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Desalination 165 (2004) 351–361

0011-9164/04/$– See front matter © 2004 Elsevier B.V. All rights reserved

Presented at the EuroMed 2004 conference on Desalination Strategies in South Mediterranean Countries: Cooperationbetween Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the EuropeanDesalination Society and Office National de l’Eau Potable, Marrakech, Morocco, 30 May–2 June, 2004.

*Corresponding author.

SWRO core hydraulic module the right concept decidesin terms of energy consumption and reliabilityPart II. Advanced pressure exchanger design

Stephan Bross*, Wolfgang KochanowskiKSB-AG, Johann-Klein-Straße 9, 67227 Frankenthal, Germany

Tel. +49 (6233) 863771; Fax +49 (6233) 863400; email: [email protected]

Received 20 February 2004; accepted 1 March 2004

Abstract

By defining the combination of high pressure seawater feed pump and energy recovery system as the corehydraulic module of SWRO plants it becomes obvious that this subsystem mainly contributes to the specific water costs by its operational costs and has to be optimized by a complete approach. In the previous paper published atthe IDA Conference 2003 [1], different energy recovery systems were analyzed with respect to theire overall lifecycle costs. Identifying the core hydraulic module using pressure exchangers as the advantageous ERS-concept thequestion of appropriate component optimization was discussed for the high pressure pump design. In the currentpaper, this approach is extended to the pressure exchanger itself, contributing to an overall optimized core hydraulicmodule. Therefore, inadequacies and limits of conventional pressure exchanger operation are explored and discussedwith respect to their impact on reliability. Based on these results, a new pressure exchanger design is presented.This design was realized in a laboratory prototype and tested on a special test rig. The arrangements of the test rigas well as the results of first prototype tests are described.

Keywords : SWRO; Pressure exchanger; Work exchanger; Core hydraulic module

1. IntroductionSeawater reverse osmosis (SWRO) desalina-

tion facilities are widely used for providingpotable water. Depending on the water demand, a

variety of designs and sizes are known, startingfrom small portable units for boat or campingapplications up to huge desalination plants for industrial or municipal water supply. While theinvestment cost is the only customer key factor for small units, the assessment of specific water



352 S. Bross and W. Kochanovski / Desalination 165 (2004) 351–361

costs becomes predominant for medium- and big-size plants. Especially in the latter ones (plant sizesabove 500 m 3/d), the seawater is normally pres-surized up to 60–90 bar by means of centrifugalpumps driven by electrical motors. Different state-of-the-art energy recovery systems (ERS), likePelton turbines (PT), turbochargers (TC) or pres-sure exchangers (PE), are widely used to reducethe specific energy consumption.

By defining the combination of high-pressureseawater feed pump (HP) and energy recovery

system as the core hydraulic module , it becomesobvious that this subsystem mainly contributesto the specific water costs by its operational costs(energy consumption, maintenance and reliability)and has to be optimized by an overall approach.

In Part I of this paper published at the IDAConference 2003 [1], different energy recoverysystems were analyzed with respect to their overalllife cycle costs. Having identified the core hyd-raulic module using pressure exchangers as theadvantageous ERS concept, the question of appropriate component optimization was dis-cussed for the high-pressure pump design.

In the current paper, this approach is extendedto the pressure exchanger itself, contributing toan overall optimized core hydraulic module.

2. SWRO core hydraulic module

2.1. Definition

The core hydraulic module of a single-stageSWRO plant is defined as the arrangement of a

Permeate 40%

HP Pump(100%) Brine

60%

a) No Energy RecoverySystemE ~ 5-6 kWh/m 3

Permeate 40%

TurbineHP Pump(100%)

Brine60%

b) State of the ArtE ~ 3.5 kWh/m 3

c) Advanced ModuleE ~ 2.4 kWh/m 3

100% Brine

Permeate 40%

HP Pump40%

PressureExchanger

Booster Pump

high-pressure pump (HP) combined with anenergy recovery system (ERS). According to thisdefinition, the core hydraulic module pressurizesthe pretreated feedwater and recovers the energycontained in the brine flow. Pretreatment fluidhandling and permeate transport are not included.

A schematic representation of three differentcore hydraulic modules using different ERSs isshown in Fig. 1. The simplest one consists of ahigh-pressure pump without using any energyrecovery device. Due to its high specific energy

consumption (~5–6 kWh/m3

), it is used only invery small plants and is mentioned here for reasons of completeness only.

