system. The commonly used drying agents used are silica gel and activated alumina, Both of which can be reactivated by heating to 140 deg cel. for a number of hrs.

4. Liquid Receivers:
It is usual to provide a welded steel pressure vessel into which liquid refrigerant from the condenser can flow. This receiver is of sufficient capacity for the 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 is only used during overhaul. A level glass if 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 pressure, a safety valve must be fitted.

4. Liquid stop valve:
An electrically operated liquid stop valve is used to cut out the liquid refrigerant when the temperature of the cold chamber has reached its lower limit or to cut in the refrigerant when the temperature has risen to its higher limit. The control of the valve is by the thermostat which is exposed to the cold chamber.

The stop valve consists of a solenoid, which when energised, releases the metal striker which springs upwards in the vertical tube, and making contact with a collar at the upper end of the valve, lifts the valve off its seat. So liquid refrigerant enters and passes out to the thermostatic expansion valve. When the cold chamber falls to its lower temperature limit, the thermostat will cut off the current from the solenoid, allowing the valve to drop with striker and shut off the supply of liquid refrigerant to the cold chamber.

5. Evaporators:
In the direct expansion system, evaporator takes place in air coolers, consisting of pipe grids, plained or finned, enclosed in a closely fitted casing, thorough which air from the holds or chamber is circulated by forced or induced draught fans.
The refrigerant flow is controlled at the expansion valve in such a way that, as it passes through the grids, it is completely vaporized and slightly superheated.

6. Desiccant:
It is used to remove moisture from the refrigeration system and also to prevent Ester oil from hydrates. Synthetic zeolite (molecular sieve etc) is an effective desiccant to use for extracting water from the refrigerant.

A molecule sieve that has a pore diameter that is smaller than the size of the refrigerant molecule is preferable so that it removes only the water molecules selectively.

7. Condenser:
Condensers are generally of “Shell and Tube” type design for high pressure. The coolant (seawater) passes through the tubes with refrigerant condensing on the outside. The shell is of welded construction and made from MS plating. Suitable bases for purging, drain plugs, relief valves, gas inlet and liquid outlet are welded to the shell. The end covers are of cast iron and fitted with a sacrificial anode. Since it is two flow types, both end covers have division plates and the circulating water inlet and outlet branches are at the one end.
The tubes plates are of mild steel clad in stainless steel and are welded circumferentially to the condenser shell. The tubes are of aluminium brass and are expended into the tube plates at both ends. The only allowance after the expansion of the tube is in the ductility of the metal of the shell and the tube plates. To keep the heat transfer coefficient in the condenser up and thereby keep the size of the condenser down it is usual to design for a water velocity of 2 m/s, after that erosion arises.

8. Evaporators:
When the brine is used as a secondary refrigerant, the evaporator may be of the shell and tube type. In a shell and tube evaporator, the area of the tube surface in contact with the liquid refrigerant determines its performance. The brine to be cooled may be circulated to the tubes with the refrigerant being on the outer side of them. This involved either a high liquid level in the shell or the plating of the tubes in the lower part of the shell only, the upper part then forming a vapour chamber.

9. Compressor shaft seal:
A compressor shaft seal is an important part of the machine in preventing the escape of gas and oil to the surrounding atmosphere. A double metallic below is fixed to the flange at one end and a rubbing face at the other. The flange makes a joint on the gland by means of a compressed asbestos fibre ring. The cup ring face of which is oil hardened and polished to a mirror finish to run against the rubbing face. inside the cup-ring is fitted a rubber composition packing ring, known as Duprene, which make the joint between the coupling and the distance piece, which is pressed against the shoulder on the crankshaft.

The cup ring, Duprene ring and the distance piece rotates with the shaft. While the helical spring fitted on the outside of the bellows, maintain the pressure between the rubbing faces. The gland is cooled and lubricated by oil delivered directly from the oil pump to the housing. From the gland housing, the oil flows below passage, drilled in the crankshaft to supply the compressor main bearing.

10. Automatic temperature control:
It is similar to LP cut out. The automatic temperature control is a two-way switch operated by temperature. This thermostat controls the opening and closing of the electric liquid stop valve. The bellow is connected by a small-bore tube to the bulb which is placed at the evaporator outlet. The bellows tubes and bulbs form a hermetically sealed system containing a gas, normally the same as used in the circuit.

11. Oil pressure cutout:
Oil pressure cutouts are differential pressure devices. As the crankcase pressure is not atmospheric and oil pressure is determined relative to crankcase pressure.

12. High pressure cut out:
It is an automatic cutout that is designed to break the electric circuit and to stop the machine. If from any cause an undue pressure is set up in the compressor discharge line such as failure of the condenser cooling water supply. The operation of the cutout is such that when excessive-high pressure is caused, the corresponding pressure in the bellow moves guide washer. The upper end of the catch is spring-loaded and supports the switch arm.

When the catch is moved by the guide washer the switch on falls, the slots prevent the return of the catch to its original position and thus electric circuit breaks. To start the machine again it should be reset.

13. Low pressure cut out:
A low pressure cut out is connected to the suction side of the compressor and breaks the electric circuit to the driving motor when the pressure reaches a predetermined low level it also “cut in” the motor on the pressure rising above the lower limit. It consists of a switch operated by a bellow unit connected by a small bore tubing to the compressor suction. Attached to the bellow is a spindle which bears against Lever.

This Lever is pivoted at one end while the other end moves a rocker arm, which in turn operates a switchblade to make or break with a fixed contact arm. In the event of the pressure in the below falling below the pressure exerted by the spring, the lever will move downward, causing the rocker arm to move the switchblade away from the fixed contact arm thus breaking the electrical circuit to the compressor motor. When the pressure in the suction line rises the lever will then be realised and a small spring will cause the arm to follow the movement and allow the switchblade to make contact with the arm and complete the circuit and start the compressor motor. The cut-out pressure can be raised or lowered as necessary by adjusting the spring.

14. Valve lifting mechanism unloader:
This mechanism can be subdivided into hydraulic and mechanical components. The hydraulic assembly consists of a housing in which is fitted a piston operated by oil pressure.

The mechanical assembly consists of a pressure ring to which are attached two activating rods on the underside. Six valve lifting pins are located around the upper face of the operating ring and the whole assembly is free to move in a vertical direction. The actuating rods pass through a supporting ring which is rigidly held in a groove around the outside of the cylinder liner. Pressure Springs are fitted around each activating rod, tending to force the operating ring in an upward direction.

To the lower end of the actuating rods is attached a semicircular Lever, the fulcrum of which is fixed to the supporting ring. The movement of this Lever is controlled by the piston stem of the hydraulic assembly bearing against the upper position thus lifting the suction valve ring. When the hydraulic piston is moved towards the cylinder liner by the high-pressure oil it bears against the lever causing the actuating rod to be drawn down against the spring-load. This causes the valve lifting pin to retract. This enables the suction valve ring to descend on its seat thereby putting the cylinder on load.

