Heat exchangers are devices used to transfer heat between two or more fluids. They are used for both heating and cooling processes. The fluids may be separated by a solid wall to prevent mixing or may be in direct contact. They are widely used in refrigeration, chemical/ petrochemical processing plants, power stations, refineries and sewage treatment plants. They are also used onboard ships for cooling and heating of lubricating oil and fuel oil used for ship propulsion. An example of a heat exchanger is an internal combustion engine where 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.
Types of heat exchangers
- Shell and Tube type
- Plate type
- Plate and Shell type
- Plate Fin type
- Adiabatic type
- Fluid heat exchanger
- Phase change type.
Shell and Tube type
The shell and tube type consist of a series of tubes which contain fluids that are heated or cooled. A second fluid flows over the tubes and either provides the heat to heat the tubes or absorbs the heat from the tubes. Shell and tube heat exchangers are typically used for high pressure applications with pressures greater than 30 bar due to their robust shape.
Premature tube failure can be the result of pollution in coastal waters or extreme turbulence due to excessive sea water flow rates. To avoid the impingement attack, care must be taken with the water velocity through tubes. For aluminium- brass, the upper limit is about 2.5m/s. A more than minimum flow is is vital to produce moderate turbulence which is essential to the heat exchange process and to reduce silting and settlement in the tubes.
Plate type heat exchanger
The outstanding feature of plate type heat exchanger is that they are easily opened for cleaning. The major advantage over the Shell and Tube type is that their higher efficiency is reflected in a smaller size for the same cooling capacity. There are many types of permanently bonded plate heat exchangers such as dip brazed, vacuum brazed and welded plate varieties, and are often specified for closed loop applications such as refrigeration.
When compared with shell and tube heat exchangers the plate type heat exchanger has lower volume and cost. Another difference is that the plate type heat exchanger typically serve low to medium pressure fluids, compared to medium and high pressure fluids for shell and tube. Thirdly the plate heat exchanger allows more concurrent flow rates than cross flow rates which allow lower approach temperature differences high temperature changes and increased efficiencies.
Plate and Shell heat exchanger.
This type combines the plate heat exchanger and the shell and tube heat exchanger technologies. The heart of the heat exchanger contains a fully welded plate pack made by pressing and cutting round plates and welding them together. Plate and shell technology offers high heat transfer, high pressure, high operating temperature. It operates completely without gaskets, which provides security against leakage at high pressures and temperatures.
Adiabatic wheel heat exchangers
This type uses an intermediate fluid or solid store to hold heat,which is then moved to the other side of the heat exchanger to be released. Examples are adiabatic wheels which consist of a large wheel with fine threads rotating through the hot and cold fluids, and fluid heat exchangers.
Plate Fin heat exchanger
This type of heat exchanger uses “sandwiched” passages containing fins to increase the effectiveness of the unit. The designs include cross flow and counterflow coupled with varies fin configurations such as straight, offset and wavy fins. Plate and ton heat exchangers are usually made of aluminum alloys which provide high heat transfer efficiency. The material enables the system to operate at a lower temperature difference and reduce the weight of the equipment. These exchangers are mostly used for low temperature services such as natural gas, helium and oxygen liquefaction plants and motor and aircraft engines.
Fluid heat exchanger
This is a heat exchanger with a gas passing upwards through a shower of fluid usually water and the fluid is taken elsewhere before being cooled. This is commonly used for cooling gases while also removing certain impurities. Commonly used in espresso machines as an energy saving method in cooling superheated water to use in the extraction of espresso.
Phase change type
These types are used in power plants. To conserve energy and cooling capacity in chemical and other plants, regenerative heat exchangers can transfer heat from a stream that must be cooled to another stream that must be heated such as distillate cooling and reboiler feed preheating. It also refers to boilers that contain materials within their structure that has a change of phase. This is usually a solid to liquid phase due to the small volume difference between these states. This change of phase effectively acts as a buffer because of occurs at a constant temperature but still allows for the heat exchanger to accept additional heat.