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Energy saving potential in humidification-dehumidification desalination system
Energy xxx (2016) 1e13
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Energy saving potential in humidification-dehumidification desalination system C. Muthusamy b, K. Srithar a, * a b
Department of Mechanical Engineering, Thiagarajar College of Engineering, Madurai 625015, Tamilnadu, India Department of Mechanical Engineering, Sethu Institute of Technology, Kariapatti 626115, Tamilnadu, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 April 2016 Received in revised form 3 September 2016 Accepted 24 October 2016 Available online xxx
Humidification dehumidification desalination (HDH) system is viewed as an auspicious technique for medium level investment and productivity. The objective of this work is to enhance the productivity with the saving of input power in a modified HDH system by various changes in its components. Inserts like short length taper twisted tape; internally finned cut out conical turbulator and half perforated circular inserts with various orientations and three different pitch ratios (PR) are used in the air heater. Two types of packing materials (Gunny bag and saw dust) are employed in humidifier section and two different dehumidifier are tested to choose the good one and it is further integrated with spring inserts of different PR to enhance its performance. The best combination is identified when the air heater equipped with divergent twisted tape of PR 3, humidifier furnished with gunny bag and dehumidifier fixed with spring insert of PR 3. Higher productivity of 0.8 kg/h with the reduction in salinity (3.2 mg/l of chloride content) attained with 40% saving of input power in the modified HDH desalination system. A noticeable saving in energy with significant development in energy and exergy efficiency is observed. The economic analysis is also carried out. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Humidification-dehumidification desalination Twisted tape Conical insert Circular insert Energy analysis Exergy analysis
1. Introduction There are several methods for purifying the saline water like Multi-stage flash distillation, Multi-effect distillation, Vapor compression distillation, Electro dialysis, Solar desalination, Humidification-dehumidification desalination and Reverse osmosis. Humidification-dehumidification (HDH) found to be suitable for medium level productivity with affordable initial and running cost. The major components of HDH desalination system are air heater, water heater, humidifier and dehumidifier. A detailed review has been carried out on the above said components. Most of the researches generally used inserts or additional devices to enhance the turbulence effect to the air flows in an air heater for different applications. Eiamsa-ard et al. [1] made a comparative study on oblique delta winglet twisted tape and straight delta winglet twisted tape in an air flow inside a circular tube. Result confirmed that due to more effective turbulence in the above said insert, heat transfer rate was inflated. Eiamsa-ard et al.
* Corresponding author. E-mail address: [email protected] (K. Srithar).
[2] also presented an experimental study on air flow characteristics in a tube fitted with full length twisted tape insert and short-length twisted tape. The short-length tape generated a strong swirl flow at the tube entry and enhanced the heat transfer rate. Tandiroglu and Ayhan [3] developed an energy dissipation analysis for the hot air flows through the tube assembled with a series of half circled baffle inserts. These inserts were examined with different pitch ratios. Baffles in the tube accelerated the dissipation energy compared to the plain tube without insert. . Akansu [4] presented a numerical analysis on heat-transfer and pressure drop in an air flow through a pipe fitted with porous rings with various pitch ratios. In the result, decrease in the pitch ratio caused an increase in friction factor and augmented the heat transfer rate. The effect of using nozzle turbulators on heat transfer rate with convergent (C) and divergent (D) mode turbulators in an air flow had been carried out by Promvonge and Eiamsa-ard [5]. It was understood that this work could effectively be utilized with divergent-type turbulators having pitch ratio of 2. Kongkaitpaiboon et al. [6] experimented the performance of perforated conical rings in an air heater. Results revealed that the heat transfer was enhanced by modifying the thermal boundary layer in the air flow.
http://dx.doi.org/10.1016/j.energy.2016.10.098 0360-5442/© 2016 Elsevier Ltd. All rights reserved.
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Some of the research works dealt with the performance of double pipe and shell and tube heat exchanger are presented below. Yildiz et al. [7] investigated the performance of double-pipe heat exchanger fitted with propellers in the flow passage and predicted that the overall heat transfer coefficient increased with increase in mass flow rate and number of propellers. Durmas et al. [8] showed that by using snail type swirl generators, the heat transfer rate in concentric tube heat exchanger was augmented at the low Reynolds number ranges. Amer et al. [9] carried out the experiment in a HDH desalination system by changing various operating conditions and different packing materials (gunny bag and wooden slates) in the humidifier and showed that the system productivity increases with increase in mass flow rate of water and wooden slates contributed the maximum productivity compared to other packing material. Chang et al. [10] investigated the operational and performance characteristics of a multi-effect solar desalination system based on HDH process. The use of porous plastic balls packed in the humidifier enhanced the evaporation rate. Narayan and Lienhard [11] conducted an energy consumption analysis in two different types of HDH system such as open air closed water and closed air open water systems. Chehayeb et al. [12] presented the mathematical model for HDH system consisting of a packed bed humidifier and a multi-tray bubble column dehumidifier. The study evaluated the effect of mass flow rate ratio on the performance of fixed size system. Garaway and Grossman [13] tested the wetting performance of different sheets such as cotton cloths, synthetic cloths, wool, paper and plastic as the packing material in the humidifier of HDH desalination system. Results revealed that the tight weave cotton cloth proved better performance and higher productivity compared to other packing materials. Al-Enezi et al. [14] presented the effect of operating parameters such as temperature and flow rate of feed water, air and cooling water on the productivity in a HDH desalination system. Maximum productivity attained for high feed water temperature of the
humidifier and air mass flow rate and low cooling water temperature of the dehumidifier. Ashrafizhadeh and Amidpour [15] conducted an exergy analysis in the HDH system and found that the mass transfer does not have any effect on the total exergy loss of HDH system. Shaobo Hou et al. [16] conducted the exergy analysis in the components of solar HDH desalination process and identified that the solar collector has the lowest exergy efficiency. Xiong et al. [17] proposed a baffled shell and tube desalination system to perform humidification and dehumidification simultaneously at tube and shell side of the single column. The baffled plate significantly enhanced the productivity of the column. Numerous research works have been carried out to enhance the heat transfer rate in the air heater and condenser by using insert for various applications. There is a research gap available to introduce such insert in the various components of HDH system. Previous researchers used half circled baffle inserts, plain cut out conical inserts and uniform width twisted tape for heat transfer enhancement for other application and leaves a gap of introducing half perforation in full circled baffle, fins in cut out conical turbulator and tapered width twisted tape for enhancing the heat transfer in the air heater of the HDH system. Two different packing materials, such as random type (saw dust) and cloth type (gunny bag) are used to augment the humidifying effect in the humidifier. Some of the works utilized the double pipe or shell and tube dehumidifier for HDH desalination system. It leaves the gap to perform the comparative study of those two types of condenser in HDH desalination system. Also the performance of dehumidifier is enhanced by using spring inserts. 2. Experimentation A laboratory scaled HDH desalination system is constucted for this study, which mainly consists of an air heater, water heater, humidifier and dehumidifier. Fig. 1 shows the schematic sketch of the HDH desalination system. The air heater made with iron pipe, 1000 mm in length and 38 mm in diameter. It is centrally covered
Fig. 1. Experimental setup of HDH desalination system.
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by an electric band heater of 500 W to supply the constant heat input. Another iron pipe with 13 mm diameter and 500 mm length is used as the water heater. A 1000 W band heater is attached centrally around the water heater unit to supply uniform heat. The humidifier chamber is a plastic tube of 152 mm diameter and 800 mm height. Outlet of the humidifier is connected to the
3
dehumidifier. The double pipe condenser, with its inner pipe made of an aluminium with 32 mm diameter and 1000 mm long. The outer tube channel has 63.5 mm diameter and 1000 mm length. Another dehumidifier is a shell and tube condenser constructed with one shell and 5 tube passes with 13 mm tube diameter and 1000 mm length and covered by the outer shell of 152 mm
Fig. 2. (a) Half perforated circular inserts (b) Angular representations(c) Cut out conical inserts-Convergent type (d) Cut out conical inserts-Divergent type (e) Conical turbulator with internal profile (f) Twisted tape e Convergent type (g) Twisted tape -Divergent type.
