Applied Energy 135 (2014) 415–422

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Applied Energy

journal homepage: www.elsevier .com/locate /apenergy

Performance study of a pilot-scale low-temperature multi-effectdesalination plant

http://dx.doi.org/10.1016/j.apenergy.2014.08.0960306-2619/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel./fax: +86 022 87898150.E-mail address: qi_chunhua@163.com (C.-h. Qi).

Qi Chun-hua ⇑, Feng Hou-jun, Lv Qing-chun, Xing Yu-lei, Li NanInstitute of Seawater Desalination & Multipurpose Utilization, SOA, Tianjin 300192, China

h i g h l i g h t s

� The performance of a 30 t/d LT-MED system is studied.� We obtain a high heat transfer coefficient and a steady operation condition.� The spray density of 240–300 L/(m h) should be adopted in actual design and operation.� Vacuum pumping in the series mode had more advantages.

a r t i c l e i n f o

Article history:Received 21 June 2014Received in revised form 21 August 2014Accepted 24 August 2014

Keywords:Seawater desalinationLow temperatureMulti-effectPilot test

a b s t r a c t

A 30 t/d low-temperature multi-effect evaporation seawater desalination (LT-MED) system was designedbased on the mathematical model, and the corresponding pilot device was constructed in Tianjin, China.Whole-process tests were carried out, and the effects of key operating parameters, including motivesteam pressure, maximum operating temperature, temperature difference, spray density, non-condens-ing gas extraction method, and steam ejector flow, on desalination performance were analyzed. Resultsshowed that the device successfully met product water design requirements; total dissolved solids wereless than 5 mg/L. Water production initially increased as motive steam pressure increased, then stabilizedwhen pressure exceeded 21% of the design value. Water production reached its maximum when heattransfer temperature difference and spray density ranged from 3 �C to 4 �C and from 240 L/(m h) to300 L/(m h), respectively. Unlike in parallel mode, water production increased by 3.64% when vacuumpumping was operated in series mode. Water production and gain output ratio increased, and systemenergy consumption reduced when a thermo-vapor compressor was introduced. The results provide auseful reference for the design of other large-scale seawater desalination systems.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The demand for freshwater is increasing dramatically with therapid growth of the global population and the general improve-ment of living standards. Limited freshwater resources obtainedfrom surface water and groundwater are wasted in many regions.As a result, freshwater shortage has become a critical problem thatlimits long-term social and economic development [1]. Fortu-nately, freshwater can be obtained from seawater through desali-nation. China has a vast marine area with more than 150 coastalcities. Moreover, nearly 6500 islands, area larger than 500 m2, arefound in the Bohai Sea, Yellow Sea, East Sea, and South Sea. There-fore, seawater desalination is an important approach to solvefreshwater shortage in coastal areas.

Different desalination methods have been developed during thepast decades, such as RO and thermal processes (e.g., MSF andMVC). However, these conventional methods usually have high ini-tial cost, operation cost, and energy consumption. Low-tempera-ture multi-effect desalination (MED) has been developed manyyears ago [2]. A thermo-vapor compressor (TVC) is added to a typ-ical MED system to reduce steam requirement (motive steam), boi-ler size, and cooling water; the TVC lowers power consumptionand pre-treatment costs [3]. This method reaches a relatively highheat transfer coefficient because of phase changes in the heatexchanger and liquid in the tube wall film flow, thereby reducingthe required heat transfer area [4–7]. At the same time, multiple-effect falling film evaporation on horizontal tubes is effectivebecause of the small heat transfer temperature difference. There-fore, low energy consumption is reached, and low-temperaturewaste heat is fully utilized [8–9]. MED–TVC with a top brine

Nomenclature

A1i the total heat transfer areas of preheating, m2

A1i the total heat transfer areas of falling film evaporation,m2

BPEi the rise in brine’s boiling point rise, �CC the salt content percentage of brine, mg/Lcp the specific heat capacity, kJ/(kg �C)D the mass flow rate of evaporation, t/hD0 the flow of the motive steam, t/hDH the flow of the TVC ejecting steam, t/hd the mass flashing flow rate of freshwater, t/hF the device surface area, m2

