Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved International Review of Mechanical Engineering, Vol. 5, N. 2 Special Issue on Heat Transfer
350 than that of active wheels. Both technologies are used in
non-residential buildings to dehumidify ventilation air. Thus, active wheels dry more deeply, achieving control
of humidity, while passive sorbents-desiccants usually dry more cheaply, helping the cooling system to
moderate the humidity.
II. Sorption Wheels Concept
The sorption wheel or desiccant wheel is a component that removes moisture from the air. In the
conventional cooling systems the dehumidification of the supplied air is achieved by cooling air below its dew
point. A sorption wheel uses a different method. It relies on the ability of hygroscopic materials such as Silica gel
or solid LiCl to adsorb water onto their surfaces. The water from the moist process air adsorbs onto the
surface of the sorption wheel. The sorption material rotates slowly and when saturated it is rotated to another
section where the desiccant material is being regenerated. This is done by the air flow of the regeneration air. More
precise, when the vapor pressure at the sorbent surface is lower than that of the air, the sorbent attracts moisture
while when the surface vapor pressure is higher than that of the surrounding air, the sorbent releases moisture.
Equilibrium is reached when the vapor pressure in the desiccant is equal to that in the air. To allow repeated use
of the desiccant, it has to be regenerated. Regeneration usually is accomplished by heating the desiccant using an
external heat source [1], [2].
A graphical representation of the sorption wheel including the air flows is shown in Fig. 1.
Fig. 1. Airflows of process and regeneration streams in a sorption wheel
State Point 1 represents the ambient conditions, which are with a corresponding dry bulb temperature T
1
and humidity ratio of w
1
. Entering air to the system State point 1 is dehumidified and heated by the sorption
wheel State Point 2 of dry bulb temperature T
2
and humidity ratio w
2
. The elevated temperature of the regeneration air stream with m
regen
mass flow rate, T
3
dry bulb temperature and w
3
humidity ratio removes moisture from the sorbent of the wheel so as to absorb
moisture again through the process air stream of m
proc
mass flow rate.
III. Modeling of Sorption Wheels
In an ideal situation the sorption wheel dries the process air and adds isenthalpically the moisture to the
regeneration air stream. In reality, the sorption and desorption processes are not isenthalpic due to the fact
that moisture, temperature and enthalpy travels in “waves” through the sorption material. Howe and
Jurinak developed a theory to calculate outlet states [3].
The process paths within the sorption wheel can be seen in Fig. 2. The numbers at the intersections indicate
entrance and exit conditions in the dehumidifier.
Fig. 2. Process and regeneration paths according to Jurinak model [3]
In this study, the model of Jurinak has been adopted and compared with measurements, considered also
flexible in the application and reliable with reference to the scope of this analysis. The model, according to the
system in Fig. 1, consists of the following set of equations:
1
12 11
18 11
f
F F
e F
F −
= −
1
2
22 21
28 21
f
F F
e F
F −
= −
2 where F11, F12, F22, F21, F18, F28 are given from the
equations bellow:
0 8624 1
1 49 1
2865 11
4 244
. .
F .
w T
⎛ ⎞
− =
+ ⎜
⎟ ⎜
⎟ ⎝
⎠
3
0 8624 2
1 49 2
2865 12
4 244
. .
F .
w T
⎛ ⎞
− =
+ ⎜
⎟ ⎜
⎟ ⎝
⎠
4
0 8624 8
1 49 8
2865 18
4 244
. .
F .
w T
⎛ ⎞
− =
+ ⎜
⎟ ⎜
⎟ ⎝
⎠
5
Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved International Review of Mechanical Engineering, Vol. 5, N. 2 Special Issue on Heat Transfer
351
1 49 0 07969
1 1
21 1 127
6360
. .
T F
. w
⎛ ⎞
= −
⎜ ⎟
⎜ ⎟
⎝ ⎠
6
1 49 0 07969
2 2
22 1 127
6360
. .
T F
. w
⎛ ⎞
= −
⎜ ⎟
⎜ ⎟
⎝ ⎠
7
1 49 0 07969
8 8
28 1 127
6360
. .
T F
. w
⎛ ⎞
= −
⎜ ⎟
⎜ ⎟
⎝ ⎠
8 Temperature and humidity ratio is known at point 1,
and by using 1, the constant F11 can be calculated and the line connecting 1-2 is fixed. This can also be done for
line 3-4, because point 3 is fixed, and therefore F12 can be calculated. The other two lines, connecting 2-3 F22
and 1-4 F28 can be calculated in the same way.
The actual performance of the wheel shifts from statepoint 2 to statepoint 2
A
. Two effectiveness values ef1 and ef2 are used to calculate the actual outlet state
2. The dashed lines in Fig. 2 can calculated based on new F12, F22 with the actual thermodynamic properties
of air in statepoint 2 and replacing the old values from 4, 7.
An iteration based on Newton Raphson method is used to find intersection of those lines. This intersection
fixes the outlet condition at the process side. The intersection of the two lines indicates the actual outlet
condition at the process side. The outlet conditions at the regeneration side can easily be calculated based on
conservation of mass and energy, 9, 10:
4 2
1 3
reg proc
proc reg
m w
m w
m w
m w
+ =
+
9
1 3
2 4
proc reg
proc reg
m h
m h
m h
m h
+ =
+
10 Required values of [ef1, ef2] = [a, b] are set, which
correspond to efficiency according to the dehumidification mode of the wheel. For enthalpy
exchange mode the above efficiencies are set [ef1, ef2] = [0.9, 0.9] while for active dehumidification mode the set
of efficiencies takes the values [ef1, ef2] = [0.16, 0.73].
The effectiveness values are calibrated using the measurements the desiccant cooling unit powered by 10
m
2
of flat plate solar liquid collectors is implemented for air-conditioning a room in the Park of Energy Awareness
PENA in Lavrio as well as from Klingenburg model [4]. The system uses a Lithium Chloride sorption wheel
and is optimized to work with auxiliary heat regeneration source.
IV. Enthalpy Exchange Wheel
