Environmental Engineering Reference
In-Depth Information
With Equation 5.22 the diffusion coefficient for water in salt solutions (m
2
s
−
1
) can
be described as a function of the film thickness
X
:
X
δ
S
ln
D
(
C
1
)
D
(
C
2
)
D
(
X
)
=
D
(
C
2
)exp
(5.24)
The mass transfer of water vapour from the surface of the liquid desiccant into
the solution film (kg s
−
1
) in direction
X
perpendicular to the desiccant flow direction
can be described as a function of the concentration drop within the solution film and
depends on the heat/mass transfer surface area
A
, the surface wetting factor
and the
concentration gradient:
X
δ
S
ln
D
(
C
1
)
D
(
C
2
)
AM
H
2
O
dC
dX
m
S
=
D
(
C
2
)exp
(5.25)
Following the mass balance, the amount of water vapour transferred from the return
air to the surface of the salt solution film calculated from Equation 5.19 must be equal
to the amount of water vapour transferred from the surface into the solution film
calculated from Equation 5.25:
x
R
m
A
=
m
S
.
Both sides of the equation still depend on the unknown water concentration
C
1
on
the surface of the liquid desiccant film. As a consequence of the numerous depen-
dencies, the water concentration
C
1
can only be calculated iteratively. Lithium chlo-
ride and calcium chloride were employed in the absorber unit as desiccant solutions.
The thermodynamic flow properties of both solutions were calculated from equations
developed and compiled by Conde (2004) and Chaudhari and Patil (2002). The moist
air properties were reported by Gl uck (1991) and Hering
et al
. (1997).
Contact Matrix Absorber Unit (CMAU)
The differential control volume includ-
ing the return air and the desiccant solution film in a typical absorber chamber of the
CMAU is shown in Figure 5.75. Heat and mass balances are developed separately for
the two nodes (ambient air node 1, cellulosematrixwall/desiccant solution node 2). All
other calculations concerning the absorption process are similar to those already men-
tioned above for theHEAUand thus the equation set is not explicitly noted. To solve the
developed equation systems for heat and mass transfer within the two absorber units,
each absorber was divided into a finite number of control volumes in two orthogonal
directions (finite element method), namely the return and ambient air flow directions,
in the case of the HEAU, and the return air and liquid desiccant flow direction, in the
case of the CMAU. The equations for the finite control volumes, each including
the heat exchanger wall and half of the two adjacent flow chambers, were solved
by the Gaussian elimination method, using the simplifications described above.
Search WWH ::
Custom Search