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What are Heat Exchangers?

10/18/2013 10:36:26 AM | by Anonymous

Heat Exchangers

Heat exchangers are commonly used for efficient transfer of heat or thermal energy between two or more mediums. In some cases, the heat exchanger may feature a solid wall in them to separate the two fluids from one another whereas on some heat exchangers the fluids may be in direct contact. Most heat exchangers feature designs that have a wall to separate the fluids as larger surface allows more fluid particles on either side to have thermal energy transferred in a shorter period of time. Some applications of these devices can be in space heating, refrigeration, and natural gas processing or sewage treatment plants. One classic application of such heat exchangers between a liquid and a gas is in the engine of cars. Cars require an engine coolant to constantly flow through the system to prevent it from over heating. Even though the coolant cools the engine, it heats itself through the process; thus, requires a heat exchanger to cool it down. This happens in the internal combustion engine where the coolant flows through radiator coils and cool air outside the coils is used to cool the coolant fluid. The cooled coolant fluid leaves the radiator coils and returns to cool the engine. Heat exchangers that feature a wall to separate the fluids are referred to as direct transfer type heat exchangers and those that allow for intermittent transfer between the fluids are called storage types or indirect transfer types.

Also, there are three primary ways of how fluid flows in heat exchangers in order to aid cooling or heating; with the three being counter-flow, cross-flow and parallel-flow. Counter-flow, as its name would suggest is when the cooling or heating fluid flows in the opposite direction from primary fluid which is to be cooled or heated. This form of flow is considered as the most efficient as it is able to transfer the greatest amount of energy to the fluid as compared to the others. Parallel-flow sees both the cooling/heating fluid as well as the primary fluid coming in from the same direction. They move in the same direction and as they move, the primary fluid is heated or cooled. Cross-flow has the cooling/heating fluid moved in the direction of the primary fluid perpendicularly.

Types of Heat Exchangers

Shell and Tube

Usually used in petroleum refining, chemical refining and heat recovery applications, these heat exchangers are made with round tubes that are mounted in cylindrical shells. Each shell can contain several tubes and the axes of these tubes are coaxial with the shell axis. This forms the basic structure of these heat exchangers, and slight variations to this design makes it usable for different applications and materials.

Not all the tubes in the heat exchanger contain the primary liquid that is required to be heated or cooled. One set of the tubes in the heat exchanger contain the liquid that has to be cooled or heated whereas the second set of tubes, contains fluid inside which would provide the primary fluid with heat or cool them, run over the tubes that contain the primary fluid. A set of tubes is referred to as the tube bundle and it can contain several different types of tubes such as longitudinally finned, u-bent, plain, etc.

Fluid Heat Exchangers

Often used for cooling gases, fluid heat exchangers are not only able to transfer thermal energy, but are also able to remove impurities. The gas to be cooled is passed upwards through a heat exchanger with a shower of fluid coming down the same channel. As the gas passes through the fluid, it cools down. The shower of fluid is usually water; thus, is able to remove any floating impurities in the gas fluid. This form of heat exchanger is often used in espresso machines as an energy saving method to cool super-heated water that would be used in the extraction of espresso.

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