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Experimental performance comparison of adiabatic and internally-cooled membrane dehumidifiers

Experimental performance comparison of adiabatic and internally-cooled membrane dehumidifiers Humidity control of indoor space using the conventional air conditioning system is energy intensive. The liquid desiccant dehumidifier, which operates on low grade energy sources, is one of the energy effi- cient alternatives for humidity control. Membrane dehumidifiers avoid the desiccant carryover and hence are preferred over the packed bed dehumidifiers. However, their performance is lower due to the add- itional resistance in the membrane. Internal cooling is one way to improve the performance of the mem- brane dehumidifier and the present study experimentally investigates such a dehumidifier. The operating parameters considered are specific humidity, mass flow rate, temperature and of inlet air. The perfor- mances of the adiabatic and internally cooled dehumidifiers are presented in terms of moisture removal rate and latent effectiveness. It is found that these are higher by 60 and 50%, respectively, for the intern- ally cooled dehumidifier. Keywords: internally cooled membrane dehumidifier; liquid desiccant dehumidification; moisture removal rate; latent effectiveness; experimental analysis *Corresponding author: mpmaiya@iitm.ac.in Received 10 January 2018; revised 6 March 2018; editorial decision 3 May 2018; accepted 10 May 2018 ......... ................. ................ ................. ................. ................ ................. ................. . ............... ................. ................. system is energy inefficient due to overcooling followed by 1 INTRODUCTION reheating of the air. Thus, alternative energy efficient systems According to the International Energy Outlook 2013, building have been studied for the control of humidity in many AC sector consumes one fifth of the total global energy consump- applications. One such prospective system is desiccant dehu- tion. India too follows the same trend. Further, more than 60% midifier which utilizes the renewable low grade energy sources of the buildings projected for 2030 are yet to be built. These are for its regeneration [3]. The hybrid AC system combines such a also expected to have increased demand of thermal comfort due desiccant dehumidifier with the conventional cooling system. to both growth in urbanization and increased aspiration for Desiccant dehumidification is the process of removing water better human comfort. Thus, the energy share required for con- vapor from air by absorbing it in the desiccant, which may be trolling the indoor conditions is expected to increase to ~45% liquid or solid. The former is selected for the present study due of the total building energy consumption from the present of to its advantages such as high moisture holding capacity, low ~25% [1]. Apart from temperature, control of humidity plays a airside pressure drop and low regeneration temperature. vital role in air conditioning (AC) at tropical climate and also Moreover, it facilitates air sterilization, operational flexibility for many special applications such as hospitals, electronic labs, and utilization of the low grade thermal energy sources such as museums etc. to maintain the required low indoor humidity solar or waste heat for its regeneration [4]. The liquid desiccant [2]. Cooling air below its dew point temperature to condense systems are classified as direct contact-packed bed and indirect the water vapor is the standard method of dehumidification contact-membrane systems. The latter is preferred to avoid the adopted in the conventional AC systems. Air has to be cooled problems associated with desiccant carryover such as health to low temperature and then heated before it enters the AC hazard and corrosion of equipment [5]. While the membrane room to control humidity. Therefore, the conventional AC avoids direct contact between air and desiccant, its micro-pores International Journal of Low-Carbon Technologies 2018, 13, 240–249 © The Author(s) 2018. Published by Oxford University Press. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/ by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com doi:10.1093/ijlct/cty020 Advance Access Publication 29 May 2018 240 Downloaded from https://academic.oup.com/ijlct/article-abstract/13/3/240/5020711 by Ed 'DeepDyve' Gillespie user on 18 October 2019 allow water vapor to get transferred between them. However, increase in such parameters increases the performance of both its mass transfer performance is lower than that of the packed adiabatic and internally cooled dehumidifiers. However, the effect bed dehumidifier due to the additional resistance imposed by of mass flow rate of desiccant is significant on the former. A the intermediate membrane. There are many ways such as numerical model of the internally cooled dehumidifier is required internal cooling, provision of nanofibrous membrane, providing to analyze its performance under various climatic conditions. micro-fins and so on to improve the performance [6]. Present Huang et al. [12] developed one such model and validated using study analyses the performance improvement of the membrane their experimental results. The governing mass, momentum and dehumidifier by internal cooling. Flat-plate configuration is energy equations were solved to find the Nusselt and Sherwood selected for the present study due to its suitability for multi- numbers for the heat and mass transfer processes of the dehu- stream applications, ease of assembly and less airside pressure midifier. Woods and Kozubal [11] analyzed the influence of air, drop over the hollow-fiber configuration [7]. desiccant and membrane on the heat and mass transfer resis- Dehumidifiers are broadly divided into two types, namely tances of internally cooled membrane dehumidifier. It is reported adiabatic and internally cooled. Cooling of the desiccant is that the air resistance accounts for 70% of the overall heat transfer essential to make it absorb water vapor from air. In the adia- resistance while the air and membrane resistances together batic dehumidifier, the desiccant is precooled in an external account for 90% of the overall mass transfer resistance. heat exchanger as shown in Figure 1a, to facilitate it to absorb From the above narratives, it can be concluded that most of water vapor from the air flowing in the adjacent channel sepa- the previous works of internally cooled membrane dehumidi- rated by the membrane. The heat of absorption raises the desic- fiers are focused only on its initial design and development. cant temperature which reducing its capacity to absorb water The parametric experimental investigations on such dehumidi- vapor. In the internally cooled type, the desiccant is continu- fiers are scarce to compare their performance with those of ously cooled as shown in Figure 1b, while it absorbs water adiabatic membrane dehumidifier. Thus, the main objective of vapor thereby improving the capacity. the present paper is to experimentally investigate the perform- Membrane dehumidifiers are developed recently as an alterna- ance of the internally cooled membrane dehumidifier for the tive to the conventional packed bed dehumidifiers. Hence, their hot and humid climatic conditions prevailing in the city of literature, especially with internal cooling arrangement is limited. Chennai, India. The operating parameters considered are mass Isetti et al. [8] developed the first prototype of such a dehumidi- flow rate, inlet temperature and specific humidity of air. The fier. Potassium formate solution (desiccant) was cooled by the performances of the adiabatic and internally cooled dehumidi- cooling water in a plate heat exchanger. Air flows in the hydro- fiers are presented in terms of moisture removal rate (MRR) phobic polypropylene membrane tubes placed in the desiccant and latent effectiveness. The presented results