CLASSIFICATION OF HEAT EXCHANGER BY CONSTRUCTION

Heat exchangers are frequently characterized by construction features into four major construction types  as follows

1. Tubular heat exchangers
2. Plate-type heat exchangers
3. Extended surface heat exchangers
4. Regenerative heat exchangers

Tubular heat exchanger

• These exchangers are generally built of circular tubes, although elliptical, rectangular, or round/flat twisted tubes.
• Tubular exchangers 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. 
• These exchangers may be classified as 

1. Shell-and tube heat exchangers
2. Double-pipe heat exchangers
3. Spiral tube heat exchangers

Shell-and-Tube Exchangers. 

• This exchanger, is generally 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. 

• A variety of different internal constructions are used in shell-and-tube exchangers, depending on the desired heat transfer and pressure drop performance and the methods employed to reduce thermal stresses, to prevent leakages, to provide for ease of cleaning, to contain operating pressures and temperatures, to control corrosion, to accommodate highly asymmetric flows, and so on. 
• The three most common types of shell-and-tube exchangers are 

1. Fixed tubesheet design
2. U-tube design
3. Floating-head type. 

The major components of this exchanger are tubes (or tube bundle), shell, frontend head, rear-end head, baffles, and tubesheets 

Tubes. 

• Round tubes in various shapes are used in shell-and-tube exchangers. 
• Most common are the tube bundles with straight and U-tubes used exchangers. 
• In most applications, tubes have single walls, but when working with radioactive,reactive, or toxic fluids and potable water, double-wall tubing is used. 
• In most applications, tubes are bare, but when gas or low-heat-transfer coefficient liquid is used on the shell side, low-height fins (low fins) are used on the shell side. 
• Tubes are drawn, extruded, or welded, and they are made from metals, plastics, and ceramics, depending on the applications.

Shells

• The shell is a container for the shell fluid.
• Usually, it is cylindrical in shape with a circular cross section, although shells of different shapes are used. 
• Although the tubes may have single or multiple passes, there is one pass on the shell side. 
• To increase the mean temperature difference and hence exchanger effectiveness, a pure counterflow arrangement is desirable for a two-tube-pass exchanger.  

Nozzles. 

• The entrance and exit ports for the shell and tube fluids, referred to as nozzles
• These are pipes of constant cross section welded to the shell and channels. 
• They are used to distribute or collect the fluid uniformly on the shell and tube sides. 

 Front- and Rear-End Heads. 

• These are used for entrance and exit of the tube fluid.
• The front-end head is stationary, while the rear-end head could be either stationary (allowing for no tube thermal expansion) or floating, depending on the thermal stresses between the tubes and shell. 
• The major criteria for selection of the front-end head are cost, maintenance and inspection, hazard due to mixing of shell and tube fluids, and leakage to ambient and operating pressures. 
• The major criteria for selection of the rear-end head are the allowance for thermal stresses, a provision to remove the tube bundle for cleaning the shell side, prevention of mixing of tube and shell fluids, and sealing any leakage path for the shell fluid to ambient. 

Baffles. 

• Baffles may be classified as transverse and longitudinal types. 
• The purpose of longitudinal baffles is to control the overall flow direction of the shell fluid such that a desired overall flow arrangement of the two fluid streams is achieved.
• Single- and double-segmental baffles are used most frequently due to their ability to assist maximum heat transfer (due to a high-shell-side heat transfer coefficient) for a given pressure drop in a minimum amount of space. 
• Triple and no-tubes-in-window segmental baffles are used for low-pressure-drop applications. 
• The choice of baffle type, spacing, and cut is determined largely by flow rate, desired heat transfer rate, allowable pressure drop, tube support, and flow-induced vibrations. 

Tubesheets. 

• These are used to hold tubes at the ends. 
• A tubesheet is generally a round metal plate with holes drilled through for the desired tube pattern, holes for the tie rods (which are used to space and hold plate baffles), grooves for the gaskets, and bolt holes for flanging to the shell and channel. 
• To prevent leakage of the shell fluid at thetubesheet through a clearance between the tube hole and tube, the tube-to-tubesheet joints are made by many methods, such as expanding the tubes, rolling the tubes, hydraulic expansion of tubes, explosive welding of tubes, stuffing of the joints, or welding or brazing of tubes to the tubesheet. 
• The leak-free tube-to-tubesheet joint made by the conventional rolling process.

Double-Pipe Heat Exchangers

• This exchanger usually consists of two concentric pipes with the inner pipe plain or finned.
• One fluid flows in the inner pipe and the other fluid flows in the annulus between pipes in a counterflow direction for the ideal highest performance for the given surface area

• However, if the application requires an almost constant wall temperature, the fluids may flow in a parallel flow direction. 
• This is perhaps the simplest heat exchanger. 
• Flow distribution is no problem, and cleaning is done very easily by disassembly. 
• This configuration is also suitable where one or both of the fluids is at very high pressure, because containment in the small-diameter pipe or tubing is less costly than containment in a large-diameter shell. 
• Double-pipe exchangers are generally used for small-capacity applications where the total heat transfer surface area required is 50m2 (500 ft2) or less because it is expensive on a cost per unit surface area basis. 
• Stacks of double-pipe or multi tube heat exchangers are also used in some process applications with radial or longitudinal fins. 

Spiral Tube Heat Exchangers. 

• These consist of one or more spirally wound coils fitted in a shell. 
• Heat transfer rate associated with a spiral tube is higher than that for a straight tube.
• In addition, a considerable amount of surface can be accommodated in a given space by spiraling. 
• Thermal expansion is no problem, but cleaning is almost impossible