Rotary Desiccant Dehumidifier
by Robert Howe
The dehumidifier used in the hybrid system is a rotary type heat
and mass exchanger containing nominal silica gel as the desiccant. Its
performance is modeled using the non-linear analogy theory proposed by
Banks(1). This method assumes that the mathematical description of the
heat and mass transfer taking place within the rotary dehumidifier is
analogous to that of heat transfer only in a rotary heat exchanger.
Potential lines, denoted F1 and F2, describe the ideal process path
for the air stream. On a graph of Temperature vs. Humidity Ratio, the ideal
process stream outlet state is at the intersection of the process stream F1
line and the regeneration stream F2 line. Jurinak(2) has developed three-
parameter curve fits to the F1 and F2 potentials for nominal silica gel. They
are functions of the air stream temperature(K) and humidity ratio (kg/kg)
given by:
-2865 .8624
F1 = ------- + 4.244 w^
1.490
T^
1.490
T^ .07969
F2 = ------- + 1.127 w^
6360
Two effectivenesses of the F1 and F2 potentials, Ef1 and Ef2, determine
the actual dehumidifier outlet state. Banks(1) defines these effectivenesses
as:
FiD - FiP
Efi = -----------, i = 1,2
FiR - FiP
where D is the actual outlet state. Figure 3.2b illustrates this effectiveness
concept. Note that for the ideal dehumidifier, Ef1=0.0 and Ef2 = 1.0.
THe dehumidifier is modeled as an infinite capacity machine. This
implies that the dehumidifier outlet state D is not affected by the mass
flow rate of the process or regenerating streams (provided, of course, that
reasonable flows are chosen). Unlike the residential desiccant systems, the
hybrid system varies the regeneration temperature to provide only the minimum
amount of latent cooling required, rather than operate at a fixed regeneration
temperature. The advantage of this is that a minimum of auxiliary thermal for
regeneration is used. The disadvantage is that at any time, the regeneration
temperature is not known. The equations for F1, F2 and Efi must be solved
implicitly using Newton's method to determine both the process outlet
temperature at state D and the regeneration temperature. The process outlet
humidity ratio is known from the building controller (Ws) and the regeneration
humidity is the same as the ambient.
REFERENCES
1) Banks, P.J. "Prediction of Heat and Water Vapour Exchanger Performance
from that of a Similar Heat Exchanger," Compact Heat Exchangers--
History, Technological Advancement, and Mechanical Design Problems
(ASME Winter Annual Meeting), HTD-10, 57, Nov. (1980)
2) Jurinak, J.J., "Open Cycle Desiccant Cooling--Component Models and
System Simulations," Ph.D. thesis, University of Wisconsin-Madison
Aug. (1982)
Howe, R.R. "Model and Performance Characteristics of a Commercially-Sized
Hybrid Air Conditioning System which utilizes a Rotary Desiccant
Dehumidifier," Masters Thesis, University of Wisconsin-Madison
Dec. 1983