are useful in the channel in cross-flow direction. The parametric study revealed optimum design of the membrane dehumidifiers. that the performance of the dehumidifier was better at low inlet temperature of the cooling water. Abdel-Salam et al. [9] experi- 2 DESCRIPTION OF MEMBRANE mentally reconfirmed the influence of inlet temperature and further found that the high water flow rate enhances the perform- DEHUMIDIFIER ance. The paper details the issues in manufacturing of the intern- ally cooled dehumidifier without leaks due to the additional Figure 2 shows the schematic diagram of the membrane dehu- cooling water channel. The results concluded that the internally midifier. It has three channels, one each for air, desiccant and cooled dehumidifier is better than the adiabatic dehumidifier. The cooling water with their flow in counter-flow direction. Upward results also indicate that the inlet temperature of cooling water entry of desiccant is adopted to avoid flow maldistribution [13]. has to be critically selected to avoid the temperature drop of des- Flow guides are provided to make the desiccant flow direction iccant in the dehumidifier. Later, the same authors extended their counter to both air and cooling water. The membrane is attached study [10] and analyzed the effect of inlet specific humidity of air to a metal mesh using double-sided foam tape and metal screws to and mass flow rate of desiccant. The results indicated that an avoid its deflection. The design details of the membrane dehumidi- fier are listed in Table 1. Experimental studies of both adiabatic (a) (b) and internally cooled dehumidifiers are carried out in the same dehumidifier, the former by switching off the cooling water pump. 3 EXPERIMENTAL SETUP AND INSTRUMENTATION The schematic diagram and photograph of the experimental Figure 1. Schematic diagram of operating conditions of the (a) adiabatic setup of membrane dehumidifier are shown in Figures 3 and 4 and (b) internally cooled membrane dehumidifiers. respectively. It consists of three circuits, namely the air, International Journal of Low-Carbon Technologies 2018, 13, 240–249 241 Downloaded from https://academic.oup.com/ijlct/article-abstract/13/3/240/5020711 by Ed 'DeepDyve' Gillespie user on 18 October 2019 G. Annadurai et al. Figure 2. Schematic diagram of the membrane dehumidifier. Table 1. Design details of the membrane dehumidifier. the dehumidifier. Temperature, relative humidity and flow rate are measured at all the key locations as shown in Figure 3. Sl. No. Parameter Value Aqueous solution of lithium chloride is used as desiccant [14] 1 Channel spacing (mm) 5 and its circuit consists of supply and storage tanks, and a peri- 2 Dehumidifier length (m) 1.1 staltic pump. Sufficient quantity of desiccant with desired con- 3 Dehumidifier height (m) 0.55 centration is filled in the supply tank. It is maintained at the 4 Membrane material PVDF 5 Membrane pore diameter (μm) 0.2 desired temperature using water from the constant temperature 6 Membrane thickness (mm) 0.22 water bath. The desiccant flow rate to the membrane dehumidi- 7 Plate thickness (mm) 1.2 fier is adjusted by controlling the speed of the pump and its flow rate is measured. Desiccant density and temperature are mea- sured both at inlet and outlet of the dehumidifier as shown in desiccant and cooling water with the provision to control the Figure 3 and the respective concentrations are calculated [15]. respective operating parameters namely, (a) flow rate, inlet tem- The cooling water circuit contains a constant temperature bath perature and specific humidity for air (b) flow rate, inlet tem- and a peristaltic pump to control the water inlet temperature to perature and concentration for desiccant and (c) flow rate and the dehumidifier and its flow rate respectively. Inlet and outlet inlet temperature for cooling water. temperatures, and flow rate of the water are measured. The The air circuit consists of an ultrasonic humidifier, cooler and details of the instruments used in the experimental setup are heater by which the desired climatic conditions (temperature listed in Table 2. The data acquisition system records all the and humidity) can be achieved. The cooler is supplied with experimental data namely temperature, relative humidity, mass water at the desired temperature from a constant temperature flow rate and density at regular intervals. water bath. The air circuit has fan, inlet static mixer and flow All the sensors and instruments are pre-calibrated. The tem- straightener before the dehumidifier. The flow rate of air is perature sensors are calibrated using a constant temperature adjusted using fan speed control. The inlet and outlet headers bath for their entire working range. The relative humidity facilitate uniform air distribution in the rectangular channel of probes are calibrated using a dew point meter in a controlled 242 International Journal of Low-Carbon Technologies 2018, 13, 240–249 Downloaded from https://academic.oup.com/ijlct/article-abstract/13/3/240/5020711 by Ed 'DeepDyve' Gillespie user on 18 October 2019 Figure 3. Schematic diagram of the experimental setup. environment chamber for their entire humidity range at differ- � Set the data acquisition system to record data from the ent temperatures. A detailed error analysis [16] has been done instruments and sensors. estimating the uncertainty in the two performance parameters, � Open Valves V2 and V3. namely MRR and latent effectiveness, which are found to be � Switch on the fan and regulate it for the desired air flow. within ±5% and ±6%, respectively. � Open Valves V8, V9 and V10, and switch on Water pump 1. � Switch on Constant temperature water bath 1 and adjust it for the required inlet water temperature to the cooler. 4 EXPERIMENTAL PLAN � Switch on the inline air heater and set the desired air tem- perature in its automatic temperature controller. 4.1 Experimental procedure � Open Valve V1. Switch on and adjust the ultrasonic humidi- The experimental procedure for the internally cooled dehumidi- fier for the desired specific humidity of air. fier is as follows. � Fill sufficient quantity of desiccant of desired concentration in the supply tank. � Check all the electrical connections for safety and switches � Open Valves V11, V12 and V13. for OFF position. � Switch on the automatic temperature controller of the desic- � Check all the valves for their closed position. cant supply tank and set it for the desired desiccant tempera- � Switch on the electric power supply to the main control panel. ture (the set temperature is maintained by on/off control � Switch on the electric power supply to the data acquisition of Water pump 2 with the signal from the temperature system, sensors and instruments. controller). International Journal of Low-Carbon Technologies 2018, 13, 240–249 243 Downloaded from https://academic.oup.com/ijlct/article-abstract/13/3/240/5020711 by Ed 'DeepDyve' Gillespie user on 18 October 2019 G. Annadurai et al. Figure 4. Photographic view of the experimental setup. Table 2. Details of the measuring instruments. Table 3. Values of the fixed parameters. Sl no. Parameter Instrument Range Accuracy Sl no. Parameters Value 1 Mass flow rate of cooling water (kg/h) 15 1 Temperature PT100 Sensors 0–100°C ±0.1°C 2 Inlet cooling water temperature (°C) 15 2 Air flow rate Turbine flow meter 0–20 m /hr ±1% 3 Mass flow rate of desiccant (kg/h) 5 3 Air relative humidity Humidity sensors 5–95 % ±1.5% 4 Inlet desiccant concentration 0.35 4 Desiccant flow rate Coriolis flow meter 0–30 kg/hr ±1% 3 3 a b 5 Inlet desiccant temperature (°C) 20 /28 5 Desiccant density Density meter 0–3 g/cm ±0.0001 g/cm 6 Water flow rate Rotameter 0–2 lpm ±3% Adiabatic dehumidifier. Internally cooled dehumidifier. � Switch on Constant temperature water bath 2 and set it for The experimental procedure for the adiabatic dehumidifier is the desired cooling water temperature. similar to the procedure mentioned above except that Steps 15, � Allow sufficient time for various parameters to reach their 19 and 20 are not to be included. respective set values such as air temperature before dehumidifier, desiccant temperature in the supply tank and cooling water tem- perature in Constant temperature water bath 2 and so on. � Open Valves V4 and V5. 