Published at : 25 Nov 2019
Volume : IJtech
Vol 10, No 6 (2019)
DOI : https://doi.org/10.14716/ijtech.v10i6.3590
Alia Sofia Norazam | Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia |
Haslinda Mohamed Kamar | Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia |
Nazri Kamsah | Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia |
Muhammad Alhamid | Department of Mechanical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia |
Solid desiccant air dehumidifier systems are widely used to supply dry
air for many industrial processes. As humid atmospheric air flows through the
system, the water vapor in the air is adsorbed by the desiccant material,
resulting in dry air leaving the system. A numerical solution has become the
preferred choice for determining the performance criteria of desiccant
materials. The aim of this study is to determine the moisture removal capacity
(MRC), dehumidification effectiveness (?DW),
and thermal effectiveness (?th)
of a solid desiccant wheel material using a numerical method. A representative
three-dimensional model of an air channel enclosed with desiccant material was
developed and meshed using triangular elements. Flow simulations were carried
out under a transient condition. The model was validated by comparing the
simulation results for moisture content and air temperature at the outlet of
the air channel with similar results using experimental data obtained from the
literature. The relative errors for the desorption process were found to be
0.14% for air temperature and 3.7% for air humidity. For the adsorption
process, they were around 3.2 and 0.01%, respectively. These figures indicate
that the numerical model has an excellent ability to estimate the desiccant
material performance. It was also found that at any given regeneration
temperature, silica gel-CaCl2 has the highest MRC, dehumidification effectiveness, and thermal effectiveness compared to silica gel B and Zeolite 13X.
Air dehumidifier; Computational fluid dynamics (CFD); Solid desiccant; Transient flow simulation
Specific
humidity or moisture content is the actual mass of water vapor in 1 kg of dry
air. Whereas relative humidity is the ratio of the actual mass of moisture in
the air at a given temperature to the maximum amount of moisture that air can
hold at the same temperature. Relative humidity and dry-bulb temperature are
two parameters used to indicate the level of personal comfort. People commonly
focus only on temperature but rarely on moisture; however, the high relative
humidity of indoor air may have serious health implications for occupants. This
is because, during the perspiration process, surrounding air that has a
relative humidity close to 100% is unable to absorb the latent heat released by
the human body. Sustained periods under these conditions can lead to people
feeling thermally uncomfortable
Malaysia is a tropical country,
characterized by high daytime temperatures of 29–34°C and relative humidity of
70–90% throughout the year (Makaremi et al., 2012). The recommended temperature
and relative humidity for the indoor environment are 23–26°C and 30–60%,
respectively, as set out by ASHRAE Standard 55 (Yang & Zhang, 2008). In
order to meet the ASHRAE requirement, air-conditioning systems are widely used
in Malaysia. The number of air-conditioning systems in use has increased from
13,000 units in 1970 to more than 250,000 units in 1991, with the number
expected to rise to around 1.5 million units by 2020 (Daou et al., 2006).
However, the growing demand for air-conditioning has contributed to the massive
consumption of electrical power. Other than sensible cooling, air-conditioning
also performs the essential task of humidity control. In conventional
air-conditioning units, the cooling process and air dehumidification are
generally driven by a cooling coil (Nguyen & Aiello, 2013). The high
humidity in Malaysia results in a significantly high air dehumidification load.
The conventional method consumes a large amount of electricity as a result of
the overcooling process to achieve lower humidity. Modern air-conditioning has
recently included separate handling of the dehumidification load and sensible
cooling capacity, which reduces its power requirement. This is usually
integrated with the air-conditioning system to provide comfort inside buildings
such as residential houses and offices, which require around 60–70% humidity,
and hospital operating rooms, which need around 50–60% humidity (Sookchaiya et
al., 2010).
A dehumidifier is a device that can be
used to reduce the humidity of the air. Some industries, such as textile,
foods, pharmaceutical, and battery production, are susceptible to moisture.
These industries require an environment with low humidity within the range of
20–55% in order to maintain the quality of their products and machines (Kamar
et al., 2016). Humid surrounding air will lead to the corrosion of metals,
deteriorated characteristics of hygroscopic material, and increased harmful
activity of micro-organisms in products (Moncmanová, 2007). The system has two
different features, i.e., compressor-based (CBD) and desiccant-based
dehumidifiers. CBD is a conventional method of removing water vapor by
condensation based on the vapor compression refrigeration system (Rambhad et
al., 2016). Humid air passes through a cooling coil where it is cooled below
its dewpoint temperature in order for condensation to occur. However, the CBD
system consumes large amounts of electrical energy during the cooling process.
The desiccant dehumidification system, meanwhile, has received much recent
attention as an alternative to the CBD type (Yamaguchi & Saito, 2013).
Here, air is dehumidified without condensation, using only sorption from
desiccant material instead. This can help reduce the electrical energy
consumption of the CBD system.
Desiccant dehumidifiers can be
characterized into two categories, i.e., liquid desiccant dehumidifier (LDD)
and solid desiccant dehumidifier (SDD). The main components in an LDD system
are the absorber and regenerator. Both parts are filled with constructed
packing materials to enhance the contact area between the desiccant solution
and process air. The absorber is concentrated with a desiccant solution to
enable it to absorb water vapor from the process air. The process air, driven
by a fan, flows in an upward direction within the liquid desiccant. Then, the
dilute liquid desiccant flows out of the absorber and is pumped to the
regenerator. In the regenerator, ambient air at a high air temperature flows in
an upward direction within the diluted liquid desiccant. The ambient air
absorbs the water vapor from the diluted liquid desiccant due to the difference
in vapor pressure between the liquid desiccant and the air. The regenerator
restores the ability of the liquid desiccant to absorb moisture for the next
process cycle. The advantage of the liquid desiccant is that regeneration can
be carried out at a lower temperature with high moisture removal capacity (MRC)
(Misha et al., 2012). However, this can lead to corrosion of the dehumidifier
components.