G0 the mass flow rate of the seawater feed to the effects, t/hGi the mass flow rate of brine blow down, t/hg the mass flashing flow rate of brine, t/hK1i the total heat transfer coefficient of preheating, W/

(m2 �C)K2i the total heat transfer coefficient of falling film evapora-

tion, W/(m2 �C)Kl2 the azimuth correction coefficient of the device surface

when wind is presentLMTD the logarithmic temperature difference in the heat

transfer, �CQ the heat transfer rate, kWQa the thermal loss through the device surface per hour,

kW/hq the thermal loss through the device surface per area,

kW/m2

T the evaporator temperature, �Ct0 the seawater temperature, �CTc,i�1 the secondary steam condensation temperature, �Cti the brine temperature, �Cts the outer wall surface temperature of the device, �Cta the air temperature, �CP the pressure, MPaC the spray density, L/(m h)Dt the total temperature difference, �CDTi the temperature difference loss caused by flow resis-

tance, �CDNEA0i the unbalanced temperature rise caused by the fresh-

water flash, �CDNEA00i the heat transfer temperature difference loss caused by

the concentration of brine, �Carc the heat transfer coefficient mixing radiation and con-

vective of the device outside, kW/(m2 �C)k the latent heat, kJ/kg

superscripts0 the fresh water00 the brine flashing

Subscriptsi the number of evaporator effects

416 C.-h. Qi et al. / Applied Energy 135 (2014) 415–422

temperature (TBT) lower than 70 �C is presently the subject ofincreased attention [10–12].

Several studies have recently focused on modeling and singleoptimization of MED–TVC systems. Bin Amer [13] optimizedMED–TVC by using smart exhaustive search and sequential qua-dratic programming. Bin Amer [13] applied an approach that max-imized the gain output ratio(GOR) of MED–TVC. Kamali et al. [11],El-Dessouky et al. [14], Zhao et al. [15], Alasfour et al. [10], andAl-Salahi and Ettouney [16] developed steady-state mathematicalmodels to represent MED–TVC and used parametric techniques todetermine the optimum operating and design conditions of the sys-tem. Sayyaadi et al. [17] performed thermodynamic and thermo-economic optimization of MED–TVC by using a hybrid stochastic/deterministic optimization approach. All the mentioned papersfocused on single optimization; few theoretical and mathematicalstudies focused on the MED–TVC. Shakouri et al. [12], Lukic et al.[18], Choi et al. [19], Ansari et al. [20], and Sharaf et al. [21] devel-oped mathematical and economic models for MED–TVC and per-formed exergy analysis. Shakouri et al. [12] optimized MED–TVCbased on minimizing unit product cost. Piacentino and Cardona[22] analyzed the MED desalination process by using the thermo-economic approach and investigated the possibility of optimizingthe six effects of MED as a case study. Sayyaadi and Saffari [23] ther-moeconomically optimized a MED desalination system with TVC.Ameri et al. [24] presented a conceptual design for a four-effectMED system with TVC with the objective of using waste heat froma gas turbine power plant to produce potable water. The resultsshowed that TBT has a minor effect on GOR, and the heat transfersurface significantly decreases as TBT increases. Aybar [25] pre-sented the results of using waste heat from a North Cyprus steampower plant to produce make-up water for boilers. System produc-tion capacity increases because of the decreasing temperature dif-ference between hot and cold side effects. El-Dessouky et al. [26]compared different configurations for MED systems. Two operatingmodes, namely, parallel and parallel/cross flow systems, were con-

sidered in the analysis. The parallel/cross feed MED system per-formed the best. However, the parallel flow system has similarperformance characteristics, and its design, construction, and oper-ation are simpler than that of the parallel/cross feed MED system.The effects of heating steam temperature and seawater salinity onGOR, specific heat transfer area, specific cooling water flow rate,and conversion ratio were also presented. El-Dessouky et al. con-cluded that specific heat transfer surface and GOR decrease withincreasing heating steam temperature. Therefore, optimizationsbased on these parameters should be performed for various appli-cations. Ashour [27] presented that GOR increases when the firsteffect temperature increases; this increase was mainly caused bythe decrease in the required sensitive heat to warm the feedwaterto saturation temperature.

