Before the advents of mechanical refrigeration, ICE, formed by natural freezing and stored until used, was the only source of refrigeration. As ice, under atmospheric pressure, always melt at 0oC (320F), it produces refrigeration as it absorbs heat in melting. Mixtures of salt and ice produce temperature lower than 00C (320F). When ordinary salt (NaCI) and finely divided ice (snow) are brought into contact, the melting (fusion) temperature is depressed to about- 21.280C (-6.30F) and heat is absorbed at this lower temperature, while the ice melts and the salt goes into solution. Certain acids and alcohols have a similar effect in depressing the melting temperature of ice. Another refrigerating material is solid carbon dioxide (dry ice), which at atmospheric pressure sublimes at-78.0C (-109.30F) and absorbs 570.97 KJ/Kg (246 Btu/lb.) of dry ice. At the present day, the production of dry ice have been reduced for the main reason that it affects the atmospheric condition of the earth through the so called “ Global Warming” or “Green House Effect” Also it was found that modern types of refrigerants, halons and some chlorinated products causes ozone depletion which in turn destroys the earth’s protective layer or shield against ultra violet radiation off settings our very own ecological balance.
To obtain fully flexible ranges of temperature or to produce refrigeration in quantity, mechanical (artificial) means must be employed. The ton of refrigeration is the absorption of heat at the rate of 12,660 KJ/hr. (12,000 Btu/hr) or 211 KJ/min. (200 Btu/min). Historically, the ton of refrigeration represented refrigeration equivalent to one-ton weight of ice melting in 24 hours. The rating or capacity of a refrigerating machine or unit is expressed in the amount of heat absorbed or rejected per unit time (Btu/hr, KJ/hr, Kcal/min. etc.) or in tons with a statement of the temperature) or temperature range) at which the machine or units are in producing its rating. Formerly all vapor refrigeration machine were rates in terms of the tons of refrigeration they could produce, when the evaporator operated at the pressures corresponding to boiling of the refrigerant at –150C 950F) and to the condensation of the refrigerant at 300C (860F). Because of the broader present-day uses of refrigeration, as in air conditioning, quick-freezing, low-temperature, and chemical process refrigeration, the- 150C (50F), + 300C (860F) rating is inadequate and a large number of rating temperatures are used.
Temperature of –23.30C (-100F), -8.70C (200F) and 4.40C (400F) are use for the evaporator and condensation temperatures of 350C (950F), 82.2) C (1000F), 40.60C) (1050F) and 43.30C (1100F) allow for the more extreme condition met when condensing with cooling tower water or with air.
But the progress of civilization and the desire for the man to control his natural environment have led to new development in applied science as related to refrigeration and air conditioning. Today, refrigeration is essential in the production and distribution of food and for the efficient operation of industry. Because of air conditioning, people live more comfortably and healthfully, and many industrial operations are conducted more effectively.