chapter 5 heat-transfer equipment...figure 12.43 can be used to estimate condensate film...
TRANSCRIPT
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Chapter 5
Heat-transfer Equipment
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Four condenser configurations are possible:
1. Condenser
1. Horizontal, with condensation in the shell, and the cooling medium in the tubes.
2. Horizontal, with condensation in the tubes.
3. Vertical, with condensation in the shell.
4. Vertical, with condensation in the tubes.
Horizontal shell-side and vertical tube-side are the most commonly used types of
condenser. A horizontal exchanger with condensation in the tubes is rarely used as
a process condenser, but is the usual arrangement for heaters and vaporizers using
condensing steam as the heating medium.
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1. Condensation outside horizontal tubes
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In a bank of tubes the condensate from the upper rows of tubes will add to
that condensing on the lower tubes. If there are Nr tubes in a vertical row and
the condensate is assumed to flow smoothly from row to row, Figure 12.42a,
and if the flow remains laminar, the mean coefficient predicted by the Nusselt
model is related to that for the top tube by:
In practice, the condensate will not flow smoothly from tube to tube, Figure 12.42b,
and the factor of Nr-1/4 applied to the single tube coefficient in equation 12.49 is
considered to be too conservative. Based on results from commercial exchangers,
Kern (1950) suggests using an index of 1/6. Frank (1978) suggests multiplying
single tube coefficient by a factor of 0.75.
1. Condensation outside horizontal tubes
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1. Condensation outside horizontal tubes
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Using Kern’s method, the mean coefficient for a tube bundle is given by:
1. Condensation outside horizontal tubes
For low-viscosity condensates the correction for the number of tube rows is generally ignored.
A procedure for estimating the shell-side heat transfer in horizontal condensers is given in the
Engineering Sciences Data Unit Design Guide, ESDU 84023.
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2. Condensation inside and outside vertical tubes
For condensation inside and outside vertical tubes the Nusselt model gives:
For a tube bundle:
Equation 12.51 will apply up to a Reynolds number of 30; above this value waves
on the condensate film become important. The Reynolds number for the
condensate film is given by:
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Above a Reynolds number of around 2000, the condensate film becomes turbulent.
The effect of turbulence in the condensate film was investigated by Colburn (1934)
and Colburn’s results are generally used for condenser design, Figure 12.43.
Equation 12.51 is also shown on Figure 12.43. The Prandtl number for the
condensate film is given by:
2. Condensation inside and outside vertical tubes
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Figure 12.43 can be used to estimate condensate film coefficients in the
absence of appreciable vapor shear. Horizontal and downward vertical vapor
flow will increase the rate of heat transfer, and the use of Figure 12.43 will
give conservative values for most practical condenser designs.
Boyko and Kruzhilin (1967) developed a correlation for shear-controlled
condensation in tubes which is simple to use. Their correlation gives the mean
coefficient between two points at which the vapor quality is known. The vapor
quality x is the mass fraction of the vapor present. It is convenient to represent the
Boyko-Kruzhilin correlation as:
Where:
and the suffixes 1 and 2 refer to the inlet and outlet conditions respectively. h'i is the tubeside
coefficient evaluated for single-phase flow of the total condensate (the condensate at point 2).
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Boyko and Kruzhilin used the correlation:
In a condenser the inlet stream will normally be saturated vapor and the vapor will
be totally condensed.
For these conditions equation 12.52 becomes:
For the design of condensers with condensation inside the tubes and downward
vapor flow, the coefficient should be evaluated using Figure 12.43 and equation
12.52, and the higher value selected.
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Example
Estimate the heat-transfer coefficient for steam condensing on the outside, and on the
inside, of a 25 mm o.d., 21 mm i.d. vertical tube 3.66 m long. The steam condensate
rate is 0.015 kg/s per tube and condensation takes place at 3 bar. The steam will flow
down the tube.
Solution
Physical properties, from steam tables:
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vertical tube loading
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Example
It is proposed to use an existing distillation column, which is fitted with a
dephlegmator (reflux condenser) which has 200 vertical, 50 mm i.d., tubes, for
separating benzene from a mixture of chlorobenzenes. The top product will be 2500
kg/h benzene and the column will operate with a reflux ratio of 3. Check if the tubes
are likely to flood. The condenser pressure will be 1 bar.
Solution
The vapor will flow up and the liquid down the tubes. The maximum flow rates of
both will occur at the base of the tube.
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Tubes should not flood, but there is little margin of safety.
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Design a condenser for the following duty: 45,000 kg/h of mixed light hydrocarbon vapors to be
condensed. The condenser to operate at 10 bar. The vapor will enter the condenser saturated at
60°C and the condensation will be complete at 45°C. The average molecular weight of the
vapors is 52. The enthalpy of the vapor is 596.5 kJ/kg and the condensate 247.0 kJ/kg. Cooling
water is available at 30°C and the temperature rise is to be limited to 10°C. Plant standards
require tubes of 20 mm o.d., 16.8 mm i.d., 4.88 m (16 ft) long, of admiralty. The vapors are to be
totally condensed and no sub-cooling is required.
Example
Solution
Only the thermal design will be done. The physical properties of the mixture will be taken as
the mean of those for n-propane (MW = 44) and n-butane (MW = 58), at the average
temperature.
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Assumed overall coefficient (Table 12.1) = 900 W/m2 °C
Mean temperature difference: the condensation range is small and the change in
saturation temperature will be linear, so the corrected logarithmic mean
temperature difference can be used.
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Try a horizontal exchanger, condensation in the shell, four tube passes. For one
shell pass, four tube passes, from Figure 12.19, Ft = 0.92.
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Significantly lower than the assumed value of 900 W/m2 °C.
Repeat calculation using new trial value of 750 W/m2 °C.
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Close enough to estimate, firm up design.
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Use pull-through floating head, no need for close clearance.
Select baffle spacing = shell diameter, 45 per cent cut.
From Figure 12.10, clearance =95 mm.
Shell-side pressure drop
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Negligible; more sophisticated method of calculation not justified.
Shell-side pressure drop
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Tube-side pressure drop
acceptable.