Page 23 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

CHAPTER

Heat transfer processes in

industrial scale 2

2.1 Introduction
Process heat transfer deals with rates of heat exchange in a heat transfer equipment commonly
employed in different engineering processes. The equipment is usually defined by the function it
performs in a process. Exchanger normally transfers heat between fluid streams. The transfer may also
be between a solid surface and a fluid or between solid particulates and a fluid. Exchangers are termed
‘heaters’ when the primary purpose is to heat a process fluid, while ‘coolers’ are employed for cooling.
Steam is the usual heating medium in heaters; circulating hot oil may also serve the same purpose in
several chemical industries. Use of synthetic fluids like ‘Dowtherm’, which is a eutectic mixture of
diphenyl and diphenyl oxide, is also common. Water is usually the cooling medium in coolers and the
water, thus heated, is in turn cooled by evaporative cooling in cooling towers. Due to the current dearth
of water, air is used as a cooling medium particularly in arid areas making fin fan coolers an attractive
option. Condensers are coolers which remove latent instead of sensible heat. Reboilers cater to the heat
requirement of a distillation process and evaporators are employed for concentration of a solution by
evaporation of water. When any fluid beside water is vaporised, the unit is a vaporiser. Heat exchangers
are common in chemical processing, power generation, metallurgical processes, air-conditioning,
refrigeration, automotive applications, etc., where the main purpose is either to cool or heat a stream,
evaporate or condense single or multicomponent fluid streams or to recover or reject heat from a
process.
The difference between furnace and heat exchanger lies in the fact that the heat energy is produced
inside the furnace at the expense of some other form of energy, usually combustion of a fuel or
electrical energy.
In most heat exchangers, heat transfer between fluids takes place through a separating solid wall
and the heat transfer is by indirect contact. In recuperators, the fluids are
separated by a surface across which heat is transferred and the fluids ideally do
not mix or leak into each other. In regenerative heat exchangers, the heat
Exchanger
transfer is intermittent. Thermal energy from the hot fluid is stored in one half
of the cycle and the stored energy is transferred to the cold fluid in the other
half of the cycle. In cases where the fluids exchanging heat are in direct
contact, the exchanger is a ‘direct contact’ type of heat exchanger. A typical example is ‘barometric
condenser’. In general, a heat exchanger consists of (i) heat transfer elements which is the core and
contains the heat transfer surface (in recuperators) or matrix (in regenerators) and (ii) fluid distribution

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