Using Diatomite Soaked in CaCl2 Solution
to Increase Dehumidification
M. Enai1, M. Ohashi2 and H. Hayama1
1
Graduate School of Engineering, Hokkaido University,
N13W8 Sapporo, 060-8628 Japan
2
Mechanical & Electrical Design Department, Obayashi Corporation
2-15-2 Konan, Minato-ku Tokyo, 108-8502 Japan
ABSTRACT
In this paper, we discuss the characteristics, of diatomite, a moisture-absorbent material used for a
new air-conditioning system. Diatomite powder is formed into grains (diameter: 8 mm) with a slantrotating processing machine. After that, the grains are baked. Small balls (diameter: 45 mm) are then
formed from the grains, and are used as moisture-absorbent materials for cells in a moistureabsorbent dehumidifying ventilation device. Gross moisture conductivity (GMC) of the small balls was
-4
measured under two conditions. The GMC of small balls soaked in CaCl2 was 1.5 *10 [kg/m s
-4
(kg/kg’)], but this was 0.45 *10 [kg/m s (kg/kg’)] for those not soaked in CaCl2. A cell, to be used as a
filter in a dehumidifying ventilation device, was packed with the soaked diatomite balls. We then tested
the characteristics of the cell’s vent resistance and its exchange efficiencies of water potential. To
study an application of the device, we numerically calculated the thermal environment during hot
summer weather of a single dwelling into which the dehumidifing ventilation device was introduced.
The quantity of moisture vapor removed by the dehumidifying ventilation device reached 2.6 kg during
3
nighttime at ventilation rates of 150 m /h. Therefore, diatomite balls soaked in CaCl2 solution have a
practical application in a new air-conditioning system.
KEYWORDS
Baked diatomite grain, Gross moisture conductivity, Exchange efficiency of water
potential, Dehumidifying ventilation device, Numerical simulation
INTRODUCTION
Because Hokkaido is a cold region, well-insulated and very airtight houses are being
built there recently, and these maintain a comfortable indoor climate that reduces
heating cost in winter. Unfortunately, however, a few of such houses actually require
an air-conditioning system in summer. Therefore, a new air-conditioning system must
be developed specifically for such well-insulated and highly air-tight buildings.
Diatomite is a very effective material for absorbing and emitting moisture vapor.
Moreover, diatomite has been tested as for use as an absorbent and emissive material in a moisture absorbing, dehumidifying, and heat-exchanging ventilation device.
The purpose of this device is to improve air quality and thermal environment inside a
home throughout the year. The ventilation device is composed of two cells of packed
with small balls formed from baked diatomite grains, two dampers to redirect airflows
and two fans. In summer, one cell absorbs moisture from fresh outdoor-air and sends
it inside a residence. The other cell simultaneously emits moisture vapor into heated
air exhausted from the residence. Since the dampers in the device are periodically
switched, a sufficient volume of air for ventilation can be continuously obtained, and
one cell reciprocally absorbs and emits moisture. In winter, this moisture-absorbent
dehumidifying ventilation device can be used as a heat-exchanging ventilation device
because the thermal capacity of diatomite is large. For example, one of the cells
heats fresh outdoor-air to send inside a residence, while the other is heated by warm
exhaust-air at the same time. As previously stated, because the dampers in the device are periodically switched, a sufficient volume of ventilation air can be
continuously obtained.
Outdoor
Occupied space
Occupied space
Dehumidification
Ball
Emission
Crawl space
Cell
Cell
Cell
Emission
Grains
Dehumidification
Outdoor
Outdoor
Cell
Outdoor
Cell
Damper
Damper
Occupied space
Figure 1: Cell filled with small balls (left), and periodical device operations (right)
In this paper, we discuss the moisture-absorbency characteristics of the baked diatomite grains. We used practical tests to measure the gross moisture conductivity
for diatomite grains and exchange efficiency of water potential for small balls made
from diatomite grains. In addition, finding the vent resistance of cells was required so
that we could determine the energy consumption of the dehumidifying ventilation device. As an application of the dehumidifying device, we introduced it into a single
dwelling, and the thermal environment in hot summer weather was numerically analized. We evaluated the relationship between the energy consumption of the proposed
air-conditioning system and the quantity of moisture vapor that it removed.
