TYPES OF HEAT EXCHANGERS

A piece of equipment often needed and encountered in heat transfer practice is the so-called "heat exchanger”. Basically, this is the device, which allows the energy transfer, because of temperature difference, between a hot fluid and a cold fluid. This occurs either across a solid barrier interposes between the two fluids to prevent their mixing, or to and from this solid wall as a result of the alternate passing of hot and cold fluids over the barrier. In the figure shown below is a schematic picture of such a general heat exchanger .In general, there are also, as shown in figure, other surfaces whose function it is to confine one or both of the fluids and which have essentially no heat transfer across them. In many heat exchangers, the barrier between the fluids will be a tube wall or a plane wall and the other containment barriers might be circular cylindrical surfaces or plan surfaces.

Heat exchangers are commonly used in air conditioning and refrigeration systems, space heating systems, power production systems, in the chemical process industry, and in engines of all the types. Specific examples include the radiator of an automobile, where outside air is used to cool the liquid water often employed as the engine coolant; the condenser of household refrigerator, where energy is being rejected from the refrigerant, such as Freon-12, to the room air; the boiler of the large power plant, where energy is being transferred from hot combustion gases to liquid water in order to generate steam; and a fuel oil cooler in jet engine, where lubrication oil is cooled by having it slightly preheat fuel going to he combustor.

Sizes of heat exchangers range from relatively small units, such as a fuel oil cooler which can be held in one hand, to extremely large units, such as power plant boilers and feed water preheaters, whose overall size may dwarf a person and which may have 5000 or 10,000 meter square of heat transfer area.

Types Of Heat Exchangers: -

                                                Heat exchangers can be grouped into the three broad classes:

1. Transfer type heat exchangers or recuperators

2. Storage type heat exchangers or regenerators

3. Direct contact type heat exchangers or mixers

1. Transfer Type Heat Exchangers: -

                                                                         In the transfer type heat exchangers, the two fluids are kept separate and they do not mix as they flow through it. Heat is transferred through the separate walls. A concentric double pipe recuperator is shown following.

FIG. (2) Concentric double pipe heat exchanger

2. Storage Type Heat Exchangers: -

                                                     In the regenerators, the hot and cold fluids flow alternatively through a solid matrix of high heat capacity. When the hot fluid flows through the matrix in an interval of time, heat is transferred from the fluid to the matrix, which stores it in the form of an increase in its internal energy. This stored energy is then transferred to the cold fluid as it flows through the matrix in the next interval of time. The matrix is then subjected to periodic heating and cooling.

Storage type heat exchangers may have the matrices, which are either (1) stationary or (2) rotating. Figure (3) shows a typical regenerator with a stationary matrix. During the heating period of the cycle when the hot fluid flows through the matrix, valves A and B are kept open and C and D are kept close. During the cooling period, valves A and B are kept close and C and D are kept open. A regenerator with a stationary matrix is used in a Stirling refrigerator, such as Philips refrigerating machine for liquefaction of air, and in a gas turbine power plant.

FIG. (3) Single matrix storage type heat exchanger

Rotary Regenerator: -

                               A rotary regenerator has a matrix rotating at a low rpm, driven by a motor through a reduction gears. The heat transfer surfaces provided in the regenerator are alternatively exposed to the hot and cold fluids (Fig.4).A typical application of this type of heat exchanger is found in a steam power plant for preheating of air, called Ljungstrom air preheater.

FIG. (4) Rotary storage type heat exchanger

A storage type heat exchanger provides a more compact arrangement than the transfer type with more surface area offered per unit volume. The major disadvantage is that some mixing of hot and cold fluids becomes inevitable, and it is quite difficult to seal the hot side from the cold side in the rotary regenerator. There are also more pressure drops in both the fluids.

Direct Contact Type Heat Exchangers: -                                 

                                                                   In direct contact type heat exchangers, the two fluids mix together and transfer heat by direct contact. Open feed water heaters, desuperheaters, cooling towers and jet condensers are examples of such heat exchangers. The heat transfer is usually accompanied by interphase mass transfer. It cannot be used for transferring of heat between two gases or between two miscible liquids. A typical direct contact heat exchanger is shown in figure (5), which gives a section through a natural draft-cooling tower.

FIG. (5)   Direct contact heat exchanger

Flow Arrangement In Recuperative Heat Exchangers: -

There are three basic flow arrangements in recuperative heat exchangers:

1. Parallel flow

2. Counter flow

3. Cross flow

If both the fluids move in the same direction, it is a parallel flow heat exchanger. If both the fluids move in the opposite direction, it is counter flow heat exchanger. If they flow normal to each other, it is a cross flow eat exchanger. The temperature of the two fluids varies from inlet to exit of the heat exchanger.

  

FIG. (6)   Schematic drawing of (a) parallel flow (b) counter flow (c) cross flow heat exchanger

Compact Heat Exchangers: -

                                                         A heat exchanger having a large surface area per unit volume is called a compact heat exchanger. The ratio of the heat transfer surface area to the volume is called the area density. The large surface area is obtained by attaching closely spaced thin plates or corrugated fins to the walls separating the two fluids. Compact heat exchangers are commonly used in gas-to-gas or gas-to-liquid heat transfer, with limitations on their weight and volume, with fins, if any, being used on the gas side where heat transfer coefficient is low. 

