Heat exchangers are the devices that are used to transfer heat from one fluid to other fluid. Many heat exchangers are been used by us in our daily lives that include condensers and evaporators that are used in air conditioning units and refrigerators.
Boilers and condensers are included in the large industrial heat exchangers in thermal power plants. The radiators and oil coolers are the form of heat exchangers in the vehicles. These heat exchangers are most abundant in all process industries and chemical industries. There are wide varieties of heat exchangers that are used for diverse purposes where the construction also differs widely. The heat exchangers can be classified into different types based on few fundamental concepts. Most frequent types of heat exchangers are used for the analysis and the design methodologies.Advertisement:
Classification of heat exchangers:
The heat exchanger classification is based on the devices basic operation, construction and the type of flow and their arrangements. The heat exchangers are classified as:
- Recuperators and Regenerators
- The direct contact transfer and indirect contact transfer processes.
- Based on geometry as tubes, plates and extended surfaces.
- The heat transfer mechanisms as single phase and two phase flows.
- Based on the flow arrangement as parallel flow, counter flow and cross flow.
Parallel flow and counter flow heat exchangers are the basic heat exchanger flow arrangements.
In a recuperator the available media are separated by a wall through which the heat is being transferred directly. Split systems are used as the intermediate medium that carries heat from a principal medium in the first heat exchanger in to the second medium in the second heat exchanger. The losses occur due to the conduction of heat through the wall in the direction of flow that can be reduced by the material that has low heat conductivity as plastics.
In a regenerator the heat from the primary medium is stored in thermal mass and it is then regenerated from that mass through a secondary medium. This thermal mass of the flow ducts through which the alternating flows of primary and secondary are led.
Direct contact heat exchangers:
Direct contact heat exchange takes place in between two process streams. These streams combination can be among solid, liquid and gas. Two direct contactors can be used in a series in order to extract heat from one gas stream and to transfer it to another. In can be easily used in all the systems. The complexity of the multiple components may overcome the economic advantage over the surface type heat exchangers.
Indirect contact heat exchangers:
Indirect contact heat exchange also takes place in between two process streams but there will not be any direct contact in between the streams.
Single phase heat exchangers:
It is a simplest form of heat transfer where the throughout media stay in the same phase in a process. SWEP BHPEs are common type of included engines that have this single phase heat exchange application. Used in many compact heat exchangers in different materials. Single phase heat transfer meets all the market demands.
Two phase heat exchangers:
This two phase heat transfer is mostly referred to condensers and evaporators. In this type of heat exchangers a back ground heat transfer mechanics and accordingly the design of heat exchanger is assumed.
Plate type, Shell type and extended surface type heat exchangers are shell and tube, plate, plate and shell, plate fin etc. all these heat exchangers almost work with the same principle of standard heat exchanger.
Parallel flow heat exchanger:
In a double pipe heat exchanger a parallel flow can be maintained by having the fluids entrance and exit on either side of the heat exchanger. Both the fluids will enter at different temperatures but they will exit at same temperature. This heat exchanger requires large surface area.
Counter flow heat exchanger:
In this type of heat exchanger the hot fluid enters from one side and the cold fluid enters from other side. A double pipe heat exchanger is used in type of heat exchanger. It is most commonly used is liquid to liquid type. It is most efficient type of heat exchanger.
These types of heat exchangers are found in car radiators and air conditioning systems where most of application is found. The air passing through the evaporator coils is cooled by a cold refrigerant present in it. gas and liquid media heat transfer takes place mostly in this type of heat exchangers.
Heat flows in between the hot and cold streams and due their differences in temperatures, a driving force acts and vary along the tube. From the heat exchanger equations the integrated average temperature difference for both parallel and counter floe can be written as:
This is used to recover the waste steam discharge from the industrial applications.
- Counter flow heat exchangers have larger LMTD value and also greater potential recovery. This permits even a smaller area of heat exchange. This will refer to a less expensive heat exchange equipment of the given application.
- In parallel flow heat exchangers the initial heating rate is very high and this advantage is used where the tube walls are to be developed with their temperature.
- The rapid initial heating will help to decrease the fluid viscosity that in turn reduces the pumping requirement which reduces the overall cost of the application.
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