PRODUCTION OF DIATOMITE GRAINS
Diatomite grains were made with the slant-rotating processing machine shown in
Figure 2.
Side scraper
Pan
Rake
Flame
Strength [10
Arm
-3
Pa]
100
Non-soaked
Soaked in water
80
60
40
20
0
400
500
600
700
Burning temperature [deg. centigrade]
Grain outlet
Figure 2: Processing machine (left), and physical strength of grains (right)
Processing conditions
The following conditions were used for producing the grains, rotating speed: 26 rpm,
depth of pan: 120 mm, angle of gradient: 0.25 pai, treatment time: 10 to 15 min.,
sample (ratio of weight) : diatomite powder (0.5 mm under) 85% and clay powder
(0.5 mm under) 15% for binder materials. During the producing process, diatomite
85% and clay 15% were continuously supplied, and simultaneously water was
sprayed on these materials. The diameter of the grains produced ranged from 2 to 20
mm. The diameter from 5 to 10 mm accounted for 40% of production.
Baking and soaking conditions
25
baked at 600! (absorption)
baked at 600! (emission)
baked at 700! (absorption)
baked at 700! (emission)
20
15
Equilibrium water content
ratio (%)
Equilibrium water content
ratio (%)
Generally, the larger the diameter of a grain, the lower its physical strength becomes.
Therefore, grains with diameter from 5 to 10 mm must be baked at of 400 to 700 degrees centigrade for 2 hours. When grains were soaked for 16 hours, their strength
was reduced by half if the baking temperature was less than 700 degrees centigrade.
At 700 degrees centigrade, the baked grains became actual ceramic materials, as
shown in Figure 2. However, when the grains were baked at 600 or 700 degrees centigrade, the difference in the absorbency of either group was very small in tems of the
equilibrium moisture content by weight as measured with an auto-vapor-absorptionmeasurement device, as is shown in Figure 3.
10
5
0
0
20
40
60
Relative Humidity (%)
80
100
40
35
30
25
20
15
10
5
0
non-soaked
(absorption)
non-soaked (absorption)
non-soaked
(emission)
non-soaked (emission)
soaked in
soaked
inCaCl2
CaCl2(absorption)
(absorption)
soaked in
soaked
inCaCl2
CaCl2(emission)
(emission)
0
20
40
60
Relative Humidity (%)
80
100
Figure 3: Equilibrium of moisture content in relation to grain weight (by BELSORP18)
In our inveatigation, we tested a CaCl2 solution of 30% density for enhancing the absorbent intensity of diatomite grains. The dehumidifying ventilation device would be
used when relative humidity was 60 to 90%. As shown in Figure 3, soaking diatomite
grains soaked in CaCl2 solution doubled the material’s absorbency.
MOISTURE CONDUCTIVITY OF BALLS FORMED FROM BAKED GRAINS
Figure 1 shows the structure of the cells for the ventilation device, small balls filled
with baked grains. The diameter of grains was about 8 mm, and the diameter of balls
was about 45 mm. The balls were enveloped in nylon nets that do not disturb the
passage of moisture vapor. This idea was that we expected normalized balls to decrease the vent resistance of the cells. Two kinds of balls were used for measuring
moisture conductivity. One was soaked in CaCl2 solution for three days, and the
other was not. After it was soaked, the former was dried at 105 degrees centigrade
for 24 hours. We measured the change in the ball’s weight continuously in a chamber
in which temperature and relative humidity were controlled.
