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Dehumidification Behavior of Cross-flow Heat Exchanger Type Adsorber Coated with Aluminophosphate Zeolite for Desiccant Humidity Control System
Abstract
A cross-flow heat exchanger type adsorber was investigated for a desiccant humidity control system. The aim of the adsorber was to improve dehumidification performance by forcibly cooling an adsorbent with flowing air. Aluminophosphate (AlPO) zeolite was coated on the heat exchanger, because it was expected to regenerate sufficiently even with a low-temperature heat source of around 333 K. Fundamental dehumidification behavior with the adsorber was experimentally investigated at various inlet absolute humidities, regeneration temperatures, and air flow velocities. Dehumidified water in an equilibrium state was kept even at a regeneration temperature of 333 K, indicating that the adsorber coated with AlPO zeolite could be driven using low-temperature heat at 333 K. The dehumidification rate was found to increase as the cooling and process air velocities increased. However, the increase in the dehumidification rate decreased when cooling air flowed at a velocity of 2 m/s or more. Heat removed by the cooling air increased as the cooling air velocity increased and the process air velocity decreased. It was also found that dehumidified air could be supplied for a longer period at a sufficiently low absolute humidity suitable for practical use as the cooling air velocity increased.
... For example, AlPO-Tric exhibited a slight reduction in water absorption capacity [75]. Furthermore, the behavior of other AlPOs structures over 10 to 30 cycles has been studied and reported a minor decline in water sorption uptake, proving their stability as well [77,83,117,118]. However, AlPOs confined with hygroscopic salts such as LiCl and CaCl 2 showed higher loss in capacities up to 5 % after only 5 cycles. ...
... It was proved that the dehumidification rate depends on the regeneration temperature, feed air velocity, feed air temperature, and cooling air velocity. Also, at least two beds for adsorption were required to generate dehumidified airflow continuously [118]. ...
... Fig. 6. Conceptual diagram of the humidity control system with cross-flow heat [118]. ...
Freshwater resources are being heavily depleted and not replenished at the same high rate, thus, atmospheric water vapor harvesting has earned growing interest. Development of high-performing desiccant or adsorbent materials offering high sorption capacity and selectivity as well as regeneration capability in atmospheric conditions is crucial to tackle water scarcity. The required properties to generate potent water sorbents include pore accessibility, high specific surface area and porosity to enable high capture capacity and kinetics, and hydrothermal resilience to resist cyclic sorption and desorption. Further, polarity, hydrogen bonding, adhesion ability of water molecules to adsorbent surface, and hydrophilic functional groups can boost water adsorption. A highly promising class of water vapor adsorbents is the aluminophosphate molecular sieves (AlPOs), which are microporous zeotype materials. In this review, AlPO-based adsorbents are discussed for water sorption applications and a link is established between performance of materials and their chemical and morphological properties. Synthesis-properties-performance relationships are elucidated in light of synthesis techniques and adsorption behavior, and prospects to enhance AlPOs' water vapor sorption performance toward large-scale water harvesting applications are highlighted.
... Thus, additional cooling devices, such as sensible cooling heat exchangers, should be installed to cool process air before it is supplied to a room or building [16]. Additionally, temperature increases are accompanied by the dehumidification process because the rotor operates in nearly adiabatic conditions, causing a decrease in the adsorption capacity of the rotor [17]. Such temperature increases affect the decrease in the relative humidity around the rotor, in turn reducing the dehumidification performance [18]. ...
... Several studies have been performed to employ cooling air in desiccant-coated heat exchangers, especially in crossflow heat exchangers (CF-DCHEs). Using aluminophosphate (AlPO) zeolite as an adsorbent, Kubota et al. developed a desiccantcoated crossflow heat exchanger that consists of primary and secondary airflow channels coated by AlPO zeolite [17]. During dehumidification, the adsorbent captures water vapor from the process air in the primary channel. ...
... Thus, the total weight of the coated AlPO zeolite is 279 g. The water vapor adsorption isotherm on the coated AlPO refers to the results of the adsorption isotherm experiment conducted by Kubota et al.[17]. ...
In this study, a crossflow-type heat exchanger coated with aluminophosphate zeolite was tested for desiccant dehumidification with direct-supplied hot water and evaporative cooling using remaining hot water. This evaporative cooling process could accelerate dehumidification by removing the heat of adsorption. Therefore, the influences of the cooling air velocity, regeneration hot water temperature, and adsorption/regeneration cycle time were experimentally investigated. The result showed that the minimum humidity of the adsorption outlet air decreased with an increase in the cooling air velocity and adsorption/desorption time, indicating that the complete regeneration of coated adsorbents is required to significantly decrease humidity. However, the maximum time-averaged amount of dehumidification appeared at an appropriate cycle time, which avoided adsorption saturation during the adsorption period. Further, in any case, increasing the cooling air velocity had a positive impact on both the momentary observed minimum humidity and time-averaged amount of dehumidification.
