A refrigeration system, no matter the size or application has one fundamental goal; the transfer of thermal energy from one place to another to produce a cooling effect. For the majority of extant systems, this ‘other place’ is out into the atmosphere, but the mechanism for how we arrive at that final step varies greatly depending on the type of heat exchange.
Rejection of heat to atmosphere is only one option of course, in many instances this waste heat may be re-utilised for other processes such Absorption Systems.
The transfer of heat occurs by one of three transfer mechanisms:
Radiation – Transfer of heat by electromagnetic waves
Convection – Heat transferred though particle movement
Conduction – Heat transferred through direct contact
In the exciting ever evolving world of industrial refrigeration, convection and conduction play the dominant role in achieving the desired outcomes, with radiation most being being a consequence rather than a tool for utilisation.
Owing to the wide scope of processes where heat transfer occurs, it follows that there is myriad substances with unique characteristics which all need to be taken into consideration. Because of this, it is simpler to design heat exchanges where the process is completely separate from the heat transfer medium as this standardises plant arrangements somewhat, simplifying design, installation and subsequent servicing.
Heat exchangers come in many configurations which are designed to be suited to a particular purpose, however in many cases the basis of design is the same. The most common arrangements for heat transfer in refrigeration systems are
Shell and tube
In making decisions on the type of heat exchange most suited to the application many factors come into play such as economic cost, available plant space, working fluid characteristics and the type of heat being transferred (latent vs. sensible).
Shell and Tube Heat Exchangers
Ideal for the transfer of heat between two fluidic substances where there is free movement of particles. Construction of these units consists of a serpentine coil housed within a large shell.
Figure 1 – Schematic View of Shell and Tube Heat Exchanger
This construction maintains a physical barrier between the two substances allowing simplified temperature control. The capacity of the unit can be increased or decreased by increasing the number of tubes within the shell thereby increasing the amount of surface area available for heat transfer. Once constructed however, the capacity of a shell and tube unit is fixed.
Shell and tube heat exchanges can also contain baffles and multiple tube passes to increase the heat transfer capability through increased fluid turbulence and increased surface area.
Benefits of shell and tube exchangers:
No moving parts
Plate Heat Exchangers
Common configuration for heat transfer between two fluids, but with an increased capacity in handling more viscous substances. Plate heat exchanges consist of a series of formed (usually metal) sheets which crate two completely separate fluid paths with a large surface area for heat exchange to take place.
These units are available in an almost endless variety of sizes. Though some plate heat exchanges are physically welded together to create a discrete unit, other configurations can be built up such that through the addition of more plates, the capacity of the unit can be increased almost infinitely.
Plate heat exchangers offer the most diverse configuration and construction styles of any exchanger, though these units are typically larger and have larger pressure drops associated with them. Flow directions through PHE’s can also be variable to suit the application. The schematic above shows a plate in counter-flow orientation, however in some instances concurrent flow is more appropriate.
Benefits of Plate Heat Exchangers
Large surface area to physical foot print ratio
Suitable for wide range of applications and materials
Most commonly seen in the form of a car radiator, a finned coil heat exchanger uses conduction and radiation as heat dissipation methods. At a basic level, construction consists of a serpentine coil which has been constructed with many thin metal plates (fins) in physical contact with the coil. Within the coil is an application specific fluent who’s primary purpose is to transport heat between, most commonly, atmosphere and the process. Air is generally drawn over the coil to absorb to the heat from the fluid, allowing the cycle to repeat.
Though a coil alone would provide a heat exchange surface, the addition of the fins creates a much larger surface area for the transfer of heat to take place and thus increases the unit capacity/efficiency. Heat is conducted along the fin surface, by virtue of being in physical contact with the tube to promote this efficiency. Depending on the application, the direction of heat transfer could be from the working fluid to the air, or visa versa as dictated by the temperature gradient between the two.
Finned coils are used extensively in refrigeration installations where the large surface areas are used to capture heat from the air being passed over it. Heat from the air is conducted through the fins to the tubes and subsequently to the fluid passing through the tube. It is in this way which most refrigerants generate their cooling effect as the cold fluid within the tube absorbs the heat energy from the room air and thus lowering its temperature.
Benefits of Finned Tube Heat Exchangers:
Large surface areas
Self contained units commonly available
Wide range of capacities