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diameter. The blower is supplied the atmospheric air to the air heater (1e2 in Fig. 1) in which air is heated and moves to the bottom of humidifier. Saline water passes through the water heater (3e4) gets heated by the electrical band heater and sprinkled from top of the humidifier. Due to low relative humidity of hot air, more water vapor absorbed by the air and the outlet air leaves with almost in saturated condition (7) and directed to the dehumidifier. The remaining salt water collected in saline water tank (9). The cold water from the cold water tank (5) flows through the outer pipe of the shell and tube condenser. The hot, humid air rejects its latent heat to the cooling water and the condensate produced is collected in the distillate tank (8). Counter flow is maintained between the above two fluids to achieve maximum heat transfer rate. The condensed water is collected at the distillate tank. The remaining salt water collected in saline water tank and hot water tank are recirculated to feed water tank to recover the waste heat. The air and water temperatures at inlet and outlet of heaters, humidifier and dehumidifier are measured at steady state. Steady state reaches within 15 min in all experiments. K type thermocouples are used to measure the temperatures. The relative humidity at the inlet and outlet of humidifier and dehumidifier are measured by hygrometer. Mass flow rate of the hot water inlet to the humidifier and the mass flow rate of hot air inlet to the humidifier are measured by the orifice meter. Manometer is used to measure the pressure drop of the air. 2.1. Modifications 2.1.1. Air flow system modifications The heat transfer enhancement in the air heater is achieved by integrating three different inserts such as, half perforated circular inserts, cut out conical turbulators and short length taper twisted tape with PR 3,4 and 5. 2.1.1.1. Half perforated circular inserts. Fig. 2 (a) shows the half perforated circular inserts made of aluminium sheet with a diameter of 33 mm. Various pitch ratios are assembled by changing the number of inserts in a fixed length of circular rod. Fig. 2(b) shows the linear arrangement of half-perforated circular inserts. Each insert is rotated with reference to the previous insert to make the three different angle of orientation in the sequence of inserts. 2.1.1.2. Internally fined cut out-conical turbulators. The tapered fins are integrated in the internal surface of the conical cut-out turbulators. Larger base of the turbulator has the diameter of 33 mm and a smaller base of turbulator has the diameter of 21 mm, with the length of 100 mm each. The conical turbulators are arranged in convergent (C-type) as well as divergent patterns (D-type) as shown in Fig. 2 (c) and (d). Fig. 2 (e) shows the internal profile of the internally finned cut out conical turbulator. 2.1.1.3. Taper twisted tape. Fig. 2 (f) and (g) shows the arrangement of twisted tape insert with convergent and divergent modes which are made of aluminium sheet has the width of 33 mm at the larger end and 14 mm at the smaller end. The twisted tape insert is set in the air heater with three PR of 3, 4 and 5. 2.1.2. Humidifier modifications Packing materials are used to increase the contact time and contact area between the feed water and air. Packing materials can be classified in to regular arrangement (plastic cylinder), random arrangement (saw dust, leaves) and cloth type packing (gunny bag,
jute). The factors influencing the choice of packing are water retaining capacity, porosity, pressure drop and cost. Based on this parameters gunny bag and saw dust packing materials are used in this present work. Two layers of packing materials are set with equal intervals in the humidifier chamber as shown in Fig. 1. 2.1.3. De-humidifier modifications The performance of shell and tube dehumidifier further augmented by using spring inserts with PR 3 and 4. Spring inserts made by winding 2 mm thick steel wire are incorporated in all the inner tubes of shell and tube condenser. 3. Error analysis The minimum error that occurs in any instrument is the ratio between its least count and the least value of the output measured. Error analysis is carried out for the thermometer, temperature indicator, manometer, voltmeter, ammeter and hygrometer. The uncertainties in the measurements is [19] involved both the fixed error of the instrument and random error observed during various measurements. The characteristics of the various measuring instruments used in the experiments are given in Table 1. Uncertainties associated with the dependent variables like Reynolds number, friction factor and Nusselt number are also estimated using the following equations [1].
DRe Re
Df f
" ¼ "
¼
2
Dm
þ
m
2
DðDpÞ Dp
Dd
(1)
d
þ
2 #0:5
2
DL L
#0:5 Dd 2 DRe þ 3 þ 2 d Re
2 2 2 ðDVÞ 2 DI Dd DTs 2 ¼4 þ þ þ þ Nu V I d Ts
DNu
DTf Tf
(2) !2 30:5 5 (3)
The calculations pointed out that the uncertainties involved are ± 0.5% for Reynolds number, ± 1.4% for friction factor and ± 0.14% for Nusselt number. The experimental results are reproducible within these uncertainty ranges. 4. Data reduction Based on the experimental results, some of the correlations used to find out various factors involved in these experiments are discussed in this chapter. 4.1. Air flow system The internal convective heat transfer coefficient can be found out by applying energy balance at the air heater. Table 1 Accuracies and ranges of measuring instruments. Instrument
Range
Accuracy
Uncertainty,%
Temperature indicator Manometer Orifice meter Ammeter Voltmeter Hygrometer
0e500 C 500 mm 500 mm 0e20 amp 0e220 V 0e100%
1 C 1 mm 1 mm 0.01 amp 0.1 V 1
± ± ± ± ± ±
3.3 4 4 0.2 0.2 1
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Energy generated in the air heater ¼ Energy convected from wall to air.Where the energy convected from wall to air is the sum of energy absorbed by air and losses through insulation. (10% of total energy convected). Energy absorbed by air ¼ Energy convected through walls to airlosses
ma Cpa ðT2 T1 Þ ¼ 0:9ha Aah Ts Tb;a þ 0:1xLosses
(4)
ma Cp;a ðT2 T1 Þ 0:9Aah Ts Tb;a
(5)
The fully developed flow is considered and the Nusselt number is evaluated [1].
Nu ¼
ha d ka
(6)
½ðT7 T6 Þ ðT8 T5 Þ ln TT87TT65
Q ¼ Uc Ac LMTDc
Uc ¼
mcw Cp;w ðT6 T5 Þ Ac LMTDc
(7)
The friction factor computed by the pressure drop across the length of the test section can be determined [1] by,
2dDР rLu2a
(8)
Thermal performance factor can be arrived from the following equation. It is defined as the ratio of the Nusselt number ratio to the friction factor ratio at the same pumping power. It can be used to evaluate the performance of air heater.
0 1 ht
hp B C TEF ¼ @ 0:333 A
(9)
ft fp
4.2. Humidifier The humidifier energy balance is with reference to Fig. 1 is given below. Energy gained by hot water in humidifier ¼ Energy absorbed by hot air
(10)
The overall mass transfer coefficient is calculated from [14]
DWA ¼¼ 0
m2 m4
ðH7 H2 Þ 1
(11)
@ðH4 H7 ÞðH9H2 ÞA ln
(14)
H4 H7 H9 H2
Effect of different packing materials analysed by using corresponding mass transfer coefficient produced in a humidifier.
4.3. Dehumidifier Applying energy balances in dehumidifier. Latent energy given by hot fluid ¼ Sensible energy gained by cold fluid
(15)
(16)
4.4. Energy and exergy analysis Energy analysis conducted to find out the efficiency of HDH desalination system. Gained output ratio is the ratio between the product of distilled water produced and the latent heat of vaporization to the thermal energy input
GOR ¼
mdw hfg QT
(17)
Exergy is defined as available useful work. Exergy is a combination property of a system and its environment depends on both system and surrounding. Exergy is always destroyed when temperature changes in the process. The use of exergetic analysis provide the details of the improvement needed for the components of HDH desalination system. The exergy analysis is conducted with the help of experimental values.
hII ¼
mw4 Cp;w ðT4 T9 Þ ¼ ma ðH7 H2 Þ
(13)
Overall heat transfer coefficient found out for the spring insert with various orientations. This equation is used to investigate the performance of spring inserts used in dehumidifier. The distilled water flow rate depends upon specific humidity
mdw ¼ ma ðW7 W8 Þ
Gleinski equation is given by Nu ¼ 0:0214 Re0:8 100 Pr 0:4
f ¼
LMTDC ¼
(12)
Overall heat transfer coefficient UC can be found out by
The heat transfer coefficient thus obtained as,
ha ¼
Q ¼ mcw Cp;w ðT6 T5 Þ ¼ ma ðH7 H8 Þ
5
Exrecovered Exinput
(18)
Exrecovered ¼ mðexo exi Þ
(19)
Exergy input ¼ Hn Ho
(20)
Exergy of air flow through the air heater; exn ¼ ðHn Ho Þ To ðSn So Þ
(21)
Specific exergy of water flows through the water heater and dehumidifier,
Tn exn ¼ Cp;w ðTn T0 Þ T0 Cp;w ln T0
(22)
Specific exergy of saturated air, Hou et al. [16]
Tn exn ¼ Cp;a þ Wo Cp;v ðTn To Þ To Cp;a þ Wo Cp;v ln To Pn ðRda þ Wn Rv Þln þ To ðRa Po 1 þ 1:6078Wo Wn þ Wn Rv Þln þ 1:6078Wa Ra ln 1 þ 1:6078Wn Wo (23)
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before water heater and dehumidifier, three different mass flow rates of humidifier feed water (42, 75 and 110 kg/h) and three flow rates of dehumidifier feed water (29, 74 and 110 kg/h) are selected. Experiment is conducted to identify the best water flow rate by keeping the dehumidifier cold water flow rate as constant and varying the humidifier feed water flow rate. The experiments is repeated by keeping constant feed water flow rate and varying different dehumidifier cooling water flow rate. In both the cases [Fig. 4 (a) & (b)] the experiments are conducted with different air flow rate ranging between 13 kg/h and 21 kg/h. Maximum productivity is obtained for higher dehumidifier water flow rate (110 kg/h) and lower humidifier feed water flow rate (42 kg/h). Higher dehumidifier water flow rate causes increase in the heat capacity (mCp) of the water. This ensures more heat absorption from the hot fluid (hot humid air) in the dehumidifier which leads to augmented productivity. By lowering the humidifier feed water flow rate, the retention time between the feed water and air is increasing and so the specific humidity increases more. This in turn increases the productivity.
Fig. 3. Test section verification.
5.3. Effect of inserts on performance of air heater The exergy efficiency of the components is calculated from the above equations. The lower the exergy efficiency represents higher the exergy loss. From the above discussion, the component in which exergy losses taken place is identified for the further improvement. 5. Results and discussions 5.1. Plain tube verification Experiments are conducted on the plain tube and the results are compared with the previous correlations given by Gnielinski. Fig. 3 displays the comparison between Nusselt number calculated from Gnielinski relation and computed values obtained from the present experiment. Also it is found that the present plain air heater tube data is in 6% of agreement with the Gnielinski correlations.
