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Experimental and theoretical investigation of decentralized desalination system

 ARTICLE · JUNE 2014

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Mohammed Elsharkawy

Mansoura University

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Hossam AbdelMeguid

University of Tabuk

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Ibrahim I. El-Sharkawy

Mansoura University, Egypt; Kyushu Universi…

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Experimental and Theoretical Investigation ofDecentralized Desalination System

ال مركلنظام تح یة میونظاختبار ع  

Mohamed Elsharkawya,1, Hossam AbdelMeguid b,2, Ibrahim El-Sharkawy b,3, and Lotfy Rabie b 

aTaba desalination plant, Holding company of water and waste water north &south Sinai,

Taba, Egypt. bMechanical Power Engineering Department, Faculty of Engineering, Mansoura University,El-Mansoura 35516, Egypt1 Email: moh.sharkawy8@gmail.com, Tel.: +2010988161822 Email: hssaleh@mans.edu.eg, Tel.: +2010664647123 Email:ielsharkawy@mans.edu.eg, Tel.: +201006107242

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ال س ارالنظریة م ن حی ث الق یم ال ط ق ةبرھنت النتائج الع یة صحة الن وذج النظرى حیث توافقت النتائج الع یة.النتائج

الكت  ة ف ى االج زاء ال خت ف ةق ال الح رارةنالنظریة بالفروض ال ثالی ة لع ی ات فسر الفرق بین النتائج الع یة.واالتجاه

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نتاجی ةث ت زداد ا/كج0.08 ل ىث/كج0.05 م نال ی اه ال غذی تصرن بزیادة معدلتزداد االنتاجیة حیث)ال غذیة

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5.16س وم ن/لت7.32 س إل ى/لت6.36  م ننتاجی ةث ت زداد /كج0.1لىث/كج0.055معدل سریان الھواء من عند درج ة ح رارة ال ی اهس/لت2.22لىس/لت1.92س ومن/لت4.08لىس/لت3.6منس/لت5.64س إلى/لتر

0.15ل ىث/كج0.08 التبری د م نبزی ادة مع  دل تص رف می. درج ة مئوی ة ع  ى الت وال70605036ال غذی ة 5.28س إل ى/لت4.68 س وم ن/لت6.84 س إل ى/لت6.12 م نخ رى ت زداد االنتاجی ث ومع ثب ات ال تغی رات /كجم7060حرارة ال ی اه ال غذی عند درجةس/لت2.04لىس/ت1.74س ومن/لت3.84لىس/لت3.24من/لتر

 .درجة مئویة ع ى التوال5036و

AbstractThis paper presents experimental and theoretical investigation of decentralized desalination system based on ahumidification-dehumidification (HDH) process. HDH is one of the methods that employed in the decentralizedsystems in remote areas where there is lack of experienced operators, maintenance and water resources. HDH is

suitable for remote areas that require small amount of fresh water. In the current work, a mathematical model is presented to investigate the effect of different operating parameters on the system productivity. A computer program based on MATLAB software is developed to solve the mathematical model and post-process thenumerical results. In order to validate the mathematical model and verify the obtained numerical results, anexperimental setup of decentralized HDH desalination system is designed and constructed. A series ofexperimental and numerical runs for different operating parameters are performed. The experimental data arecompared with the numerical results and a good agreement in both values and trend is observed. The transientstate behaviour of each component of the system and the influence of the different operating parameters on thesystem productivity are presented theoretically and experimentally.

Keywords: Desalination; Humidification; Dehumidification; Decentralized. 

