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116 Applied Process Design for Chemical and Petrochemical Plants

Spiral Coils in Vessels m 0.1 ( 8.621 10 ) 0.21
5
C p heat capacity, Btu/(lb) (°F)
Spiral coils can be useful in transferring heating and cool- D impeller diameter, ft
ing from the helical or nonhelical coil to and from a volume d o tube diameter, ft
of liquid in a process vessel or storage tank. These coils in a d t tube O.D., ft
stagnant or noncirculating tank are almost useless; there- h o outside (process fluid side) heat transfer coefficient
2
k thermal conductivity of liquid, Btu/(hr) (ft ) ((F/ft)
fore, the best arrangement is to use the coil in an agitated/
m experimental exponent, usually 0.14.
mixing tank. See Chapter 5 of Volume 1, 3rd Edition of this
N impeller speed, rev/hr
series.
T tank diameter, ft
viscosity, bulk fluid, lb/(ft) (hr)
Tube-Side Coefficient s viscosity of fluid at film temperature at heat transfer
surface, lb/(ft) (hr)
70
Kern reports that tube-side coefficients can be approxi- liquid density, lb/ft 3
mately 20% greater in a spiral coil than in a straight pipe or U o overall heat transfer coefficient based on outside
tube using the same velocities. The Sieder-Tate correlation is tube area
shown in Equations 10-44 and 10-45 and for streamline flow
is DG/ 2,100. For transition and turbulent flow, see Condensation Outside Tube Bundles
Equation 10-46 and Figure 10-46 or Figure 10-50A and 10-
81
50B for straight pipes and tubes. McAdams suggests multi- Film-type condensation is considered to be the usual con-
plying the h value obtained by (1 3.5 (D/D H ), when D is dition for most pure vapors, although drop-type condensa-
the inside diameter of the tube and D H is the diameter of the tion gives transfer coefficients many times larger when it
helix, in ft. 70 does occur. For practical purposes, film-type is considered in
design.
Outside Tube Coefficients Figure 10-66 indicates the usual condensing process,
which is not limited to a vertical tube (or bundle) as shown,
This design is not well adapted to free-convection heat
but represents the condensing/cooling mechanism for any
transfer outside a tube or coil; therefore, for this discussion
tube. The temperature numbers correspond to those of Fig-
only agitation is considered using a submerged helical coil, ure 10-28.
70
Oldshue 241 and Kern .
2
L N 2>3 C p 1>3 0.14 Vertical Tube Bundle 70
h c D j
0.87a b a b a b (10-68)
k k w
See Figure 10-67A and 10-67B.
Using the nomenclature of Equation 10-44, in addition: Figure 10-67A has been initially represented by
82
McAdams from several investigators. This figure represents
h c heat transfer coefficient for outside of coil, the mean coefficient for the entire vertical tube for two val-
2
Btu/(hr) (ft )((F) ues of the Prandtl number, Pr f , which c /k.
D j diameter of inside of vessel, ft
L tube length, ft where
N agitator speed, rev/hr c specific heat of fluid, Btu/(lb) (°F)
density, lb/ ft 3 fluid viscosity, lb/(ft) (hr)
2
viscosity, lb/ft-hr k thermal conductivity, Btu/(hr) (ft ) (°F/ft)
2
k thermal conductivity of liquid, Btu/(hr) (ft ) (°F/ft)
C p specific heat, Btu/(lb) (°F) Note that the break at Point A on Figure 10-67B at Re c
2,100 indicates where the film is believed to become turbu-
A related but somewhat more recent work by Oldshue 241 lent. 172 McAdams discusses the two regions on the figure,
82
presents heat transfer to and from helical coils in a baffled streamlined at the top and turbulent on the way down, with
tank, using standard baffling of T/12 located either inside a transition region in between:
the coil diameter or outside: 4
Re c (10-70)
d N 0.67 C p 0.37 0.1 0.5 m l
2
d t D d
h o 1coil2 0.17a b a b a b a b a b where
k k T T s
G w/ t , condensate loading for each vertical tube,
(10-69)
lb/(hr) (ft)
where w flow rate, rate of condensation per tube, W/N t ,
(use conventional units for symbols) lb/(hr)(tube), from lowest point of tube(s)