Singh, S. and Singh, K. / Mechanica Confab
ISSN: 2320-2491
AN EXPERIMENTAL STUDY ON THE PERFORMANCE OF
LIQUID DESICCANT DEHUMIDIFICATION SYSTEM
a
b
Satwinder Singh , Karnvir Singh
a
Assistant Professor, Department of Mechanical Engineering, Chandigarh Group of CollegesCollege of Engineering, Landran, Mohali, Punjab, India
b
Assistant Professor, Department of Mechanical Engineering, Chandigarh Engineering
College, Landran, Mohali, Punjab, India
singhsatwinder371@gmail.com, karnvirsinghcheema@gmail.com
Abstract
The performance of liquid desiccant dehumidification system is presented in this paper. A
solution of CaCl2 is used as desiccant in the experimental study. The experimentation is done
with a PVC packing of honey comb structure in the dehumidifier. The performance of
dehumidifier is studied by changing the inlet parameters like air flow rate, air inlet
temperature and desiccant solution flow rate. The effect of these parameters on outlet
parameters on specific humidity, air outlet temperature, moisture removal rate is studied in
this experimental study.
Keywords: Dehumidification system, Desiccant, Calcium Chloride, Liquid desiccant
systems.
1. Introduction
Moisture removal is very much essential for maintaining healthy and comfortable conditions
for various industrial and agricultural processes. For a comfortable and healthy environment
the two essential requirements are: adequate ventilation and good humidity control. But in
very humid climates, which include much of the densely populated regions of the world, it is
difficult to meet both these requirements without using a lot of electricity. One of the most
common methods of dehumidification is the vapour compression refrigeration system. But
the major problem associated with vapour compression system is that it causes the depletion
of ozone layer and it also requires a lot of electricity for its operation. Desiccant
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Singh, S. and Singh, K. / Mechanica Confab
ISSN: 2320-2491
dehumidification is an alternate method to dehumidify the air. Desiccant is a substance that
can attract and hold the water vapours.
Desiccant systems work on the principle of removing the moisture from the air due to
difference in vapour pressure of desiccant and water vapour present in air. Vapour pressure of
a liquid desiccant is directly proportional to its temperature and inversely proportional to its
concentration. As the concentration of the desiccant in the solution increases its vapor
pressure decreases. This difference in vapour pressure allows the desiccant solution to absorb
moisture from air whenever the vapour pressure of air is greater than that of the desiccant
solution.
The equilibrium vapour pressure of the desiccant solution depends on its temperature and
concentration. The higher is the concentration and lower is the temperature, the higher will be
the moisture absorbed and lower will be the humidity. But as the concentration increases and
temperature decreases, then the crystallization of the desiccant takes place due to its
solubility. In desiccant dehumidifiers, the desiccants attract moisture from the air by creating
an area of low vapour pressure at the surface of the desiccant. As the desiccant comes in
contact with the humid air, the desiccant releases heat and it warms the air the air due to
which the air gets dried and dehumidified. This dry air can then be cooled to desired comfort
conditions by using evaporator coolers, heat exchangers etc.
The most commonly used liquid desiccants are the aqueous solutions of calcium chloride,
lithium chloride, lithium bromide and triethylene glycol [3]. The most important component
in a liquid desiccant dehumidification system is the dehumidifier where air and desiccant
solution comes in direct contact with each other and air loses its moisture to the desiccant.
The dehumidifier is packed with packing material. Different packings made of various
materials and sizes are generally used in the dehumidifier. Out of these the usually used ones
are flexi rings, berl saddles, intalox saddles, raschig rings [4].
Liquid desiccant dehumidification systems are widely used in various industrial applications.
D.Seenivasan et al. studied the performance of an adiabatic dehumidifier using calcium
chloride as a liquid desiccant [5]. Agung Bakhtiar et al. studied the effect of air velocity and
desiccant flow rate on a structured packed tower of lithium chloride dehumidifier system [6].
S. Bouzenada et al. analyze the effect of temperature for regeneration process using an
injected air through the liquid desiccant solution (calcium chloride dihydrate) by a dryer [7].
Min-Hwi Kim et al. discussed the integration of a liquid desiccant system into an evaporative
cooling-assisted 100% outdoor air system and detailed energy simulation is performed for
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ISSN: 2320-2491
estimating the impact of the liquid desiccant system in indirect and direct evaporative cooler
operations [8]. G.I Sultan et al. studied the effect of inlet parameters on the performance of
packed tower regenerator [9]. Muhammad Sultan et al. compared the performance of
desiccant dehumidification and conventional air conditioning system [10]. M. Mujahid
Rafique et al. discussed the technology of desiccant based evaporative cooler and also
discussed some modern and modified evaporative coolers [11]. Ahmed and Ahmed studied
the rate of moisture removal in a solar powered liquid desiccant system [12]. Hesamoddin
Salarian et al. studied the size and performance of a dehumidification tower by simulating
various operating conditions and presented the performance of a packed tower absorber for a
lithium chloride desiccant dehumidification system. They showed the effects of the main
variables like air flow rate, liquid desiccant flow rate and inlet air temperature on the rate of
dehumidification [13]. F. N Ani et al. tested the performance of a hybrid liquid desiccant
system for various packing heights [14]. P. Gandhidasan et al. investigated the effect of
various independent variables such as air inlet absolute humidity, flow rate and its
concentration on the performance of a liquid desiccant dehumidifier [15]. Esam Elsarrg
investigated the effect of varying various design parameters like air and liquid flow rate,
humidity ratio and packing height on the moisture removal rate [16].