At all cooling air velocity, the temperature of the hot water contributed to the increase in the time-averaged amount of dehumidification. However, when the hot water temperature was 55 °C, the cooling effect of evaporative was not observed, even when the cooling air velocity reached 1.5 m/s. Considering that the actual temperature of the coated adsorbent was expected to be much lower than that of the supplied hot water, the hot water temperature should have been ≥65 °C.
... Silica gel and some zeolites are well-known porous adsorbents for these systems. [9][10][11][12][13][14][15][16][17] Silica gel adsorbs water vapor almost linearly with relative humidity (RH) and can be operated over a wide range of RH. [9][10][11] FAM-Z01 is a zeolite with a unique sigmoid water vapor adsorption isotherm. 12,13) Organic sorbents, which are bridge complexes of sodium polyacrylate, have higher water adsorptivities than silica gel over the entire RH range. ...
... [9][10][11][12][13][14][15][16][17] Silica gel adsorbs water vapor almost linearly with relative humidity (RH) and can be operated over a wide range of RH. [9][10][11] FAM-Z01 is a zeolite with a unique sigmoid water vapor adsorption isotherm. 12,13) Organic sorbents, which are bridge complexes of sodium polyacrylate, have higher water adsorptivities than silica gel over the entire RH range. The sorbents are considered a suitable material for high RH conditions for desiccant humidity control systems. ...
Desiccant humidity control systems have been garnering considerable attention in the attempt to achieve highly efficient utilization of low-temperature heat exhausted from various industries at temperatures less than 373 K. We have focused on thermosensitive polymers as new desiccants because a large amount of dehumidified water would be expected in the system because of their thermosensitivity. Our previous study focused on the water adsorption behavior of poly(N-isopropylacrylamide) (poly(NIPA)), which has a low critical solution temperature (LCST) of 306 K. In this study, poly(N-isopropylmethacrylamide) (poly(NIPMA)) and poly(2-(dimethylamino)ethyl methacrylate) (poly(DMAEMA)) cross-linked with N,N′-methylenebisacrylamide (MBAA) were investigated. These polymers are known to exhibit thermosensitivity in the temperature range of 313–319 K in water, which is a higher LCST than that of poly(NIPA). Poly(NIPMA) adsorbed water vapor linearly with increasing relative humidity. It was also observed that poly(NIPMA) prepared at MBAA concentrations of 200 mol/m³ exhibited a thermosensitivity in the temperature range of 303–313 K in water vapor adsorption. Meanwhile, poly(DMAEMA) adsorbed little water vapor up to a relative humidity (RH) of 40%; however, it exponentially adsorbed water at RH levels higher than 40%. From the estimation results of effective water adsorptivity, we found that poly(DMAEMA) is applicable in desiccant humidity control systems when the dehumidification process is performed at high RH.
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... Zeolite coatings may be beneficial for catalysis, separation/purification, adsorption, ion exchange, surface protection, etc. They may act as catalytic reactors [1,2], membranes [3], dehumidifiers [4], sensors [5], adsorption heating/cooling devices [6] as well as antimicrobial [7] and anticorrosive [8] materials. Various substrates, including metals [6,9], silicon wafers [10], carbon fibers [11], and ceramic surfaces [12], have been used for the direct crystallization of zeolite coatings. ...
Induction heating was used as a tool to prepare zeolite coatings on stainless steel substrates. The substrates were double-layer wire meshes which were placed in a Teflon reactor and were heated from a distance by the alternating magnetic field produced by a generator. The reaction mixture, also heated to some extent in the reactor, was circulated in the system to be cooled in a water bath to a temperature lower than that of the mixture at the exit of the reactor. Different synthesis conditions were investigated and the coatings obtained initially using two previously published synthesis compositions were characterized by X-ray diffraction (XRD) and thermogravimetry (TG). Induction heating was shown to allow the formation of crystalline zeolite A and X coatings. Relatively thick coatings of these zeolites could be prepared in much shorter times than by conventional synthesis, and even the substrate heating method using conduction. The cylindrical geometries of the induction coil and the substrate allowed the uniform heating of the substrate, leading to the formation of uniformly thick coatings. Utilizing substrates consisting of two layers of wire meshes, also played significant role in obtaining thick and highly stable coatings by the induction heating synthesis method.
... The electric power consumption for the air blowing through the smaller beds is expected to decrease, accordingly. Moreover, employing modern desiccant coated heat exchangers [77][78][79] instead of granulated adsorbent beds helps in further reducing both hydraulic resistance and electricity consumption [62]. The authors of [80] prepared the AB in the shape of air-channelled porous discs and tested them at various air flow rates, cycle times, and outdoor air conditions down to −15°C. ...