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 NomenclatureA heat transfer area [m2]As surface area [m2]a width [m]β temperature reciprocal [ C¯ ]

 b length [m]Cpw  specific heat of water [J/kg.K]c height [m]Gr Grashof number (The ratio of the

 buoyancy to viscous force)g gravitational acceleration [m/s2]h convective heat transfer coefficient

[W/m2.K]ha enthalpy of air [J/kg]k thermal conductivity [W/m.K]Lc characteristic length [m]loss losses of component [Watt]MCp heat capacity [J/K]ṁ mass flow rate [kg/s]

 Nu Nusselt Number (convective heattransfer / Conductive heat transfer)

Pr Prandtl number (Viscose diffusionrate / Surface diffusion rate)(Cp.µ/k)

 p perimeter [m]Q heater capacity [Watt]Ra Rayleigh number (Grashof number

* Prandtl number) (Gr. Pr)

T temperature [o

C]Tf average temperature [oC]

U Overall heat transfer coefficient[W/m2.K]

w specific humidity [kgw/kga]Z insulation thickness [m]

Subscripts

a airamb ambienta1 air outlet from dehumidifiera2 air inlet to humidifiera3 air outlet from humidifiercw cooling waterDH dehumidifierH humidifierhor horizontali inletins insulationLc characteristic lengthloss lossesmw make up watero outletost outside storage tank

 prod productst storage tankver verticalw1 feed water inlet to humidifier

w2 water outlet from humidifier

1. Introduction

Interest in seawater desalination goes backto the fourth century BC, when Greeksailors used to obtain drinking water fromseawater. The first work published on solardesalination was that of Arab alchemists in1551 [1]. However, the first solar still was

designed and constructed in Chile by theSwedish engineer, Carlos Wilson, in 1872[1].

World-wide water scarcity, especially inthe developing world, indicates a pressingneed to develop inexpensive, decentralizedsmall-scale desalination technologieswhich use renewable resources of energy.

Combining the principle ofhumidification–dehumidification (HDH)with solar desalination decreases theconsumption of conventional energyresources in desalination plants andtherefore appears to be the best method ofwater desalination in remote areas.

This section provides a comprehensive

review of the state-of-the-art in one of themost promising of these technologies,HDH desalination technology.

A number of studies on solar desalinationsystems using HDH technique were

 presented in the literature [2-4]. Mostly,these studies include the performanceevaluation of the HDH desalinationsystems under different systems operating

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 parameters. In addition, in some of thesestudies, different types of solar collectors(water and air), humidifying towers anddehumidifying exchangers were designedand constructed, and their effects on thesolar desalination system productivity were

evaluated theoretically and experimentally[5-7]. Moreover, the influence of differentsystem configurations, (such aswith/without solar air and water heaters)on the solar desalination units productivitywere investigated [8, 9]. As well, naturaland forced air circulation solar HDHdesalination were investigated [10]. Inaddition, a hybrid HDH-Flashingevaporating system and HDH-Compression systems were proposed andstudied [11, 12].

During the last decade, a number of HDHdesalination units were developed andtested [13, 14]. Dai and Zhang [15] studiedexperimentally a solar desalination systemwith HDH. The used process was based onthe principle of open air-closed watercycles and forced air circulation. Mainly, asolar water heater, humidifier andcondenser configure the system. The mostimportant characteristic of the system isthe ability to utilize efficiently low-grade

heat resources, such as waste heat,geothermal, gas/oil/coal burning, etc. Itwas found that the performance of thesystem is strongly dependent on the rate offeed water temperature, the mass flow rateof salt water and the mass flow rate of the

 process air. In addition, it is reported thatwater productivity of the described solardesalination system is about 6.2 kg/m2.dayunder optimal operating conditions.

Chafik [16] presented a seawater

desalination process based on the stepwiseheating-humidifying technique. The mainfeature of the suggested process is that theair is successively loaded with the watervapour up to relative high humidity, suchas 10 or 15%. It was reported that, by thisway, the airflow rate through the plantcould be reduced and low investment andoperating costs could be achieved. In

addition, he developed two different typesof the humidifier, which are pad humidifierand U-tube spray chamber humidifier. Theexperimental results of the humidifiersindicated that their efficiency is about95%, and also, he tried to find out an

optimum number of requiredheating/humidifying stages.

