Abstrict A method and apparatus for conditioning air for an enclosure is
disclosed in which a supply air stream, preferably from the atmosphere
is cooled by the cooling coil of a refrigerant cooling system to
reduce the temperature and humidity thereof to first predetermined
level. The thus cooled and dehumidified air is then passed through
a segment of a rotating desiccant wheel under conditions which reduce
moisture content and increase temperature to a second predetermined
temperature range. The supply air is then delivered from the desiccant
wheel to the enclosure. The desiccant wheel is regenerated by heating
a separate regeneration air stream, also preferably from the atmosphere,
using the condensing coil of the refrigerant system in order to
increase the regeneration air stream temperature to a third predetermined
temperature range. The thus heated regeneration air stream is then
passed through another segment of the rotating desiccant wheel to
regenerate the wheel.
Claims What is claimed is:
1. A method for conditioning ambient air for supply to an enclosure
comprising the steps of cooling an ambient supply air stream having
a temperature range of between 65.degree. F.-95.degree. and above
and a moisture content of between 90-180 gr/lb with a refrigerant
system cooling coil to reduce the moisture content and temperature
thereof to a first predetermined moisture content saturation level
and saturation temperature range, passing the thus cooled and dried
ambient supply air stream through a segment of a rotating desiccant
wheel under conditions which increase its temperature to a second
predetermined temperature range of about 73.degree. F.-78.degree.
F. and reduces its moisture content further to a predetermined humidity
level of between 55-71 gr/lb; and then delivering the thus treated
air to said enclosure; and regenerating the desiccant wheel by heating
an ambient regeneration air stream whose temperature and moisture
content are substantially the same as that of the ambient supply
air stream with the condensing coil of the refrigerant system to
increase its temperature to a third predetermined temperature range
of 105.degree. F.-135.degree. F. while decreasing its relative humidity
and then passing the heated regeneration air stream through another
segment of the rotating desiccant wheel to regenerate the desiccant
in the wheel.
2. The method of claim 1 including the step of exhausting the
regeneration air stream leaving the desiccant wheel to the atmosphere.
3. The method as defined in claim 1 wherein said step of cooling
the supply air stream to said first predetermined temperature range
comprises the step of cooling the air supply stream to a temperature
range of between 45.degree. and 68.degree. F.
4. The method as defined in claim 3 wherein said step of cooling
the supply air stream to said first predetermined temperature range
comprises the step of cooling the air supply stream to a temperature
of between 60.degree. and 65.degree. F.
5. The method as defined in claim 1 wherein said second predetermined
temperature range is between 73.degree. to 78.degree. F. and said
predetermined humidity level is between 55 and 71 gr/lb.
6. The method as defined in claim 3 wherein said second predetermined
temperature range is between 73.degree. to 78.degree. F. and said
predetermined humidity level is between 55 and 71 gr/lb.
7. The method for conditioning air as defined in claim 1 wherein
said step of heating the regeneration air stream to said third predetermined
temperature range comprises the step of heating the regeneration
air stream to a temperature range of 105.degree. to 135.degree.
F.
8. The method for conditioning air as defined in claim 3 wherein
said step of heating the regeneration air stream to said third predetermined
temperature range comprises the step of heating the regeneration
air stream to a temperature range of 105.degree. to 135.degree.
F.
9. The method for conditioning air as defined in claim 5 wherein
said step of heating the regeneration air stream to said third predetermined
temperature range comprises the step of heating the regeneration
air stream to a temperature range of 105.degree. to 135.degree.
F.
10. The method as defined in claim 1 wherein said step of heating
the regeneration air stream to said third predetermined temperature
range includes the step of selectively drawing a portion of the
regeneration air stream over said condensing coil for the refrigeration
system and exhausting said portion of the regeneration air stream
to the atmosphere upstream of the desiccant wheel whereby only a
portion of the regeneration air stream is passed through the desiccant
wheel.
11. The method as defined in claim 1 wherein said step of selectively
drawing a portion of the regeneration air stream over the condensing
coil is performed when the condensing head pressure exceeds a predetermined
pressure limit.
