Paisley college

radiation. Any surface underneath the glass cover can then be heated. However, the re-radiated wavelengths from the heated surface are such that little radiant energy can be transmitted back through the glass and hence there is a temperature rise in the enclosure. This effect is put to use in solar stills to produce fresh water from brackish or saline water by evaporation, energy being supplied by the incoming solar radiation.

The mechanism of solar radiation transmission through glass is shown schematically in Fig. 6.4. It is seen that the bulk of the solar radiation is transmitted directly through the glass. The net solar heat gain as a percentage of incident radiation is given in Fig. 6.5, based on data from Pilkingtons. [5] For

Directly transmitted solar radiation

Directly transmitted solar radiation

Incident short wave solar radiation

Angle of incidence

Reflected short wave radiation

Reradiation (long wave) convection and conduction

Heat loss by radiation (long wave) convection and conduction

Fig. 6.4. Mechanism of solar radiation transmission through glass.

Incident short wave solar radiation

Angle of incidence

Reflected short wave radiation

Reradiation (long wave) convection and conduction

Heat loss by radiation (long wave) convection and conduction

Fig. 6.4. Mechanism of solar radiation transmission through glass.

angles of incidence up to 35° the total solar heat gain is as high as 85 per cent of incident radiation. The areas of applicability of solar distillation are roughly banded by latitudes 35°N to 35°S which embraces all the world's arid zones. There is obviously a wide range of average solar radiation intensities in this band and Table 6.1 from Lof [6] gives the monthly and average values (cal/m2/day) for various locations throughout the world. Conversion to W/m2 = insolation (4.83 x 10-5).

Still output per unit area is determined by solar radiation intensity - all other things being equal. Thus from Table 6.1 a solar still located in Alice Springs would have a much greater output than one located in Cambridge, UK - although solar water heaters, based on the greenhouse principle, for swimming pools are sold in the UK.

Still construction

Still construction is or should be a simple affair, as sophistication is not desired or desirable in the usual geographical locations where this method of desalination is

Location

Lat

Long

elev (m)

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Ann'l

Aden

12°50'N

45°01'E

4

415

481

541

591

573

530

503

535

541

544

500

444

516

Australia

Alice Springs

23°48'S

133°53'E

546

646

620

556

466

369

337

361

460

551

592

623

641

518

Dry Creek, SA

34°50'S

138°35'E

4

691

601

487

426

233

197

209

286

397

479

600

662

439

Ceylon Batticaloa

07°43'N

81°42'E

3

440

510

550

540

540

510

520

530

540

500

470

420

507

France

Montpellier

43°35'N

3°50'E

150

230

300

420

480

600

640

490

380

240

170

120

352

Greece

Athens

37°58'N

23°43'E

107

180

276

334

457

516

577

572

498

396

276

187

161

369

India

Baroda

22°15'N

73°15'E

450

520

580

650

690

600

450

400

540

470

480

420

521

Kenya Nairobi

01°18'S

36°45'E

1 799

558

595

559

486

416

397

324

366

464

492

486

522

472

Peru

Huangayo

12°02'S

75°19'W

3 313

670

518

574

540

499

491

519

568

597

634

629

596

570

Spain Almeria

37°N

2-5°W

215

296

403

503

552

588

595

541

443

337

241

190

409

UK

Cambridge

52°13'N

00°06'E

23

60

102

190

284

398

428

412

320

246

151

67

42

225

USA

Boston

42°21'N

71°04'W

102

139

198

293

364

472

499

496

425

341

238

145

119

311

Phoenix

33°26'N

112°0l'W

102

297

408

521

643

724

740

652

612

568

452

339

280

520

Conversion to W/m2 = insolation (4.83 x 10~s)

Conversion to W/m2 = insolation (4.83 x 10~s)

Angle of incidence (degrees)

Fig. 6.5. Net solar heat gain as a percentage of incident radiation.