Amara et al. [17] carried out anexperimental and theoretical study of a padhumidifier used in a multi-stage solardesalination process in Tunisia underdifferent ambient and operating conditions.The mathematical formulation of the padhumidifier was developed and the non-linear system of equations was solved byusing the numerical method, Newton-

Raphson. The numerical results werecompared to the experimental data tovalidate the numerical model and there wasa good agreement between them.

 Nafey et al. [18, 19] carried out atheoretical and experimental study of aHDH process using solar energy at theweather conditions of Suez City, Egypt. Inthe theoretical study, a mathematicalmodel was developed, simulating thesystem, to study the effects of the differentsystem configurations and operatingconditions. The results showed that the

 productivity of the system increases withthe increasing of air mass flow rate, solarair and water collector areas, and inputsolar energy. On the other hand,

 productivity of the unit decreases with theincreasing of water mass flow rate. Finally,they reported that the wind speed andambient temperature have very smallinfluence on the system performance.

This study aims to develop a mathematicalmodel and validate it by usingexperimental data, and investigatetheoretically and experimentally the effectof different operating parameters, such asmass flow rates of air and water and feedwater temperature, on the productivity of adecentralized air HDH desalination system.

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Humidifier Dehumidifier

StorageTank 

Heater 

Valve

Flow

Meter 

Tw1, ṁw1

ṁw2

Tw2

ṁmwTmw

Ta3, ṁawa3,ha3

Ta2, ṁa ,ha2

wa2=wa1

Tcwoṁcw

Tcwi

Product

Tank 

T prod

ṁ prod

Air Loop

Water Loop

To drain

P

 

F

ṁcw

Water

Figure 1 A schematic diagram of HDH desalination system. 

2. Experimental set upA schematic diagram of the HDH (Open

water and closed air cycles) system is

shown in Figure 1.  The hot water is

 pumped (373 Watt centrifugal pump) from

the storage tank and sprayed into the air

humidifier. The air enters the humidifier

from the bottom, where it contacts thefalling hot water film at large mass transfer

area of the packing. The remaining water is

drained. The humid air passes through a

dehumidifier (heat exchanger) for

dehumidification (condensing air water

contents) using cooling water, and then air

is re-circulated to the humidifier again.

2.1 Humidifier

The humidifier unit contains five cassettes

in series made of plastic material(polypropylene), which forms the wetted

surface. The cross-sectional area of the

cassette is 720 mm x 520 mm, and the

thickness of each cassette is 170 mm. The

 packed pad has been mounted horizontally

in a metal rack of dimensions: aH= 530 mm

width, bH= 730 mm length and cH= 1400

mm height. The rack has been made of

galvanized steel of 1.5 mm thickness and

has been insulated by foam of thickness 10

mm. Figure 2 shows a section view of the

humidifier.

Figure 2 section view of the humidifier.

2.2  Dehumidifier

Air cooler heat exchanger of copper tubes,

corrugated aluminium fins has been used

as a dehumidifier. The total surface area of

the dehumidifier is about 11 m2. To prevent air leakage and heat gain, the

dehumidifier heat exchanger has been

 packed by a metal case. The case has been

constructed of 1.5 mm thick galvanized

steel of dimensions aDH= 400 mm width,

 bDH= 950 mm length and cDH= 330 mm

height.

Fan

Pump

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2.3  Water storage tankThe water storage tank has beenconstructed of 5 mm thick plastic tank ofdimensions: 600 mm diameter, and 920mm height. The tank is thermally insulated

 by glass wool of thickness 10 mm. Itconsists of one electric heater of capacityQ= 3 kW with a built-in thermostat. Theheater was horizontally placed at the

 bottom of the tank. The heater was used toheat the water inside the tank to a certaintemperature to investigate the effect of thefeed water temperature on the system

 productivity. A constant temperature andhead of water inside the storage tankduring the experimental runs have beenmaintained.

3. Mathematical modelling

In order to apply the energy and mass balance equations for each of the systemcomponents, the following assumptions areapplied to the present model:

1) 

Either laminar or turbulent flow is fullydeveloped.

