SECONDARY REFRIGERANTS

In the vapour compression or absorption system the cooling effect produced may be applied directly to the bodies or fluids which are to be refrigerated, such as when the evaporator carrying boiling refrigerant is placed inside a cold room. This practice is known as direct expansion. On the other hand, the refrigeration may be needed at places more or less distant from the evaporator, and a secondary refrigerant is used to distribute the refrigeration. The secondary refrigerant itself is cooled in the evaporator arid is then pumped to the various places where refrigeration is wanted. The advantage of using the secondary refrigerant is that the refrigerant in the primary system can be confined to a small area, the plant room, with reduced risk of leakage because of the smaller number of pipe joints needed; the whole refrigerating plant can be factory assembled, so that only pipework for the secondary refrigerant need be done on the site; and the problem of controlling and distributing the refrigeration is simplified.

The secondary refrigerant must remain liquid at the temperature desired. For air-conditioning installations, water is suitable liquid. Chilled water at about 6-7°C is produced in the plant room and distributed to exchanger throughout the building. For lower temperatures, various non-freezing solutions or pure liquids are used.

Solutions of inorganic salts in water.

The most widely used is calcium chloride, but magnesium and sodium chlorides are also used for certain purposes. Calcium chloride brine can be used down to about -40°C, provided that it is strong enough.

Methanol (methyl alcohol) and ethanol (ethyl alcohol) alone or in aqueous solution.

Methanol freezes at -97°C and ethanol at - 117°C. The densities and specific heat capacities of the pure liquids are less than those of calcium chloride brine, but the liquids are usable to lower temperatures. Because of their flammability precautions are required, especially when the plant is not running, with the methanol or ethanol at room temperature. The aqueous solutions are rather viscous.

Aqueous solution of higher alcohols, principally ethylene and propylene glycol.

These have an eutectic temperature of about - 60°C. The densities and specific heat capacities are relatively high, but the solutions are viscous. They are also rather corrosive and must be inhibited.

Pure organic liquids such as trichlorethylene (freezing point -186°C) and Rll(freezing point -111^oС). These have high densities and low viscosities but their specific heat capacities are low.

Anhydrous ammonia can be used down to temperatures near its freezing point of -77.7°C, but the vessels and pipework must be capable of withstanding the vapour pressure of the ammonia at ambient temperature when the plant is not in use. Liquid ammonia has a higher specific heat capacity than water, but a lower density.

Solutions of ammonia in water are also used. These have a lower vapour pressure which does not require pressure-tight construction, but they are rather viscous at low temperature.

Supplementary text:

MONTREAL PROTOCOL ON SUBSTANCES THAT DEPLETE THE OZONE LAYER

The Conference of the Parties for adopting the Protocol on chlorofluorocarbons to the Vienna Convention for the Protection of the Ozone Layer took place on September 14-16, 1987 in Montreal, Canada.

All countries were invited to take part in the Conference. 53 countries participated in the work of the Conference, among them was Ukraine. 6 countries took part as observers. EEC took part in the work of the Conference as well as observers of UN, specialized enterprises, intergovernmental and non-governmental organizations.

The Conference adopted the Protocol on Substances that Deplete the Ozone Layer.