Fig. 5 shows the variation of heat transfer coefficient with mass flow rate for different types of inserts. The increase of Reynolds number indicates the turbulence enhancement and due to that heat transfer rate is augmented. Extreme heat transfer rate attained for low PR due to utmost flow disturbance. The heat transfer coefficient increases with the increase in orientation angle and reaches to a maximum value at 180 due to the abrupt change in flow direction and more turbulence effect. Conical turbulator with divergent mode creates more flow reversion along the flow direction due to sudden enlargement and shows better heat transfer enhancement. The twisted tape inserts create radial and axial flow with strong turbulence intensity leading to higher heat transfer augmentation. Also centrifugal force is generated in fluid flow in the tube with twisted tape. Pressure loss developed was much higher due to increase in friction surfaces of these turbulators and interruption to the flow.
5.2. Effect of water flow rate 5.4. Effect of air heater inserts on productivity First phase of experiments are conducted to identify and fix the best mass flow rate of feed water to humidifier and cold water to dehumidifier. Depends on available specification of valves fitted
Fig. 6 shows the effect of various orientations of air heater inserts on heat transfer, relative humidity and output distilled water.
Fig. 4. Effect of feed water and cooling water.
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Fig. 5. Effect of inserts in performance of air heater.
The usage of inserts increases the air temperature which simultaneously reduces the relative humidity of the air enters into the humidifier. Due to the low relative humidity of air, the capability to absorb more amount of moisture from feed water is increased and hence high humid air come from humidifier. The maximum productivity of 0.6 kg/h, 0.52 kg/h and 0.4 kg/h is attained for twisted tape with divergent, circular insert (180 ) and cut out conical insert with divergent respectively. These selected orientations are used for further analysis. 105 sets of experiments are conducted using all the inserts with all possible orientations and PR to identify these best PR and orientation.
5.5. Performance of packing materials on productivity Packing materials helps to increase the dispersion of water droplets, contact time and contact area. About 100 sets of experiments have been conducted and based on data reduction, Fig. 7 is drawn which shows the variation of mass transfer coefficient in the humidifier for the air heater inserts with twisted tape in divergent
mode for PR 3 and two different packing material in the humidifier. Comparing to both type of packing materials, gunny bag has the best water retaining capacity due to its nature of porous void. Even though the saw dust has the good water retaining capacity, it block the air flow after some time. The mass transfer coefficient also increases with temperature and mass flow rate of air. The increase of mass transfer rate increases the productivity. The maximum productivity of 0.640 kg/h is attained for the system with air heater equipped by the twisted tape insert of pitch ratio 3 and gunny bag in humidifier. From the discussion in Section 5.4, among all the air heater inserts studied in the experiment, PR 3 is found to be better and in the current study of the two packing materials gunny bag is found better. These parameters are considered for further analysis.
5.6. Effect of spring inserts on productivity Spring inserts enhances the turbulent flow motion of the humidified air and there by better cooling between hot humid air and
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Fig. 6. Effect of air heater inserts in productivity.
Fig. 7. Effect of packing materials on performance of humidifier.
cold water which helps for higher production. Secondary flow or vortices are generated due to curvature of the coils. It has a significant effect on heat transfer enhancement. Fig. 8 (a) reveals the overall heat transfer coefficient of the humid air flows through the inner pipe of shell and tube condenser equipped with spring inserts. Condensation depends on enhancement of heat transfer rate in dehumidifier in addition to that cooling effect produced by the cooling water circuit. The mass flow rate and the relative humidity of hot and humid air passing through the inner tube of the condenser have an effect over the mass of condensate produced. Fig. 8 (b) shows the productivity of the HDH desalination system corresponding to the effect of spring inserts with the pitch ratios of 3 and 4. The overall heat transfer coefficient increases with the increase in mass flow rate of air. This effect is higher in case of low
PR of 3. The maximum productivity obtained is about 0.8 kg/h at the air flow rate of 21 kg/h. 5.7. Gained output ratio It is the overall performance indicator of HDH desalination system compared with the input power. Gained output ratio is maximum for the modified HDH system compared to the normal HDH system without any inserts. It indicates that the saving of power is attained in the modified HDH desalination system Fig. 9. 5.8. Exergy losses Exergy analysis is a process of evaluating the losses takes places in its components. Fig. 10 represents the second law efficiency of
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Fig. 8. Effect of spring inserts on performance of de humidifier.
the individual components of HDH system when it is provided with previously identified orientation of inserts in its components. Exergetic efficiency of the air heater progressively increases with the reduction in pitch ratio, increase in orientation angle for circular insert and with divergent type for remaining two type of inserts. It is observed that maximum exergy efficiency obtained in air heater, dehumidifier and humidifier are in the subsequent order and minimum exergy efficiency observed in water heater. So, maximum scope is available for improvement of productivity by concentrating on water heater, humidifier and dehumidifier. Air heater equipped with twisted tape of PR 3 gives highest exergy efficiency. The exergy efficiency increased by an increase in evaporation rate.
5.9. Effect of inserts on energy and exergy Fig. 11 shows the comparision between variation of Reynolds number with the first and second law efficiency. First law efficiency represents the effective utilization of input power. Second law efficiency is the ratio between reversible work to the useful work for the work consuming devices. The increase of exergy efficiency pointed out that the maximum productivity attained in the process with minimum useful work input. The increase of Reynolds number creates lesser effect on second law efficiency compared to first law efficiency and the usage of inserts increases the exit temperature of air which simultaneously increases the humid air generation with high temperature. 5.10. Productivity of modified HDH system Fig. 12 shows that the productivity of modified HDH desalination system. In which the twisted tape (Divergent) in air heater, gunny bag in humidifier and spring insert in dehumidifier with PR 3 shows two times of higher productivity compared to conventional system for the same input power. Comparing the productivity, normal system produces the maximum distillate of 0.40 kg/h with the first law efficiency of 20% and second law efficiency of 15% but the enhanced system equipped with inserts shows energy efficiency of 44% and second law efficiency of 38% with 50% increase of its productivity. The energy and exergy efficiency enhanced to gain a high energy recovery rate. 5.11. Correlation of experimental results
Fig. 9. Performance of Modified HDH desalination system.
The taguchi regression analysis with a level of L8 is used to predict the productivity of HDH system using previously selected best performed inserts in air heater, dehumidifier and gunny bag in humidifier for any intermediate value between the designed range. Eq. (24) represents the productivity of system with half perforated circular inserts in air heater for the above said conditions. Most of the predicted correlated values lies within ± 9% when compared with experimental productivity. The regression equation for the distilled water productivity in terms of the various parameters is given below. This regression equation is applicable for the range of
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Fig. 10. Exergy efficiency at various location with various inserts.
Fig. 11. Effect of inserts on the energy and exergy efficiency of HDH system.
mass flow rate 14 kg/h to 21 kg/h, circular inserts (180 ) pitch ratio ranging between 3 and 5, relative humidity between 55% and 65% and spring inserts pitch ratio 3 to 4.
mdw ¼ 810 þ 7:14ma 45PRAH þ 0:5F 55PRSP
(24)
Similar procedure repeated for cut out conical inserts and the newly arrived equation is given below. This regression equation is applicable for the range of mass flow rate 14 kg/h to 21 kg/h, conical insert (D) pitch ratio ranging between 3 and 5, relative humidity between 60% and 80% and spring inserts pitch ratio 3 to 4.
mdw ¼ 493 þ 13:6ma 17:5PRAH 1:75F 5PRSP
(25)
Following equation represent the regression equation for twisted tape turbulators (D). This regression equation is applicable for the range of mass flow rate 14 kg/h to 21 kg/h, twisted tape (D) pitch ratio ranging between 3 and 5, relative humidity between 30%
to 45% and spring inserts pitch ratio 3 to 4.
mdw ¼ 1053 þ 7:68ma 56:9PRAH 1:75F 86:3PRSP
(26)
Fig. 13, reveal the most of the values fall in the acceptable range with the variation of ± 9% and ± 6%. The productivity depends upon mass flow rate and relative humidity of air. 5.12. Comparison of previous results The present results are compared with the previous related works in the humidification dehumidification desalination system and given in Fig. 14. Based on mass flow rate ranges, two previous researchers work has been selected. Cemil yamali introduced the concept of integrating a double pass solar air heater with HDH desalination. Agouz conducted an experiment in bubble column HDH desalination system.
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Fig. 14. Comparison with previous results [20,21].
Table 2 Water analysis. Type of water
Fig. 12. Productivity of modified HDH system.
For the same mass flow rate ranges, Cemil yamali's work shows a larger productivity of 0.3 kg/h and Agouz obtained the distillate of 1.3 kg/h whereas current work shows an yield of 0.8 kg/h with in the twisted tape in divergent mode, 0.7 kg/h in the circular insert of 180 and 0.650 kg/h for the cut out conical insert in divergent mode with gunny bag in humidifier and spring insert in dehumidifier with divergent mode of PR 3. Maximum deviation of productivity between our work is 38% lesser than the highest value of Cemil yamali's work and 62% higher than the lowest productivity of Agouz's work. Present results follow the same trend of previous results. Mostly the productivity increased in this present work is moderately higher compared to the related works.
pH
Total suspended Hardness Chloride Turbidity Solids mg/l mg/l mg/l NTU
Salt water 8.2 40 Distilled water 7.8 8 Drinking water 6.5 to 8.5 100e200 (As per IS 10500)
158 147.5 300
32.90 3.19 250
11 7 10
5.13. Water analysis The chemical analysis of input salt water and final condensed distilled water are conducted in the Sethu Institute of Technology, Madurai, to identify the suitability of the distillate for various purposes. The analysis details are given in Table 2. 5.14. Economic analysis The Payback period of the experimental setup depends on the
Fig. 13. Correlation using air heater inserts.
