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Experimental investigation of packed-bed cross-flow humidifier
Applied Thermal Engineering 117 (2017) 584–590
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Experimental investigation of packed-bed cross-flow humidifier Mostafa H. Sharqawy a,⇑, Ibrahim Al-Shalawi b, Mohamed A. Antar b, Syed M. Zubair b a b
School of Engineering, University of Guelph, Guelph, Ontario N1G 2W1, Canada King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
h i g h l i g h t s The performance of a cross-flow packed-bed humidifier is presented. Capacity, saturation efficiency, and specific energy consumption are calculated. The specific energy consumption is almost constant for the same packing material. Effectiveness-NTU model is adopted to estimate the effectiveness of the humidifier.
a r t i c l e
i n f o
Article history: Received 5 August 2016 Revised 11 February 2017 Accepted 14 February 2017 Available online 16 February 2017 Keywords: Humidifier Cross-flow Packed-bed Experimental Modelling Effectiveness Humidification capacity Saturation efficiency
a b s t r a c t This paper presents the experimental performance of a cross-flow packed-bed humidifier. In this humidifier, hot water is sprayed over packing material where air flows though it in a cross flow arrangement. The air is heated and humidified while flowing through the humidifier duct. The humidification capacity, saturation efficiency, and specific energy consumption are calculated using the experimental data of the inlet and outlet conditions for both air and water streams. The variation of these performance indicators with the mass flow rate ratio of water-to-air, the water inlet temperature, and the packing volume; is investigated. It was found that the specific energy consumption is almost constant with variation of the mass flow rate ratio, packing thickness, and water inlet temperature within the investigated range of these parameters. An effectiveness model originally developed for cross-flow packed-bed cooling tower is adopted to estimate the effectiveness of the humidifier. In addition, the effectiveness and the number of transfer units of the humidifier are determined using the experimental data. The model was found to be in a good agreement with the experimental measurement at high capacity ratio with a deviation of 6%. However, the model underestimates the effectiveness at lower capacity ratio with a maximum deviation of 30%. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Humidifiers are devices which increase the moisture content of air or a carrier gas. They are widely used in heating, ventilation, and air conditioning (HVAC) systems to ensure a comfortable and healthy environment for workers and residents [1]. Industrial humidifiers are used to adjust the humidity level to prevent static electricity buildup in industries such as packaging, plastics, textiles, electronics, semiconductors, automotive manufacturing, pharmaceuticals, electrostatic painting, and powder coating. Humidifiers are used also to preserve material properties such as in paper industry to prevent shrinkage and paper curl and in food industry to maintain the freshness of food against the dryness
⇑ Corresponding author. E-mail address: [email protected] (M.H. Sharqawy). http://dx.doi.org/10.1016/j.applthermaleng.2017.02.061 1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.
caused by cold temperatures in cold storage rooms. In addition, humidifiers are used in medical ventilators and hospital operating rooms. Recently, humidifiers are used in small-scale water desalination systems such as the humidification dehumidification desalination (HDH) process [2] as well as in proton exchange membrane (PEM) fuel cells [3]. There are different types of humidifiers however, they can be classified as steam or heating element humidifiers, atomizing or spray humidifiers, wetted element or packed bed humidifiers [4,5], and bubble columns or spargers [6]. Steam humidifiers inject steam directly into air or add heat to evaporate supplied water to the conditioned space. Steam and heating element humidifiers consume large energy generated from gas, fossil fuel, or electricity to evaporate the liquid water in a steam generator [4]. Atomizing or spray humidifiers spray fine water droplets into air stream which are evaporated and added to the air. There are different
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Nomenclature Ac As CR cw f h Km _ m P T v
cross sectional area (m2) total surface area (m2) capacity ratio (–) specific heat of water (kJ kg1 K1) slope of the saturated air enthalpy versus temperature (kJ kg1 K1) specific enthalpy (kJ kg1) mass transfer coefficient (kg m2 s1) mass flow rate (kg s1) pressure (kPa) temperature (°C) velocity (m s1)
Greek letters e effectiveness (–) g efficiency (–) q density (kg m3) r correction factor defined by Eq. (5) (kJ kg1) x humidity ratio (kgv kg1 a )
mechanisms of atomizing such as ultrasonic, centrifugal atomizing, pneumatic atomizing, and air washers. Wetted element humidifiers force air to flow over a water film so that water diffuses into the air stream as vapor. Packed bed humidifier is a wetted element humidifier where the elements are made of random or structured packing material which has high surface area-to-volume ratio that increases the diffusion surface area. The principle of operation for all of these devices is same. When water is brought into contact with air that is not saturated with water vapor, water diffuses into air and raises the humidity. If the water temperature is higher than the air temperature, the air will be heated and humidified. If the water temperature is lower than the dew point temperature of the air (i.e. chilled water is used), the air will be cooled and dehumidified. If the water is circulated adiabatically (i.e. without being heated or cooled), it will approach the wet bulb temperature and the air will be cooled and humidified which is known as the air washer or evaporative cooling process. Packing material is typically used in wetted element humidifiers to increase the dispersion of water droplets, the contact area, and the contact time. Devices that contain packing material are known as packed bed towers with water sprayed at the top and air flows in counter or cross flow arrangement. Special types of packed bed towers that are used to cool water are called cooling towers. The performance of humidifiers is usually measured by its humidification capacity which is the rate of water vapor added to the air [4]. This capacity is limited when the air becomes saturated. Therefore, the saturation efficiency is also used as a performance index for humidifiers which is defined as the increase in the humidity ratio to the maximum increase in the humidity when the air is fully saturated at its exit temperature. The energy effectiveness (or simply effectiveness) of the humidifier is an important performance indicator which is defined as the ratio of the total energy transferred to the air (both latent and sensible) to the energy that will be consumed if the exit air is fully saturated at the water inlet temperature [7]. There are few analytical models that could be used to design or evaluate the performance of humidifiers. These models were originally derived for counter-flow wet cooling towers such as Merkel’s model [8], Poppe and Rogener [9] (known as Poppe’s model), Braun’s effectiveness model [10], and the e-NTU (effectiveness – number of transfer units) model by Jabber and Webb [11]. Critical
Subscripts a dry air av average c corrected in inlet min minimum max maximum out outlet sat saturated air condition th theoretical v vapor w water Abbreviations MR mass flow rate ratio NTU number of transfer units SEC specific energy consumption (kW h kg1)
assumptions were made in some of these models in order to obtain an analytical solution. These assumptions are such as neglecting the water loss due to evaporation, a Lewis number of unity, and a linear relationship for saturation air enthalpy with respect to water temperature. Klopper and Kröger [12,13] gave a detailed review and comparisons of these models and found that Poppe’s model is more accurate than the Merkel and e-NTU ones for both counter and cross-flow cooling tower. However, Poppe’s model equations must be solved numerically and iteratively and it is relatively complex. On the other hand, e-NTU model by Jabber and Webb [11] can be employed under normal ambient conditions if only the water outlet temperature is an important consideration in the design of a cooling tower. In this paper, cross-flow humidifier is investigated experimentally to evaluate its performance. The inlet air is heated and humidified using hot water which is sprayed at the top of structure packing material. The effect of the mass flow rate ratio of the water and air as well as the water inlet temperature and packing volume on the humidification capacity, saturation efficiency, and specific energy consumption is presented. Finally, the e-NTU model of Jabber and Webb [11] originally developed for counter- and crossflow cooling tower is adopted to predict the humidifier effectiveness which is compared with the measured one. The reason for choosing this model to validate its accuracy for the cross-flow humidifier, is that it is has an analytical expression for the effectiveness and does not need complex numerical and iterative procedures. 2. Experimental details A schematic of the packed-bed cross-flow humidifier used in this study is shown in Fig. 1. The humidifier consists of a duct with cross sectional dimensions of 30 cm 30 cm and a length of 90 cm. Three structured packings (Brentwood XF125 cross-fluted film fill) of 10 cm thickness are installed inside the duct and separated by a distance of 20 cm. The purpose of using three packings in-series is to study the effect of increasing the packing volume (or thickness) which consequently increases the total surface area and the number of transfer units. An axial flow fan is installed at the humidifier entrance so that the air at room temperature is blown through the humidifier and passes through the packing material. Hot water is sprayed over the packing material and flows downward to a small
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Fig. 1. Schematic of the cross-flow humidifier used in the present work.