2)  The effectiveness of the humidifying process is assumed to be unity, thus:a)

 

The air leaving the humidifier is atsaturation condition. Therefore, thewet-bulb and dry-bulb temperaturesare identical.

 b)  The temperature of water leavingthe humidifier is equal to the wet-

 bulb temperature.3)  The dehumidification process lies on

the saturation curve.4)  Temperature gradient inside the water

storage tank is neglected (a perfect

mixing is assumed).5)  The temperature of inlet water (feedwater) is equal to the water temperaturein the storage tank (no heat loss in thelines is assumed).

6)  Cooling water temperature is constantduring the day (infinite source ofcooling is assumed).

7)  Exit temperature of the condensatewater and the outlet cooling water areequal to the dry-bulb temperature ofthe dehumidifier exit.

Applying the thermodynamic laws for the

humidifier, dehumidifier, and storage tankas shown in Figure 1, the energy and mass balance equations can be written as:

3.1 Humidifier

Mass balance:ṁ  + ṁw  = ṁ  + ṁw 

(1)Heat balance:

MCp dT

dt   = m 

Cp

T

 + m  h

− m  CpT  − m   h−loss 

(2)MCp  = MCp  + MCp +MCp + MCp  

(3)The humidity and saturated air enthalpyare given by Al-Hallaj et al [20];

w

  = 2.19∗10

T

 − 1.85 ∗ 10

T

+ 7.06 ∗ 10T−0.077 

(4)h   = 0.00585T  −0.497T  + 19.87T−207.61 (5)

3.2 DehumidifierMass balance:

ṁ  = ṁ  ∗ (w  − w ) 

(6)Heat balance:

MCp dTdt   =ṁ

h  +ṁ

CpT− ṁh  − ṁCpT− ṁCpT  − loss  

(7)MCp  = MCp +MCp+MCp  + MCp  

(8)

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3.3 Water storage tank

Mass balance:ṁ  = ṁ  − ṁ 

(9)Heat balance:

MCp dT(t)dt   = Q  + ṁCpT+ ṁCpT− ṁCpT  − loss  (10)MCp  = MCp  + MCp  

(11)

3.4 Heat losses from the system  The heat lost from storage tank , [21]:

loss  = A U(T  − T ) (12)U  =  1   (1 h ⁄   + Z   k )⁄  

(13)The relation between Nu and Ra forcylindrical shape, [22]:

Nu  = ⎩⎨⎧

0.6 +  0.387Ra

1 + (0.559/Pr) ⎭⎬⎫

 

(14)

  The heat lost from the humidifier:loss  = 2h ab(T  − T ) +2hcb(T  − T ) +2hca(T  − T ) (15)

  The heat lost from the dehumidifier:

loss

 = 2h

a

b

(T

 − T

) +2hcb(T  − T ) +2hca(T  − T ) (16)

The relation between Nu and Ra forhorizontal plate, [22]:

Nu  = 0.27Ra / 

(17)

For vertical plate, Churchill and Chu [22] proposed:Nu

= 0.825+  0.387Ra /

1 + (0.492/Pr) /

 

(18)Ra  =   β() ν   Pr (19)Lc = As/p  (20)

β  = 1/T    (21)T   =     (22)h =  ∗   (23)

4. Numerical solution

A computer simulation program to solvethe mathematical model of the system has

 been developed by means of MATLABsoftware to investigate the effect of thevarious operating parameters on the

 productivity of decentralized HDHdesalination system. The transient statelumped parameter model has been solvednumerically by employing the functionode45.

ode45 implements a version of the Runge-

Kutta(RK) 4

th

/5

th

  order algorithm [23].That means the numerical solver ode45combines a fourth-order method and afifth-order method, both of which aresimilar to the classical fourth-order RKmethod. The modified RK varies the stepsize, choosing the step size at each step inan attempt to achieve the desired accuracy.Therefore, the solver ode45 is suitable fora wide variety of initial value problems in

 practical applications.

