MsoLMfoatx UAATxa

and the total evaporative area is (5X + S2_„). The feed heaters are assumed to have equal area and thus the total feed heater area becomes where Uh is the overall feed heater heat transfer coefficient. The total heat transfer area is thus given by the sum of equations (5.14), (5.15) and (5.16). Now forMF0 = 3Md/2 and setting ATx = ATj/n the specific area Ay/MD becomes

Thus At/Md can be obtained for fixed values of 7T, ATj, and n. L, Ue, Uh are fixed for any given plant though the selection of Ue is a design variable in that fluted tubes with a greatly improved Ue value over that of plain tubes may be specified.

From equation (5.17) it is seen that the specific area required is strongly influenced by the overall temperature range of the plant.

A comparison has been made by Burley [2] of the heat transfer areas required for both multiple-effect evaporation (LTV and fluted tube) and multi-stage flash processes respectively in terms of performance ratio, operating temperatures and heat transfer coefficients. The conditions used by Burley in his analysis are given in Table 5.1 (British units) and the results are shown in Fig. 5.3.

Figure 5.3 demonstrates why the LTV process has a superior chance of market penetration compared to MSF. MSF clearly requires more heating surface than LTV and the improvement with fluted tubes is even more significant. Obviously the use of a greater number of stages in MSF will reduce the area required, however the optimum number is roughly 3R. The development of fluted tubes has led to projections of 20 per cent reduction in product costs compared with MSF and it is with plants of this type that considerable development is now taking place.

Table 5.1. Comparison of MSF and LTV distillation processes

Fluted tubes

A7V(°F) aC F) IBtu/lb IsBtu/lb U Btu/h/ft2 °F Ue Btu/h/ft2 °F t/h Btu/h/ft2 °F

992 934 500

500 500

1 500 500

Performance ratio R

Fig. 5.3. Comparison of specific areas for MSF and LTV processes versus performance ratio.

Fluted tubes

The concept of enhanced heat transfer surface using thin film techniques is not new. Figure 5.4 shows a pictorial representation of a fluted tube VTE module. The incoming brine flows across an upper tube plate, through a distributor to form a film on the inside of the tube, which falls under the action of gravity. Steam from the previous effect or boiler condenses on the outside of the tubes forming the product distillate in a film. Heat transfer takes place across the condensate film, through the tube wall to the evaporating brine as shown in Fig. 5.5 which depicts the various resistances to heat transfer encountered. The two-phase mixture leaving the bottom of the tube is separated and the steam passed to the next effect.

The heat transfer resistance constraints are such that the tube wall resistance is fixed by the material used and its thickness which is dictated by design life. The designer then has to consider film resistances and modes of heat transfer which are, respectively, condensation on one side of the tube and boiling or liquid heating on the other. The principal resistance to heat transfer lies in the film thicknesses and many techniques have been tried to obtain thin film. The most successful, as it employs no moving parts or turbulence promoter, is the fluted tube which produces and maintains thin films in both the evaporation and condensation modes.

The effect of the flutes is to create surface tension forces inversely proportional to the flute radius of curvature which causes the condensate film to drain from the crests into the grooves. The result is that a substantial portion of the crest has a

Upper tubeplate

Brine distributor

Steam from previous effect (or boiler)

Condensate

Steam + brine

Upper tubeplate

Brine distributor

Steam from previous effect (or boiler)

Condensate

Steam + brine

Lower brine plenum Fig. 5.4. Fluted tube VTE module.

Fouling

Fouling

Fig. 5.5. Heat transfer in the LTV evaporator.

Evaporation side Thiri film region

Evaporation side Thiri film region

Thin film region Condensate

Heat flow

Condensing side

Fig. 5.6. VTE fluted tube heat transfer.

Thin film region Condensate

Heat flow

Condensing side

Fig. 5.6. VTE fluted tube heat transfer.

very thin film of condensate which enhances heat transfer in this area as illustrated in Fig. 5.6. The condensate in the grooves drains by gravity with reduced heat transfer in this area. The falling feed film also drains into the grooves under the action of surface tension. Boiling takes place in both crests and grooves and the crests are kept constantly wet by the boiling action in the grooves. The flutes also assist the feed distribution inside the tube and ensure uniform distribution along the length. The net result is that the high condensing and evaporating coefficients combine to produce overall heat transfer coefficients three to four times those of a

Fig. 5.7. Fluted tubes.
Fig. 5.8. Fluted tube heating chamber assembly being connected to vapour chamber sump on Gibraltar 1 360 m3/day plant.

plain tube. Tubes have been developed which can be readily manufactured and give coefficients of 1.4 x 104 W/m2 °C (2 470 Btu/ft2/h °F) based on the original tube diameter. Approximate fluted tube costs are 30 per cent above the cost of a normal tube which is not significant compared to a three- or four-fold increase in overall heat transfer coefficient. Figure 5.7 shows one form of commercially available fluted tube and Fig. 5.8 shows a tube assembly being connected to a vapour chamber sump.

