Two-phase heat exchangers

R. L. Webb, Performance Evaluation Criteria for Enhanced Tube Geometries Used in Two-Phase Heat Exchangers, in Heat Transfer Equipment Design, R. K. Shah, E. C. Subbarao, and R. A. Mashelkar eds., Hemisphere, New York, 1988. 

C. B. Panchal, Condensation Heat Transfer in Plate Heat Exchangers, in J. T. Pearson and J. B. Kitto, Jr. (eds.), Two-Phase Heat Exchanger Symposium, HTD-Vol. 44, ASME, New York, pp. 45-52,1985. 

In order to illustrate the systems benefit that can be achieved, it is helpful to classify the main types of equipment using two-phase heat exchangers as follows ... 

The domain of the search for an improved separation process was defined by certain criteria (a) isotopic fractionation should be achieved by means of a two-phase, chemical exchange reaction which was amenable to countercurrent operation in a multistage contactor at ambient temperature and pressure (b) the single-stage isotopic fractionation factor for the reaction should be appreciably larger than that for the distillation of Me20 BF3 (c) the molecular species in each process stream should be thermally refluxable—i,e, convertible from one species to the other by the addition or removal of heat alone (d) process materials should be more stable with respect to irreversible decomposition than those used in the (CH3)20 process and (e) the chemical form of the product should permit a ready, quantitative conversion of the separated isotopes to the elemental state. 

The flow maldistribution problem in evaporators is most severe when the major part of the pressure drop occurs in the two-phase heated region and cannot be prevented in some cases (reboilers and evaporators). However, in most heat exchangers with such problems, the economic penalty for maldistribution is small, and deterioration of the thermal performance is slightly affected [115]. The flow instabilities can be controlled by restricting the liquid circulation [2], Falling film evaporators are particularly prone to flow maldistribution. This effect can be reduced through design modifications (the top tube plate must be exactly horizontal, addition of inserts into the top of each tube must be secured, etc.) [120]. 

R. K. Shah and M. S. Bhatti, Assessment of Correlations for Single-Phase Heat Exchangers, in Two-Phase Flow Heat Exchangers Thermal Hydraulic Fundamentals and Design, S. Kaka(, A. E. Bergles, and E. O. Fernandes (eds.), pp. 81-122, Kluwer Academic Publishers, Dordrecht, Netherlands, 1988. 

Small tubes are commonly employed where the reaction is rapid and/or the heat of reaction must be removed rapidly. The two conventional types of tubular reactors are (1) coils immersed in a constant-temperature bath and (2) a jacketed pipeline in which the inner tube is designed to withstand the reaction pressure. A modification of the conventional jacketed-pipe reactor can be used where it is desirable to minimize the thickness of the inner tube in order to reduce the area required for heat transfer. An example of this type of equipment is the liquid-phase heat exchanger of the Bureau of Mines, in which the outside pipe has an outside diameter of 4 2 ill- wall thickness of 1.005 in. The outside diameter of the inner tube is in., but the wall thickness is only 0.16 in. The worldng... 

As discussed in the previous two sections, heat exchange is commonly used in conjunction with separation and reaction steps to change the temperature and/or phase condition of a process stream. When using a process simulator to perform heat exchange calculations, it is necessary to select a method of heat exchange from the following six possibilities, where all but the last two involve the separation, by a solid wall, of the two streams exchanging heat... 

For companies who rely heavily on simulation software for process examination, there will be a gradual transition to incorporate the capabilities of PI plant in these. As an example, ANSYS Huent is involved in a major project that is developing correlations for two-phase heat transfer at the micro-scale, suitable for micro-reactor and micro-heat exchanger design and sizing, see Web 1 (2007). 

The shape of the coohng and warming curves in coiled-tube heat exchangers is affected by the pressure drop in both the tube and shell-sides of the heat exchanger. This is particularly important for two-phase flows of multicomponent systems. For example, an increase in pressure drop on the shellside causes boiling to occur at a higher temperature, while an increase in pressure drop on the tubeside will cause condensation to occur at a lower temperature. The net result is both a decrease in the effective temperature difference between the two streams and a requirement for additional heat transfer area to compensate for these losses. 

Expansion turbines—three in Phase 1 and two in Phase 2—cool the gas to -90°C, at which point more liquid drops out in another separator. In this system, the cold methane gas emerges from the expansion turbines at a reduced pressure of 325 psi and is recirculated to the original heat exchanger to reduce the temperature of the natural gas entering the plant. 

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