The state-of-the-art core hydraulic module(Fig. 1b) uses a turbine recovering the brine energycoupled to the motor of the high-pressure pump.Different arrangements, like the Grundfos BMET[i] (two standard high-pressure pumps, onedirectly coupled to the turbine, one driven by anelectric motor) or the TURBO [ii] (hydraulicturbocharger integral turbine-driven centrifugalpump in combination with an additional elect-rically driven HP pump), are known. All are basedon the same principle of energy transfer frompressure (brine) into mechanical (shaft) and back into pressure (feedwater) energy. Thereby, therequired specific energy consumption is reducedto about 3–4 kWh/m 3.

In advanced core hydraulic modules (Fig. 1c),high-pressure pumps are combined with pressureor work exchangers (PE) for energy recoverypurposes. Within these devices, the pressure

Fig. 1. Schematic representation of different core hydraulic modules.



S. Bross and W. Kochanovski / Desalination 165 (2004) 351–361 353

energy of the brine is transferred directly to thefeed stream, thus avoiding the losses of convertingfrom one energy form to another. Pressure or work exchangers are based on the positive displacementtechnology with net energy transfer efficienciesof up to 96%. Accordingly, the specific energyconsumption can be reduced further to about 2– 3 kWh/m 3. Commercial examples of such sys-tems, viz. Energy Recovery Inc.’s PressureExchanger PX [iii], DesalCo’s Work Exchanger Energy Recovery (DWEER) [iv], are described

in more detail in [3] and [4]. In contrast to thestate-of-the-art module, where the high-pressurepump has to handle the whole feedwater stream,the size of the HP pump is reduced to an extent tohandle only the permeate flow rate, with anadditional recirculation pump (BP), to balance themembrane’s and the PE’s pressure drop.

The core hydraulic module mainly contributesto the specific water costs by its operational costs(energy consumption, maintenance and reliability)and has to be optimized overall. While consideringonly the energy consumption, it is obvious thatthe advanced core hydraulic module (HP-BP-PE)is advantageous. On the other hand, maintenanceand reliability are important and directly con-nected to an appropriate and matched HP-ERSdesign. It is well known that features decreasingthe overall investment costs or increasing themodule reliability may have some shortcomingsin the available peak efficiency. Knowing for the

different core hydraulic modules the sensitivityof specific water costs to peak efficiency, mainte-nance or reliability answers the question as towhich feature the component design has to focuson. In Part I of this paper [1], simple equationswere evaluated to calculate the specific energyconsumption of each arrangement and to work outtheir sensitivities to pump and ERS efficiencychanges. Based on these equations, the specificenergy consumption for a 5000 m 3/d single-stageRO plant was calculated. The plant parameters

and assumed operating conditions are listed inTable 1.The results of these calculations are presented

in Fig. 2, varying the recovery rate κ . The mem-brane working pressure ∆p and the membranepressure drop ∆pM have been considered to be afunction of the recovery rate and were obtainedfrom membrane design software. Additionally,Fig. 2 contains the permeate salinity TDS [ppm]obtained for the specified recovery rate. The

Table 1Operating conditions

Plant parameters Efficiencies

QPermeate = 5000 m 3/d ηHP = 82%T = 20°C ηT = 85%TDS Feed = 38,500 ppm η BP = 82%∆p = 55–75 bar ηPE = 94%∆pM = 1.6–0.4 bar ηMot = 95%

0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60Recovery Rate κ

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Specific Energy E [kWh/m

3]

200

300

400

500

600

700

800

Permeate Salinity TDS [ppm]

η HP=80%η HP=80%η HP=82%η HP=82% Turbine-KitTurbine-Kit

η HP=84%η HP=84%

η HP=80%η HP=80%η HP=82%η HP=82% PE-KitPE-Kit

η HP=84%η HP=84%

TDS PermeateTDS Permeate

Fig. 2. Specific energy consumption and water quality for the turbine and the PE core hydraulicmodule; variation of HP efficiency.




specific energy consumption is obviously higher for the turbine kit than for the PE kit ( ∆E ~0.5– 1 kWh/m 3). While the specific energy consump-tion is minimum for κ ~0.45 in the turbine con-figuration, it is shifted to lower recoveries ( κ ~0.35)in the case of the PE kit. Taking into account thatpermeate TDS rises with increasing recovery rate,production of a high permeate quality at lowrecoveries and low specific energy consumptionis possible with the PE technology only. Inaddition, variation in specific energies for a varia-tion of ∆η HP = ±2% in high-pressure pump effici-encies is shown in Fig. 2. It can be seen clearlythat the range of specific energy deviation is muchhigher for the state-of-the-art (turbine kit) thanfor the advanced core hydraulic module (PE kit).