Suction Port block type unloader(Loading state)

Suction Port Block type unloader (Unloading state)

Another type of unloader is the Suction valve open type. High-pressure gas is used to keep the suction valve in the open condition so that the compression in the unit will not take place. The gas is controlled with a solenoid valve.

15. Cylinder heads with cooling water jacket if compression temperature is higher than 120 degrees Celsius

16. High-pressure safety valve:
This valve is a spring-loaded relief valve fitted between delivery and suction manifolds. It is set at a pressure of 17 kg per metre square.

17. Thermostatic Expansion valve:
The purpose of a Thermostatic Expansion Valve(TEV) is to pass the refrigerant from the HP side to the LP side while regulating the flow.
Details of TEV is explained under a separate heading "Thermostatic Expansion valve".

Thermostatic Expansion valve

The figure shows the TEV through which the refrigerant passes from the HP to the LP side of the system.

Liquid refrigerant as it passes through valve seat restriction dropped in pressure causes the evaporating temperature of the refrigerant to fall below that of the evaporator.

The flow of the liquid through the valve seat is controlled by a needle valve which is actuated by a push rod linking with the diaphragm against spring pressure. The underside of the diaphragm is a direct influence of the evaporating pressure. The top side of the diaphragm is a sealed unit incorporating a length of the capillary tube with a bulb heaving the same refrigerant. The thermal bulb is clipped on the outlet pipe of the evaporator. So the pressure above the diaphragm in relation to temperature in the compressor suction while under it is the evaporating pressure with the corresponding temperature.
If the pressure above and below the diaphragm equalises, no superheat is obtained. This is overcome by means of a bias spring pressure equivalent to superheated temperature 5-6 deg (approx). The expansion valve is not passing enough refrigerant. The temperature of the suction pipe will be unduly high. The corresponding pressure above the bulb increases tends to open the valve. If excess pressure drop in the low-pressure side between the expansion valve and feeler bulb resulting excessive superheat. An excess superheat reduces the performance of the plant, explained by the pressure enthalpy curve and coefficient of performance. To counteract this an external equalizer connection is fitted from the suction line (between bulb & compressor).
Ideally the gas should leave the evaporator with 5° or 6°C of superheat. This ensures that the refrigerant is being used efficiently and that no liquid reaches the compressor. A starved condition in the evaporator will result in a greater superheat which through expansion of the liquid in the bulb and capillary, will cause the-valve to open further and increase the flow of refrigerant. A flooded evaporator will result in lower superheat and the valve will decrease the flow of refrigerant by Closing in as pressure on the top of the bellows reduces. Saturation temperature is related to pressure but the addition of superheat to a gas or vapour occurs after the latent heat transaction has ended. The actual pressure at the end of an evaporator coil is produced inside the bellows by the equalizing line and this is in effect more than balanced by the pressure in the bulb and capillary acting on the outside of the bellows. The greater pressure on the outside of the bellows is the result of saturation temperature plus superheat. The additional pressure on the outside of the bellows resulting from superheat, overcomes the spring loading which tends to close the valve.
In the smaller plants, there is an internal equalizer connection fitted. Here the pressure of refrigerant leaving the TEV acts underside the diaphragm. This connection is used where there is no pressure difference between the TEV outlet and evaporator outlet that means evaporation has taken place at constant pressure which is an ideal case. To adjust the expansion valve cap is removed and the spindle is turned in an anticlockwise direction (locking end of the spindle) to pass more liquid and vice versa. After adjustment cap should be replaced tightly to avoid the possibility of leakage.

Functions of the Thermostatic Expansion Valve:
The thermostatic expansion valve performs the following functions:
(1) Reduce the pressure of the refrigerant: The first and 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 remains 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 to this the temperature of the refrigerant also drops down suddenly and it produces a 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 a 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.

Refrigeration plant Troubleshooting & Maintenance

Troubleshooting
	Trouble	Cause
1	Delivery pressure is too high. Check the pressure gauge. (H.P cut out activated) Temp on Del pressure gauge>C.W Outlet by 6deg.	Air in the system. Condenser tubes blocked or dirty, partition wall is corroded. Cooling water temperature is very high or water flow is short. Too much refrigerant in the condenser, so the heat exchanging area is reduced. Delivery stop valve not open wide enough. Gas suction pressure is too high. (During starting up period only or otherwise)
2	Delivery pressure is too low. Check the pressure gauge.	Condenser water temperature is very low or water flow rate is excessive. Liquid refrigerant returns. Clearance between piston and cylinder is large. Suction stop valve is not open wide enough. Leaking HP safety valve. Suction pressure, lower than normal.
3	Suction pressure is too high. Check the pressure gauge.	Suction or delivery valve defective/leaks. Leaking HP safety valve. Capacity of the compressor is insufficient/ defective. Opening of the expansion valve is excessive. Degree of superheat is low. Excessive leakage past piston.
4	Suction pressure is too low. Check the pressure gauge. (L.P cut out activated).	Suction stop valve is not open wide enough. Liquid refrigerant flow rate is limited.(Dryer filter is clogged). Less refrigerant in the system. Refrigerant suction strainer choked. Expansion valve is clogged with ice/oil particles. Opening of the expansion valve is small. Degree of superheat is high. Capacity of the evaporator coil is lowered. (Check fan, dust deposit on cooling coil, frost). The refrigerant leaks out of the feeler bulb of the expansion valve.
5	Refrigeration efficiency is insufficient.	Refer Trouble (1), (3), (4)
6	Dew or frost gathers on the crankcase or liquid stroke.	The expansion valve is opened excessively. The charged refrigerant is excessive. Capacity control out of order.
7	Warm suction side of H.P safety valve.	Leaking safety valve.
8	The cylinder head cover is overheated.	Discharge pressure is high. Suction gas temperature is very high. The Discharge valve leaks.
9	It smells Odor.	The refrigerant leaks in a large volume. One of electric devices are overheated. The belt is overheated.
10	Abnormal noise is heard.	Anchor bolts/belt or pulley are loosened. Liquid stroke cases knocking or oil stroke. Suction/Discharge valve, piston pin, con. Rod brg etc. are damaged or worn. The refrigerant is throttled by the condenser inlet stop valve.
11	Oil pressure is too high. (Running up period oil pressure is always high) Check the pressure gauge.	Oil pressure regulator defective or maladjusted.
12	Oil pressure is too low. Check the pressure gauge.	Oil pressure regulator defective or maladjusted. Insufficient oil in the crankcase. Oil suction and/or delivery strainer blocked. Slack bearings. Blocking of oil ways in the crankshaft. Gas suction pressure is too low.
13	The consumption is too high.	Worn piston rings. Oil separator automatic oil return valve defective. The liquid refrigerant returns.
14	The compressor does not start	Low voltage, overload relay, electric wiring. Contract points of the pressure switch/thermostat are open. Condenser water pump/evaporator fan not running. HP cut out, oil pressure cut out switch activated, not reseted.

