Page 74 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

4.1 Introduction 71

Tube size is specified typically in terms of diameter and wall thickness. Smaller diameter tubes
yield higher heat transfer coefficient and result in a more compact exchanger,
while larger diameter tubes are easier to clean and are more rugged. For
mechanical cleaning, the smallest practical size is 19.05 mm, while for
Size and number
chemical cleaning, smaller tubes can be used provided plugging does not
occur. Detailed dimensions of heat exchanger tubes are provided in Sections
4.4 and 4.6.2. One may note that unlike commercial pipes, the nominal
diameter of heat exchanger or condenser tubes is the actual outside diameter (usually specified in
inches) within a very strict tolerance and the tube thickness is expressed as BWG (Birmingham wire
gauge) and not Schedule number used for commercial pipes.
The number of tubes in an exchanger depends on the fluid flow rates and available pressure drop
(Section 4.4). When solids are present, the velocity is kept high enough to prevent settling. If the tubes
are too close to each other, the tube sheet becomes weak. The number of tubes that can be placed
within a shell is function of tube layout, tube outside diameter, pitch, number of passes and shell
diameter. This information for square and triangular layouts is provided in Table 4.8.
Tube layout is characterised by the included angle between tubes. The angle is defined with respect
to flow direction. Two standard layouts are square and equilateral triangle. The
equilateral triangular layout can be oriented at 30 degrees or 60 degrees to the
flow direction and the square layout at 45 degrees and 90 degrees. 30 degrees,
Pitch and layout
45 degrees and 60 degrees are staggered and 90 degrees is in line. Choice
between different tube layouts can be made based on their features listed in
Table 4.1. Under comparable conditions of flow and tube size, the heat transfer
coefficient for triangular arrangement is about 25% higher than that for square arrangement. The
triangular layout also provides a more compact arrangement, usually resulting in smaller shell
diameter and stronger header sheet for a specified shell-side flow area. It is preferred when the
operating pressure difference between the two fluids is large. Square pitch (45 degrees and 90 degrees
layouts) is adopted for ease of jet or mechanical cleaning of tube outer surface and a minimum
cleaning lane of 6 mm (1/4 ) is provided. The 90 degrees layout is preferred for vaporising applica-
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tions as it provides vapour escape lanes. This also provides the lowest heat transfer coefficient and the
lowest pressure drop among the different arrangements.
If mechanical cleaning is not required, the 30 degrees layout is preferred for single-phase laminar
or turbulent flow and condensing applications involving a high DT range. The 60 degrees layout is
preferred for condensing application involving a low DT range and for boiling applications. Square
layout is generally not used in the fixed header sheet design since mechanical cleaning is anyhow
infeasible under this condition.
The shortest centre to centre distance between successive tubes is tube pitch (P T ) and the minimum
distance between adjacent tubes is tube ‘clearance’ (tube pitch minus tube outside diameter ¼ P T D o ).
The selection of tube pitch is a compromise between a close pitch (small values of P T /D o ) for increased
shell-side heat transfer and surface compactness and an open pitch (large values of P T /D o ) for decreased
tendency of shell-side plugging and easy shell-side cleaning. According to IS 4503:1967, tubes are
spaced with a minimum centre to centre distance of 1.25 times the tube outer diameter. The basis of
limiting P T /Do as 1.25 stems from the consideration that header plate (tube sheet) becomes too weak for
proper rolling of the tubes and causes leaky joints when tubes are too close. Though the designer has the
freedom to choose a suitable pitch, usually a standard tube pitch and layout is chosen based on Table 4.8.