Page 209 - Design and Operation of Heat Exchangers and their Networks

198 Design and operation of heat exchangers and their networks

ð
½
2 0:4l c, eff 0:8 l c d s d ctl Þ=2Š
N rw ¼ int ¼ int (5.14)
s l s l
where 0.4l c,eff is the penetration distance for crossflow in the tube bundle in
the window section.
The shell-to-baffle clearance between the shell inside diameter d s and
the baffle diameter d b is designated as δ sb and is given by Thulukkanam
(2013) as

δ sb ¼ d s d b ¼ 0:0031 + 0:004d s (5.15)
The baffle hole-to-tube clearance between the baffle hole diameter d bt
and tube outside diameter d o is designated as δ bt :

δ bt ¼ d bt d o (5.16)
The TEMA standards (TEMA, 2007) recommend the clearance as
δ bt ¼0.8mm, where the maximum unsupported tube length is 36in.
(914mm) or less, or for tubes larger in diameter than 1 1/4in. (31.8mm)
OD; δ bt ¼0.4mm, where the unsupported tube length exceeds 36in.
(914mm) for tubes 1 1/4in. (31.8mm) diameter and smaller.
The central angle of baffle cut, θ ds , is the angle subtended at the center by
the intersection of the baffle cut and the inner shell wall:
θ ds ¼ 2cos 1 ð 1 2l c =d s Þ (5.17)

The upper central angle of baffle cut, θ ctl , is the angle in radians between
the baffle cut and two radii of a circle through the centers of the outermost
tubes:
θ ctl ¼ 2cos 1 ð ½ d s 2l c Þ=d ctl Š (5.18)

The baffle spacing can be determined by the shell inside diameter:
(5.19)
l bc ¼ 0:4d s
The number of baffles can be specified or calculated from the geometric
data by

L L bi L bo
N b ¼ int + 1 (5.20)
L bc

The fraction of the number of tubes in one window section equals to the
ratio of the window area to the area of the tube bundle:
1
F w ¼ ð θ ctl sinθ ctl Þ (5.21)
2π