Profiling the temperatures in the regeneration airstream for a
desiccant bed yields higher thermal efficiency. In staged regeneration,
distinct heating elements are thermostatically controlled to attain
progressively higher temperatures toward the hotter end of the bed.
Stratified heat recovery can be used to transfer heat from process
air from zones on the process side to regeneration airstreams headed
for thermally corresponding zones on the regeneration side of the
desiccant bed. A recirculation system diverts process air from the
hottest process zone or zones directly to the hottest corresponding
zone or zones on the regeneration side.
What is claimed is:
1. In a regeneration system for a desiccant bed having at least
one desiccant bed divided into a process side and a regeneration
side, means for flowing process air through said process side of
said bed, and means for flowing regeneration air through said regeneration
side of said bed, the improvement comprising means for spatially
profiling the temperature of the regeneration air nonuniformly over
the surface area of the regeneration side of said bed, said profiling
means including stationary heat exchanger means for transferring
heat between corresponding zones in the process and regeneration
2. The system of claim 1 wherein said corresponding zones are
parallel, rectilinear strata.
3. In a desiccant-based regeneration system having at least one
desiccant bed having a process side which includes a first hottest
sector and a regeneration side which utilizes a last hottest sector,
means for flowing process air through the process side of said bed,
and means for flowing regeneration air through the regeneration
side of said bed, the improvement comprising
means for diverting the process air flowing out of said first hottest
sector of the process side of said desiccant directly to said last
hottest sector of the regeneration side of said bed as regeneration
means for heating said diverted process air during its passage
from said process side to said regeneration side.
4. The system of claim 3 wherein said diverting means includes
means for diverting process air from a plurality of zones of said
process side to corresponding zones of said regeneration side to
provide regeneration air.
5. The system of claim 4 wherein said process side zones are contiguous
zones of declining temperature, respectively, and said regeneration
side zones are corresponding contiguous zones of declining temperature,
6. The system of claim 3 further including means for spatially
profiling the temperature of at least a portion of said regeneration
7. In a regeneration system for a desiccant bed having desiccant
wheel means for providing a desiccant bed, a partition for dividing
said bed into a process side and a regeneration side, means for
imparting relative rotation between said wheel means and said partition,
means for flowing process air through said process side of said
bed to produce a process outlet stream, the improvement comprising:
stationary heat exchanger means having a process side inlet for
receiving said process outlet stream and a regeneration side outlet,
means for flowing regeneration air to the regeneration side of
said bed via said regeneration side of said stratified heat exchanger
said heat exchanger means including strata means for associating
regions of said process side with thermally corresponding regions
on said regeneration side of said bed, and
regeneration heating means interposed between said regeneration
side outlet of said heat exchanger means and the regeneration side
of said bed for heating the regeneration airstream.
8. The system of claim 7 wherein said stratified heat exchanger
means includes means defining a plurality of corresponding parallel
rectilinear strata on process and regeneration sides.
9. The system of claim 7 wherein said stratified heat exchanger
means includes a plurality of stationary heat pipes bent in a radial
configuration conforming approximately to corresponding isotherms
of the regeneration and process sides of said bed.
CROSS-REFERENCE TO RELATED APPLICATION
The subject matter of the present application is related to that
of the abandoned application entitled "Nonhomogeneous Desiccant
Bed for Dehumidifying or Cooling System", Ser. No. 837850
filed Mar. 16 1986 by Cohen et al, assigned to the assignee of
the present application and incorporated herein by reference.
BACKGROUND OF THE INVENTION
The invention relates generally to environmental cooling systems
based on regenerative desiccant dehumidifiers and more particularly
to the means by which the desiccant is regenerated or restored.