The strategic goal of reducing GHG emissions opens a niche for applying an emerging technology of adsorptive heat conversion/storage (AHCS). Currently, the AHCS technologies are mainly used in countries with a hot and humid climate. However, there are many territories of the Earth with quite a cold weather. The aim of this review has been to survey the application of AHCS in cold countries. The thermodynamic indicators of AHCS cycle – the first and second law efficiencies - are briefly described. For obtaining a high first law efficiency, the adsorbate exchange per cycle has to be increased, the inert masses - decreased, and a working fluid with a large latent heat should be used. The SLT efficiency for heat amplification increases at colder ambient. It opens up new possibilities for using AHCS in cold countries.
The issues of which climate is deemed to be cold and how low ambient temperature can affect thermodynamic and dynamic features of the AHCS cycle/unit are considered. Traditional and recently proposed cycles/units are surveyed, and their applicability for cold territories is analyzed for closed and open AHCS systems. Special attention is paid on two new approaches intently proposed for cold climates, namely, a “Heat from Cold” cycle for upgrading the ambient heat temperature and a Ventireg process for regeneration of heat and moisture in ventilation systems. The performed analysis shows the prospects of AHCS technologies for cold countries and outlines the boundaries of their applicability.
... A large number of Desiccant Coated Heat Exchangers (DCHEs) based on silica gel [13], FAM [11,14], silica-alumina-phosphate SAPO-34 [15], metal-organic framework MIL-100(Fe) [16] have been developed. Their feasibility has been successfully demonstrated for open desiccant air cooling [17] and heating systems [18], desiccant enhanced vapour compressed heat pumps [19], desiccant humidity control systems [20], and air dehumidification [21] systems. ...
Due to global climate change and fossil fuel depletion, the rational use of thermal energy has attracted great research interest. Large differences between indoor and outdoor temperatures in cold regions results in huge amounts of heat waste and drop in indoor humidity. Ventireg, an adsorption method, has been often recommended for heat and humidity regeneration in cold countries. In this research work, VentireC, an advanced method employing two thermally coupled adsorbent beds is discussed. It allows the heat released during adsorption of moisture in one adsorber to be transferred to another adsorber to facilitate water desorption. The VentireC approach is comprehensively analysed and described in this paper. A composite adsorbent based on LiCl in silica gel pores, which can exchange up to 0.5 g-H2O/g-sorbent, is selected for VentireC processes under cold Western Siberia conditions. Mathematical simulation of humidity recuperation, employing the selected sorbent with and without thermal coupling, demonstrates the advantages of the VentireC process.
... They reported that the SAPO-34 system could outperform commonly used silica gel system and it shows a high potential for dehumidification. In our previous study [18,19], two DCCFHEs were fabricated by coating silica gel and aluminophosphate (AlPO) zeolite on the surface of plate-fin heat exchangers respectively. Experimental study demonstrated that the cooling air effectively enhances the heat and mass transfer as well as promotes the removal of moisture. ...
Air-cooled desiccant coated cross-flow heat exchanger (DCCFHE) system provides a simple and effective approach to improve dehumidification performance by using cooling air to remove adsorption heat during adsorption process. In this study, a novel numerical model validated by experimental data was established to predict the performance of a conventional silica gel DCCFHE system. The fin efficiency, as a variable due to the release of adsorption heat, was taken into account in the model. Parametric study for various air velocity and structural parameters was conducted to reveal their effects on the dehumidification performance. Results show that the smaller process air velocity is conducive to improve dehumidification performance, but the performance increases with the increase in cooling air velocity. Compared to the operation case without cooling air, the moisture removal capacity and dehumidification coefficient of performance can obtain about 35% improvement at the cooling air velocity of 1 m/s. The results also indicate that thicker desiccant layer thickness and smaller fin pitch improve moisture removal capacity and dehumidification coefficient of performance but result in greater total pressure drop ΔP, while the performance is insensible to fin thickness and fin height. Besides, effect of heat transfer performance in dehumidification and cooling sides on dehumidification performance was also analyzed. It was found that the heat transfer performance in dehumidification side is the dominant factor affecting dehumidification performance.
... Our present study focuses on mass transfer resistance in the adsorbent layer. Kubota investigated dehumidification performance of cross-flow heat exchange type adsorber [8]. Ramy H. Mohammed investigated the relationship between physical properties and adsorption kinetics of silica-gel/water [9,10]. ...
