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The humidification-dehumidification desalination process is a promising technique to desalinate seawater. To improve the output of a desalination unit the baffle plates are placed in the dehumidifier. The effect of variables on freshwater production was studied experimentally. Two packing materials are used and compared with previous researchers. Operating variables include the air and water temperature at the entry to the humidifier, the humidifier water flow rate, airflow rate and the cooling water flow rate into the dehumidifier. The experimental setup, which includes a geyser for water heating, air heater, humidifier and a baffle plates in the dehumidifier. The results show that by enhancing the temperature up to 60 ºC of inlet water supplied to the humidifier,0.785 kg/hr-m 2 freshwater was produced. Also, increase in the water flow rate in the humidifier, and the flow rate of cooling water in the dehumidifier, which in turn increases the output of the desalination system with increased production of distilled water. The inlet temperature of cooling water reduced from 30 ºC to 20 ºC enhances the production of distilled water significantly. The productivity increases by 60 % with baffle plates inside dehumidifier as compared to without baffles.
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Journal of Engineering Science and Technology
Vol. 15, No. 2 (2020) 768 - 777
© School of Engineering, Taylor’s University
769
EXPERIMENTAL STUDY ON DESALINATION SYSTEM USING
HUMIDIFICATION- DEHUMIDIFICATION PROCESS WITH
BAFFLES IN THE DEHUMIDIFIER
KUMARA1,*, G. VEERSHETTY2, D. H. ASHEBIR3
1,2Department of Mechanical Engineering, National Institute of Technology,
Karnataka, Surathkal, 575025, India
3Department of Mechanical Engineering, Bule Hora University, Bule Hora, Ethiopia
*Corresponding Author: kumsmechs@gmail.com
Abstract
The humidification-dehumidification desalination process is a promising technique
to desalinate seawater. To improve the output of a desalination unit the baffle plates
are placed in the dehumidifier. The effect of variables on freshwater production was
studied experimentally. Two packing materials are used and compared with
previous researchers. Operating variables include the air and water temperature at
the entry to the humidifier, the humidifier water flow rate, airflow rate and the
cooling water flow rate into the dehumidifier. The experimental setup, which
includes a geyser for water heating, air heater, humidifier and a baffle plates in the
dehumidifier. The results show that by enhancing the temperature up to 60 ºC of
inlet water supplied to the humidifier,0.785 kg/hr-m2 freshwater was produced.
Also, increase in the water flow rate in the humidifier, and the flow rate of cooling
water in the dehumidifier, which in turn increases the output of the desalination
system with increased production of distilled water. The inlet temperature of
cooling water reduced from 30 ºC to 20 ºC enhances the production of distilled
water significantly. The productivity increases by 60 % with baffle plates inside
dehumidifier as compared to without baffles.
Keywords: Humidification-dehumidification, potable water, heat transfer, baffle
plates.
769 Kumara et al.
Journal of Engineering Science and Technology April 2020, Vol. 15(2)
1. Introduction
Seventy-five percent of the earth is covered with water, however, there is a crisis for
drinking water in many parts of the world. It is a basic need to have sufficient quality
and quantity of water for today and tomorrow. The consequences of industrialisation
and urbanisation results in an increased requirement of freshwater. In the earlier days,
living things such as humans, plants, and animals have depended on natural resources
such as ponds, rivers, and underground water for their basic need. There are many
technologies to produce freshwater, however, they are consuming more energy and
affects the environment.
Desalination of seawater seems to be a promising technology, and this can be done
by various processes. While many processes like Reverse Osmosis (RO),
Electrodialysis, and Multi-Stage flash distillation seem promising they involve a higher
cost for set-up and maintenance, which cannot be afforded by developing countries. In
this regard, desalination by Solar-heated humidification-dehumidification process
seems to have an upper hand. This paper studies the effects of operating variables on
the quantity of freshwater produced by experimentation and tries to increase the yield
by a certain construction inside a dehumidifier.