5. Results and discussionThis section presents the experimentalresults, the validation of the developedmathematical model and a parametricstudy for the decentralized HDHdesalination system. The parametric studyof the developed mathematical model

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includes the effects of feed water massflow rate and temperature, air mass flowrate, and cooling water mass flow rate onthe system productivity.

5.1 Model validation

The numerical results and experimentaldata of the steady-state system productivityat different values of the feed water massflow rate at temperatures of 36 °C and 50°C are compared in Figure 3. It can be seenthat the results obtained show a goodagreement in both values and trend withrelative error ranges from 3.9% to 6% forfeed water temperature of 50 °C, and 6.8%to 12.1% for feed water temperature of 36°C. This deviation can be attributed to the

assumptions 3, 4, and 5 that assume ideal processes.

Figure 4 shows a comparison between thenumerical results and experimental data ofthe steady-state system productivity atdifferent values of the feed water massflow rate at temperatures of 36 °C and 50°C. There is a good agreement in bothvalues and trend between the theoreticaland experimental data. The deviation

 between the numerical results and theexperimental data varies from 1.3% to10.8% for feed water temperature of 50 °C,and from 6.3% to 9.2% for feed watertemperature of 36 °C. These differencesare due to the assumed perfect processesand ignoring the thermal and frictionallosses in the system.

The numerical results and experimentaldata of steady-state productivity of thesystem at different values of the coolingwater mass flow rate are compared inFigure 5. It can be seen from the figure that

 both results are matching with each other.Due to the assumed ideal processes andneglecting both frictional and thermallosses (assumptions 3, 6, and 7), adeviation of 3.6% to 7.1% for feed watertemperature of 50 °C, and 6.8% to 8.1%for feed water temperature of 36 °C areobserved.

0.02 0.04 0.06 0.08Feed water mass flow rate, mw1[kg/s]

1

2

3

4

5

     P    r    o     d    u    c     t      i    v      i

     t    y     [     l      i     t      /     h    r     ]

Exper.at Tw1=36 oC

Model.at Tw1=36 oC

Exper.at Tw1=50 oC

Model.at Tw1=50 oC

 

Figure 3 Comparison between experimental andnumerical results of the steady-state production atdifferent values of the feed water mass flow rate. 

0.02 0.04 0.06 0.08 0.1

Air mass flow rate, ma[kg/s]

1

2

3

4

5

     P    r    o     d    u    c     t      i    v      i     t    y     [     l      i     t      /     h    r     ]

Exper.at Tw1=36 oC

Model.at Tw1=36 oC

Exper.at Tw1=50 oC

Model.at Tw1=50 oC

mw1

=0.06 kg/s, mcw

=0.11 kg/s

Tcwi=19oC, Tamb=17oC

 

Figure 4 Comparison between experimental andnumerical results of the steady-state production at

different values of the air mass flow rate.

5.2 Experimental Results

In order to measure the experimental datathermocouples connected to a temperaturerecorder have been used to measure thevarious temperatures, a water flow meter(rotameter) to measure the mass flow rateof water, and an anemometer is used tomeasure the mass flow rate of air.

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0.06 0.08 0.1 0.12 0.14 0.16Cooling water mass flow rate, m

cw[kg/s]

1

2

3

4

5

     P    r    o     d    u    c     t      i    v      i     t    y     [     l      i     t      /     h    r     ]

Exper.at Tw1=36 oC

Model.at Tw1=36 oC

Exper.at Tw1=50 oC

Model.at Tw1=50 oC

ma=0.055 kg/s, m

w1=0.06 kg/s

Tcwi

=19oC, Tamb

=17oC

 

Figure 5 Comparison between experimental andnumerical results of the steady-state production at

different values of the cooling water mass flow rate.The effect of feed water mass flow rate onthe system productivity is presented inFigure 6. It depicts that the productivity ofthe system increases from 5.76 to 8.04lit/hr and from 4.32 to 5.76 lit/hr for feedwater temperature of 70 and 60°C,respectively. The results show that the

 productivity of the system increases withincreasing the mass flow rate of feed waterand the feed water temperature.