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C. Muthusamy, K. Srithar / Energy xxx (2016) 1e13
overall cost of fabrication, operating cost and maintenance cost.
The overall fabrication cost
¼
Daily productivity of the desalination unit (8 h) Cost of water produced
¼ 6.8 litres ¼
Electrical power used per hour (¼1.9 units) Cost of electricity per day (8 h)
¼
Total cost of mineral per day
¼
Payback period ¼ Investment/net earnings
¼
7500 (136$)
68 (1.25$)
22.80 (0.412$) 1.36(0.024$) 7500 (136$)/
. 43.84 (0.81$)
¼ 171days
Using economic analysis, cost of distilled water is estimated. In addition to the capital cost of the HDH desalination system (136 $), other parameters such as sinking fund factor, annual salvage value and maintenance cost and interest rate per year (i) should be considered. The capital recovery factor is defined interms of interest per year and also number of life year (n) of the system and is given by Ref. [18].
CRF ¼
ið1 þ iÞn ð1 þ iÞn 1
(27)
Here interest rate per year (i) is taken as 12% for life year (n) of 10 years. Fixed annual cost (FAC) is the product of capital cost (P) and capital recovery factor.
FAC ¼ P ðCRFÞ
(28)
By taking the salvage value of system S equal to 20% of capital cost; Sinking fund factor (SFF) and annual salvage value (ASV) can be expressed by Ref. [18].
SFF ¼ 1 ðð1 þ iÞn 1
(29)
ASV ¼ SFF S
(30)
The AMC which is annual maintenance operational cost of the system. Here, 15% of fixed annual cost is considered as maintenance cost:
AMC ¼ 0:15FAC
(31)
M is the average annual yield of the HDH system. In this study the amount of M has been calculated based on daily water productivity. It is found that using inserts with a negligible cost results in a cost reduction by nearly 9% and actual cost (AC) is given by, Cost per litre, is determined by [18].
CPL ¼ AC=M
(32)
AC ¼ FAC þ AMC ASV
(33)
The cost of fresh water produced with this modified HDH desalination system is decreased when compared to the normal HDH desalination system of without any inserts. 6. Conclusion The productivity of HDH desalination system enhanced by changing the flow behavior in their components using the insertion of new type of inserts and also two different types of packing
material in humidifier. The efficiency of each individual components augmented and its effect utilized for further enhancement of overall system productivity. With the attainment of induced heat transfer rate in an air heater, using twisted tape in divergent mode of PR 3 helps to increase the evaporation rate in humidifier. The performance enhancement of humidifier further augmented by utilising gunny bag. Suitable type of condenser selected to condense the enhanced quantity of saturated air. With the chosen shell and tube dehumidifier performance further accelerated by using the spring inserts with PR 3 and productivity of 0.8 kg/h attained with 40% saving of input power in the modified HDH desalination system. Energy and exergy analysis are identified the utilization level of input energy and the available and unavailable level of energy in the components. Present work with different best configurations is compared with two previous results. Present results follow the same trend of previous results. Mostly the productivity moderately increased in this present work is higher compared to the related works. Payback period of such a system is found as 171 days and it is one time low cost investment method. References [1] Eiamsa-ard S, Wongcharee S, Eiamsa-ard K, Thianpong P. Heat transfer enhancement in a tube using delta-winglet twisted tape inserts. Appl Therm Eng 2010;30:310e8. [2] Eiamsa-ard S, Thianpong C, Eiamsa-ard P, Promvonge P. Convective heat transfer in a circular tube with short-length twisted tape insert. Int Commun Heat Mass Transf 2009;36:365e71. [3] Tandiroglu A, Ayhan T. Energy dissipation analysis of transient heat transfer for turbulent flow in a circular tube with baffle inserts. Appl Therm Eng 2006;26:178e85. [4] Akansu SO. Heat Transfer and Pressure drops for porous-ring turbulators in a circular pipe. Appl Energy 2006;83:280e98. [5] Promvonge P, Eiamsa-ard S. Heat transfer and turbulent friction in a circular tube fitted with conical nozzle turbulators. Int Commun Heat Mass Transf 2007;34:72e82. [6] Kongkaitpaiboon V, Nanan K, Eiamsa-ard S. Experimental investigation of heat transfer and turbulent flow friction in a tube fitted with perforated conical rings. Int Commun Heat Mass Transf 2010;37:560e7. [7] Yildiz C, Yaar Biger, Dursun Pehllvan. Influence of fluid rotation on the heat transfer and pressure drop in double pipe heat exchanger. Appl Energy 1996;54:49e56. [8] Durmus Aydin, Durmus Ayla, Esen Mehet. Investigation of heat transfer and pressure drop in a concentric heat exchanger with snail entrance. Appl Therm Eng 2002;22:321e32. [9] Amer EH, Kotb H, Mostafa GH, El-Ghalban AR. Theoretical and experimental investigation of humidification e dehumidification desalination unit. Desalination 2009;249:949e59. [10] Chang Z, Zheng H, Yang Y, Su Y, Zhanchun Duan Z. Experimental investigation of a novel multi-effect solar desalination system based on humidificationdehumidification process. Renew Energy 2014;69:253e9. [11] Narayanan GP, Lienhard JH. Thermal design of Humidificationdehumidification systems for affordable small scale desalination. IDA J 2014;4(3):24e34. [12] Chehayeb KM, Narayan GP, Zubair SM, Lienhard JH. Thermodyamicbalancing of a fixed-size two-stage humidification dehumidification desalination. Desalination 2015;369:125e39. [13] Garaway I, Grossman. Investigation of a solar-powered desalination system employing regeneration. Desalination 2006;197(1e3):63e74. [14] Al-Enezi G, Ettoney H, Fawzy N. Low temperature humidificationdehumidification desalination system. Energy Convers Manag 2006;47: 470e84. [15] Ashrafizadeh SA, Amidpour M. Exergy analysis of humidificationedehumidification desalination systems using driving forces concept. Desalination 2012;285:108e16. [16] Hou Shaobo, Zeng Dongqi, Yeb Shengquan, Zhang Hefei. Exergy analysis of the solar multi-effect humidificationedehumidification desalination process. Desalination 2007;203:403e9. [17] Xiong RH, Wang SC, Xie LX, Wang Z, Li PL. Experimental investigation of a baffled shell and tube desalination column using the humidification dehumidification process. Desalination 2005;180:253e61. [18] Kianifar A, Zeinali heris S, Mahian O. Exergy and economic analysis of a pyramid shaped solar water purification system: active and passive cases. Energy 2012;38:31e6. [19] Barford NC. Experimental measurements: precision, error and truth. second ed. Newyork: John Wiley&sons; 1990. [20] Yamali Cemil, Solmus Ismail. Theoretical investigation of a humidification and dehumidification desalination system configured by a double pass flat plate
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C. Muthusamy, K. Srithar / Energy xxx (2016) 1e13 solar air heater. Desalination 2007;205:163e77. [21] EI-Agouz SA. A new process of desalination by air passing through sea water based on humidificationedehumidification process. Energy 2010;35: 5108e14.