Rajaseenivasan and Sridhar [1] investigated experimental work on humidification
dehumidification (HDH) desalination integrated with dual-purpose solar collectors,
which are used to heat simultaneously air and water. Air is heated with different
tubular configuration to increase the turbulence so that the efficiency of the system is
improved. Highest productivity with tubular collector was 15.23 kg/m2-day. Kumara
and Veershetty [2] studied theoretical desalination process using humidification
dehumidification and its performance using mathematical programming under the
climatic conditions of Surathkal, India. Soufari et al. [3] performed nonlinear
programming based on humidification dehumidification to solve for 3 objectives
improving the production and minimising the energy consumption. They concluded
that the entry humidifier water temperature and the ratio of air to water are significant
parameters.
Yildirim and Solmus [4] investigated the study on humidification
dehumidification desalination for the climatic condition of Antalya, Turkey. They
compared theoretical and experimental work and found that heating of water has a
significant effect on the production of water. The finite difference method was used
to design the humidification and dehumidification by applying thermodynamics laws
within the humidifier and dehumidifier was carried out by El Kader et al. [5]. Similar
works were carried out by Nematollathi et al. [6], Amer et al. [7] and Bourouni et al.
[8] and they concluded that air and water temperature have the most effect in the
production of freshwater.
Theoretical investigation on the HDH system is carried out using double pass solar
air heater with two glass covers in the region of Ankara, Turkey. The energy equations
are solved numerically. The productivity of the system is increased up to 8% by using
double pass solar air heater and decreased by about 30% without double pass solar air
heater under the same operating conditions [9]. A review of solar energy-based
technologies has been done [10]. Similar work had been done by Alnaimat [11],
Hussein [12, 13], Hussein et al. [14, 15] and Li et al. [16]. Ahmed et al. [17] carried
experimental work using aluminium sheets packing material. The literature shows that
the humidification dehumidification desalination system has better technology to
Experimental Study on Desalination System using Humidification-. . . . 770
Journal of Engineering Science and Technology April 2020, Vol. 15(2)
produce freshwater when compared to the solar still. It is observed that the ability of
the system to produce freshwater increases with the humidifier inlet temperature.
An attempt has been made to conduct an experiment on humidification-
dehumidification with baffle plate plates in the dehumidifier. The detailed schematic
diagram is shown above Fig. 1. The air is blown to the humidifier after heating with the
heating coil, which is placed at the bottom of the humidifier. Seawater is heated in the
Geyser and sprayed through the water sprinklers. Air gets humidified and sent to a
dehumidifier with baffle plates wherein moistures contained in the hot humid air get
condensed by cooling water supplied from the separate water tank.
Fig. 1. Detailed schematic humidifier and dehumidifier desalinator
of the experimental setup T1-T6: K-type thermocouples, v1, v2:
Control valves, Twb, Tdb-wet and dry bulb temperature.
2. Experimental setup
The experimental setup is shown in Fig. 2(a) has three main parts namely humidifier
(evaporator), dehumidifier (condenser) and electric heater. The system works on a
closed cycle for water and open cycle for the air stream. The facility is used for the
distillation of water. The humidifier constructed from M S material of rectangular
section of 0.4 m by 0.4 m and height 1 m with a provision of air inlet from the bottom
and includes the spray water from the sprinklers at the top and includes distribution
of air to the top and water collection tank at the bottom. The humidifier is connected
using flange with nuts and bolts to dehumidifier through the rectangular bent duct as
shown in the following figure. The polypropylene packing material is placed in the
humidifier to improve the wettability of air and water particles. The airflow rate is
measured using the anemometer.