Desiccant based cooling systems have been known for some time to
have potential as heat-actuated space-cooling devices. Desiccant
systems operate entirely on heat and mass transfer processes. An
open cycle desiccant cooling system is essentially a hybrid of two
fundamental air processors: a desiccant dehumidifier and an adiabatic
evaporative cooler. In the simplest embodiment, outdoor ambient
air passes through a drying wheel of hygroscopic material which
absorbs or adsorbs moisture from the air, accompanied by heating
of the air. The dried warmed air then flows through a heat exchanger,
usually a second rotating wheel, where it is cooled by the transfer
of sensible heat. The dried cooled air is then further cooled and
reconstituted to a desired humidity by passing it through an evaporative
humidifier. In the desiccant wheel, the dehumidification process
converts the latent heat of water vapor to sensible heat by means
of absorption or adsorption. Thus, the desiccant wheel is sometimes
referred to as the L-wheel while the heat exchanger, if rotational,
is referred to as the S-wheel, alluding to the transfer of sensible
There are numerous ways to configure the layout of these various
components, namely, the L-wheel, S-wheel and evaporators, in order
to modify or improve performance characteristics. In all of the
systems, however, one of the main features is the regeneration of
the desiccant. Typically the L-wheel is divided into a process side
and a regeneration side by means of diametrical and circumferential
seals. Heated air is blown through the regeneration side to dry
out the desiccant so that when it rotates into the process side,
it is available again for sorption of water from ambient air.
Open cycle desiccant cooling systems based on natural adiabatic
and heat transfer processes offer a set of characteristics which
make them potentially more cost effective than vapor compression
electric air conditioning systems in certain applications. For example,
larger roof top installations represent an excellent application,
particularly where ventilation of the building is also a requirement.
Here, the lower energy consumption and lack of high pressure coolant
lines and seals make desiccant cooling systems attractive, especially
where added heat for regenerating the desiccant is provided by gas
burners, gas costing much less than electricity in the summer.
As with any energy related product, however, one of the focal points
for cost effectiveness is high performance component design. From
a system standpoint the desiccant regeneration system is quite important
in terms of increasing the overall efficiency or so called coefficient
of performance ("COP"), (namely, the cooling capacity
divided by the thermal energy input of the cooling system) without
sacrificing the specific cooling capacity ("SCC") (BTU/lb.sub.da).
The rotating L wheel, divided in two by sliding diametrical stationary
seals, exhibits a static angular temperature profile or gradient
which is approximately symmetrical with respect to the plane of
the partition between the process and regeneration sides. At any
given moment of operation after equilibrium is reached, the wheel
is hottest where it leaves the regeneration side and enters the
process side and declines in temperature to the coolest region where
it leaves the process side and enters the regeneration side. In
general, in prior art systems, air on the regeneration side leaves
the heat exchanger (S wheel) in a uniform, mixed stream and is relatively
uniformly heated by the regeneration heater before passing through
the desiccant wheel. The temperature of the regeneration air immediately
prior to entering the desiccant wheel is approximately the same
throughout the semi-circular area.
SUMMARY OF THE INVENTION
Accordingly, the general purpose of the invention is to improve
the coefficient of performance (COP) without sacrificing the specific
cooling capacity (SCC) in a desiccant-based cooling system by optimizing
the regeneration system. Another object of the invention is to enhance
the usage of recovered heat by optimizing the heat exchanger design.
These and other objects of the invention are achieved by tailoring
the regeneration temperature profile at the entrance to the regeneration
side of the desiccant wheel. A spatial distribution of temperatures
in either the heater or the heat exchanger or both is matched to
the pre-existing spatial temperature distribution across the surface
on the regeneration side of the desiccant wheel. Two distinct methods
of marrying the spatial temperature distribution in the regeneration
stream with the temperature distribution in the wheel are: (1) staged
or profiled regenerative heating and (2) stratified heat recovery.
In staged regeneration, the heater is arranged to produce a temperature
gradient on the regeneration side in either discrete steps or a
continuously uniformly or nonuniformly varying temperature profile.