In Fig. 3, the same analysis is extended for avariation of considered ERS efficiencies. Itappears that a) the influence of ERS efficiencyvanishes with increased recovery rates and b) the

variation of PE efficiency dominates at lower recoveries.The consequences of this analysis become

evident if the change in energy costs per year isillustrated for the above-mentioned 5000 m 3/d ROplant. Assuming (i) the operating conditions givenin Table 1, (ii) a recovery rate of κ = 0.4 and (iii)an energy price level of C E = US$0.05/kWh, thedeviation in energy costs for a variation of effici-ency by 2% in the HP or the ERS device is

Fig. 3. Specific energy consumption and water

quality for the turbine and the PE corehydraulic module; variation of ERS efficiency.0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60

Recovery Rate κ

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Specific Energy E [kWh/m

3]

200

300

400

500

600

700

800

Permeate Salinity TDS [ppm]

η T=83%η T=83%η T=85%η T=85% Turbine-KitTurbine-Kit

η T=87%η T=87%

η PE=92%η PE=92%

η PE= 94%η PE= 94%

PE-KitPE-Kit

η PE=96%η PE=96%

TDSPermeateTDSPermeate

12242 4897 4747 5933

0

2000

4000

6000

8000

10000

12000

14000

Variation of HP-Efficiency

Variation of ERS-Efficiency

Turbine-KitPE-Kit

1%-Reliability Improvement

Energy Savings [$US/a]

represented in Fig. 4. Based on overall energycosts of approx. US$288,000/a for the state-of-the-art module (turbine kit), savings of aboutUS$12,242/a can be achieved by improving thehigh-pressure pump, while it is only US$4,747/ain the case of turbine improvements.

For the advanced core hydraulic module,overall energy costs of ~US$218,000/a are incur-red, which is 25% reduced as compared to theturbine kit. In this case, savings of only US$4,897/acan be achieved by HP efficiency improvements,which is comparable to what can be reached byPE efficiency enhancements (US$5,933/a).According to [5], track re-liability of more than90% has to be guaranteed for BOOT (Build, Own,Operate, Transfer) contracts with an increasing

Fig. 4. Energy savings by efficiency improvements of the HP and ERS of a 5000 m 3/d RO plant.




trend. Based on a permeate price of ~ US$0.6/m 3,shortcomings of about 1% in reliability will causeproduction losses of about US$10,500/a, whichis twice the savings through HP or PE efficiencyimprovements . Therefore, it is obvious that for the advanced core hydraulic module, the specificwater costs can be minimized by increasing thereliability and reducing the maintenance costs (of all components: HP, BP and PE) more than byoptimizing the peak efficiency.

Following this philosophy, a new high-

pressure pump design concept was developed andpresented in Part I of this paper [1]. In the presentpaper, this philosophy is extended to the pressureexchanger design itself, taking into account therequirements of high efficiency, high reliabilityand low maintenance.

3. Appropriate pressure exchanger design

3.1. General considerations

The following discourse is mainly on medium-to large-size RO plants ( QPermeate > 2000 m 3/d)equipped with stationary-type pressure ex-changers (stationary vessels and switched valves,

V1

V2

PressureExchanger

Membrane

Qnew =40%Qaltered =36%



∆ pnew =3bar ∆ paltered =5bar

Qnew =40%Q

altered=36%

∆ pnew =70 bar ∆ paltered =72 bar

Booster Pump

HP Pump

Feedwater Pump

Vnew =0,5 m/svaltered =0,45 m/s Control

Valve

∆ pnew =3bar ∆ paltered =5bar

cf. DWEER system [iv]). In principle, thesedeliberations also apply to smaller-sized unitsequipped with rotating pressure exchangers (e.g.ERI [iii]), but there, the approaches to solutionare different, so that they are not treated here butwill be dealt with in one of the upcoming articles.

