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Introduction
A heat exchanger is a device that is used to transfer thermal energy (enthalpy) between two or more fluids, between a solid surface and a fluid, or between solid particulates and a fluid, at different temperatures and in thermal contact.
In heat exchangers, there are usually no external heat and work interactions
Components of Heat Exchanger
Heat transfer elements: core or matrix containing the heat transfer surface
Is in direct contact with fluids and through which heat is transferred by conduction.
Fluid distribution elements such as headers, manifolds, tanks, inlet and outlet nozzles or pipes, or seals.
ACCORDING TO TRANSFER PROCESS
Indirect-Contact Heat Exchangers
The fluid streams remain separate and the heat transfers continuously through an impervious dividing wall or into and out of a wall in a transient manner.
Thus, ideally, there is no direct contact between thermally interacting fluids. This type of heat exchanger, also referred to as a surface heat exchanger, can be further classified into
direct-transfer type
storage type
fluidized-bed exchangers
ACC TO SURFACE COMPACTNESS
1. Tubular heat exchangers
It can be designed for high pressures relative to the environment and high-pressure differences between the fluids.
Tubular exchangers are used primarily for liquid-to-liquid and liquid-to-phase change (condensing or evaporating) heat transfer applications.
They are used for gas-to-liquid and gas-to-gas heat transfer applications primarily when the operating temperature and/ or pressure is very high or fouling is a severe problem on at least one fluid side and no other types of exchangers would work.
2.Direct-Contact Exchanger
Two fluid streams come into direct contact, exchange heat, and are
then separated.
Common applications of a direct-contact exchanger involve mass
transfer in addition to heat transfer, such as in evaporative cooling
and rectification.
Compared to indirect contact regenerators, in direct-contact heat
exchangers are
1. Very high heat transfer rates are achievable
2. The exchanger construction is relatively inexpensive
3. The fouling problem is generally non existent, due to the
absence of a heat transfer surface (wall) between the two
fluids.
ACCORDING TO NUMBER OF FLUIDS
Most processes of heating, cooling, heat recovery, and heat rejection involve transfer of heat between two fluids. Hence, two-fluid heat exchangers are the most common.
Three fluid heat exchangers are widely used in cryogenics and some chemical processes (e.g., air separation systems, a helium–air separation unit, purification and liquefaction of hydrogen, ammonia gas synthesis).
Heat exchangers with as many as 12 fluid streams have been used in some chemical process applications. The design theory of three- and multi fluid heat exchangers is algebraically very complex .
1.1 Shell and tube heat exchanger
These exchangers are built of a bundle of round tubes mounted in a cylindrical shell with the tube axis parallel to that of the shell.
One fluid flows inside the tubes, the other flows across and along the tubes.
The major components of this exchanger are tubes (or tube bundle), shell, frontend head, rear-end head, baffles, and tubes sheets.
2.Plate heat exchanger
Heat exchangers consist of thin plates joined together, with a small amount of space between each plate, typically maintained by a small rubber gasket.
The surface area is large, and the corners of each rectangular plate feature an opening through which fluid can flow between plates, extracting heat from the plates as it flows.