A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contact. They are widely used in space heating, refrigeration, air conditioning, power stations, chemical plants, petrochemical plants, petroleum refineries, natural-gas processing, and sewage treatment. The classic example of a heat exchanger is found in an internal combustion engine in which a circulating fluid known as engine coolant flows through radiator coils and air flows past the coils, which cools the coolant and heats the incoming air. Another example is the heat sink, which is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant.
Flow arrangement
According to their flow arrangement, there are three primary classifications of heat exchangers:
In parallel-flow heat exchangers, the fluids enter the exchanger at the same end, and travel in parallel to one another to exit the other side. This configuration is preferable at the expense of efficiency when the two fluids are intended to reach exactly the same temperature, as it reduces thermal stress and produces a more uniform rate of heat transfer. In counter-flow heat exchangers the fluids enter the exchanger from opposite ends. The counter current design is the most efficient, roughly 70-90% depending on flow rate and temperature differential, in that it can transfer the most heat from the heat (transfer) medium per unit mass due to the fact that the average temperature difference along any unit length is higher. See countercurrent exchange. In a cross-flow or cross-plate heat exchanger, the fluids travel roughly perpendicular to one another the with efficiencies between 50 and 70% in air.
To increase efficiency, heat exchangers are designed to maximize the surface area of the wall between the two fluids, while minimizing resistance (pressure drop) to fluid flow through the exchanger. Heat exchanger's efficiency can be increased by the addition of heat transfer features: fins, corrugations, dimples, or louvres in one or both directions, which increase surface area and may channel fluid flow or induce turbulence. However, care must be taken when adding heat transfer features that may trap or attract particles over operating time; resulting in clogging, fouling and increased resistance that reduces and may possibly eliminate over time any initial efficiency benefits vs. a heat exchanger without such features. To restore heat exchanger efficiency in the original clean state, preventative maintenance may be required to restore designed efficiency.
The driving temperature across the heat transfer surface varies with position, but an approximate 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.
Design, manufacturing regulations and validation The design and manufacturing of heat exchangers has numerous regulations, which vary according to the region in which they will be used.
Design and manufacturing codes include: ASME Boiler and Pressure Vessel Code (US); PD 5500 (UK); BS 1566 (UK); EN 13445 (EU); CODAP (French); Pressure Equipment Safety Regulations 2016 (PER) (UK); Pressure Equipment Directive (EU); NORSOK (Norwegian); TEMA; API 12; API 560; API 660.
Temperature Ratings In application heat exchangers can be roughly divided into low-temperature or high-temperature by maximum operating temperature in application. The former work up to 500–650 °C and generally do not require special design and material considerations. The latter work up to 1000 or even 1400 °C but require special design and material considerations.
Validation in Manufacturing Heat exchanger construction processes may need to be validated before use in application using the following tests:
thermodynamic performance (SAE J3094, etc.) burst at selected multiple of max pressure (expected or measured) in application leak (helium, gas, or hydrostatic filled) pressure pulsation, # of cycles at selected target pressures and temperatures heat soak (ASTM D3045, etc.) at selected target temperature cold impact (ASTM D5420, etc.) at selected target temperature thermal shock, # of cycles at selected target temperature range corrosion internal corrosion analysis with specified fluid external salt spray vibration on shaker table resonance, at range of frequencies using given amplitude and direction highly accelerated life test, using vibration profile (historical or measured) expected to be produced when installed in equipment
Construction Types Heat exchangers come in unique designs but generally fall into the following broad categories:
Double-pipe Shell and tube Tube and fin (finned plate) Plate and fin Plate and shell Adiabatic wheel Pilliow plate Direct contact
Double-pipe Double pipe heat exchangers are the simplest exchangers allowing for the lowest cost and maintenance. In the past they were widely used in application but now are mostly used to teach students heat exchanger design basics to students as the fundamental physics rules for all heat exchangers are the same. Their low efficiency coupled with comparatively large size, has led modern industries to shift use to more efficient heat exchangers like shell and tube or plate.