Nomenclature I: Current, A d: Diameter of the test tube, m H: Enthalpy, J/kg S: Entropy, J/K Ex: Exergy flow rate, J/s f: Friction factor R: Gas constant, J/kgK h: Heat transfer coefficient, W/m2K Q: Heat transfer rate, W A: Heat transfer surface area, m2 hfg: Latent heat of vaporization, J/kg K L: Length of the test tube, m LMTD: Logarithmic mean temperature difference, K m: Mass flow rate, kg/ s DWA: Mass transfer coefficient, kg/s NTU: Nephelometric turbidity units Nu: Nusselt number U: Overall heat transfer coefficient, W/m2 K pd: Partial pressure of dry air, Pa pv: Partial pressure of vapor, Pa PR: Pitch ratio (pitch length/diameter) Pr: Prandtl number P: Pressure, Pa Re: Reynolds number ex: Specific exergy, J/kg Cp: Specific heat capacity at constant pressure, J /kg K W: Specific humidity, kg water/kg dry air T: Temperature, K k: Thermal conductivity of air, W/mK TEF: Thermal enhancement factor QT: Total input power, W ua: Velocity, m/s V: Voltage, Volts
13
D: Change in parameter h: Efficiency b: Orientation angle, degree
ɸ: Relative humidity, % ℮: Density, kg/m3 Subscripts
a: Air ah: Air heater 1: Air heater input 2: Air heater output o: Ambient atm: Atmosphere cw: Cooling water 5: Cooling water inlet 6: Cooling water outlet c: condenser con: Convection dehu: Dehumidifier dw: Distilled water 8: Distilled water output from condenser ex: Exergy gen: generated hu: Humidifier 7: Humid air inlet at condenser in: Inlet b: Mean out: Outlet P: Plain tube n: Location sat: Saturated 9: Salt water exit from humidifier s: Surface sp: Spring insert t: Tube with insert v: Vapor w: Water wh: Water heater 3: Water heater input 4: Water heater output
Greek Symbols
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Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Energy saving potential in humidification-dehumidification desalination system C. Muthusamy b, K. Srithar a, * a b
Department of Mechanical Engineering, Thiagarajar College of Engineering, Madurai 625015, Tamilnadu, India Department of Mechanical Engineering, Sethu Institute of Technology, Kariapatti 626115, Tamilnadu, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 April 2016 Received in revised form 3 September 2016 Accepted 24 October 2016 Available online xxx
Humidification dehumidification desalination (HDH) system is viewed as an auspicious technique for medium level investment and productivity. The objective of this work is to enhance the productivity with the saving of input power in a modified HDH system by various changes in its components. Inserts like short length taper twisted tape; internally finned cut out conical turbulator and half perforated circular inserts with various orientations and three different pitch ratios (PR) are used in the air heater. Two types of packing materials (Gunny bag and saw dust) are employed in humidifier section and two different dehumidifier are tested to choose the good one and it is further integrated with spring inserts of different PR to enhance its performance. The best combination is identified when the air heater equipped with divergent twisted tape of PR 3, humidifier furnished with gunny bag and dehumidifier fixed with spring insert of PR 3. Higher productivity of 0.8 kg/h with the reduction in salinity (3.2 mg/l of chloride content) attained with 40% saving of input power in the modified HDH desalination system. A noticeable saving in energy with significant development in energy and exergy efficiency is observed. The economic analysis is also carried out. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Humidification-dehumidification desalination Twisted tape Conical insert Circular insert Energy analysis Exergy analysis
1. Introduction There are several methods for purifying the saline water like Multi-stage flash distillation, Multi-effect distillation, Vapor compression distillation, Electro dialysis, Solar desalination, Humidification-dehumidification desalination and Reverse osmosis. Humidification-dehumidification (HDH) found to be suitable for medium level productivity with affordable initial and running cost. The major components of HDH desalination system are air heater, water heater, humidifier and dehumidifier. A detailed review has been carried out on the above said components. Most of the researches generally used inserts or additional devices to enhance the turbulence effect to the air flows in an air heater for different applications. Eiamsa-ard et al. [1] made a comparative study on oblique delta winglet twisted tape and straight delta winglet twisted tape in an air flow inside a circular tube. Result confirmed that due to more effective turbulence in the above said insert, heat transfer rate was inflated. Eiamsa-ard et al.
* Corresponding author. E-mail address: [email protected] (K. Srithar).
[2] also presented an experimental study on air flow characteristics in a tube fitted with full length twisted tape insert and short-length twisted tape. The short-length tape generated a strong swirl flow at the tube entry and enhanced the heat transfer rate. Tandiroglu and Ayhan [3] developed an energy dissipation analysis for the hot air flows through the tube assembled with a series of half circled baffle inserts. These inserts were examined with different pitch ratios. Baffles in the tube accelerated the dissipation energy compared to the plain tube without insert. . Akansu [4] presented a numerical analysis on heat-transfer and pressure drop in an air flow through a pipe fitted with porous rings with various pitch ratios. In the result, decrease in the pitch ratio caused an increase in friction factor and augmented the heat transfer rate. The effect of using nozzle turbulators on heat transfer rate with convergent (C) and divergent (D) mode turbulators in an air flow had been carried out by Promvonge and Eiamsa-ard [5]. It was understood that this work could effectively be utilized with divergent-type turbulators having pitch ratio of 2. Kongkaitpaiboon et al. [6] experimented the performance of perforated conical rings in an air heater. Results revealed that the heat transfer was enhanced by modifying the thermal boundary layer in the air flow.
http://dx.doi.org/10.1016/j.energy.2016.10.098 0360-5442/© 2016 Elsevier Ltd. All rights reserved.
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Some of the research works dealt with the performance of double pipe and shell and tube heat exchanger are presented below. Yildiz et al. [7] investigated the performance of double-pipe heat exchanger fitted with propellers in the flow passage and predicted that the overall heat transfer coefficient increased with increase in mass flow rate and number of propellers. Durmas et al. [8] showed that by using snail type swirl generators, the heat transfer rate in concentric tube heat exchanger was augmented at the low Reynolds number ranges. Amer et al. [9] carried out the experiment in a HDH desalination system by changing various operating conditions and different packing materials (gunny bag and wooden slates) in the humidifier and showed that the system productivity increases with increase in mass flow rate of water and wooden slates contributed the maximum productivity compared to other packing material. Chang et al. [10] investigated the operational and performance characteristics of a multi-effect solar desalination system based on HDH process. The use of porous plastic balls packed in the humidifier enhanced the evaporation rate. Narayan and Lienhard [11] conducted an energy consumption analysis in two different types of HDH system such as open air closed water and closed air open water systems. Chehayeb et al. [12] presented the mathematical model for HDH system consisting of a packed bed humidifier and a multi-tray bubble column dehumidifier. The study evaluated the effect of mass flow rate ratio on the performance of fixed size system. Garaway and Grossman [13] tested the wetting performance of different sheets such as cotton cloths, synthetic cloths, wool, paper and plastic as the packing material in the humidifier of HDH desalination system. Results revealed that the tight weave cotton cloth proved better performance and higher productivity compared to other packing materials. Al-Enezi et al. [14] presented the effect of operating parameters such as temperature and flow rate of feed water, air and cooling water on the productivity in a HDH desalination system. Maximum productivity attained for high feed water temperature of the
humidifier and air mass flow rate and low cooling water temperature of the dehumidifier. Ashrafizhadeh and Amidpour [15] conducted an exergy analysis in the HDH system and found that the mass transfer does not have any effect on the total exergy loss of HDH system. Shaobo Hou et al. [16] conducted the exergy analysis in the components of solar HDH desalination process and identified that the solar collector has the lowest exergy efficiency. Xiong et al. [17] proposed a baffled shell and tube desalination system to perform humidification and dehumidification simultaneously at tube and shell side of the single column. The baffled plate significantly enhanced the productivity of the column. Numerous research works have been carried out to enhance the heat transfer rate in the air heater and condenser by using insert for various applications. There is a research gap available to introduce such insert in the various components of HDH system. Previous researchers used half circled baffle inserts, plain cut out conical inserts and uniform width twisted tape for heat transfer enhancement for other application and leaves a gap of introducing half perforation in full circled baffle, fins in cut out conical turbulator and tapered width twisted tape for enhancing the heat transfer in the air heater of the HDH system. Two different packing materials, such as random type (saw dust) and cloth type (gunny bag) are used to augment the humidifying effect in the humidifier. Some of the works utilized the double pipe or shell and tube dehumidifier for HDH desalination system. It leaves the gap to perform the comparative study of those two types of condenser in HDH desalination system. Also the performance of dehumidifier is enhanced by using spring inserts. 2. Experimentation A laboratory scaled HDH desalination system is constucted for this study, which mainly consists of an air heater, water heater, humidifier and dehumidifier. Fig. 1 shows the schematic sketch of the HDH desalination system. The air heater made with iron pipe, 1000 mm in length and 38 mm in diameter. It is centrally covered
Fig. 1. Experimental setup of HDH desalination system.
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by an electric band heater of 500 W to supply the constant heat input. Another iron pipe with 13 mm diameter and 500 mm length is used as the water heater. A 1000 W band heater is attached centrally around the water heater unit to supply uniform heat. The humidifier chamber is a plastic tube of 152 mm diameter and 800 mm height. Outlet of the humidifier is connected to the
3
dehumidifier. The double pipe condenser, with its inner pipe made of an aluminium with 32 mm diameter and 1000 mm long. The outer tube channel has 63.5 mm diameter and 1000 mm length. Another dehumidifier is a shell and tube condenser constructed with one shell and 5 tube passes with 13 mm tube diameter and 1000 mm length and covered by the outer shell of 152 mm
Fig. 2. (a) Half perforated circular inserts (b) Angular representations(c) Cut out conical inserts-Convergent type (d) Cut out conical inserts-Divergent type (e) Conical turbulator with internal profile (f) Twisted tape e Convergent type (g) Twisted tape -Divergent type.