Geyger
Humidifier
Blower
Packing
material
Hot air
cold water
out
Hot water
spray
water storage tank
pump
Air flow
Water flow
heater
T3
T5
T1
T2
T6
baffles
cold water in
dehumidifier
fresh water
T4
v1
v2
water tank
T7
T8
Twb1,Tdb1
Twb2,Tdb2 Twb3,Tdb3
humid air
771 Kumara et al.
Journal of Engineering Science and Technology April 2020, Vol. 15(2)
The dehumidifier is of the rectangular cross-section of 0.2 m by 0.2 m fabricated
from M S material, and total length 0.8 m. The copper coils having 100 mm outer
diameter and 90 mm inner diameter with a total length of 4.4 m is placed. The 6 baffle
plates are placed in a skewed manner in between to have better condensation. The
dehumidifier is connected using flange having nuts and bolts. The cold water is supplied
through copper coils to dehumidify the humid air coming out from humidifier. The
fabrication of dehumidifier with baffle plates is shown in Fig. 2(b). The baffle plates
are placed to improve dehumidifier performance. The temperature of the inlet and outlet
air is measured by using two thermocouples.
The packing material is used in the humidifier to increase the contact time
between the water and the air. The two-packing material such a polypropylene and
paddy grass are shown in Fig. 3.
(a) Photo of the experimental
setup without insulation.
(b) Photo of the baffle plates in the
dehumidifier without copper tubes.
Fig. 2. Photo of the experimental setup without insulation.
(a) Paddy grass.
(b) Polypropylene.
Fig. 3. Packing material in the humidifier.
Experimental Study on Desalination System using Humidification-. . . . 772
Journal of Engineering Science and Technology April 2020, Vol. 15(2)
Measuring instruments
Many parameters are measured while conducting experiments to evaluate the
freshwater production such as temperatures of air and water at the inlet and outlet
of the humidifier and dehumidifier, mass flow rates of the air and water, relative
humidity of air at inlet and outlet of the humidifier and dehumidifier and the
freshwater production. Rotameters are used to measure the flow rate water (range:
0.01 - 5 LPM) with an accuracy of ±0.01 LPM. The calibration is done by
comparing the amount of water collected for a particular time interval. The digital
anemometer with an accuracy of ±0.1 m/s is used to measure air velocity. Three
Psychrometer (to measure wet and dry bulb temperature K-type thermocouple are
used with error range being 1.75 %) set up was used to measure humidity at
different locations (accuracy: ±2 RH). The freshwater production is measured by
collecting the amount of fresh water collected in a graduated flask during the
prescribed time interval. The stopwatch was used to note down the time interval.
The error analysis is shown in Table 1. The measuring instruments used in the
experimentation is shown in Fig 4.
Table 1. Experimental error analysis.
Instruments
Accuracy
Range
% Error
Thermocouple
± 0.15 0C
0 - 100 0C
2
Humidity sensor
± 2.0 RH
0 - 100 % RH
1.75
Air flow meter
± 0.1 m/s
0.02 - 5
1.5
Water flow meter
± 0.1 m/s
0.01 - 4
0.5
Fig. 4. Photo images of measuring instruments.
773 Kumara et al.
Journal of Engineering Science and Technology April 2020, Vol. 15(2)
3. Results and Discussion
The parameters, which affect the freshwater production of the system are discussed
one by one like water flow rate (Mw, kg/s), inlet water temperature (Twi, ºC), airflow
rate (Ma, kg/s), inlet air temperature (Tai, ºC), cooling water temperature, cooling
water flow rate (Mcw, kg/s) and comparison of productivity with and without baffle
plates is finally discussed.
3.1. Effect of humidifier inlet water flow rate
Observations were made to study the effectiveness of humidifier, as shown in Fig. 5.
With the variation in the supply of feed water to the humidifier from 0.01 kg/s to 0.04
kg/s, the productivity was reduced, with the reason being that the temperature of feed
water supplied is less, and thereby decreasing the evaporation rate. While as the mass
flow rate of air into the humidifier was increased from 0.01 kg/s to 0.04 kg/s, the
humidifier performance improved.
