Chapter 5 Heat-transfer Equipment 12.43 can be used to estimate condensate film coefficients in the...

Chapter 5

Heat-transfer Equipment

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.

1. Condensation outside horizontal tubes

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

1. Condensation outside horizontal tubes

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.

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:

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

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).

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.

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:

vertical tube loading

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.

Tubes should not flood, but there is little margin of safety.

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.

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.

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.

Significantly lower than the assumed value of 900 W/m2 °C.

Repeat calculation using new trial value of 750 W/m2 °C.

Close enough to estimate, firm up design.

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

Negligible; more sophisticated method of calculation not justified.

Shell-side pressure drop

Tube-side pressure drop

acceptable.