water basin at the bottom of the humidifier duct through which the water returns back to a water tank. The water in the tank is heated by electric heating elements that have a total heating capacity of 8 kW. The electric heaters are connected to an AC power source through a voltage controller to regulate the power and provide a controlled water inlet temperature to the humidifier. The hot water is returned back to the humidifier by a centrifugal pump and a ball valve is used to regulate the water flow rate. Drift eliminators are installed at the air downstream side of each fill to strip the water droplets that are carried out by the humidified air. Instruments are installed to measure the temperatures and flow rates of both air and water streams. K-type thermocouples are installed at the inlet and outlet of the air and water streams to measure the dry bulb, wet bulb, and water temperatures. For the wet-bulb temperature measurement, the thermocouple junction was wrapped by a wet wick supplied by water from a gravity feeding syringe. Water flow rate is measured using a glass tube rotameter (Omega FL-50004A) of ±5.0% accuracy and a range of 1.8–18 L/ min. The air velocity is measured using a digital anemometer (Smart Sensor AR 836) of ±3.0% accuracy. A handheld temperature data logger is used to display and record the measured temperatures from the thermocouples. All thermocouples were calibrated using a liquid-in-glass thermometer while the rotameter and anemometer were calibrated by their manufacturers. The uncertainty of the calculated performance parameters (i.e. the humidification capacity, saturation efficiency, and effectiveness) is determined using the method described by Holman [17] and knowing the uncertainty of the individual measuring instruments. Water inlet temperature is adjusted at specific values of 35, 45, and 55 °C using the voltage controller connected to the electric heaters in the water tank. The water flow rate is adjusted using a ball valve after the pump and the flow rate is adjusted at different values to ensure a range of mass flow rate ratio of the water-to-air. The air fan draws ambient air to the humidifier at almost constant volumetric flow rate. The air flow rate is measured at the humidifier exit using a digital anemometer. All temperatures are recorded automatically by digital data loggers every 5 s until a steady state condition is approached at which the variables are changing only within their uncertainty band. All measurements are taken at room condition which has an air temperature of 22 °C ± 2 °C and atmospheric pressure which is assumed to be 101.325 kPa. During the experiment, hot water is sprayed over the packing material such that the surface area would be large enough for interaction with
Fig. 2. Representation of the heating and humidification process on the psychrometric chart.
the air stream that passes in a cross flow direction such that air would be heated and humidified as shown in Fig. 2. The driving potential for heat transfer is temperature difference between both streams whereas another driving potential for mass transfer due to the difference in water-vapor concentration. 3. Mathematical analysis The purpose of this analysis is to use the experimental results to calculate the performance parameters of the cross-flow humidifier. The psychrometric properties of moist air [14] and the thermodynamics properties of water [15] are used in these calculations. Fig. 2 shows the humidification processes applied to air on the psychrometric chart. As shown in this figure, the air is heated while it is humidified and this happens because the sprayed water is at a higher temperature than the air inlet one. This humidification and heating process is similar to the process occurs in a cooling tower. However, in the cooling tower the aim is to cool the hot process water. Therefore, the analysis of both devices is nearly alike except that the performance parameters are different. The humidification capacity is one of the important parameters that characterize any humidifier. It is the amount of water vapor
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that is added to the flowing air. It is important to select or design the humidifier with a humidification capacity that matches with the humidifying load required for the space or process so as to maintain a predetermined relative humidity. The humidification capacity of the humidifier can be calculated as:
_ a ðxout xin Þ _v ¼m m
ð1Þ
_ a is the mass flow rate of dry air and xout and xin are the where m outlet and inlet air humidity ratio, respectively. The dry air mass flow rate is calculated using the continuity equation knowing the average air velocity, density, and cross-sectional area as given by,
_ a ¼ qa Ac v m
ð2Þ
The saturation efficiency is another performance index that characterizes humidifiers. It is defined as the amount of water vapor transferred to the air to the maximum amount of water vapor when the exit air is saturated. This efficiency is used also with air washers and evaporative coolers as represented by,
gsat ¼
xout xin xout;sat xin
ð3Þ
The maximum humidity difference in the saturation efficiency definition (i.e. the denominator of Eq. (3)) assumes that the outlet air is saturated at the exit air temperature. Therefore, this definition ignore the possibility of increasing the air temperature (hence the saturation humidity) to the inlet hot water temperature. In this case, the saturation efficiency defined by Eq. (3) will be higher compared with the one calculated using the maximum humidity ratio occurs when the air temperature reaches the inlet water temperature. In order to combine both the heat and mass transfer effects occurring in the humidifier, the energy effectiveness should be used [7]. This energy effectiveness is given by [11],
_ a ðha;out ha;in Þ m e¼ _ min ðha;sat T r ha;in Þ m
ð4Þ
w;in
where ha;sat T
w;in
is the saturated air enthalpy at the water inlet tem-
perature, r is a correction factor to correct for the non-linearity relationship between the temperature and the enthalpy of saturated air. This correction factor is given by Jaber and Webb [11]. It can be written as,
r ¼ ha;sat T w;in þ ha;sat T w;out 2ha;sat T w;av
. 4
ð5Þ
The minimum mass flow rate given in Eq. (4) is the minimum of the dry air mass flow rate and a corrected mass flow rate of water, which is given by;
_ w cw =f _ w;c ¼ m m
ð6Þ
where f is the slope of the saturated air enthalpy versus temperature curve given by;
f ¼
ha;sat T
w;in
ha;sat T w;out
ð7Þ
T w;in T w;out
The purpose of calculating the energy effectiveness by Eq. (4) is to adapt Jaber and Webb’s model for the cross-flow cooling tower for the current cross-flow humidifier. This model calculates the effectiveness using the number of transfer units (NTU) and the capacity rate ratio (CR). The number of transfer unit in this model is given by,
NTU ¼
N K m As X Dh a;i ¼ _ min m ha;sat ha;i i¼1
ð8Þ
T w;i
where Km is the overall mass transfer coefficient, and As is the total surface area of the packing material. The number of transfer units
could be also calculated by performing the numerical integration given in Eq. (8). In this integration scheme, an energy balance is applied between the air and water after dividing the total water temperature difference (or the range) into N segments. The enthalpy of saturated air is calculated at the water temperature divisions and the summation of the ratio of the air enthalpy increase to the potential enthalpy is calculated (as given in Eq. (8)). In this study the experimental NTU is calculated by performing the numerical integration given above since the overall mass transfer coefficient is not known. On the other hand, the capacity ratio (CR) is given by [11].