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diameter. The blower is supplied the atmospheric air to the air heater (1e2 in Fig. 1) in which air is heated and moves to the bottom of humidifier. Saline water passes through the water heater (3e4) gets heated by the electrical band heater and sprinkled from top of the humidifier. Due to low relative humidity of hot air, more water vapor absorbed by the air and the outlet air leaves with almost in saturated condition (7) and directed to the dehumidifier. The remaining salt water collected in saline water tank (9). The cold water from the cold water tank (5) flows through the outer pipe of the shell and tube condenser. The hot, humid air rejects its latent heat to the cooling water and the condensate produced is collected in the distillate tank (8). Counter flow is maintained between the above two fluids to achieve maximum heat transfer rate. The condensed water is collected at the distillate tank. The remaining salt water collected in saline water tank and hot water tank are recirculated to feed water tank to recover the waste heat. The air and water temperatures at inlet and outlet of heaters, humidifier and dehumidifier are measured at steady state. Steady state reaches within 15 min in all experiments. K type thermocouples are used to measure the temperatures. The relative humidity at the inlet and outlet of humidifier and dehumidifier are measured by hygrometer. Mass flow rate of the hot water inlet to the humidifier and the mass flow rate of hot air inlet to the humidifier are measured by the orifice meter. Manometer is used to measure the pressure drop of the air. 2.1. Modifications 2.1.1. Air flow system modifications The heat transfer enhancement in the air heater is achieved by integrating three different inserts such as, half perforated circular inserts, cut out conical turbulators and short length taper twisted tape with PR 3,4 and 5. 2.1.1.1. Half perforated circular inserts. Fig. 2 (a) shows the half perforated circular inserts made of aluminium sheet with a diameter of 33 mm. Various pitch ratios are assembled by changing the number of inserts in a fixed length of circular rod. Fig. 2(b) shows the linear arrangement of half-perforated circular inserts. Each insert is rotated with reference to the previous insert to make the three different angle of orientation in the sequence of inserts. 2.1.1.2. Internally fined cut out-conical turbulators. The tapered fins are integrated in the internal surface of the conical cut-out turbulators. Larger base of the turbulator has the diameter of 33 mm and a smaller base of turbulator has the diameter of 21 mm, with the length of 100 mm each. The conical turbulators are arranged in convergent (C-type) as well as divergent patterns (D-type) as shown in Fig. 2 (c) and (d). Fig. 2 (e) shows the internal profile of the internally finned cut out conical turbulator. 2.1.1.3. Taper twisted tape. Fig. 2 (f) and (g) shows the arrangement of twisted tape insert with convergent and divergent modes which are made of aluminium sheet has the width of 33 mm at the larger end and 14 mm at the smaller end. The twisted tape insert is set in the air heater with three PR of 3, 4 and 5. 2.1.2. Humidifier modifications Packing materials are used to increase the contact time and contact area between the feed water and air. Packing materials can be classified in to regular arrangement (plastic cylinder), random arrangement (saw dust, leaves) and cloth type packing (gunny bag,
jute). The factors influencing the choice of packing are water retaining capacity, porosity, pressure drop and cost. Based on this parameters gunny bag and saw dust packing materials are used in this present work. Two layers of packing materials are set with equal intervals in the humidifier chamber as shown in Fig. 1. 2.1.3. De-humidifier modifications The performance of shell and tube dehumidifier further augmented by using spring inserts with PR 3 and 4. Spring inserts made by winding 2 mm thick steel wire are incorporated in all the inner tubes of shell and tube condenser. 3. Error analysis The minimum error that occurs in any instrument is the ratio between its least count and the least value of the output measured. Error analysis is carried out for the thermometer, temperature indicator, manometer, voltmeter, ammeter and hygrometer. The uncertainties in the measurements is [19] involved both the fixed error of the instrument and random error observed during various measurements. The characteristics of the various measuring instruments used in the experiments are given in Table 1. Uncertainties associated with the dependent variables like Reynolds number, friction factor and Nusselt number are also estimated using the following equations [1].
DRe Re
Df f
" ¼ "
¼
2
Dm
þ
m
2
DðDpÞ Dp
Dd
(1)
d
þ
2 #0:5
2
DL L
#0:5 Dd 2 DRe þ 3 þ 2 d Re
2 2 2 ðDVÞ 2 DI Dd DTs 2 ¼4 þ þ þ þ Nu V I d Ts
DNu
DTf Tf
(2) !2 30:5 5 (3)
The calculations pointed out that the uncertainties involved are ± 0.5% for Reynolds number, ± 1.4% for friction factor and ± 0.14% for Nusselt number. The experimental results are reproducible within these uncertainty ranges. 4. Data reduction Based on the experimental results, some of the correlations used to find out various factors involved in these experiments are discussed in this chapter. 4.1. Air flow system The internal convective heat transfer coefficient can be found out by applying energy balance at the air heater. Table 1 Accuracies and ranges of measuring instruments. Instrument
Range
Accuracy
Uncertainty,%
Temperature indicator Manometer Orifice meter Ammeter Voltmeter Hygrometer
0e500 C 500 mm 500 mm 0e20 amp 0e220 V 0e100%
1 C 1 mm 1 mm 0.01 amp 0.1 V 1
± ± ± ± ± ±
3.3 4 4 0.2 0.2 1
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Energy generated in the air heater ¼ Energy convected from wall to air.Where the energy convected from wall to air is the sum of energy absorbed by air and losses through insulation. (10% of total energy convected). Energy absorbed by air ¼ Energy convected through walls to airlosses
ma Cpa ðT2 T1 Þ ¼ 0:9ha Aah Ts Tb;a þ 0:1xLosses
(4)
ma Cp;a ðT2 T1 Þ 0:9Aah Ts Tb;a
(5)
The fully developed flow is considered and the Nusselt number is evaluated [1].
Nu ¼
ha d ka
(6)
½ðT7 T6 Þ ðT8 T5 Þ ln TT87TT65
Q ¼ Uc Ac LMTDc
Uc ¼
mcw Cp;w ðT6 T5 Þ Ac LMTDc
(7)
The friction factor computed by the pressure drop across the length of the test section can be determined [1] by,
2dDР rLu2a
(8)
Thermal performance factor can be arrived from the following equation. It is defined as the ratio of the Nusselt number ratio to the friction factor ratio at the same pumping power. It can be used to evaluate the performance of air heater.
0 1 ht
hp B C TEF ¼ @ 0:333 A
(9)
ft fp
4.2. Humidifier The humidifier energy balance is with reference to Fig. 1 is given below. Energy gained by hot water in humidifier ¼ Energy absorbed by hot air
(10)
The overall mass transfer coefficient is calculated from [14]
DWA ¼¼ 0
m2 m4
ðH7 H2 Þ 1
(11)
@ðH4 H7 ÞðH9H2 ÞA ln
(14)
H4 H7 H9 H2
Effect of different packing materials analysed by using corresponding mass transfer coefficient produced in a humidifier.
4.3. Dehumidifier Applying energy balances in dehumidifier. Latent energy given by hot fluid ¼ Sensible energy gained by cold fluid
(15)
(16)
4.4. Energy and exergy analysis Energy analysis conducted to find out the efficiency of HDH desalination system. Gained output ratio is the ratio between the product of distilled water produced and the latent heat of vaporization to the thermal energy input
GOR ¼
mdw hfg QT
(17)
Exergy is defined as available useful work. Exergy is a combination property of a system and its environment depends on both system and surrounding. Exergy is always destroyed when temperature changes in the process. The use of exergetic analysis provide the details of the improvement needed for the components of HDH desalination system. The exergy analysis is conducted with the help of experimental values.
hII ¼
mw4 Cp;w ðT4 T9 Þ ¼ ma ðH7 H2 Þ
(13)
Overall heat transfer coefficient found out for the spring insert with various orientations. This equation is used to investigate the performance of spring inserts used in dehumidifier. The distilled water flow rate depends upon specific humidity
mdw ¼ ma ðW7 W8 Þ
Gleinski equation is given by Nu ¼ 0:0214 Re0:8 100 Pr 0:4
f ¼
LMTDC ¼
(12)
Overall heat transfer coefficient UC can be found out by
The heat transfer coefficient thus obtained as,
ha ¼
Q ¼ mcw Cp;w ðT6 T5 Þ ¼ ma ðH7 H8 Þ
5
Exrecovered Exinput
(18)
Exrecovered ¼ mðexo exi Þ
(19)
Exergy input ¼ Hn Ho
(20)
Exergy of air flow through the air heater; exn ¼ ðHn Ho Þ To ðSn So Þ
(21)
Specific exergy of water flows through the water heater and dehumidifier,
Tn exn ¼ Cp;w ðTn T0 Þ T0 Cp;w ln T0
(22)
Specific exergy of saturated air, Hou et al. [16]
Tn exn ¼ Cp;a þ Wo Cp;v ðTn To Þ To Cp;a þ Wo Cp;v ln To Pn ðRda þ Wn Rv Þln þ To ðRa Po 1 þ 1:6078Wo Wn þ Wn Rv Þln þ 1:6078Wa Ra ln 1 þ 1:6078Wn Wo (23)
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before water heater and dehumidifier, three different mass flow rates of humidifier feed water (42, 75 and 110 kg/h) and three flow rates of dehumidifier feed water (29, 74 and 110 kg/h) are selected. Experiment is conducted to identify the best water flow rate by keeping the dehumidifier cold water flow rate as constant and varying the humidifier feed water flow rate. The experiments is repeated by keeping constant feed water flow rate and varying different dehumidifier cooling water flow rate. In both the cases [Fig. 4 (a) & (b)] the experiments are conducted with different air flow rate ranging between 13 kg/h and 21 kg/h. Maximum productivity is obtained for higher dehumidifier water flow rate (110 kg/h) and lower humidifier feed water flow rate (42 kg/h). Higher dehumidifier water flow rate causes increase in the heat capacity (mCp) of the water. This ensures more heat absorption from the hot fluid (hot humid air) in the dehumidifier which leads to augmented productivity. By lowering the humidifier feed water flow rate, the retention time between the feed water and air is increasing and so the specific humidity increases more. This in turn increases the productivity.
Fig. 3. Test section verification.
