Chapter 5 Heat-transfer Equipment

Chapter 5 Heat-transfer Equipment

1. Condenser Four condenser configurations are possible: 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

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 N r 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: 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:

2. Condensation inside and outside vertical tubes 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:

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.

Example 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. 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/m 2 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, F t = 0.92.

Significantly lower than the assumed value of 900 W/m 2 C. Repeat calculation using new trial value of 750 W/m 2 C.

Close enough to estimate, firm up design.

Shell-side pressure drop 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.

Tube-side pressure drop acceptable.