Page 46 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

42 Chapter 2 Heat transfer processes in industrial scale

Table 2.5 Typical overall heat transfer coefficient in industrial tubular heat exchangers (Includes
fouling factor).dcont’d
Typical overall heat transfer
2
Fouling Inside fluid Outside fluid coefficient (W/m K)
Aqueous vapours Water 1001 1501
Organic vapours Water 700 1001
Condensers
Organics with Water 500 700
noncondensable
Vacuum condenser Water 199 500
Steam Aqueous solutions 1001 1501
Vaporisers Steam Light organics 899 1200
Steam Heavy organics 603 899

for a similar system. The tabulated values can be used for the preliminary sizing of equipment for
process evaluation and also as initial guesses for a detailed thermal design. The guess value needs to be
checked with the overall design coefficient, U D obtained from Eqns. (2.13a,b and 2.14a,b) once the
values of individual heat transfer coefficients are known.
The magnitude of the individual coefficients depends on (a) the heat transfer process (conduction,
convection, boiling, condensation, radiation), (b) physical properties and flow rate of the fluid and
(c) geometrical configuration of heat transfer surface. Some of the commonly used correlations
available for estimation of the individual heat transfer coefficients (h) under different input conditions

are presented in Table 2.6. The correlation usually express h as Nusselt number ðNu ¼ hD e =kÞ
or Stanton number ðSt ¼ Nu =RePr ¼ h =ðrUC p ÞÞ where k; r; U; C p refer to the thermal conductivity
3
(W/mK), density (kg/m ), velocity (m/s) and specific heat capacity (J/kgK) of the fluid, all properties

evaluated at the average bulk fluid temperature T fi þT fo or at the respective caloric
2
T f ;avg ¼
temperature (discussed at the end of this chapter) and D e is the equivalent diameter (m). Table 2.7
presents the correlations commonly used for boiling and condensation under different flow conditions.
In Table 2.6,
rUD e GD e
(2.20)
m m
Re ¼ ¼
C p m
(2.21)
k
Pr ¼
m is fluid viscosity evaluated at T f;avg . Refer to the following section for the concept of caloric
temperature for evaluating m.
D e ¼ tube inside diameter for flow inside tube, hydraulic diameter for flow in annuli or shell and
twice the gap between the plates in plate-type exchangers.
T w1 þ T w2
m ¼ fluid viscosity evaluated at average wall temperature T w;avg ¼ .
w
2