When one fluid flows through the smaller pipe, the other flows through the annular gap between the two pipes. These flows may be parallel or counter-flows in a double pipe heat exchanger.
The figure above illustrates the parallel and counter-flow flow directions of the fluid exchanger.
Shell and tube
Shell and tube heat exchangers consist of a series of tubes which contain fluid that must be either heated or cooled. A second fluid contained in an outer shell runs over the tubes that are being heated or cooled so that it can either provide the heat 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, bent, U-tubes etc. Shell and tube heat exchangers are typically used for high-pressure, high-temperature applications (with pressures greater than 30 bar and temperatures greater than 260 °C)Saunders, E. A. (1988). Heat Exchanges: Selection, Design and Construction. New York: Longman Scientific and Technical. because the shell and tube heat exchangers are robust due to their shape. There can be many variations on the shell and tube design; however construction should be implemented using industry standards: TEMA. Typically, the ends of each tube are connected to plenums (sometimes called water boxes) through holes in tube-sheets.
Shell and tube heat exchangers have many variations due to the variety of available design features: Tube diameter: Using a small tube diameter makes the heat exchanger both economical and compact. However, it is more likely for the heat exchanger to foul up faster and the small size makes mechanical cleaning of the fouling difficult. To prevail over the fouling and cleaning problems, larger tube diameters can be used. Thus to determine the tube diameter, the available space, cost and fouling nature of the fluids must be considered. Tube shape: The tube shape can be more oval shaped or an aerofoil to improve the heat transfer while reducing pressure drop. Tube thickness: The thickness of the wall of the tubes is usually determined to ensure: There is enough room for corrosion That flow-induced vibration has resistance Axial strength Availability of spare parts Hoop strength (to withstand internal tube pressure) Buckling strength (to withstand overpressure in the shell) Tube length: heat exchangers are usually cheaper when they have a smaller shell diameter and a long tube length. Thus, typically there is an aim to make the heat exchanger as long as physically possible whilst not exceeding production capabilities. However, there are many limitations for this, including space available at the installation site and the need to ensure tubes are available in lengths that are twice the required length (so they can be withdrawn and replaced). Also, long, thin tubes are difficult to take out and replace. Tube pitch: when designing the 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 of tubes, mainly used for the inner tubes, increases the turbulence of the fluids and the effect is very important in the heat transfer giving a better 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 piping. 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 support the tubes and direct fluid in a natural manner or to create turbulence (to increase fluid mixing) across the tube bundle. They run perpendicularly to the shell and hold the bundle, preventing the tubes from sagging over a long length. They can also prevent the tubes from vibrating. The most common type of baffle is the segmental baffle. The semicircular segmental 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 consideration for the conversion of pressure drop and heat transfer. For thermo economic optimization it is suggested that the baffles be spaced no closer than 20% of the shell's inner diameter. 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 doughnut baffle, which consists of two concentric baffles. An outer, wider baffle looks like a doughnut, whilst the inner baffle is shaped like a disk. This type of baffle forces the fluid to pass around each side of the disk then through the doughnut baffle generating a different type of fluid flow. Baffles are used to support the tubes, direct the fluid flow to the tubes in an approximately natural manner, and maximize the turbulence of the shell fluid. There are many various kinds of baffles, and the choice of baffle form, spacing, and geometry depends on the allowable flow rate of the drop in shell-side force, the need for tube support, and the flow-induced vibrations. There are several variations of shell-and-tube exchangers available; the differences lie in the arrangement of flow configurations and details of construction. Tubes & fins Design: in application to cool air with shell-and-tube technology (such as intercooler / charge air cooler for combustion engines), the difference in heat transfer between air and cold fluid can be such that there is a need to increase heat transfer area on air side. For this function fins can be added on the tubes to increase heat transfer area on air side and create a tubes & fins configuration.