In the discrete system, for example, unheated air from the heat
exchanger may be passed directly to the first sector on the regeneration
side, moderately heated air is passed to an intermediate sector
and the most heated air is passed to the last sector before the
In stratified heat recovery, the rotary S wheel of the prior art
is replaced with a stationary heat exchanger with radial or rectilinear
strata which associate the temperature zones of the process air
out of the desiccant wheel with the corresponding temperature zones
of the regeneration side. Thus, for example, the hottest process
air is used to heat regeneration air headed for the hottest zone
of the regeneration side of the L wheel. Both direct and indirect
heat exchanger designs are disclosed, along with a novel method
of manufacturing the direct stratified heat exchanger by folding
flat stock accordion-fashion in an overall rectilinear shape, abutting
aligned partitions to opposite ends and sealing alternate end faces
of the resulting flat channels.
A further refinement can be achieved by a recirculation system
which shunts the hottest process air from the first process zone
or zones directly to the corresponding final zone or zones of the
regeneration side after having elevated its temperature by heating.
In one embodiment, multiple contiguous sectors on process and regeneration
sides are coupled with different corresponding heat inputs.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a functional, schematic, perspective view representing
a typical open cycle desiccant air conditioning system.
FIG. 2 is a schematic plan diagram of the layout of the components
of the system of FIG. 1.
FIG. 3 is a schematic diagram illustrating staged regeneration
in discrete steps.
FIG. 3A is a graph of temperature versus angle illustrating the
different regeneration temperatures.
FIG. 4 is a diagram illustrating staged regeneration with continuously
FIG. 5 is a graph of temperature versus angle illustrating the
temperature variation in the design of FIG. 4.
FIG. 6 is a diagrammatic plan view of a heating coil system for
achieving nonuniform temperature distribution in the regeneration
FIG. 7 is a diagrammatic plan view of another embodiment of the
continuously varying profiled temperature regeneration heater having
a uniform distribution.
FIG. 8 is a diagrammatic plan view of another embodiment of the
continuously variable profiled temperature regeneration heater having
a circumferentially, nonuniformly distributed heat exchanger.
FIG. 9 is a schematic diagram illustrating the concept of stratified
heat recovery for profiling the regeneration temperatures.
FIG. 9A is a graph of temperature versus angle illustrating the
temperature profile yielded by the design of FIG. 9.
FIG. 10 is a schematic perspective view of a desiccant based cooling
system having a stratified heat exchanger according to the invention.
FIG. 10A is a graph of temperature versus angle illustrating temperature
profiles at the indicated points with respect to the system of FIG.
FIGS. 11A-F are schematic perspective views illustrating four consecutive
steps, respectively, in the construction of a preferred form of
stratified heat exchanger.
FIG. 12 is a schematic plan view of an indirect heat exchanger
composed of heat pipes bent in a radial design to associate corresponding
temperature zones according to the invention.
FIGS. 13A and 13B are graphs of typical temperature and humidity
versus angle at the proces outlet.
FIGS. 14 and 15 are schematic perspective views of desiccant wheels
equipped, respectively, with single sector and double sector shunts
for recirculation regeneration between the process and regeneration
sides according to the invention.
FIGS. 14A and 15A are graphs of temperatures versus angle in the
corresponding embodiments of FIGS. 14 and 15 respectively.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following description relates to three conceptually different
designs for profiling the regeneration temperatures at the inlet
to the regeneration side of the desiccant wheel. Desiccant wheels
are useful by themselves as dehumidifiers and separators, or as
components of a desiccant based cooling system. As the following
embodiments are directed specifically to cooling systems, an understanding
of the typical desiccant air conditioning system is helpful.
Numerous component configurations have been proposed for recirculating
and ventilation mode systems in the past. Current investigation
has focussed on ventilation mode systems of the type shown in FIGS.