Desiccant-coated heat exchangers (DCHEs) in air conditioning offer advantages like independent temperature
and humidity control, reduced energy consumption, and high performance. However, the availability of desiccants
with high water adsorption and low thermal energy requirements is limited. The present study proposes
that aluminum fumarate (Al-Fum) MOF-coated heat exchangers will outperform conventional desiccants, especially
at low regeneration temperatures (around 50 °C). In order to evaluate this hypothesis, the performance of
DCHE with validated analytical models was evaluated using different solid desiccants: silica gel, zeolite, and Al-
Fum MOF. Water vapor sorption characteristics, heat and mass transfer, and adsorption and desorption capacities
were determined. The results show that the Al-Fum MOF-coated heat exchanger outperforms silica gel and
zeolite at low regeneration temperatures under adiabatic conditions. The MOF exhibits superior water uptake capacity
(0.39 g/g) between 20% and 80% RH compared to conventional desiccants. Moreover, the MOF achieves
the lowest outlet air temperature. At 19.5 g/kgda, the MOF demonstrates higher adsorption rates (4 % and 0.3 %)
than zeolite and silica gel. These findings highlight the potential of Al-Fum MOFs as energy-efficient solutions for
controlling indoor air humidity, particularly for latent-sensible heat separation systems with conventional vapor
compression bus air-conditioning systems.
Desiccant coated heat exchanger (DCHE) systems have significant potential in improving dehumidification performance. The moisture transfer occurring in one single air channel of DCHE can dominate the overall system performance. In this study, the adsorber with an approximate air channel of DCHE was designed to experimentally investigate the kinetics of silica gel and two aluminophosphate zeolites (FAM Z01 and FAM Z05). Their dehumidification performances also were simulated at regeneration temperature range of 50–80 °C to reveal the applicability in air-cooled cross-flow DCHE. Results showed that linear driving force (LDF) model could well describe the dynamic water uptake behaviors. Adsorption rate coefficient kads increased with increasing inlet air humidity ratio and velocity, but decreased with increasing coating thickness. It is worth noting that increasing regeneration temperature had a little effect on kads, while desorption rate coefficient kdes increased significantly. Simulation results showed that moisture removal capacity (MRC) and dehumidification performance of coefficient (DCOP) of FAM Z05 coated DCHE could reach 0.495 g/kg and 0.57 at regeneration temperature of 50 °C, which were 2.3 and 15.4 times higher than these of silica gel and FAM Z01 coated DCHEs. It demonstrated that FAM Z05 was more promising to utilize lower-grade heat energy.
Heat and mass transfer characteristics (HMTCs), which defined the latent and sensible load, are essential for developing an effective desiccant dehumidification system (DSDHS). Therefore, the current work provides an investigation on the HMTCs, in the form of the Nusselt and Sherwood numbers. Under constant adsorption temperature of 293 K, the HMTCs were analyzed with three different regeneration temperatures of 308 K, 318 K, and 328 K. While the velocities were kept constant of 0.1 kg/s both for adsorption and regeneration processes. The DSDHS used the polymer material having the highest adsorption capacity of 0.78 kg/kg. Using the thermal and mass resistance approaches, the heat, and mass transfer coefficients were able to predict. The results show that the highest regeneration temperature revealed the most top differences between the humidity ratio inlet and outlet, which was 2.2 g/kg. The Nusselt number values, as well as Sherwood number, tend to increase with the rise of regeneration temperature. The proposed empirical correlation for predicting the HMTCs was followed by the experimental data with a maximum discrepancy of 17%. It is suggested that the properties of the interface must be carefully acquired to reduce the discrepancy between the experimental data and analytical.
Solid desiccant dehumidification is a promising alternative to vapor compression-based air-dehumidification to reduce energy consumption and improve air quality. Desiccant coated heat exchanger (DCHE), as one type of solid desiccant dehumidification system, can improve system performance and efficiency. The thermal performance and moisture removal capability of the DCHE greatly influence its dehumidification performance.
The present work aims to critically analyze the designs, materials, and manufacturing methods of heat exchangers used for solid desiccant coating and look into recent developments in regular heat exchangers, potentially deployed as a substrate for DCHE applications. A comprehensive literature review of publications regarding solid desiccant materials, heat exchangers, manufacturing and coating methods, binder materials, and the performance of heat exchangers has been developed. Alternative heat exchangers are described as better options than fin and tube heat exchangers that have been considered thus far for DCHE applications. Additionally, the heat exchanger's manufacturing process and materials over their thermal and mass exchange performance have been analyzed. Despite the widespread use of fin and tube heat exchangers for solid desiccant coating, recent developments in heat exchanger design, and related fields such as manufacturing methods and materials, open the possibilities for their application in solid desiccant dehumidification systems.
Finally, the authors provide their outlook on possible developments of DCHE technology, aiming to increase systems' energy and dehumidification performance.