(a) Sketch and describe high-pressure cut-out in a refrigeration system.

when there is an overpressure on the condenser side of the compressor, the high-pressure cut-out will cause the compressor to shut down. The device is required to be reset by hand which will allow restarting the compressor.
The bellows in the cut-out are connected by a small-bore pipe between the compressor discharge and the condenser. The bellows tend to be expanded by the pressure and this movement is opposed by the spring. The adjustment screw is used to set the spring pressure.
During normal running, the switch arm is held by the switch arm catch and holds the electrical circuit in place. Excessive pressure expands the bellows and moves the switch arm catch around its pivot. The upper end slips to the right of the step and releases the switch arm so breaking the electrical contact and causing the compressor to cut out. The machine cannot be restarted until the trouble has been remedied and the switch reset by hand.

(b) The refrigeration compressor has stopped due to the operation of the h.p cut-out.
(i) The possible cause:-
The H.P cut-out shall be activated if the delivery pressure is too high, as the H.P cut-out is fitted in the delivery line. The delivery pressure should be confirmed at the delivery pressure gauge.
Air in the system:
condenser tubes dirty impaired cooling or partition wall corroded.
cooling water temperature is too high or water flow is short.
too much refrigerant in the system so the heat exchanging area is reduced.
the delivery stop valve is not open wide enough.
gas suction pressure too high (this may be during start-up only)
(ii) finding causes and possible remedies.
-frosting can be handled by defrosting
-low refrigerant if found, then recharge of same to be carried out.
-if found trip settings are incorrect then check and adjust them all.
-too much cooling water in condenser if flowing, then adjust the same.
-air found in the condenser then needs to be purged.
-if TEV is found partially blocked that the filters are to be cleaned.
- check for leaking solenoid valve and overhaul the same.
-check for discharge valve malfunctioning and overhaul the same.

(c) What steps are taken if the compressor “short-cycle” on low-pressure cut-out? causes of LP cut-out and actions.
the suction stop valve is not open wide enough.
The liquid refrigerant flow rate is limited. The dryer filter is clogged. less refrigerant in the system.
suction strainer choked.
the expansion valve is clogged with ice/oil particles.
The opening of the expansion valve is small. degree of super-heat is high.
the capacity of the evaporator coil is lowed. (check the fan, duct deposit on cooling coil, frosted coil).
the refrigerant leaks out of the feeler bulb of the expansion valve.
Regular leak tests and monitoring of parameters is important in keeping the plant running efficiently. Unless the charge is right, it will work poorly. When there isn't enough refrigerant, high and low sides pressures drop, which drops the system capacity. Over-charge will create excessive-high pressures which also results in a drop in cooling efficiency. Before charging, it is essential to locate and fix all leakages. No matter how well the piping joints are made or how good the quality of materials used, expect to find leaks. The only way to ensure good performance is to stop those leaks first, which can cause endless headaches to the operating engineer. To check for leaks, usually, Nitrogen is charged in the system, and left to stand. Monitor the pressure gauge, which should remain steady, if the system is tight. When it is confirmed that there are no leaks, charge the refrigerant from the gas bottle, which is usually weighed to give a precise quantity of charge, which depends on the size of the system. The first step in charging is to weigh the refrigerant and connect it to the charging valve on the liquid line, using a good quality flexible tube that does not kink. Crack open the bottle valve to purge out air from the line.
Open compressor discharge and suction valves, condenser hot gas and liquid valves and liquid solenoid valves. Then open the charging valve. After charging is complete, close the liquid valve and check the scale for the weight of gas-charged. The refrigerant level should show in the receiver glass. Keep cooling water circulation going in the condenser, to assist in collecting gas.

Routine maintenance The best way to avoid costly and annoying shut-downs is to test all the safeties are working, which will prevent damage to the machine. Adjust the low-pressure switch so that it starts the compressor whenever the suction pressure rises above the desired setting. (Check this from the manual). Adjust the differential or cut-out points as low as possible, without having short-cycling. Check the high-pressure cut-off; which guards the system against cooling water failure. It is important to try out the interlocks — the compressor should be `locked out' if the lube oil pressure is lower than the set value.
Thermostatic expansion valve This is probably the single most important component in the system, without which it would be impossible to carry out efficient cooling. Check the thermostatic expansion valve for the correct amount of superheat. Put a thermometer on the suction line near the remote bulb of the thermostatic valve. Read the refrigerant temperature at a point as near the remote bulb as possible, but on the compressor side of the bulb. Read the suction pressure from the gauge. The difference between the thermometer reading on the suction line and the temperature calculated from the suction pressure gives the superheat. The main job of the thermostatic expansion valve is to control the flow of refrigerant to the evaporator. If too little liquid enters your evaporator, it quickly flashes to a gas without absorbing much heat. When too much liquid enters the coil, not all of it is vaporized, with the result that liquid floods back to the compressor suction.

Either way, you are in trouble. With insufficient liquid, the cooling capacity falls off, while too much liquid returning can wreck your compressor valves and pistons, because the clearances are small, and liquid is incompressible. The superheat of the gas leaving the evaporator ensures that there is no liquid return. To control this, there is a thermal bulb placed at the exit, whose pressure is affected by the temperature of the gas leaving the evaporator. In effect, this is a means of sensing the outlet, which is used to control the inlet, i.e. the setting of the thermostatic expansion valve from the bulb. Ideally, the liquid entering the evaporator coil should completely flash off to gas with a slight degree of superheat. At some point in the coil, say A, saturated gas starts absorbing heat to become superheated at the point of exit. When the load falls off, there isn't enough heat absorbed between point A and the bulb to get the required degree of superheat — so the bulb cools, closing the expansion valve (due to the pressure difference) and reduces the liquid flow proportionately. In this way, the required degree of superheat is maintained, and liquid is prevented from entering the compressor. Too high a superheat means that your cooling capacity is reduced (since more of the coil is used for superheating), while too low a superheat could lead to liquid flooding back to the compressor. The correct amount is specified on the valve and in the manual. Large pressure drop through the system low side cuts compressor capacity and can also reduce the expansion valve capacity. The pressure from the thermal bulb regulates the valve opening by acting against spring and evaporator pressure.
With large pressure drops through the evaporator coil, the saturation temperature of the gas near the outlet is lower (higher drop in pressure). With such low saturation temperatures, more superheating would be required to hold the expansion valve open, which would result in a drop in capacity. One way of avoiding this is through an external equalizer connection so that the expansion valve is not affected by the pressure drop, and so responds only to the superheat of the gas. This is connecting the underside of the diaphragm to the evaporator outlet. The exact location of the equalizer depends on the layout but is usually at the point of greatest pressure drop in the coil.

Maintenance planning
In carrying out routine maintenance, ensure that you follow the PMS on board. Given below are only general guidelines :
Every three months :

Check for leakages.
Grease bearings, if required.
Check safety cut-outs.
Check belt tension.

