Methods of Refrigeration
There are number of methods by which the refrigeration can be achieved. They are broadly classified into two categories: Non-Cyclic and Cyclic methods of Refrigeration. Firstly, let us see non-cyclic methods.
In the non-cyclic method of refrigeration there is no thermodynamic cycle followed for creating the cooling effect. There are two methods of non-cyclic refrigeration process as described below:
1) Ice Refrigeration: In this method the ordinary ice is used for keeping the space at temperature below the surrounding temperature. The temperature of ice is considered to be 0 degree Celsius hence it can be used to maintain the temperatures of about 5 to 10 degree Celsius. To use the ice for refrigerating effect a closed and insulated chamber is required. On one side of the chamber ice is kept while on the other side there is a space which is to be cooled where some material to be cooled can be placed. If the temperature below 0 degree Celsius is required, then the mixture of ice and salt is used. This method of cooling is still being used for cooling the cold drinks, keeping the water chilled in thermos, etc.
2) Dry ice refrigeration: Dry ice is the solid carbon dioxide having the temperature of -78 degree Celsius. Dry ice converts directly from solid state to gaseous; this process is called as sublimation. Dry ice can be pressed into various sizes and shapes as blocks or slabs. Dry ice is usually packed in the frozen food cartons along with the food that has to be kept frozen for long intervals of time. When the dry ice gets converted into vapor state it keeps the food frozen. The process of dry ice refrigeration is now-a-days being used for freezing the food in aircraft transportation.
The non-cyclic methods of refrigeration can be used only in places where small amount of refrigeration is required in places like laboratories, workshops, water coolers, small old drink shops, small hotels etc. In fact the ordinary ice and dry ice used for the refrigeration purposed have to be manufactured by the cyclic methods of refrigeration which we shall see in the next article. However, in the earlier days the ice used for the cooling purposes was usually harvested during the winter seasons from the ponds and lakes and stored in large insulated ice houses for the use throughout the year.