5.3. Effect of inserts on performance of air heater The exergy efficiency of the components is calculated from the above equations. The lower the exergy efficiency represents higher the exergy loss. From the above discussion, the component in which exergy losses taken place is identified for the further improvement. 5. Results and discussions 5.1. Plain tube verification Experiments are conducted on the plain tube and the results are compared with the previous correlations given by Gnielinski. Fig. 3 displays the comparison between Nusselt number calculated from Gnielinski relation and computed values obtained from the present experiment. Also it is found that the present plain air heater tube data is in 6% of agreement with the Gnielinski correlations.
Fig. 5 shows the variation of heat transfer coefficient with mass flow rate for different types of inserts. The increase of Reynolds number indicates the turbulence enhancement and due to that heat transfer rate is augmented. Extreme heat transfer rate attained for low PR due to utmost flow disturbance. The heat transfer coefficient increases with the increase in orientation angle and reaches to a maximum value at 180 due to the abrupt change in flow direction and more turbulence effect. Conical turbulator with divergent mode creates more flow reversion along the flow direction due to sudden enlargement and shows better heat transfer enhancement. The twisted tape inserts create radial and axial flow with strong turbulence intensity leading to higher heat transfer augmentation. Also centrifugal force is generated in fluid flow in the tube with twisted tape. Pressure loss developed was much higher due to increase in friction surfaces of these turbulators and interruption to the flow.
5.2. Effect of water flow rate 5.4. Effect of air heater inserts on productivity First phase of experiments are conducted to identify and fix the best mass flow rate of feed water to humidifier and cold water to dehumidifier. Depends on available specification of valves fitted
Fig. 6 shows the effect of various orientations of air heater inserts on heat transfer, relative humidity and output distilled water.
Fig. 4. Effect of feed water and cooling water.
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Fig. 5. Effect of inserts in performance of air heater.
The usage of inserts increases the air temperature which simultaneously reduces the relative humidity of the air enters into the humidifier. Due to the low relative humidity of air, the capability to absorb more amount of moisture from feed water is increased and hence high humid air come from humidifier. The maximum productivity of 0.6 kg/h, 0.52 kg/h and 0.4 kg/h is attained for twisted tape with divergent, circular insert (180 ) and cut out conical insert with divergent respectively. These selected orientations are used for further analysis. 105 sets of experiments are conducted using all the inserts with all possible orientations and PR to identify these best PR and orientation.
5.5. Performance of packing materials on productivity Packing materials helps to increase the dispersion of water droplets, contact time and contact area. About 100 sets of experiments have been conducted and based on data reduction, Fig. 7 is drawn which shows the variation of mass transfer coefficient in the humidifier for the air heater inserts with twisted tape in divergent
mode for PR 3 and two different packing material in the humidifier. Comparing to both type of packing materials, gunny bag has the best water retaining capacity due to its nature of porous void. Even though the saw dust has the good water retaining capacity, it block the air flow after some time. The mass transfer coefficient also increases with temperature and mass flow rate of air. The increase of mass transfer rate increases the productivity. The maximum productivity of 0.640 kg/h is attained for the system with air heater equipped by the twisted tape insert of pitch ratio 3 and gunny bag in humidifier. From the discussion in Section 5.4, among all the air heater inserts studied in the experiment, PR 3 is found to be better and in the current study of the two packing materials gunny bag is found better. These parameters are considered for further analysis.
5.6. Effect of spring inserts on productivity Spring inserts enhances the turbulent flow motion of the humidified air and there by better cooling between hot humid air and
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Fig. 6. Effect of air heater inserts in productivity.
Fig. 7. Effect of packing materials on performance of humidifier.
cold water which helps for higher production. Secondary flow or vortices are generated due to curvature of the coils. It has a significant effect on heat transfer enhancement. Fig. 8 (a) reveals the overall heat transfer coefficient of the humid air flows through the inner pipe of shell and tube condenser equipped with spring inserts. Condensation depends on enhancement of heat transfer rate in dehumidifier in addition to that cooling effect produced by the cooling water circuit. The mass flow rate and the relative humidity of hot and humid air passing through the inner tube of the condenser have an effect over the mass of condensate produced. Fig. 8 (b) shows the productivity of the HDH desalination system corresponding to the effect of spring inserts with the pitch ratios of 3 and 4. The overall heat transfer coefficient increases with the increase in mass flow rate of air. This effect is higher in case of low
PR of 3. The maximum productivity obtained is about 0.8 kg/h at the air flow rate of 21 kg/h. 5.7. Gained output ratio It is the overall performance indicator of HDH desalination system compared with the input power. Gained output ratio is maximum for the modified HDH system compared to the normal HDH system without any inserts. It indicates that the saving of power is attained in the modified HDH desalination system Fig. 9. 5.8. Exergy losses Exergy analysis is a process of evaluating the losses takes places in its components. Fig. 10 represents the second law efficiency of
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Fig. 8. Effect of spring inserts on performance of de humidifier.
the individual components of HDH system when it is provided with previously identified orientation of inserts in its components. Exergetic efficiency of the air heater progressively increases with the reduction in pitch ratio, increase in orientation angle for circular insert and with divergent type for remaining two type of inserts. It is observed that maximum exergy efficiency obtained in air heater, dehumidifier and humidifier are in the subsequent order and minimum exergy efficiency observed in water heater. So, maximum scope is available for improvement of productivity by concentrating on water heater, humidifier and dehumidifier. Air heater equipped with twisted tape of PR 3 gives highest exergy efficiency. The exergy efficiency increased by an increase in evaporation rate.
5.9. Effect of inserts on energy and exergy Fig. 11 shows the comparision between variation of Reynolds number with the first and second law efficiency. First law efficiency represents the effective utilization of input power. Second law efficiency is the ratio between reversible work to the useful work for the work consuming devices. The increase of exergy efficiency pointed out that the maximum productivity attained in the process with minimum useful work input. The increase of Reynolds number creates lesser effect on second law efficiency compared to first law efficiency and the usage of inserts increases the exit temperature of air which simultaneously increases the humid air generation with high temperature. 5.10. Productivity of modified HDH system Fig. 12 shows that the productivity of modified HDH desalination system. In which the twisted tape (Divergent) in air heater, gunny bag in humidifier and spring insert in dehumidifier with PR 3 shows two times of higher productivity compared to conventional system for the same input power. Comparing the productivity, normal system produces the maximum distillate of 0.40 kg/h with the first law efficiency of 20% and second law efficiency of 15% but the enhanced system equipped with inserts shows energy efficiency of 44% and second law efficiency of 38% with 50% increase of its productivity. The energy and exergy efficiency enhanced to gain a high energy recovery rate. 5.11. Correlation of experimental results
Fig. 9. Performance of Modified HDH desalination system.
The taguchi regression analysis with a level of L8 is used to predict the productivity of HDH system using previously selected best performed inserts in air heater, dehumidifier and gunny bag in humidifier for any intermediate value between the designed range. Eq. (24) represents the productivity of system with half perforated circular inserts in air heater for the above said conditions. Most of the predicted correlated values lies within ± 9% when compared with experimental productivity. The regression equation for the distilled water productivity in terms of the various parameters is given below. This regression equation is applicable for the range of
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Fig. 10. Exergy efficiency at various location with various inserts.
Fig. 11. Effect of inserts on the energy and exergy efficiency of HDH system.
mass flow rate 14 kg/h to 21 kg/h, circular inserts (180 ) pitch ratio ranging between 3 and 5, relative humidity between 55% and 65% and spring inserts pitch ratio 3 to 4.
mdw ¼ 810 þ 7:14ma 45PRAH þ 0:5F 55PRSP
(24)
Similar procedure repeated for cut out conical inserts and the newly arrived equation is given below. This regression equation is applicable for the range of mass flow rate 14 kg/h to 21 kg/h, conical insert (D) pitch ratio ranging between 3 and 5, relative humidity between 60% and 80% and spring inserts pitch ratio 3 to 4.
mdw ¼ 493 þ 13:6ma 17:5PRAH 1:75F 5PRSP
(25)
Following equation represent the regression equation for twisted tape turbulators (D). This regression equation is applicable for the range of mass flow rate 14 kg/h to 21 kg/h, twisted tape (D) pitch ratio ranging between 3 and 5, relative humidity between 30%
to 45% and spring inserts pitch ratio 3 to 4.
mdw ¼ 1053 þ 7:68ma 56:9PRAH 1:75F 86:3PRSP
(26)
Fig. 13, reveal the most of the values fall in the acceptable range with the variation of ± 9% and ± 6%. The productivity depends upon mass flow rate and relative humidity of air. 5.12. Comparison of previous results The present results are compared with the previous related works in the humidification dehumidification desalination system and given in Fig. 14. Based on mass flow rate ranges, two previous researchers work has been selected. Cemil yamali introduced the concept of integrating a double pass solar air heater with HDH desalination. Agouz conducted an experiment in bubble column HDH desalination system.
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Fig. 14. Comparison with previous results [20,21].
Table 2 Water analysis. Type of water
Fig. 12. Productivity of modified HDH system.