Angle of incidence (degrees)

Fig. 6.5. Net solar heat gain as a percentage of incident radiation.

used. A land-based solar still installation essentially comprises a brine pool with associated radiation-absorbent liner, a supporting structure and a glass or plastic cover. Provision is made for the introduction of feed water, removal of distillate and disposal of reject brine. Storage of the product may also be required. The brine is usually contained in a level quiescent pool, hence the name 'basin-type' still is given to this form of installation. Continuous flow may be obtained by providing a gentle gradient on the base and allowing the feed to trickle in and the reject brine out.

Figure 6.6 shows a cross-section of a glass-roof basin still constructed by

Aluminium ridge bar and support column

Aluminium ridge bar and support column

Fig. 6.6. Basin-type still.

Porteous [7] for the Royal Society Research Station on the island of Aldabra in the Indian Ocean. This was commissioned in July 1970. The design is based on a series of standard prefabricated modules which were readily erected on the island by local fishermen. Each module has a brine pool area of 42 m2. The Aldabra installation has four such modules to provide a reliable means of water supply on this remote coral atoll. Rainwater catchment gutters are also provided, thus augmenting the still output considerably in wet weather when solar radiation may be insufficient for significant distillate output.

In operation, the incoming solar radiation heats the brine pool and evaporation takes place when the brine reaches the temperature range 50°-65°C (122°-149°F). As the roof is transparent to solar radiation it is below the saturation temperature of the mixture of air and water vapour in the enclosure and condensation takes place on the sloping surface. The condensate forms in drops or rivulets, and because of the wetting characteristics of the glass it runs into condensate channels, whence it is led out of the system. The normal mode of operation is to allow evaporation of the brine to a concentration of twice that of the incoming feed and then drain and refill. In most stills this is done once every three to seven days, depending on depth of brine pool and insolation.

Several points are worth mentioning. Many solar still designs have been mooted Bloemer et al.\ [8] Howe and Tleimat; [9] Morse [10] most with emphasis on cheap construction through the use of plastics for the roof and very light supporting structures, often with polythene-lined brine pools. In the writer's opinion, which has been confirmed in practice, there is no substitute for glass, a well-supported structure, and a heavy gauge pool liner if the object of the design is long life, reliability and ease of maintenance. These statements are borne out by the fact that the Symi still constructed by Delyannis and Piperoglou [11] has had its plastic roof replaced and their subsequent Patmos installation used glass instead of plastic. In Australia, prototype continuous stills have met with difficulty in performance maintenance through ruptured plastic liners and other cooling problems. There are two inherent problems with plastics. They possess adverse wetting characteristics, thus condensate drainage is poor and the subsequent 'fogging' impedes the transmission of solar radiation. Their life is also short under the high radiation conditions normally prevalent as decomposition takes place under the action of ultraviolet radiation.

A form of continuous still has been under development in Australia for about ten years, but little information is available on the current design or its performance, but the Mark II (1966) design has been documented. [12] The major problem with continuous distillation is the control of a very thin film of saline water as it flows down a slight incline (1 in 60) under a glass covered enclosure some 20 m long. If 'dry patching' occurs on the pool liner, a hot spot develops and scale build-up is rapid.

Basin-type stills are relatively simple affairs as shown in Fig. 6.6, and it is not surprising that they form the majority of the world-installed solar still capacity.

The output of a still is determined by the intensity of solar radiation and the area covered, all other things being equal. Table 6.2 gives details of a few

Location

Brine pool areafm2 )

Date of Type erection

Average annual distillate output (based on pool area) llm2lday

Corresponding (average insolation) kcal/m2/day

W/m2

Data reference

Indian Ocean

167

1970

Basin

3.8

5 160

250

Author's data from

(Aldabra Is.)

Aldabra Research

Station

Chile: Andes

(Elevation

1 300 m)

4 700

1872

Basin

5.76 (max.)

