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falling film learning Engineering Data Book III Falling Film Evaporation 14-1 Chapter 14 Falling Film Evaporation (This chapter was updated in 2009.) Summary: Horizontal, shell-side falling film evaporators have a significant potential to replace flooded evaporators i...

falling film learning
Engineering Data Book III Falling Film Evaporation 14-1 Chapter 14 Falling Film Evaporation (This chapter was updated in 2009.) Summary: Horizontal, shell-side falling film evaporators have a significant potential to replace flooded evaporators in large refrigeration systems and also to be used in place of vertical tube-side falling evaporators in the petrochemical industry. The main advantages for the first application are higher heat transfer performance and reduced refrigerant charge. For petrochemical applications, shell-side falling film evaporation on horizontal tube bundles is advantageous because enhanced boiling tubes, such as the Turbo-Bii or Turbo-Biii, can be utilized and hence a much more compact design is obtained compared to vertical plain tube units. Additionally, such a unit can also take advantage of multiple tube passes for the heating fluid, further improving heat transfer performance and compactness. In this chapter, the state-of-the-art of evaporation on horizontal single tubes and horizontal tube bundles is reviewed. Emphasis is placed on recent work on enhanced tubes. The effects of lubricating oils on thermal performance are also described and some test results presented. Numerous experimental tests have been made for plain, low fin and enhanced geometry tubes in the past decade, primarily for fluids such as water, refrigerants and ammonia. Based on these test data, enhanced tubes have demonstrated significant heat transfer augmentation for falling film evaporation on horizontal bundles with respect to plain tube units. In fact, some enhanced tubes demonstrate higher performance functioning with evaporating falling films than for their original pool boiling performance. Heat transfer design methods for horizontal units are presented and discussed as well as methods for predicting the film flow mode transitions between vertical tube rows (droplet, column and sheet) in horizontal bundles. No comprehensive design method is yet available, however. The relative benefits of horizontal falling film units compared to vertical intube units, and to a lesser extent to flooded evaporators, are addressed. Some remarks on liquid distributors and minimum overfeed rates are also presented. 14.1 INTRODUCTION TO FALLING FILM EVAPORATION Falling film evaporation is a process controlled by two different heat transfer processes. First of all, thin film evaporation is a heat transfer mechanism controlled by conduction and/or convection across the film where phase change is at the interface and whose magnitude is directly related to the thickness of the film and whether or not the film is laminar or turbulent. If the heat flux is above that required for onset of nucleation (nearly always the case for enhanced boiling tubes), nucleate boiling is also present, where bubbles grow in the thin film at the heated wall (or in the re-entrant channels of an enhanced structured surface) and migrate to the interface. The film normally flows downward under the force of gravity. Therefore, except for the nucleate boiling mechanism, this process is quite similar to falling film condensation and in fact many analogies can be drawn between these two processes. For instance, falling film evaporation can occur in a laminar film flowing down the outside of a horizontal tube, similar to the Nusselt (1916) theory for laminar film condensation. In both cases, heat transfer is mainly dictated by film thickness. Furthermore, this film may develop surface waves or become turbulent, depending on the local film Reynolds number. On the contrary, a falling evaporating film often also has nucleate boiling occurring in it, which further increases the heat transfer coefficient. Furthermore, the formation of dry patches on the tube of an evaporating film may occur, which is very detrimental to heat transfer since heat transfer is then only to the vapor-phase on those parts of the surface. Figure 14.1 shows a schematic illustration of falling film evaporation on a single horizontal tube, with nucleate boiling occurring in the falling film. Hence, both thin falling film evaporation and nucleate Engineering Data Book III Falling Film Evaporation 14-2 boiling play a role in the heat transfer process. In a falling film evaporator, an array of horizontal tubes arranged in a matrix is used with the liquid falling from tube to tube. Vertical falling film evaporators have been used for many years in the petrochemical industry. As evidence, Kern (1950) included the design method of Bays and McAdams (1937) for vertical intube units in his widely used book. In the chemical industry, vertical