CR ¼
_ min m _ max m
ð9Þ
where the minimum and maximum flow rates of Eq. (9) are the ones for the dry air flow rate (given by Eq. (2)) and the corrected water flow rate (given by Eq. (6)). Using the NTU and the capacity rate ratio defined above, Jaber and Webb [11] recommended to use the correlation of cross-flow heat exchanger with both fluids unmixed to calculate the theoretical (or model) effectiveness. This correlation is given in Kays and London’s book [16] as,
eth ¼ 1 exp
1 NTU 0:22 ðexpðC R NTU 0:78 Þ 1Þ CR
ð10Þ
The specific energy consumption (SEC), is the energy consumed per unit water vapor added to the dry air during the humidification process. This is an important performance index for the humidifier and could be used to compare different humidifiers from the energy consumption prospective. The specific energy consumption can be calculated by,
SEC ¼
_ w ðhw;in hw;out Þ T w;in T w;out m ¼ MR cw _ a ðxout xin Þ xout xin m
ð11Þ
where MR is the mass flow rate ratio of the water and dry air, and cw is the average specific heat of water. As noticed in Eq. (11), SEC depends on the mass flow rate ratio and the change in water temperature per change in air humidity ratio. 4. Results and discussion The experimental performance of the cross flow humidifier is evaluated by calculating the humidification capacity, saturation efficiency, specific energy consumption, and the energy effectiveness using the correlations presented in the previous section. The variation of the performance with the operating conditions is investigated by changing the operating parameters. The parameters which are investigated in this study are: The mass flow rate ratio (MR) of water to air is changed by varying the water flow rate only while the volume flow rate of air is almost constant from the fan. However, the mass flow rate of air is changing due to changes in the air density. The mass flow rate ratio is changed within a range of 0.5–3.5. The inlet water temperature sprayed on the packing material is varied at 35, 45, and 55 °C. The volume of the packing material is changed by using 1, 2, and 3 packings of 10 cm thickness each. Increasing the packing volume in the air flow direction, increases the total surface area. Fig. 3 shows the humidification capacity in kg/hr as it changes with the mass flow rate ratio for 1, 2, and 3 fills and at different water inlet temperatures. As shown in this figure, the humidification capacity increases with the mass flow rate ratio until approaches an asymptotic value. That is when the air exiting the humidifier is saturated with water vapor. The increase of mass flow rate ratio results in higher water heating capacity which ensure keeping the water temperature high and hence heats and humidify
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7 6
1 1 Fill Tw,in = 35 °C Tw,in = 45 °C Tw,in = 55 °C
Saturation Efficiency (ηs)
Humidification Capacity, kg/hr
8
5 4 3 2
0.9 0.8 0.7 0.6
1 Fill Tw,in = 35 °C Tw,in = 45 °C Tw,in = 55 °C
0.5
1 0 0.5
1
1.5
2
2.5
3
0.4 0.5
3.5
1
6
2.5
Tw,in = 35 °C Tw,in = 45 °C Tw,in = 55 °C
4 3 2
0.8
0.7
2 Fills Tw,in = 35 °C
0.6
Tw,in = 45 °C Tw,in = 55 °C
1
1.5
2
2.5
3
0.5 0.5
3.5
1
8
2
2.5
3
3.5
1 3 Fills
Saturation Efficiency (ηs)
Tw,in = 35 °C Tw,in = 45 °C Tw,in = 55 °C
5 4 3 2 1 0 0.5
1.5
Mass Flow Rate Ratio (MR)
Mass Flow Rate Ratio (MR)
6
3.5
0.9
1
7
3
1
5
0 0.5
Humidification Capacity, kg/hr
2
2 Fills
Saturation Efficiency (ηs)
Humidification Capacity, kg/hr
8 7
1.5
Mass Flow Rate Ratio (MR)
Mass Flow Rate Ratio (MR)
0.9
0.8
0.7
3 Fills Tw,in = 35 °C
0.6
Tw,in = 45 °C Tw,in = 55 °C
1
1.5
2
2.5
3
3.5
Mass Flow Rate Ratio (MR)
0.5 0.5
1
1.5
2
2.5
3
3.5
Mass Flow Rate Ratio (MR)
Fig. 3. Effect of mass flow rate ratio on humidification capacity.
Fig. 4. Effect of mass flow rate ratio on saturation efficiency.
the air. We notice also that the humidification capacity increases with the water temperature since the driving force for the heat and mass transfer processes increases with the water temperature. On the other hand, by increasing the packing material thickness (i.e. by adding more packing), the total heat and mass transfer surface area increases which lead to higher humidification capacity. In Fig. 4, the saturation efficiency of the humidifier is plotted vs. the mass flow rate ratio at all packing thicknesses (i.e. with 1, 2, and 3 fills) and water inlet temperatures. Generally, the saturation efficiency increases with MR and approaches unity at higher MR values. That is, when the air exiting the humidifier is saturated with water vapor. The figure also shows that the saturation efficiency increases with the water inlet temperature and thickness of the packing material (or the number of fills). These results match with the finding of the humidification capacity shown in previous figure. How-
ever, it was noticed that the effect of the water inlet temperature is not significant at higher packing thickness (i.e. when using 3 fills). Since the air has longer time and flow distance within the packing material so that to get saturated. Therefore, the water temperature has less significant effect than the packing thickness. Fig. 5 shows the variation of the specific energy consumption with the mass flow rate ratio for all packing thicknesses. The specific energy consumption as defined by Eq. (11) varies between 0.85 and 0.95 with an average value of almost 0.9 kW h/kg. This could be explained as the mass flow rate ratio increases, the water temperature difference per unit humidity ratio difference decreases, which cancels the MR effect. This is clearly shown in Fig. 6 and is obvious from Eq. (11). It is an important finding of having almost constant specific energy consumption for cross flow humidifier. Therefore, both the energy and humidification capacity increases with almost the same rate when larger humidifier is designed or
589
1.2 1.1
14
1 Fill 2 Fills 3 Fills
1 Fill
12
2 Fills 3 Fills
10
1
% Heat Loss
Spc. Energy Consumption, kWh/kg
M.H. Sharqawy et al. / Applied Thermal Engineering 117 (2017) 584–590
0.9 0.8
8 6 4
0.7 0.6 0.5
2 1
1.5
2
2.5
3
0 0.5
3.5
1
Mass Flow Rate Ratio (MR)
1.5
2
2.5
3
3.5
Mass Flow Rate Ratio (MR)
Fig. 5. Effect of mass flow rate ratio on specific energy consumption.
Fig. 8. Percentage heat loss from the humidifier surface.
1
C R = 0.5 and 0.6
Effectiveness
0.8
0.6
0.4
0.2
0
0
0.5
1
1.5
2
2.5
3
Number of Transfer Units (NTU) Fig. 6. The variation of the water temperature difference per air humidity ratio difference with the mass flow rate ratio.
Fig. 9. Measured and predicted air effectiveness vs. the number of transfer units at capacity ratio range of 0.5–0.6.
1 1
C R = 0.7 and 0.8
2 Fills
0.8
3 Fills
0.8
Effectiveness
Cross Flow Effectiveness
1 Fill
0.6
0.4
0.6
0.4
0.2 0.2 0
0
0.2
0.4
0.6
0.8
1
Measured Effectiveness
0
0
0.5
1
1.5
2
2.5
3
Number of Transfer Units (NTU) Fig. 7. A comparison between the predicated and the measured air effectiveness.
selected. However, it is expected that SEC will have a different constant value when different packing with higher or lower specific surface area is used. Increasing the surface area will increase the humidification capacity for the same energy input. This prediction requires further experimental investigation with different packing materials. Fig. 7 shows a comparison between the experimental values of the effectiveness (calculated by Eq. (4)) and the model estimates
Fig. 10. Measured and predicted air effectiveness vs. the number of transfer units at capacity ratio range of 0.7–0.8.