In the present study the effect of various inlet parameters are studied on the performance of
liquid desiccant dehumidifier. There are several parameters that affect the performance of a
dehumidifier e.g. air flow rate, desiccant flow rate, air inlet temperature, desiccant
concentration and temperature, packing height. The packing height in the present study is
kept constant and the performance of liquid desiccant dehumidifier is measured in terms of
humidity ratio, moisture removal rate, air outlet temperature.
2. Experimental Setup
The setup used for the experiment consists of a desiccant solution tank, desiccant pump,
rotameter, orifice to measure the air flow rate, dehumidifier, air blower, air duct,
psychrometer, rubber pipes, PVC packing of honey comb structure etc. The flow rate of air
can be controlled by controlling the speed of air blower. The dimensions of the dehumidifier
are taken as 900 mm × 600 mm × 300 mm and the size of the air inlet duct is 750 mm × 150
mm × 150 mm. An air duct is provided at the top of the dehumidifier through which the
dehumidified air flows. Two PVC zigzag packing of size 600mm×300mm×150mm were
stacked one over another to reduce desiccant carry over rate and to increase the air–desiccant
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ISSN: 2320-2491
contacting surface area. An orifice of diameter 50 mm is inserted in the outlet air duct to
measure the pressure head across the dehumidifier. By measuring this pressure head flow rate
of air can be measured.
Fig. 1. Experimental Setup
3. Operation Procedure
The desiccant solution of CaCl2 of desired concentration is prepared in the desiccant solution
tank. The pump and air blower is started and the system is allowed to run for 15 minutes to
bring it to the operating steady state conditions. The desiccant solution is pumped to the top
of the dehumidifier where it falls over the packing and flows through the packing. The air
from the air blower enters the dehumidifier and flows upwards in an opposite direction to the
flow of desiccant solution. The air loses its moisture to the desiccant solution and gets
dehumidified. The dehumidified air flows through the orifice plate and comes out. Various
inlet and outlet parameters of air and solution are recorded.
4. Results and Discussion
A series of experiments were performed on the set up at various inlet parameters. The input
parameters used in the experiment are air inlet temperature, solution inlet temperature,
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solution flow rate, air flow rate. The measured outlet parameters are air outlet temperature,
specific humidity or humidity ratio, solution outlet temperature, rate of evaporation.
4.1. Effect of solution flow rate
In this experiment the effect of solution flow rate is studied on moisture removal rate, change
in specific humidity and air outlet temperature. Figure 2 shows the effect of solution flow rate
on rate of moisture removal rate for different air flow rate. From the figure we see that the
moisture removal rate increases with increases solution flow rate. . This is due to the reason
that with increasing solution flow rate, more desiccant comes in contact with the air and it
absorbs more latent heat as such it absorbs more moisture and hence moisture removal rate
increases. e.g. for an air flow rate of 0.00112 kg/s, the moisture removal rate increases from
0.001344 to 0.004928 g/s as the solution flow rate is increased from 0.0195 to 0.0662 kg/s.
An increase in air flow rate leads to increase in vapor pressure on solution surface, which
results in increase in evaporation rate and consequently increase in desiccant concentration.
Fig. 2. ms vs mw
Effect of solution flow rate on change in specific humidity is shown in figure 3. The solution
o
temperature is kept as 40 C with concentration of 40%. The air flow rate is varied from
0.0007085 kg/s to 0.0121 kg/s. . It is seen that with the increase in solution flow rate change
in specific humidity also incraeses. For example, for an air flow rate of 0.00112 kg/s, the
change in specific humidtiy is 2.8 g/kg for a solution flow rate of 0.0428 kg/s and it incraeses
to 4.2 g/kg when the solution flow rate is increased to 0.0662 kg/s.
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Singh, S. and Singh, K. / Mechanica Confab
ISSN: 2320-2491
Fig. 3. ms vs ∆ W
The air outlet temperature also incraeses with increase in flow rate of desiccant. Effect of air
outlet temperature with solution flow rate is shown in figure 4. This is due to the reason that
latent heat absorption capacity of the desiccant increases with increase in solution flow rate,
thus the temperature of the solution increases. Due to this the air that comes in contact with
the solution also increases and hence air outlet temperature increases. For an air flow rate of
0.00112 kg/s as the solution flow rate changes from 0.0195 kg/s to 0.0778 kg/s, the air outlet
temperature changes from 34oC to 38.5oC and for an air flow rate of 0.00121 kg/s, the air
outlet temperature changes from 35oC to 41oC. It can also be seen that air outlet temperature
also increases with increase in air flow rate.
