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Choice of refrigerants for ship refrigeration systems

Choice of refrigerant: Mechanical refrigeration makes possible the control of the pressure and therefore, the temperature at which a refrigerant boils (within the limits of critical pressure and temperature). The closed circuit ensures repeated use of the same refrigerant with little or no loss to atmosphere.

Theoretically, almost any liquid can be used as a refrigerant if its pressure/temperature relationship is suitable for the conditions. Although no perfect refrigerant is known, there are certain factors which determine a refrigerant's desirability for a particular duty and the one selected should possess as many as possible of the following characteristics:

• 1 Moderate condensing pressure, obviating the need for heavily constructed compressors, condensers and high pressure piping.
• 2 High critical temperature, as it is impossible to condense at a temperature above the critical, no matter how much the pressure is increased.
• 3 Low specific heat of the liquid. This is desirable as throttling at the expansion valve causes liquid refrigerant to be cooled at the expense of partial evaporation.
• 4 High latent heat of vaporization, so that less refrigerant may be circulated to perform a given duty.
• 5 The refrigerant should be non-corrosive to all materials used in the construction of the refrigerating machinery and systems.
• 6 It should be stable chemically.
• 7 It should be non-flammable and non-explosive.
• 8 World wide availability, low cost and ease of handling are desirable.
• 9 The problem of oil return to the compressor crankcase is simplified when an oil-miscible refrigerant is used, by the admixture of the oil and the refrigerant in the system. With non-miscible refrigerants, once oil has passed to the system, its return to the crankcase can only be effected with difficulty.
• 10 The current concern with depletion of the ozone layer has resulted in a new requirement that refrigerants should be environmentally friendly. Strong pressure exerted to phase out CFCs and in particular R12, resulted in the Montreal Protocol adopted in 1987 after ratification by 70 countries and additional conventions seeking to phase out these gases.

Finally the refrigerant should preferably be non-toxic, have satisfactory heat transfer characteristics, and leakages should be easy to detect either by odour or by the use of suitable indicators.

It is not proposed to list or deal with all known refrigerants, only those likely to be encountered on board. These refrigerants are referred to by their trade name, chemical name or their internationally recognized numbers,

Commonly used refrigerants and possible replacements

Rll is a CFC which is included in the Montreal protocol having an ozone depletion potential (ODP) of 1 and a greenhouse potential (GP) of 3300. The formula for Rll is CC13F and it has the chemical name trichloromonofluoromethane. It was found suitable for air conditioning installations.

R12 is a CFC which, with an ODP of 1 and a GP of 10 000 is also to be phased out by the Montreal protocol. The formula for R12 is CC12F2 and the chemical name dichlorodifluoromethane.

R22 is an HCFC with a much lower ODP of 0.05, is much less of a threat to the ozone layer and has been used in place of R12 in some recent installations. The formula for R22 is CHC1F2 and its chemical name is rnonochlorodifluoromethane. It has a GP of 1100.

R502 is an azeotropic mixture composed of 48.8% R22 and 51.2% of Rl.15. Details of R22 are given above and R115 is a CFC with the formula CC1F^CF3 which is used only in R502. As a CFC, with an ODP of 0.6 and a GP of 25 000 the R115 is included in the Montreal convention and this implies that R502 is also included. R502 is particularly suited for use with hermetic compressors.

Replacement refrigerants

A number of new refrigerants have been developed by the large chemical companies as potential replacements for refrigerants commonly in service. Three refrigerants considered as candidates to replace R12 are referred to as HFC-134a, Blend MP33 and Blend MP39. The two blends are both made up from the same three other compounds but in different proportions by weight.

Thus Blend MP33 consists of 40% HCFC-22 plus 17% HFC-152a plus 43% HCFC-124 and Blend MP39 is made up from 52% HCFC-22 plus 15% HFC-152a plus 33% HCFC-124. These gases are stated to have very low ozone depletion potential when compared with R12 and the two blends are less toxic with TLVs of 750 ppm for Blend MP33 and 800 for MP39, The HFC-134a has a TLV of 1000 ppm which is the same as that of R.12.

Any replacement refrigerant must compare in performance with existing gases, for direct economy of operation as well as in respect of the amount of fossil fuel consumed.

