Page 160 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10D 5/30/2001 4:31 PM Page 123

Heat Transfer 123

liquid and allows it to overflow through the outlet. Here Condenser Design Procedure
also, the subcooling area is calculated separately from the
condensing area. The two are then added to obtain the The usual total condenser will follow the following design
total. Usually the liquid held to be subcooled covers only steps:
about 15—30% of the total surface, although some units may
run as high as 50%. If the quantity of the liquid is very large, 1. Establish condensing temperature of vapors, either by
handling it in a separate liquid cooler where higher coeffi- the conditions of other parts of the process (distillation
cients can be obtained is possibly a better solution. column, vacuum jet, etc.) or by the temperature
The cooling of the condensate by free convection is 70 approach to cooling water, remembering that a close
approach will require relatively large surface area.
3
k f p t c t t 0.25 Select the cooling water temperature to ensure
h c 116 ca ba bd (10-91)
f ¿ d o performance in the summer months and consider the
conditions during the winter (see step 8q).
where
2. Establish film temperature for condensation from
f viscosity in centipoise
Equation 10-26 or 10-28.
t temperature difference between tube surface and
3. Establish physical properties of fluids, shell side at a dif-
fluid, °F
ferent temperature than tube side.
d o O.D. tube, in.
4. Calculate the heat load of condensation from latent
coefficient of thermal expansion, %/°F
density, lb/ft 3 heat. (This may be a weighted value for a mixture.)
2
k f thermal conductivity of film, Btu/hr (ft ) (°F/ft) 5. Set an allowable temperature rise for the cooling water.
c f specific heat, Btu/lb (°F) at film conditions 6. Calculate water rate:
t f (t w t a )/2, °F, average, film temperature
t a bulk fluid temperature, °F W Q/c p t, lb/hr (10-94)
g acceleration of gravity, ft/(hr) 2 Q Btu/hr
G o = condensate mass flow per unit tube outside circumfer- t temperature rise of water, °F
ence, vertical tubes, lb./(hr) (ft) c p Btu/lb °F
The usual range of film coefficient values is 40—50 for
organic solvents and light petroleum fractions such as hexa- gpm W (10-95)
nes; 25 for heavier materials such as aniline, straw oil, etc.; 18.3321602
and 0.5—3 for low temperature (10—40°F) subcooling of
heavier organics and inorganics such as chlorine. (for water, otherwise correct 8.33 lb/gal for sp. gr. of coolant)
gpm
3
ft >sec cfs (10-96)
Film Temperature Estimation for Condensing 17.4821602
70
Kern recommends the temperature to use in estimating 7. Estimate the number of tubes per pass to maintain
or determining fluid properties: minimum water velocity.
Set minimum velocity in tubes at 3.5—6 ft/sec v.
t f 1>2 1t b t w 2 (10-92)
2
Water flow area cfs/v, ft a
Select tube size and calculate flow area available:
where
t b bulk temperature of fluid, °F.
t w wall temperature of tube surface, °F tube cross-section, in 2
2
Flow area>tube ft >tube (10-97)
144
t f t f t w
a
Estimated no. tubes>pass n¿ (10-98)
82
2
McAdams recommends: ft >tube 1cross-sect.2
t f t sv 3>4 t (10-93) 8. Assume a unit:
(a) Estimate overall coefficient, U, from Tables 10-15,
t t sv t w
10-17, and 10-18 or by your own experiences.
where (b) Roughly calculate a log mean temperature differ-
t sv saturation or dew point temperature, °F. ence T.
(c) Estimate area Q/U t, ft 2
In most instances the effect of the difference on physical (d) The total tube footage required A/(ft surface/ft
2
properties will be small. tube length), ft F 1.