12. The method as defined in claim 11 wherein said predetermined
pressure limit is between 250 and 350 psi.
13. The method as defined in claim 1 including selectively varying
the rotational speed of the desiccant wheel to vary dehumidification
capacity.
14. The method as defined in claim 1 including the step of varying
dehumidification capacity by passing a portion of the supply air
stream from said cooling coil around the desiccant wheel directly
to the enclosure.
15. The method as defined is claim 1 including the step of selectively
deactivating the refrigerant system and supplying return air to
the regeneration segment of the desiccant wheel whereby the desiccant
wheel selectively serves as an energy recovery ventilator.
16. The method as defined in claim 1 including the step of rotating
the desiccant wheel at a rate of 6-20 rph.
17. The method as defined in claim 15 including the step of rotating
the desiccant wheel at a rate of 10-30 rpm when the refrigerant
system is deactivated.
18. The method as defined in claim 1 including the step of drawing
a first portion of the regeneration air stream over said condensing
coil and exhausting it to the atmosphere upstream of the desiccant
wheel whereby only a second portion of the regeneration air stream
is passed through the desiccant wheel.
19. The method as defined in claim 18 wherein said condensing
coil includes a first section for heating regeneration air and a
second section for increasing cooling of the coil; and said step
of drawing a first portion of the regeneration air stream over said
condensing coil comprises the step of drawing said first portion
only over the second section of the condensing coil.
20. The method as defined in claim 18 drawing a first portion
of the regeneration air stream over the second section of the condensing
coil is performed when the condensing head pressure in the condensing
coil exceeds a predetermined pressure limit.
21. The method as defined in claim 19 wherein said predetermined
pressure limit is between 250 and 35 psi.
22. An apparatus as defined in claim 1 wherein said desiccant
wheel includes means for preventing odors from being released in
the enclosure.
23. The method as defined in claim 10 wherein the step of selectively
drawing a portion of the regeneration air stream over said condensing
coil is performed to control a predetermined head pressure of refrigerant
temperature in the refrigerant system.
24. The method as defined in claim 10 wherein said step of selectively
drawing a portion of the regeneration air stream over the condensing
coil includes the step of varying the volume of said portion of
the regeneration air stream to maintain a predetermined head pressure
or condenser leaving air temperature.
25. The method as defined in claim 1 including the step of precooling
the regeneration air stream before passing it over the condenser
coil when the temperature of air leaving the condenser coil is above
a predetermined temperature or when the compressor head pressure
in the refrigerant system is above a predetermined pressure.
Description BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates to air conditioning and dehumidification
equipment, and more particularly to an air conditioning method and
apparatus using desiccant wheel technology.
It is well known that traditional air conditioning designs are
not well adapted to handle both the moisture load and the temperature
loads of a building space. Typically, the major source of moisture
load in a building space comes from the need to supply external
make-up air to the space since that air usually has a higher moisture
content than required in the building. In conventional air conditioning
systems, the cooling capacity of the air conditioning unit therefore
is sized to accommodate the latent (humidity) and sensible (temperature)
conditions at peak temperature design conditions. When adequate
cooling demands exists, appropriate dehumidification capacity is
achieved. However, the humidity load on an enclosed space does not
vary directly with the temperature load. That is, during morning
and night times, the absolute humidity outdoors is nearly the same
as during higher temperature midday periods. Thus, at those times
there often is no need for cooling in the space and therefore no
dehumidification takes place. Accordingly, preexisting air conditioning
systems are poorly designed for those conditions. Those conditions,
at times, lead to uncomfortable conditions within the is building
and can result in the formation of mold or the generation of other
microbes within the building and its duct work, leading to what
is known as Sick Building Syndrome. To overcome these problems,
ASHRAE Draft Standard 62-1989 recommends the increased use of make-up
air quantities and recommends limits to the relative humidity in
the duct work. If that standard is properly followed, it actually
leads to a need for even increased dehumidification capacity independent
of cooling demands.
A number of solutions have been suggested to overcome this problem.
