Page 197 - Applied Process Design For Chemical And Petrochemical Plants Volume III

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160 Applied Process Design for Chemical and Petrochemical Plants

Note: If the metal resistance (r w ) had been consid- Pressure drops from DOWTHERM A heat transfer media
2
ered, it would equal 0.00024 (hr) (ft ) (°F)/Btu flowing in pipes may be calculated from Figure 10-137. The
(Table 10-13A) and would have made U c 142 effective lengths of fittings, etc., are shown in Chapter 2 of
2
Btu/(hr) (ft ) (°F). The metal resistance is thus Volume 1. The vapor flow can be determined from the
shown to be insignificant and may be neglected. latent heat data and the condensate flow. With a liquid sys-
tem, the liquid flow can be determined using the specific
4. Design Overall Heat Transfer Coefficient (U d ) heat data.
In the design of all parts of a system, special consideration
should be given to the large amount of flash vapor liberated
1 1
r io r o on the reduction of pressure. Because of the high ratio of
U d U c
specific heat to latent heat, much more flash vapor is liber-
d i ated with DOWTHERM A than with steam. Consequently, all
r io r i 0.003 0.827
d o constrictions that would cause high pressure drops should
2
r io 0.00248 1hr2 1ft 2 1°F2>Btu be avoided.
In addition to steam and controlled-temperature water, a
2
r o 0.001 1hr2 1ft 2 1°F2>Btu 1given2 number of different heat transfer fluids for a wide range of
temperatures from 100—700°F are supplied by (a) the Dow
2
1>U c 1>147 0.00681 1hr2 1ft 2 1°F2>Btu Chemical Co., (b) Monsanto Chemical Co., (c) Multitherm
Corp., (d) Union Carbide Corp., (e) Exxon Chemical Co.,
1>U d 0.00681 0.00248 0.00100 (f) Mobil Chemical Co., (g) Calfo division of Petro Canada,
and (h) others with qualified products.
2
1>U d 0.01029 1hr2 1ft 2 1°F2>Btu
Falling Film Liquid Flow in Tubes
2
U d 97.2 Btu>1hr2 1ft 2 1°F2
The liquid runs in a film-like manner by gravity down the
or if read from Figure 10-39 at (r o r io ): inner walls of the vertical tubes in a falling film exchanger.
The tubes do not run full, and, therefore, the film coeffi-
cient is greater than for the same liquid rate in a full tube
2
U d 97 Btu>1hr2 1ft 2 1°F2 81
by
2
D i
h1film gravity2 h1full2a b (10-134)
4d¿1D i d¿2
5. Surface Area
where
D i I.D. of tube, ft
Q U d
t LM
d film thickness, ft
360,000
A Q>1U d 21 t LM 2 66.3 ft 2
197.22155.92 For water: 81
w 1>3
h m 120a b 1201G¿2 1>3 (10-135)
Pressure Drop
D i
where
When a fluid flows over a stationary or moving surface, G mass flow rate/unit circumference, lb/(hr) (ft)
the pressure of the fluid decreases along the length of the G w/( D
surface due to friction. This is commonly called the pressure w mass flow rate, lb/hr
drop of the system. Of particular interest are the pressure
drops in pipes (tubes) and in heat exchanger shells. For other liquids in turbulent flow:
81
The Sieder and Tate equation for the pressure drop in
tubes is h m c 1>3 4G¿ 1>3
0.01a b a b (10-136)
2 3 2 2 k
fG Ln¿ 2k t g> f f
p
10
5.221102 1D i 21s21 > w 2 0.14
Properties are evaluated at the length mean average tem-
The Sieder and Tate equation for the pressure drop in
perature.
shells is
2
fG D i 1N 12 where
p f viscosity of liquid at film temperature, t f , lb/(hr)(ft)
10
5.221102 1D e 21s21 > w 2 0.14
8
g acceleration due to gravity 4.17 10 ft/(hr)
Values of f versus Re number are given in Figure 10-140. lb/ft 3