There have always been minor problems with refrigerant and oil miscibility, Some problems with lubricants are being experienced with the new gases. Rl34a is seriously considered as the best replacement for R12, It is an HFC with the chemical formula CF3CH2F an OOP of 0 and a GP of 900.

R123 is being considered as a replacement for Rl 1. It is an HCFC with the chemical formula CHC12CF3 an OOP of 0.02 and a GP of only 50.

R125 is being tested as a potential replacement for R22 and the gas with similar properties R502. The gas R125 is an HFC with the chemical formula CHF2CF? an OOP of 0 and a GP of 1900.

Ozone

An oxygen enriched gas may be introduced to eliminate odours by oxidising the offending molecules. Ozone is produced by an electrically operated generator in the fan space and the gas is circulated throughout the affected area. It should be noted that the ozone will only deal with airborne odours. Those which emanate from spills will continue until the spill has been cleared up and any damage to insulation or dunnage which has absorbed the spill quickly removed or renewed.

Furthermore, all traces of ozone should be removed by fresh air ventilation before the space is entered or sensitive cargo worked through and/or loaded into the space. Particular care should be exercised before entering a fan space to attend to or remove an ozone machine where there is liable to be a concentration of gas.

Liquid indicators

These can be either cylindrical or circular glasses installed in the liquid line, providing a means of ascertaining 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. Some types incorporate a moisture indicator which, by changing colour indicates the relative moisture content of the liquid passing through.


Summarized below various refrigeration system components, working process and maintenance guideline:

1. Automatic direct expansion refrigeration- vapour compression

The basic components of any refrigeration system (Figure 11.1) working on the vapour compression cycle, are the compressor, condenser, expansion valve, evaporator and the refrigerant fluid which is alternately vaporized and liquefied during the refrigeration cycle. The temperature at which a fluid boils or condenses, is known as the saturation temperature and varies with pressure....more


2. Choice of refrigerants

Theoretically, almost any liquid can be used as a refrigerant if its pressure/temperature relationship is suitable for the conditions. Although no perfect refrigerant is known, there are certain factors which determine a refrigerant's desirability for a particular duty and the one selected should possess as many as possible of the following characteristics.....more


3. Refrigeration systems - Chamber cooling arrangements

To avoid having an extended refrigeration circuit for cargo cooling, a brine system can be used. The brine is cooled by the evaporator and in turn cools grids or batteries. Grids provide cooling which relies on convection and conduction but air circulated through brine batteries provides a positive through cooling effect. .....more


4. Refrigeration system components

Marine condensers are generally of the shell and tube type, designed for high pressures. There may a few coil-in-casing or other types still in use. The coolant passes through the tubes with refrigerant condensing on the outside......more


5. Refrigeration system compressors

Refrigeration compressors are usually either reciprocating, or of the rotary screw displacement type. Centrifugal and rotary vane compressors have also been used.....more


6. Refrigeration systems expansion valves

The expansion valve is the regulator through which the refrigerant passes from the high pressure side of the system to the low pressure side. The pressure drop causes the evaporating temperature of the refrigerant to fall below that of the evaporator. .....more


7. Monitoring instruments,CO2 measurement & Heat leakage and insulation test

All necessary cargo temperature readings are obtained on modern reefers and container ships on a data logger which makes an automatic record. The temperatures and pressures relating to refrigerant gas and liquid, cooling water, brine and the ambient are also required. Most of these are obtained from direct reading instruments. .....more


8. Marine condenser assembly

The temperature of the refrigerated spaces with a direct expansion system is controlled between limits through a thermostatic switch and a solenoid valve which is either fully open to permit flow of refrigerant to the room evaporator, or closed to shut off flow. The solenoid valve is opened when the sleeve moving upwards due to the magnetic coil hits the valve spindle tee piece and taps the valve open.....more


9. Comparison between refrigerants R717 ammonia & R744 carbon dioxide

The ammonia used for refrigeration systems based on the use of a compressor, condenser, expansion valve and an evaporator (Figure 11.2) is dry (anhydrous) in that there is no water in solution with it. It has the chemical formula NH3 but as a refrigerant, it is coded with the number R717....more


10. Container cooling system

The air is cooled either by brine or direct expansion batteries and the containers are arranged so that one cooler can maintain a stack of containers at a given temperature. The temperature of the return air duct for each container is monitored.....more

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