Cooling

Before the invention of the mechanical refrigeration system, the ice was used successfully cooling method for thousands of years. It is not surprising, then, that the ice was also the first of cooling agent used in household and commercial refrigerators. Before the development of die mechanical cooling system, ice blocks were placed in isolated offices to act as the evaporator. Ice die of cold surface needed for the absorption of heat from the environment. A typical refrigerator ice is depicted in Figure 9-1. Heat entering the refrigerated space through the walls of the Cabinet, air and food products is transferred on the ice due to natural convection currents. Currents develop as the air in contact with the warm product and wall Cabinet absorb heat by conduction and expand. As its density decreases, the warm air rises to die ice compartment is located in the upper part of the Cabinet. Warm air comes in contact with the cooler surface of the die ice, transfer of heat energy by heat conduction. In the process of heat transfer decreases die temperature and volume of air die. The higher the density of cold air sinks to die floor of the Cabinet. On his trip to the bottom of the closet, he comes into contact with warmer surface absorbs the heat-and the cycle continues.

To ensure proper air circulation inside the Cabinet, the ice was located in the upper part of the refrigerator. Unclear was in direct air flows in and out of the district, where the ice was located. Pallets was placed beneadi ice water collection, which was prepared as a by-product in the process of heat transfer. This water was working fluid, handed the heat energy absorbed from the interior of a room and brought him through the boundaries of the system to the outside of the Cabinet. The ice is still used for storage and marketing purposes. Fresh vegetables, fish and birds, often packaged and shipped in ice chips to reduce dehydration and maintain their appearance. Ice also has a very great attraction and is used to display fresh foods and beverages in restaurants and stores.

Ice has a number of shortcomings that limit its usefulness as a cooling agent. Its melting point requires the Cabinet for operation at temperatures above 32F (0C). The melting temperature of ice can be lowered to about 0 F (17.7C) by adding a solution of sodium chloride or calcium chloride die of the water before it is frozen in a block of ice. This range is only acceptable for very short-term storage of frozen food. The need freguently replenishment of ice in the office was also one of the main isadvantage ice cold.

Another, less obvious disadvantage was the difficulties in controlling the rate of heat transfer during cooling (BTU/lir, J/h). Rate of ice absorbs heat energy is directly proportional to the surface area and the temperature difference between the die space and ice surface. Consequently, the rate of heat absorption decreases as the die surface melting of the ice decreases. When the refrigerating rate decreases to the point lower the speed with which heat is transferred to the Cabinet, the temperature of space, starts to grow. This response makes it difficult to maintain the desired temperature inside the refrigerated space and has a negative impact on the shelf life and quality of stored products food...