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\M-therm\Th15-3.pm5820 ENGINEERING THERMODYNAMICS(iii)Multiple shell and tube passes. Multiple shell and tube passes are used for enhancing
the overall heat transfer. Multiple shell pass is possible where the fluid flowing through the shell is
re-routed. The shell side fluid is forced to flow back and forth across the tubes by baffles. Multiple
tube pass exchangers are those which re-route the fluid through tubes in the opposite direction.
(iv)Compact heat exchangers. These are special purpose heat exchangers and have a very
large transfer surface area per unit volume of the exchanger. They are generally employed when
convective heat transfer sufficient associated with one of the fluids is much smaller than that asso-
ciated with the other fluid.
Example : Plate-fin, flattened fin tube exchangers etc.
- Physical state of fluids
Depending upon the physical state of fluids the heat exchangers are classified as follows :
(i) Condensers (ii) Evaporators
(i)Condensers. In a condenser, the condensing fluid remains at constant temperature
throughout the exchanger while the temperature of the colder fluid gradually increases from inlet to
outlet. The hot fluid loses latent part of heat which is accepted by the cold fluid (Refer Fig. 15.35).
tc 1th 11 2 Lt = constanth θ(= t – t )hcth 2
tc 2t=th 1 h 2t tLth 1tc 1th 2
tc 2t = constantc
θ= (t – t )hc(^12)
Fig. 15.35. Temperature distribution in a condenser. Fig. 15.36. Temperature distribution in an evaporator.
(ii)Evaporators. In this case, the boiling fluid (cold fluid) remains at constant temperature
while the temperature of hot fluid gradually decreases from inlet to outlet. (Refer Fig. 15.36).
15.4.3. Heat exchanger analysis
For designing or predicting the performance of a heat exchanger it is necessary that the total
heat transfer may be related with its governing parameters : (i) U (overall heat transfer coefficient
due to various modes of heat transfer), (ii) A total surface area of the heat transfer, and (iii) t 1 , t 2 (the
inlet and outlet fluid temperatures). Fig. 15.37 shows the overall energy balance in a heat exchanger.
Let, m& = Mass flow rate, kg/s,
cp = Specific heat of fluid at constant pressure J/kg°C,
t = Temperature of fluid, °C, and
∆t = Temperature drop or rise of a fluid across the heat exchanger.
Subscripts h and c refer to the hot and cold fluids respectively ; subscripts 1 and 2 correspond
to the inlet and outlet conditions respectively.
Assuming that there is no heat loss to the surroundings and potential and kinetic energy
changes are negligible, from the energy balance in a heat exchanger, we have :
Heat given up by the hot fluid, Q = mh cph (^) (–)tthh 12 ...(15.46)
Heat picked up by the cold fluid, Q = mc cpc (^) (–)ttcc 21 ...(15.47)