This can be attributed to the increase in the heat and mass transfer coefficients
and hence, increases the moisture carried by air. The freshwater production decreases
with an increase of flow rate of air when it is maintained flow rate of the water
constant. This may be because increasing the airflow rate decreases the heat absorbed
by the air and hence, its temperature inside the humidifier, thereby it is capacity to
carry vapour.
3.1.1. Effect of inlet water temperature
Figure 6 provides an insight into the variation of productivity of the HDH system
with the variation of inlet water temperature, i.e., the feed water. It was noted that
with the increase in feed water temperature up to 60 ºC, the productivity of the
humidifier increased to 0.785 kg/hr/m2, this can be attributed to the increased
ability of air to get humidified at a higher temperature. Also, when the feed water
rate is increased the productivity of humidifier increased for the same temperature
of feed water.
0.010 0.015 0.020 0.025 0.030 0.035 0.040
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Mfw / kg-1 m-2
l/kgs-1
Ma=0.04 kg/s
Ma=0.03 kg/s
Ma=0.02 kg/s
Ma=0.01 kg/s
30 35 40 45 50 55 60
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Mfw/kg h-1 m-2
T1 / 0C
Mw=0.04kg/s
Mw=0.03kg/s
Mw=0.02kg/s
Mw=0.01kg/s
Fig. 5. Effect of feed water
flow rate on unit productivity.
Fig. 6. Effect of inlet temperature
on unit productivity.
Experimental Study on Desalination System using Humidification-. . . . 774
Journal of Engineering Science and Technology April 2020, Vol. 15(2)
3.1.2. Effect of inlet air temperature
Figure 7 presents the mass rate of freshwater collected when the temperature of inlet air
was varied for a different mass flow rate of air (0.01 kg/s to 0.04 kg/s). For a higher mass
flow rate of air, there is higher productivity for a given inlet temperature of the air. The
productivity increases as there is an increased supply of air, which absorbs more vapour,
in other words, there is more air to absorb moisture. For a fixed supply of air, the ability
to absorb moisture increased when the temperature of the air was increased, this is because
the ability of air to absorb moisture is more at higher temperatures from psychrometric
principles.
3.1.3. Effect of inlet cooling water temperature
Figure 8 presents the change in freshwater productivity with a change in cooling water
temperature flowing through the copper pipes in the dehumidifier for different mass flow
rates (0.05 kg/s to 0.15 kg/s) of cooling water. It is seen that decreasing cooling water inlet
temperature from 30 ºC to 20 ºC the productivity gradually increases. This may be
possible because of decreased heat transfer rate between air and water through the coil as
a result of lower temperature difference according to Newtons law of cooling. Also, the
increase in the cooling water rate reduces the production of freshwater due to less time
for the heat transfer between the humid air and the cold water.
30 32 34 36 38 40
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Mfw/ kg h-1m-2
T4/ 0C
Ma=0.01 kg/s
Ma=0.02 kg/s
Ma=0.03 kg/s
Ma=0.04 kg/s
20 25 30 35 40
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Mfw/ kg h-1 m-2
Cooling water inlet temperature/0C
Mcw=0.15 kg/s
Mcw=0.01kg/s
Mcw=0.05kg/s
Fig. 7. Effect of inlet air
temperature on unit productivity.
Fig. 8. Effect of cooling water inlet
temperature on unit productivity.
3.1.4. Effect of baffle plates
The non-condensable gas, which accounts ranging from 40-90% in the moisture leads
to a significant reduction in the heat transfer in the dehumidification process.
Therefore, it is necessary to have baffle plates in the condenser to have better
condensation. Hence, baffle plates are placed in the dehumidifier to increase heat
transfer. The experimental results with baffle plates in the dehumidifier were
recorded and compared with the dehumidifier without baffle plates. Figure 9 shows
the effect of baffle plates on the unit performance of the system. The productivity is
increased with baffle plates and this may be due to turbulence generated in the humid
air and increased contact time of moist air with the copper pipe inside the
dehumidifier. As a result, there is a better condensation heat transfer coefficient,
hence, improved unit productivity.
