Presented at the Conference on Desalination and the Environment, Las Palmas, Gran Canaria, November 9–12, 1999.European Desalination Society and the International Water Association.

0011-9164/99/$– See front matter © 1999 Elsevier Science B.V. All rights reserved

Desalination 125 (1999) 243–249

Plate heat exchangers — the new trend in thermal desalination

John B. Tonner*, Steen Hinge, Carlos LegorretaAlfa Laval Water Technologies A/S, Maskinvej 5, 2860 Soeborg, Denmark

Tel. +45 (39) 53 60 00; Fax +45 (39) 53 65 00

Abstract

In recent years thermal desalination using multiple effect processes, often augmented by thermo-compressors, hasbecome increasingly popular. The basic process has remained unchanged since the late 1800s when scientists such asRilleaux developed the concepts we use today. At the turn of the century these multiple-effect processes almostexclusively used submerged tube designs. Over the years several other equipment configurations have been used withhorizontal tube falling film (HTFF) finding new popularity in the last decade. The low pumping power consumptionof falling film processes has led to the decline of forced circulation processes, such as MSF, especially in plant sizesless than 10,000 t/d. Plate heat transfer surfaces are the latest development in falling film technology. Variousconfigurations of plates have been tried in the past. Among them have been dimpled plates and corrugated plates. Thelatest type of plates are pressed plate used in a falling film (PPFF). This new configuration has several advantages neverbefore available to any thermal desalination process. The use of PPFF leads to higher heat transfer coefficients due tothe combination of plate pattern and thickness which is based on 40 years of proprietary manufacturing technology. ThePPFF system incorporates a patented distribution system, which provides greater control of fluid distribution andwetting of the surface as well as turbulent boundary layers promoted at low velocities due to the plate pattern.Conversely, all HTFF systems have a vapour space between the distribution system and the top row of tubes thensuccessive gaps between each row. The new PPFF system removes these potential sources of scaling due to inadequatewetting found in HTFF. Perhaps the most practical new feature is the flexibility and access to the heat transfer surface.Normal cleaning procedures involve the typical cleaning in place (CIP) circulation of mild acid solution used by mostdesalination processes. However, the possibility of full access to the heating surfaces in the PPFF means that virtuallyno irreversible scaling will take place. Even the dreaded calcium sulphate scale, should it arise due to operational errors,can be readily and completely removed. The PPFF configuration can also be configured to allow future addition of theheat transfer surface. This makes it possible to plan for future plant capacity expansion with minimal investment today.While membrane systems may be able to allow for the addition of extra membrane for planned increases in capacity,this has not before been possible with any thermal desalination process, thereby the new PPFF configuration offers avery high design flexibility.

Keywords: Seawater desalination; Innovation; Development; Reduction in energy consumption

*Corresponding author.

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1. History of multiple effect as a process

1.1. Sugar industry

In the 1800s the colonization of the Caribbeanincluded the development of sugar as a cash crop.Sugar beet or cane must have a large amount ofwater evaporated from it to yield crystals or evena concentrated syrup. To achieve this, waste caneby-products were burned to provide heat for theevaporation processes. Yet there was still apressing need for more energy-efficientevaporators. Low-temperature evaporation waspreferred to avoid charring the sugar or baking itinto hard cakes.

This industrial demand led to the developmentof an evaporator industry and improvedefficiencies of operation. Howard is credited withthe development of vacuum assisted evaporationwhile Cleland, Degand, Péclet, Pecuer, andfinally Reillieux all worked on the multiple effectprocess [1]. They discovered the improvedefficiencies related to sequential effects atsuccessively lower pressures. This becameknown as Rillieux’s First Principle: “In a multi-effect, one pound of steam applied to theapparatus will evaporate as many pounds ofwater as there are bodies in the set” [2]. Todaythis principle is known as gained output ratio(GOR), performance ratio, or economy.

The thermodynamic process has essentiallyremained unchanged since the days of thesepioneers. However, the configuration of theequipment has changed over the years in an effortto improve heat transfer, avoid scaling, andimprove reliability.

Many of the early configurations weresubmerged tube coil designs. Tubes in verticalconfigurations have been used with rising films(VTRF) and with falling films (VTFF). Each ofthese had features which, while promising, led tocontinuous investigation of improvements.

In the 1950s horizontal tubes with a fallingfilm on the outside was first used in the pulpindustry and later desalination (HTFF). This

configuration seemed to offer benefits of thin-film evaporation and greater control of vapourflow for venting of non-condensable gases and is,at this time, the prevalent configuration usedtoday for multiple-effect systems.

1.2. New configurations

Other configurations have been tried withinthe last two decades. Examples of these includethe use of plates as heat transfer surfaces. Thepossibility exists to include a profile on plates toenhance heat transfer. As a result, large surfaceareas can be developed more economically andhigher-grade materials of construction can becompetitive with lower cost materials. With plateconfigurations the technology, the processknowledge and the manufacturing expertise ofthe heat transfer surface lie with the equipmentmanufacturer.