1 and 2. Operating with humid outside air, these systems compare
favorably to electric vapor compression systems where interior ventilation
is also required so that credit is given for cooling from ambient
A desiccant wheel 10 is mounted for rotation on axis a. A stationary
partition 12 (shown as vertical in FIG. 1), running from one end
of the system to the other with diametrical and circumferential
seals where necessary, divides the exposed face of the desiccant
wheel 10 into two semi-circular areas. The right-hand area 10a of
the wheel 10 as viewed in FIG. 1 defines the entrance to the so-called
process side of the system. The left-hand side 10b of the face of
the desiccant wheel as viewed in FIG. 1 defines the exit point of
the so-called regeneration side of the system. A heat exchanger
or S-wheel 14 is coaxially spaced from the desiccant wheel, the
sliding seal vertical partition 12 again defining corresponding
process and regeneration sides 14a and 14b of the heat exchanger
14. Typically the S-wheel and L-wheel rotate in opposite directions.
On the other side of the heat exchanger 14 air is forced through
a humidifer 16 having water-laden evaporator media 16a and 16b lying
respectively in the process and regeneration sides of the system.
A heater 18 lies in the regeneration side between the heat exchanger
14 and the desiccant wheel 10 as shown in FIG. 2. Electric motor
driven low velocity fans or blowers (not shown) are typically utilized
to cause air to move through the desiccant wheel to the humidifier
on the process side and in the opposite direction from the humidifier
to the desiccant wheel on the regeneration side as indicated by
the arrows in FIGS. 1 and 2.
In operation, hot moist outdoor air is drawn into the exposed process
side face 10a of the desiccant wheel 10 at point No. 1. Passing
through the desiccant wheel, the air is dehumidified by condensation
accomplished by adsorption of the water vapor in microporous materials
such as silica gel or molecular sieves or by absorption of the water
vapor forming inorganic salt hydrates or salt solutions, using chemicals
such as lithium chloride. As disclosed in the above-referenced copending
application, nonhomogeneous desiccants can also be employed.
Besides drying the air, condensation also heats the air by releasing
the latent heat of the water vapor as sensible heat. At point 2
the dry air, now hotter than the ambient air, encounters the heat
exchanger 14 a heat capacity structure of aluminum, for example
which transfers a portion of the heat in the air at point 2 to the
regeneration side of the system. Leaving the heat exchanger 14 at
a lower temperature and still dry, the air next encounters the evaporator
16a at point 3. Taking on water vapor absorbs heat from the air
stream thus resulting in cooler humidified air at point 4 entering
the conditioned space or room to be cooled.
On the regeneration side, return air from the room (point 5) is
drawn through evaporator 16b where it is cooled and further humidified
before being passed at point 6 to the heat exchanger 14. Heat released
by dehumidification on the process side is transferred to the air
stream on the regeneration side so that return air exiting the heat
exchanger 14 at point 7 is warmer than the return air at point 6.
However, the air at point 7 is not yet hot enough to drive the moisture
out of the desiccant wheel 10. Accordingly, thermal energy is added
by heater 18. The hot relatively dry air at point 8 passes through
the regeneration side 10b of the desiccant wheel 10 dries out the
desiccant by absorbing water, and exits at point 9 as warm humid
air which is exhausted to the atmosphere.
The desiccant wheel 10 is a moisture exchanger whose sole function
is to dry the ambient air. The above-referenced copending application
describes a nonhomogeneous desiccant bed design in which a rotary
L wheel is divided into two or more axial layers with corresponding
desiccants or mixtures of desiccants arranged in order of decreasing
optimal operating range of humidity and increasing optimal operating
temperature range in the direction of process air flow. The present
application is compatible with the nonhomogeneous desiccant bed
The specific point of departure of the present application is in
the regeneration side upstream of the desiccant wheel 10 and involves
either the regeneration heater 18 or heat exchanger 14 (or both)
in an attempt to match the temperature profile of the regeneration
airstream at points 7 and/or 8 with the spatial distribution of
temperatures in the regeneration half of the wheel 10b as shown
in FIG. 2. Two conceptually different approaches called staged regeneration
and stratified heat recovery are shown in FIGS. 1-8 and 9-12 respectively.