(calculated by Eq. (10)). The NTU and CR provided for the model estimates were calculated using the experimental data and Eqs. (8) and (9), respectively. As shown in Fig. 7, the maximum and minimum deviations between the model and the experimental results of the effectiveness are 15% and 30%, respectively. This is considered as large over and under estimates which is due to the
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1
C R = 0.9 and 1.0
Effectiveness
0.8
0.6
0.4
0.2
0
0
0.5
1
1.5
2
2.5
3
Number of Transfer Units (NTU) Fig. 11. Measured and predicted air effectiveness vs. the number of transfer units at capacity ratio range of 0.9–1.0.
assumptions made in this e-NTU model. The major assumptions of this model are neglecting the water loss due to evaporation, a Lewis number of unity (defined as the ratio of thermal diffusivity to mass diffusivity which relates the heat and mass transfer coefficients), and a linear relationship for saturation air enthalpy with respect to temperature. In addition, this bias may be also associated to the errors in the measurements of the air outlet state, in particular with the wet bulb temperature. In addition, there is heat loss from the humidifier duct surface due to the hot air inside. This heat loss is calculated and found to be in the range of 4–10% from the supplied energy by the hot water as shown in Fig. 8. However, the model assumes that there is no heat loss. Since the model tends to under predict and over predict the effectiveness at different operating conditions, the comparison made in Fig. 7 does not give which operating conditions the model has large deviation from the experimental results. Therefore, Figs. 9–11 give detailed comparison between the model and results for the range of capacity ratio and number of transfer units measured. In Fig. 9, the model effectiveness at CR = 0.5 and 0.6 are plotted as continuous lines, and the experimental results at the same range of CR are plotted as marking points. Similar results are given in Figs. 10 and 11 but for different ranges of CR. These three figures show that the deviation between the model and experimental results increases at lower CR values. The maximum deviation is 30% for CR = 0.5–0.6 (shown in Fig. 9), 19% for CR = 0.7–0.8 (shown in Fig. 10), and 6% for CR = 0.9–1.0 (shown in Fig. 11). The reason for that may be explained from the fact that at lower CR, the mass flow rate ratio is higher which indicates more water evaporation and higher humidification capacity (see Fig. 3). Therefore, at lower CR values the assumption of neglecting the water loss due to evaporation becomes significant, this gives large deviation in the effectiveness value. On the other hand, at higher CR values (i.e. higher MR) the water temperature difference is higher and this makes the assumption of linear relationship for saturation air enthalpy with respect to temperature significant. 5. Conclusion Experimental performance measurements and model validity comparisons of a packed-bed cross-flow humidifier are conducted at different mass flow rates, water inlet temperature, and packing volumes. The experimental performance shows that the humidification capacity and saturation efficiency both increase with the mass flow rate ratio, water inlet temperature, and packing volume. This is basically due to driving force for the heat and mass transfer processes increases with the increase of; (a) the water
temperature, (b) the mass flow rate ratio, and (c) the total transfer area (by increasing the packing volume). The results also show that the specific energy consumption of the humidifier is almost constant at all varied operating conditions with an average value of 0.9 kW h/kg. However, it is expected that this value will be different if a packing material with different specific surface area is used (i.e. surface area per unit volume). Since increasing the surface area will increase the humidification capacity for the same energy input. This prediction requires further experimental investigation with different packing materials. On the other hand, the effectiveness of the humidifier is calculated from the measured data and compared with the predicted one using Jaber and Webb’s effectiveness model originally derived for packed-bed cross-flow cooling tower. It was found that the model underestimates the effectiveness to a maximum of 30%. This large deviation occurs at lower capacity ratio (i.e. 0.5–0.6) and decreases to about 6% at higher capacity ratio values (i.e., 0.9–1). The reason for this large deviation is due to major assumptions of the model that neglects the water loss due to evaporation and assumes a linear relationship for the saturation air enthalpy with respect to temperature. A more robust and precise analytical model that does not make these assumptions is required for this types of humidifiers. Acknowledgements The authors would like to thank King Fahd University of Petroleum and Minerals in Dhahran, Saudi Arabia, for funding the research reported in this paper through National Science, Technology and Innovation Plan office, NSTIP Project # 11-WAT1625-04. References [1] ASHRAE Handbook - HVAC Applications, ASHRAE Inc., 2015. [2] N.G. Prakash, M.H. Sharqawy, E.K. Summers, J.H. Lienhard, S.M. Zubair, M.A. Antar, The potential of solar-driven humidification-dehumidification desalination for small-scale decentralized water production, Renew. Sustain. Energy Rev. 14 (2010) 1187–1201. [3] H. Zhang, Z. Qian, D. Yang, J. Ma, Design of an air humidifier for a 5 kW proton exchange membrane fuel cell stack operated at elevated temperatures, Int. J. Hydrogen Energy 38 (2013) 12353–12362. [4] S.K. Wang, Air Systems: Components - Fans, Coils, Filters, and Humidifiers, in: Handbook of Air Conditioning and Refrigeration, McGraw-Hill Education, 2000, pp. 1424. [5] R.E. Treybal, Mass Transfer Operations, third ed., McGraw-Hill, New York, NY, 1980. [6] H. Liu, M.H. Sharqawy, Experimental performance of bubble column humidifier and dehumidifier under varying pressure, Int. J. Heat Mass Transfer 93 (2016) 934–944. [7] N.G. Prakash, K.H. Mistry, M.H. Sharqawy, S.M. Zubair, J.H. Lienhard, Energy effectiveness of simultaneous heat and mass exchange devices, Front. Heat Mass Transfer 1 (2010). [8] F. Merkel, Verdunstungskühlung, VDI-Zeitschrift 70 (1925) 123–128. [9] M. Poppe, H. Rogener, Berechnung von Rückkühlwerken, VDI-Warmeatlas (1991) Mi 1–Mi 15. [10] J.E. Braun, S.A. Klein, J.W. Mitchell, Effectiveness models for cooling towers and cooling coils, ASHRAE Trans. 95 (1989) 164–174. [11] H. Jaber, R.L. Webb, Design of cooling towers by the effectiveness-NTU method, J. Heat Transfer 111 (1989) 837–843. [12] J.C. Kloppers, D.G. Kröger, A critical investigation into the heat and mass transfer analysis of counterflow wet-cooling towers, Int. J. Heat Mass Transfer 48 (2005) 765–777. [13] J.C. Kloppers, D.G. Kröger, A critical investigation into the heat and mass transfer analysis of cross-flow wet-cooling towers, Numer. Heat Transfer, Part A 46 (2004) 785–806. [14] R.W. Hyland, A. Wexler, Formulations for the thermodynamic properties of dry air from 173.15 K to 473.15 K, and of saturated moist air from 173.15 K to 372.15 K, at pressures to 5 MPa, ASHRAE Trans. 2 (1983) 520–535. [15] Release on the IAPWS Formulation 1995 for the Thermodynamic Properties of Ordinary Water Substance for General and Scientific Use, in: International Association for the Properties of Water and Steam, 1996. [16] W.M. Kays, A.L. London, Compact Heat Exchangers, third ed., McGraw-Hill, New York, 1984. [17] J.P. Holman, Experimental Methods for Engineers, seventh ed., McGraw-Hill, New York, 2001.