Every six months :

Check all couplings.
Check the oil filter and oil of the compressor.
Clean strainers.
Check dryers.

Every twelve months :

Renew lube oil and filter.
Examine all valves.
Calibrate thermometers and gauges.
Weigh spare gas bottles.
Examine fans.
Recharge dryers.

Every twenty-four months :

Over-haul reciprocating compressor (screw type run for longer periods).
Pistons, bearings and shaft to be examined.
Coolers to be checked and cleaned.
Pressure testing of heat exchangers.

Air Conditioning.
Air conditioning consists of maintaining the room temperature within the Comfort zone. Most package units range in capacity from 3 to 15 tones. The Condensers of larger units are usually sea water-cooled, while the Compressor is air-cooled, and is a hermetically sealed unit, running on 380/400V three-phase A.C.
The thermostat, with its sensing bulb fixed on the air entry side, serves to cut in or cut out the Compressor, to maintain the temperature within the desired limits. High/Low-pressure switches, overload relays and water flow switches form the 'protective circuit', while the hermetically sealed compressor has a winding protection thermostat embedded inside. All controls are connected in series with the hold-on coil of the Starter. The circuit is interlocked with the Evaporator fan motor Starter, to ensure that the Fan is running before the Compressor cuts in.

Air Conditioning problems
Trouble-shooting of problems :
1. Increase in Discharge pressure.

This could be due to clogging of the cooling coils (if air-cooled) or fouling of the sea water side (water-cooled).
Scaling will decrease the heat transfer rate. The remedy is cleaning.

2. Reduction in Suction pressure.

Fouling of the Evaporator surface due to dirt leads to an increased pressure drop. This combined with fouling of the filter can lead to a drop in the suction pressure.
Erratic operation/maladjustment of the Expansion valve can also cause a drop in the suction pressure. Reduction of air quantity over the Evaporator coil, due to choked inlet filter especially in a dusty atmosphere, can be a major cause of the drop in the suction pressure.
Shortage of refrigerant leads to starving or drop in the suction pressure. This also causes overheating of the compressor crankcase, cylinder, valves and cylinder head. The discharge valve manifold will be excessively hot.
Shortage of refrigerants can also cause an oil return problem.
In the case of hermetically sealed compressors, where the refrigerant is used to cool the winding, shortage of refrigerant can lead to overheating of the windings, break-down of insulation or even motor burn-out.
Low pressures at suction also lead to possible air ingress, with further deterioration of the performance.

3. Suction vapour superheating.
Excess superheat of the refrigerant at the suction of the compressor is undesirable, as it can cause overheating of the compressor. This could be due to a malfunctioning TEV. The bad effects of overheating have already been explained under the shortage of refrigerant.

4. Compressor burn-out.
Voltage fluctuation over ± 10 % is not permissible, as the windings are not designed for excessive voltages.
The low refrigerant charge can cause overheating and burn-out, as already explained. Quality of oil — oil with higher moisture content and acidity, or having insufficient dielectric strength can cause burn-out.

Container refrigeration

(A) sketch of ship's indirect refrigeration arranged for cooling container stowed in stacks in the hold.

(B). Description of the refrigeration system sketched above:-Special porthole container ships are equipped with refrigeration units permanently installed below deck which supply the containers with the cold air they require. Originally, individual refrigeration ducts were used to supply cold air to up to 48 refrigerated containers and were arranged horizontally and vertically. Subsequently, only a vertical arrangement was used. This was better suited to onboard handling, enabling a clear division of containers and temperatures and producing smaller batches of cargo.

Working principle of indirect refrigeration arrangement:-

In cooling systems permanently installed on the ship, heat is dissipated from the porthole containers via fixed ducts in the cooling system. The vertical cooling ducts are divided into two channels (supply air duct and return air duct).
The supply air duct is connected to the lower aperture in the end wall of the container via switchable couplings. The cold air flows through this duct into the container below the grating, through the cargo and then back over the cargo through the upper opening via the coupling to the return air duct.
The heated air is drawn off from the return air channel by the refrigeration duct fan and then conveyed back into the supply air channel by the fan. The figure shows an indirect system with an intermediate brine circuit that connects the cooling system which is located centrally in the engine room to the cold air system in the hold. The supply air temperature is here controlled by adjusting the three-way valve in the brine circuit.
Since the installed refrigeration units are relatively large, porthole containers may thus achieve a higher level of efficiency than integral containers.
The arrangement of the refrigeration ducts with the couplings in the hold of a container ship on which up to seven refrigerated containers can be stacked and connected to a single refrigeration duct.
Sometimes, porthole containers are also transported on deck. If this is done, a "clip-on unit" is attached to the container which supplies it with cold air. Clip-on units are also used to refrigerate the containers at terminals or when transporting them by truck.
Working principle of direct refrigeration arrangement:-
In contrast to portable containers, integral refrigerated containers are equipped with their own refrigeration unit. This normally relies on a three-phase electrical power supply. Cold air flows through and around the goods in the container. This air is blown in through the gratings in the floor and then drawn off again below the container ceiling. The circulating fans then force the air through the air cooler, which also acts as the evaporator in the cold circuit, and back through the gratings into the cargo. Integral containers are dominating the porthole type containers. Thus the direct cooling system is dominating over indirect cooling. Direct cool containers provide flexibility while loading cargo on board while indirectly cooled containers are required in load in a manner to keep similar cargo type-together.

Banana transportation

Recommended transport conditions:
• Desired transit temperature: 56° to 58°F (13° to 14°C)
• Desired relative humidity: 90 to 95 %.
• Highest freezing point: 30.6°F (-0.8°C)

Bananas are shipped green and usually ripened at the destination, although some in-transit ripening is also done. They are very temperature sensitive; lower than desired temperatures will cause chilling injury, and higher than desired temperatures may cause rapid and improper ripening or ‘cooking’. Proper air circulation is required to maintain uniform temperatures throughout the load since fluctuating temperatures are detrimental. Provide a fresh air vent at a minimum to prevent ethylene gas buildup inside the container or trailer or use an ethylene scrubber. Ethylene is produced by bananas and will cause premature ripening. Also, do not ship bananas with other cargo that is not temperature compatible or that produces high amounts of ethylene.

Recommended loading methods:
• Fiberboard cartons — Nearly all bananas are packed in heavy-duty, plastic film-lined fiberboard cartons at the country of origin. The gross weight of the cartons is 40 pounds (18 kg). The cartons are usually palletized and secured by glue, straps, or netting. They then are transported under carefully controlled temperature and humidity conditions in refrigerated marine containers or, less commonly, on break-bulk ships.

Since bananas are easily bruised, do not throw or drop the cartons during handling. Place the cartons on their bottoms, and do not invert or stack them on their sides. Use airflow or centre loading with spacer blocks for pallets in trailers; block stow individual cartons and use 9-11 loading of pallets in marine containers, being sure to use spacer blocks in units with flat walls. If not palletized, block stow the cartons crosswise or lengthwise and stack tightly together to get a dense load. In extremely cold weather, transfer the cartons onto floor racks or pallets in vehicles without deep T-rail floors to prevent freezing or chilling injury.

