Heat Exchangers

Heat Exchangers: A heat exchanger is a device built for efficent heat transfer from one medium to another, whether the media are separated by a solid wall so that they never mix, or the media are in direct contact.

Shell and Tube Heat Exchangers: Shell and tube heat exchangers consist of a series of tubes.  One set of these tubes contains the fluid that must be either heated or cooled.  THe second fluid runs over the tubes that are being heated or cooled so that it can either proved the heat or absorb the heat required.  A set of tubes is called the tube bundle and can be made up of several tpes of tubes: Plain, logitudinally finned etc.  Shell and tube Heat Exchangers are typically used for high pressure applications (with pressures greater than 30 bar and temperatures greater than 260 degree C.  This is because the shell and tube heat exchangers are robust due to their shape. 


Plate Heat Exchanger: Another type of heat exchanger is the plate heat exchanger.  One is composed of multiple, thin, slightly-seperated plates that have very large surface areas and fluid flow passages for heat transfer.  This stacked-plate arrangement can be more effective, in a given space, than the shell and tube heat exchanger.  Advances in gasket and brazing technology have made the plate-type heat exchanger increasingly practical.  In HVAC applications, large heat exchangers of this type are called plate-and-frame; when used in open loops, these heat exchangers are normally of the gasketed type to allow periodic disassembly, cleaning and inspection.  There are many types of permanently-bonded plate heat exchangers, such as dip-brazed and vacuum-brazed plate varieties, and they are often specified for closed-loop applications such as refrigeration.  Plate Heat Exchangers also differ in the types of plates that are used, and in the configurations of those plates.  Some plates may be stamped with "chevron" or other patterns, where others may have machined fins and/or grooves.

Regenerative Heat Exchanger: A third type of heat exchanger is the regenerative heat exchanger.  In this, the heat from a process is used to warm the fluids to be used in the process, and the same type of fluid is used either side of the heat exchanger (these heat exchangers can be either plate-and-frame or shell-and-tube construction).  These exchangers are used only for gases and not for liquids.  The major factor for this is the heat capacity of the heat transfer matrix. 

Adiabatic Wheel Heat Exchanger: A fourth type of heat exchanger 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.  Two examples of this are adiabatic whells, which consist of a large wheel with fine threads rotating through the hot and cold fluids, and fluid heat exchangers.  This type is used when it is acceptable for a small amount of mixing to occur between the two streams.

Fluid Heat Exchangers: This is a heat exchanger with a gas passing upwards through a shower of fluid (often water), and the fluid is then taken elsewhere before being cooled.  This is commonly used for cooling gases whilst also removing certain impurities, thus solving two problems at once.  It is widely used in espresso machines as an energy saving method of cooling super-heated water to be used in the extraction of espresso. 

Dynamic Scraped surface heat exchanger: Another type of heat exchanger is called "dynamic heat exchangers" or "scraped-surface heat exchanger".  This is mainly used for heating or cooling with high-viscosity products, crystallisation processes, evaporation and high fouling applications.  Long running times are achieved due to the continuous scraping of the surface, thus avoiding fouling and achieving a sustainable heat transfer rate during the process. 

Phase-change Heat Exchanger: In addition to heating up or cooling down fluids in just a single phase, heat exchangers can be used either to heat a liquid to evaporate (or boil) it or used as condensers to cool a vapor and condense it to a liquid.  In chemical plants and refineries, reboilers used to heat incoming feed for distillation towers are often heat exchangers.  

HVAC air coils: One of the widest uses of heat exchangers is for air conditioning of building and vehicles.  This class of heat exchangers is commonly called air coils, of just coils due to their often-serpentine internal tubing.  Liquid-to-air, or air-to-liquid HVAC coils are typically of modified crossflow arrangement.  In vehilces, heat coils are often called heater cores.

Spiral Heat Exchangers: A spiral heat exchanger (SHE), may refer to a helical (coiled) tube configuration, more generally, the term refers to a pair of flat surfaces that are coiled to form the two channels in a counter-flow arrangement.  Each of the two channels has long long curved path.  A pair of fluid ports are connected tangentially to the outer arms of the spiral, and axial ports are common, but optional.