An SDD consists of a desiccant material
constructed in the form of a wheel that rotates at a low speed, an air heater,
and a drive motor. The wheel comprises a process air section and a regeneration
air section. The humid process (ambient) air flows through the process air
section, during which time the desiccant material adsorbs moisture from the
air. The temperature of the air increases slightly since the adsorption process
releases heat. As a result, the process air leaves the desiccant wheel with
lower humidity and a somewhat higher temperature. On the other hand, hot
regeneration air flows through the regeneration section of the wheel in the
opposite direction to the process air. As this happens, the water vapor sitting
on the desiccant material surface is desorbed by the hot air. This
desorption process also involves heat transfer, and as a result, the
regeneration air exits the wheel with higher humidity and a slightly lower
temperature. The regeneration section of the desiccant wheel then becomes dry,
ready for the new process air to flow through it. Since the desiccant wheel is
rotating slowly, the above sequences of processes are continuous. This system
consumes less electrical energy due to the lack of pump usage and moving parts
(Wu & Wang, 2006). It is also more straightforward than liquid desiccants
(Misha et al., 2012) because it has a low risk of crystallization and only a
slight risk of damage due to high temperatures. Also, the solid desiccant
material is environmentally friendly.
Two of the most critical components in the
SDD system are the desiccant wheel and solid desiccant material itself, which
is corrugated in the numerous channels inside the rotary wheel. Cheng et al.
(2016) carried out a study on the influence of desiccant material properties on
dehumidification effectiveness and showed that it is influenced by the thermal
conductivity, specific heat, porosity, tortuosity, and thickness of desiccant
materials. Jia et al. (2007) compared the effect of silica gel and composite
materials on the coefficient of performance (COP) and MRC. It was found that
the composite desiccant wheel adsorbed more moisture than the conventional one.
Zhang et al. (2014) investigated the effects of ten types of desiccant
materials on COP, specific dehumidification power (SDP), and dehumidification
efficiency. The performance of the desiccant wheel is affected by several
parameters, including wheel geometry, rotation wheel speed, inlet process air
properties, inlet regeneration temperature, and velocity (Yamaguchi & Saito,
2013; Cheng et al. 2016). Jia et al. (2007) conducted a similar study using an
experimental method. Others used a numerical method as an alternative to the
experiment (Misha et al., 2012). This approach is less complicated, much
cheaper, less time-consuming, and less laborious. Cheng et al. (2016)
established a three-dimensional (3D) single-channel model representing
desiccant material and found the model to be essential in obtaining accurate
predictions. For simplification, many researchers have used a single-channel
model to describe the airflow through the desiccant wheel. This may be for
reasons of geometric similarity and in order to avoid prohibitive computation
costs, and it is reasonable to use a single channel to represent the multiple
channels in the desiccant wheel (Yadav et al., 2014).
Recent research and developments of SDD
have focused on improving energy efficiency by using a low-grade heat source
such as solar energy so that the regeneration temperature can be reduced. The
regeneration temperature is determined by the properties of desiccant material,
which should have high adsorption capacity and good regeneration ability.
Though some novel materials have improved the performances of SDD systems, no
material currently available can perfectly satisfy the entire demand for an
energy-efficient, environmentally friendly, and affordable material. Therefore,
more studies on the development of desiccant materials are needed in order to
meet the requirements of industry. Traditionally, these have involved
conducting experiments on the SDD system. However, a series of tests have
needed to be performed as part of the experiments and clearly, due to the need
to install a variety of desiccant wheel models, this practice is highly costly
and time-consuming. In order to ensure efficiency in carrying out the
parametric analysis, numerical modeling should be used where it can promote
energy and cost-saving. There are still only limited studies in
three-dimensional (3D) modeling representing solid desiccant material, where
major past researchers have only developed simplified models in 1D or 2D. These
models reduce the validity of simplified models of the SDD. Therefore, this
study aims to examine the effects of air regeneration temperature and desiccant
material on the performance criteria of solid desiccant material using
numerical modeling. A 3D model of a single air channel was developed to
represent a flow path of the process and regeneration air through the solid
desiccant material. Flow simulations were carried out under a transient state
to predict the average temperature and humidity of the process air at the
channel exit. The model was validated by comparing the simulation results with
experimental data obtained from the literature. The performance criteria
considered are MRC, dehumidification effectiveness, and thermal effectiveness. This research
produces an economical method for determining the performance criteria of solid
desiccant materials. Thus, it could identify the most suitable materials that
give the lowest possible humidity of process air at any given regeneration air
temperature.
A numerical method was used to perform a
transient simulation of moisture adsorption and desorption processes in a
single air channel of a solid desiccant material. The objective was to
determine the MRC, dehumidification effectiveness, and thermal effectiveness of
the desiccant material. The numerical model was validated by comparing the
variation of air humidity and air temperature at the process air channel outlet
with similar data from the literature. For the adsorption process, the relative
errors for the air temperature and air humidity were found to be 0.14% and
3.7%, respectively. While for the desorption process, the relative errors for
the air temperature and air humidity were 0.01% and 3.2%, respectively. It was
also found that at any given regeneration temperature, gel-CaCl2 has
the highest MRC, dehumidification effectiveness, and thermal effectiveness compared
to silica gel B and Zeolite 13X.
The authors are grateful to Universiti
Teknologi Malaysia for providing the funding for this study, under vote number
20H44. The financial support was managed by the Research Management Centre (RMC),
Universiti Teknologi Malaysia.
Filename | Description |
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R1-ME-3590-20191022101747.pdf | Copyright Form |
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