Fig. 5 shows the arrangement of a stationarypressure exchanger in a single-stage RO plant. Asdescribed in [4] and [5], the pressure is transferredfrom the brine to the feedwater inside the twovessels which can be switched by means of a

control valve on one side of the vessel. On theother vessel side, check valves provide thenecessary flow, whose switching is influencedexclusively by the pressure conditions prevailingin the vessels and, hence, indirectly by the positionof the control valve.

The vessels themselves are filled with brineand fresh water in alternation, and the directionof flow goes into reverse at every alternation, i.e.the water column inside the vessels has to beaccelerated or decelerated permanently. Theassociated problem of non-stationary pressurefluctuations is well-known from plant engineeringand construction, cf. [6]. If the flow inside a pipeis broken by quick closing of a globe or gate valve,

Fig. 5. Changes of system conditions due to membrane alteration.




pressure surges with a maximum amplitude of

Shock p a v∆ = ρ ⋅ ⋅ (1)

can be caused in the worst case. In Eq. (1), ρrepresents the density, a the speed of sound and vthe flow velocity of the medium. A pressureexchanger whose vessels are filled at a mean flowvelocity of approx. v = 1m/s can induce pressurefluctuations of up to ± 10bar. During operation of a pressure exchanger, the check valves are reactingto the pressure fluctuations, which is audible as

loud hammering and is the cause of frequentmaintenance.As per Eq. (1), the amplitude of the pressure

fluctuations can be reduced, for instance, by areduction of the mean velocity v inside the vessels.But a reduction of the amplitude to 25% of theinitial value is associated with a doubling of thevessel diameters, which results in higher invest-ment cost. In order to reduce the hammering of the check valves, complex constructions are oftenused. Also these cost-intensive measures combatthe symptom, but they do not eliminate the causeof the problem. As is known from literature,pressure fluctuations and surges in pipes can beavoided when the shut-off elements used arematched to the individual plant conditions, thatmeans when their closing behavior is designedfor that particular case, cf. [6].

The second demand placed on a pressure ex-changer to ensure trouble-free operation is itscontrollability. Different system conditions dueto alteration of the membranes, changes in thefeedwater quality (salt content, temperature), or

different modes of operation lead to changeddesign parameters (flow rate Q, pressures ∆p,recovery rate, cf. Fig. 5) which pressure ex-changers have to be matched to during operation.

3.2. Basic design of pressure exchanger

The design of a pressure exchanger is basedon four objectives which ultimately all have areduction of the RO plant’s operating costs incommon.

1. Increased reliability of the plant by using aproduct that is designed for continuous opera-tion without maintenance work.

2. Controllability of the pressure exchanger byan integral control unit which adjusts thepressure exchanger independently and prompt-ly to the operating conditions of the RO plant.

3. Avoidance of pressure fluctuations in the pres-sure exchanger and in the RO plant.

4. Low specific energy consumption for allcomponents of the core hydraulic module.

For the basic design, a two-vessel system waschosen that is on one end equipped with a drivenrotating valve that controls the flow rate of thepressure exchanger and the adaptation to theoperating conditions (Fig. 6). The other end of the pressure exchanger is fitted with check valves.The overall length of the pressure exchanger isabout 7 m in order to allow its positioning under-neath the membranes. The rotating valve is drivenby a servo motor. Both the number of deliveringcycles and the opening times of the free passagecross-sections in the rotating valve can be changed.This makes for a high flexibility in realizing themost diverse control times and makes it possibleto govern all of the pressure exchanger’s operatingconditions in the RO plant.

The rotor of the rotating valve is supported intwo medium-lubricated bearings. The materialcombination of the bearings is resistant to seawater and is used for heavy-duty applica-tions in boiler feed pumps and in cooling water pumps up toinstalled power ratings of 10 MW. Operating expe-riences over more than seven years demonstrate

that there has been no wear on these bearings.The experiences made in the field of pumpsand valves for power stations and MSF installa-tions have been incorporated in the design of therotating valve. The reliability of pumps for power stations has to be such high that operation withoutany maintenance under the most different, evenvery severe operating conditions is assured. Thisphilosophy also governs the design features of therotating valves. All components of the rotating




Fig. 6. Pressure exchanger with advanced valve technology; basic design.

valve are made of duplex steel and dimensionedsuch that the deformations occurring under operating conditions in the casing and the rotor are smaller than 50 µm. This makes it possible tokeep the leakage losses very small and to attain acompetitive efficiency.

