Types of Heat Exchangers
Heat transfer equipments are found to be useful in a wide range of industries. They can be broadly defined as equipments used to transfer heat from a hot medium to cold medium. The heat exchanger design and structure varies a lot depending on the application where they are used and the heat exchange media involved.
Heat transfer equipments can be classified in various types based on structure, phases of fluid involved and principle of operation.
Plate and Frame Exchangers - Plate exchangers are compact in structure and much cheaper compared to shell and tube heat exchangers. As shown in the schematic below, these exchangers consist of a series of corrugated metal plates parallel to each other held together by gaskets located in corners of each rectangular plate (indicated by holes in the following schematic). These gaskets also contain hot and cold fluids, indicated in the schematic by red and blue colors respectively. This series of plates forms chambers which are alternatively filled with hot and cold fluids, so that each corrugated plate is separating hot and cold fluid providing large area for heat transfer between these fluids. The space between two plates determines the heat transfer coefficients and also the pumping cost. Although the pressure drop to push liquid through the this space may be expected to be quite high, usually pressure drop per unit heat exchanged turns out to be lower than that for shell and tube heat exchangers.
These exchangers can be used for fluids at low or medium pressures and are very effective for applications involving space constraints.
Spiral Heat Exchangers – Spiral heat exchangers are very basic in their structure. They consist of two separate spiral chambers as shown in the schematic below. These two chambers house hot and cold liquids separated from each other by spiral metal sheet. Heat transfer coefficients on both sides are high. The hot and cold fluid flows are countercurrent to each other all the way through the exchangers. These factors lead to much lower surface area requirements than shell and tube exchangers. These exchangers can be used for highly viscous fluids at low, medium pressures.
Plate-Fin Heat Exchangers – In principle, these exchangers are similar to the plate and frame exchanges, involving alternating chambers of hot and cold fluids separated by thin metal sheets. The difference is that, two metal sheets which form one chamber are separated by wavy, perforated metallic fins which form channels to allow the passage of fluid. Two opposite sides of each chamber are sealed and other two sides allow the inflow and outflow of fluid. The sealed sides are rotated at 90 degrees for alternating chambers. So the hot and cold fluid flows are always at 90 degrees to each other. These exchangers can be efficiently used for a wide range of applications, wide range of temperature and pressure conditions.
Air coolers – utilize the ambient air for cooling the fluids which have a significantly higher temperature than ambient air (around 150C). The hot fluid flows through finned tubes across which air is blown by fans. Depending on whether the fans are located above or below the tubes, the coolers can be classified as induced draft coolers or forced draft coolers respectively. Induced draft coolers pull the flow of air across tubes. Forced draft coolers with fans located below the tubes, push the flow of air across the tubes. For sufficiently high heat duties it may be more economical to use cooling water instead of cooling air provided adequate water supply is available.
Double Pipe Exchangers – These exchangers have a 'tube in tube heat exchanger' structure which gives them this name. Often the double pipes in these exchangers have a ‘U-tube or Hairpin structure’. The reason behind this structure is to mainly accommodate the thermal expansion of the tubes without involving expansion joints. These exchangers can be used for high pressure, high temperature and highly viscous service. Usually more corrosive fluid is pushed through the inner tube which can be equipped with scraping blades for easy cleaning. The inner heat exchanger tube can be finned on the outer surface to have increased area for enhanced heat transfer.
Shell and Tube Exchangers – are the most widely used exchangers in a variety of industries. This has given rise to certain standards for the design of heat shell and tube exchangers. TEMA (Tubular Exchangers Manufacturers Association) standards are most widely recognized heat exchangers design standards. The structure of these exchangers is made of a shellside and multiple smaller tubes passing through the shell. The shell may contain baffles to induce turbulence and to increase heat transfer by means of crossflow of fluids. The tubes are held together by tubesheets at both ends. There may be multiple shellside and tubeside passes in any exchanger.
The possible flexibility in design of these exchangers is mainly responsible for their wide use in a range of applications. These exchangers do not have design limitations in terms of temperature and pressure. These exchangers are easy to maintain. These exchangers can be easily designed to accommodate thermal stresses.