Page 88 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

4.4 Design e F T method 85

4.4 Design e F method
T
The steps in a design procedure using F T method are given below. This procedure leads to w10%
overdesign.
1. Compile the required fluid physical properties namely density, viscosity and thermal
conductivity. These properties vary with temperature and since the formulae involve a single
value, either an average property value or a correction to the average is used. Weighted average
values may be taken for fluid mixtures. The concept of average temperature and caloric
temperature in this regard is discussed in Chapter 2.
2. Define duty: heat transfer rate Q, fluid flow rates m c , m h (subscripts c and h refer to the cold and
hot fluid, respectively) and terminal temperatures. The heat transfer between the phases in a
shell and tube exchanger can involve transfer of sensible heat, latent heat or both. For no phase
change in any of the fluids, the heat balance equation is

Q ¼ m c C pc T c;out T c;in ¼ m h C p;h T h;in T h;out (2.1)
Subscripts in and out indicate the inlet and outlet condition. In case of phase change, the balance
equation is expressed in terms of enthalpy.
3. Preliminary selection of shell- and tube-side fluid as per the general guidelines provided in
Table 3.2. In horizontal thermosiphon reboiler, the process fluid is in the shell and the heating
stream or steam in the tubes. This is to lower pressure drop in the thermosiphon circuit. Low
flow of cooling water leads to deposits and fouling and it is usually chosen as tube-side fluid. In
most industries using cooling water, nonferrous tubes, commonly brass and copper, are used
because cooling water is corrosive to steel especially for high tube wall temperatures and in
presence of dissolved air. Shells are usually fabricated from steel.
4. Select a value for the design overall heat transfer coefficient, U D . Table 2.5 may be referred to
for providing an educated guess based on values for a similar system.
5. Calculate area required from Eq. (2.7) expressed as

Q
(2.7)
A o ¼
F T U D DT LMTD;counterflow
2
where A o is heat transfer area, i.e., effective tube outside area (m ); Q is total heat duty (W);
DT LMTD;counterflow is the log mean temperature difference ( C) assuming that the heat transferred is a

linear function of temperature difference and F T , the LMTD correction factor can be obtained as
discussed below.
Generalised expressions of F T for n shell passes
As discussed in Chapter 2, F T for multipass exchangers (Fig. 4.7) is expressed as function of

m c C p;c T h;in T h;out T c;out T c;in
and .
R ¼ ¼ S ¼
m h C p;h T c;out T c;in T h;in T c;in