Many types of heat exchangers have been developed to meet the widely varying applications. Based upon their,
i. Operating principle
ii. Arrangement of flow path
iii. Design and certain constructional features,
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The heat exchangers can be classified into the following categories:
1. Nature of Heat Exchange Process:
Based upon the nature of heat exchange process, the heat exchangers are classified into direct contact, regenerators and recuperators.
In direct contact or open heat exchangers, the energy transfer between the hot and cold fluids is brought about by their complete physical mixing; there is simultaneous transfer of heat and mass. Use of such units is restricted to the situations where mixing between the fluids is either harmless or is desirable.
Examples are water cooling towers and jet condensers in steam power plants. Figure 13.1., represents a direct contact heat exchanger. Steam is being bubbled into water; steam gets condensed and releases heat that warms up the water.
In a regenerator, the hot fluid is passed through a certain medium called matrix. The heat is transferred to the solid matrix and accumulates there; the operation is called heating period.
The heat thus stored in the matrix is subsequently transferred to the cold fluid by allowing it to pass over the heated matrix. The regenerators are quite often used in connection with engines and gas turbines. Other applications are- regenerators of open hearth and glass melting furnaces and air heaters of blast furnaces.
The operation of a regenerator is intermittent; the matrix alternately stores heat extracted from the hot fluid and then delivers it to the cold fluid. However in some of the regenerators the matrix is made to rotate through the fluid passages arranged side by side and that renders the heat exchange process continuous. The effectiveness of a regenerator depends upon the heat capacity of the regenerating material and the rate of absorption and release of heat.
In a recuperator, the fluids flow simultaneously on either side of a separating wall; the heat transfer occurs between the fluid streams without mixing or physical contact with each other.
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The wall provides an element of thermal resistance between the fluids and the heat transfer consists of:
i. Convection between the hot fluid and the wall
ii. Conduction through the wall
iii. Convection between wall and the cold fluid
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Such exchangers are used when the two fluids cannot be allowed to mix, i.e., when the mixing is undesirable. Majority of the industrial applications have exchangers of the recuperator type.
Notable examples are:
(i) Boilers, super-heaters and condensers; economisers and the air preheaters in steam power plants
(ii) Automobile radiators
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(iii) Condensers and evaporators in refrigeration units
(iv) Oil heaters for an airplane
(v) Heat exchanger inside a gas furnace etc.
The open-type (direct contact) heat exchangers and the recuperators operate under steady state conditions; and the transfer of heat inside a regenerator takes place essentially under transient conditions.
2. Relative Direction of Motion of Fluids:
According to the direction of flow of fluids, the heat exchangers are classified into three categories: parallel flow, counter flow and the cross flow.
In the co-current or parallel flow arrangement, the fluids (hot and cold) enter the unit from the same side, flow in the same direction and subsequently leave from the same side. Obviously the flow of fluids is unidirectional and parallel to each other.
In the counter-current or counter-flow arrangement, the fluids (hot and cold) enter the unit from opposite ends, travel in opposite directions and subsequently leave from opposite ends. Obviously the flow of fluids is opposite in direction to each other. For a given surface area, the counter-flow arrangement gives the maximum heat transfer rate and is naturally preferred for the heating and cooling of fluids.
In the cross-flow arrangement, the two fluids (hot and cold) are directed at right angles to each other. Figure 13.5 shows two common arrangements of cross-flow heat exchangers. In Figure 13.5 (a) the fluid A flows inside the separate tubes and its different streams do not mix. The fluid B flows over the tube banks and gets perfectly mixed. In Figure 13.5 (b), each of the fluid stays in prescribed paths and are not allowed to mix as they flow through the heat exchanger.
When mixing occurs, the temperature variations are primarily in the flow direction. When unmixed, there is temperature gradient along the stream as well as in the direction perpendicular to it. Apparently, temperatures of the fluids leaving the unit are not uniform for the unmixed streams. The cross flow heat exchangers are commonly employed in air or gas heating and cooling applications, e.g., the automobile radiator and the cooling unit of a air-conditioning duct.
3. Mechanical Design of Heat Exchange Surface:
(a) Concentric Tubes:
Two concentric pipes are used, each carrying one of the fluids. The direction of flow may correspond to unidirectional or counter flow arrangement.
(b) Shell and Tube:
One of the fluids is carried through a bundle of tubes enclosed by a shell. The other fluid is forced through the shell and flows over the outside surface of tubes. The direction of flow for either or both fluids may change during its passage through the heat exchanger.
(c) Multiple Shell and Tube Passes:
The two fluids may flow through the exchanger only once (single pass), one or both fluids may traverse the exchanger more than once (multi-pass). By suitable header design, the fluid within the tubes (tube side fluid) can be made to traverse back and forth from one end of the shell to the other.
Quite often longitudinal baffles are provided within the shell which cause the fluid surrounding the tubes (shell side fluid) to travel the length of shell a number of times. An exchanger having n-shell passes and m-tubes passes is designated as n-m exchanger.
A multiple shell and tube exchanger is preferred to ordinary counter-flow design due to its low cost of manufacture, easy dismantling for cleaning and repair and reduced thermal stresses due to expansion.
4. Physical State of Heat Exchanging Fluids (Condensation and Evaporation):
(a) Condenser:
The hot fluid (condensing steam) remains at constant temperature all along its passage through the heat exchanger, whilst the temperature of the other fluid (coolant water) gradually increases from inlet to outlet. Obviously the hot fluid loses only part of its heat.
(b) Evaporator:
During heat exchange in the evaporation of water into steam, the cold- fluid (boiling water) evaporates at constant temperature whilst the temperature of hot gases continuously decreases from inlet to outlet.
The heat exchangers can be further classified on the basis of following design parameters:
(i) Temperature and pressure levels of the fluid
(ii) Corrosiveness, toxicity and scale forming tendency of the fluids.
(iii) Economic considerations such as cost, ease of manufacture, necessary space and required life etc. The cost considerations may, however, be subordinate to weight and size limitation in space and aeronautical limitations. Exchangers of compact design are employed where weight, space and cost limitations are severe.