The recirculation system shown in FIGS. 14 and 15 illustrates another
form of stratified heat recovery without using a heat exchanger.
Recirculation regeneration is also compatible with staged regeneration.
Constant regeneration temperatures with low heat capacity desiccant
beds can easily supply too much regeneration energy. The passage
of heated air through a cool substance medium creates a wave-like
effect wherein the medium is heated more deeply with time. The thermal
wave or regeneration temperature wave can propagate to almost complete
breakthrough of the bed. The result is an average outlet air temperature
during regeneration significantly higher than desired for high efficiency.
The thermal wave can and should be delayed or retarded by gradually
increasing regeneration temperature with time (staging), rather
than holding it constant over the entire regeneration period. This
strategy shows important increases in the COP with little accompanying
reduction in SCC.
The following embodiments show different ways of raising COP by
profiling the temperature of the regeneration air stream. In all
of them the objective is to slow down the regeneration wave so that
it does not break through the bed. The lower the heat capacity of
the wheel, the steeper the waves will be and therefore the easier
it will be to confine them within the bed by the use of nonuniform
As shown in FIG. 3 the face of the desiccant wheel facing the
regeneration heater and heat exchanger is partitioned into two semicircles,
the regeneration side being designated 0.degree. to 180.degree.
and the process side being designated 180.degree. to 360.degree..
Rotating counterclockwise as viewed in FIG. 3 (in the same direction
as increasing angle), the hot end of the wheel will be at 180.degree.
and the cool end of the wheel will be at 0.degree.. FIGS. 3 and
3A show a stepwise increase in the regeneration stream temperature
from 0.degree. to 180.degree.. The first stage of heating is accomplished
by using only heat exchanger effluent which is at a temperature
higher than the bed initially due to the averaging of the temperatures
out of the process side in the heat exchanger 14 (FIG. 2). Subsequently,
air is heated to progressively higher temperatures in stages by
heating elements A and B as shown in FIG. 3. If T.sub.0 is the outlet
temperature of the heat exchanger, the first heat level T.sub.1
will be approximately equal to T.sub.0 as indicated in FIGS. 3 and
3A for the first sector in the regeneration side. The middle sector
in the regeneration side receives an air stream heated to temperature
T.sub.2 by heating element B. The final sector of the regeneration
side receives a stream heated to temperature T.sub.3 by heating
element A. The temperatures have the relationship T.sub.3 >T.sub.2
A hydronic heater with two coils may be useful in providing the
heating elements A and B of FIG. 3. Alternatively, heat pipe heating
systems may be used and may be more economical than hydronic heating
systems. Heat pipe heating systems are recommended for markets that
do not require a yearly cycle of heating since the heat pipe system
does not readily permit the desiccant cooling system to perform
in a space heating mode.
FIGS. 4 and 5 illustrate the alternate embodiment of staged regeneration
in which the temperature nonuniformity is established in a continuous
fashion using a heat exchanger designated C carrying a moving fluid
which has a low enough total heat capacity that it varies its temperature
with angular displacement as heat is extracted thus varying the
temperature to which the varying points in the regeneration air
stream are heated. As in the discrete staged regenerator of FIG.
3 the first sector receives effluent from the heat exchanger 14
directly. The term T.sub.f,in represents the temperature of the
heating fluid entering the heat exchanger C while T.sub.f,out represents
the temperature of the cooled fluid at the outlet of the heat exchanger
The fan-shaped heating structures of FIGS. 6 7 and 8 represent
alternate means of achieving continuously variable temperatures
in the regeneration stream. As shown in FIG. 6 a heated fluid is
carried through a serpentine, fan-shaped, heat exchanger tube 36
which is intersected by equally spaced radial baffles 38. In FIG.