8 150

395

Harding (1883)

Greece:

Patmos

8 667

1967

Basin

3.0

Not given

-

Delyannis and Piperoglou

(1967)

Symi

2 700

1964

Basin

2.62

3 743

182

Delyannis and Piperoglou

(1968)

Australia:

Coober Pedy

3 500

1966

Continuous

3.22

5 100

246

Morse (1967)

Muresk

416

1936

Continuous

2.2

5 100 (author's est.) 246

Morse (1967)

Pacific Is.

4-65

1966

Basin

4.3 (max)

6 300 (author's est. 305

Howe and Tleimat (1967)

(plastic roof)

from Lof 1966)

installations where published figures are available. It is noted in passing that there is in principle little difference in construction between a 50 m2 and a 25 000 m2 still. The relevant parameters by which performance can be judged are average insolation and output per unit area and these are given in the table. An order of magnitude for a well-designed basin still is 51 m2/day (1 gal/day 10 ft2) for an insolation of 340 W/m2 (7 000 kcal/m2/day).

Performance

The theoretical performance of solar distillation has been analysed by Porteous [13] and is mainly a function of insolation and base loss, i.e. the heat losses through the base have a strong effect on still output. The results of the theoretical

Insolation (W/m2 x 100)

Fig. 6.7. Variation of output with insolation.

Insolation (W/m2 x 100)

Fig. 6.7. Variation of output with insolation.

analysis are summarised in Fig. 6.7 which shows still output plotted against insolation with base loss (hb) as parameter. It is seen that the effect of hb is quite marked at high insolation, e.g. at 300 W/m a reduction in hb from 5.7 to 1 W/m2 °C will give an approximately 20 per cent increase in output. A typical loss coefficient for a base of sand and gravel is 5.7 W/m2 °C.

Practical comparison is possible as the performance of several solar still installations have been monitored; Symi (Delyannis and Piperoglou [10]), Coober Pedy (Morse and Read [12]) and Florida (Bloemer et al. [8]) respectively. Data on the Aldabra installation was also collected by the research station staff and this along with the data for the Symi, Florida and Coober Pedy stills is shown in Fig.

6.8 as a plot of output versus insolation. It is seen that the performance of the Aldabra, Florida and Symi stills are quite similar. This is to be expected as these were basin-type solar stills. The Coober Pedy installation was continuous, and again, as would be expected, the performance is substantially less due to the characteristics of these plants.

Over the usual operational range of 200 to 350 W/m2 there is a good measure of agreement between the predicted and actual outputs. Confidence can therefore be taken in the theoretical predictions given that the base loss coefficient can be estimated for the installation. The main utility of the theory, however, is to demonstrate the influence on output of the various parameters under the designer's control.

Solar distillation plants are finding favour with many isolated communities and can successfully deal with borehole water on which many distillation plants are unable to operate. The process is not usually a solution where skilled labour and cheap fuel are available but it is capable of great dependability for the provision of water at a cost comparable with other processes in its range of applicability. Its use may be expected to increase as the ready availability of cheap fuel supplies decrease.

Low temperature difference distillation

Low Temperature Difference (LTD) distillation is a means of utilising the low-grade thermal energy content of reject cooling streams from processing plants, diesel engine water jackets, etc. The premise is that this energy will go to waste anyway and if a fresh water supply is required, e.g. shipboard or boiler make-up then a simple flash installation is capable of doing this using the heated effluent as feed.

The design procedures employed for LTD plants vary considerably from those outlined in Chapter 4 for MSF. A conventional MSF plant will operate over a temperature range 127°-32oC(250o-90°F)andheat recovery is of prime importance to keep fuel consumption down. The LTD plant has no such constraint. It commonly may have a heated effluent stream as feed at a temperature of 38°-54°C (100°-130°F). Sea water at c. 26.5°C (80°F) may be available as coolant, this gives a maximum temperature differential of 11°C and 27.6°C (20° and 50°F) respectively for heat rejection purposes.