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Experimental investigation of packed-bed cross-flow humidifier Mostafa H. Sharqawy a,⇑, Ibrahim Al-Shalawi b, Mohamed A. Antar b, Syed M. Zubair b a b
School of Engineering, University of Guelph, Guelph, Ontario N1G 2W1, Canada King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
h i g h l i g h t s The performance of a cross-flow packed-bed humidifier is presented. Capacity, saturation efficiency, and specific energy consumption are calculated. The specific energy consumption is almost constant for the same packing material. Effectiveness-NTU model is adopted to estimate the effectiveness of the humidifier.
a r t i c l e
i n f o
Article history: Received 5 August 2016 Revised 11 February 2017 Accepted 14 February 2017 Available online 16 February 2017 Keywords: Humidifier Cross-flow Packed-bed Experimental Modelling Effectiveness Humidification capacity Saturation efficiency
a b s t r a c t This paper presents the experimental performance of a cross-flow packed-bed humidifier. In this humidifier, hot water is sprayed over packing material where air flows though it in a cross flow arrangement. The air is heated and humidified while flowing through the humidifier duct. The humidification capacity, saturation efficiency, and specific energy consumption are calculated using the experimental data of the inlet and outlet conditions for both air and water streams. The variation of these performance indicators with the mass flow rate ratio of water-to-air, the water inlet temperature, and the packing volume; is investigated. It was found that the specific energy consumption is almost constant with variation of the mass flow rate ratio, packing thickness, and water inlet temperature within the investigated range of these parameters. An effectiveness model originally developed for cross-flow packed-bed cooling tower is adopted to estimate the effectiveness of the humidifier. In addition, the effectiveness and the number of transfer units of the humidifier are determined using the experimental data. The model was found to be in a good agreement with the experimental measurement at high capacity ratio with a deviation of 6%. However, the model underestimates the effectiveness at lower capacity ratio with a maximum deviation of 30%. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Humidifiers are devices which increase the moisture content of air or a carrier gas. They are widely used in heating, ventilation, and air conditioning (HVAC) systems to ensure a comfortable and healthy environment for workers and residents [1]. Industrial humidifiers are used to adjust the humidity level to prevent static electricity buildup in industries such as packaging, plastics, textiles, electronics, semiconductors, automotive manufacturing, pharmaceuticals, electrostatic painting, and powder coating. Humidifiers are used also to preserve material properties such as in paper industry to prevent shrinkage and paper curl and in food industry to maintain the freshness of food against the dryness
⇑ Corresponding author. E-mail address: [email protected] (M.H. Sharqawy). http://dx.doi.org/10.1016/j.applthermaleng.2017.02.061 1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.
caused by cold temperatures in cold storage rooms. In addition, humidifiers are used in medical ventilators and hospital operating rooms. Recently, humidifiers are used in small-scale water desalination systems such as the humidification dehumidification desalination (HDH) process [2] as well as in proton exchange membrane (PEM) fuel cells [3]. There are different types of humidifiers however, they can be classified as steam or heating element humidifiers, atomizing or spray humidifiers, wetted element or packed bed humidifiers [4,5], and bubble columns or spargers [6]. Steam humidifiers inject steam directly into air or add heat to evaporate supplied water to the conditioned space. Steam and heating element humidifiers consume large energy generated from gas, fossil fuel, or electricity to evaporate the liquid water in a steam generator [4]. Atomizing or spray humidifiers spray fine water droplets into air stream which are evaporated and added to the air. There are different
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585
Nomenclature Ac As CR cw f h Km _ m P T v
cross sectional area (m2) total surface area (m2) capacity ratio (–) specific heat of water (kJ kg1 K1) slope of the saturated air enthalpy versus temperature (kJ kg1 K1) specific enthalpy (kJ kg1) mass transfer coefficient (kg m2 s1) mass flow rate (kg s1) pressure (kPa) temperature (°C) velocity (m s1)
Greek letters e effectiveness (–) g efficiency (–) q density (kg m3) r correction factor defined by Eq. (5) (kJ kg1) x humidity ratio (kgv kg1 a )
mechanisms of atomizing such as ultrasonic, centrifugal atomizing, pneumatic atomizing, and air washers. Wetted element humidifiers force air to flow over a water film so that water diffuses into the air stream as vapor. Packed bed humidifier is a wetted element humidifier where the elements are made of random or structured packing material which has high surface area-to-volume ratio that increases the diffusion surface area. The principle of operation for all of these devices is same. When water is brought into contact with air that is not saturated with water vapor, water diffuses into air and raises the humidity. If the water temperature is higher than the air temperature, the air will be heated and humidified. If the water temperature is lower than the dew point temperature of the air (i.e. chilled water is used), the air will be cooled and dehumidified. If the water is circulated adiabatically (i.e. without being heated or cooled), it will approach the wet bulb temperature and the air will be cooled and humidified which is known as the air washer or evaporative cooling process. Packing material is typically used in wetted element humidifiers to increase the dispersion of water droplets, the contact area, and the contact time. Devices that contain packing material are known as packed bed towers with water sprayed at the top and air flows in counter or cross flow arrangement. Special types of packed bed towers that are used to cool water are called cooling towers. The performance of humidifiers is usually measured by its humidification capacity which is the rate of water vapor added to the air [4]. This capacity is limited when the air becomes saturated. Therefore, the saturation efficiency is also used as a performance index for humidifiers which is defined as the increase in the humidity ratio to the maximum increase in the humidity when the air is fully saturated at its exit temperature. The energy effectiveness (or simply effectiveness) of the humidifier is an important performance indicator which is defined as the ratio of the total energy transferred to the air (both latent and sensible) to the energy that will be consumed if the exit air is fully saturated at the water inlet temperature [7]. There are few analytical models that could be used to design or evaluate the performance of humidifiers. These models were originally derived for counter-flow wet cooling towers such as Merkel’s model [8], Poppe and Rogener [9] (known as Poppe’s model), Braun’s effectiveness model [10], and the e-NTU (effectiveness – number of transfer units) model by Jabber and Webb [11]. Critical
Subscripts a dry air av average c corrected in inlet min minimum max maximum out outlet sat saturated air condition th theoretical v vapor w water Abbreviations MR mass flow rate ratio NTU number of transfer units SEC specific energy consumption (kW h kg1)
assumptions were made in some of these models in order to obtain an analytical solution. These assumptions are such as neglecting the water loss due to evaporation, a Lewis number of unity, and a linear relationship for saturation air enthalpy with respect to water temperature. Klopper and Kröger [12,13] gave a detailed review and comparisons of these models and found that Poppe’s model is more accurate than the Merkel and e-NTU ones for both counter and cross-flow cooling tower. However, Poppe’s model equations must be solved numerically and iteratively and it is relatively complex. On the other hand, e-NTU model by Jabber and Webb [11] can be employed under normal ambient conditions if only the water outlet temperature is an important consideration in the design of a cooling tower. In this paper, cross-flow humidifier is investigated experimentally to evaluate its performance. The inlet air is heated and humidified using hot water which is sprayed at the top of structure packing material. The effect of the mass flow rate ratio of the water and air as well as the water inlet temperature and packing volume on the humidification capacity, saturation efficiency, and specific energy consumption is presented. Finally, the e-NTU model of Jabber and Webb [11] originally developed for counter- and crossflow cooling tower is adopted to predict the humidifier effectiveness which is compared with the measured one. The reason for choosing this model to validate its accuracy for the cross-flow humidifier, is that it is has an analytical expression for the effectiveness and does not need complex numerical and iterative procedures. 2. Experimental details A schematic of the packed-bed cross-flow humidifier used in this study is shown in Fig. 1. The humidifier consists of a duct with cross sectional dimensions of 30 cm 30 cm and a length of 90 cm. Three structured packings (Brentwood XF125 cross-fluted film fill) of 10 cm thickness are installed inside the duct and separated by a distance of 20 cm. The purpose of using three packings in-series is to study the effect of increasing the packing volume (or thickness) which consequently increases the total surface area and the number of transfer units. An axial flow fan is installed at the humidifier entrance so that the air at room temperature is blown through the humidifier and passes through the packing material. Hot water is sprayed over the packing material and flows downward to a small
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Fig. 1. Schematic of the cross-flow humidifier used in the present work.