Increasing the water production with thefeed water mass flow rate can be explainedclearly with the aid of Figures 7 and 8. Itcan be seen that the difference between theoutlet temperatures of the air leaving thehumidifier, Ta3, and inlet temperatures, Ta1,increases as the feed water mass flow rateincreases, thus relative humidity of the airincreases. This is due to the increasing incontact surface area between the feedwater and process air.

Figure 7 illustrates the transient variationsof the humidifier inlet air temperatures,Ta1, at different feed water mass flow rates.For constant feed water temperature, theinlet air temperature increases graduallyfrom the initial temperature of about 17°C(ambient temperature) to reach a steady-state operation in about 20 min. As well, itcan be seen from the figure that as the feed

water mass flow rate increases, the steady-state operation of the inlet air temperatureincreases.

Figure 8 depicts the transient trajectory ofthe humidifier outlet air temperatures, Ta3,at different feed water mass flow rates. The

outlet air temperature rises from the initialtemperature of about 17 °C to reach asteady-state operation in about 20 min forconstant feed water temperature of 50 °C.In addition, the results showed thatincreasing the feed water mass flow rateincreases the steady-state operation of theoutlet air temperature.

0.04 0.05 0.06 0.07 0.08

Feed water mass flow rate, mw1 [kg/s]

2

4

6

8

10

     P    r    o     d    u    c     t      i    v      i     t    y     [     l      i     t      /     h    r     ]

Tw1=60 oC

Tw1=70 oC

ma=0.055 kg/s, mcw=0.11 kg/s

Tcwi=19 °C, Tamb=17 °C

 

Figure 6 Effect of feed water mass flow rate on thesystem steady-state productivity.

0 10 20 30Time [min]

16

18

20

22

24

26

28

     T    a      1

     [    o      C     ]

ma= 0.055 kg/s, mcw= 0.11 kg/s

Tcwi= 19 oC, Tamb= 17  oC,

Tw1=50  oC

 Figure 7 Transient variations of the inlet air

temperatures, Ta1, at different feed water mass flowrates.

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0 10 20 30

Time [min]

16

20

24

28

32

36

40

     T    a      3

     [    o      C     ]

ma= 0.055 kg/s, mcw= 0.11 kg/s

Tcwi= 19 oC, Tamb= 17  oC, Tw1=50 oC

 Figure 8 Transient variation of the outlet air

temperature,Ta3, at different feed water mass flowrates. 

5.3 Parametric studyA number of runs have been performed inorder to assess the influence of thefollowing operating parameters on thetransient and steady-state productivity ofthe system:

  Mass flow rates of the feed waterṁw1;

  Mass flow rate of the process airṁa;

  Mass flow rate of the cooling waterṁcw; and

 

Feed water temperature.

Figure 9  depicts the transient variation ofthe system productivity at different valuesof feed water mass flow rate at constantfeed water temperature of 50 °C. Thesystem reaches a steady-state productionafter about 20 minutes due to thermalinertia of the system.

The effect of the feed water mass flow rate

on the system steady-state productivity forair mass flow rate of 0.055 kg/s andcooling water of 0.11 kg/s at 19 °C is

 presented in Figure 10. It illustrates thatthe productivity of the system increaseswith increasing feed water mass flow rate.The theoretical results show that thedecentralized HDH desalination systemconsidered in this study produces 8.04

lit/hr in the steady-state operation at feedwater temperature of 70 °C and mass flowrate of 0.08 kg/s.

Figure 11 shows the transient variation ofthe humidifier inlet air temperature, Ta1,

with constant air mass flow rate at differentfeed water mass flow rates and constantinlet water temperature of 50 °C. Theincrease of feed water mass flow rateincreases inlet air temperature. The steady-state operation is achieved after 20 min dueto thermal inertia of the system.