Thus LTD plants commonly operate in the region where conventional processes terminate due to flash chamber sizing, vapour flow problems, etc. The LTD plant is a chamber or series of chambers with provision for feed entry and distribution in a dispersed phase such as a spray or film as hydrostatic head effects dominate at these low temperatures and must be minimised. Cooling is by a simple tube bundle arrangement. The circulation of ambient sea water as coolant may restrict the temperature difference available for operation and the exchanger may require careful design. As the vapour specific volume is correspondingly large at the low saturation temperatures employed efficient demisting arrangements may be required.

Product removal and blowdown can be accomplished by extraction pumps but on land-based installations the plant may be elevated by roughly 10 m and extraction problems eliminated by the use of the resulting hydrostatic head. Such a plant has been designed by the A. Ahlstrom Osakeyhitio Company [15] and

Feed water

4ft.

Annular Deaerator Chamber

4ft.

40ft. 1

Condenser

Product-trough

Spray nozzle

Demist er shield

-Brine to waste

\Cooling water o1—

Precondenser

Vacuum pump

Product

Fig. 6.9. Schematic diagram of low temperature difference plant.

Water flow

Water flow

produces 100 m3 water per day. The costs claimed for a write-off factor of 15 per cent per year are $0.138/m3 ($0.25 per 1 000 gal) for a 20°C (36°F) temperature differential.

Howe et ah [161 have published details of an LTD pilot plant successfully run at the University of California whose details are given in Figs 6.9 and 6.10 which show plant layout and the flow distribution nozzles used respectively.

The simplicity of plants of this type has much to commend it, though as with solar distillation their adoption is a function of the specific situation. The maximum output will be constrained by the feed rate available and permissible flash drop. For a 5.5°C (10°F) flash drop 100 m3 (100 gal) of feed will be required per m3 (gal) of product. The LTD process is another step in the chain of total energy and has little reliance on capital fuel resources given that a heated effluent is being discharged to waste from some other process. Similarly geothermal energy may be employed in LTD installations either as the energy source if steam is vented or as the flashing feed stream.

Controlled flash evaporation (CFE)

Controlled flash evaporation is an attempt to eliminate the hydrostatic head effects which obtain in conventional multi-stage flash distillation. The flashing brine is cascaded down a vertical tower which contains chutes or channels enabling a brine film c. 1 mm in thickness to form. The result is controlled equilibration with the vapour substantially in equilibrium with the brine at every point. Although the CFE process is analogous to the LTD process described previously, its use, however, is projected over the conventional temperature range for MSF. The chutes are arranged usually in analogous fashion to the horizontal stages in an MSF plant though the overall stage drop can be 8.3°C (15°F) as opposed to 1.67°-2.78°C (3°-5°F).

The process has been described in detail by Roe and Othmer [ 17] who see it as a replacement to conventional MSF due to its almost 100 per cent equilibration operation and projected lower construction costs. Simultaneously these authors see it as a potential LTD process operating on heated effluents discharged at 8.3°-14°C (15°-25°F) above ambient. The substantial temperature gradients in tropical seas with surface water up to 40°C above that of the depths are also seen as being suitable for CFE applications - no commercial installations utilise this supposedly . free energy source as yet.

Non-metallic heat transfer surfaces

As roughly 40 per cent of the capital costs of most distillation plants is tied up in the heat transfer surface, it is to be expected that many attempts have been made at its elimination. Two routes have been tried, namely the use of an immiscible liquid as a heat transfer medium and non-metallic heat transfer surfaces, immiscible liquids have foundered technically for many reasons and so far are still at the laboratory stage. Non-metallic surfaces on the other hand are receiving intensive investigation and an assessment of the possible use of glass and thermoplastics in flash plants has been done by Wood [18] and Table 6.3 below (from [18]) summarises the characteristics of typical alloys and non-metallics. Table 6.4 (also from [18]) summarises the assessment for relative costs and characteristics of glass condensers and conventional surface for a (5 m.g.d.) flash plant.