Flow distribution

In VTE plants, the film should be evenly distributed around the tube periphery and the brine should be equitably distributed to every tube in the effect. Maldistribution of the total flow could lead to the calcium sulphate solubility limit being exceeded in individual tubes causing scale deposition and a drop in performance ratio. Similarly uneven flow distribution leads to dry patch formation with a drop in performance ratio. The precipitation of calcium sulphate scale is to be avoided at all costs as its removal calls for severe treatment which in extremes means tube replacement.

Several varieties of flow distribution exist. One common method is to use a projection of the top edge of the tube as a weir and allow the brine to flow over. This has the disadvantage that fluctuations in head may cause maldistribution. Hawes [1] has reported the development of a nozzle shown in Fig. 5.9 which incorporates metering orifices that in turn form brine jets which hit a deflector and

Metering orifice

Metering orifice

Fig. 5.9. VTE flow distribution nozzle.

thus produce a film on the tube inside walls. It is claimed that this nozzle arrangement is insensitive to head fluctuations and the brine flow was distributed to within ±10 per cent of the mean on a bank 68 tubes x 75 mm dia. x 95 mm triangular pitch.

Horizontal tube evaporator: multiple-effect evaporator (HTE)

One of the disadvantages of the VTE is that each effect requires its own pump to circulate the brine to the respective upper brine plenum. A possible multiple-effect arrangement which obviates this difficulty is the horizontal tube evaporator shown schematically in the two-effect arrangement in Fig. 5.10.

The principle of operation is exactly the same as in VTE, steam condenses on the inside of the tubes imparting its latent heat of condensation to the evaporating brine which cascades over the outside. The steam formed in the first effect is sent on to the next effect and the brine falls under gravity to feed the next effect, thus eliminating the intereffect pump. Effects can be stacked vertically and so only one pump instead of n is required for the brine.

The heat transfer rates in the HTE have been investigated by the Office of Saline Water and reported by Cox. [3] It was found that condensation inside the tubes was filmwise and that overall coefficients were approximately double those of smooth LTV tubes. Bubble nucleation on the evaporating side took place on the top of the tubes with rapid bubble growth and a sliding around the tube circumference taking place simultaneously. This mechanism caused rapid local rate of heat transfer to take place and it was concluded that the evaporative heat

Fig. 5.11. Multiple-effect, horizontal, falling film plant showing feed distribution and boiling in progress.

transfer coefficient was independent of brine thickness but dependent upon the number and size of the bubbles. The principal resistance to heat flow was the condensing coefficient and in an analogous fashion to the use of fluted tubes in the LTV process it was found that a significant improvement could be obtained by the use of internal grooves which experimentation set at 0.254 mm x 0.8 mm (0.010 in deep x 0.031 in) spacing. The result was an overall HTE coefficient roughly three times that for an LTV plant using plain tubes which is a comparable improvement with that of fluted tubes in the latter process.

The operation of HTE was found to be very susceptible to the presence of non-condensable gases. Their blanketing effects were minimised by maintaining an exit velocity of greater than 2.45 m/sec (8 ft/sec) to sweep the gases out with the excess steam. The desired exit velocity is provided by locating the sea-water feed heater downstream of the effects, i.e. the steam condensed in any feed heater must first pass through the horizontal tubes of any effect.

The large-scale development of the HTE process is proceeding, Rhodes and Mills [4] have reported the successful operation of two test modules at Dungeness 'A' Nuclear Power Station. It is claimed that the HTE is the best distillation plant design when acid pretreatment is used as the formation of scale or tube attack and the boiling process can be readily observed through the inspection ports. Figure 5.11 shows feed distribution and boiling in progress in an HTE module.

Multiple-effect plant layout and operation

The layout of a multiple-effect plant is best illustrated with reference to Fig. 5.12 from the paper by Rhodes and Mills [4] (on which the following description is based) for the Gibraltar LTV fluted tube plant. This plant has 13 effects, a performance ratio of 10 and an output of 1 360 m3/day (300 000 gal/day). Equal area preheaters are used except preheater 13 which absorbs part of the last effect vapour. All heating chambers were made identical in size except for the last effect where a lower heat transfer coefficient required an increased tube length to provide the additional area. The heating chambers were mounted on common sumps, fabrication requirements dictating that four be used. The common sump technique enables brine and vapour to be separated. Brine feed and distillate streams can also flow from one effect to the next with the elimination of most of the interconnecting pipework required if each effect were totally separate.