Not all plate configurations have incorporatedthe same approach. Perhaps one of the mostcommon plate types is the dimpled plate which isstill used in the pulp industry but has beenunsuccessful in desalination.

Shown in Fig. 1, the dimpled plate ismanufactured by spot welding two flat platestogether and then pressurizing the internal sectionto puff it up and create a dimple. This createsinternal channels for the heating vapour to flowinto and condense. The evaporating fluid fallsover the external surface of the dimples.

Modules of these plates are installed insideevaporator vessels including devices to conductvapour in and out the plate pack. This type ofdimpled plate pack cannot be disassembled wheninstalled but can be removed, as shown in Fig. 2.Great care must be taken when creating thedimples to avoid stresses and cracking around thespot weld.

While dimpled plates have been tested ondesalination, the correct materials of constructionand investigation of appropriate fluid wettingrates does not appear to have been completed.

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Fig. 1. Dimpled plate.

Fig. 3. Thin-plate assemblies.

Another configuration using a plate conceptwas to assemble thin plate assemblies into packssimilar to radiators or fin/fan coolers as shown in

Fig. 2. Dimpled plate pack.

Fig. 4. The plate evaporator.

Fig. 3. There are two configurations of this typeof plate pack. In the first configuration the packis oriented with the cold fluid inside and thecondensing vapour outside. This configuration issuitable for forced circulation processes and hasbeen used for a small number of MSF systems. A

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second configuration used the internal flow pathsoriented vertically similar to vertical tubeevaporator (VTE) systems. For this VTE typeconfiguration the evaporating fluid follows afalling film path inside the plate pack with thecondensing heating vapour on the outside. Theprimary attraction of this configuration is that itcan be used to create inexpensive heat transfersurface.

The corrugated plate concept can be movedtowards a more technically elegant and efficientsolution by pressing a more complex pattern onthe plate and by incorporating an adequatedistribution system. This has been done forseveral years in rising film configurations forboth water desalination and the concentration ofother process fluids as shown in Fig. 4.

Using advanced manufacturing techniques,these systems have been commercially competi-tive incorporating titanium as the heat transfersurface and today dominate the marine andoffshore desalination markets.

The same plate technology can be applied toa falling film concept, which is better suited tolow differential temperature applications andyields lower pumping costs than traditional multi-stage flash (MSF) systems.

This pressed plate falling film (PPFF)concept, shown in Fig. 5, has similar processcharacteristics to HTFF systems but with thefollowing distinct benefits:C The distribution system is incorporated

directly into the heat transfer surface; there isno gap as found between spray devices andtubes in HTFF.

C The thin film created at the top of the plateremains intact in direct contact down the fullheight of the plate. Conversely with HTFF thethin film must be re-established each timewater drops fall from a higher tube row to alower row (in an uncontrolled manner).

The pattern on the new PPFF configurationenhances heat transfer coefficient (HTC) via

Fig. 5. Pressed plate falling film concept.

increased boundary layer turbulence within boththe evaporating and condensing films. This samefeature has led to rapid growth in market share ofplate heat exchangers over shell and tubeexchangers in liquid-liquid (and other)applications.

Different concepts operate with differentHTCs. There are many changes which may bemade to any of the heat transfer configurations toenhance heat transfer. Examples of this wouldinclude enhancing heat transfer with the use offoaming agents, which is most commonly donewith rising film. There is no doubt that PPFF heattransfer coefficients are higher as this is seen inmore compact systems with less heat transferarea.

The real question regarding HTCs should be,can they be sustained and are they so high thatthe plant becomes too sensitive to operate? Formany prior enhancements, such as extended orfluted surfaces on tubes, the answer has been no.

The HTCs cannot be sustained. For PPFF theanswer is yes due to the impact of the plate

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pattern in the promotion and maintenance of aturbulent boundary layer. The ability of the PPFFconfiguration to accept additional heat transferareas, even after plant commissioning, alsomeans that little or no technical risk exists in thisregard.

This particular PPFF configuration utilizes aproprietary venting scheme that removes non-condensable gases from each condensingchamber in parallel. This ensures there is nodegradation in HTC due to build-up on non-condensing gases on the heat transfer surfaces.

The heat transfer area can be adjusted with theaddition or removal of plates. This allows forgreater operational flexibility including theability to have phased capacity increases. Toachieve this benefit, only the evaporator vessel,including the plate support devices, and someancillaries need to be prepared to allow for futureexpansion. The cost of heat transfer surface canbe deferred until it is required at which time asimple installation of extra plates will result inincreased output.

The pressed plates used in this configurationincorporate sealing gaskets to segregate thecondensing and evaporating sections. The gasketsseal a differential pressure of only approximately200 mbar and are not under significantcompressive load since there is plate-to-platecontact points in the design. These gaskets arehigh quality Nitrile rubber which, in anenvironment free of air and UV rays inside theevaporator vessel, have proven to have a trouble-free life in excess of 10 years.