0 10 20 30Time [min]

0

1

2

3

4

5

     P    r    o     d    u    c     t      i    v      i     t    y     [     l      i     t

      /     h    r     ]

ma= 0.055 kg/s, mcw= 0.11 kg/s

Tcwi= 19 oC, Tamb= 17 oC, Tw1=50 oC

 

Figure 9 Transient production at different massflow rates of feed water at a constant temperature of50°C.

0.02 0.04 0.06 0.08Feed water mass flow rate, mw1[kg/s]

2

4

6

8

10

     P    r    o     d    u    c     t      i    v      i     t    y     [     l      i     t      /     h    r     ]

Tw1=60 oC

Tw1=70 oC

 

Figure 10 Effect of inlet hot water mass flow rateon productivity.

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0 10 20 30Time [min]

16

18

20

22

24

26

28

     T    a      1     [

      o      C     ]

mcw

= 0.11 kg/s, ma= 0.055 kg/s

Tcwi= 19  oC, Tamb= 17 oC

 Figure 11 Inlet air temperatures, Ta1, of the

humidifier at different feed water mass flow rates

and inlet water temperature of 50°C.

0 10 20 30Time [min]

15

20

25

30

35

40

     T    a      3

     [      o      C     ]

mcw= 0.11 kg/s, ma= 0.055 kg/s

Tcwi= 19  o C, Tamb= 17  o C

 

Figure 12 Outlet air temperatures, Ta3, of the

humidifier at different feed water mass flow ratesand inlet water temperature of 50 °C.  

Figure 12 depicts the transient variation ofthe humidifier outlet air temperature, Ta3,with constant air mass flow rate at differentfeed water mass flow rates and constantinlet water temperature of 50 °C. Theincrease of feed water mass flow rateincreases the wet-bulb temperature of theair leaving the humidifier and approachesto the inlet feed water temperature.Consequently, the moisture content of theair leaving the humidifier increases and asignificant improvement on the system

 productivity can be achieved by increasingthe feed water mass flow rate. 

Figure 13 illustrates the transient variationof the system productivity with differentvalues of air mass flow rate at constant

feed water flow rate of 0.06 kg/s, constantinlet water temperature of 50 °C, coolingwater of constant mass flow rate of 0.11kg/s, and constant inlet cooling watertemperature of 19 °C. The system reachesthe steady-state production after about 20minutes due to thermal inertia of thesystem. The results also show that thesystem is capable of producing 4.26 lit/hrunder the previously mentioned conditionsand air mass flow rate of 0.1 kg/s.

0 10 20 30Time [min]

0

1

2

3

4

5

     P    r    o     d    u    c     t      i    v      i     t    y     [     l      i     t      /     h    r     ]

ma=0.03 kg/s

ma=0.055 kg/s

ma=0.08 kg/s

ma=0.1 kg/s

mw1= 0.06 kg/s, m cw= 0.11 kg/s

Tcwi

= 19   oC, Tamb

= 17  o C

 

Figure 13 Transient production at different air massflow rates and inlet water temperature of 50°C. 

0.02 0.04 0.06 0.08 0.1Air mass flow rate, ma[kg/s]

4

5

6

7

8

     P    r    o     d

    u    c     t      i    v      i     t    y     [     l      i     t      /     h    r     ]

Tw1=60 oC

Tw1=70 oC

 

Figure 14 Effect of air mass flow rate on steady-state system productivity.

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It can be observed from Figure 14 that the productivity of the system increases byincreasing the air mass flow rate. This may

 be due to the fact that increasing the airmass flow rate increases the entrainedwater vapour and, therefore, increases the

 productivity. In addition, increasing the airmass flow rate increases the mass and heattransfer inside the humidifier, whichincreases the rate of vaporization of waterand, hence, the productivity. 