With the much smaller raw feed rates employed in ME plants, compared with MSF, great care has to be exercised in scale prevention measures if acid dosing is employed. The volume of sea water requiring treatment is substantially less, thus accidental overdosing is more probable which can lead to acid corrosion. The Gibraltar plant has automatic regulation by means of a feedback loop from a pH indicator on the degasser inlet which controls the injection pumps. Acid is diluted before use in order to suit the pump size. One interesting feature is the use of an acid blend box to ensure proper pH control and accurate pH readings at the sensor. The feed is extracted from the sea-water coolant stream for the excess vapour

I Boiler feedi—1 .A|ar^ tank (1st effect|1steffect* J \* distillate) «Alarm Lo Alarm Lo

Circulation pumps

Boiler feed pumps

Circulation pumps

Boiler feed pumps o = orifice Hi = Alarm High Lo = Alarm Low

Fig. 5.12. Part of flow sheet for LTV sea-water distillation plant for the Government of Gibraltar.

condenser, it then passes in succession through No. 13 preheater, the ejector condenser and No. 12 preheater before acid dosing to pH 4.5. Degassing follows with stripping steam extracted from the 12th effect. After degassing the feed flows through 11 U-type preheaters in series. After the feed has passed the preheater chain it enters the No. 1 effect sump where it is recirculated to the top of the heating chamber.

Each heating chamber has a top water box fastened to the tube plate which serves as the brine plenum. Recirculated brine passes through a wire mesh to prevent scale clogging the tube distributor orifices and flows down the inside of the fluted tubes. Non-condensable gases are extracted through a central tube with orifices along its length. The condensate from the first effect cascades into an extension of the sump which acts as a boiler feed tank. The vapour separates from the brine which is then transferred to the second effect sump through an orifice and recirculated. Vapour from the preheater is extracted from the top of the heating chamber so that velocities are maintained throughout its length.

The steam is supplied by a packaged boiler and is used in a 200 kW turbo alternator before exhausting to the No. 1 effect. Part of the boiler steam is used for the ejector and a dump condenser is available for balancing the system during start up and shut down.

The materials of construction are similar to those for MSF plants. Main effect sumps are carbon steel with a large corrosion allowance. All water boxes are aluminium bronze except those on the heating chambers where low velocities are employed and carbon steel is therefore satisfactory. Tubes are aluminium brass except for the ejector condenser and non-condensable gas cooler where titanium is employed because of the corrosive duties. All pipework is carbon steel except sea-water services before the degasser which are PVC or rubber-lined steel. The degasser is rubber-lined carbon steel.

Scaling in VTE plants

As with any other distillation process, scale prevention is of paramount importance. Experimental work by Hodgson et al. [5] shows that the CaS04 prevention guidelines discussed in Chapter 3 apply, with one proviso. Heat flux as well as temperature and concentration must be taken into account in the VTE falling film evaporator. The results of Hodgson et al. show the importance of heat flux on CaS04 formation, e.g. at 'low' heat flux CaS04 light deposition was encountered at 130°C (266°F), whereas a 'high' heat flux deposition was observed at 115°C (240°F). With high heat flux typical experimental values were

Concentration factor 1.05

Temperature 115°C (240°F)

CaS04 formation recorded with low heat flux

Concentration factor 1.0 Temperature 130°C (266°F)

Light CaS04 deposition The work further showed that at a given temperature, scaling of the heating tube surface occurs at a lower concentration in VTE compared with MSF. This is explained by the presence (presumed) of areas of high concentration forming within the film as a result of uneven evaporation. This work enables VTE plants to be designed for high temperature operation.

Present position

Both VTE and HTE systems are commercially available. VTE fluted tube ship-board installations have been in use for several years. A commercial VTE fluted tube

Fig. 5.13. 1 360 m3/day (300 000 gal/day) VTE plant for Gibraltar.
Fig. 5.14. 4 282 m3/day (942 000 gal/day) VTE installation at Europort Rotterdam.

installation with a performance ratio of 10 and 1.36 x 103 m3/day is in operation in Gibraltar with a claimed reduction in water costs compared with an equivalent MSF plant. The improved multiple-effect processes as embodied in the LTV and HTE concepts will assume growing importance in water supplies by desalination in the future. Figures 5.13 and 5.14 show two VTE installations completed by Aitons Ltd. for Gibraltar and Europort Rotterdam, respectively.

References

1. Hawes, R. I. 'Sea-water distillation studies in the U.K.A.E.A.', Paper C/18/73, pp. 33-41.1. Mech. E. Conf. on Water Distillation, London, Jan. 1973.

2. Burley, M. J. 'Analytical comparison of the multi-stage flash and long tube vertical distillation processes', 2nd European Symp. on Fresh Water from the Sea, Athens, May 1967.

3. Cox, R. B. 'Some factors affecting heat transfer coefficients in the horizontal tube multiple effect (HTME) distillation process', Proc. 3rd Int. Symp. on Fresh Water from the Sea, Vol. 1, pp. 247-63, Djbrovnik, 1970.

4. Rhodes, C. and Mills, K. E. 'The Gibraltar multiple-effect VTE falling film plant', Paper 6/19/73. I. Mech. E. Conf. on Water Distillation, London, 1973, pp.43-49.

5. Hodgson et al. 'Calcium sulphate scaling in falling film evaporators', Proc. 4th Int. Symp. on Fresh Water from the Sea, Vol. 2, pp. 143-59 Heidelberg, 1973.

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