HEAT EXCHANGERS
In general, heat exchangers are the devices used to exchange heat between same or different forms of matter through conduction, convection or radiation. Industrially, the term ‘heat exchangers’ is used for devices that assist in exchanging heat between two fluids at different temperatures without physically mixing them. For example, room or water heaters are used to heat the ambient air or water by exchanging heat between the heating element and the surrounding air or water respectively. Also, refrigerators or air conditioners have heat exchangers to exchange the heat and cool the surrounding medium. Likewise, they have diverse applications in wide range of fields such as automobile industry, food and beverage industry, nuclear power production etc.
1. Shell and Tube heat exchangers
Shell and tube heat exchangers are the most commonly discussed heat exchangers. They consist of shell and a tube where fluids at two different temperatures are circulated parallel (co-current), perpendicular (cross flow), or anti-parallel (counter-current) to each other. A typical double pipe heat exchanger consists of only one tube inside a shell and is shown in Figure 1. A tube can have multiple passes before exiting a shell depending on the requisite amount of heat load to be transferred.2. Plate heat exchangers
These heat exchangers consist of several metal plates that are arranged together (brazed or connected by gaskets) such that hot and cold fluids pass through alternate plates. They are more compact in size than tube heat exchangers for the same amount of heat transferred between two fluids but suffer from the drawback of having limited utility at high pressures or temperatures. Usually, the maximum pressure and temperatures that can be used are 3MPa and 260°C, respectively. (Shah, 1994)
3. Plate-fin/ Tube-fin heat exchangers
The exchangers are similar to the ones mentioned above except that the plates/tubes have fins attached to them. The fins provide extended surface area for the heat transfer and enhance further the quantity of heat transferred. The construction cost of these exchangers is higher than exchangers discussed earlier.
Figure 1: Schematic showing a simple parallel flow shell and tube heat exchanger.
Heat exchanger design
The amount of heat absorbed/released by fluids is represented by
where 
, and 
are the flow rates of hot and cold fluids, respectively. Specific heats of hot and cold fluids are cph and cpc respectively while ΔTh and ΔTc are the absolute changes in temperatures between inlet and exit for hot and cold fluids, respectively. In shell and tube heat exchangers, amount of heat transferred per unit time, q is represented by (Holman, 2010),
where U is overall heat transfer coefficient, A is effective surface area for heat transfer, F is correlation factor and ΔTm is log mean temperature difference between two fluids.
Overall heat transfer coefficient depends on various properties such as type of exchangers. As well as physical properties of fluid such as density, viscosity etc, turbulence in flow, thickness of tubes/plates, thermal conductivity of the design material and fouling. For example, in case of a shell and tube heat exchanger overall heat transfer coefficient can be represented in terms of individual resistances to heat transfer on both inside and outside of the tube. In a simpler case, where U stays constant in the process, it can be represented as
where A, r and h represent total surface area of the tube, radius of the tube and heat transfer coefficient in the fluid, respectively. Suffixes o and i represent outside and inside of the tubes, respectively. L and k are the length of the tubes and thermal conductivity of the design material of the tubes, respectively. ΔTm, can be represented in terms of inlet and exit temperatures of fluids as
Apart from desired heat load, factors such as:
- Construction cost
- Design material cost
- Pressure loss in fluid while pumping through the exchangers
- Compatibility of tube/plate material with the fluids
Maintenance in heat exchangers
Fouling in heat exchangers is a major factor which reduces performance of heat exchangers in due course by reducing the overall heat transfer coefficient. Fouling is the modification of surface of plates/tubes over time due to several factors. For example, corrosion, magnesium/calcium deposits or biological factors such as algae settlements. Mechanical cleaning, treatment of inlet water or circulating cleaning fluids are some of the methods used for maintenance of heat exchangers. Some design materials such as stainless steel or titanium are more resistant to corrosion while copper alloys reduce biological fouling, thereby having higher performance.
Market Share/Demand
[1] Shah, R. K., 1994, Heat exchangers, in Encyclopedia of Energy Technology and the Environment, Wiley, New York, pp. 1651–1670.
[2] JP Holman, 2010, Heat Transfer, 10th Edition, McGraw Hill, New York.
[3] “Global Heat Exchangers Market Size, Share, Development, Growth and Demand Forecast to 2020”, P&S market research.
（content from the wattco）