4.2 Fixed parameters Experiments are carried out to explore the influence of operat- � Switch on the desiccant pump and adjust its speed to main- tain the desired desiccant flow rate. ing parameters pertaining only to air on the performance of the � Open Valves V4 and V5. dehumidifiers. Therefore, other potential operating parameters pertaining to cooling water and desiccant are held constant as � Switch on the cooling water pump and adjust its speed to maintain the desired cooling water flow rate. listed in Table 3. The desiccant is precooled in the case of the � Allow sufficient time for the experimental setup to attain adiabatic dehumidifier (Figure 1). Hence, its inlet temperature is lower than that in the case of the internally cooled steady state condition. � Record all the final data for the performance analysis. dehumidifier. 244 International Journal of Low-Carbon Technologies 2018, 13, 240–249 Downloaded from https://academic.oup.com/ijlct/article-abstract/13/3/240/5020711 by Ed 'DeepDyve' Gillespie user on 18 October 2019 Table 4. Range and default values of the airside operating parameters. Sl no. Parameters Default value Range 1 Inlet specific humidity (g/kg ) 22.5 15–25 da 2 Inlet temperature (°C) 36 28–40 3 Mass flow rate (kg/h) 5 3.5–8.7 Converted from volume flow rate. 4.3 Orating parameters The selected operating parameters are mass flow rate, tempera- ture and specific humidity of air at the inlet of the dehumidifier. The range and default values of these parameters are listed in Table 4. The conditions of inlet temperature and specific humidity of air are selected based on the standard climatic con- ditions of Chennai (13.0827° N, 80.2707° E), India [17]. While the membrane dehumidifier has many air channels, its testing needs only one channel with corresponding air flow rate. Each operating parameter is varied to study its effect by keeping the rest at their respective default value during the experimentation. 4.4 Performance parameters Performance comparison between the adiabatic and internally cooled dehumidifiers is presented using the following two para- metric indices. 4.4.1 Moisture removal rate The MRR is defined as the total amount of water vapor trans- ferred from air to the liquid desiccant [14]. Thus, MRR=× m (W − W ) (1) a a,in a,out where m is the mass flow rate of air while W and W are a a,in a,out the inlet and outlet specific humidities of air, respectively. 4.4.2 Latent effectiveness (ε ) The latent effectiveness is defined as the ratio of total specific humidity drop of air in the dehumidifier to the maximum pos- Figure 5. Effect of inlet specific humidity of air on (a) moisture removal rate sible drop that can take place [14]. Thus, and (b) latent effectiveness. WW − a,in a,out ε = () 2 WW − a,in s,in (MRR) is found to increase linearly with the inlet specific where W is the inlet equivalent specific humidity calculated s,in humidity of air for both the dehumidifiers. This is due to as function of temperature and concentration of desiccant [15]. increase in the mass transfer potential, i.e. pressure difference between the partial pressure of water vapor in the air and that in the air–desiccant interface of the dehumidifier. When the 5 RESULTS AND DISCUSSION inlet specific humidity of the air increases, the partial pressure of water vapor in the air also increases, which in turn increases The performances of the adiabatic and internally cooled dehu- the mass transfer potential for the dehumidifiers. Moreover, midifiers are compared for the varying specific humidity, mass Figure 5(a) shows that the MRR of the internally cooled dehu- flow rate and temperature of inlet air. midifier is not only higher but also increases at a higher rate than that of the adiabatic dehumidifier. It increases from 3.7 to 5.1 Effect of inlet specific humidity of air 10.6 g/s (186%) when the inlet specific humidity of air is Figure 5 shows the effect of inlet specific humidity of air on the increased from 15 to 25 g/kg . This is due to the continuous da performance of the dehumidifiers. The Moisture Removal Rate removal of the exothermic heat (heat of absorption) by the International Journal of Low-Carbon Technologies 2018, 13, 240–249 245 Downloaded from https://academic.oup.com/ijlct/article-abstract/13/3/240/5020711 by Ed 'DeepDyve' Gillespie user on 18 October 2019 G. Annadurai et al. cooling water that is released during the mass transfer process. This continuous cooling restricts the desiccant from heating up thereby limiting the equivalent specific humidity (i.e. specific humidity of air in equilibrium with the desiccant) of the desiccant. Thus the average mass transfer potential of the internally cooled dehumidifier is higher than that of the adiabatic dehu- midifier. Therefore, the increment in MRR of the latter is com- paratively less at 160% (2.5–6.5 g/s) for the same variation in the inlet specific humidity. As discussed, the increase in the inlet specific humidity of air increases the mass transfer poten- tial. This increases the drop in specific humidity of air WW − while it passes through the dehumidifier. This in a,in a,out turn increases the heat of absorption, which increases the tem- perature of the desiccant. Consequently, the equivalent specific humidity of desiccant also increases thereby its absorption cap- acity reduces. Therefore, the increase in the inlet specific humidity of air simultaneously increases the specific humidity drop of air and decreases the absorption capacity of the desic- cant. The effect of the former is slightly higher in the present case. Therefore, the latent effectiveness increases slightly by 17% and 15%, respectively, for the internally cooled and adia- batic dehumidifiers as shown in Figure 5(b). As expected, the latent effectiveness of the former is relatively higher due to the continuous removal of heat of absorption. 5.2 Effect of mass flow rate of air Figure 6 shows the effect of mass flow rate of air on the per- formance of the dehumidifiers. It illustrates that the increase in the mass flow rate of air enhances the MRR for both the dehu- midifiers. While it is more for the internally-cooled dehumidi- fier as discussed above, its rate of increase is lower (102%) than that of adiabatic dehumidifier (128%) for the increase in mass flow rate of air from 3.5 to 8.7 kg/h. An increase in the mass flow rate of air decreases its residence time in the dehumidifier which in turn decreases the drop in specific humidity of air Figure 6. Effect of mass flow rate of air on (a) moisture removal rate and WW − while it passes through the dehumidifier. As a ai,, n a out (b) latent effectiveness. result, the average specific humidity of air in the dehumidifier increases. Consequently, the mass transfer potential of the dehumidifier also increases. Even though the air flow regime is laminar, the increase in its mass flow rate is expected to decreases the mass transfer potential at high mass flow rate of increase the mass transfer coefficient between the air and desic- air. As a result, the rate of increase in MRR gradually decreases cant due to the flow disturbance caused by the membrane sup- with mass flow rate of air as shown in Figure 6(a). As discussed, port [18]. It is observed from Figure 6(a) that the rate of an increase in the mass flow rate of air decreases the drop in increase of MRR in the internally cooled dehumidifier is lower specific humidity of airWW − in the dehumidifier due to ai,, n a out at higher mass flow rates of air. This is due to increase in the its less residence time. cooling requirement of the desiccant. At high mass flow rate of Therefore, the latent effectiveness decreases by 28% and air, the amount of water vapor