Boiling

Pool boiling data are the foundation for correlating the performance of commercial boiling equipment. A typical pool boiling curve, from the HTRI Design Manual, is shown in Figure 200-5. It applies to boiling water at atmospheric pressure. This type of boiling is obtained when the heated surface is surrounded by a fluid that is not flowing. Agitation is produced by natural convection currents and bubble motion.

The physical condition associated with various parts of the curve is illustrated in Figure 200-5 and briefly described below.

A-B Natural convection (no boiling)
B-C Incipient boiling (surface boiling with subcooled bulk fluid)
C-D Nucleate boiling (bulk fluid at saturation temperature)
D-E Transition to film boiling (unstable)
E-F-G Stable film boiling (heated surface is not wetted)

Commercial boiling equipment is intended to operate in the incipient boiling or nucleate boiling regions. Operation in the nonboiling A-B region or the D-E-F-G film boiling regions may result in severe fouling and/or mechanical failure of the equipment.

The pool boiling curves for single component fluids have been correlated in terms of critical pressure (Pc) and reduced pressure (Pr) and are given in Appendix D.

Critical pressure is the pressure above which the distinction between liquid and vapor vanishes. Reduced pressure is the ratio of operating pressure to critical pressure. Boiling is not possible at or above the critical pressure. The critical pressure of most hydrocarbons is between 400 and 600 psia.

fig 1 3 - Boiling

The nucleate boiling heat flux for multicomponent mixtures is less than that of pure components at the same surface-to-bulk-temperature difference. More volatile components boil first, concentrating less volatile components near the heat transfer surface. This reduction in heat flux correlates with boiling range (dewpoint—bubble point) and critical temperature. The maximum nucleate boiling heat flux for mixtures (point D in Figure 200-5) satisfactorily correlates to mixture critical pressure.

Boiling curves for horizontal tube bundles are markedly different than for single tubes. Figure 200-6 shows an example from the HTRI Design Manual.

The bundle curve was obtained by boiling normal pentane in the shell at saturation temperature with a large excess of saturated steam in the tubes to minimize tube side resistance. The Overall Temperature Difference in Figure 200-6 is the steam saturation temperature minus the n-pentane saturation temperature.

Nucleate boiling heat flux for bundles is much higher than for single tubes, but maximum bundle heat flux (dryout) is much lower. Vapor generated by each tube enhances circulation and heat transfer for neighboring tubes at low heat flux. Excessive vapor generation, however, prevents wetting some interior tubes above the incipient dryout heat flux. Incipient dryout of central tubes usually occurs at 50% to 70% of the maximum bundle heat flux. Extensive dryout in the interior of the bundle exists at the maximum bundle heat flux.

fig 1 4 - Boiling

Enhanced nucleate boiling heat flux, incipient dryout heat flux, and maximum bundle heat flux correlate with the ratio of heat transfer area to peripheral inflow/outflow area. These correlations are given in Appendix D.

At or above incipient dryout heat flux, a fraction of the liquid entering the lower part of the bundle becomes completely vaporized in the interior. Any solids in the liquid will deposit at this point. Solids deposition in the bundle restricts flow, reduces dryout heat flux, and extends the deposition region. Because most commercial streams contain a few parts per million solids, the usual result of operation above the incipient dryout heat flux is plugging of the bundle.

Operation of natural circulation boilers in a nonboiling region (A-B in Figure 200-5) results in a similar plugging problem. Fluid shear is usually not adequate to keep trace amounts of solids in the liquid from accumulating and adhering to the heated surface and eventually stopping circulation. Vigorous agitation associated with nucleate boiling is more than adequate to keep solids suspended. The minimum heat flux to ensure nucleation is about 2000 Btu/hr × ft2. This may impose a turn-down limit, particularly for grossly oversized boilers.

Reboilers should be designed reasonably close to but not exceeding incipient dryout heat flux. This minimizes the size and cost of the reboiler, maximizes allowable turndown, and minimizes the required temperature of the heat medium.


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