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heat exchanger is a system used to transfer heat between two or more fluids. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contact. They are widely used in space heating, refrigeration, air conditioning, power stations, chemical plants, petrochemical plants, petroleum refineries, natural-gas processing, and sewage treatment. The classic example of a heat exchanger is found in an internal combustion engine in which a circulating fluid known as engine coolant flows through radiator coils and air flows past the coils, which cools the coolant and heats the incoming air. Another example is the heat sink, which is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant.

One of the widest uses of heat exchangers is for refrigeration and air conditioning. This class of heat exchangers is commonly called air coils, or just coils due to their often-serpentine internal tubing, or condensers in the case of refrigeration, and are typically of the finned tube type. Liquid-to-air, or air-to-liquid HVAC coils are typically of modified crossflow arrangement. In vehicles, heat coils are often called heater cores. On the liquid side of these heat exchangers, the common fluids are water, a water-glycol solution, steam, or a refrigerant. For heating coils, hot water and steam are the most common, and this heated fluid is supplied by boilers, for example. For cooling coils, chilled water and refrigerant are most common. Chilled water is supplied from a chiller that is potentially located very far away, but refrigerant must come from a nearby condensing unit. When a refrigerant is used, the cooling coil is the evaporator, and the heating coil is the condenser in the vapor-compression refrigeration cycle. HVAC coils that use this direct-expansion of refrigerants are commonly called DX coils. Some DX coils are "microchannel" type. On the air side of an HVAC coil, there is a significant difference between coils used for heating and coils used for cooling. Because of the hygrometer, the air being cooled will often condense out moisture, unless it is a very dry airflow. Heating some air increases the airflow's ability to hold moisture. Therefore, the heating coil does not need to consider moisture condensation on the air side, but the cooling coil must be adequately designed and selected to handle its specific latent (moisture) and explicit (cooling) loads. The water removed is called condensate.

Heat exchangers are generally made of metal materials, among which carbon steel and low alloy steel are mostly used to manufacture medium and low pressure heat exchangers; in addition to stainless steels mainly used for different corrosion-resistant conditions, austenitic stainless steels can also be used as heat exchangers. High and low temperature materials; copper, aluminum and their alloys are mostly used to manufacture low temperature heat exchangers; nickel alloys are used under high temperature conditions; in addition to gasket parts, non-metallic materials have also begun to be used to manufacture non-metallic materials, corroding heat exchangers, such as Graphite heat exchanger, fluoroplastic heat exchanger, glass heat exchanger, etc.

pressure vessel is a container designed to hold gases or liquids at a pressure substantially different from the ambient pressure. Construction methods and materials may be chosen to suit the pressure application, and will depend on the size of the vessel, the contents, working pressure, mass constraints, and the number of items required. Pressure vessels can be dangerous, and fatal accidents have occurred in the history of their development and operation. Consequently, pressure vessel design, manufacture, and operation are regulated by engineering authorities backed by legislation. For these reasons, the definition of a pressure vessel varies from country to country. 

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