transferred from air to the desic- 15%, respectively, for the internally cooled and adiabatic dehu- cant increases, which in turn increases the amount of exother- midifiers as shown in Figure 6(b). As expected, the latent effect- mic heat released from the desiccant. However, the cooling iveness of the former is higher due to the continuous removal water cannot remove all the heat and therefore, the average of heat of absorption. In addition, it decreases faster for the temperature of the desiccant increases. Consequently, it increases internally cooled dehumidifier due to the increase in the cool- the average equivalent specific humidity of the desiccant, which ing requirement of desiccant at higher mass flow rate of air. 246 International Journal of Low-Carbon Technologies 2018, 13, 240–249 Downloaded from https://academic.oup.com/ijlct/article-abstract/13/3/240/5020711 by Ed 'DeepDyve' Gillespie user on 18 October 2019 temperature of air on the desiccant temperature difference 5.3 Effect of inlet temperature of air (−TT ) is low. As illustrated in Figure 8, when the inlet Figure 7 shows the effect of inlet temperature of air on the per- s,out s,in temperature of air increases from 28°Cto40°C, the increase in formance of the dehumidifiers. As the inlet temperature of air the desiccant temperature difference is less than 1°C for both increases, the amount of heat transfer from the air to the desic- the cases. Therefore, the increase in inlet temperature of air cant also increases due to the increase in the temperature differ- does not significantly increase the equivalent specific humidity ence. This increases the temperature of the desiccant and of the desiccant. Consequently, the mass transfer potential of consequently its equivalent specific humidity. Thus, the mass the dehumidifiers remains almost unaffected. As a result, MRR transfer potential of both the dehumidifiers decreases which in and latent effectiveness of both the membrane dehumidifiers turn decreases their MRR and latent effectiveness. However, become independent of the inlet temperature of the air as these performance parameters are independent of the inlet tem- shown in Figure 7. It can be concluded that such membrane perature of air in the present study as shown in Figure 7. This dehumidifiers are suitable for the regions where the ambient is due to the fact that the intermediate membrane which is temperature fluctuates over a wide range. It is also observed made up of polyvinylidene difluoride has low thermal conduct- from Figure 7 that both MRR and latent effectiveness of the ivity and therefore the heat transfer potential of the dehumidi- internally cooled dehumidifier are higher than those of the fiers is almost unaffected. As a result, the influence of inlet adiabatic dehumidifier. This is due to the continuous removal of heat of absorption in the former. The liquid desiccant system requires a control system to ensure desirable temperature and specific humidity of air from the dehumidifier irrespective of the variation in ambient tem- perature and specific humidity. The performance of either dehumidifier is found to remain unchanged with variation in the inlet temperature of air as shown in Figure 7. As a result, the liquid desiccant system requires a control system only for variation in the specific humidity of ambient air. This, in turn, increases its reliability and reduces its size and cost. 5.4 Performance comparison at equal heat transfer area Cooling of the desiccant is essential to make it absorb water vapor from air. Therefore, it is continuously cooled during the mass transfer process in the case of internally cooled dehumidi- fier whereas, in the case of adiabatic dehumidifier, it is Figure 7. Effect of inlet temperature of air on (a) moisture removal rate and Figure 8. Effect of inlet temperature of air on the temperature difference of (b) latent effectiveness. desiccant in the dehumidifiers. International Journal of Low-Carbon Technologies 2018, 13, 240–249 247 Downloaded from https://academic.oup.com/ijlct/article-abstract/13/3/240/5020711 by Ed 'DeepDyve' Gillespie user on 18 October 2019 G. Annadurai et al. in Chennai, India. The operating parameters considered are specific humidity, mass flow rate and temperature of air. The performances of the dehumidifiers are presented in terms of MRR and latent effectiveness. It is found that while the per- formance trends with the operating parameters are similar, the performance of the internally cooled dehumidifier is better than that of the adiabatic dehumidifier at all the operating condi- tions. Both inlet specific humidity and mass flow rate of air are found to increase the MRR. For the fixed mass flow rate of air, the latent effectiveness of the dehumidifiers is found to be inde- pendent of change in the ambient conditions, i.e. both tempera- ture and specific humidity. The observations pertaining to the effect of inlet temperature of air confirm that the membrane dehumidifiers are suitable for regions where the ambient tem- perature fluctuates over a wide range. Figure 9. Performance comparison of the dehumidifiers at both level playing ACKNOWLEDGEMENTS ground and practical conditions. This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors. precooled in the external heat exchanger (Figure 1). Therefore, in the present study, the inlet temperatures of desiccant for the former and latter are selected as 28°C and 20°C, respectively. In NOMENCLATURE addition, these are the recommended values for the high humid climatic conditions [19]. However, the performance comparison c Specific heat capacity (kJ/kg.K) of the dehumidifiers with these desiccant inlet temperatures m Mass flow rate (kg/h) will not be on an equal basis. The level playing ground would T Temperature (°C) be 15°C and 28°C, respectively, the latter with 15°C chilled W Specific humidity (kg/kg ) da water for internal cooling. With 15°C chilled water, it is theor- Greek symbol etically possible to precool the desiccant to 15°C for the adia- ε Latent effectiveness Subscripts batic dehumidifier. In the case of membrane-based internally a Air cooling dehumidifier, the heat exchanging area between the a,in Air inlet desiccant and chilled water is equal to that of the membrane. a,out Air outlet This is larger than that of the external heat exchanger. If one cw Cooling water provides the same area for the both, the effectiveness of the cw,in Cooling water inlet s Desiccant exchanger will be close to 1. With (mc ) > (mc ) , the ter- p d p cw s,in Desiccant inlet minal temperature difference at the cold end will be zero and s,out Desiccant outlet thereby the assumption for the level playing ground is justified. W Latent Figure 9 compares the performance of internally cooled and Abbreviations adiabatic dehumidifiers, the latter with the inlet desiccant tem- AC Air conditioning MRR Moisture removal rate perature of both 15°C (level playing ground) and 20°C(practical). Even with the inlet desiccant temperature of 15°C, the perform- ance of the adiabatic dehumidifier falls short by 13% of that of the internally cooled one. The performance of the adiabatic dehu- midifier for practical cases is still poorer. Thus, the provision for REFERENCES internal cooling arrangement is desirable for the membrane dehu- midifier to improve its mass transfer performance. [1] A technical report of energy and buildings by Centre for Science and Environment, http://www.cseindia.org/userfiles/Energy-and-%20buildings. pdf [Accessed 5th June 2017]. 6 CONCLUSIONS [2] Enteria N, Mizutani K. The role of the thermally activated desiccant cool- ing technologies in the issue of energy and environment. 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Experimental performance comparison of adiabatic and internally-cooled membrane dehumidifiers

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Oxford University Press
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© The Author(s) 2018. Published by Oxford University Press.
