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.

Types of heat exchangers

1.      Shell and Tube heat exchangers

2.      Plate  heat exchangers

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.

Also read: Kettle Reboiler- heat exchangers

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 c_ph and c_pc respectively while ΔT_h and ΔT_c 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 ΔT_m 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. ΔT_m, can be represented in terms of inlet and exit temperatures of fluids as

 
where T_h1 and T_h2 are the inlet and exit temperatures of hot fluid, respectively; while T_c1 and T_c2 are the inlet and exit temperatures of cold fluid, respectively. Value of correlation factor, F, depends on construction design of the heat exchangers such as number of shells or number of passes of tubes inside the shell of the exchangers and is equal to one for simple case of double pipe heat exchanger.

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

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.

Also read: Understanding the Benefits of Kettle-Type Heat Exchangers in Industrial Applications

 Market Share/Demand

According to the research conducted by P&S market research (P & S market research, 2016), market size of heat exchangers was valued at 14.1 billion dollars in 2014 and is estimated to grow with a CAGR of 6.5 % during the period of 2015-2020 with chemical industry expecting the highest growth at CAGR of 9.2% and Europe being the biggest market for heat exchangers.

[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, 10^th Edition, McGraw Hill, New York.
[3] “Global Heat Exchangers Market Size, Share, Development, Growth and Demand Forecast to 2020”, P&S market research.