water basin at the bottom of the humidifier duct through which the water returns back to a water tank. The water in the tank is heated by electric heating elements that have a total heating capacity of 8 kW. The electric heaters are connected to an AC power source through a voltage controller to regulate the power and provide a controlled water inlet temperature to the humidifier. The hot water is returned back to the humidifier by a centrifugal pump and a ball valve is used to regulate the water flow rate. Drift eliminators are installed at the air downstream side of each fill to strip the water droplets that are carried out by the humidified air. Instruments are installed to measure the temperatures and flow rates of both air and water streams. K-type thermocouples are installed at the inlet and outlet of the air and water streams to measure the dry bulb, wet bulb, and water temperatures. For the wet-bulb temperature measurement, the thermocouple junction was wrapped by a wet wick supplied by water from a gravity feeding syringe. Water flow rate is measured using a glass tube rotameter (Omega FL-50004A) of ±5.0% accuracy and a range of 1.8–18 L/ min. The air velocity is measured using a digital anemometer (Smart Sensor AR 836) of ±3.0% accuracy. A handheld temperature data logger is used to display and record the measured temperatures from the thermocouples. All thermocouples were calibrated using a liquid-in-glass thermometer while the rotameter and anemometer were calibrated by their manufacturers. The uncertainty of the calculated performance parameters (i.e. the humidification capacity, saturation efficiency, and effectiveness) is determined using the method described by Holman [17] and knowing the uncertainty of the individual measuring instruments. Water inlet temperature is adjusted at specific values of 35, 45, and 55 °C using the voltage controller connected to the electric heaters in the water tank. The water flow rate is adjusted using a ball valve after the pump and the flow rate is adjusted at different values to ensure a range of mass flow rate ratio of the water-to-air. The air fan draws ambient air to the humidifier at almost constant volumetric flow rate. The air flow rate is measured at the humidifier exit using a digital anemometer. All temperatures are recorded automatically by digital data loggers every 5 s until a steady state condition is approached at which the variables are changing only within their uncertainty band. All measurements are taken at room condition which has an air temperature of 22 °C ± 2 °C and atmospheric pressure which is assumed to be 101.325 kPa. During the experiment, hot water is sprayed over the packing material such that the surface area would be large enough for interaction with
Fig. 2. Representation of the heating and humidification process on the psychrometric chart.
the air stream that passes in a cross flow direction such that air would be heated and humidified as shown in Fig. 2. The driving potential for heat transfer is temperature difference between both streams whereas another driving potential for mass transfer due to the difference in water-vapor concentration. 3. Mathematical analysis The purpose of this analysis is to use the experimental results to calculate the performance parameters of the cross-flow humidifier. The psychrometric properties of moist air [14] and the thermodynamics properties of water [15] are used in these calculations. Fig. 2 shows the humidification processes applied to air on the psychrometric chart. As shown in this figure, the air is heated while it is humidified and this happens because the sprayed water is at a higher temperature than the air inlet one. This humidification and heating process is similar to the process occurs in a cooling tower. However, in the cooling tower the aim is to cool the hot process water. Therefore, the analysis of both devices is nearly alike except that the performance parameters are different. The humidification capacity is one of the important parameters that characterize any humidifier. It is the amount of water vapor
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that is added to the flowing air. It is important to select or design the humidifier with a humidification capacity that matches with the humidifying load required for the space or process so as to maintain a predetermined relative humidity. The humidification capacity of the humidifier can be calculated as:
_ a ðxout xin Þ _v ¼m m
ð1Þ
_ a is the mass flow rate of dry air and xout and xin are the where m outlet and inlet air humidity ratio, respectively. The dry air mass flow rate is calculated using the continuity equation knowing the average air velocity, density, and cross-sectional area as given by,
_ a ¼ qa Ac v m
ð2Þ
The saturation efficiency is another performance index that characterizes humidifiers. It is defined as the amount of water vapor transferred to the air to the maximum amount of water vapor when the exit air is saturated. This efficiency is used also with air washers and evaporative coolers as represented by,
gsat ¼
xout xin xout;sat xin
ð3Þ
The maximum humidity difference in the saturation efficiency definition (i.e. the denominator of Eq. (3)) assumes that the outlet air is saturated at the exit air temperature. Therefore, this definition ignore the possibility of increasing the air temperature (hence the saturation humidity) to the inlet hot water temperature. In this case, the saturation efficiency defined by Eq. (3) will be higher compared with the one calculated using the maximum humidity ratio occurs when the air temperature reaches the inlet water temperature. In order to combine both the heat and mass transfer effects occurring in the humidifier, the energy effectiveness should be used [7]. This energy effectiveness is given by [11],
_ a ðha;out ha;in Þ m e¼ _ min ðha;sat T r ha;in Þ m
ð4Þ
w;in
where ha;sat T
w;in
is the saturated air enthalpy at the water inlet tem-
perature, r is a correction factor to correct for the non-linearity relationship between the temperature and the enthalpy of saturated air. This correction factor is given by Jaber and Webb [11]. It can be written as,
r ¼ ha;sat T w;in þ ha;sat T w;out 2ha;sat T w;av
. 4
ð5Þ
The minimum mass flow rate given in Eq. (4) is the minimum of the dry air mass flow rate and a corrected mass flow rate of water, which is given by;
_ w cw =f _ w;c ¼ m m
ð6Þ
where f is the slope of the saturated air enthalpy versus temperature curve given by;
f ¼
ha;sat T
w;in
ha;sat T w;out
ð7Þ
T w;in T w;out
The purpose of calculating the energy effectiveness by Eq. (4) is to adapt Jaber and Webb’s model for the cross-flow cooling tower for the current cross-flow humidifier. This model calculates the effectiveness using the number of transfer units (NTU) and the capacity rate ratio (CR). The number of transfer unit in this model is given by,
NTU ¼
N K m As X Dh a;i ¼ _ min m ha;sat ha;i i¼1
ð8Þ
T w;i
where Km is the overall mass transfer coefficient, and As is the total surface area of the packing material. The number of transfer units
could be also calculated by performing the numerical integration given in Eq. (8). In this integration scheme, an energy balance is applied between the air and water after dividing the total water temperature difference (or the range) into N segments. The enthalpy of saturated air is calculated at the water temperature divisions and the summation of the ratio of the air enthalpy increase to the potential enthalpy is calculated (as given in Eq. (8)). In this study the experimental NTU is calculated by performing the numerical integration given above since the overall mass transfer coefficient is not known. On the other hand, the capacity ratio (CR) is given by [11].