Transport reefer units are equipped with microprocessors that are interfaced with temperature sensors controlling the supply air and return air temperatures that are located inside the reefer unit (not inside the insulated cargo box). Supply and return air temperatures and other parameters are generally controlled and measured from inside the refrigeration units and not inside the insulated cargo box. These sensors can control the conditioned air as it is delivered to the refrigerated cargo space in the insulated box and returns back to the reefer unit to be re-cooled (or heated) and then re-circulated back to the cargo space. These sensors do not control the micro-environments that develop inside the cargo compartments of insulated boxes.

A Controlled Atmosphere reefer technology is used for sensitive fruits and vegetables, because for the following reasons:
• After harvesting, fruits and vegetables consume oxygen and release carbon dioxide& ethylene gas
• Release of moisture and heat generation provide specific challenges
• Process of respiration results in deterioration of the product
• Fruits and vegetables are still “alive” after harvesting
• In a standard reefer container such products start to degrade and deteriorate immediately
• Fruits and vegetables require an adjusted environment to preserve their condition
• Atmosphere control slows down the ripening process and increases the shelf life of perishable products
The technologies used especially for live cargo represent the following specific advantages to prolong shelf life
• Slow down of ripening process and reduce risk of decay
• Decrease of dehydration (no weight loss)
• Extension of market range from harvest to consumption
• Maintenance of freshness, colour and condition to maintain product quality
• Prevention/minimization of wastage and financial losses
• Ocean transport to become a viable alternative to airfreight
For banana or similar live cargo transportation containers with special fittings are being used for example Liventus and Maxtend fitted containers. They are a special supplement for reefer containers. They create a tailor-made atmosphere via pre-trip gas injections. An additional controller is monitoring & recording the atmospheric composition inside the reefer. Such “Controlled Atmosphere” containers are ideal for low respiring cargo.
• Pre-injection gas mixture (N2 and CO2) the system can modify and preserve the desired atmosphere.
• Equipped with oxygen (O2) and carbon dioxide (CO2) sensors.
• Equipped with automatic ventilation to let in the fresh air, when O2 levels drop below-set point.
• Ethylene scrubbers are available for sensitive cargoes.
Air Exchange Management is an automated ventilation system designed to regulate the oxygen and carbon dioxide level inside the container via the fresh air vents. This technology takes advantage of the natural respiration processes of the fruit or vegetables and therefore does not require gas injections.

A refrigeration cooling system is basically comprised of a refrigeration compressor, an evaporator, a condenser, an expansion valve, piping, and refrigerant. The refrigeration system operates on the principle of the vapour-compression cycle. This refers to the physical principle that all substances must gain energy (heat) to change from solid to liquid and liquid to gas phases, and release energy (heat) when changing phase in the opposite direction. The temperature at which phase changes occur are characteristic of the substance and vary according to pressure – increasing the pressure increases the temperature at which the phase change occurs. Refrigerants are chosen based on their high heat capacity and ability to change phase between liquid and gas at the desired temperature.
Mechanical refrigeration units are rated according to their ability to remove or produce heat. The cooling capacity of a unit is expressed in the number of Btu’s per hour a unit can remove. Today’s refrigeration units are equipped with microprocessors programmed to control the operation of the unit so that both refrigeration and fuel efficiency is maximized. Air temperatures are monitored at the discharge and return locations and adjusted to demand refrigeration at the thermostat set-point.
This reduces temperature spread around the thermostat set-point, which reduces dehydration and maintains product quality. The microprocessors also can be programmed to provide diagnostic tests and automatically run through a ‘Pre-Trip’ mode. Some of the microprocessors are radio-equipped and may be contacted via satellite to monitor the performance of the refrigeration unit, pinpoint the geographic location of the trailer, monitor product temperatures, and perform other functions.

Refrigerated Containers

A refrigerated container, also known as an integral reefer container, is an intermodal shipping container that is used for the transportation of temperature-sensitive cargoes. The container’s reefer unit is located inside the cargo compartment Refrigerated containers can be transported over land and sea by trains, trucks, and vessels.
The three main components of a refrigerated container are the refrigeration system, the microprocessor controller, and the air circulation system (chilled or heated air system). A reefer container is a metal box with polyurethane insulation and an attached electric-powered cooling (refrigeration) unit. Reefer containers are equipped with a microprocessor/data logger function that controls the operation of the reefer machinery, documents the pre-trips (system maintenance and diagnostic checks), and records temperatures and various events during transit.
Reefer containers are equipped with a bottom-air delivery system (also known as reverse airflow). The conditioned air is delivered from the reefer unit located at the front of the reefer container, along the floor, through the ‘T’ floor channels, and up vertically through the cargo so that the air is driven around and through the load to maintain product temperatures. This type of airflow system pressurizes the ‘T’ floor with conditioned air when the cargo is properly stowed in the container. The pressurized air is forced a short distance up and through the cargo vertically from the ‘T’ floor to the top of the load and back to the reefer unit. A bulkhead (plenum or false wall) at the front of the container directs the discharge air out at floor level and allows the return air to flow back to the reefer only at ceiling level.
Reefer containers have an adjustable fresh air exchange opening that is set to avoid potentially injurious depletion of oxygen (O2) or build-up of carbon dioxide (CO2) and ethylene (C2H4) gases. The fresh air exchange is usually set manually, but automated systems are also available that are controlled by the refrigeration system microprocessor. Automated fresh air exchange systems can keep the fresh air exchange closed as much as possible to improve temperature pull-down and reduce fuel usage, maintain a beneficial atmosphere of O2 and CO2, or avoid infiltration of freezing or chilling outside air that may damage the products being carried.
The reefer unit is self-contained, but it does require a power source. The operation of integral reefer units relies on electrical power sources and is connected to an electrical supply through the container’s electrical plug. The electrical power supply must be either 380 volts/50 hertz or 440 volts/60 hertz and the power cables must have standard ISO plugs.
Transport temperature management
Temperature management plays the most significant role in extending the market life of perishable foods. Bringing the product to its desired carrying temperature as quickly as possible and maintaining uniform temperatures is paramount for domestic and global distribution. The practice of maintaining optimum temperatures throughout the perishables distribution system without any breaks is often referred to as ‘maintaining the cool chain.’ Refrigeration units for containers, trailers, and railcars are equipped with computerized controls and recording capabilities. The computer or microprocessor is sometimes referred to as a Data Management System (DMS) or a data logger. Computerized refrigeration units offer significant benefits to transportation companies, drivers, shippers, and receivers. To wit, the microprocessor controls, tracks, and records operations of the refrigeration system, including, in part, the temperature setpoint, return air sensing, discharge air sensing, operating modes, safety alarms, and probe sensing. Temperatures can be recorded in either degrees Celsius (°C) or Fahrenheit (°F). The microprocessor also performs pre-trips and diagnostics of the refrigeration unit and records operational events and alarm codes. The refrigeration unit’s computer system offers various levels of guarded access, thereby protecting the refrigeration unit from tampering and unwelcome changes. Trip data can be retrieved (but not erased) from the microprocessor memory.
The DMS also permits the operator to set up pre-determined temperature management conditions including upper and lower critical control limits for perishable foods. Product storage guidelines can be utilized to help set up carrying temperature parameters for perishable and temperature-sensitive products transported using containers, trailers, and rail cars equipped with these refrigeration units.

