When the resistance of the tube wall is neglected, the overall coefficient can be written in the following form
Single-pass r, because it has one shell-side pass and one tube-pass, is a 1-1 exchanger.
In an exchanger the shell-side and tube-side heat-transfer coefficients are of comparable important, and both must be large if a satisfactory overall coefficient is to be attained.
An even number of tube-side passes are used in multipass exchangers. The shell side may be either single-pass or multipass.
In multipass exchangers, floating heads are frequently used.
2-4 exchanger The 1-2 exchanger has an important
limitation. Because of the parallel-flow pass, the exchanger is unable to bring the exit temperature of one fluid very near to the entrance temperature of the other.
In the second, called contact condensers, the coolant and vapor streams are physically mixed.
Extended-surface equipment
Difficult heat-exchange problem arise when one of two fluid streams has a much lower heattransfer coefficient than the other.
The fluid stream having the lower coefficient is brought into contact with the extended surface and flows outside the tubes , while the other fluid, having high coefficient, flows through the tubes.
Metal plate, usually with corrugated faces, are supported in a frame; hot fluid passes between alternate pairs of plates, exchanging heat with the cold fluid in the adjacent spaces.
In simple devices these quantities can be evaluated easily and with considerable accuracy,
but in complex processing units the evaluation may be difficult and subject to considerable uncertainty.
Multipass construction increases the fluid velocity, with a corresponding increase in the heat-transfer coefficient.
The disadvantages for a multipass construction are that (1) the exchanger is slightly more complicated ;
The heat recovery of a 1-2 exchanger is inherently poor.
A better recovery can be obtained by adding a longitudinal baffle to give two shell passes.
Correction of LMTD in multipass exchangers
4.7 Heat-Exchange Equipment
In industrial processes heat energy is transferred by a variety of methods.
Including conduction-convection in exchangers, boilers, and condensers; radiation in furnaces and radiant heat dryer.
In multipass exchangers which have more tube passes than shell passes, the flow is countercurrent in some sections and parallel in others.
The LMTD, as given by Eq 5.4-27 does not apply in this case, and it is customary to define a correction factor f.
multipass exchanger
The 1-1 exchanger has limitations, because when the tube-side flow is divided evenly among all the tubes, the velocity may be quite low, giving a low heat transfer coefficient.
Special heat-transfer devices used to liquefy vapors by removing their latent heats are called condensers.
Condensers fall into two classes. In the first, called shell-and-tube condenser, the condensing vapor and coolant are separated by a tube wall.
Z T1 T2 t2 t1
The factor Z is the ratio of the fall temperature of the hot fluid to the rise in temperature of the cold fluid.
And the abscissas are values of the dimensionless ratio η
t2 t1
T1 t1
The factor η is the heating effectiveness.
From the numerical values of Z and η, factor f is read from Figure, and multiplied by the LMTD for countercurrent flow to give the true mean temperature
From material and energy balances, the required heat-transfer rate is calculated. Then, using the overall coefficient and the average T, the required heat-transfer area is determined.
The velocity and turbulence of the shell-side liquid are as important as those of the tubeside fluid.
To promote crossflow and raise the average velocity of the shell-side fluid, baffles are installed in the shell.
Extended surfaces have been developed in which the outside area of tube is multiplied.
Types of extended surface: (a) longitudinal fins; (b) transverse fins.
Δtm = f • LMTD
Figure shows factor f for 1-2 exchangers,
Figure shows factor f for 2-4 exchangers,
Plate-type exchanger
For heat transfer between fluids at low or moderate pressure, below about 20 atm, plate–type exchangers are competitive with shell-and-tube exchangers, especially where corrosion-resistant materials are required
The plates are typically 5mm apart.