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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.
Types
Double pipe heat exchangers are the simplest exchangers used in industries. On one hand, these heat exchangers are cheap for both design and maintenance, making them a good choice for small industries. On the other hand, their low efficiency coupled with the high space occupied in large scales, has led modern industries to use more efficient heat exchangers like shell and tube or plate. However, since double pipe heat exchangers are simple, they are used to teach heat exchanger design basics to students as the fundamental rules for all heat exchangers are the same.
1. Double-pipe heat exchanger
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.
(a) Parallel flow, where both hot and cold liquids enter the heat exchanger from the same side, flow in the same direction and exit at the same end. This configuration is preferable 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.
(b) Counter-flow, where hot and cold fluids enter opposite sides of the heat exchanger, flow in opposite directions, and exit at opposite ends. This configuration is preferable when the objective is to maximize heat transfer between the fluids, as it creates a larger temperature differential when used under otherwise similar conditions.[citation needed]
The figure above illustrates the parallel and counter-flow flow directions of the fluid exchanger.
2. Shell-and-tube heat exchanger
In a shell-and-tube heat exchanger, two fluids at different temperatures flow through the heat exchanger. One of the fluids flows through the tube side and the other fluid flows outside the tubes, but inside the shell (shell side).
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.
In application to cool air with shell-and-tube technology (such as intercooler / charge air cooler for combustion engines), fins can be added on the tubes to increase heat transfer area on air side and create a tubes & fins configuration.
3. Plate Heat Exchanger
A plate heat exchanger contains an amount of thin shaped heat transfer plates bundled together. The gasket arrangement of each pair of plates provides two separate channel system. Each pair of plates form a channel where the fluid can flow through. The pairs are attached by welding and bolting methods. The following shows the components in the heat exchanger.
In single channels the configuration of the gaskets enables flow through. Thus, this allows the main and secondary media in counter-current flow. A gasket plate heat exchanger has a heat region from corrugated plates. The gasket function as seal between plates and they are located between frame and pressure plates. Fluid flows in a counter current direction throughout the heat exchanger. An efficient thermal performance is produced. Plates are produced in different depths, sizes and corrugated shapes. There are different types of plates available including plate and frame, plate and shell and spiral plate heat exchangers. The distribution area guarantees the flow of fluid to the whole heat transfer surface. This helps to prevent stagnant area that can cause accumulation of unwanted material on solid surfaces. High flow turbulence between plates results in a greater transfer of heat and a decrease in pressure.
4. Condensers and Boilers Heat exchangers using a two-phase heat transfer system are condensers, boilers and evaporators. Condensers are instruments that take and cool hot gas or vapor to the point of condensation and transform the gas into a liquid form. The point at which liquid transforms to gas is called vaporization and vice versa is called condensation. Surface condenser is the most common type of condenser where it includes a water supply device. Figure 5 below displays a two-pass surface condenser.
The pressure of steam at the turbine outlet is low where the steam density is very low where the flow rate is very high. To prevent a decrease in pressure in the movement of steam from the turbine to condenser, the condenser unit is placed underneath and connected to the turbine. Inside the tubes the cooling water runs in a parallel way, while steam moves in a vertical downward position from the wide opening at the top and travel through the tube. Furthermore, boilers are categorized as initial application of heat exchangers. The word steam generator was regularly used to describe a boiler unit where a hot liquid stream is the source of heat rather than the combustion products. Depending on the dimensions and configurations the boilers are manufactured. Several boilers are only able to produce hot fluid while on the other hand the others are manufactured for steam production.
Heat exchangers are used to give control over the temperature in various processes to improve efficiency, prevent overheating or other potential hazards, and to improve safety.
For example, an oil cooler cools down hot oil by passing cold water next to the hot oil tube. The heat from the oil is transferred into the cold water, reducing the temperature of the oil.
Wherever heat is being generated in a process, heat exchangers can be used to keep the process safe, as well as use the heat energy most efficiently. As there are so many different places they can be used, there are a lot of different varieties.
If you don't understand any terms that we're using, refer to our heat exchanger glossary
heat exchangers work
Heat exchangers work differently according to the specific type. They use a combination of conduction and convection to move energy, in the form of heat, from one location or flow to another. They generally operate in a sensible heat range and occasionally engage the stored capacity of state change, enthalpy.
This could mean heating one medium by conducting heat into it. Alternatively, it could be cooling something by transferring heat from it into another medium, such as air or water. The goal is to improve efficiency or safety of a process. Some processes wouldn't work at all without a heat exchanger.
Read how each of the common types of heat exchanger works below.
Different types of heat exchanger
Shell and tube
Shell and tube heat exchangers involve one tube which contains the medium that needs cooling or heating. This is surrounded by the 'shell' which contains the fluid that cools or heats the first medium.
There are a few different varieties of shell and tube heat exchangers including counterflow, parallel flow and cross flow.
It depends on the medium being cooled or heated as to which would work best. When we're recommending a heat exchanger, we always discuss the application and goals to decide which variety will be most effective.
Finned Tube
Finned tube heat exchangers maximise surface area to improve the speed and efficiency of the heat transfer. This is especially helpful when the fluid or medium has a very low thermal conductivity, such as air.
They consist of a stack of evenly spaced plates, which are the fins. The tubes are placed through pre-cut holes in the fins. Thanks to their large surface area, finned tube heat exchangers are sometimes referred to as extended surface heat exchangers.
Plate heat exchangers are very similar to shell and tube heat exchangers. They maximise surface area through a stack of plates. On the whole, finned tube is used much more often now due to increased efficiency over standard plate type
Heat exchanger applications include power generation in power plants, heating and air con systems, refrigeration, manufacturing, food processing, chemical processing, car radiators and many others.
Heat exchangers are used in thousands of different places.
Around the home, they're commonly found in central heating combi boilers and help to heat and cool down the water efficiently and safely. They're also found in your refrigerator, ensuring it stays at a stable, cool temperature.
You're also likely to have benefited from heat exchangers in public places. Your local swimming pool would be much colder without a heat exchanger helping to keep the water warm.
Car engines produce a lot of heat and this needs to be managed effectively to prevent dangers. Cars often use a combination of fans and air flow, with fins to dissipate heat, and the use of a coolant fluid.
Heat exchangers are also used widely in different industrial applications. This includes power generation, the manufacture and storage of food, chemical engineering, and even in the running of air and marine transport, for example.
Sterling TT works with a range of industries to provide specialist heat exchangers. Find out more about the markets we serve.
Even in the defence sector, we find heat exchangers. They are installed, for example, on the navy surface and auxiliary ships as well as on submarines. They cool nuclear submarine propulsion motors.
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