7 instead of a single serpentine fluid path, a plurality of approximately
concentric semicircular tubes 40 are connected between aligned inlet
and outlet manifolds 42 and 44 connected to a hydronic boiler system,
for example. The concentric heat exchanger tubes 40 intersect equally
spaced radial baffles 38 as in FIG. 6.
The systems of FIGS. 6 and 7 depend on fluid temperature variation
to control the temperature to which the air stream is heated. The
system of FIG. 8 however, has the same baffles 38 and concentric
fluid paths 40 but uses a nonuniform distribution of heat exchanger
elements such as fins 50 to nonuniformly transfer heat to the air
stream. In the embodiment of FIG. 8 with nonuniform heat exchangers
distributed over the 0.degree. to 180.degree. arc, the surface area
of the heat exchanger in each sector governs the amount of heat
transferred to the stream and thus the temperature of the stream
in a given sector. In FIG. 8 ideally the fluid velocity through
the lines 40 is high so that the temperature drop from inlet to
outlet manifold 42 and 44 is not as influential as the heat transfer
surface area variation as a function of angle. In contrast, the
fluid temperature controlled embodiments of FIGS. 6 and 7 may require
significantly lower flow rates to achieve the desired temperature
gradient. Other patterns and sizes of heat exchanger components
for the embodiments of FIGS. 6 7 and 8 can be tailored to attain
desired temperature profiles.
FIGS. 9-12 illustrate stratified heat recovery. This system seeks
to establish a nonuniform regeneration stream before encountering
the heater 18. It is typical for the process air to exit from the
desiccant wheel 10 with a nonuniform outlet temperature which is
hot initially and progressively cooler as rotation angle increases
from 180.degree. to 360.degree.. On the other hand, it has been
shown that the regeneration side, which sees the coolest desiccant
first, would prefer to use lower temperature heating first, increasing
the temperature as a function of angular displacement from 0.degree.
to 180.degree.. FIG. 9 illustrates the techniques of correlating,
by heat exchange, points on the process side with points on the
regeneration side. That is, strata are created by baffles or other
means which provide section to section heat exchange rather than
having the process stream transfer heat to the regeneration stream
in bulk, the mixing being inherently thermodynamically inefficient.
Thus, as shown in FIG. 9 stream l.sub.P from the first sector of
the process side, a hot stratum, transfers heat with sector l.sub.R,
a point on the regeneration side preferring hot air. This stratified
heat exchange continues around the wheel. For each stratum, a small
amount of heat is added (Q in FIG. 9) since regeneration must occur
at a temperature slightly higher than process.
FIG. 9A shows the resulting temperature profile which exhibits
a staircase pattern of graduated temperatures as desired.
The effect of the stratified heat recovery system is to return
temperatures in a shape similar to the mirror image of the natural
process outlet profile, thereby first heating the bed with air only
slightly higher than the minimum process outlet temperature. By
reflecting the outlet temperature profile on the process side to
the regeneration side, thereby preserving both high and low ranges
of temperature, with the appropriate cycle time, a thermodynamically
superior situation results. The goal of optimization is achieved
in that low process humidities are generated while rejecting a minimum
amount of heat to the ambient. Properly scheduled regeneration retards
breakthrough by allowing only an amount of heat to be added to the
bed sufficient to cause regeneration of the desiccant.
A stratified heat exchanger 14' suitable for stratified heat recovery
is shown in FIG. 10 and a specific embodiment is shown in FIG. 11D.
In this design, ductwork (not shown) would take the radial geometry
at the process outlet into a rectilinear air-to-air channel-type
heat exchanger whereby no crossover between channels would be permitted,
thereby retaining stratification. Essentially, each stratum of air
goes through its own individual heat exchanger as shown schematically
in FIG. 9.