The use of medium-lubricated bearings and of an electric drive has enabled a simple design of the rotating valves with no necessity of further auxiliary equipment or additional lubricants (e.g.oil).

3.3. Control unit of pressure exchanger.The control unit is integrated in the pressure

exchanger that means the pressure exchanger controls itself independently and does not needany control signals or additional measurementinstruments from the RO plant. The operating con-dition inside the pressure exchanger is measured,and the necessary control demands are deriveddirectly. The principal items of the operating

condition recognition are the position and thevelocity of the two pistons one in each vessel

fitted with magnets; the pistons’ position isdetermined by four sensors (Fig. 7). The positionof the pistons and their velocities are the inputparameters of the control unit’s logical diagram.

In order to guarantee trouble-free operation of the RO membranes, continuous delivering of thefeed stream ahead of the membrane must beensured. This is achieved by moving the piston inthe low-pressure vessel with a higher velocity thanthe piston in the high-pressure vessel (Fig. 7).

Therefore, the piston in the low-pressure vesselis the first to reach the turning point. The rotatingvalve switches and “exchanges pressures” (lowpressure–high pressure). The low-pressure pistonis accelerated in the opposite direction, so thatboth pistons deliver high-pressure brine in thedirection of the booster pump for about 1.5 sbefore the second piston reaches the turning point.Further sensors are positioned on the so-calleddead ends of the vessels. Should one of the pistons

Check Valves

Rotating Valve

Pressure Vessel

Casting

Rotor

Pressure Vessel

Medium

Lubricated Bearing

Rotating Valve




reach such a position due to changed operatingconditions, the control unit receives an alarmsignal, and a new synchronization cycle of thepistons is initiated.

The velocities of the pistons are measured for each cycle so that, in principle, all operatingconditions can be derived from changes invelocity. This also applies to the start-up procedureand the shutdown of the plant.

The use of a servo motor bears the advantagethat the control unit reacts from cycle to cycleand even within a cycle. Another advantage of the control unit is its ability to operate and syn-chronize several pressure exchangers in parallelor in cascade connection. The permanent velocitymeasurements of the flows in the vessels of thepressure exchanger can be exploited to forward

these data via an interface and to also address thepumps in the hydraulic module, e.g. by a variablespeed drive.

4. Prototype test results

4.1. Test bed setup

Fig. 8 shows a test bed that was set up in 2003.It serves to simulate for the components of thehydraulic module the most diverse operating

Fig. 7. Schematic representation of sensor integration.

HP FeedOutlet

LP

HP

Step Motor

LP FeedInlet

Piston PositionSensor

LP BrineOutlet

HP BrineInlet

v = 1.02 m/s

v = 0.98 m/s

Check Valves

RotatingValve

Piston

Check Valves

conditions in an RO plant and to optimize thecomponents. The test bed is a complete reverseosmosis unit with feedwater pump, high-pressurepump, pressure exchanger and booster pump.Membranes are not installed in the test bed. Thepressure losses of the membranes are simulatedby changing the heads of the high-pressure pumpwhich, like all other pumps in the circuit, is speed-controlled.

It is also possible to measure the efficienciesof the components and the mixing factor of thepressure exchanger. The pressure exchanger’scontrol unit has been developed and tested withthe help of this test bed. In addition, it is possibleto simulate the specific energy consumption of the facility for different operating conditions andto deduce optimizations for the control of all com-

ponents in the facility. Moreover, examinationsregarding pressure fluctuations can be conducted.

4.2. Test bed results

The above-mentioned pressure fluctuations areclearly audible and visible during operation of conventional pressure exchangers. For the exam-ination of this phenomenon, the first prototype of the new pressure exchanger was manufactured so




Fig. 8. RO-test bed with installed PE-prototype.

that its operating behavior corresponded to thatof known systems. Very loud noises developed atthe check valves (valve hammering), jerky moving

as well as “breathing” (cyclic contraction) of thebrine outlet line (hose) were noticeable.

Then, the design of the pressure exchanger’srotating valve was changed in a way to preventthe pressure fluctuations described above. Internalbypass lines avoid pressure fluctuations on openingand closing of the rotating valve. These measuresmake for absence of pressure fluctuations in thehigh-pressure part. The measured pressure charac-teristics of the low-pressure inlet and outlet floware shown in Figs. 9 and 10.

