A heat exchanger is a device that allows heat from a fluid (a liquid or a gas) to pass to a second fluid (another liquid or gas) with the two fluids at different temperatures and in thermal contact. In heat exchangers, there are usually no external heat and work interactions. Heat exchangers are classified according to transfer processes, several fluids, and degree of surface compactness, construction features, flow arrangements, and heat transfer mechanisms.
Applications Heat Exchangers
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- Heating or cooling of a fluid stream.
- Evaporation or condensation of single- or multi-component fluid streams.
- To recover or reject heat.
- Sterilize, pasteurize, fractionate, distill, concentrate, crystallize, or control process fluids.
- Chemical and petrochemical plants.
- Air conditioning systems.
- Power production.
- Waste heat recovery.
- Automobile radiator.
- Central heating system.
- Electronic parts.
Classification of heat exchangers
In general, based on the relative flow direction of the two fluid streams heat exchangers are classified into two general classes as follows.
(i) Crossflow exchangers: Both the fluid streams cross each other in space at a right angle, Fig.1(a).
(ii) Parallel flow exchangers: Both the fluid streams move in a parallel direction in space. For example, shell and tube heat exchanger. If the fluid flows in the parallel direction, two situations may arise.
- Fluids flow in the same direction.
- Fluids flow in opposite directions.
When the fluid flow is in the same direction it is called “Parallel-flow” heat exchangers and when it is in the opposite direction called “Co-current flow” heat exchangers, Fig.1.
Shell and Tube Heat Exchanger
The shell and tube heat exchangers are the most commonly used heat exchangers in the chemical process industries. This type of heat exchanger consists of a bundle of tubes properly secured at the ends of tube sheets. The metal sheets have holes into which the tubes are fixed up to leak-proof joints. The entire tube bundle is placed inside a closed shell in such a way that it forms two immiscible zones for hot and cold fluids. One fluid flows through the tubes whereas the other fluid flows around the outside of the tubes within the space between the tube sheets and is enclosed by the outer shell. The proper fluid distribution of the shell side fluid is achieved by placing baffles normal to the tube bundle. Baffle creates turbulence in the shell side fluid and enhances the transfer coefficients for the shell side flow.
The tube heat exchanger, Fig.2, has one shell and one tube pass since both the shell and tube side fluid make a single traverse through the heat exchanger. Thus, this type of heat exchanger is designated as a 1-1 exchanger. If the tube fluid passes twice it is designated as 1-2 exchangers. Similarly, if the heat exchanger has 2 shell passes and 4 tube passes, it is designated as 2-4 exchangers. The number of a pass on the tube side is done by the pass partition plate. A pass partition plate is shown in Fig.3. The shell side pass can be created by a flat plate as shown in Fig.4.
In reality, this type of shell and tube heat exchanger is used in the process industry and is quite complex and is improved in design for thermal expansion stresses, tube fouling due to contaminated fluids, ease of assembling, and disassembling, size, weight, etc. The area available for the flow of the tube side fluid is inversely proportional to the number of passes. Thus, on increasing the number of passes the area reduces, and as a result, the velocity of the fluid in the tube increases, and henceforth the Reynolds number also increases. It results in an increased heat transfer coefficient but it is at the cost of a high-pressure drop. Generally, even numbers of tube passes are preferred for the multi-pass heat exchangers.
The commonly used method of classifying heat exchangers is indirect contact type or direct contact type heat exchangers.
(A) Indirect contact type heat exchangers:
In many heat exchangers, the fluids are separated by a heat transfer surface, and ideally, they do not mix or leak. Such exchanges are referred to as direct transfer types.
- Direct transfer type
- Storage type
- Fluidized bed type
(B) Direct contact type:
Exchangers in which there is intermittent heat exchange between the hot and cold fluids via thermal energy storage and release through the exchanger surface are referred to as indirect transfer types.
- Immiscible fluids
- Gas-liquid
- Liquid-vapour
Indirect-Contact Heat Exchangers
In an indirect-contact heat exchanger, the fluid streams remain separate and the heat transfers continuously through a dividing wall or into and out of a wall. There is no direct contact between thermally interacting fluids. This type of heat exchanger is also called a surface heat exchanger. These types of heat exchangers can be classified into direct-transfer type, storage type, and fluidized-bed type heat exchangers as described below.
(a) Direct-transfer type exchangers:
In this type, heat transfers continuously from the hot fluid to the cold fluid through a dividing wall, Fig.5. Although a simultaneous flow of two fluids is required in the exchanger, there is no direct mixing of these fluids because each fluid flows in separate fluid passages. In general, there are no moving parts in most such heat exchangers. This type of exchanger is designated as a recuperative heat exchanger or simply as a recuperator. Recuperator is a form of heat exchanger in which heating air is waste gases.