Few studies focused on seawater desalination pilot tests. Moststudies only focused on single-tube evaporation, atmospheric evap-oration, or working fluid desalination, which is different from distil-lation. Consequently, the results proved unconvincing and difficultto use to guide the actual design and optimization of MED systems.

In this work, a 30 t/d low-temperature multi-effect evaporationseawater desalination system was designed based on a mathemat-ical model, and the corresponding pilot device was constructed inTianjin, China. Whole-process tests were carried out, and theeffects of key operating parameters, including motive steam, max-imum operating temperature, temperature difference, spray den-sity, non-condensing gas extraction method, and steam ejectorflow, on desalination performance were analyzed. The results areexpected to provide a useful reference for the design of large-scaleseawater desalination systems.

2. System design model

(1) The pilot test material balance flow is shown in Fig. 1.Previous publications presented design procedures and mathe-

matical models for different MED systems [24,26,28]. Mass and

Fig. 1. Material balance flow.

C.-h. Qi et al. / Applied Energy 135 (2014) 415–422 417

energy balance equations for each effect are similar to those pre-sented by El-Dessouky et al. [26] for the parallel/cross flow system.We provide material balance in one evaporator in this paper.

(2) The mathematical definition of material and energy balancefor each effect is given as follows:

G0 � Di ¼ Gi � Gi�1 ð1Þ

ðGi�1 � Gi�2Þ � Ci�1 ¼ ðGi � Gi�1Þ � Ci ð2Þ

Q i ¼ Di�1 � ki�1 þ di�1 � k0i�1 þ gi�1 � k00i�1 ¼ G0 � cp;iðTi � t0ÞþDi � ki

ð3Þ

(3) The mathematical definition of energy balance for freshwa-ter flashing in each effect is given as follows:

di � k0i ¼X

Di�1 � ½c0p;i�1 Tc;i�1 � c0p;iðTi þ DNEA0iÞ� ð4Þ

(4) The mathematical definition of energy balance for brineflashing in each effect is given as follows [29]:

gi � k00i ¼ Gi�1 � ½c00p;i�1 � ti�1 � c00p;i � ðti þ DNEA00i Þ� ð5Þ

(5) The heat transfer equation of each effect is shown as follows[30,31]:

Q i ¼ K1i � A1i � ðLMTDÞi þ K2i � A2i � ðTci � tiÞ ð6Þ

(6) The rise in boiling point and temperature difference loss aredefined as follows [32]:

Fig. 2. Schematic diagram of pilot device. (1–3: evaporator, 4: condenser, 5: vacuum pumproduct pump, 11: raw water pump, 12: inhibitor dosing system).

Ti ¼ ti � BPEi ð7Þ

Tci ¼ Ti � DTi ð8Þ

(7) The GOR equation is given as follows:

GOR ¼X

Di=D0 ð9Þ

(8) Device thermal losses are calculated as follows:

Qa ¼ q� F ð10Þ

q ¼ arc � ðts � taÞ � Kl2 ð11Þ

where Kl2 adopts a different value. For example, vertical surface is 1;level up is 1.3; and level down is 0.7.

(9) TVC ejection ratio can be calculated as follows:

l ¼X

DH=D0 ð12Þ

TVC ejection ratio represents the performance of second steamreuse, and the high ejection ratio, the large GOR.

3. MED pilot device

An MED pilot device was designed and constructed by the Insti-tute of Seawater Desalination and Multipurpose Utilization basedon the above mathematical model. The pilot was located at theDatang Lubei power plant in Tianjin, China. Fig. 2 shows a simpli-fied schematic of the device. The device consists of three-effectevaporators, condenser, steam ejector, pumps, vacuum pumps,connecting pipes, valves, and electronic control instrumentation.Raw water was obtained from the cooling seawater of the powerplant, with a temperature of 20–28 �C. The device successfullyran for more than two years since it began operating in October2011. The device produces freshwater at an average rate of 30 t/d. Fig. 3 shows the experimental pilot device.