Fig. 9 depicts the pressure pattern in the low-pressure feedwater and low-pressure brine water pipes and shows the pattern of the pressure fluc-tuations for the conventional design. At a flowvelocity of 0.5 m/s, a maximum pressure fluctua-tion of ∆p = 8 bar occurs. The brine outlet line issubjected to both low pressure (under 1 bar) anda strong pressure fluctuation, which leads to theabove-mentioned “breathing” and moving of thehose.

Then, the logical diagram was improved withthe purpose of minimizing the above pressurefluctuations.

Fig. 10 clearly shows that these pressurefluctu-ations do not occur any more, which hasnot been achieved by additional equipment (surgetank, for example) in the plant, but solely bychanging the logical diagram of the rotating valve.

Thanks to this flexible, freely programmablecontrol unit and the permanent velocity measure-ments of the pistons, it can be ensured that thepressure exchanger adjusts to each system con-dition and to changed operating conditions with-out any failure.

Actually, the installation of a basic-desingprototype in a plant on the Sinai Peninsula in order to gain field test experiences is on the way.

Furthermore, this test unit will also be equip-ped with the other components of the hydraulicmodule, so that it will be possible to measure theoperating conditions for pressure exchanger, high-pressure pump and recirculation pump (booster pump) and gain experiences for their optimaladjustment. The specific energy consumption of




Fig. 11. Measured pressure characteristics at the low pressure side advanced design: (a) feed water; (b) brine.

rotation an le

p F

eed in

measured valuemeasured valuereference valuereference value

open valveclosed valverotation an le

p B

rine out


open valveclosed valve

measured valuereference value


closedvalve

openvalve

closedvalve

openvalve

rotation anglerotation angle

pFeed

in

pBrine

out8 bar

(a) (b)

Fig. 10. Measured pressure characteristics at the low pressure side conventional design: (a) feed water; (b) brine.

rotation angle

p Feed in


open valveclosed valverotation angle

p B

rine out

measured valuereference valuereference value

open valveclosed valve



closedvalve

openvalve

closedvalve

openvalve

rotation angle rotation angle

pFeed

in

pBrine

out

(a) (b)

the entire hydraulic module at plant operation canthen be documented.

5. Conclusion

The use of a pressure exchanger as energyrecovery system enables a very low energy con-sumption for the reverse osmosis process. Lowoperating costs are a prerequisite for low drinkingwater prices. Especially for the regions of theMiddle East and North Africa receiving littlerainfall, the provision of drinking water willbecome more and more affordable.

An operationally reliable pressure exchanger

that can be incorporated in any reverse osmosisfacility and can cope with each facility’s operatingconditions is an important precondition. Thedesign of the product presented has been matchedto satisfy these requirements. A new rotating valvetechnology with an integrated control system wasdeveloped, eliminating the burdens of pressurefluctuations with occurring conventional designs.Tests in a test bed simulating a reverse osmosisfacility have been conducted with positive results.The test and documentation of the pressureexchanger’s operational reliability in a field testis on the way. The next step will be the instalationof this solution in different projects.




Acknowledgement

The authors would like to thank the whole ROteam of the KSB development center for their commitment and care in carrying out the design,manufacture and testing of the pressure exchanger prototype.

References

[1] S. Bross, W. Kochanowski, M. Ellegaard and G.Schwarz, SWRO-core-hydraulic-module; the right

concept decides in terms of energy consumption andreliability, IDA World Congress on Desalination andWater Reuse, Paradise Island, Bahamas, 2003.

[2] E. Oklejas, Energy efficiency considerations for ROplants: a method for evaluation, InternationalDesalination & Water Reuse Quarterly, 11(4) (2002)26–34.

[3] L. Hauge, The pressure exchanger, InternationalDesalination & Water Reuse Quarterly, 9(1) (2000)54–60.

[4] S.A. Shumway, The work exchanger for SWROenergy recovery, International Desalination & Water Reuse Quarterly, 8(4) (1999) 27–33.

[5] W.T. Andrews, A twelve-year history of large scaleapplication of work-exchanger energy recoverytechnology, Desalination, 138 (2001) 201–206.

[6] J. Parmakian, Water Hammer Analysis. Prentice HallInc., New York, 1955.

Commercial references

[i] BMET, Grundfos, DK-8850 Bijerringbro.[ii] TURBO, Pump Engineering, Willington, Delaware.[iii] PX, Energy Recovery, Inc., San Leandro.[iv] DWEER, DesalCo Ltd, Cayman Islands.

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