The small balls were formed into a spherical body. We evaluated moisture conductivity with the simultaneous heat and moisture transfer equations given below and a
method of estimating parameters under the hygroscopic zone. Assumed conditions
were (Le)2/3 = 0.817 (Le: Lewis number [-]), number of partitions = 40, and time step
= 0.02 s. The difference in the moisture conductivities for the two kinds of balls was
Physicality values for numerical calculations:
3
Specific gravity [kg/m ]
Void ratio [-]
Specific heat [J/kgK]
Thermal conductivity [W/mK]
2
Heat transfer coefficient [W/m K]
Non-soaked
927
0.371
671
0.375
50.0
Soaked
1028
0.371
677
0.375
50.0
Change of weight (g)
that the soaked balls had three times the moisture conductivity of those not soaked,
as is shown in Figure 4. Therefore, impregnating the balls with CaCl2 solution made
them more effective for moisture conductivity. Gross moisture conductivity (GMC) of
small balls not soaked in CaCl2 was 0.45 *10-4 [kg/m s (kg/kg’)]. The GMC of small
balls soaked in CaCl2 was 1.5 *10-4 [kg/m s (kg/kg’)].
3
2.5
non-soaked grains measurement value
non-soaked grains analytical value
soaked grains measurement value
soaked grains analytical value
2
1.5
1
0.5
0
0
5
10
15
20
25
30
Elapsed time (h)
Figure 4: Analysis results
dV
(P0gair+k)
-rk
-N
dX/dt
=
(cg+rN) dT/dt
R’Sn2(Xn+1-Xn)/Ln2-R’Sn1(Xn-Xn-1)/Ln1
(1)
R Sn2(Tn+1-Tn)/Ln2-R Sn1(Tn-Tn-1)/Ln1
Where,
X:absolute humidity [kg/kg’]
T:temperature [C]
t:time [s]
L:distance between centers in a subject area [m]
R’:gross moisture conductivity [kg/ms(kg/kg’)]
R:thermal conductivity [W/mK]
3
3
k:change of moisture content to X [kg/m (kg/kg’)]
N:change of moisture content to T [kg/m K]
3
3
3
P0:void ratio [m /m ]
gair :specific gravity of air [kg/m ]
r:latent heat of phase change [J/kg]
c:specific heat [J/kgK]
3
2
g:specific gravity of material [kg/m ]
S:surface area that a divided area touches a neighbouring [m ]
VENT RESISTANCE OF CELL
140
120
100
80
60
40
20
0
golf balls
balls filled with beads
analytical value
0
50
Vent Resistance (Pa)
Vent Resistance (Pa)
The form of balls filled with artificial beads (diameter: 8 mm) was like that of a golf
ball. We measured the vent resistances of cells packed with actual golf balls and
those packed with balls filled with the beads. The results matched the theoretical figure (Steinour formula) as shown in Figure 5. Therefore, we could see relationships
between vent resistance and the number of layers of small balls. If we used the produced grains and small balls, vent resistance decreased as shown in Figure 5.
100 150 200
Air Volume (m3/h)
250
300
250
measurement value of primary diatomite
1 layer (analytical value)
2 layers (analytical value)
3 layers (analytical value)
4 layers (analytical value)
5 layers (analytical value)
200
150
100
50
0
0
50
Figure 5: Vent resistance of cells
100 150 200
Air Volume (m3/h)
250
300
EXCHANGE EFFICIENCY OF WATER POTENTIAL
The cells absorbent and emissive characteristics could be more understood by examining their water potential than by examining relative humidity. Therefore, we
0.25
Maximum value appears in
these bounds.
0.2
0.15
0.1
4 layers
3 layers
2 layers
0.05
0
0
0.0001 0.0002 0.0003 0.0004 0.0005
Dimensionless time (-)
Absolute humidity [kg/kg’]
0.3
!%
Before passing
through a cell
(absorbent process)
!$
./0*+123145
Exchange efficiency of
water potential
measured the ventilation device’s exchange efficiency of water potential. Three kinds
of cells were packaged; those with two layers, three layers or four layers. The exchange efficiency of water potential can be expressed in dimensionless time (the
length of time of the air/changeover period for operating dampers) as a maximum
point on a curved line. The aspects of air after passing it through the cells could be
assumed to be those on the psychrometric diagram as shown in Figure 6.
!#
After passing
through a cell
(emissive process)
!"