The result of the assessment in Table 6.4 shows a potential 11.8 per cent saving on capital investment compared with the conventional plant. Cheap plastic condensers are so far restricted to a maximum temperature of 71°-75°C (160°-170°F) and the savings are not so marked as those for glass. However, if some of the new varieties of plastics (polyisobutylene, polyphenylene oxide) come down in price the cost picture may change radically.

To date the only reported plant in a commercial size using non-metallic heat exchangers is the 'Kogan-Rose' process, [19] see Fig. 6.11, which uses both direct contact condensation and plastic tubed heat exchangers. The process is in most aspects conventional MSF except that the flashing vapour is cooled by direct contact with a counter current stream of cold product. The heated product is then led to a separate plastic film heat exchanger where it is used to heat the re-cycled brine and sea water make-up stream. By using product water as the coolant stream in the flash chambers costly tubing and water boxes with their attendant problems

Table 6.3. Relative costs and characteristics of plastic and conventional 2 m.g.d. flash plants

Plant

Conventional

Plastic condensers

Flash range

°F

195-105

195-105

Performance ratio

lb/1 000 Btu

7

7

Total number of

stages

35

(Conventional 11)

(Plastic 24)

Mean overall U

Btu/ft2/h °F

600

80

LMTD in recovery

stages

°F

9.6

9.6

Total heat transfer

surface

ft2

0.143 x 106

(Conventional 0.045 x 106)

(Plastic 0.750 x 106)

Brine flow rate

lb/ft/h

650 000

650 000

Table 6.4. Relative costs and characteristics of glass and conventional 5 m.g.d.

flash plants

Plant

Conventional Glass condensers

Flash range

°F

240-100

i 200-100

Mean overall U

Btu/ft2/h°F.

550

150

LMTD in recovery

stages

°F

8.4

6.1

Total heat transfer

surface

ft2

0.556 x 106 2.9 xlO6

Brine flow rate in

shell

lb/ft/h

400 000

300 000

No. of decks

2

1

Condenser arrangement

Cross-tube Through-tube

Wt of shell steelwork

tons

650

775

Approximate shell

size

1 x b x h, ft

80x45x33 110x 70x 18

Plant capital cost*

£M

1 144

1 009

*Erected but excluding civil works.

Saving of glass plant over conventional = 11.8% on capital.

*Erected but excluding civil works.

Saving of glass plant over conventional = 11.8% on capital.

Product recycle P = 8.26 T = 76.2 CR = 0(0) W = 143.8

Product condensate P =9.26 T =195.5 CR = 0 (0) W =161.2

Blowdown -P = 0.57 CR = 2.75 (9.66) W =9.90 T =83.5

Product recycle P = 8.26 T = 76.2 CR = 0(0) W = 143.8

Product condensate P =9.26 T =195.5 CR = 0 (0) W =161.2

Product recirculating pumps AH = 80.7 ft. 2 x 4110 h.p.

Recycle brine P =8.53 T = 83.5 CR = 2.75 (9.66) W = 148.4

Sea water intake P = 4.87 T 65 CR = 1 (3.52) W =84.7 Sea water return. P = 3.30 T =83.5 CR = 1 (3.52) W = 57.4

Sea water intake pumps AH = 55 ft 2 x 1470 h.p.

Sea water intake pumps AH = 55 ft 2 x 1470 h.p.

Economy = 6.52

Btu/hr Ibs

Heat exchanger

Condensate pumps AH = 230 ft. 2 x 80 h.p. P =9.26 T =202.0 CR = 0 (0)

- Heat recovery-

—Brine recirculation pumps AH = 32 ft. 2 x 1770 h.p.