The test aimed to simulate operating conditions of large-scale,low-temperature, multi-effect distillation desalination devices.Another objective was to study the effects of key technical param-eters and operating factors on device performance under largeoperating parameters. First, the device was initiated in cold-stateoperating mode when motive steam passed. The running parame-ters (i.e., evaporating pressure, operating temperature, heatingtransfer temperature difference, and spray density) were thengradually adjusted to a specified value. Second, the automaticoperation mode was selected, and the system reached a steady

p, 6: steam ejector, 7: circulating pump, 8: cooling water pump, 9: brine pump, 10:

Fig. 3. Experimental pilot device.

Fig. 5. Time variation of specific conductance of product water.

Table 1Detected product quality results.

418 C.-h. Qi et al. / Applied Energy 135 (2014) 415–422

state after two hours of operation. Finally, the influencing param-eters were tested. The pneumatic valve, electric valve, and trans-ducer were accurately adjusted to alter the experimentalparameters [i.e., temperature difference, spray density, vacuumpumping mode, and TVC (steam ejector)] by using the host com-puter system.

We used measuring parameters, including temperature, pres-sure, flow rate, fluid level, online conductivity, and pH. Sensorson the device body and pipes tested the temperature and pressure.Pipe-installed electromagnetic flow meters measured the flow rateof seawater, concentrated water, and product water. Motive steamtemperature and pressure fluctuations affected the measurementaccuracy of the steam flow rate. In our experiment, the Orifice plateflowmeter is used to measure steam flow rate with temperatureand pressure compensation to ensure a measurement error of lessthan ±0.1%. Product water quality was monitored by using anonline conductivity meter and a pH meter installed on the productpump outlet. The host computer records and stores all in situ oper-ation data automatically.

4. Experimental results and discussion

4.1. Whole-process test results

The whole-process test included two procedures: cold-state testand hot-state test. The difference between the cold-state test andthe hot-state test was based on the motive steam injected intothe system. Cold-state tests were generally conducted first; hot-state tests were initiated when the regulated technical require-ments were met. Adjusting vapor pressure, operating temperature,heat transfer temperature difference, and spray density to the cor-responding design values enabled the main devices (i.e., vacuumpump, steam ejector, circulating pump, cooling water pump, brine

Fig. 4. Time variation of specific conductance and temperature of raw water.

pump, and product pump) to meet the technical requirements andremain stable for 168 h at a full operation load.

Figs. 4 and 5 show the variation of the specific conductance andtemperature of raw water and the specific conductance of productwater during stable operation, respectively. The specific conduc-tance of raw water was within the range of 37,900–44,650 ls/cm, while the specific conductance of product water graduallydecreased and was maintained at 7.5 ls/cm. Therefore, the presentpilot test has a strong adaptive capacity to raw water and obtainsgood product water quality.

The analysis results of product water samples provided by theNational Quality Supervision and Inspection Center for Seawaterand Brackish Water Utilization Products indicate that the productwater meets the national drinking water criterion. The total dis-solved solids are less than the design valve of 5 mg/L [33], asshown in Table 1.

4.2. Variations of motive steam flow with pressure

The motive steam used in the pilot test was the industrial steamfrom the power plant; vapor pressure and flow can be adjusted byusing valves. Fig. 6 shows the variation of motive steam flow withits pressure. The steam flow trendline shows that motive steamflow increases linearly on average with its pressure. Although thepressure and flow can be adjusted by using valves, they cannotbe regulated individually.

