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Types of heat exchangers

Heat exchangers, cross flow, parallel flow, and counter flow.
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Overview

A heat exchanger is a device built for efficient heat transfer from one medium to another. The media may be separated by a solid wall so that they never mix, or they may be in direct contact. They are widely used in space heating, refrigeration, air conditioning, power plants, chemical plants, petrochemical plants, petroleum refineries, natural gas processing, and sewage treatment. One common example of a heat exchanger is the radiator in a car. The heat source is the hot engine-cooling fluid (water in most cases), that transfers heat to air flowing through the radiator - the heat transfer medium.

There are two primary classifications of heat exchangers according to their flow arrangement.
  • In parallel-flow heat exchangers, the two fluids enter the exchanger at the same end, and travel parallel to one another to the other side.
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  • In counter-flow heat exchangers the fluids enter the exchanger from opposite ends. The counter current design is most efficient, in that it can transfer the most heat from the heat (transfer) medium.
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  • In a cross-flow heat exchanger, the fluids travel roughly perpendicular to one another through the exchanger.
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For efficiency, heat exchangers are designed to maximize the surface area of the wall between the two fluids, while minimizing resistance to fluid flow through the exchanger. The exchanger's performance can also be affected by the addition of fins or corrugations in one or both directions, which increase surface area and may channel fluid flow or induce turbulence.

The driving temperature across the heat transfer surface varies with position, but an appropriate mean temperature can be defined. In most simple systems this is the "log mean temperature difference" (LMTD). Sometimes direct knowledge of the LMTD is not available and the NTU method is used.

Shell And Tube Heat Exchanger

Shell and tube heat exchangers consist of a series of tubes. One set of these tubes contains the fluid that must either be heated or cooled. The second fluid runs over the tubes that are being heated or cooled so that it can either provide or absorb the heat required. A set of tubes is called the tube bundle and can be made up of several types of tubes; plain, longitudinally finned, etc. Shell and tube heat exchangers are typically used for high-pressure applications (with pressures greater than 30 bar and temperatures greater than 260 degrees Celsius). This is because shell and tube heat exchangers are robust due to their shape. There are several thermal design features that must be taken into account when designing the tubes in the shell and tube heat exchangers. These include:

  • Tube diameter: Using a small tube diameter makes the heat exchanger both economical and compact. However, it is more likely to cause the heat exchanger to foul up, and the small size makes mechanical cleaning of the fouling difficult. To overcome the fouling and cleaning problems, larger tube diameters can be used. Thus, to determine the tube diameter, the available space, cost, and the fouling nature of the fluids must be considered.
  • Tube thickness: The thickness of the wall of the tube is usually determined to ensure:
1. There is enough room for corrosion

2. That flow-induced vibration has resistance

3. Axial strength

4. Availability of spare parts

5. Hoop strength (to withstand internal tube pressure)

6. Buckling strength (to withstand overpressure in the shell)

  • Tube length: heat exchangers are usually more cost effective when they have a smaller shell diameter and a long tube length. Thus typically, the aim is to make the heat exchanger as long as physically possible whilst not exceeding production capabilities. However, there are many limitations for this, including the space available at the site where it is going to be used and the need to ensure that there are tubes available in lengths that are twice the required length (so that the tubes can be withdrawn and replaced). Also, it has to be remembered that long, thin tubes are difficult to take out and replace.
  • Tube pitch: when designing tubes, it is practical to ensure that the tube pitch (i.e. the centre-centre distance of adjoining tubes) is not less than 1.25 times the tubes' outside diameter. A larger tube pitch leads to a larger overall shell diameter which leads to a more expensive heat exchanger.
  • Tube corrugation: this type, used mainly for the inner tubes, increases the turbulence of the fluids. This effect is very important in the transfer of heat to obtain improved performance.
  • Tube Layout: refers to how tubes are positioned within the shell. There are four main types of tube layout, which are, triangular (30), rotated triangular (60), square (90) and rotated square (45). The triangular patterns are employed to give greater heat transfer as they force the fluid to flow in a more turbulent fashion around the tubing. Square patterns are employed where high fouling is experienced and cleaning is more regular.
  • Baffle Design: baffles are used in shell and tube heat exchangers to direct fluid across the tube bundle. They run perpendicular to the shell and hold the bundle, preventing the tubes from sagging over a extended length. They can also prevent the tubes from vibrating. The most common type of baffle is the segmented baffle. The semicircular segmented baffles are oriented at 180 degrees to the adjacent baffles, forcing the fluid to flow upward and downwards between the tube bundle. Baffle spacing is of large thermodynamic concern when designing shell and tube heat exchangers. Baffles must be spaced with due consideration to the pressure drop and heat transfer. For thermo-economic optimization, it is suggested that the baffles be spaced no closer than 20% of the inner diameter of the shell. Having baffles spaced too closely causes a greater pressure drop because of flow redirection. Consequently, having the baffles spaced too far apart means that there may be cooler spots in the corners between baffles. It is also important to ensure the baffles are spaced close enough that the tubes do not sag. The other main type of baffle is the disc and donut baffle, which consists of two concentric baffles. The outer wider baffle looks like a donut, whilst the inner baffle is shaped like a disk. This type of baffle forces the fluid to pass around each side of the disk, and then through the donut baffle, generating a different type of fluid flow.

Plate Heat Exchanger

Another type of heat exchanger is the plate heat exchanger. This is composed of multiple, thin, slightly-separated plates that have very large surface areas and fluid flow passages for heat transfer.This stacked plate arrangement can be more effective, in a given space, than the shell and tube heat exchanger. Advances in gasket and brazing technology have made the plate-type heat exchanger increasingly practical. In HVAC applications, large heat exchangers of this type are called plate-and-frame; when used in open loops, these heat exchangers are normally of the gasketed type to allow periodic disassembly, cleaning, and inspection. There are many types of permanently- bonded plate heat exchangers, such as dip-brazed and vacuum-brazed plate varieties, and they are often specified for closed-loop applications such as refrigeration.

Adiabatic Wheel Heat Exchanger

A third type of heat exchanger uses an intermediate fluid or solid store to hold heat, which is then moved to the other side of the heat exchanger to be released. Two examples of this are adiabatic wheels, which consist of a large wheel with fine threads rotating through the hot and cold fluids, and fluid heat exchangers.

Plate Fin Heat Exchanger

This type of heat exchanger uses "sandwiched" passages containing fins to increase the effectiveness of the unit. The designs include crossflow and counterflow coupled with various fin configurations such as straight fins, offset fins and wavy fins. Plate and fin heat exchangers are usually made of aluminium alloys which provide higher heat transfer efficiency. The material enables the system to operate at a lower temperature and reduce the weight of the equipment. Plate and fin heat exchangers are mostly used for low temperature services such as natural gas, helium and oxygen liquefaction plants, air separation plants and transport industries such as motor and aircraft engines.

Advantages of plate and fin heat exchangers:
  • High heat transfer efficiency especially in gas treatment
  • Larger heat transfer area
  • Approximately 5 times lighter in weight than that of shell and tube heat exchanger
  • Able to withstand high pressure

Disadvantages of plate and fin heat exchangers:
  • Might cause clogging as the pathways are very narrow
  • Difficult to clean the pathways