Legend: P Flow in pounds per pound product T Temperature in °F

CR - Concentration ratio and (% TDS) W - Flow in 106 lbs/hr

Fig. 6.11. 'Kogan-Rose' 50 m.g.d. (US) process flow sheet.

are eliminated. The heat transfer problems are transferred to the design of an external shell and tube heat exchanger. The savings claimed are a 25-30 per cent reduction over conventional desalting plant costs and if the results from a 45 m3/day (10 000 gal/day) pilot plant are successful, the MSF process should have a strong and continuing role in sea-water distillation. As this process has now been running at pilot plant level for over 3 years, it is worthwhile giving the process description and detailed flow sheet for a 50 m.g.d. installation (see 6.11) - the obsession with large-plant capacities seems to be a tendency in most conceptual designs - however, the relevant scale sensitive values may be reduced by a factor of 10 to bring the plant down to present-day capacities.

The plant process is a single-effect multi-stage flash evaporator - direct contact condenser

Sea water at 18.3°C (65°F) is pumped into the plant through a 2.7 m (9 ft) diameter intake pipe from a point located 750 m (2 500 ft) offshore. The sea water is treated with chlorine and screened before entering the sea-water pump.

Approximately two-thirds of the incoming sea water is used for heat rejection. It is returned to the sea at 29°C (83.5°F) after passing through the heat rejection section of a plastic film heat exchanger.

The remaining one-third of the incoming sea water serves as a process make-up stream. It is acidified with sulphuric acid to prevent scale deposition in the plant. It is then partly deaerated in a spray-type deaerator by scrubbing with vapour extracted from the concentrated brine reject stream. Its temperature is raised in the heat rejection section of the heat exchanger to 29°C (83.5°F), the temperature level of the concentrated brine emerging from the last flash stage. The temperature of the sea water make-up stream and of the recirculated brine stream is further raised in flowing through the heat recovery section of the heat exchanger. The two streams emerging from the hot end of the heat exchanger at 95°C (200°F) enter as a combined brine stream into the first flash stage. A small amount of additional deaeration is accomplished in this stage. The hot brine stream flows through the flash evaporator stages which are situated at different elevations, the stage elevation increasing progressively from the hot end to the cold end of the evaporator-condenser. In passing through the successive flash stages the hot brine is progressively concentrated and cooled down due to adiabatic vaporisation. The particular streamlined design of the interstage passages enables the brine stream flowing through each interstage passage to liberate vapour in a quasi-continuous and orderly manner. The mechanical energy liberated during the flash process is spent in lifting the brine level. The brine stream elevation is increased by 3.55 m (11.75 ft) in passing through the flashing stages of the evaporator-condenser. Brine leaves the last stage of the evaporator-condenser at 29°C (83.5°F) and a concentration ratio of 2.75. Of this stream 93.5 per cent flows into the cold end of the heat recovery section of the heat exchanger, while the remaining 6.5 per cent is rejected to the sea after passage through the evaporation section of the deaerator.

The vapour released in the successive flash stage is condensed by direct contact with a stream of desalted water which enters the condensation section of the last (cold end) stage of the evaporator-condenser and flows down through the condensation sections of the successive stages in a direction counter-current to the flashing brine stream. The desalted water stream enters the last stage of the evaporator-condenser at 24.4°C (76.2°F) and leaves the first stage at 90°C (195.8°F). Its elevation is decreased by 10.4 m (34.25 ft).

The distillate emerging from the first stage of the evaporator-condenser flows through a conventional steam heater to the hot end of the heat exchanger. In passing through the heater the temperature of the desalted water stream is raised to 95°C (204°F). In the heat recovery section of the heat exchanger heat is exchanged between the hot distillate stream and the cold brine stream. The temperature of the distillate stream is thereby decreased to 30.5°C (87.5°F). Additional cooling of the distillate is achieved in the heat rejection section of the heat exchanger and it is then pumped from the cold end of the heat exchanger at a temperature of 24.4°C (76.2°F). Of this stream 87.5 per cent is recirculated to the cold end of the evaporator-condenser while the remaining 12.5 per cent is discharged as product.

