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Refrigeration plants on board - how to troubleshoot system faults

Refrigeration is a process in which the temperature of a space or its contents is reduced to below that of their surroundings. Heat is a form of energy and is indestructible and it is not a substance and can be dealt with only from its effect on substances.



When two substances are brought into thermal contact, the heat starts to flow one into the other until they are both at the same temperature, at which no more heat can flow. At this point, where no more heat can be extracted known as absolute zero.

In all refrigeration, heat is collected in a suitable fluid and this fluid is removed from the space substance being cooled carrying the heat with it. Such fluid known as refrigerant.

Example in melting ice produces refrigeration under easily controlled conditions and with a minimum expenditure of the refrigerant. It because large quantities of latent heat are absorbed by ice in melting. Similarly a liquid in changing its state to vapour absorbs large quantities at latent heat.

The vapour pressure corresponding to its freezing temperature and it critical temperature that above which the vapour cannot be liquefied no matter what pressure is applied (regardless of pressure) to it and its known as saturated temperature and the vapour known as dry saturated vapour.

If however, dry saturated vapour is not in contact with its liquid, it will rise in temperature and becomes a superheated gas.

However, if it is cooled with no vapour present, it becomes sub-cooled liquid.

Thus both superheated gas and sub-cooled liquid of any refrigerant have specific heats and the heat absorbed during evaporation considered as latent heat (when no further change in temperature) and the temperature at which a liquid evaporates depends directly on the pressure existing at its surface.

There are three main physical law on refrigeration principle which depends on:-

i) Any refrigerant in evaporating takes in heat forms it surrounding means latent heat of evaporation (no further change in temperature)
ii) The temperature at which a refrigerant evaporates depends on the pressure existing at the inter-face between the vapour and the liquid.
iii) Any vapour or gas can be condensed to a liquid by suitably compressing and cooling it latent heat of condensation.


During operation a number of particular problems can occur which will affect the plant performance. An overcharge or excess refrigerant in the system will be seen as a high condenser pressure.



The refrigerant should be pumped to the condenser and the excess released from there. Air in the system will also show as a high condenser pressure. With the condenser liquid outlet closed the refrigerant charge should be pumped in and cooled. Releasing the purge valve will vent off the air which will have collected above the refrigerant.

Under-charge will show as a low compressor pressure and large bubbles in the liquid line sightglass. A leak test should then be carried out over the system to determine the fault and enable its rectification. A leak detector lamp for Freon refrigerants may be of the methylated spirit type, but more commonly uses Calor gas (butane/propane). The Freon is drawn into the flame and the flame will change colour, going from green to blue depending on the concentration of the gas.

When charging the system with more gas the main liquid valve should be closed and gas introduced before the regulating valve until the system is correctly charged. (It is possible to charge on the outlet side of the regulating valve and is quicker, but this requires a good amount of experience to prevent liquid carrying over and damaging the compressor.)

Moisture in the system may change to ice and close up the regulating valve, resulting in a drop in pressure on the evaporator side and a rise in pressure on the condenser side. The drier should be examined and the drying chemicals will probably require replacing. A correctly operating regulating valve will have frosting on the outlet side but not on the inlet side.


Refrigerant Gas Losses

It is important that gas losses are minimized in systems. Gas used in this system is both expensive and a danger to health. The system is therefore to be kept in an absolutely gas-tight condition. On no account should the refrigerant be blown off to atmosphere. Before carrying out repairs the refrigerant should be pumped down to the liquid receiver or the condenser. The remaining gas should be vented off and the area well ventilated.

It must be mentioned, that certain refrigerant Freon gases used in compressors have proved to contribute to the "global warming effect" and destruction of the ozone layer. New rules are now in force and gas dumping to atmosphere must be avoided at all costs.



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