CR ¼
_ min m _ max m
ð9Þ
where the minimum and maximum flow rates of Eq. (9) are the ones for the dry air flow rate (given by Eq. (2)) and the corrected water flow rate (given by Eq. (6)). Using the NTU and the capacity rate ratio defined above, Jaber and Webb [11] recommended to use the correlation of cross-flow heat exchanger with both fluids unmixed to calculate the theoretical (or model) effectiveness. This correlation is given in Kays and London’s book [16] as,
eth ¼ 1 exp
1 NTU 0:22 ðexpðC R NTU 0:78 Þ 1Þ CR
ð10Þ
The specific energy consumption (SEC), is the energy consumed per unit water vapor added to the dry air during the humidification process. This is an important performance index for the humidifier and could be used to compare different humidifiers from the energy consumption prospective. The specific energy consumption can be calculated by,
SEC ¼
_ w ðhw;in hw;out Þ T w;in T w;out m ¼ MR cw _ a ðxout xin Þ xout xin m
ð11Þ
where MR is the mass flow rate ratio of the water and dry air, and cw is the average specific heat of water. As noticed in Eq. (11), SEC depends on the mass flow rate ratio and the change in water temperature per change in air humidity ratio. 4. Results and discussion The experimental performance of the cross flow humidifier is evaluated by calculating the humidification capacity, saturation efficiency, specific energy consumption, and the energy effectiveness using the correlations presented in the previous section. The variation of the performance with the operating conditions is investigated by changing the operating parameters. The parameters which are investigated in this study are: The mass flow rate ratio (MR) of water to air is changed by varying the water flow rate only while the volume flow rate of air is almost constant from the fan. However, the mass flow rate of air is changing due to changes in the air density. The mass flow rate ratio is changed within a range of 0.5–3.5. The inlet water temperature sprayed on the packing material is varied at 35, 45, and 55 °C. The volume of the packing material is changed by using 1, 2, and 3 packings of 10 cm thickness each. Increasing the packing volume in the air flow direction, increases the total surface area. Fig. 3 shows the humidification capacity in kg/hr as it changes with the mass flow rate ratio for 1, 2, and 3 fills and at different water inlet temperatures. As shown in this figure, the humidification capacity increases with the mass flow rate ratio until approaches an asymptotic value. That is when the air exiting the humidifier is saturated with water vapor. The increase of mass flow rate ratio results in higher water heating capacity which ensure keeping the water temperature high and hence heats and humidify
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7 6
1 1 Fill Tw,in = 35 °C Tw,in = 45 °C Tw,in = 55 °C
Saturation Efficiency (ηs)
Humidification Capacity, kg/hr
8
5 4 3 2
0.9 0.8 0.7 0.6
1 Fill Tw,in = 35 °C Tw,in = 45 °C Tw,in = 55 °C
0.5
1 0 0.5
1
1.5
2
2.5
3
0.4 0.5
3.5
1
6
2.5
Tw,in = 35 °C Tw,in = 45 °C Tw,in = 55 °C
4 3 2
0.8
0.7
2 Fills Tw,in = 35 °C
0.6
Tw,in = 45 °C Tw,in = 55 °C
1
1.5
2
2.5
3
0.5 0.5
3.5
1
8
2
2.5
3
3.5
1 3 Fills
Saturation Efficiency (ηs)
Tw,in = 35 °C Tw,in = 45 °C Tw,in = 55 °C
5 4 3 2 1 0 0.5
1.5
Mass Flow Rate Ratio (MR)
Mass Flow Rate Ratio (MR)
6
3.5
0.9
1
7
3
1
5
0 0.5
Humidification Capacity, kg/hr
2
2 Fills
Saturation Efficiency (ηs)
Humidification Capacity, kg/hr
8 7
1.5
Mass Flow Rate Ratio (MR)
Mass Flow Rate Ratio (MR)
0.9
0.8
0.7
3 Fills Tw,in = 35 °C
0.6
Tw,in = 45 °C Tw,in = 55 °C
1
1.5
2
2.5
3
3.5
Mass Flow Rate Ratio (MR)
0.5 0.5
1
1.5
2
2.5
3
3.5
Mass Flow Rate Ratio (MR)
Fig. 3. Effect of mass flow rate ratio on humidification capacity.
Fig. 4. Effect of mass flow rate ratio on saturation efficiency.
the air. We notice also that the humidification capacity increases with the water temperature since the driving force for the heat and mass transfer processes increases with the water temperature. On the other hand, by increasing the packing material thickness (i.e. by adding more packing), the total heat and mass transfer surface area increases which lead to higher humidification capacity. In Fig. 4, the saturation efficiency of the humidifier is plotted vs. the mass flow rate ratio at all packing thicknesses (i.e. with 1, 2, and 3 fills) and water inlet temperatures. Generally, the saturation efficiency increases with MR and approaches unity at higher MR values. That is, when the air exiting the humidifier is saturated with water vapor. The figure also shows that the saturation efficiency increases with the water inlet temperature and thickness of the packing material (or the number of fills). These results match with the finding of the humidification capacity shown in previous figure. How-
ever, it was noticed that the effect of the water inlet temperature is not significant at higher packing thickness (i.e. when using 3 fills). Since the air has longer time and flow distance within the packing material so that to get saturated. Therefore, the water temperature has less significant effect than the packing thickness. Fig. 5 shows the variation of the specific energy consumption with the mass flow rate ratio for all packing thicknesses. The specific energy consumption as defined by Eq. (11) varies between 0.85 and 0.95 with an average value of almost 0.9 kW h/kg. This could be explained as the mass flow rate ratio increases, the water temperature difference per unit humidity ratio difference decreases, which cancels the MR effect. This is clearly shown in Fig. 6 and is obvious from Eq. (11). It is an important finding of having almost constant specific energy consumption for cross flow humidifier. Therefore, both the energy and humidification capacity increases with almost the same rate when larger humidifier is designed or
589
1.2 1.1
14
1 Fill 2 Fills 3 Fills
1 Fill
12
2 Fills 3 Fills
10
1
% Heat Loss
Spc. Energy Consumption, kWh/kg
M.H. Sharqawy et al. / Applied Thermal Engineering 117 (2017) 584–590
0.9 0.8
8 6 4
0.7 0.6 0.5
2 1
1.5
2
2.5
3
0 0.5
3.5
1
Mass Flow Rate Ratio (MR)
1.5
2
2.5
3
3.5
Mass Flow Rate Ratio (MR)
Fig. 5. Effect of mass flow rate ratio on specific energy consumption.
Fig. 8. Percentage heat loss from the humidifier surface.
1
C R = 0.5 and 0.6
Effectiveness
0.8
0.6
0.4
0.2
0
0
0.5
1
1.5
2
2.5
3
Number of Transfer Units (NTU) Fig. 6. The variation of the water temperature difference per air humidity ratio difference with the mass flow rate ratio.
Fig. 9. Measured and predicted air effectiveness vs. the number of transfer units at capacity ratio range of 0.5–0.6.
1 1
C R = 0.7 and 0.8
2 Fills
0.8
3 Fills
0.8
Effectiveness
Cross Flow Effectiveness
1 Fill
0.6
0.4
0.6
0.4
0.2 0.2 0
0
0.2
0.4
0.6
0.8
1
Measured Effectiveness
0
0
0.5
1
1.5
2
2.5
3
Number of Transfer Units (NTU) Fig. 7. A comparison between the predicated and the measured air effectiveness.
selected. However, it is expected that SEC will have a different constant value when different packing with higher or lower specific surface area is used. Increasing the surface area will increase the humidification capacity for the same energy input. This prediction requires further experimental investigation with different packing materials. Fig. 7 shows a comparison between the experimental values of the effectiveness (calculated by Eq. (4)) and the model estimates
Fig. 10. Measured and predicted air effectiveness vs. the number of transfer units at capacity ratio range of 0.7–0.8.