Fig. 4. ms vs Tao
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Singh, S. and Singh, K. / Mechanica Confab
ISSN: 2320-2491
4.2. Effect of air inlet temperature
The experiment is carried out to study the effect of air inlet temperature on outlet parameters
like specific humidity at outlet, moisture removal rate, air outlet temperature. The air flow
rate, solution, desiccant concentration, solution inlet temperature is kept as constant. Figure 5
shows the variation of moisture removal rate with air inlet temperature. It is observed that the
evaporation rate or moisture removal rate increases with increase in air inlet temperature.
This is due the reason that with increase in air inlet temperature, the solution temperature
increases which further increase the vapor pressure on the surface of the solution. Due to this
vapor pressure difference between the solution and the air the rate of moisture removal
increases. For example for a solution flow rate of 0.0662 kg/s, as the air inlet temperature
changes from 30oC to 34oC the moisture removal rate decreases from 0.00248 kg/s to
0.006534 kg/s and for a solution flow rate of 0.0662 kg/s, the moisture removal rate increases
from 0.00248 kg/s to 0.006534 kg/s and for a solution flow rate of 0.0778 kg/s, it increases
from 0.002976 kg/s to 0.007502 kg/s.
Fig. 5. Tai vs mw
Specific humidity also incraeses with increase in air inlet temperature. This is again due to
the reason of increasing vapor pressure over the solution surface. From figure 6 we can see
that for a solution flow rate of 0.0428 kg/s, the specific humidity increases from 1.2 g/kg to
4.2 g/kg as the air inlet temperature changes from 30oC to 34oC. The temperature of air at
outlet also increases with increase in air inlet temperature (shown in figure 7). This is due to
the reason that with increase in air inlet temperature the solution temperature increases due to
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Singh, S. and Singh, K. / Mechanica Confab
ISSN: 2320-2491
which the temperature of the air which comes in direct contact with the desiccant solution
also increases. For a solution flow rate of 0.0778 kg/s, the air outlet temperature changes
o
o
o
o
from 36 C to 41 C with an increase in air inlet temperature from 30 C to 34 C.
Fig. 6. Tai vs Wo
Fig. 7. Tai vs Tao
4.3. Effect of air flow rate
Effect of mass flow rate of air on specific humidity at outlet and moisture removal rate is
shown in fig. 8 and fig. 9 respectively. This experiment is carried out for different solution
temperatures of desiccant solution keeping other parameters like solution flow rate, air inlet
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Singh, S. and Singh, K. / Mechanica Confab
ISSN: 2320-2491
temperature and specific humidity at inlet as constant. The air flow rate is varied from
o
o
0.007085kg/s to 0.00121kg/s while the solution temperature is varied from 55 C to 75 C.
Fig. 8. ma vs Wo
o
Figure 8 shows that for desiccant temperature of 55 C the humidity ratio increases from
o
0.0208 kg/kg to 0.0242 kg/kg but as the desiccant temperature is increased to 75 C the
specific humidity increases at a faster rate from 0.032 kg/kg to 0.0371 kg/kg.
Fig. 9. ma vs mw
This shows that specific humidity changes more rapidly for higher desiccant’s temperatures
o
along with air flow rate. From figure 9 it can be seen that for desiccant temperature of 55 C
the moisture removal rate increases from 0.000314 g/s to 0.03454 g/s as the air flow rate
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ISSN: 2320-2491
o
changes from 0.0007085 kg/s to 0.00121 kg/s while for desiccant temperature of 70 C it
increases from 0.001432 g/s to 0.005217 g/s for the same change in mass flow rate of air.
5. Conclusions
Experiments were conducted to study the effect of various parameters like air flow rate, air
inlet temperature and desiccant flow rate to on the performance of dehumidifier in terms of
outlet parameters like specific humidity, air outlet temperature, moisture removal rate. From
the experimental results it is concluded that for a given flow rate of air, the specific humidity
at outlet and moisture removal rate increases as the solution flow rate increases. Also increase
in air inlet temperature shows an incremental impact upon specific humidity at outlet,
moisture removal rate and air outlet temperature for a given flow rate of desiccant solution.
With increase in air inlet temperature and mass flow rate of air, the rate of heat transfer was
from hot desiccant to the air stream, high temperature desiccant is having higher vapor
pressure on its surface, which is the driving force for mass transfer and it leads to increase in
rate of moisture removal. It also increases the desiccant concentration.
6. Nomenclature
ma: Air flow rate, mw: Moisture removal rate, ms: Solution flow rate, Tai: Dry bulb
temperature of air at inlet, ΔW: Change in specific humidity, Wo: Specific humidity at outlet.
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