Types of Heat Exchangers
The two most common types of heat exchangers are the shell-and-tube and plate-fin. In shell-and-tube heat exchangers, one fluid flows through a set of metal tubes while the second fluid passes through a sealed shell that surrounds them. The fluids can flow in the same direction (parallel flow), in opposite directions (counter-flow) or at right angles (cross flow).
The plate-fin heat exchanger is a design that uses plates and fins with a large surface area to transfer heat between fluids. It is often referred to as a compact heat exchanger to emphasize its relatively high heat transfer surface area to volume ratio.
The plate-fin heat exchanger design is widely used in many industries. Aluminum alloy plate (brazed) fin heat exchangers have been used in the aircraft and aerospace industries since the 1940s, and shortly afterward, they were implemented in cryogenic processes at natural gas chemical plants. They are also used in railway engines and vehicles (radiators). Stainless steel plate-fin heat exchangers, as well, are used in both the aircraft and process industries.
Compact exchangers, initially developed for the aerospace industry, were driven by requirements for a large heat transfer surface area, a lightweight material and a relatively small volume. Aluminum was the material of choice since it is easy to machine, relatively lightweight and has a high thermal conductivity.
For the process industry, the compact exchanger’s large heat transfer surface area to weight and volume ratio is not as important as the following considerations:
- The material, aluminum, is attractive for its superior mechanical properties at cryogenic temperatures.
- Brazed exchanger construction produces nearly ideal countercurrent flow among the process streams for optimum heat transfer.
- Even with relatively clean process fluids, brazed exchangers are constrained by operating pressure and temperature limitations. Aluminum rapidly loses its mechanical strength above ambient temperatures, although operating temperatures to 400°F are possible. Stainless steel is usually used in process applications up to 1000°F.
The basic construction consists of alternate layers of corrugated metal fins and plates that form a honeycomb structure, providing a high resistance to vibration and shock. At the edges of the plates are bars, which confine each fluid to the space between adjacent plates.
The structure is joined together by brazing, which is the process of bonding by heating and melting a filler (alloy) between the two pieces of metal. The filler has a melting temperature below that of the metal pieces. Brazing can seam dissimilar metals such as aluminum, silver, copper, gold and nickel.
The design may contain different corrugation and bar heights mounted between the plates. For a liquid stream application, a low height corrugation with high heat transfer coefficient and lesser surface area is often used. For low-pressure stream, a high corrugation height with matching low coefficient, higher surface area and a lower pressure drop is appropriate.
Corrugations are also made in various configurations and characteristics.
Types of corrugation:
Plain corrugation is the basic form and is used normally for low-pressure drop streams.
Perforated corrugation produces a slight increase in performance over plain corrugation, but this is reduced by the loss of area due to perforation. Perforated corrugation permits migration of fluid across fin channels, usually in boiling duties.
Serrated corrugation is made by cutting the fins every 3.2 mm and displacing the second fin to a point halfway between the preceding fins. This results in a significant increase in heat transfer.
Herringbone corrugation is made by displacing the fins sideways every 9.5 mm to make a zig-zag path. Performance characteristics fall somewhere between the plain and serrated forms. The friction factor continues to fall at high Reynolds numbers, resulting in superior performance at higher velocities and pressures than the serrated configuration.
Plate-fin heat exchangers offer several advantages over competing designs:
- High thermal effectiveness and close temperature approach (as low as 3K temperature approach between single phase fluid streams and 1K between boiling and condensing fluids is common)
- Large heat transfer surface area per unit volume (typically 1000 m2 /m3)
- Low weight
- Multi-stream operation (ten process streams can exchange heat in a single heat exchanger)
- True counter-flow operation (unlike the shell and tube heat exchanger, where the shell side flow is usually a mixture of cross and counter flow)
The principal limitations of the plate fin geometry are:
- Reduced range of temperature and pressure
- Difficult to clean passages, which limits its application to clean and relatively non-corrosive fluids
- Hard to repair in case of failure or leakage between passages
The plate-fin heat exchanger engineer can optimize the design by configuring for counterflow or crossflow and varying fin passage sizes, changing fin heights, fin pitch and fin thickness for each of the four standard fin types.
Although plate-fin heat exchangers should only be used with relatively clean process streams, the advantages of close temperature approaches, true countercurrent flow and a unique ability to exchange heat with multiple streams make them viable alternatives to traditional shell-and-tube exchangers in a variety of applications.