"&
"'
*+
)
,-
(&
('
Air temperature [deg. centigrade]
Figure 6: Efficiency of water potential exchange, and psychrometric diagram
SIMULATION MODEL, SCHEMATIC DIAGRAM AND NUMERICAL RESULTS
Figure 7 presents the plan and cross section of the simulation model. The dehumidifying ventilation device was set up in a crawl space, because the device is large and
has an underground tube for pre-cooling outdoor air.
Simulation model
As the device is large, it will
be set up in a crawl space
One room model
Figure 7: House plan and cross section (schematic diagram)
Schematic diagram and basic conditions for numerical simulation
Temperature [C],
Relative humidity [%]
& Persons in a house
2
1000
(2)
80
800
(6)
60
(3)
40
600
400
(1)
(4)
(5)
20
(7)
0
200
0
0
3
6
Solar radiation [M/m ]
& Heat gain [W]
Pattern 1
100
9 12 15 18 21 24
(1) Outdoor Temperature
(2) Relative Humidity
Underground tube Device
(3) Normal Solar
Radiation
(4) Horizontal Solar
Electric heater
Radiation
(5) Person Staying in
Occupied Spaces
(6) Heat Gain in
Underground tube Device
Occupied Spaces
(7) Illumination
Heat pump
Solar
collector
Pattern 2
Elapsed time [h]
Figure 8: Boundary conditions and numerical simulation models
Solar
collector
In this research, two patterns were simulated. One had an underground tube to precool outdoor air and heat coils to pre-heat air exhausted from occupied spaces, Pattern 1. The other has a heat pump system for cooling outdoor air and for heating air
exhausted from occupied spaces, especially during the nighttime, Pattern 2. If the
heat pump is used at the same time, the system could obtain air condition below 70%
RH when the occupants of a home are sleeping during the hottest period in summer.
Temperature (C)
28
26
Outdoor air
Switch off the device
Pattern 1
Pattern 2
24
22
20
0:00
4:00
Relative humidity (%)
100
30
Outdoor air
Switch off the device
Pattern1
Pattern2
90
80
70
60
50
0:00
8:00 12:00 16:00 20:00 0:00
Time
4:00
8:00 12:00 16:00 20:00 0:00
Time
Amount of
dehumidification (kg/h)
0.3
Pattern1
Pattern2
0.25
0.2
0.15
0.1
0.05
0
0:00 4:00 8:00 12:00 16:00 20:00 0:00
Time
Energy consumption
[kJ/day]
Figure 9: Room air temperature and relative humidity on a typical summer day
30000
25000
20000
15000
10000
5000
0
Pattern 1 (2.24 kg/day)
Pattern 2 (4.17 kg/day)
1
2
3
1:Total energy, 2:Thermal load of pre-heat,
3:Electric energy of fan
Figure 10: Changes in rate of dehumidification and energy consumption
As this system requires some of heat for its emissive cycle, Pattern 2 was more efficient than Pattern 1. This was because the heat pump could heat and cool at the
same time. If the heat pump was used with a passive system, for example, the underground tube and solar panels, the moisture moved could be increased to 86% and
energy consumption decreased by 30 to 50% because electric heat coils to pre-heat
the exhaust air from occupied spaces would be unnecessary.
CONCLUSION
Of course, we can perfectly control indoor climate when we used a recent heat pump
system. In cold region, however, hot summer season is very short. Therefore, we
could produce better indoor climate by dehumidifying moisture if we combined a convenient device and with the backup of a small heat pump. An environmental comfort
for sleeping can be achieved by reducing humidity by 2.6 kg during the night.
References
1) Ohashi, M., Enai, M., Honma, Y., Hayama, H. and Mori, T. (2003). Development of a Device for Moisture-absorbent Dehumidifying and Heat-exchanging Ventilation by Using the Diatomite Soaked in CaCl2 Solution. Journal of Architectural Planning
and Environmental Engineering, AIJ No. 565, 25-31
2) Enai, M., Ohashi, M., Aratani, N., Hayama, H. and Mori, T. (2003). Development of Ventilation Devices for Moistureabsorbent Dehumidification and Heat Exchange by Using the Diatomite Soaked with CaCl2. AIJ Journal of Technology and
Design, No.17, 243-246