A plate core heat exchanger accomplishing stratified heat recovery
is shown in FIG. 11D. FIGS. 11A through C show the four-step construction
process. In the first step a flat, elongated sheet stock 60 of construction
material, preferably aluminum, is cut to the desired length and
width. In step 2 as shown in FIG. 11B, the flat stock 60 is repeatedly
folded in an accordion fashion to form alternate plates of side
A and side B. In FIG. 11C the pleated structure 62 is placed inside
a hollow, rectangular duct or box 64 so that a series of parallel,
flat channels are defined. Next, sealing partitions 66 and 68 are
aligned with opposite ends of the open channel work. Partitions
66 and 68 are part of the partition system 12 shown in FIG. 10.
Note that the partitions 66 and 68 do not extend into the box 64
but form static seals along opposite open end faces of the box.
The end faces of alternate channels are sealed as shown in FIG.
11C so that there can be no communication between the regeneration
and process sides other than heat transfer through the flat stock
60. Addition of an internal, corrugated (or pleated) structure 70
(FIG. 11F) to the core makes additional heat transfer area available.
This internal structure also provides the needed structural support
to prevent the walls of the plates from collapsing under positive
and negative blower pressure. The result is a compact, inexpensive,
pure counterflow, direct-type, stationary static seal heat exchanger
with high calculated effectiveness.
As shown in FIG. 12 closed heat pipes 74 can be used to transfer
heat indirectly between strata. One way of accomplishing this is
to bend the heat pipes to form approximately radial pipe sections
as shown. The diameter of each pipe or the size of its fins 75 may
increase with distance from the seal 12 to help keep the temperature
of each zone uniform with distance from the seal in each sector.
The result is a spatial correlation of corresponding temperature
zones on the process and regeneration sides of the desiccant wheel.
Another system (not shown) is to duct the strata from the process
outlet of the desiccant wheel through a transition from a radial
to a rectilinear heat pipe arrangement. A nearly constant heat rate
could be applied while maintaining the same mirror image stratification.
A fixed heat exchanger such as that shown in FIGS. 10 11C and
12 overcomes the seal problem with rotary heat exchangers. The use
of heat pipes in particular has an advantage over static heat exchanger
structures like that shown in FIG. 11C since heat pipes provide
good conductivity of heat over extended distances with good thermal
contact and transfer properties due to mass transfer by boiling
and condensation in the heat pipes. A subset of this concept is
to use multiple arrays in the flow direction (not shown) to increase
As shown in FIGS. 13A and l3B the process outlet temperature and
humidity profiles from the desiccant wheel, i.e., the thermodynamic
states of the air with respect to the displacement angle, are nonuniform.
Air which is best for cooling or dehumidification is produced in
the middle zone ("MZ"). From 180.degree. to the angle
.theta..sub.1 i.e., the first process sector, the air is typically
hot and humid. The sector of the wheel that has just exited the
regeneration stream is at a high temperature and has a high equilibrium
vapor pressure since it still has not cooled sufficiently to allow
adsorption. The air coming out of this sector will therefore be
hot and wet, thus reducing the average specific capacity of the
air and the overall capacity of the unit. This air can be rejected.
It is useless as a product of the cycle either in a cooling device
or in a dehumidifier. It does, however, contain a significant energy
availability which could be used to regenerate the desiccant, a
process requiring hot air. If any of its useful energy content can
be recovered, it reduces the regeneration energy input while increasing
the cooling capacity of the unit.
As shown in FIG. 14 a single recirculation loop can be used to
shunt process air from the first, hottest process sector to the
last, hottest sector of the regeneration side. A small boost in
temperature level (Q) may be all that is needed. A further refinement
uses multiple recirculation loops to couple a plurality of corresponding
sectors in the hotter zones of the desiccant wheel. Slightly higher
heat Q.sub.1 is added to the top adjacent sectors and relatively
lower heat Q.sub.2 is added to the next adjacent corresponding sectors
as shown in FIG. 15. The result is stratified recirculation wherein
air of different temperatures is provided in different sectors of
the regeneration side of the desiccant wheel. Adjusting the cycle
time or bed heat capacity can change the outlet profile to make
it more favorable for this technique. Use of this recirculation
system is seen to benefit designs with large heat capacities more
than designs with small heat capacities. This is because the thermal
wave is steeper for the latter designs, resulting in a shorter,
useful recirculation period. If the recirculation period is shorter,
the effect of recirculation on the performance of the wheel will,
of course, be lower.