(calculated by Eq. (10)). The NTU and CR provided for the model estimates were calculated using the experimental data and Eqs. (8) and (9), respectively. As shown in Fig. 7, the maximum and minimum deviations between the model and the experimental results of the effectiveness are 15% and 30%, respectively. This is considered as large over and under estimates which is due to the
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Number of Transfer Units (NTU) Fig. 11. Measured and predicted air effectiveness vs. the number of transfer units at capacity ratio range of 0.9–1.0.
assumptions made in this e-NTU model. The major assumptions of this model are neglecting the water loss due to evaporation, a Lewis number of unity (defined as the ratio of thermal diffusivity to mass diffusivity which relates the heat and mass transfer coefficients), and a linear relationship for saturation air enthalpy with respect to temperature. In addition, this bias may be also associated to the errors in the measurements of the air outlet state, in particular with the wet bulb temperature. In addition, there is heat loss from the humidifier duct surface due to the hot air inside. This heat loss is calculated and found to be in the range of 4–10% from the supplied energy by the hot water as shown in Fig. 8. However, the model assumes that there is no heat loss. Since the model tends to under predict and over predict the effectiveness at different operating conditions, the comparison made in Fig. 7 does not give which operating conditions the model has large deviation from the experimental results. Therefore, Figs. 9–11 give detailed comparison between the model and results for the range of capacity ratio and number of transfer units measured. In Fig. 9, the model effectiveness at CR = 0.5 and 0.6 are plotted as continuous lines, and the experimental results at the same range of CR are plotted as marking points. Similar results are given in Figs. 10 and 11 but for different ranges of CR. These three figures show that the deviation between the model and experimental results increases at lower CR values. The maximum deviation is 30% for CR = 0.5–0.6 (shown in Fig. 9), 19% for CR = 0.7–0.8 (shown in Fig. 10), and 6% for CR = 0.9–1.0 (shown in Fig. 11). The reason for that may be explained from the fact that at lower CR, the mass flow rate ratio is higher which indicates more water evaporation and higher humidification capacity (see Fig. 3). Therefore, at lower CR values the assumption of neglecting the water loss due to evaporation becomes significant, this gives large deviation in the effectiveness value. On the other hand, at higher CR values (i.e. higher MR) the water temperature difference is higher and this makes the assumption of linear relationship for saturation air enthalpy with respect to temperature significant. 5. Conclusion Experimental performance measurements and model validity comparisons of a packed-bed cross-flow humidifier are conducted at different mass flow rates, water inlet temperature, and packing volumes. The experimental performance shows that the humidification capacity and saturation efficiency both increase with the mass flow rate ratio, water inlet temperature, and packing volume. This is basically due to driving force for the heat and mass transfer processes increases with the increase of; (a) the water
temperature, (b) the mass flow rate ratio, and (c) the total transfer area (by increasing the packing volume). The results also show that the specific energy consumption of the humidifier is almost constant at all varied operating conditions with an average value of 0.9 kW h/kg. However, it is expected that this value will be different if a packing material with different specific surface area is used (i.e. surface area per unit volume). Since increasing the surface area will increase the humidification capacity for the same energy input. This prediction requires further experimental investigation with different packing materials. On the other hand, the effectiveness of the humidifier is calculated from the measured data and compared with the predicted one using Jaber and Webb’s effectiveness model originally derived for packed-bed cross-flow cooling tower. It was found that the model underestimates the effectiveness to a maximum of 30%. This large deviation occurs at lower capacity ratio (i.e. 0.5–0.6) and decreases to about 6% at higher capacity ratio values (i.e., 0.9–1). The reason for this large deviation is due to major assumptions of the model that neglects the water loss due to evaporation and assumes a linear relationship for the saturation air enthalpy with respect to temperature. A more robust and precise analytical model that does not make these assumptions is required for this types of humidifiers. Acknowledgements The authors would like to thank King Fahd University of Petroleum and Minerals in Dhahran, Saudi Arabia, for funding the research reported in this paper through National Science, Technology and Innovation Plan office, NSTIP Project # 11-WAT1625-04. References [1] ASHRAE Handbook - HVAC Applications, ASHRAE Inc., 2015. [2] N.G. Prakash, M.H. Sharqawy, E.K. Summers, J.H. Lienhard, S.M. Zubair, M.A. Antar, The potential of solar-driven humidification-dehumidification desalination for small-scale decentralized water production, Renew. Sustain. Energy Rev. 14 (2010) 1187–1201. [3] H. Zhang, Z. Qian, D. Yang, J. Ma, Design of an air humidifier for a 5 kW proton exchange membrane fuel cell stack operated at elevated temperatures, Int. J. Hydrogen Energy 38 (2013) 12353–12362. [4] S.K. Wang, Air Systems: Components - Fans, Coils, Filters, and Humidifiers, in: Handbook of Air Conditioning and Refrigeration, McGraw-Hill Education, 2000, pp. 1424. [5] R.E. Treybal, Mass Transfer Operations, third ed., McGraw-Hill, New York, NY, 1980. [6] H. Liu, M.H. Sharqawy, Experimental performance of bubble column humidifier and dehumidifier under varying pressure, Int. J. Heat Mass Transfer 93 (2016) 934–944. [7] N.G. Prakash, K.H. Mistry, M.H. Sharqawy, S.M. Zubair, J.H. Lienhard, Energy effectiveness of simultaneous heat and mass exchange devices, Front. Heat Mass Transfer 1 (2010). [8] F. Merkel, Verdunstungskühlung, VDI-Zeitschrift 70 (1925) 123–128. [9] M. Poppe, H. Rogener, Berechnung von Rückkühlwerken, VDI-Warmeatlas (1991) Mi 1–Mi 15. [10] J.E. Braun, S.A. Klein, J.W. Mitchell, Effectiveness models for cooling towers and cooling coils, ASHRAE Trans. 95 (1989) 164–174. [11] H. Jaber, R.L. Webb, Design of cooling towers by the effectiveness-NTU method, J. Heat Transfer 111 (1989) 837–843. [12] J.C. Kloppers, D.G. Kröger, A critical investigation into the heat and mass transfer analysis of counterflow wet-cooling towers, Int. J. Heat Mass Transfer 48 (2005) 765–777. [13] J.C. Kloppers, D.G. Kröger, A critical investigation into the heat and mass transfer analysis of cross-flow wet-cooling towers, Numer. Heat Transfer, Part A 46 (2004) 785–806. [14] R.W. Hyland, A. Wexler, Formulations for the thermodynamic properties of dry air from 173.15 K to 473.15 K, and of saturated moist air from 173.15 K to 372.15 K, at pressures to 5 MPa, ASHRAE Trans. 2 (1983) 520–535. [15] Release on the IAPWS Formulation 1995 for the Thermodynamic Properties of Ordinary Water Substance for General and Scientific Use, in: International Association for the Properties of Water and Steam, 1996. [16] W.M. Kays, A.L. London, Compact Heat Exchangers, third ed., McGraw-Hill, New York, 1984. [17] J.P. Holman, Experimental Methods for Engineers, seventh ed., McGraw-Hill, New York, 2001.