Desiccant coated heat exchanger (DCHE) has shown great potential for dehumidification in vapor compression cooling systems and has been widely concerned in the research field of air conditioning, as well as heat and mass transfer in recent years. This paper is aimed at providing fundamental knowledge and research progresses on DCHE for dehumidification and cooling. Initially, DCHE dehumidification is chosen as the research object in this review due to its superiority of the principle by comparing with other two typical dehumidification processes, which are condensation dehumidification and rotary-wheel dehumidification. Then, intermittent dehumidification and continuous dehumidification as two operation modes of DCHE are compared. Finally, the variation characteristics of temperature and humidity during operation are discussed. The current studies of DCHE are reviewed from four aspects, including desiccant coating, cooling source, regeneration heating source and basic heat exchanger. The performance analysis of DCHE is then conducted, according to the different effects of above aspects acting on dehumidification performance, cooling performance and energy efficiency of DCHE. The ongoing researches indicate that the rational construction of DCHE is an effective way to achieve its multi-objective performance optimization. The significant progress of DCHE lays the foundation for applications on other fields. However, further research is required in the field of the selection of desiccant coatings and the structure of heat exchangers to broaden its application market.
Solid desiccant heat pump system (SDHP) can handle sensible and latent heat load simultaneously with high coefficient of performance (COP). However, in such system, supply air temperature is relatively high and unstable due to the frequent switch between desiccant coated heat exchanger as evaporator and condenser. In order to overcome these drawbacks, the concept of semi-coupled solid desiccant heat pump (SCSDHP) system is proposed in this paper, then a mathematical model is established to investigate the feasibility of this novel system. Calculation shows that under ARI (Air-conditioning and Refrigeration Institute) summer condition silica gel coating with density of 0.25 kg m⁻² is optimal, and area ratio between desiccant coated evaporator and conventional evaporator of 1:1 is recommended. Furthermore, compared with conventional SDHP system, supply air temperature can be 4.5~7°C lower and the variation range of supply air temperature decreases by 16~21°C. Also, effects of main operating parameters including the temperature and relative humidity of outdoor air on the performance of SCSDHP system are investigated.
Nowadays, advanced technologies for rational use of energy in dwellings have aroused considerable interest. In cold countries huge amounts of heat and moisture are wasted through the air infiltration due to the large difference between indoor and outdoor temperatures. In this work, an advanced adsorption approach to heat and moisture recuperation in ventilation, called VentireC, is suggested. In this approach, the moisture and sensible heat from outgoing air are absorbed on the adsorbent and heat storing beds and then withdrawn into the inflowing outdoor air, thus, maintaining the indoor temperature and humidity balance. Thermal coupling between two adsorbent beds, which work out of phase, allows latent and sensible loads to be managed separately to enhance the humidity recuperation. For harmonizing the adsorbent properties with the operating conditions of the VentireC process, the requirements for optimal sorbents are formulated based on the thermodynamic analysis of the process. New sorbents based on LiCl incorporated in four matrices with the various mesoporous structure are synthesized and investigated. The water sorption/desorption equilibrium for the most promising sorbent is reported. This composite exchanges over 0.5 g-H2O/g under a typical VentireC cycle, which is promising for effective heat and moisture regeneration.
Desiccant coated aluminum fins are an important part of desiccant coated heat exchangers (DCHEs). The overall performance of DCHE-based systems benefits from improved heat and mass transfer characteristics of desiccant coated aluminum fins. In this paper, a new type of nano-silver powder supported FAPO-34 composite fin is proposed. Samples with different mass percentages of nano-silver powder were fabricated and related characteristics including thermal conductivity, cycle water uptake and adsorption/desorption performance were investigated. Our experimental results show greatly improved thermal conductivity of nano-silver powder supported composite sheets. Analysis of the adsorption testing confirmed that the nano-silver powder improved dynamic adsorption performance, and adsorption rate coefficients of composite sheets increasing by 6–103% compared with that of a pure FAPO-34 coated sheet. The composite samples also exhibited better dynamic desorption performance, in that their desorption rate coefficients increased to be 1.3–2.3 times as great as those of the pure FAPO-34 sample.
Desiccant dehumidification systems provide an environment-friendly and energy-saving option for air humidity regulation. The dehumidification wheel as the key component occurring mass transfer process determines the dehumidification performance of the systems. In this study, a two-dimension numerical model was developed to conduct parameter studies on the mass transfer characteristics of dehumidification process for the desiccant-coated dehumidification wheel. The mass transfer coefficient Kd and Ktotal, the integral-average of mass transfer coefficient K¯d,eff and K¯total,eff determined by the effective adsorption time (teff) and the dimensionless parameter χ(t) were introduced to evaluate the mass transfer performance. Effects of main operational and structural parameters on the mass transfer performance were analyzed and discussed. The results show that these evaluation indices significantly depend on the various parameters, and the preferable dehumidification channel shape is related to cycle time. Research efforts have been made to obtain a correlation for the mass transfer coefficient, which provides an effective tool to predict the dehumidification characteristics of the dehumidification systems.