falling film evaporators are utilized to evaporate fluids under vacuum conditions where the liquid static head from the distillation column would otherwise create too much subcooling for efficient operation as a vertical or horizontal thermosyphon reboiler. They are also used to evaporate temperature sensitive fluids and to remove volatiles from mixtures. The falling liquid film is placed on the tube-side of these vertical, plain tube exchangers so that uniform liquid distribution to the tubes on the top tubesheet can achieved by placing liquid distributor caps on each tube hole. The flow rate of liquid is controlled so as to inundate the tubes with liquid but rather to form a uniformly distributed falling liquid film inside each tube. Hence, it is very important to install these units with a strict tolerance to the vertical plumb line since a small inclination would result in dryout at the lower end of the tubes on the higher side. Figure 14.1. Falling film evaporation on a heated horizontal tube with nucleate boiling. Figure 14.2. Ammonia falling film evaporator at EPFL (view of head and tube sheet). Falling film evaporation has also been used on the shell-side of large heat pump systems. For example, an ammonia-cycle heat pump system at the Swiss Federal Institute of Technology in Lausanne (EPFL) takes thermal energy from Lake Geneva for central heating of the campus buildings, utilizing two 2 m diameter and 10 m long, horizontal shell-and-tube units that function with a significant amount of immiscible lubricating oil in the working fluid (refer to Figure 14.2). The desalination industry also exploits falling film evaporators, typically utilizing plain, horizontal tube bundles. This allows for closer temperature approaches and very significant energy savings. In large tonnage air-separation plants, massive vertical coiled-tube-in-shell units with shell-side falling film evaporation are used to take advantage of the close Engineering Data Book III Falling Film Evaporation 14-3 temperature approaches that can be attained to save on energy consumption. Here the tubes are nearly horizontal in their spiral within the coil. Falling film evaporators have also been tested in Ocean Thermal Energy Conversion (OTEC) pilot plants, again to achieve a closer temperature approach between the evaporating fluid and the heating fluid, and hence attain higher cycle thermal efficiency. Falling film evaporation has also been exploited in absorbers and vapor generators of absorption heat pump systems. Falling film evaporators are also sometimes referred to as spray-film evaporators. With respect to large refrigeration systems, one major U.S. refrigeration company brought out a complete new line of refrigeration units operating with R-134a at the beginning of 1998 based on enhanced tube falling film evaporators, achieving significant performance improvements compared to traditional enhanced tube flooded evaporators. A company in Texas has experience in using an inhouse enhanced tube for ammonia falling film evaporators since about 1992. One of the significant advantages of falling film operation is the large reduction in liquid charge in the evaporator. Furthermore, higher heat transfer performance can also be attained. Also, for falling film evaporators applied to refrigeration units, the bottom tube rows can be purposely flooded to evaporate the access liquid reaching the bottom of the bundle and thus minimize the liquid that must be pumped back to the inlet. Horizontal falling film evaporators are to some extent similar to kettle-type steam generators in that the liquid is fed to the bundle overhead, using sprinklers or trays. The difference is that to achieve a falling film, the liquid holdup in the shell is reduced to a minimum and the liquid flow rate is limited to that required to wet the entire bundle without the formation of dry patches, rather than flooding the shell in a pool of liquid. The unevaporated liquid can be removed from the bottom of the bundle by placing a nozzle at the bottom of the shell as shown in Figure 14.3. Figure 14.3. A horizontal shell-and-tube falling film evaporator. 14.2 AN ASSESSMENT OF ADVANTAGES/DISADVANTAGES Before looking at the scientific details of falling film evaporation, let’s first consider their overall attributes. Comparing horizontal falling film evaporators to flooded evaporators, the former have the following potential benefits: • Reduction in working fluid to about 1/3 that of a comparable flooded unit; • Higher heat transfer performance; Engineering Data Book III Falling Film Evaporation 14-4 • More uniformity in the overall U within the bundle; • Closer temperature approach in some cases; • More compact evaporator design; • Improved oil removal (oil holds up in a flooded bundle but drains to the bottom of a falling film unit). Comparing horizontal falling film evaporators to flooded evaporators, the former also have potential disadvantages: • Less