Airflow Management
In a refrigerated transport unit, a temperature differential exists between the air entering the refrigeration unit, (i.e., the return air), and the air exiting the unit, (i.e., the supply air). This temperature differential continues into the cargo space, creating micro-environments within the cargo and the air surrounding the cargo. Air circulation is one of the most important factors in protecting refrigerated loads of perishable foods. Air circulation is important to ensure uniform temperatures throughout the load. Refrigeration capabilities are meaningless if the refrigerated air is not properly circulated to maintain product temperature. Air circulation carries product heat and the heat which penetrates the walls, floors, and ceiling of the trailer to the refrigeration unit where it can be removed. The heated air may be circulated to keep fresh produce from incurring chilling or freezing injury. There are two major methods of circulating air in refrigerated vehicles. Top air delivery is the conventional method for refrigerated trucks and rail cars. The second method is bottom-air delivery, which has been employed extensively in seagoing van containers for several decades but only to a limited extent in highway trailers. Ribbed (fluted) sidewalls or spacers at least 1 inch (3 cm) thick to allow top airflow down the sides of the load. This reduces the amount of heat conducted across the walls to or from the product. Up to 20% of the top airflow should bleed off down the sidewalls. Vertical channels or fluted rear doors and sidewalls facilitate airflow and prevent the blocking of air circulation between the load and the rear doors and sidewalls.

Refrigeration gases Hazards

The selection and use of refrigerants mainly in response to the environmental issue of "holes in the ozone layer" & "global warming or greenhouse effect" The only one of these fluid to be considered environment friendly is ammonia, but it is not readily suited to commercial or air conditioning refrigerant applications because of its toxicity, flammability and attack by copper. Ammonia is less denser than air.
A direct contact to R-22 liquid refrigerant may result in frost bite due to the rapid evaporation of the liquid. The liquid refrigerant squirt directly into the eyes. Avoid rubbing of the eyes. If large quantity of refrigerant escapes into a poor ventilatated room, risk of suffocation. R-22 is heavier than air, ventilation should be carried out from floor level. Non flammable, but refrigerant vapour coming into contact with temperature of 316 deg celcius and above will decompose to form phosgene, hydrogen fluoride and hydrogen chloride, extemely harmful physiological effects on human being as well as heighly corrosive.

A. Explain the Ozone Depleting Potential (ODP) of conventional refrigerant gases. The chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are referred to as ozone-depleting substances (ODS) because once these gases are released into the environment and reach the stratosphere, they interact with the ozone layer and destroy ozone molecules. ODS lifetime in the stratosphere is between 100 and 400 years. An ODS molecule has the potential to destroy ozone molecules during its entire lifetime. Therefore, various CFCs and HCFCs are assigned Ozone Depletion Potentials (ODP) depending on their potential (specified relative to CFC-11) to cause ozone depletion in the stratosphere. Ozone is a gas composed of three bonded oxygen atoms (O3). In the Earth’s atmosphere, ozone is formed from molecular oxygen (O2) in the reactions initiated by UV light. Ozone can be found in two levels, at ground level and in the Earth’s upper atmosphere, referred to as the stratosphere. At ground level, ozone is a significant air pollutant, forming smog. In the stratosphere, it is referred to as the ozone layer. The ozone layer encircles the stratosphere at approximately 10 km above ground level. It filters ultraviolet (UV) radiation reducing the amount of radiation reaching ground level. The depletion of the ozone layer exposes living organisms to high levels of harmful UV-B radiation.

Ozone Depletion Potential
Ozone-depleting substances (ODS) vary in their capacity to destroy ozone molecules. So scientists have developed a method of characterizing the relative depletion caused by different ODS. The ODP or Ozone Depletion Potential is the potential for a single molecule of the refrigerant to destroy the Ozone Layer. All of the refrigerants use R11 as a datum reference and thus R11 has an ODP of 1.0.
The less the value of the ODP the better the refrigerant is for the ozone layer and therefore the environment.
The ozone layer provides a filter for ultaviolate radiation to enter in our atmosphere. Research has found that the ozone layer is thinning, due to emission into the atmosphere of chloroflurocarbons (CFCs), halons and Bromides. The Montreal protocol in 1987 agreed that the production of these chemicals would be phased out by 1995 and alternative fluids developed. they have all ceased production within thouse countries which are signaturies to th Montreal protocol. The situation is not so clear cut, because there are countries like Russia, India, China etc. who are not signaturies and who could still be poducing these harmfull chemicals. R22 is an HCFC and now regarded as a transitional refrigerant, in that it will be completely phased out of production by 2020 as agreed under the Montreal protocol.

Global warming potential
Global warming potential (GWP) is a measure of how much a given mass of gas contributes to global warming. GWP is a relative scale that compares the amount of heat trapped by greenhouse gas to the amount of heat trapped in the same mass of CO2.
Global warming is the increase in the world's temperature, which results in melting of polar ice caps and rising sea levels. It is caused by the release into the atmosphere of so-called 'Green House' gases, which form a blanket and reflecct heat back to the earth's surface or hold heat in the atmosphere. The most common green house gas in CO2, which once released remains in atmosphere for 500 years, so there is a constant build up as time progresses.
The newly developed refrigerant gases also have a global warming potential if released into the atmosphere. For example R134a has a GWP of 1300, which means that emission of 1kg of R134a is equivalant to 1300kg of CO2 but R134a if released remains in atmosphere for 15years.
Therefore overall GWP and represented by the term "total equivalent warming impact" (TEWI) is more in CO2 comparing R134a. Hydrocarbons such as propane and butane are being successfully used as replacement, But they have flammable characterstics which have to be taken into account by health and safety requirements. However, there is a market for their use in sealed refrigerant systems such as domestic refrigeration..

ODP and GWP of conventional refrigerant gas are as follows:-

Gas	ODP	GWP
R-11	1.0	4000
R-12	1.0	2400
R-22	0.05	1700
Halon	1211	4

Because of much higher ODP halon is banned.

B. Name the alternate refrigerant gases available and being used onboard.
Alternate gases available onboard which are used in refrigeration systems are:-
R 134a (ODP=0)is long term replacement for R-12 and is best performed in medium and high-temperature applications.
R 410A (ODP=0) is twice as efficient as R-22 but is recommended for new systems only.
R 407C (ODP=0) is suitable for medium and high-temperature applications. And is suitable for the new system and for R-22 changeover.
R 404A (ODP=0) is suitable for low and medium temperature applications. It is suitable for the new marine system.

