Page 57 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

3.2 Design 53

3.2.3 Codes and standards
Common standards for double-pipe heat exchangers are TEMA and API 660. There is no Indian (BIS)
code. Hairpin sections are specially designed units, which are normally not built to any industry
standard other than ASME Code. However, TEMA tolerances are normally incorporated wherever
applicable.

3.2.4 Guidelines to select inner and outer fluid
The guideline for selecting the inner and outer fluid is the same for a shell and tube exchanger and a
double-pipe exchanger. The general guidelines for preliminary selection are presented in Table 3.2.
They are general in nature and not rigid rules. Optimal fluid placement depends on several service-
specific factors, as well.

Table 3.2 General guidelines for selecting the shell and tube side fluids.
Tube side fluid Shell side fluid
· High-temperature fluid · More viscous fluid
At higher temperature the allowable stress is lower. The critical Reynolds number for turbulent flow is
Since tubes have a much lower diameter as compared 200 on the shell side. Thus for the same Re, when the
to a shell, they can withstand higher pressure at the flow is laminar in tubes, the shell flow may be tur-
same temperature. This makes the design safer. bulent. However, if the flow is still laminar in the
Further, this ensures lower heat losses from the shell, it is directed through the tubes, as this ensures a
exchanger to the surroundings and lower cost of more accurate prediction of both heat transfer and
exchanger insulation. flow distribution.
· Dirty and fouling fluids · Liquid with a lower flow rate
Tubes are easier to clean. Fouling tendency is lower To avoid multi-pass construction that will have
due to fewer stagnation points. Usually cooling water LMTD correction factor below unity. Turbulent flow
is used in tubes for this reason. may also result due to lower critical Reynolds number
Also, the tube fluid, mostly flowing at a higher ve- for the shell side.
locity, would have lower fouling (less deposit). Me-
chanical cleaning is easier for tubes. Slurry is
preferred in the tube side for this reason.
· More hazardous or expensive fluid · Fluid undergoing a phase change, e.g., condensing
The chance of leaking out is less. steam/vapour
Shell side offers a lower pressure drop. Vapour-liquid
mixtures resulting from vapour condensation is
allowable in vertical condensers.
· Fluid at a higher pressure · Fluid for which pressure drop limit is lower, or there is
The lower diameter of tubes calls for a lower wall a chance of exceeding the same, e.g., fluid of high
thickness compared to the shell. viscosity.
· Corrosive fluid · Fluid that has poorer heat transfer characteristics:
Only the tubes and not the shell is exposed to the As the critical Reynolds number for turbulent flow is
corrosive environment. A corrosive fluid in the shell 200 on the shell side.
would affect both the shell and the tubes. In addition,
it is cheaper to fabricate tubes from expensive
corrosion-resistant materials.
· Streams with low flow rates · Fluid with large DT(>40 C)

These are placed in tubes to obtain increased velocity
and turbulence.