775 Kumara et al.
Journal of Engineering Science and Technology April 2020, Vol. 15(2)
Fig. 9. Effect of baffle plates on unit performance.
3.1.5. Comparison with previous work
Paddy grass packing material is unique because the production of freshwater is more
when compared to other materials. Table 2 shows the comparison of present work
with the previous work with different packing materials like paddy grass and
polypropylene. The freshwater production is better for paddy grass packing material
due to the increase in the contact time between the air and water.
Table 2. Comparison between present work with previous literature results.
Mw
(kg/s)
Ma
(kg/s)
T1
C)
T4
C)
Packing specification
(m3)
Dehumidifier
area (m2)
Production
Reference
kg/hr
kg/m2-
hr
kg/m3
0.01
0.04
68.9
43
Cellulose packing
V = 0.3 × 0.3 x 0.4
3.5
1.45
0.41
40
Ahmed et al.
[17]
0.04
0.01
60
40
Paddy grass packing
V = 0.25 × 0.25 × 0.25
1.25
0.735
0.58
47
Current study
0.04
0.01
60
40
Polypropylene
packing
V = 0.25 × 0.25 × 0.25
1.25
0.465
0.372
29
Current study
0.031
0.0316
44
28
Metal packing
V = 0.33 × 0.3 × 1.7
1.6
3.2
2
19
Ahmed et al.
[17]
4. Conclusions
The humidifier and dehumidifier with baffle plates were fabricated and tested. The
parameters, which affect the production of freshwater is studied experimentally using
humidification-dehumidification technology with the baffle plates in the dehumidifier.
The results indicate that by increasing the temperature of inlet water (up to 60 ºC)
supplied to the humidifier, the water flow rate in the humidifier, and the flow rate of
cooling water in the dehumidifier increases, which in turn increases the output of the
desalination system with increased production of distilled water. It is found that there
is an increased yield of 0.785 kg/hr/m2 in the system with the baffles. The productivity
increases by almost 60 percent with baffle plates inside dehumidifier as compared to
without baffles. The inlet temperature of cooling water temperature significantly
0.010 0.015 0.020 0.025 0.030 0.035 0.040
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Mfw/ kg h-1 m-2
FR per unit length kg s-1 m-2
without baffles
with baffles
Ma=0.01 kg/s,Mw=0.04 kg/s
T1=60 0C
Experimental Study on Desalination System using Humidification-. . . . 776
Journal of Engineering Science and Technology April 2020, Vol. 15(2)
enhances the production of freshwater. Also, the comparison of two different packing
materials is done with the previous literature work.
Nomenclatures
A
Area, m2
Ma
Mass flow rate of air, kg/s
Mcw
Mass of cooling water flow rate, kg/s
Mw
Mass flow rate of water, kg/s
Mfw
Mass of freshwater, kgh-1m-2
T1
Inlet humidifier water temperature, ºC
T2
Bottom humidifier water temperature, ºC
T3
Water storage tank temperature, ºC
T4
Humidifier inlet air temperature, ºC
T5
Inlet humidifier temperature, ºC
T6
Outlet humidifier temperature, ºC
T7
Inlet cooling water temperature, ºC
T8
Outlet cooling water temperature, ºC
Tdb
Dry bulb temperature, ºC
Twb
Wet bulb temperature, ºC
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The aim of this study is to investigate theoretically the effect of different system operating conditions, types of air heater, and some different design parameters and a weather condition on a solar water desalination system performance under the climatological conditions of Ankara (40 °N,33 °E), Turkey. For this purpose, a computer simulation program based on the mathematical model is developed by means of MATLAB software. In this simulation program, the fourth order Runge-Kutta method is used to solve the energy balance equations numerically. The desalination unit is configured by a double-pass flat plate solar air heater with two glass covers, humidifying tower, storage tank and dehumidifying exchanger. The system used in this work is based on the idea of closed water and open air cycles. Air is heated by using a double-pass solar air heater whereas water is not heated.