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1748-1317
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1748-1325
DOI
10.1093/ijlct/cty020
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Abstract

Humidity control of indoor space using the conventional air conditioning system is energy intensive. The liquid desiccant dehumidifier, which operates on low grade energy sources, is one of the energy effi- cient alternatives for humidity control. Membrane dehumidifiers avoid the desiccant carryover and hence are preferred over the packed bed dehumidifiers. However, their performance is lower due to the add- itional resistance in the membrane. Internal cooling is one way to improve the performance of the mem- brane dehumidifier and the present study experimentally investigates such a dehumidifier. The operating parameters considered are specific humidity, mass flow rate, temperature and of inlet air. The perfor- mances of the adiabatic and internally cooled dehumidifiers are presented in terms of moisture removal rate and latent effectiveness. It is found that these are higher by 60 and 50%, respectively, for the intern- ally cooled dehumidifier. Keywords: internally cooled membrane dehumidifier; liquid desiccant dehumidification; moisture removal rate; latent effectiveness; experimental analysis *Corresponding author: mpmaiya@iitm.ac.in Received 10 January 2018; revised 6 March 2018; editorial decision 3 May 2018; accepted 10 May 2018 ......... ................. ................ ................. ................. ................ ................. ................. . ............... ................. ................. system is energy inefficient due to overcooling followed by 1 INTRODUCTION reheating of the air. Thus, alternative energy efficient systems According to the International Energy Outlook 2013, building have been studied for the control of humidity in many AC sector consumes one fifth of the total global energy consump- applications. One such prospective system is desiccant dehu- tion. India too follows the same trend. Further, more than 60% midifier which utilizes the renewable low grade energy sources of the buildings projected for 2030 are yet to be built. These are for its regeneration [3]. The hybrid AC system combines such a also expected to have increased demand of thermal comfort due desiccant dehumidifier with the conventional cooling system. to both growth in urbanization and increased aspiration for Desiccant dehumidification is the process of removing water better human comfort. Thus, the energy share required for con- vapor from air by absorbing it in the desiccant, which may be trolling the indoor conditions is expected to increase to ~45% liquid or solid. The former is selected for the present study due of the total building energy consumption from the present of to its advantages such as high moisture holding capacity, low ~25% [1]. Apart from temperature, control of humidity plays a airside pressure drop and low regeneration temperature. vital role in air conditioning (AC) at tropical climate and also Moreover, it facilitates air sterilization, operational flexibility for many special applications such as hospitals, electronic labs, and utilization of the low grade thermal energy sources such as museums etc. to maintain the required low indoor humidity solar or waste heat for its regeneration [4]. The liquid desiccant [2]. Cooling air below its dew point temperature to condense systems are classified as direct contact-packed bed and indirect the water vapor is the standard method of dehumidification contact-membrane systems. The latter is preferred to avoid the adopted in the conventional AC systems. Air has to be cooled problems associated with desiccant carryover such as health to low temperature and then heated before it enters the AC hazard and corrosion of equipment [5]. While the membrane room to control humidity. Therefore, the conventional AC avoids direct contact between air and desiccant, its micro-pores International Journal of Low-Carbon Technologies 2018, 13, 240–249 © The Author(s) 2018. Published by Oxford University Press. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/ by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com doi:10.1093/ijlct/cty020 Advance Access Publication 29 May 2018 240 Downloaded from https://academic.oup.com/ijlct/article-abstract/13/3/240/5020711 by Ed 'DeepDyve' Gillespie user on 18 October 2019 allow water vapor to get transferred between them. However, increase in such parameters increases the performance of both its mass transfer performance is lower than that of the packed adiabatic and internally cooled dehumidifiers. However, the effect bed dehumidifier due to the additional resistance imposed by of mass flow rate of desiccant is significant on the former. A the intermediate membrane. There are many ways such as numerical model of the internally cooled dehumidifier is required internal cooling, provision of nanofibrous membrane, providing to analyze its performance under various climatic conditions. micro-fins and so on to improve the performance [6]. Present Huang et al. [12] developed one such model and validated using study analyses the performance improvement of the membrane their experimental results. The governing mass, momentum and dehumidifier by internal cooling. Flat-plate configuration is energy equations were solved to find the Nusselt and Sherwood selected for the present study due to its suitability for multi- numbers for the heat and mass transfer processes of the dehu- stream applications, ease of assembly and less airside pressure midifier. Woods and Kozubal [11] analyzed the influence of air, drop over the hollow-fiber configuration [7]. desiccant and membrane on the heat and mass transfer resis- Dehumidifiers are broadly divided into two types, namely tances of internally cooled membrane dehumidifier. It is reported adiabatic and internally cooled. Cooling of the desiccant is that the air resistance accounts for 70% of the overall heat transfer essential to make it absorb water vapor from air. In the adia- resistance while the air and membrane resistances together batic dehumidifier, the desiccant is precooled in an external account for 90% of the overall mass transfer resistance. heat exchanger as shown in Figure 1a, to facilitate it to absorb From the above narratives, it can be concluded that most of water vapor from the air flowing in the adjacent channel sepa- the previous works of internally cooled membrane dehumidi- rated by the membrane. The heat of absorption raises the desic- fiers are focused only on its initial design and development. cant temperature which reducing its capacity to absorb water The parametric experimental investigations on such dehumidi- vapor. In the internally cooled type, the desiccant is continu- fiers are scarce to compare their performance with those of ously cooled as shown in Figure 1b, while it absorbs water adiabatic membrane dehumidifier. Thus, the main objective of vapor thereby improving the capacity. the present paper is to experimentally investigate the perform- Membrane dehumidifiers are developed recently as an alterna- ance of the internally cooled membrane dehumidifier for the tive to the conventional packed bed dehumidifiers. Hence, their hot and humid climatic conditions prevailing in the city of literature, especially with internal cooling arrangement is limited. Chennai, India. The operating parameters considered are mass Isetti et al. [8] developed the first prototype of such a dehumidi- flow rate, inlet temperature and specific humidity of air. The fier. Potassium formate solution (desiccant) was cooled by the performances of the adiabatic and internally cooled dehumidi- cooling water in a plate heat exchanger. Air flows in the hydro- fiers are presented in terms of moisture removal rate (MRR) phobic polypropylene membrane tubes placed in the desiccant and latent effectiveness. The