At some time during normal operation thesystem will require cleaning of the heat transfersurfaces. Normal operation of PPFF includes theinjection of a typical polymer anti-scalant thathas the primary objective of ensuring any scalecrystals that deposit are soft and readilyremovable. This, of course, applies to the typicalmineral scales that are expected to form in minoramounts within any desalination system.

Cleaning is achieved in place (CIP), with the

circulation of mild, inhibited acids such assulphamic acid. No dismantling of equipment orunusual equipment is required. This should beeffective within a few hours for normalconditions. The PPFF heat transfer pack can bedisassembled for cleaning should normal CIP notprove effective due to changing feed-waterquality or poor operation and maintenance. Amajor benefit from this plate pack configurationis the ability to separate the plates and removeeasily severe scaling deposits. This is notpossible with any other thermal or membraneprocess. This feature also allows the system tooperate closer to scaling limits without risk ofpermanent loss of production or to evaporatefluids not currently treated via evaporation —even if the design point is not close to scalinglimits. This means the PPFF systems areinherently less prone to problems associated withoperational errors and over-concentration. Aprime example of this is how the system canhandle sulphate scalings. Normally sulphate scaleoccurs in membrane or evaporator systems whenbrine concentration limits are exceeded either bymisoperation, by changing the feed-waterconditions or a combination of both.

Sulphate scales are impervious to normal CIPprocedures and can only be partially cleanedusing EDTA or other expensive, slow andhazardous chemicals.

Testing has been carried out using pressedfalling film plates that have been deliberatelyforced to scale with calcium sulphate. Asexpected, normal CIP procedures did not work.The system was opened for inspection andmanual removal of the calcium sulphate. As canbe seen from Figs. 6 and 7, the scale readilyseparates from the heat transfer plate. Thisseparation readily occurs due to the difference inflexibility between the plate and the scale, alsodue to the smooth surface of the titanium plates.

The ability to fully access the heat transfersurfaces is currently unrivalled and creates somenew possibilities in extending the operating range

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Fig. 6. Scale from a plant which was deliberately scaledup with calcium sulphate in order to successfully test andprove cleanability.

Fig. 7. As in Fig. 6.

of existing facilities or in pushing the perfor-mance of new projects. Several systems areoperating in feed-waters that were unsuitable forRO or are using RO reject as feed-water toincrease overall water recovery.

Additional benefits are currently beinginvestigated. Among the most interesting is theperformance of the system in the presence of oilin the feed-water. Both organic and inorganic oilshave been tested with great success.

While the amount of oil carried over into thedistilled water is a function of the type and

weight of oil, the thermodynamic performance ofthe plate remains very close to design values.This has been tested with significant quantities ofoils in the range of thousands of parts per million.Further work is being carried out to determineapplication guidelines. It is believed that thisperformance is possible due to the turbulencecreated in the boundary layer on the plate surfaceacting to keep the surface clean.

2. Significant features of this PPFFconfiguration

1. Full access to the heat transfer surface ispossible for maximum cleaning and recovery ofperformance without detriment to plate materials,pattern and surface quality.

2. It is possible to recover from operationalerrors back to 100% of design.

3. There is flexibility for future expansion ofcapacity.

4. There is the ability to use high-gradematerials at competitive marketplace.

5. The distribution system is an integral partof each plate.

6. No uncontrolled gaps exists betweendistribution device and heat transfer surface.

7. The thin film is continuous as it passesover the entire heat transfer surface.

8. There is low differential pressure acrossgaskets.

9. There is a low compressive load on gasketsas the PPFF plates have contact points betweeneach other plate.

3. Conclusions

It is expected that PPFF systems will continueto grow in popularity and market share since theconfiguration successfully addresses the twomost commonly quoted operational issues withany desalination process:

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C corrosion — this is overcome by the selectionof high grade materials

C scaling — this is overcome by a configurationwhich allows full access to the surfaces

Only a few studies using plate heatexchangers for thermal desalination have beenmade, primarily at research level by universitiesand state-owned research centers; and theseworks so far have only barely approached themost elementary benefits of this technology.

The most important aspects on plate heatexchanger technology relay on economic issuesin order to produce efficient plate heatexchangers at commercially competitive prices.The pressed plate technology is highly capitalintensive and, therefore, so far only pursued by

few companies able to maintain a proprietaryknow-how based on sophisticated and a cost-efficient manufacturing process plus totallycontrolled logistics in terms of availability andsourcing of raw materials.

It is, therefore, not surprising to find that pastand present research done by public institutionscontains elementary mistakes and wrongconclusions in terms of technical characteristicsof plate heat exchangers and their commercialapplications.

References

[1] J.D. Birkett, personal correspondence.[2] A.L. Webre and C.S. Robinson, Evaporation,

Chemical Catalog Company, New York, 1926.