Figure 15 depicts the transient variation ofthe system productivity with differentvalues of cooling water mass flow rate atconstant feed water mass flow rate of 0.06kg/s and inlet water temperature of50°C.The system reaches the steady-state

 production after about 20 minutes due tothermal inertia of the system. Thenumerical results illustrate that the systemcould produce 3.98 lit/hr under the

 previously mentioned conditions andcooling water mass flow rate of 0.15 kg/s.

The results shown in Figure 16  illustratethe effect of cooling water mass flow rateon the steady-state system productivity. Byincreasing the cooling water mass flowrate, significant drop in the surface

temperature of the cooler coil can beachieved and consequently, an increase ofthe rate of the condensation of the watercontent carried by the flowing air isachieved on the dehumidifier coil surface.

6. ConclusionsAn experimental study was performed toinvestigate the effect of different operatingconditions on the productivity of thedesalination system using the

humidification-dehumidification (HDH) process and to validate the developedmathematical model of the system.

The numerical results accord with theexperimental data in values and trend withdeviation ranges from 1.3% to 12.1% inthe steady-state system productivity fordifferent operating conditions.

0 10 20 30Time [min]

0

1

2

3

4

5

     P    r    o     d    u    c     t      i    v      i     t    y     [     l      i     t      /     h    r     ]

mw1= 0.06 kg/s, ma= 0.055 kg/s

Tcwi = 19  o C, T amb = 17  o C

 

Figure 15Transient production at different massflow rates of cooling water for a constant feed watermass flow rate of 0.06 kg/s and constant inlet water

temperature of 50°C.

0.04 0.08 0.12 0.16 0.2Cooling water mass flow rate, mcw[kg/s]

4

5

6

7

8

     P    r    o     d    u    c     t      i    v      i     t    y     [     l      i     t      /     h    r     ]

Tw1=60 oC

Tw1=70 oC

 Figure 16 Effect of cooling water mass flow rate on

the steady-state system productivity. 

Both experimental data and numericalresults show that the production increases

 by increasing feed water mass flow rate,air mass flow rate, cooling water massflow rate, and feed water temperature.

For the considered system in the presentstudy, the numerical results depict that thedecentralized HDH desalination systemwith feed water mass flow rate of 0.08 kg/sat constant inlet water temperatures of 70°C, 60 °C, 50 °C and 36 °C, is capable of

 producing steadily 8.42 lit/hr, 6.18 lit/hr,4.32 lit/hr and 2.31 lit/hr, respectively.

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While the experimental data show that thesystem produces 8.04 lit/hr, 5.76 lit/hr,4.08 lit/hr and 2.16 lit/hr for the sameoperating conditions.

Table 1 shows a comparison between this study andKabeel et al [10].

Researcher namePresentstudy

Kabeel[10]

Max. Product[kg/h]

8.04 23

Feed watertemperature,

Tw1[⁰C]70 50:90

Feed water massflow rate,ṁw1[kg/s]

0.080.01667

:0.06667

Deviation between Exp &

Theo

12.1% /5.9 %(max

/mean)

13% /7-8%

(max/mean)

Appendix

The values of constants used are;

Cpw=Cpwater=Cpcw=Cpprod=4.2kJ/kg.K

Cpair=Cpa= 1.007 kJ/kg.KCppacking= 1.9 kJ/kg.K

Cpmetal=0.42 kJ/kg.K for steel, 1.67kJ/kg.K for plastic

The parameters measured whileexperimental runs;

Tw1 (Feed water temperature)

ṁw1 (Feed water mass flow rate)

ṁa (Air mass flow rate)

ṁcw (Cooling water mass flow rate)ṁ prod (Production mass flow rate)

Ta3 (Outlet air temperature of humidifier)

Ta1 (Inlet air temperature of humidifier)

Tcwi (Cooling water inlet temperature)

Tcwo (Cooling water outlet temperature)

Tamb (Ambient air temperature)

The parameters Computed while numericalruns;

All other parameters computednumerically while using the numericalmodel.

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