presented results are useful in the channel in cross-flow direction. The parametric study revealed optimum design of the membrane dehumidifiers. that the performance of the dehumidifier was better at low inlet temperature of the cooling water. Abdel-Salam et al. [9] experi- 2 DESCRIPTION OF MEMBRANE mentally reconfirmed the influence of inlet temperature and further found that the high water flow rate enhances the perform- DEHUMIDIFIER ance. The paper details the issues in manufacturing of the intern- ally cooled dehumidifier without leaks due to the additional Figure 2 shows the schematic diagram of the membrane dehu- cooling water channel. The results concluded that the internally midifier. It has three channels, one each for air, desiccant and cooled dehumidifier is better than the adiabatic dehumidifier. The cooling water with their flow in counter-flow direction. Upward results also indicate that the inlet temperature of cooling water entry of desiccant is adopted to avoid flow maldistribution [13]. has to be critically selected to avoid the temperature drop of des- Flow guides are provided to make the desiccant flow direction iccant in the dehumidifier. Later, the same authors extended their counter to both air and cooling water. The membrane is attached study [10] and analyzed the effect of inlet specific humidity of air to a metal mesh using double-sided foam tape and metal screws to and mass flow rate of desiccant. The results indicated that an avoid its deflection. The design details of the membrane dehumidi- fier are listed in Table 1. Experimental studies of both adiabatic (a) (b) and internally cooled dehumidifiers are carried out in the same dehumidifier, the former by switching off the cooling water pump. 3 EXPERIMENTAL SETUP AND INSTRUMENTATION The schematic diagram and photograph of the experimental Figure 1. Schematic diagram of operating conditions of the (a) adiabatic setup of membrane dehumidifier are shown in Figures 3 and 4 and (b) internally cooled membrane dehumidifiers. respectively. It consists of three circuits, namely the air, International Journal of Low-Carbon Technologies 2018, 13, 240–249 241 Downloaded from https://academic.oup.com/ijlct/article-abstract/13/3/240/5020711 by Ed 'DeepDyve' Gillespie user on 18 October 2019 G. Annadurai et al. Figure 2. Schematic diagram of the membrane dehumidifier. Table 1. Design details of the membrane dehumidifier. the dehumidifier. Temperature, relative humidity and flow rate are measured at all the key locations as shown in Figure 3. Sl. No. Parameter Value Aqueous solution of lithium chloride is used as desiccant [14] 1 Channel spacing (mm) 5 and its circuit consists of supply and storage tanks, and a peri- 2 Dehumidifier length (m) 1.1 staltic pump. Sufficient quantity of desiccant with desired con- 3 Dehumidifier height (m) 0.55 centration is filled in the supply tank. It is maintained at the 4 Membrane material PVDF 5 Membrane pore diameter (μm) 0.2 desired temperature using water from the constant temperature 6 Membrane thickness (mm) 0.22 water bath. The desiccant flow rate to the membrane dehumidi- 7 Plate thickness (mm) 1.2 fier is adjusted by controlling the speed of the pump and its flow rate is measured. Desiccant density and temperature are mea- sured both at inlet and outlet of the dehumidifier as shown in desiccant and cooling water with the provision to control the Figure 3 and the respective concentrations are calculated [15]. respective operating parameters namely, (a) flow rate, inlet tem- The cooling water circuit contains a constant temperature bath perature and specific humidity for air (b) flow rate, inlet tem- and a peristaltic pump to control the water inlet temperature to perature and concentration for desiccant and (c) flow rate and the dehumidifier and its flow rate respectively. Inlet and outlet inlet temperature for cooling water. temperatures, and flow rate of the water are measured. The The air circuit consists of an ultrasonic humidifier, cooler and details of the instruments used in the experimental setup are heater by which the desired climatic conditions (temperature listed in Table 2. The data acquisition system records all the and humidity) can be achieved. The cooler is supplied with experimental data namely temperature, relative humidity, mass water at the desired temperature from a constant temperature flow rate and density at regular intervals. water bath. The air circuit has fan, inlet static mixer and flow All the sensors and instruments are pre-calibrated. The tem- straightener before the dehumidifier. The flow rate of air is perature sensors are calibrated using a constant temperature adjusted using fan speed control. The inlet and outlet headers bath for their entire working range. The relative humidity facilitate uniform air distribution in the rectangular channel of probes are calibrated using a dew point meter in a controlled 242 International Journal of Low-Carbon Technologies 2018, 13, 240–249 Downloaded from https://academic.oup.com/ijlct/article-abstract/13/3/240/5020711 by Ed 'DeepDyve' Gillespie user on 18 October 2019 Figure 3. Schematic diagram of the experimental setup. environment chamber for their entire humidity range at differ- � Set the data acquisition system to record data from the ent temperatures. A detailed error analysis [16] has been done instruments and sensors. estimating the uncertainty in the two performance parameters, � Open Valves V2 and V3. namely MRR and latent effectiveness, which are found to be � Switch on the fan and regulate it for the desired air flow. within ±5% and ±6%, respectively. � Open Valves V8, V9 and V10, and switch on Water pump 1. � Switch on Constant temperature water bath 1 and adjust it for the required inlet water temperature to the cooler. 4 EXPERIMENTAL PLAN � Switch on the inline air heater and set the desired air tem- perature in its automatic temperature controller. 4.1 Experimental procedure � Open Valve V1. Switch on and adjust the ultrasonic humidi- The experimental procedure for the internally cooled dehumidi- fier for the desired specific humidity of air. fier is as follows. � Fill sufficient quantity of desiccant of desired concentration in the supply tank. � Check all the electrical connections for safety and switches � Open Valves V11, V12 and V13. for OFF position. � Switch on the automatic temperature controller of the desic- � Check all the valves for their closed position. cant supply tank and set it for the desired desiccant tempera- � Switch on the electric power supply to the main control panel. ture (the set temperature is maintained by on/off control � Switch on the electric power supply to the data acquisition of Water pump 2 with the signal from the temperature system, sensors and instruments. controller). International Journal of Low-Carbon Technologies 2018, 13, 240–249 243 Downloaded from https://academic.oup.com/ijlct/article-abstract/13/3/240/5020711 by Ed 'DeepDyve' Gillespie user on 18 October 2019 G. Annadurai et al. Figure 4. Photographic view of the experimental setup. Table 2. Details of the measuring instruments. Table 3. Values of the fixed parameters. Sl no. Parameter Instrument Range Accuracy Sl no. Parameters Value 1 Mass flow rate of cooling water (kg/h) 15 1 Temperature PT100 Sensors 0–100°C ±0.1°C 2 Inlet cooling water temperature (°C) 15 2 Air flow rate Turbine flow meter 0–20 m /hr ±1% 3 Mass flow rate of desiccant (kg/h) 5 3 Air relative humidity Humidity sensors 5–95 % ±1.5% 4 Inlet desiccant concentration 0.35 4 Desiccant flow rate Coriolis flow meter 0–30 kg/hr ±1% 3 3 a b 5 Inlet desiccant temperature (°C) 20 /28 5 Desiccant density Density meter 0–3 g/cm ±0.0001 g/cm 6 Water flow rate Rotameter 0–2 lpm ±3% Adiabatic dehumidifier. Internally cooled dehumidifier. � Switch on Constant temperature water bath 2 and set it for The experimental procedure for the adiabatic dehumidifier is the desired cooling water temperature. similar to the procedure mentioned above except that Steps 15, � Allow sufficient time for various parameters to reach their 19 and 20 are not to be included. respective set values such as air temperature before dehumidifier, desiccant temperature in the supply tank and cooling water tem- perature in Constant temperature water bath 2 and so on. � Open Valves V4 and V5. 