4.3. Effects of motive steam pressure P1 on water production and GOR

Figs. 7 and 8 show the effects of motive steam pressure on waterproduction and GOR, respectively. The experimental operating

Number Test item Unit Result Standard limited valueof drinking water [34]

1 pH / 6.78 6.5–8.52 Fluoride mg/L <0.2 1.03 Chroma / 5 154 Copper lg/L 144.3 10005 Total dissolved solids mg/L 3.5 10006 Volatile phenol mg/L <0.002 0.0027 Calcium mg/L <0.10 Total hardness 64508 Magnesium mg/L 0.119 Sulfate mg/L 0.35 25010 Iron mg/L <0.10 0.311 Aluminum mg/L <0.10 0.212 Sodium mg/L 0.57 20013 Potassium mg/L <0.05 /14 Chloride mg/L 0.89 25015 Sulfide mg/L <0.01 0.02

Fig. 6. Variation of motive steam flow with pressure.

Fig. 7. Effects of motive steam pressure P1 on water production.

Fig. 8. Effects of motive steam pressure P1 on GOR.

Fig. 9. Effects of operating temperature T0 on water production.

Fig. 10. Effects of operating temperature T0 on GOR.

C.-h. Qi et al. / Applied Energy 135 (2014) 415–422 419

temperature was T0 = 72 �C, temperature difference was Dt = 6 �C,and total spray density was C = 300 L/(m h).

Water production increased along with increased motive steampressure, as shown in Fig. 7. The motive steam pressure stabilizedwhen steam pressure continued to increase and exceeded0.95 MPa (i.e., 121% of the designed value).

The trend of GOR with motive steam pressure was similar tothat of the TVC ejection ratio with the motive steam pressure(Fig. 8). However, the TVC ejection ratio and GOR exhibited a slightdownward trend when the motive steam pressure exceeded thedesign value. This downward trend was due to the fact that themotive steam pressure increased beyond its optimum operatingconditions, which do not match the TVC design parameters.

4.4. Effect of maximum operating temperature T0 on water productionand GOR

The maximum operating temperature for low-temperatureMED is usually less than 70 �C. This temperature is expected to

increase with the development of anti-scaling and heat transfertechnologies. Experiments on water production variation lawswere performed at a higher operating temperature (above 70 �C)to investigate the effects of maximum operating temperature onwater production and GOR. The experimental conditions were asfollows: P1 = 0.85 MPa, C = 300 L/(m h), and Dt = 6 �C. The highestoperating temperatures used in the test were T0 = 78 �C, 72 �C,and 66 �C. The results are shown in Figs. 9 and 10.

Fig. 9 shows that water production increased along with themaximum operating temperature because of the horizontal fallingfilm evaporator; a high operating temperature resulted in lowliquid viscosity and surface tension. Liquid film flow rate wasincreased, and liquid film fluctuation was enhanced. The liquid filminside and outside the tube was thinned and shed. Finally, theoverall heat transfer coefficient of falling film evaporation wasincreased. As the highest operating temperature up to 78 �C, themaximum water production is up to 1.42 t/h and the overall heattransfer is supreme, about 2618 w/(m2 �C).

As shown in Fig. 10, GOR increased from 7% to 12% when themaximum operating temperature increased by 6 �C. Two possiblereasons can explain this result. First, the overall heat transfer coef-ficient for the given motive steam parameters of falling film evap-oration increases as operation temperature increases. Second, theejected secondary steam pressure and temperature increase asoperating temperature increases. Consequently, the TVC ejectionratio is increased. Moreover, the GOR can be enhanced becauseof the improved secondary steam utilization ratio.

4.5. Effect of temperature difference (Dt) on water production andGOR

It is tested that the effects of temperature difference (Dt) onwater production, GOR, and ejection ratio l. The experimental

Fig. 11. Effects of temperature difference Dt on water production.

420 C.-h. Qi et al. / Applied Energy 135 (2014) 415–422

operating temperatures were T0 = 72 �C, P1 = 0.85 MPa, andC = 300 L/(m h). Temperature differences Dt that were used were3 �C, 4 �C, 5 �C, and 6 �C. The results are shown in Figs. 11 and 12.

