CLASSIFICATION OF HEAT EXCHANGERS BASED ON CONSTRUCTION

Classification of heat exchangers based on construction

Plate-Type Heat Exchangers

• Plate-type heat exchangers are usually built of thin plates (all prime surface). 
• The plates are either smooth or have some form of corrugation, and they are either flat or wound in an exchanger. 
• Generally, these exchangers cannot accommodate very high pressures,temperatures, or pressure and temperature differences. 

• Plate heat exchangers (PHEs) can be classified as gasketed, welded (one or both fluid passages), or brazed, depending on the leak tightness required. 
• Other plate-type exchangers are spiral plate, lamella, and platecoil exchangers.

Extended Surface Heat Exchangers

• The tubular and plate-type exchangers are all prime surface heat exchangers, except for a shell-and-tube exchanger with low finned tubing. 
• In some applications, much higher (up to about 98%) exchanger effectiveness is essential, and the box volume and mass are limited so that a much more compact surface is mandated.
• Also, in a heat exchanger with gases or some liquids, the heat transfer coefficient is quite low on one or both fluid sides. This results in a large heat transfer surface area requirement.
• One of the most common methods to increase the surface area and exchanger compactness is to add the extended surface (fins) and use fins with the fin density ( fin frequency, fins/m or fins/in.) as high as possible on one or both fluid sides, depending on the design requirement. 
• Addition of fins can increase the surface area by 5 to 12 times the primary surface area in general, depending on the design. 
• The resulting exchanger is referred to as an extended surface exchanger. 
• Flow area is increased by the use of thingauge material and sizing the core properly.
• The heat transfer coefficient on extended surfaces may be higher or lower than that on unfinned surfaces. 

• Plate-fin and tube-fin geometries are the two most common types of extended surface heat exchangers.

Regenerators

• The regenerator is a storage-type heat exchanger. 
• Regenerator, is a type of heat exchanger where heat from the hot fluid is intermittently stored in a thermal storage medium before it is transferred to the cold fluid. 
• To accomplish this the hot fluid is brought into contact with the heat storage medium, then the fluid is displaced with the cold fluid, which absorbs the heat.

• The operation of regenerative heat exchangers is cyclic.
• In the first cycle hot gases/fluids passing through the media heat up the media. In the following cycle, the cold gases pass through the media and they are heated by the already hot media.
• Typical medias used are packed towers of ceramic material with required gaps for the gases to pass through them.
• In regenerative heat exchangers, the fluid on either side of the heat exchanger can be the same fluid.