p> Досліджені експлуатаційні характеристики адсорбційного регенератора низько-потенційного тепла та вологи на основі композитних сорбентів «силікагель – натрій сульфат», синтезованих золь – гель методом. Розроблені математична модель та алгоритм визначення базових параметрів процесів експлуатації адсорбційного регенератора в умовах сектора житлово-комунального господарства. Запропонований алгоритм включає розрахунок об’єму повітря, який пройшов через шар теплоакумулюючого матеріалу, концентрації води в повiтрi на виході з теплового акумулятора, адсорбції, теплоти адсорбції, кінцевої температури холодного повітря, температури повітря після змішування холодного повітря з вулиці та теплого повітря в приміщенні при подачi, розрахунок концентрації води в повiтрi на виході з теплового акумулятора, об’єму повітря, який пройшов через шар теплоакумулюючого матеріалу, адсорбції та теплоти адсорбції, кінцевої температури теплого повітря, температури повітря після змішування холодного повітря з вулиці та теплого повітря з приміщення при викиді, визначення температурного коефіцієнта корисної дії, сумарної адсорбції та часу досягнення максимальної адсорбції. Підтверджена кореляція температур повітря біля теплого та холодного кінці регенератора, а також теплових коефіцієнтів корисної дії, встановлених за результатами розрахунків згідно запропонованого алгоритму та дослідним шляхом. Проведено математичне моделювання процесів експлуатації адсорбційних регенераторів на основі композитів «силікагель – натрій сульфат» в умовах типової системи вентиляції житлових приміщень. Показана залежність теплового коефіцієнта корисної дії від часу перемикання потоків повітря, швидкості руху потоків повітря, а також температур зовнішнього та внутрішнього повітря в стаціонарних умовах. Максимальні значення теплових коефіцієнтів корисної дії встановлені при швидкості вологого повітря близько 0,22 – 0,32 м/с та часу перемикання потоків 5 – 10 хв.. Виявлено вплив швидкості руху потоків вологого повітря на час досягнення максимальної адсорбції. Результати проведеного дослідження можуть бути використані при розробці енергоефективних вентиляційних систем та пристроїв для житлових та складських приміщень.
The performance characteristics of the adsorptive regenerator of the low-potential heat and moisture on basis of the composite sorbents ‘silica gel - sodium sulfate’ synthesized by sol-gel method are studied. The mathematical model and algorithm for determining the basic parameters of adsorptive regenerator exploitive processes in the housing and communal services sector are developed. The proposed algorithm includes calculating the volume of air passed through the layer of heat-accumulating material, the concentration of water in the flow at the exit from the regenerator, the adsorption, the heat of adsorption, the final temperature of the cold air, the air temperature after mixing the cold air from the street and the warm air in the room at the warm end of the regenerator during inflow, calculation of the final concentration of water in the flow at the cold end of the regenerator, the volume of air passing through the layer of heat-accumulating material, adsorption and heat of adsorption, the final temperature of the air at the cold end of the regenerator, the air temperature after mixing of the cold air from the street and the warm air from the room at the cold end of regenerator during outflow, determining the temperature efficiency coefficient, summarized adsorption and maximal adsorption time. The correlation of air temperatures near the warm and cold end of the regenerator, as well as the thermal efficiency coefficients stated by the results of calculations according to the proposed algorithm and experimental way is confirmed. The mathematical modeling of the processes of operation of adsorption regenerators on the basis of ‘silica gel - sodium sulfate’ composites in the conditions of the typical ventilation system of residential premises is carried out. The dependences of the temperature efficiency coefficient vs. the time of switching air flows and the velocity of air flow, as well as the temperatures of external and internal air under stationary conditions are shown. The results of the research can be used in the development of energy-efficient ventilation systems and devices for residential and warehouse premises </p
Conventional air source heat pump system faces several challenges when adopted in winter season. Solid desiccant air conditioning system can provide humidification and heating power simultaneously and can be driven by low grade thermal energy; it provides a good alternative for air source heat pump systems. However, conventional solid desiccant air conditioning system adopts desiccant wheel with high cost as core component, which hinders the development of such system. Recently, desiccant coated heat exchanger (DCHE) with low initial cost and high efficiency was developed and this paper aims to investigate performance of DCHE air conditioning system adopted in Shanghai winter season. Performance of the system is predicted by a developed mathematical model where supply air states, mass of humidification and coefficient of performance (COP) are adopted as performance indices to evaluate the feasibility and energy utilization ratio of the system. Effects of regeneration water temperature on system performance are analyzed. It is found that under the simulation condition, relatively low regeneration temperature (such as 20 °C) cannot meet the designed standard and relatively high regeneration temperature (such as 40 °C) provides too much extra heating power, thus moderate regeneration temperature around 30 °C is recommended. Meanwhile, switch time is a crucial operation parameter for the system to obtain satisfied supply air, switch time from 40 s to 80 s and from 70 s to 240 s are recommended for transient and average supply air states, respectively. Both mass of humidification and COP increase with increasing regeneration temperature under simulation condition. Also, influences of ambient air temperature and humidity ratio on system performance are discussed to study the feasibility of such system regarding different climatic conditions. Results show that higher humidity ratio of ambient air results in increased humidity ratio of supply air, temperature of ambient air has neglectable effect on supply air. In conclusion, DCHE air conditioning system can be adopted for winter operation with moderate selection of regeneration temperature as well as switch time.