design experience with falling film units; • Uniformity of liquid distribution onto the top tube row; • Less tolerance for undercharging of refrigerant to the unit. For petrochemical applications, the potential advantages of using horizontal falling film units as opposed to vertical units can be summarized as follows: • Heat transfer coefficients on plain horizontal tubes are higher than those for vertical tubes since the heated flow length is much shorter; • External enhancements are available for tubes in copper, copper-nickel, carbon and stainless steels, etc. for up to a 10-fold increase in boiling coefficients; • The temperature approach between the evaporating fluid and the heating fluid can be reduced to an absolute minimum for maximum thermal efficiency; • A horizontal bundle can have multiple tube passes of the heating fluid to significantly increase its heat transfer coefficient compared to a single shell pass in vertical units; • Longer, smaller diameter horizontal shell-and-tube units can be designed rather than short, large diameter shells often required in vertical units to avoid dryout and flooding in the tubes; • Two-pass floating head (or even U-tube) designs can be specified in the horizontal units, which are much more convenient to maintain and cheaper than one-pass floating heads in vertical units; • The reduced flow length of the liquid film minimizes the liquid holdup and residence time of temperature sensitive fluids; • A horizontal orientation reduces the height of the liquid nozzle inlet with respect to grade and hence may reduce the amount of piping and the required elevation of the distillation column with respect to the top of the exchanger. The principal disadvantage of horizontal units is for corrosive applications where alloy tubes are needed, which would mean placing the corrosive fluid on the shell-side. However, on the shell-side an enhanced surface may be applicable, which would greatly reduce the size of the unit with the potential of being more economical. In some materials, including carbon steel and copper-nickel alloys, doubly-enhanced tubes with internal helical ribs are available for augmenting the heating fluid side of horizontal falling film evaporators while little experience is available in using internal helically ribbed tubes for falling film evaporation in vertical units. 14.3 THERMAL DESIGN CONSIDERATIONS Before looking at the state-of-the-art of falling film heat transfer, let’s first consider what special aspects must be taken into account when designing a horizontal falling film evaporator. Indeed, much less experience and know how is generally available for designing horizontal falling film evaporators and thus new challenges face thermal designers, such as: Engineering Data Book III Falling Film Evaporation 14-5 • Choice of the most appropriate enhanced tube for the fluid to be handled. Note that conventional low finned tubes should not be used since they tend to inhibit longitudinal spreading of the liquid film along the tube. On the other hand, enhanced pool boiling tubes and also enhanced falling film condensation tubes perform very well in the falling film evaporation mode. • Choice of the optimum tube bundle layout (number of tubes and their length, bundle width and height, tube pitch and layout and number of tube passes). Optimizing the bundle size depends significantly on all these factors. • Selection and proper placement of the spray nozzles or sprinklers or distribution trays to achieve uniform liquid distribution on the top row of tubes in the bundle. These systems are not readily available and most likely the designer will have to come up with his own solution. • Minimum liquid overfeed necessary for proper operation of the unit. This involves avoiding the formation of dry patches (very low thermal performance) while at the same time limiting the flow rate of the liquid on the top of the bundle. Normally, the intertube flow mode between tubes should be sheet mode or staggered column mode, with the second one requiring a lower flow rate to achieve. The minimum liquid feed rate is hence that which still gives the staggered column mode on the lowest tube row. • Vapor escape from the bundle. The tube layout should include a consideration of how to best facilitate the escape of vapor from the bundle. • Local modeling of the heat transfer coefficients and mass transfer effects (important if the fluid is a zeotropic mixture) plus the influence of viscous components on performance, such as a lubricating oil in a refrigeration system; • Proper oil separation in the liquid pool beneath the bundle in refrigeration evaporators to avoid oil builds up in the unit. These points would also be relevant if designing a horizontal falling film evaporator in place of a horizontal flooded evaporator, a kettle reboiler, or a horizontal thermosyphon reboiler for petrochemical applications. In summary, there are numerous aspects to be considered