Heat Exchanger Basic – Types (Part 1)
Many of us have heard or perhaps work with heat exchangers in the industry. However, do you know the basics of heat exchanger? What is it all about? What is the principal behind it?
Heat exchangers are an equipment targeted to the efficient transfer of heat from a hot fluid flow to a cold fluid flow, in most cases through an intermediate metallic wall and without moving parts. We here focus on the thermal analysis of heat exchangers, but proper design and use requires additional fluid-dynamic analysis (for each fluid flow), mechanical analysis (for closure and resistance), materials compatibility, and so on.
Heat losses or gains of a whole heat exchanger with the environment can be neglected in comparison with the heat flow between both fluid flows; i.e. a heat exchanger can be assumed globally adiabatic. Thermal inertia of a heat exchanger is often negligible too (except in special cases when a massive porous solid is used as intermediate medium), and steady state can be assumed, reducing the generic energy balance to:
where the total enthalpy ht has been approximated by enthalpy (i.e. negligible mechanical energy against thermal energy), and D means output minus input.
Although heat flows from hot fluid to cold fluid by thermal conduction through the separating wall (except in direct-contact types), heat exchangers are basically heat convection equipment, since it is the convective transfer what governs its performance. Convection within a heat exchanger is always forced, and may be with or without phase change of one or both fluids. When one just relies in natural convection to the environment, like in the space-heating hot-water home radiator, or the domestic fridge back-radiator, they are termed ‘radiators’ (in spite of convection being dominant), and not heat exchangers. When a fan is used to force the flow of ambient air (or when natural or artificial wind applies, like for car radiator) the name heat exchanger is often reserved for the case where the ambient fluid is ducted. Other names are used for special cases, like ‘condenser’ for the case when one fluid flow changes from vapour to liquid, ‘vaporiser’ (or evaporator, or boiler) when a fluid changes from liquid to vapour, or the ‘cooling tower’ dealt with below. Devices with just one fluid flow (like a solar collector, a spacecraft radiator, a submerged electrical heater, or a simple pipe with heat exchange with the environment) are never named heat exchangers.
To be continued…
