A heat exchanger is a device that transfers heat between two fluids without them directly coming into contact or mixing with each other. It can be used to cool or heat air, liquids, or gases without contaminating them.
These are constructed in shell & tube or plate type. The objective could be to cool compressed air or gas designed to reduce the temperature or extract the heat from source for utlisation in required applications.
Classification of Heat Exchangers by Flow Configuration
There are four basic flow configurations:
- Counter Flow
- Concurrent Flow
- Crossflow
- Hybrids such as Cross Counterflow and Multi Pass Flow
Counter flow:
Below figure illustrates an idealized counterflow exchanger in which the two fluids flow parallel to each other but in opposite directions. This type of flow arrangement allows the largest change in temperature of both fluids and is therefore most efficient (where efficiency is the amount of actual heat transferred compared with the theoretical maximum amount of heat that can be transferred).
Concurrent flow: In cocurrent flow heat exchangers, the streams flow parallel to each other and in the same direction as shown in below figure, This is less efficient than counter current flow but does provide more uniform wall temperatures.
Cross flow: Crossflow heat exchangers are intermediate in efficiency between counter current flow and parallel flow exchangers. In these units, the streams flow at right angles to each other as shown in below figure.
Hybrid: In industrial heat exchangers, hybrids of the above flow types are often found. Examples of these are combined crossflow/counterflow heat exchangers and multi pass flow heat exchangers. See below figure as an example.
Classification of Heat Exchangers by Construction
Heat exchangers are also classified by their construction, as per below chart. The first level of classification is to divide heat exchanger types into recuperative or regenerative. A Recuperative Heat Exchanger has separate flow paths for each fluid and fluids flow simultaneously through the exchanger exchanging heat across the wall separating the flow paths. A Regenerative Heat Exchanger has a single flow path, which the hot and cold fluids alternately pass through.
Regenerative heat exchangers
In a regenerative heat exchanger, the flow path normally consists of a matrix, which is heated when the hot fluid passes through it (this is known as the “hot blow”). This heat is then released to the cold fluid when this flows through the matrix (the “cold blow”). Regenerative Heat Exchangers are sometimes known as Capacitive Heat Exchangers.
Regenerators are mainly used in gas/gas heat recovery applications in power stations and other energy intensive industries. The two main types of regenerators are Static and Dynamic. Both types of regenerators are transient in operation and unless great care is taken in their design there is normally cross contamination of the hot and cold streams. However, the use of regenerators is likely to increase in the future as attempts are made to improve energy efficiency and recover more low-grade heat. However, because regenerative heat exchangers tend to be used for specialist applications recuperative heat exchangers are more common.
Recuperative heat exchangers
There are many types of recuperative exchangers, which can broadly be grouped into indirect contact, direct contact and specials. Indirect contact heat exchangers keep the fluids exchanging heat separate by the use of tubes or plates etc.. Direct contact exchangers do not separate the fluids exchanging heat and in fact rely on the fluids being in close contact.
Other classifications of Heat Exchangers:
Indirect heat exchangers:
Shell and Tube type; Plate type
Shell and Tube type: consists of a number of tubes mounted inside a cylindrical shell. Below figure illustrates a typical unit that may be found in a petrochemical plant. Two fluids can exchange heat, one fluid flows over the outside of the tubes while the second fluid flows through the tubes. The fluids can be single or two phase and can flow in a parallel or a cross/counter flow arrangement. The shell and tube exchanger consists of four major parts:
- Front end–this is where the fluid enters the tube side of the exchanger.
- Rear end–this is where the tube side fluid leaves the exchanger or where it is returned to the front header in exchangers with multiple tube side passes.
- Tube bundle–this comprises of the tubes, tube sheets, baffles and tie rods etc. to hold the bundle together.
- Shell—this contains the tube bundle.
Plate type: consist of two rectangular end members which hold together a number of embossed rectangular plates with holes on the corner for the fluids to pass through. Each of the plates is separated by a gasket which seals the plates and arranges the flow of fluids between the plates, see below figure. This type of exchanger is widely used in the food industry because it can easily be taken apart to clean. If leakage to the environment is a concern it is possible to weld two plate together to ensure that the fluid flowing between the welded plates can not leak. However, as there are still some gaskets present it is still possible for leakage to occur. Brazed plate heat exchangers avoid the possibility of leakage by brazing all the plates together and then welding on the inlet and outlet ports.
All heat exchangers types must undergo some form of mechanical design. Any exchanger that operates at above atmospheric pressure should be designed according to the locally specified pressure vessel design code such as ASME VIII (American Society of Mechanical Engineers) or EN 13445 (European standard”Unfired pressure vessels”). These codes specify the requirements for a pressure vessel, but they do not deal with any specific features of a particular heat exchanger type. In some cases specialist standards exist for certain types of heat exchanger.
(Also see, ‘After cooler’)