Some examples of direct transfer type heat exchangers are tubular, plate-type, and extended surface exchangers. The term recuperator is not commonly used in the process industry for shell-and-tube and plate heat exchangers, but they are considered recuperators. Recuperators are further sub-classified as prime surface exchangers and extended-surface exchangers. Prime surface exchangers do not use fins or extended surfaces on any fluid side. Plain tubular exchangers, shell-and-tube exchangers with plain tubes, and plate exchangers are good examples of prime surface exchangers.
(b) Storage type exchangers:
In the storage type exchanger, Fig.6, both the fluids flow alternatively through the same flow passages and thus the heat transfer is intermittent. The heat transfer surface is generally cellular in structure and is referred to as a matrix or it is a permeable solid material, referred to as a packed bed. When hot gas flows over the heat transfer surface the thermal energy from the hot gas is stored in the matrix wall, and thus the hot gas is cooled during the matrix heating period. As cold gas flows through the same passages later, the matrix wall gives up thermal energy, which is absorbed by the cold fluid. Thus, heat is not transferred continuously through the wall as in a direct-transfer type exchanger, but the corresponding thermal energy is alternately stored and released by the matrix wall. This storage type heat exchanger is also referred to as a regenerative heat exchanger, or simply as a regenerator.
To operate continuously and within a desired temperature range, the gases, headers, or matrices are switched periodically so that the same passage is used for hot and cold gases. The actual time required for hot gas to flow through a cold regenerator matrix is called the hot period or hot blow. Whereas the time required for cold gas to flow through the hot regenerator matrix is called the cold period or cold blow. It is not necessary to have hot- and cold-gas flow periods of equal duration. There is some unavoidable carryover of a small fraction of the fluid that remained in the passage to the other fluid stream after switching of the fluids; this is known as carryover leakage. If the hot and cold fluids are at different pressures, the leakage is from the high-pressure fluid to the low-pressure fluid past the radial, peripheral, and axial seals, or across the valves, referred to as pressure leakage. These leakages being unavoidable, regenerators are used exclusively in gas-to-gas heat and mass transfer applications with sensible heat transfer. In some applications, regenerators may transfer about 5% moisture from humid air to dry air.
(c) Fluidized-bed heat exchangers:
In a fluidized-bed heat exchanger, one side of a two-fluid exchanger is immersed in a bed of finely divided solid material, as shown in Fig.7. If the upward fluid velocity on the bedside is low, the solid particles will remain fixed in position in the bed and the fluid will flow through the interstices of the bed. If the upward fluid velocity is high, the solid particles will be carried away with the fluid. At a ‘‘proper’’ value of the fluid velocity, the upward drag force is slightly higher than the weight of the bed particles. This causes the solid particles to float with dilation of bed behave like a liquid and is referred to as a fluidized condition. In this condition, the fluid pressure drop through the bed remains almost constant, independent of the flow rate, and a strong mixing of the solid particles occurs. This causes the uniform temperature in the whole bed with an apparent thermal conductivity of the solid particles as infinity.
Very high heat transfer coefficients are achieved on the fluidized side compared to particle-free or dilute-phase particle gas. A chemical reaction is common on the fluidized side in many process applications, and combustion takes place in coal combustion fluidized beds. The common applications of the fluidized-bed heat exchanger are drying, mixing, adsorption, reactor engineering, coal combustion, and waste heat recovery. The initial temperature difference between the inlet temperature of the hot fluid and the fluidized bed is reduced by fluidization and thus the exchanger effectiveness is lowered.
Direct-Contact Heat Exchangers
In a 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 and heat transfer, such as in evaporative cooling and rectification. The enthalpy of phase change in such an exchanger represents a significant portion of the total energy transfer. The phase change generally enhances the heat transfer rate. Direct-contact heat exchangers have very high heat transfer rates, the exchanger construction is relatively inexpensive, and the fouling problem is generally non-existent due to the absence of a heat transfer surface between the two fluids. These exchangers are classified as follows.
(a) Immiscible Fluid Exchangers:
(i) Liquid-liquid exchanger: In this type, two immiscible fluid streams are brought into direct contact. These fluids may be single-phase fluids, or they may involve condensation or vaporization. For example, condensation of organic vapors and oil vapors with water or air.
(ii) Gas-liquid exchangers: In this type, one fluid is a gas (usually air) and the other a low-pressure liquid (commonly, water) and is readily separable after the energy exchange. In either cooling of liquid (water) or humidification of gas (air) applications, liquid partially evaporates and the vapor is carried away with the gas. In these exchanges, more than 90% of the energy transfer is by mass transfer due to the evaporation of the liquid and convective heat transfer is a minor mechanism. For example, water cooling tower with forced- or natural-draft airflow, air-conditioning spray chamber, spray drier, spray tower, and spray pond.
(b) Liquid–Vapour Exchangers:
In this type, steam is partially or fully condensed using cooling water, or water is heated with waste steam through direct contact in the exchanger. Non-condensable and residual steam and hot water are the outlet streams. For example, de-superheaters and open feedwater heaters (de-aerators) in power plants.
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