A silica gel coated heat exchanger based air conditioning system driven by the evacuated tube solar water heater has been experimentally investigated. The system has been operated for two different modes namely cooling with dehumidification mode and heating with humidification mode in summer and winter season respectively. The system performance is analyzed in terms of regeneration rate, dehumidification rate and thermal coefficient of performance (COPth). Experimental results demonstrated that, for cooling and dehumidification mode, the process air is cooled by an average temperature of 8.5 degrees C. A better dehumidification rate can be achieved by using pre-cooling before dehumidification process. Post-cooling after dehumidification process is found to be advantageous for cooling capacity and COPth. For heating with humidification mode, the process air is heated by an average temperature of 13.3 degrees C with an average increment in humidity ratio of 1.9 g/kg. It is found that the average COPth of the system is 0.45 and 0.87 for cooling and heating mode respectively.
A DCHE (desiccant coated heat exchanger) is a novel solid desiccant cooling component with desiccant coated onto the surface of a fin-tube heat exchanger. In the paper, carbon based composite desiccants were developed and studied for DCHE systems. Composite desiccants were fabricated by impregnating LiCl into pores of activated carbon and activated carbon fiber. Due to impregnated salt, composite desiccants were found to have smaller surface area and pore volume. Sorption isotherms were measured and simulated based on Polanyi potential theory. Water sorption isotherms showed that composite desiccants possessed enhanced sorption quantity. Sorption kinetics was also investigated and fitted with the linear driving model. Composite desiccants showed higher dynamic water uptakes and reasonable rate coefficients. Finally, to predict dehumidification performance of composite desiccants in DCHE systems, a mathematical model was built. Simulation results showed that composite desiccant coated DCHEs can remove more moisture from the process air.
Desiccant dehumidification system is an alternate option against conventional dehumidification system in hot and humid climates. Conventional dehumidification systems have many drawbacks that include high power consumption and increase the chlorofluorocarbon (CFC) level in the environment and major contribute to depletion of ozone layer. This paper discuss the functioning of dehumidification, cooling and air-conditioning systems using various solid desiccant with focus on the use of solar energy for dehumidification of humid air and regeneration of solid desiccant wheel. A comparative study of various dehumidification, cooling and air-conditioning systems show that solid desiccant has low operating and maintenance cost and is environment friendly.
In this paper, the heat and moisture transfer characteristics of a desiccant dehumidification unit using fin-tube heat exchanger with silica gel coating have been experimentally investigated. The present study focuses on the analysis of air side heat and moisture transfer performance in dehumidification mode. The empirical formula on Nusselt and Sherwood numbers were deducted and curve fitted from the test results under various operation conditions, using the non-linear least square method. The typical operation condition is that the air side Reynolds number ranges between 220 and 550 and tube side cooling water flow rate is about 7.7 L/min. Further tests indicate that the correlations of Nusselt and Sherwood number on air side of the unit agree well with the test results, and the mean errors are about ±10% and ±12%, respectively. Finally, the improvements of the operation and configuration parameters of this novel heat and moisture transfer components are discussed with those formulas. It is found that the correlated formulas provide the theoretical basis and practical experience for design and optimization of the desiccant dehumidification unit using fin tube heat exchanger with silica gel coating.
A DCHE (desiccant coated heat exchanger) is a novel solid desiccant component with desiccant coated onto the surface of a fin-tube heat exchanger, in which desiccant coated aluminum-fin is an important part. It has an impact on overall performance of the whole desiccant cooling system. In this study, different zeolite-like powders (SAPO-34 and FAPO-34) coated aluminum sheets were fabricated. Their performance was tested and analyzed to investigate the feasibility in a DCHE system. Experimental values of surface area and pore parameters for desiccant coatings had a slight reduction because of the existence of binder solution. Adsorption kinetics indicated that the binder solution would not affect water adsorption capacity or adsorption kinetics of original desiccant powder. Moreover, SAPO-34 and FAPO-34 coated aluminum sheets had higher adsorption capacity than that of silica gel. Sorption isotherms were also measured and fitting equations were developed based on Polanyi principle. Finally, their dehumidification performance in DCHE systems was predicted with a mathematical model. Simulation results indicated that the DCHE prepared from FAPO-34 can have 2–3 times larger dehumidification capacity than those from SAPO-34 and silica gel at low regeneration temperatures.