4.2 Fixed parameters Experiments are carried out to explore the influence of operat- � Switch on the desiccant pump and adjust its speed to main- tain the desired desiccant flow rate. ing parameters pertaining only to air on the performance of the � Open Valves V4 and V5. dehumidifiers. Therefore, other potential operating parameters pertaining to cooling water and desiccant are held constant as � Switch on the cooling water pump and adjust its speed to maintain the desired cooling water flow rate. listed in Table 3. The desiccant is precooled in the case of the � Allow sufficient time for the experimental setup to attain adiabatic dehumidifier (Figure 1). Hence, its inlet temperature is lower than that in the case of the internally cooled steady state condition. � Record all the final data for the performance analysis. dehumidifier. 244 International Journal of Low-Carbon Technologies 2018, 13, 240–249 Downloaded from https://academic.oup.com/ijlct/article-abstract/13/3/240/5020711 by Ed 'DeepDyve' Gillespie user on 18 October 2019 Table 4. Range and default values of the airside operating parameters. Sl no. Parameters Default value Range 1 Inlet specific humidity (g/kg ) 22.5 15–25 da 2 Inlet temperature (°C) 36 28–40 3 Mass flow rate (kg/h) 5 3.5–8.7 Converted from volume flow rate. 4.3 Orating parameters The selected operating parameters are mass flow rate, tempera- ture and specific humidity of air at the inlet of the dehumidifier. The range and default values of these parameters are listed in Table 4. The conditions of inlet temperature and specific humidity of air are selected based on the standard climatic con- ditions of Chennai (13.0827° N, 80.2707° E), India [17]. While the membrane dehumidifier has many air channels, its testing needs only one channel with corresponding air flow rate. Each operating parameter is varied to study its effect by keeping the rest at their respective default value during the experimentation. 4.4 Performance parameters Performance comparison between the adiabatic and internally cooled dehumidifiers is presented using the following two para- metric indices. 4.4.1 Moisture removal rate The MRR is defined as the total amount of water vapor trans- ferred from air to the liquid desiccant [14]. Thus, MRR=× m (W − W ) (1) a a,in a,out where m is the mass flow rate of air while W and W are a a,in a,out the inlet and outlet specific humidities of air, respectively. 4.4.2 Latent effectiveness (ε ) The latent effectiveness is defined as the ratio of total specific humidity drop of air in the dehumidifier to the maximum pos- Figure 5. Effect of inlet specific humidity of air on (a) moisture removal rate sible drop that can take place [14]. Thus, and (b) latent effectiveness. WW − a,in a,out ε = () 2 WW − a,in s,in (MRR) is found to increase linearly with the inlet specific where W is the inlet equivalent specific humidity calculated s,in humidity of air for both the dehumidifiers. This is due to as function of temperature and concentration of desiccant [15]. increase in the mass transfer potential, i.e. pressure difference between the partial pressure of water vapor in the air and that in the air–desiccant interface of the dehumidifier. When the 5 RESULTS AND DISCUSSION inlet specific humidity of the air increases, the partial pressure of water vapor in the air also increases, which in turn increases The performances of the adiabatic and internally cooled dehu- the mass transfer potential for the dehumidifiers. Moreover, midifiers are compared for the varying specific humidity, mass Figure 5(a) shows that the MRR of the internally cooled dehu- flow rate and temperature of inlet air. midifier is not only higher but also increases at a higher rate than that of the adiabatic dehumidifier. It increases from 3.7 to 5.1 Effect of inlet specific humidity of air 10.6 g/s (186%) when the inlet specific humidity of air is Figure 5 shows the effect of inlet specific humidity of air on the increased from 15 to 25 g/kg . This is due to the continuous da performance of the dehumidifiers. The Moisture Removal Rate removal of the exothermic heat (heat of absorption) by the International Journal of Low-Carbon Technologies 2018, 13, 240–249 245 Downloaded from https://academic.oup.com/ijlct/article-abstract/13/3/240/5020711 by Ed 'DeepDyve' Gillespie user on 18 October 2019 G. Annadurai et al. cooling water that is released during the mass transfer process. This continuous cooling restricts the desiccant from heating up thereby limiting the equivalent specific humidity (i.e. specific humidity of air in equilibrium with the desiccant) of the desiccant. Thus the average mass transfer potential of the internally cooled dehumidifier is higher than that of the adiabatic dehu- midifier. Therefore, the increment in MRR of the latter is com- paratively less at 160% (2.5–6.5 g/s) for the same variation in the inlet specific humidity. As discussed, the increase in the inlet specific humidity of air increases the mass transfer poten- tial. This increases the drop in specific humidity of air WW − while it passes through the dehumidifier. This in a,in a,out turn increases the heat of absorption, which increases the tem- perature of the desiccant. Consequently, the equivalent specific humidity of desiccant also increases thereby its absorption cap- acity reduces. Therefore, the increase in the inlet specific humidity of air simultaneously increases the specific humidity drop of air and decreases the absorption capacity of the desic- cant. The effect of the former is slightly higher in the present case. Therefore, the latent effectiveness increases slightly by 17% and 15%, respectively, for the internally cooled and adia- batic dehumidifiers as shown in Figure 5(b). As expected, the latent effectiveness of the former is relatively higher due to the continuous removal of heat of absorption. 5.2 Effect of mass flow rate of air Figure 6 shows the effect of mass flow rate of air on the per- formance of the dehumidifiers. It illustrates that the increase in the mass flow rate of air enhances the MRR for both the dehu- midifiers. While it is more for the internally-cooled dehumidi- fier as discussed above, its rate of increase is lower (102%) than that of adiabatic dehumidifier (128%) for the increase in mass flow rate of air from 3.5 to 8.7 kg/h. An increase in the mass flow rate of air decreases its residence time in the dehumidifier which in turn decreases the drop in specific humidity of air Figure 6. Effect of mass flow rate of air on (a) moisture removal rate and WW − while it passes through the dehumidifier. As a ai,, n a out (b) latent effectiveness. result, the average specific humidity of air in the dehumidifier increases. Consequently, the mass transfer potential of the dehumidifier also increases. Even though the air flow regime is laminar, the increase in its mass flow rate is expected to decreases the mass transfer potential at high mass flow rate of increase the mass transfer coefficient between the air and desic- air. As a result, the rate of increase in MRR gradually decreases cant due to the flow disturbance caused by the membrane sup- with mass flow rate of air as shown in Figure 6(a). As discussed, port [18]. It is observed from Figure 6(a) that the rate of an increase in the mass flow rate of air decreases the drop in increase of MRR in the internally cooled dehumidifier is lower specific humidity of airWW − in the dehumidifier due to ai,, n a out at higher mass flow rates of air. This is due to increase in the its less residence time. cooling requirement of the desiccant. At high mass flow rate