Page 304 - Fluid mechanics, heat transfer, and mass transfer

HEAT EXCHANGERS 285

should not be used for fouling service as cleaning
becomes difficult. Therefore, small tube diameters
are used for clean fluids. Tubes with diameter
12.7 mm (0.5 in.) are used for small exchangers
2
with heat transfer areas less than 20–30 m .
& Tubes with diameter 19 mm (0.75 in.) and 4.9 m
(16 ft) length on a 25.4 mm (1 in.) triangular pitch
are common. Tubes with diameter 25.4 mm (1 in.) are
also common.
& Normally, tube lengths are 2.44, 3.66, 4.88, and
6.10 m (8, 12, 16, and 20 ft). Tube length of 4.9 m
(16 ft) is common.
. Under what circumstances, use of 19 mm (0.75 in.)
FIGURE 10.15 Types of tube pitch.
tubes is cheaper than use of 25.4 mm (1 in.)?
& An exchanger using 19 mm (0.75 in) tubes will be
cheaper than that using 25.4 mm tubes when the tube
side has a much lower heat transfer coefficient than
the outside of the tubes, under conditions of laminar
flow in a two-tube pass exchanger. If four tube
passes are used, the 19 mm tubes will have to be
significantly shorter than allowed in order to meet
pressure drop requirements. On the other hand, the
25.4 mm (1 in.) tube design uses the full allowable
tube length.
. How much tube surface can be accommodated in shells
of (i) 30 cm (1 ft) diameter, (ii) 60 cm (2 ft) diameter,
and (iii) 90 cm (3 ft) diameter?
& A 1 ft (30 cm) shell will contain about 9.3 m 2
2
(100 ft ).
& A 2 ft (60 cm) shell will contain about 37.2 m 2
2
(400 ft ). FIGURE 10.16 Shell side flow patterns in triangular pitch.
& A 3 ft (90 cm) shell will contain about 102 m 2
2
(1100 ft ).
. Compare heat transfer and pressure drop characteris-
tics of arranging tubes in square pitch and triangular
pitch.
& Triangular Pitch: Higher turbulence and hence
10.1.1 Tube Pitch
higher h and higher DP.
. What is meant by tube pitch? Define.
& Square Pitch: Less turbulence and hence poorer heat
& Pitch is the shortest center-to-center distance be-
transfer and lower DP.
tween two adjacent tubes in a heat exchanger.
. Arrange the following in the order of increasing DP for
. What are the different ways by which tube pitch is
the case of flow outside tube banks, flow rate remaining
arranged in a heat exchanger? Illustrate.
the same.

& (i) Square pitch (90 ), (ii) rotated square pitch (45 ),

1. Tube arrangement with triangular pitch (staggered).

(iii) triangular pitch (30 ), and (iv) rotated triangular
2. Tube arrangement for square pitch (in-line).
pitch (60 ).

3. Use of finned tubes with staggered arrangement.
& Figures 10.15 and 10.16 illustrate types of tube pitch.
& 2, 1, 3.
. ‘‘Square or rotated square pitch accommodates
. What are the advantages of square tube pitch in a heat
more tubes than triangular or rotated triangular pitch.’’
True/False? exchanger?
& False: It is the other way round. Triangular or & Lower DP.
rotated triangular pitch accommodates more tubes. & Ease of mechanical cleaning.