Page 77 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

74 Chapter 4 Shell and tube heat exchanger

D s =5 causes large pressure drop and leakages that nullify the heat transfer advantage of closer
spacing. Baffle spacing and not the baffle cut determines the effective velocity of the shell-side fluid
and hence has the greatest influence on shell-side pressure drop.
Baffle orientation is important for shell-side phase change. In case of vaporisation/boiling on the
shell side, the baffle cut may be either vertical or horizontal depending on the service. For shell-side
condensation, the cut is vertical to allow for condensate flow towards the outlet without significant
liquid holdup due to baffle.
The triple segmental baffles are used for low pressure applications. Sealing strips are used to
minimise channelling of fluid between outer row of tubes and shell. They usually consist of metal
strips attached to baffles and running between shell and outer tubes.
Disc-and-doughnut baffles (Fig. 4.2C) have alternate outer rings and inner discs to direct the flow
radially across the tube bundle. Generally, the disk diameter is more than D s =2, and the diameter of the
doughnut hole is less than D s=2. This arrangement eliminates the potential bundle-to-shell gap bypass
stream and provides a lower pressure drop as compared to single segmental baffles for the same un-
supported tube span. The disadvantages are (i) the tie rods to hold baffles are within the tube bundle
and (ii) the central tubes are supported by disk baffles which in turn are supported only by tubes in the
overlap of the larger diameter disk over the doughnut hole.
In an orifice baffle, shell-side fluid flows through the clearance between tube outside diameter and
baffle-hole diameter (Fig. 4.2D). Rod baffles are used for services where
pressure drop can become a constraint with other baffle types. In this case,
flow is parallel to the tubes and flow-induced vibrations are practically
Other baffle types
eliminated by baffle support of the tubes.

Tie rods and spacers
The baffles are a part of the tube bundle assembly and are held in position by tie rods and spacer
sleeves on the rods. Tie rods are fixed at one end in the blind tapped holes in the tube sheet. Other end
of the tie rod ends passes through baffles and are fixed with a nut after the last baffle.

Impingement baffle
Some of the tubes in the outer layer of tube bundle face impingement by fluid entering through the inlet
nozzle. These tubes get eroded particularly when kinetic energy of the fluid is high and also when the
entering fluid is a mix of vapour and liquid. Such tubes are protected against erosion by a plate barrier
called ‘impingement plate’ or ‘impingement baffle’. Impingement plates are fitted immediately facing
the shell-side fluid entry nozzle and are fixed to the tie rods. The location of this baffle within the shell
is critical to minimise the associated pressure drop and the high velocity of the shell-side fluid after the
baffle. Accordingly, sufficient area needs to be provided between the nozzle and plate and between
plate and tube bundle. This is often achieved either by omitting some of the tubes from the circular
bundle or by modifying the nozzle to form an expanded section.

Multipass exchanger
Based on the number of shell and tube passes, shell and tube exchangers are designated as nem
exchanger. The first number n refers to the number of shell passes and the second number m refers to
the number of tube passes. The simplest configuration is 1e1, with one shell and one tube pass. Use of