The effect of temperature difference increases on the falling filmevaporator heat transfer coefficient has two aspects. First, thesuperheat degree increases, and liquid viscosity decreases whentemperature difference increases; this reduction in viscosity playsa role in enhancing heat transfer [35]. Second, the temperature dif-ference increase results in a great heat flux density and condensa-tion liquid flow, which increases liquid film thickness. Thecondensation heat transfer coefficient is obviously reduced,whereas condensation liquid flow is increased. The proportion ofthe condensation liquid heat transfer area (at the bottom of thetubes) to the total heat transfer area increases as the condensationflow increases; the average heat transfer coefficient on the surfacedecreases [36–38]. The total heat transfer coefficient decreases andproduct water flow gradually reduces with the increase of Dtbecause of the influence of the two aspects (Fig. 11). However,the brine boiling point elevation and flow resistance cause a lossin temperature difference when the temperature difference is toosmall, which results in poor device stability. Therefore, tempera-ture difference should be restricted to the range of 3–4 �C, whichis critical to obtaining a high heat transfer coefficient and a steadyoperating condition.

The ejection ratio and GOR decrease as temperature differenceincreases, as shown in Fig. 12. A great temperature differenceresults in a low secondary steam pressure in the third-effect andsmall TVC ejection ratio under certain operating temperaturesand motive steam pressure. Consequently, secondary steam andGOR utilization decrease accordingly.

Fig. 12. Effects of temperature difference Dt on GOR and l.

Fig. 13. Effect of spray density C on water production.

Fig. 14. Effect of spray density C on product conductivity.

4.6. Effect of spray density C on water production and productconductivity

The effect of spray density C on device performance was mea-sured under steady conditions to select the spray density basis forthe MED plant design. The experimental conditions were as fol-lows: P1 = 0.85 MPa, T0 = 72 �C, and Dt = 6 �C. Spray density wasadjustable within the range of 147–425 L/(m h). Figs. 13 and 14show the effect of spray density on water production and productconductivity. The experimental results indicate that spray densityis an important factor that influences the evaporation surface heattransfer coefficient under falling film horizontal tube evaporation.

The evaporation surface heat transfer coefficient and total heattransfer coefficient increased as spray density increased. Thisincrease can be attributed to the fact that the average thicknessof the liquid film increases with increased liquid load. Thus, morepreheat steam will be consumed, which is unfavorable for the heattransfer process. Moreover, liquid flow velocity outside the tubeincreases, which aggravates the fluctuation of the liquid film. Thus,the convective heat transfer is strengthened [39]. Based on Fig. 13,the largest water production was obtained when spray density was290 L/(m h); beyond this value, water production decreased.

Fig. 14 shows that the product conductivity stabilized at 10 ls/cm when the spray density was within the range of 240–300 L/(m h). Moreover, product conductivity increased gradually withincreased spray density. The nozzle outlet pressure increased,and spray splash and droplet atomization intensified when spraydensity (which was greater than the design value) increased, whichresulted in increased steam-entrained droplets. Consequently,product conductivity was increased. A spray density value between240 L/(m h) and 300 L/(m h) should be adopted for the actualdesign and operation. Installing a baffle before the steam passageinlet prevents seawater droplet splashing, which greatly affectsproduct water quality.

Fig. 16. Effects of vacuum pumping in series mode and in parallel mode on GOR.

Fig. 17. Effects of different valve opening of TVC on water production.

C.-h. Qi et al. / Applied Energy 135 (2014) 415–422 421

4.7. Effect of vacuum pumping modes on water production and GOR

The operation performances were conducted when the vacuumpump was in the parallel and in the series modes. Experimentalconditions are similar to the relationship between water produc-tion and GOR with the different NCG extraction modes, as shownin Figs. 15 and 16.

Comparisons between the effects of vacuum pumping in seriesmode and in parallel mode on water production and GOR are pre-sented in Figs. 15 and 16, respectively. The results demonstratethat vacuum pumping in the series mode had stronger adaptabilityto steam fluctuation compared with that in the parallel mode. Thepressure difference between effects was easily maintained, and theoperational conditions were stable because every evaporator andcondenser depended on vacuum pumping methods. Non-condens-able gas (NCG) is extracted from the former effect to the nextthrough pressure difference between the two effects. Finally, theNCG is extracted by the vacuum pump in the condenser. However,in the case of vacuum pumping in parallel mode, every evaporatorand condenser was independent, which resulted in inconsistentpressure change. Thus, motive steam pressure fluctuations easilyaffected the device, which made steady operation difficult.