For the same mass flow rate ranges, Cemil yamali's work shows a larger productivity of 0.3 kg/h and Agouz obtained the distillate of 1.3 kg/h whereas current work shows an yield of 0.8 kg/h with in the twisted tape in divergent mode, 0.7 kg/h in the circular insert of 180 and 0.650 kg/h for the cut out conical insert in divergent mode with gunny bag in humidifier and spring insert in dehumidifier with divergent mode of PR 3. Maximum deviation of productivity between our work is 38% lesser than the highest value of Cemil yamali's work and 62% higher than the lowest productivity of Agouz's work. Present results follow the same trend of previous results. Mostly the productivity increased in this present work is moderately higher compared to the related works.
pH
Total suspended Hardness Chloride Turbidity Solids mg/l mg/l mg/l NTU
Salt water 8.2 40 Distilled water 7.8 8 Drinking water 6.5 to 8.5 100e200 (As per IS 10500)
158 147.5 300
32.90 3.19 250
11 7 10
5.13. Water analysis The chemical analysis of input salt water and final condensed distilled water are conducted in the Sethu Institute of Technology, Madurai, to identify the suitability of the distillate for various purposes. The analysis details are given in Table 2. 5.14. Economic analysis The Payback period of the experimental setup depends on the
Fig. 13. Correlation using air heater inserts.
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overall cost of fabrication, operating cost and maintenance cost.
The overall fabrication cost
¼
Daily productivity of the desalination unit (8 h) Cost of water produced
¼ 6.8 litres ¼
Electrical power used per hour (¼1.9 units) Cost of electricity per day (8 h)
¼
Total cost of mineral per day
¼
Payback period ¼ Investment/net earnings
¼
7500 (136$)
68 (1.25$)
22.80 (0.412$) 1.36(0.024$) 7500 (136$)/
. 43.84 (0.81$)
¼ 171days
Using economic analysis, cost of distilled water is estimated. In addition to the capital cost of the HDH desalination system (136 $), other parameters such as sinking fund factor, annual salvage value and maintenance cost and interest rate per year (i) should be considered. The capital recovery factor is defined interms of interest per year and also number of life year (n) of the system and is given by Ref. [18].
CRF ¼
ið1 þ iÞn ð1 þ iÞn 1
(27)
Here interest rate per year (i) is taken as 12% for life year (n) of 10 years. Fixed annual cost (FAC) is the product of capital cost (P) and capital recovery factor.
FAC ¼ P ðCRFÞ
(28)
By taking the salvage value of system S equal to 20% of capital cost; Sinking fund factor (SFF) and annual salvage value (ASV) can be expressed by Ref. [18].
SFF ¼ 1 ðð1 þ iÞn 1
(29)
ASV ¼ SFF S
(30)
The AMC which is annual maintenance operational cost of the system. Here, 15% of fixed annual cost is considered as maintenance cost:
AMC ¼ 0:15FAC
(31)
M is the average annual yield of the HDH system. In this study the amount of M has been calculated based on daily water productivity. It is found that using inserts with a negligible cost results in a cost reduction by nearly 9% and actual cost (AC) is given by, Cost per litre, is determined by [18].
CPL ¼ AC=M
(32)
AC ¼ FAC þ AMC ASV
(33)
The cost of fresh water produced with this modified HDH desalination system is decreased when compared to the normal HDH desalination system of without any inserts. 6. Conclusion The productivity of HDH desalination system enhanced by changing the flow behavior in their components using the insertion of new type of inserts and also two different types of packing
material in humidifier. The efficiency of each individual components augmented and its effect utilized for further enhancement of overall system productivity. With the attainment of induced heat transfer rate in an air heater, using twisted tape in divergent mode of PR 3 helps to increase the evaporation rate in humidifier. The performance enhancement of humidifier further augmented by utilising gunny bag. Suitable type of condenser selected to condense the enhanced quantity of saturated air. With the chosen shell and tube dehumidifier performance further accelerated by using the spring inserts with PR 3 and productivity of 0.8 kg/h attained with 40% saving of input power in the modified HDH desalination system. Energy and exergy analysis are identified the utilization level of input energy and the available and unavailable level of energy in the components. Present work with different best configurations is compared with two previous results. Present results follow the same trend of previous results. Mostly the productivity moderately increased in this present work is higher compared to the related works. Payback period of such a system is found as 171 days and it is one time low cost investment method. References [1] Eiamsa-ard S, Wongcharee S, Eiamsa-ard K, Thianpong P. Heat transfer enhancement in a tube using delta-winglet twisted tape inserts. Appl Therm Eng 2010;30:310e8. [2] Eiamsa-ard S, Thianpong C, Eiamsa-ard P, Promvonge P. Convective heat transfer in a circular tube with short-length twisted tape insert. Int Commun Heat Mass Transf 2009;36:365e71. [3] Tandiroglu A, Ayhan T. Energy dissipation analysis of transient heat transfer for turbulent flow in a circular tube with baffle inserts. Appl Therm Eng 2006;26:178e85. [4] Akansu SO. Heat Transfer and Pressure drops for porous-ring turbulators in a circular pipe. Appl Energy 2006;83:280e98. [5] Promvonge P, Eiamsa-ard S. Heat transfer and turbulent friction in a circular tube fitted with conical nozzle turbulators. Int Commun Heat Mass Transf 2007;34:72e82. [6] Kongkaitpaiboon V, Nanan K, Eiamsa-ard S. Experimental investigation of heat transfer and turbulent flow friction in a tube fitted with perforated conical rings. Int Commun Heat Mass Transf 2010;37:560e7. [7] Yildiz C, Yaar Biger, Dursun Pehllvan. Influence of fluid rotation on the heat transfer and pressure drop in double pipe heat exchanger. Appl Energy 1996;54:49e56. [8] Durmus Aydin, Durmus Ayla, Esen Mehet. Investigation of heat transfer and pressure drop in a concentric heat exchanger with snail entrance. Appl Therm Eng 2002;22:321e32. [9] Amer EH, Kotb H, Mostafa GH, El-Ghalban AR. Theoretical and experimental investigation of humidification e dehumidification desalination unit. Desalination 2009;249:949e59. [10] Chang Z, Zheng H, Yang Y, Su Y, Zhanchun Duan Z. Experimental investigation of a novel multi-effect solar desalination system based on humidificationdehumidification process. Renew Energy 2014;69:253e9. [11] Narayanan GP, Lienhard JH. Thermal design of Humidificationdehumidification systems for affordable small scale desalination. IDA J 2014;4(3):24e34. [12] Chehayeb KM, Narayan GP, Zubair SM, Lienhard JH. Thermodyamicbalancing of a fixed-size two-stage humidification dehumidification desalination. Desalination 2015;369:125e39. [13] Garaway I, Grossman. Investigation of a solar-powered desalination system employing regeneration. Desalination 2006;197(1e3):63e74. [14] Al-Enezi G, Ettoney H, Fawzy N. Low temperature humidificationdehumidification desalination system. Energy Convers Manag 2006;47: 470e84. [15] Ashrafizadeh SA, Amidpour M. Exergy analysis of humidificationedehumidification desalination systems using driving forces concept. Desalination 2012;285:108e16. [16] Hou Shaobo, Zeng Dongqi, Yeb Shengquan, Zhang Hefei. Exergy analysis of the solar multi-effect humidificationedehumidification desalination process. Desalination 2007;203:403e9. [17] Xiong RH, Wang SC, Xie LX, Wang Z, Li PL. Experimental investigation of a baffled shell and tube desalination column using the humidification dehumidification process. Desalination 2005;180:253e61. [18] Kianifar A, Zeinali heris S, Mahian O. Exergy and economic analysis of a pyramid shaped solar water purification system: active and passive cases. Energy 2012;38:31e6. [19] Barford NC. Experimental measurements: precision, error and truth. second ed. Newyork: John Wiley&sons; 1990. [20] Yamali Cemil, Solmus Ismail. Theoretical investigation of a humidification and dehumidification desalination system configured by a double pass flat plate
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Nomenclature I: Current, A d: Diameter of the test tube, m H: Enthalpy, J/kg S: Entropy, J/K Ex: Exergy flow rate, J/s f: Friction factor R: Gas constant, J/kgK h: Heat transfer coefficient, W/m2K Q: Heat transfer rate, W A: Heat transfer surface area, m2 hfg: Latent heat of vaporization, J/kg K L: Length of the test tube, m LMTD: Logarithmic mean temperature difference, K m: Mass flow rate, kg/ s DWA: Mass transfer coefficient, kg/s NTU: Nephelometric turbidity units Nu: Nusselt number U: Overall heat transfer coefficient, W/m2 K pd: Partial pressure of dry air, Pa pv: Partial pressure of vapor, Pa PR: Pitch ratio (pitch length/diameter) Pr: Prandtl number P: Pressure, Pa Re: Reynolds number ex: Specific exergy, J/kg Cp: Specific heat capacity at constant pressure, J /kg K W: Specific humidity, kg water/kg dry air T: Temperature, K k: Thermal conductivity of air, W/mK TEF: Thermal enhancement factor QT: Total input power, W ua: Velocity, m/s V: Voltage, Volts
13
D: Change in parameter h: Efficiency b: Orientation angle, degree
ɸ: Relative humidity, % ℮: Density, kg/m3 Subscripts
a: Air ah: Air heater 1: Air heater input 2: Air heater output o: Ambient atm: Atmosphere cw: Cooling water 5: Cooling water inlet 6: Cooling water outlet c: condenser con: Convection dehu: Dehumidifier dw: Distilled water 8: Distilled water output from condenser ex: Exergy gen: generated hu: Humidifier 7: Humid air inlet at condenser in: Inlet b: Mean out: Outlet P: Plain tube n: Location sat: Saturated 9: Salt water exit from humidifier s: Surface sp: Spring insert t: Tube with insert v: Vapor w: Water wh: Water heater 3: Water heater input 4: Water heater output
Greek Symbols
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