C. What steps are taken to minimize the release of refrigerant gases from the plant during normal operation and maintenance activities.
As per Annex VI, Regulation 12:- Ozone Depleting Substances (ODS)
1. Existing systems and equipment using ODS are permitted to continue in service and may be recharged as necessary. However, the deliberate discharge of ODS to the atmosphere is prohibited.
2. Maintenance, servicing and repair work shall be carried out without releasing any substantial quantity of refrigerant.
3. When servicing or decommissioning systems or equipment containing ODS the gases are to be duly collected in a controlled manner and, if not to be reused on board, are to be landed at appropriate reception facilities for banking or destruction.
4. Any redundant equipment or material containing ODS is to be landed ashore for appropriate decommissioning or disposal. The latter also applies when a ship is dismantled at the end of its service life.

To minimize the release of refrigerant gas from plants following steps should be taken: During operation:-
Most important is to maintain a daily log of referent commonly called reefer log. This is to be maintained by the duty engineer and cross-checked by the chief engineer to ensure effective monitoring and early detection of any abnormality which can lead to gas leak to atmosphere. During normal operation loss of refrigerant from leaking joints, seals, gaskets and cracked pipe should be checked.
Loss of refrigerant from the safety relief valve, to overcome this pressure to be maintained in range.
Damaged mechanical seals on open type compressors are a frequent source of refrigerant leaks. A clean dry system is essential for prolonged mechanical seal effectiveness to eliminate emission. Compressor oils used for HCFC and HFC will absorb moisture readily and must keep dry to prevent refrigerant decomposition.
Excessive vibration and excess water pressure should not be allowed in the condenser to avoid tube failure.
Leak testing should be carried out regularly. Testing can be done by bubble testing with soap solution or by electronic leak detection.
During maintenance:-
Loss of small quantities of refrigerant from charging lines during charging has to be avoided by taken proper care of the connection.
Before doing any maintenance gas should be recovered and not leaked into the environment. There is a recovery cylinder is there onboard and a vacuums pump to recover gas from the system. During maintenance, compatible gaskets should be used which are compatible with the gas and oil used in the system.
Evacuate the hoses before disconnecting temporary equipment
Practice recovery and recycling when recharging dryers and filters.

Records and documents to be maintained:-
a) A list of equipment containing ODS should be maintained.
b) If the ship has any rechargeable system containing ODS, then an ODS record book should be maintained. This record book shall be approved by the administration.
c) Check for gas leaks to be carried out regularly and recorded.
d) Entries in the ODS record book shall be recorded in terms of mass (kg) of substance in respect of-
Recharge of equipment
Repair or maintenance
Discharge of ODS to the atmosphere either deliberate or non-deliberate
Discharge of ODS to land-based facilities
Supply of ODS to ship.

Choice of refrigerant

a. Low boiling point otherwise a high vacuum is necessary.
b. Moderate condensing pressure and thus heavily constructed compressor and leakages avoided.
c. High critical temperature otherwise impossible to condense.
d. High specific enthalpy of vaporization to reduce the quantity of refrigerant in circulation and lower machine speed and size.
e. Low specific volume in vapor state reduces size and increases efficiency.
f. Non-corrosive, chemically stable, non-flammable, non-explosive, worldwide availability, low cost, non-toxic, easily detectable (Odor).
g. Oil Miscible.
h. Environment friendly.

Properties	R-404A	R-22 (CHClF)	R-12 (CCl2F2)	R-744 (CO2)	R-717 (NH3)
Boiling temp. @ 1.013bar	-46.8℃	-40.8℃	-30℃	-78℃	-33℃
Suc Press.	4.0 Bar	3.0Bar	1.8Bar	23Bar	2.4Bar
Disc Press.	16.0Bar	12 Bar	7.4 Bar	72 Bar	11.7Bar
Critical temp.	72℃	96.15℃	112℃	31℃	133℃
ODP (CFC-11=1)	0	0.055	1		0
GWP (CO2 =1)	3300	1700	10000	1.0	0

'R' followed by a two-digit number - Refrigerant derived from methane. The first digit is the number of hydrogen atoms, the second digit is the number of fluorine atoms.
'R' followed by a three-digit number - The first digit is the type of compound, the second digit is the number of hydrogen atoms and the third is the number of fluorine atoms.
Letters at the end, Lower-case letters describe the structure of the molecule. Capital letters describe specific mixing proportions of different components.

Ammonia (NH3) reacts with copper, zinc and their alloy. It also corrodes brass, bronze and similar alloys if water is present. So steel only be used in ammonia plants. It also attacks natural rubber, so correct gasket material to be used. It is extremely toxic with a long term threshold limit of 35ppm and may be lethal at concentrations of 2500ppm and above.
It has a pungent odour, detectable below 10ppm, provides a warning. Ammonia is flammable in air at concentrations 16%-27% and may form an explosive mixture, However, it is easily expelled by Boiling. The action makes the vapour absorption refrigerator, do not require a compressor for operation, only a heat source is sufficient.

Air conditioning system and trouble spots

Sketch of the basic system.

Trouble spots

Defects
Causes
Remedies
Low air circulation
Chocking air inlet filter
Renew or clean the filters

Defective fans
Rectify defect

Fan motor direction reversed
Make correct connections
Overloading of air handling unit
Air leakage at access doors
Replace gasket and fasteners.

Mixing of fresh air with the circulated air in large quantities.
Reduce the mixing of fresh air.
Compressor start-stop too frequently
L.P cut out activated
Defrost

The refrigerant charge is low.
Charge refrigerant

Low and high-pressure trips wrongly set
Check and adjust all trips correctly.

Too much cooling water
Reduce cooling water

Air in the condenser
Purge air

Expansion valve partly blocked
Clean filters

Leakage solenoid valve
Overhaul solenoid valve

Discharge valve malfunctioning
Overhaul discharge valve
Compressor running continuously
Refrigerant charge low
Recharge

The cooling capacity is too small for the load
Check fresh air leakage and insulation

Too much loading of the plant
Reduce the load

Suction and discharge valve malfunctioning
overhauling
Condenser pressure too high
Too much refrigerant in the system.
Drain off excessive refrigerant

Too little or too warm cooling water
Supply more or colder water.