Furthermore, vacuum pumping in the parallel mode facilitatedsteam extraction from the system. Condensation inside NCG flowmeters of one- to three-effect evaporators appeared during the testprocess, which reduced system efficiency. However, steam thatwas extracted from the former evaporator is reused in the nexteffect evaporator for vacuum pumping in the series mode, therebyimproving system efficiency, water production, and GOR. Vacuumpumping in the series mode increased water production and GORby 3.64% and 3.28%, respectively, on average.

Fig. 18. Effects of different valve opening of TVC on GOR and ejecting ratio.

4.8. Effect of TVC valve openings on water production and GOR

Figs. 17 and 18 show the effects of the TVC valve openings (i.e.,TVC ejection ratio l) on water production and GOR, respectively.The experimental motive steam pressure was P1 = 0.85 MPa,T0 = 72 �C, and Dt = 6 �C. The TVC ejection ratio was adjustedduring the test by changing the different TVC valve openings.

As shown in Figs. 17 and 18, the TVC ejector flow affects waterproduction, GOR, and ejection ratio. For the given motive steampressure, water production was about 0.8 t/h and GOR was 3.2when the ejector flow was zero. Water production increased from0.923 t/h to 1.32 t/h when secondary TVC steam flow increased;the GOR and ejection ratio also increased. The GOR had a highervalue when TVC was used compared with using standalone MEDsystems [7,28]. However, water production, GOR, and ejection ratiostabilized when the TVC valve opening exceeded 44�.

Fig. 15. Effects of vacuum pumping in series mode and in parallel mode on waterproduction.

5. Conclusion

In this paper, a 30 t/d low-temperature multi-effect evaporationseawater desalination system was designed based on the mathe-matical model, and a corresponding pilot device was constructed.Whole-process tests were conducted, and the effects of key operat-ing parameters were analyzed. The following conclusions can bedrawn from the test results:

(1) Motive steam pressure fluctuations strongly influence thestability of the device, water production, and GOR. Waterproduction increased as motive steam pressure increased.However, the motive steam pressure stabilized when steampressure continued to increase and exceeded 0.95 MPa (i.e.,121% of the designed value).

(2) Device water production increases as the maximum operat-ing temperature increases. The tests indicate that water pro-duction increased by 7–12% when the maximum operating

422 C.-h. Qi et al. / Applied Energy 135 (2014) 415–422

temperature increased by 6 �C. Hence, the set value of themaximum operating temperature should be slightlyincreased, which is dependent on the progress of anti-scaleand heat transfer technologies.

(3) Water production increases as heat transfer temperature dif-ference decreases. However, water production can no longerincrease when the temperature difference decreases to acertain value. Temperature difference was restricted to therange of 3–4 �C in this paper; this limited range was criticalto obtaining a high heat transfer coefficient and a steadyoperating condition.

(4) Device maximum water production was obtained whenspray density was between 240 L/(m h) and 300 L/(m h).These values should be adopted for actual design and oper-ation. Installing a baffle in front of the steam passage inletprevents seawater droplet splashing, which can greatlyaffect the quality of product water.

(5) Vacuum pumping in the series mode had more advantagesthan vacuum pumping in the parallel mode. These advanta-ges include good adaptability to motive steam fluctuationsand high system efficiency, water production and GOR.Water production and GOR increased by 3.64% and 3.28%on average, respectively, in the series mode.

(6) TVC increases water production and GOR as well as reducessystem energy consumption. An adjustable TVC nozzle isrecommended to improve device stability and efficiency.

Acknowledgments

The authors are grateful for the support of special fund forresearch institutes of the Ministry of Science and Technology(No. 013EG149277), China. This paper is also supported by specialfund of Operating Expense of Fundamental Scientific Researches ofScientific Research Institutions of Public Welfare (No. K-JBYWF-2013-T1).

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