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CHAPTER
3
Double pipe heat exchanger
3.1 Introduction
The double-pipe heat exchanger is probably the simplest to construct and is used for low heat load
applications. As shown in Fig. 3.1, these may comprise of a single tube or multiple tubes inside a shell.
Commercially available single tube double-pipe sections range from 50 mm through 100 mm
(2 through 4 ) pipe size shells with inner tubes varying
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from 19 mm to 65 mm (3/4 to 2 1/2 ) pipe size.
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The exchanger with a bundle of U tubes inside a pipe of
Multi-tube double-pipe exchanger 150 mm diameter and above uses segmental baffles (discussed
in Chapter 4) and is referred to as hairpin or jacketed U tube
exchanger. Multi tubular double-pipe sections may contain 7 to
64 tubes within the outer tube. Nevertheless, sections containing more than seven tubes per section are
rarely used since they have limited economic advantage for most services. If the particular service
requires fractional portions or short tube lengths of a multi-tube section, single tube sections are more
economical. One end of the tube element is free-floating for thermal expansion. The two fluids usually
flow in countercurrent mode for the highest thermal performance for a given surface area. However, for
an almost constant wall temperature, the flow can be co-current. Double-pipe
sections have been designed for pressure up to 165 bar (g) (2400 psig) on the
shell side and up to 1033 bar (g) (15,000 psig) on the tube side. Metal-to metal
Finned tube ground joints, ring joints or confined ‘O’-rings are used in the front end clo-
sures at lower pressures.
In general, the tubes used are plain, but some applications use low-fin
tubes for the inner pipes that provide about 2.5 times the external surface
area. Finned tubes in double-pipe exchangers are economical if the heat transfer coefficient for the
fluid flowing in the annular area is less than 75% of the tube side coefficient. The fins are
longitudinally attached to the inner tube either by welding, brazing, or mechanical bonding.
Usually, single tubes have longitudinal fins while multi-tubes have radial fins. The fins 16 to 48 per
tube are 12.5e25 mm (1/2 to 1 inch) high and 0.9e1.3 mm (35e50 miles) thick and the fin height
is dictated by the clearance between the inner and the outer pipe. Shorter fins have higher fin
efficiency. The minimum thickness is rarely below 0.8 mm. Fin efficiency increases with
decreasing annular coefficient and increasing fin thermal conductivity. Low-fin tubes are costlier
by 50%e70% compared to plain tubes.
Process Equipment and Plant Design. https://doi.org/10.1016/B978-0-12-814885-3.00003-8 49
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