In compact heat exchangers, the two fluids usually move perpendicular to each other, and such flow configuration is called cross flow, as stated earlier. The cross flow is further classified as unmixed flow and mixed flow. Figure (6) (a) the cross flow is said to be unmixed, since the plate fins forced the fluid to flow through a particular interfin spacing and prevent it from moving in the transverse direction. The cross flow in (b) is said to be mixed since the fluid flow is free to move in the transverse direction. 

FIG. (7) Cross flow heat exchanger (a) Fluids unmixed; (b) One fluid mixed, other unmixed

Perhaps the most common type of heat exchanger in industrial application is the shell-and-tube heat exchanger (Fig.7).A large number of tubes is packed inside a shell with their axes parallel to that of the shell. Heat transfer takes place as one fluid flows inside the tubes while the other fluid flows outside the tubes through the shell. Baffles are commonly placed in the shell to force the shell side fluid to flow across the shell to enhance heat transfer and to maintain uniform spacing between the tubes. Because of their relatively large size and weight, shell and tube heat exchangers are not suitable for use in automotive, aircraft and marine applications. At both ends of the shell there are headers where the fluid accumulates before entering the tubes and after leaving them.

  

  

FIG. (8)  Shell and tube heat exchanger (one shell pass and one tube pass)

Shell and tubes heat exchangers are further classified according to the number of shell and tube passes involved. Heat exchangers in which all the tubes make one U-turn in the shell, are called one-shell pass and two-tube pass heat exchangers. Likewise, a heat exchanger that involves two passes in the shell and four passes in the tubes is called a two-shell and four-tube pass heat exchanger (Fig.8).

FIG. (9) Multiple pass heat exchangers; (b) one shell pass and two tube pass; (c) two shell pass and four tube pass

An innovative type of heat exchanger, which has found widespread use, is the plate heat exchanger, which consists of a series of plates with corrugated flow passages (Fig. 9). The hot and cold fluids flow in alternate passages, and thus two hot fluid streams, resulting in very effective heat transfer, surround each cold fluid stream. The heat transfer capacity can be enhanced by simply adding more plates in series. They are well suited for liquid-to-liquid heat transfers applications, provided that the hot and cold fluid streams are at about the same pressure.

FIG. (10)  Plate heat exchanger

Shell and Coil Heat Exchangers: -

                                                                               The shell and coil heat exchangers are constructed using circular layers of helically corrugated tubes placed inside a light compact shell. The fluid in each layer flows in the opposite direction to the layer surrounding it, producing a criss-cross pattern. The large number of closely packed tubes creates a significant heat transfer surface within a light compact shell. The alternate layers create a swift uniform heating of fluids increasing the total heat transfer coefficient. The corrugated tubes produce a turbulent flow where the desired feature of fluctuating velocities is achieved. This haphazard movement of fluid particles reduces deposit buildup by performing a "scoop and lift" action. The connection locations and angle of entry is specially selected to reduce the probability of debris buildup.

Shell and Coil Construction Features: -

The shell and coil tube series are manufactured as a singe unit with no removable parts. The coiled tube bundles are welded to a compact tube sheet located within the entry and exit connections. The cylindrical shell is terminated by hemi-spherical heads. Design variations include smooth or grooved tubes, angled or 90 connections in flanged or NPT termination, and 316L stainless or titanium components.

Spiral Coil Heat Exchanger: -

                                                  A spiral coil heat exchanger (SCHE) consists of a number of horizontal coil layers of spirally wound, finned tubes (Figure 1). Each coil is formed by bending straight circular-section tubes into circular spiral turns of reducing radii. The SCHE unit is composed of layers of the spiral coils stacked on top of one another. The inner and outer ends of the layers of the spiral coils are attached to headers parallel to the axis of the coils. In operation, hot gases can be directed to flow outwards radically from the inside or central core to the outside, or in the opposite direction. Liquid flow in the coiled tubes is countercurrent to the gas flow, whichever gas flow path is selected. The SCHE system may be designed using bare or finned tubes, depending on the nature of the hot gas flow.

An advantage, which the SCHE has over conventional heat exchanger systems, is its ability to accommodate differential thermal expansion. The system is more compact. For the same heat transfer surface, the spiral coil arrangement has a volume 15% smaller than that of a cross-flow heat exchanger. The SCHE system is formed by attaching the ends of each finned spiral layer to the vertical inlet and outlet manifolds. In this manner, the spiral layers may be readily removed for cleaning of the fouled heat transfer surfaces. This feature is particularly desirable in air-conditioning applications where clean air is important.

Theoretical models of the SCHE have been developed to describe the performance when it is operating as a sensible heat transfer unit for industrial waste heat recovery applications and as a cooling and dehumidifying unit for air-conditioning applications. Extensive experiments have been performed in the laboratory to verify the thermal performance of the SCHE in each of the two applications. Design charts are now available for the sizing of the SCHE unit to suit a particular application. A prototype of the SCHE had been installed in the Air-Handling Unit (AHU) plant room at the Faculty of Engineering and operated to supply conditioned air to one floor (19 rooms) of lecturer’s offices.

 Heat exchangers are often given specific names to reflect the specific application for which they are used. For example, a condenser is a heat exchanger in which one of the fluids gives up heat and condenses as it flows through the heat exchanger. A boiler is another heat exchanger in which one of the fluids absorbs heat and vaporizes.