and not all of them well understood, and some of them can only be resolved with experimental tests on prototype units. There are also various thermal mechanisms and flow phenomena specific to falling film evaporation on horizontal tube bundles that must be kept in mind during thermal design: • prediction of liquid film flow mode transitions between tubes; • vapor shear effects on the liquid film flow in a tube bundle; • crossflow effects of vapor flow on film flow modes between tubes; • nucleate boiling in the film and its onset; • prediction of heat transfer coefficients by tube row and intertube flow regime; • prediction of the onset of dry patch formation; • critical heat flux for nucleate boiling in thin films; • effect of enhancement geometry on the above processes; • effect of lubricating oil on the above processes. All of these have an important influence on proper operation of these units and their thermal optimization, and essentially all require further study (in particular for fluids other than water and refrigerants). 14.4 INTERTUBE FALLING FILM MODES Before discussing the research work on falling film evaporation heat transfer, it is instructive to first review the literature on the prediction of intertube falling film modes for plain and enhanced tubes. Figure Engineering Data Book III Falling Film Evaporation 14-6 14.4 shows photographs of the five flow modes. The intertube flow modes are classified from observations as follows: A. Droplet mode. The flow is in droplet mode when there is only a flow of liquid in the form of distinct droplets between the tubes. B. Droplet-Columns mode. This intermediate mode is present when at least one stable column exists between the tubes in addition to falling droplets. A column is a continuous liquid link between tubes. A column can move horizontally along the tubes but has to be continuous to define this mode. C. Column mode. This mode is simply when there is only liquid flow in columns between the tubes. At lower flow rates in this mode the columns tend to be inline while at higher flow rates they are staggered from one tube to the next. D. Column-Sheet mode. In this intermediate mode, both columns and a liquid sheet are simultaneously flowing between the tubes at different locations along the tubes. It is reached when at least one small sheet is visible. This small sheet is formed by the merging of two nearby columns and typically has a triangular profile. E. Sheet mode. This mode is when the fluid flows uniformly between the tubes as a continuous film or sheet. The flow progresses from mode A to mode E as the mass flow rate is increased. There is normally some hysteresis in these transitions when observing them for increasing and decreasing flow rates. For practical purposes, it is probably best to ignore this secondary effect on thermal design. Figure 14.4. Photographs of flow modes on plain tubes (from left to right, top to bottom): droplet, droplet-column, column (inline), column (staggered), column-sheet, and sheet. Engineering Data Book III Falling Film Evaporation 14-7 Despite numerous observations of condensation and falling film evaporation on rows of tubes, apparently no generalized flow mode map is currently available, although Honda et al. (1987) have made some transition expressions for individual fluids condensing on low finned tubes. These processes however are similar to adiabatic falling films of liquid fed onto the top of a tube array, which has been studied extensively by Hu and Jacobi (1996a) for a variety of fluids, tube diameters, tube pitches and flow rates and with/without cocurrent gas flow. Based on their observations, they proposed a flow mode map with coordinates of film Reynolds number versus modified Galileo number, ReΓ vs. GaL. The mixed mode zones of column-sheet and droplet-column are transition zones between the three dominant modes of sheet, column and droplet in which both modes are present. Their four flow transition expressions, between these five zones, are given below (valid for passing through the transitions in either direction and hence the symbol ⇔): Droplet⇔Droplet-Column: [14.4.1] 302.0LGa074.0Re =Γ Droplet-Column⇔Column: [14.4.2] 301.0LGa096.0Re =Γ Column⇔Column-Sheet: [14.4.3] 233.0LGa414.1Re =Γ Column-Sheet⇔Sheet: [14.4.4] 236.0LGa448.1Re =Γ The modified Galileo number GaL is defined as g Ga 4 L 3 L L μ σρ= [14.4.5] The film Reynolds number ReГ is defined in these transition equations as L L4Re μ Γ=Γ [14.4.6] where ΓL is the flow rate of liquid on one side of the tube per unit length of tube in kg/ms, so that the total flow rate on both sides of the tube is 2ΓL. This definition is consistent with ReГ for a vertical plate where the flow rate in the film is ΓL. Their map is applicable to plain tubes for air velocities less than 15 m/s. The modified Galileo number is sometimes referred to as the “film number” and is in fact the inve
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