Mechanical Freezing Systems

Mechanical Freezing System

Any system that uses electrical power to produce chilled air, and relies on a refrigerated cooling system. The chilled air is continuously passed over the food product, and in doing so, it removes heat. Mechanical freezing systems are characterized by a large capital investment, a significant ongoing preventive maintenance cost, and a sizeable permanent commitment of plant space. On the other hand, the resulting refrigeration is produced at a fraction of the consumable cost of cryogenic refrigeration. Refrigerated cooling systems are widely used technology and present in some form in virtually every food processing plant. In some cases, mechanical refrigeration systems tend to dehydrate, or strip moisture from that product. For some products, this is not an issue. In others, it is a major quality or yield concern.

Mechanical Refrigeration

Refrigeration is the withdrawal of heat from a chamber (refrigeration load) to achieve temperatures lower than ambient temperatures. After heat is withdrawn, it is transferred to a condenser and dissipated to air or water. The purpose of refrigerated cooling systems in food processing is to preserve quality and delay spoilage; in volatile organic compounds recovery, is to condense and capture harmful vapor emissions; and in the liquid natural gas industry, to facilitate natural gas storage and transportation.

90 percent of U.S. industrial refrigeration is provided by mechanical systems

More than 90 percent of U.S. industrial refrigeration is provided by mechanical systems using ammonia as the refrigerant. Mechanical refrigeration units are dedicated systems, installed at individual industrial facilities and owned and operated by the industrial companies. Refrigeration is achieved when the refrigerant, circulating in the system, withdraws heat energy from the chamber to be cooled (load). Heat energy (latent heat of evaporation) is absorbed as liquid refrigerant undergoes a phase change to a gaseous state. Systems are composed of four basic elements connected with piping into a closed loop that re-circulates refrigerant. Compressors (generally) use motor-driven rotating impellers to generate gas pressure. Gaseous refrigerant enters the compressor at low pressure and temperature and exits at high pressure and temperature. Inside condenser coils gaseous refrigerant condenses to liquid state. To facilitate phase change, the condenser dissipates heat energy to ambient air or water. High pressure refrigerant exits at lower temperature. An expansion valve controls the flow of high pressure liquid refrigerant to the evaporator. As refrigerant passes through the expansion valve it is further cooled by the Joule Thompson effect, the scientific principle that the temperature of a stream is reduced when forced through a narrow nozzle and allowed to expand. Inside the evaporator, liquid refrigerant vaporizes into a gaseous state. Vaporization requires heat energy, which is extracted from the industrial process load (food items to be cooled). The refrigerant is returned to the compressor to repeat the cycle.

High Pressure Core Heat Exchanger

Heat exchangers are devices that transfer heat from a hot to a cold fluid. The barrier between the two fluids is a metal wall, such as that of a tube or pipe. In many engineering applications it is desirable to increase the temperature of one fluid while cooling another. This double action is economically accomplished by coils, evaporators, condensers, and coolers that may all be considered heat exchangers. Heat exchangers are designed with various flow arrangements. The concentric tubes design uses one pipe placed inside another. Cold fluid flows through the inner tube and the warm fluid in the same direction through the annular space between the outer and the inner tube. Heat is transferred from the warm fluid through the wall of the inner tube (the so-called heating surface) to the cold fluid. Concentric tube heat exchangers can also be operated in counter-flow, in which the two fluids flow in parallel but opposite directions. The shell and tube design utilizes a bundle of tubes through which one of the fluids flows. These tubes are enclosed in a shell with provisions for the other fluid to flow through the spaces between the tubes. In most designs of this type, the free fluid flows roughly perpendicular to the tubes containing the other fluid in what is known as a cross flow exchange. The plate-fin design uses metal sheets brazed together into internal channels to carry warmer fluid stream, which has to be cooled. Fins, brazed to the outside surface of these channels, facilitate faster and more efficient heat transfer to the cold fluid stream on the outside of these channels.


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