of Therefore, the latent effectiveness decreases by 28% and air, the amount of water vapor transferred from air to the desic- 15%, respectively, for the internally cooled and adiabatic dehu- cant increases, which in turn increases the amount of exother- midifiers as shown in Figure 6(b). As expected, the latent effect- mic heat released from the desiccant. However, the cooling iveness of the former is higher due to the continuous removal water cannot remove all the heat and therefore, the average of heat of absorption. In addition, it decreases faster for the temperature of the desiccant increases. Consequently, it increases internally cooled dehumidifier due to the increase in the cool- the average equivalent specific humidity of the desiccant, which ing requirement of desiccant at higher mass flow rate of air. 246 International Journal of Low-Carbon Technologies 2018, 13, 240–249 Downloaded from https://academic.oup.com/ijlct/article-abstract/13/3/240/5020711 by Ed 'DeepDyve' Gillespie user on 18 October 2019 temperature of air on the desiccant temperature difference 5.3 Effect of inlet temperature of air (−TT ) is low. As illustrated in Figure 8, when the inlet Figure 7 shows the effect of inlet temperature of air on the per- s,out s,in temperature of air increases from 28°Cto40°C, the increase in formance of the dehumidifiers. As the inlet temperature of air the desiccant temperature difference is less than 1°C for both increases, the amount of heat transfer from the air to the desic- the cases. Therefore, the increase in inlet temperature of air cant also increases due to the increase in the temperature differ- does not significantly increase the equivalent specific humidity ence. This increases the temperature of the desiccant and of the desiccant. Consequently, the mass transfer potential of consequently its equivalent specific humidity. Thus, the mass the dehumidifiers remains almost unaffected. As a result, MRR transfer potential of both the dehumidifiers decreases which in and latent effectiveness of both the membrane dehumidifiers turn decreases their MRR and latent effectiveness. However, become independent of the inlet temperature of the air as these performance parameters are independent of the inlet tem- shown in Figure 7. It can be concluded that such membrane perature of air in the present study as shown in Figure 7. This dehumidifiers are suitable for the regions where the ambient is due to the fact that the intermediate membrane which is temperature fluctuates over a wide range. It is also observed made up of polyvinylidene difluoride has low thermal conduct- from Figure 7 that both MRR and latent effectiveness of the ivity and therefore the heat transfer potential of the dehumidi- internally cooled dehumidifier are higher than those of the fiers is almost unaffected. As a result, the influence of inlet adiabatic dehumidifier. This is due to the continuous removal of heat of absorption in the former. The liquid desiccant system requires a control system to ensure desirable temperature and specific humidity of air from the dehumidifier irrespective of the variation in ambient tem- perature and specific humidity. The performance of either dehumidifier is found to remain unchanged with variation in the inlet temperature of air as shown in Figure 7. As a result, the liquid desiccant system requires a control system only for variation in the specific humidity of ambient air. This, in turn, increases its reliability and reduces its size and cost. 5.4 Performance comparison at equal heat transfer area Cooling of the desiccant is essential to make it absorb water vapor from air. Therefore, it is continuously cooled during the mass transfer process in the case of internally cooled dehumidi- fier whereas, in the case of adiabatic dehumidifier, it is Figure 7. Effect of inlet temperature of air on (a) moisture removal rate and Figure 8. Effect of inlet temperature of air on the temperature difference of (b) latent effectiveness. desiccant in the dehumidifiers. International Journal of Low-Carbon Technologies 2018, 13, 240–249 247 Downloaded from https://academic.oup.com/ijlct/article-abstract/13/3/240/5020711 by Ed 'DeepDyve' Gillespie user on 18 October 2019 G. Annadurai et al. in Chennai, India. The operating parameters considered are specific humidity, mass flow rate and temperature of air. The performances of the dehumidifiers are presented in terms of MRR and latent effectiveness. It is found that while the per- formance trends with the operating parameters are similar, the performance of the internally cooled dehumidifier is better than that of the adiabatic dehumidifier at all the operating condi- tions. Both inlet specific humidity and mass flow rate of air are found to increase the MRR. For the fixed mass flow rate of air, the latent effectiveness of the dehumidifiers is found to be inde- pendent of change in the ambient conditions, i.e. both tempera- ture and specific humidity. The observations pertaining to the effect of inlet temperature of air confirm that the membrane dehumidifiers are suitable for regions where the ambient tem- perature fluctuates over a wide range. Figure 9. Performance comparison of the dehumidifiers at both level playing ACKNOWLEDGEMENTS ground and practical conditions. This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors. precooled in the external heat exchanger (Figure 1). Therefore, in the present study, the inlet temperatures of desiccant for the former and latter are selected as 28°C and 20°C, respectively. In NOMENCLATURE addition, these are the recommended values for the high humid climatic conditions [19]. However, the performance comparison c Specific heat capacity (kJ/kg.K) of the dehumidifiers with these desiccant inlet temperatures m Mass flow rate (kg/h) will not be on an equal basis. The level playing ground would T Temperature (°C) be 15°C and 28°C, respectively, the latter with 15°C chilled W Specific humidity (kg/kg ) da water for internal cooling. With 15°C chilled water, it is theor- Greek symbol etically possible to precool the desiccant to 15°C for the adia- ε Latent effectiveness Subscripts batic dehumidifier. In the case of membrane-based internally a Air cooling dehumidifier, the heat exchanging area between the a,in Air inlet desiccant and chilled water is equal to that of the membrane. a,out Air outlet This is larger than that of the external heat exchanger. If one cw Cooling water provides the same area for the both, the effectiveness of the cw,in Cooling water inlet s Desiccant exchanger will be close to 1. With (mc ) > (mc ) , the ter- p d p cw s,in Desiccant inlet minal temperature difference at the cold end will be zero and s,out Desiccant outlet thereby the assumption for the level playing ground is justified. W Latent Figure 9 compares the performance of internally cooled and Abbreviations adiabatic dehumidifiers, the latter with the inlet desiccant tem- AC Air conditioning MRR Moisture removal rate perature of both 15°C (level playing ground) and 20°C(practical). Even with the inlet desiccant temperature of 15°C, the perform- ance of the adiabatic dehumidifier falls short by 13% of that of the internally cooled one. The performance of the adiabatic dehu- midifier for practical cases is still poorer. Thus, the provision for REFERENCES internal cooling arrangement is desirable for the membrane dehu- midifier to improve its mass transfer performance. [1] A technical report of energy and buildings by Centre for Science and Environment, http://www.cseindia.org/userfiles/Energy-and-%20buildings. pdf [Accessed 5th June 2017]. 6 CONCLUSIONS [2] Enteria N, Mizutani K. The role of the thermally activated desiccant cool- ing technologies in the issue of energy and environment. 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International Journal of Low-Carbon Technologies – Oxford University Press

Published: Sep 1, 2018

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