As an additional possibility the main regeneration stream temperature
T.sub.2 in FIG. 14 can be raised to T.sub.2 ' to assist in low temperature
reqeneration. T.sub.2 ' can also be a function of angle as in stated
regeneration shown in FIGS. 3 and 5.
The benefits of the recirculation regeneration system are manifested
throughout the system. Since the recirculated flow does not pass
through the entire system, the volume of air moving through the
entire system is reduced, resulting in reduction of the size of
all components including, for example, lower fan power. In a standard
ventilation cycle desiccant cooling system like that shown in FIGS.
1 2 and 10 the heat exchanger operates typically with laminar
flow and requires a certain effectiveness. It is sized to handle
the full air flow even though some of the air is useless or may
even have a negative effect. Thus the heat exchanger 14 and humidifiers
16 may be reduced in size to handle only the net useful flow if
the hot sectors are short circuited to the regeneration side.
Recovery of the highest temperature heat available in the cycle
is beneficial to the COP and the stratified or multiple recirculation
design is akin to stratified heat recovery as shown in FIGS. 9-12
but is more efficient, suffering no heat exchange loss. The hottest
process air, although humid compared to other process air, is dry
or at best equivalent when compared to the regeneration air which
is humidified room air. Thus, the recirculation streams can do an
effective job of regeneration if they are only slightly heated.
The recirculation regeneration system is also compatible with either
staged regeneration or stratified heat recovery heat exchangers.
The advantages of the invention, many of which have already been
pointed out in the foregoing description, are numerous. By tailoring
the temperature profile of the regeneration air stream, the invention
controls the thermal wave through the desiccant so that a minimum
amount of heat is rejected to the ambient. The symmetry in the temperature
profile matches hot with hot and cool with cool for optimum heat
transfer over a relatively small uniform difference in temperature,
unlike prior systems which expose hot and cool sectors alike to
the same temperature regeneration air stream. Because the regeneration
profiling system transfers heat more efficiently, it lessens the
dependency of the system on the design of the heat exchanger, while
at the same time retaining a low mass desiccant wheel. Thus, smaller
wheels can be used with less pressure drop, lowering the blower
The foregoing description of preferred embodiments is intended
to be illustrative and not restrictive. Many variations are possible
without departing from the spirit and scope of the invention. For
example, nonhomogeneous beds composed of layered distinct desiccants
as in the copending application can yield improved COP. Staged regeneration
and stratified heat recovery are compatible with high performance
multi-desiccant beds. The stratified heat exchanger design offers
the advantage of recovering heat while preserving an image of the
temperature profile on the process side which can be amplified in
its entirety by uniform or staged heating to feed back a compatible
profile of temperatures to the regeneration side.
A further option exists to close the loop on the regeneration temperature
profile by using temperature sensors to monitor the outlet temperature
distribution on the regeneration side of the wheel and then use
this data to control the set points of a multistage regeneration
heater to optimize the parallel profile of the regeneration air
stream temperatures. The opportunity exists, on a dynamic basis,
to control regeneration temperature in response to a changing load
and/or changing ambient conditions. This would provide more efficient
operation on long-term basis, e.g., over a period having seasonal
While the embodiments have been described in reference to rotary
desiccant wheels, the invention is also applicable in an analogous
manner to reciprocating and traveling belt desiccant designs. In
a reciprocating pair of desiccant beds, the periodicity corresponds
to time rather than angles of displacement, thus the temperature
profile would be staged time-wise rather than spatially distributed.
The scope of the invention is indicated by the appended claims
and equivalents thereto.