Summary

Only those processes which have strong applicability and are commercially available or feasible have been described. There are many laboratory variants around, ranging from multiple-effect humidification to the elimination of heat transfer surfaces by the use of an immiscible liquid. Until these are proven commercially a healthy scepticism is usually advisable. Vapour compression and solar distillation are capable of filling a niche in the desalination plant spectrum left untouched by large-scale VTE and MSF plants. The use of solar energy (income), reject thermal effluent or geothermal sources has much to commend it. Capital fuel resources will become scarcer and commensurately more expensive and the solar distillation and LTD processes will gain wider acceptance as the scarcity grows.

References

1. Silver, R. S. 'Fresh water from the sea\Proc. I. Mech. E„ Vol. 179, part 1. No. 5.1964-65.

2. Stormer, R. and Lowes, I. 'A study into nuclear single-purpose MSF plant with vapour recompression', Paper SM-113/52, I.A.E.A. Symp. on Nuclear Desalination, Madrid, Nov. 1968.

3. Wood, F. C. and Herbert, R. 'The characteristics of dual process distillation plant combining vapour compression and multi-stage flash evaporators', Paper SM-113/44, I.A.E.A. Symp. on Nuclear Desalination, Madrid, Nov. 1968.

4. Canberra Department of National Development. A survey of water desalination and the relevance to Australia, 1965.

5. Pilkingtons (Flat Glass Works, St. Helens), Communication, April 1971.

6. Lof, G. 'World distribution of solar radiation', Report No. 21, Engineering Experimental Station, University of Wisconsin, 1966.

7. Porteous, A. 'The design of a prefabricated solar still for the island of Aldabra, 1970', Desalination 8,93-8.

8. Bloemer, J. W. et al. Proc. 1st Int. Symp. on Water Desalination. Washington D.C., Oct. 1965, pp. 609-21.

9. Howe, E. D. and Tleimat, B. W. 'Solar distillers for use on coral islands', Paper 99, 2nd European Symp. on Fresh Water from the Sea, Athens, May 1967.

10. Morse, R. N. 'The construction and installation of solar stills in Australia', Paper 101, 2nd European Symp. on Fresh Water from the Sea, Athens, May 1967.

11. Delyannis, A. and Piperoglou, E. Sol. Energy, 12, 1968, 113.

12. Morse, R. N. and Read, W. R. W. Sol. Energy, 12, 1968, 5.

13. Porteous, A. 'The theory, practice and economics of solar distillation', Chem. Engr., 255, 1971,406-11.

14. Delyannis, A. and Piperoglou, E. 'Solar distillation developments in Greece', Sun at Work, 1st quarter 1967, pp. 14-19.

15. Anonymous. 'Finnish flash distillation', Engineer's Notebook, Chartered Mechanical Engineer, March 1973.

16. Howe, E. D. et al. 'Distillation schemes using low grade heat energy', Proc. 3rd Int. Sym. on Fresh Water from the Sea, Vol. 1, pp. 691-701, Djbrovnik, 1970.

17. Roe, R. C. and Othmer D. F. 'Controlled flash evaporation on improved multi-flash system', Proc. 3rd Int. Symp. on Fresh Water from the Sea, Vol. 1, pp. 169-90, Djbrovnik, 1970.

18. Wood, F. C. 'An assessment of the possible use of non-metallic heat transfer surfaces in multi-stage flash distillation', Proc. 3rd Int. Symp. on Fresh Water from the Sea, Vol. 1, pp. 363-402. Djbrovnik, 1970.

19. Kogan, A. and Lavie, A. Conceptual design of a 50 m.g.d. MSF, direct-contact condensation desalination plant. Communication from A. Kogan, The Tech-nion, Israel, Aug. 1973.

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