In this paper, an experimental setup of a desiccant dehumidification unit silica gel coated fin-tube heat exchanger is installed and investigated. In this unit, two silica gel coated heat exchangers (SCHE) are adopted and switched to provide continuous dehumidification capacity. Meanwhile, hot water from vacuum tube solar collector is used to regenerate silica gel. System performance is evaluated in terms of moisture removal mass and thermal COP. Influences of major parameters on system performance are tested and analyzed under Shanghai summer conditions. It is found that system performance is affected significantly by cycle time between dehumidification and regeneration, and optimal cycle time is 600 s under test conditions. For actual system, the unit can be independently used as dehumidifier with 100% fresh air under mild conditions. But when the outside humidity is too large, it can be used in combination with primary return air-conditioning to provide satisfied supply air.
The paper focuses on development of new adsorbent material of aluminophosphate (FAM-Z05) which could be subsequently used in utilization of waste beat below 373 K. To evaluate this new material, water vapor adsorption isotherms of FAM-Z05 were measured at several temperatures. Furthermore, an attempt was made to improve the heat exchanger by coating FAM materials onto rin tube type heat exchanger. Then, the dehumidification performance of direct cooling and heating desiccant system was evaluated. It was found that the water vapor adsorption isotherms of FAM-Z05 were characterized by S-shape and the adsorption character of FAM-Z05 was dependent on temperature. Further, the new adsorbent of FAM-Z05 was able to adsorb water vapor in lower regeneration temperature than FAMZ02 and FAMZ01, which was attributed to FAM-Z05 ability to adsorb water vapor in high humidity range. However, when FAM-Z05 is used in low humidity range, the dehumidification performance of FAM-Z05 might be lower than those of FAM-Z01 and FAM-Z02.
AFI-type structure ferroaluminophosphate zeolite (FAPO-5) was examined as an AHP adsorbent. FAPO-5 with iron content of 2-8mol% (Functional Adsorbent Material-Zeolite 01; FAM-Z01) was selected for further testing. The water vapor adsorption isotherm of FAM-Z01 was S-shaped and highly dependent on temperature, and almost no hysteresis was observed with adsorption/desorption at 333 K and 348 K. No changes were observed in the properties of FAM-Z01 after 200,000 cycles of water vapor adsorption-desorption, indicating that FAM-Z01 is suitably durable for practical use. When the AHP was operated under conditions of TL/TM/TH= 283 K/303 K/333 K, the adsorption capacity of FAM-Z01 was 4 times that of silica gel.
The cross-cooled dehumidifier prototype was constructed in the form of a cube with a side dimension of 0.6 m. The cross-cooling was achieved by passing air through rectangular channels perpendicular to process channels which are lined with desiccant sheets consisting of 9..mu..m Syloid 63/sup TM/ silica gel held in a Teflon web. The process for the manufacture of the silica gel sheets was developed at the Illinois Institute of Technology. The dehumidifier prototype was installed in a test system that simulated the performance of a cross-cooled desiccant cooling system and monitored the performance of the prototype. The dehumidifier prototype was operated over a wide range of operating conditions which would be typically encountered in the field installation of such a system. Variables measured were the moisture cycled, cooling capacity, total cooling capacity and coefficient of performance.
A modified version of the cross cooled desiccant bed is presented. The parallel flow channels are subdivided longitudinally and packed with silica gel particles instead of being glued to the side walls. The bed is regenerated utilizing the rejected heat from a LiBrH2O absorption cooling machine and both are integrated in a solar powered air conditioning system. Two beds are alternately working in the humidification/dehumidification cycle.Treating the matrix and the desiccant separately, a five governing equation model, describing the heat and mass transfer between the passing air flow, the desiccant material and the matrix metallic structure, is introduced and given. This is numerically solved for both the humidification mode and the dehumidification mode with each occupying half of the bed cyclic duration.For a case study in Jeddah, with a cyclic duration of 15 minutes, the results showed an overall coefficient of performance of 1.2 which is 53% higher than that for the absorption machine alone. Moreover, adding the desiccant bed to the air conditioning system raised the evaporator temperature to 19.3°C compared with the 11.5°C required for the absorption machine alone.
A new modified cross-cooled compact solid desiccant dehumidifier (CCCD) is proposed and its mathematical model describing heat and mass transfer processes between the moist air and the desiccant is presented. The dehumidifier is constructed on the basic structure of plate fin heat exchanger with silica gel elaborately glued on all the metal surfaces of the process-flow channels. The cyclic dynamic performance of the CCCD is simulated by implicit finite difference method and several important variables that strongly influence the performance of the CCCD are identified and analyzed. The simulated dehumidification performance is compared with the experimental results, and showing good agreement within 7% deviation. It is shown that the performance of the CCCD proposed in this paper with cooling effect is much better than that without cooling. It proves that dehumidification by silica gel with cooling can greatly improve its performance.