Dirty condenser
Clean condenser
Condenser pressure too low
Refrigerant charge low
Charge refrigerant

Too much cooling water/cooling water temperature is low
Reduce cooling water

Defective piston rings liner.
Overhauling

Defective valve malfunctioning
Overhauling
Suction pressure too high
Oil pressure regulating valve malfunctioning
Overhauling oil pressure regulating valve

The liquid in the suction line
Inspect and adjust expansion valve

Suction and discharge the malfunction
Overhauling suction and disc valve.
Suction pressure too low
The refrigeration charge is low
Charge refrigerant

Too much cooling water/ cooling water temperature too low
Reduce cooing water

Filter in the liquid suction line is chocked
Clean the filter

Solenoid valve leaks
Overhaul the same

Too much loading of the plant
Reduce load
Discharge temperature too high
Too high superheating
Adjust suction gas superheating

Faulty compressor valves
Inspect overhaul or change if required

Insufficient cooling water or too high cooling water temperature
Increase cooling water

Air in the system
Purging

Dirty condenser
Clean the Condenser

Discharge valve malfunctioning
Overhaul the valve
Oil temperature too high
Too high superheating
Adjust superheating

Capacity control is too low. ( lower capacity gives higher oil temperature)
Switch off the compressor and adjust the proper capacity requirement.

Faulty compressor valve
Inspect overhaul or change if required
Oil in crankcase disappears
Oil separator return valve malfunctioning
Overhaul oil separator return valve.

Defective piston ring and liner
Overhaul
Oil in crankcase foams
The liquid in the suction line
Inspect and clear thermostatic expansion valve

The defective heating element in the crankcase
Rectify the heating element.
Oil pressure too low
Oil charge is less
Charge oil

Oil strainer/filter clogged
Inspect and clean

Worn out or defective bearing
Replace defective bearing.

Temperature and Relative humidity

The four points defining the extreme corners of the comfort zone in the psychrometric chart.
20° @ 70% R.H cold
27° @ 70% R.H too warm
22° @ 40% R.H too cold
29° @ 40% R.H comfertable
The surrounding temperature of air and humidity are important factors that influence human comfort. The atmosphere air is treated in the AHU. In summer, the cooling coil extracts the heat and humidity from the air. In winter, the heating coil and humidifier treat the air to the required condition. The air from the AHU is circulated throughout the ship through the air duct.
Air conditioning involves control of humidity, temperature, cleanliness, air moition etc. Saturated air is defined as the air that holds the maximum possible weight of water vapour at a particular temperature. To measure R.H in Air conditioning system wet and dry bulb thermometers are used. When air passes through the moist wick evaporation of water takes place and this depresses the temperature in the wet bulb compared to dry bulb. This temperature difference is called as wet bulb depression.

1. Humidity control of cooled air:-
Specific humidity:-$\displaystyle \mathrm{\frac{Mass\ of\ water\ vapor}{Mass\ of\ dry\ air}}$ @ given volume of the mixture.

R.H %:-$\displaystyle \mathrm{\frac{Mass\ of\ water\ vapor\ per\ meter\ cube\ of\ actual\ air}{Mass\ of\ water\ vapor\ per\ meter\ cube\ of\ standard\ air}}$ @ constant temperature.

or, R.H %:- $\displaystyle \mathrm{\frac{Partial\ pressure\ of\ actual\ air}{Partial\ pressure\ of\ saturated\ air}}$ @ constant temperature.

As the temperature of the air is reduced; its capacity for carrying water vapor is also significantly reduced.
Referring to the psychrometric chart - air at temperature 20°C and R.H 75%, when cooled to 15°C will have an R.H 100% i.e saturated. Further cooling the air will precipitate excess moisture. Air-cooled up to 10° with 100% R.H, when reheated to 25°C, will have an R.H of 50%. Thus to remove excess moisture from the air, cooling below the saturation and reheating is used.
2. Humidity control of heated air:- air at 5°C with R.H 50%, when heated to a temperature of 20°C. will have only 30% R.H. This does not fall in the comfort zone in the Psychrometric chart. The rise in the temperature of air increases its capacity to carry the moisture in suspension. This air will draw air from the body through nasal passage and throat or by perspiration. which is uncomfortable for the crew and passengers. Thus the air is required to be humidified by steam injection or hot water spray.

Points to Remember

Marine refer plants are Vapor compression type.
The principle of refrigeration is alternate Liquification and evaporation.
The compressor circulates the refrigerant by pumping and raises its pressure causing its saturation temperature to rise above the cooling water temperature.
In the condenser, refrigerant is subcooled below the saturation temperature by the cooling water. The heat of the compression and the cooling chamber is also removed.
TEV controls the flow of refrigerant from the LP side to the HP side, so saturation temperature falls.
An equalizer connection facilitates a control on the degree of superheat. A higher degree of superheat reduces the plant efficiency.
The refrigerant evaporates in the evaporator coils by taking the latent heat from the coils and convert into dry saturated vapour.
The degree of superheat is the difference between the temperature of the refrigerant leaving the evaporator and the temperature of saturated vapour at compressor suction.
Moisture causes ice formation at the expansion valve. Also, the reaction of moisture and freon produces acid which causes corrosion and blocking the TEV.
In the shaft seal, one end of the double metallic bellow is fixed and the other end has a rubbing surface that rubs against the cup ring. The Cup ring contains duprene and the distance piece pressed against the shoulder of the crankshaft.
In HP cut out the gas pressure excess to spring pressure moves the guide washer. That moves the catch and thus switch arm falls. That need a manual reset.
In LP cut out if the gas pressure gets lower than the spring pressure, the lever will move downwards. this will move the lever so as to move the switchblade and break the contact. The contact will make again by itself when the gas pressure restore.
Loading in the refer compressor takes place when the oil pressure acts on the piston and move the piston towards the cylinder liner. This cause the lever to move in such a way so as to pull the actuating rod downwards against the spring. Thus retract the valve lifting pins and suction valve descend on its seat.

Q. For a fully automated Provision Refrigerating System incorporating a number of rooms
a) Explain how room temperature is set;
b) Describe the sequence of events following a demand for increased refrigerant flow from any room;
c) State the reasons the devices incorporated in to the system to protect the machineries and equipments against malfunction;
d) State how the satisfactory operation of the plant can be established.
Ans. a) The main purpose of ship’s refrigeration plant is to avoid any damage to the cargo or perishable material so that the cargo in transported in good and healthy condition. Refrigeration prevents growth of micro-organisms, oxidation, fermentation and drying out of cargo etc.

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. Drops the pressure, Maintains a degree of superheat at the evaporator outlet and ensures no liquid flow to the compressor suction.
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 refeer plant.
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.
Ans.b) With the increase in load in one room, the degree of superheat at the evaporator outlet of that room starts to rise, the thermostatic bulb pressure starts increasing above the expansion valve diaphragm, expansion valve moves down in the opening direction, admitting more fluid refrigerant to enter the evaporator, absorbing more latent heat of evaporation thus extracting heat from surrounding room, restoring the operating condition and stabilizing the system again.
Ans. c) The main safeties adopted for refrigeration plants are :
Low Suction 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.
High Suction Pressure Cut In : After LP Cut Off , 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.

Ans. d) Satisfactory operation of the plant can be ensured if the Co efficient of Performance (COP) i.e. the ratio of heat extracted in the evaporator to the drive motor input, closely matches with the Manufacturer’s recommended COP.COP = (h2 – h1) / h3 – h2.

Comments

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LINUS PROJECTS (INDIA) 10 October 2024 at 12:56
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