One solution, known as an "Energy Recovery Ventilator (ERV),"
is shown in FIG. 1 of the drawings and utilizes a conventional desiccant
coated enthalpy wheel to transfer heat and moisture from the make-up
air stream to an exhaust air stream. These devices are effective
in reducing moisture load, but require the presence of an exhaust
air stream nearly equal in volume to the make-up air stream in order
to function efficiently. ERVs are also only capable of reducing
the load since the delivered air will always be at a higher absolute
humidity in the summer months than the return air. Without active
dehumidification in the building, the humidity in the space will
rise as the moisture entering the system exceeds the moisture leaving
in the exhaust stream. However, ERVs are relatively inexpensive
to install and operate.
Other prior art systems use so-called cool/reheat devices as shown
schematically in FIG. 2. In these devices the outside air is first
cooled to a temperature corresponding to the desired building internal
dew point. The air is then reheated to the desired temperature,
most often using a natural gas heater. Occasionally, heat from a
refrigerant condenser system is also used to reheat the cooled and
dehumidified air stream. Such cool/reheat devices are relatively
expensive and inefficient, because excess cooling of the air must
be done, followed by wasteful heating of air in the summer months.
A third category of prior art device has also been suggested using
desiccant cooling systems, as shown for example in FIG. 3. In these
devices supply air from the atmosphere is first dehumidified using
a desiccant wheel or the like and the air is then cooled using a
heat exchanger. The heat from this air is typically transferred
to a regeneration air stream and is used to provide a portion of
the desiccant regeneration power requirements. The make-up air is
delivered to the space directly, as is, or alternatively is cooled
either by direct evaporative means or through more traditional refrigerant-type
air conditioning equipment. The desiccant wheel is regenerated with
a second air stream which originates either from the enclosure being
air conditioned or from the outside air. Typically, this second
air stream is used to collect heat from the process air before its
temperature is raised to high levels of between 150.degree. F. to
350.degree. F. as required to achieve the appropriate amount of
dehumidification of the supply air stream. Desiccant cooling systems
of this type can be designed to provide very close and independent
control of humidity and temperature, but they are typically more
expensive to install that traditional systems. Their advantage is
that they rely on low cost sources of heat for the regeneration
of the desiccant material.
U.S. Pat. No. 3401530 to Meckler, U.S. Pat. No. 5551245 to
Carlton, and U.S. Pat. No. 5761923 to Maeda disclose other hybrid
devices wherein air is first cooled via a refrigerant system and
dried with a desiccant. However, in all of these disclosures high
regeneration temperatures are required to adequately regenerate
the desiccant. In order to achieve these high temperatures, dual
refrigerant circuits are needed to increase or pump up the regeneration
temperature to above 140.degree. F. In the case of the Meckler patent,
waste heat from an engine is used rather than condenser heat. U.S.
Pat. No. 4180985 to Northrup discloses a device wherein refrigerant
condensing heat is used to regenerate a desiccant wheel or belt.
In the Northrup system, the refrigerant circuit cools the air after
it has been dried. As discussed below, this cycle is not as effective
or efficient as the cycle proposed in accordance with the present
invention.
It is an object of the present invention to treat outside supply
air and condition it to so-called space neutral conditions in an
efficient and economic manner.
Yet another object of-the present invention is to provide a desiccant
based dehumidification and air conditioning system which is relatively
inexpensive to manufacture and to operate.
A further object of the present invention is to provide an air
conditioning system which enables the operator to vary the latent/sensible
cooling ratios provided by the system.
The present invention is particularly suited to take outside air
of humid conditions, such as are typical in the South and Southeastern
portions of the United States and in Asian countries and render
it to a space neutral condition. This condition is defined as ASHRAE
comfort zone conditions and typically consists of conditions in
the range of 73-78.degree. F. and a moisture content of between
55-71 gr/lb or about 50% relative humidity. In particular, the system
is capable of taking air of between 85-95.degree. F. and 130-145
gr/lb of moisture and reducing it to the ASHRAE comfort zone conditions.
However, as will be understood by those skilled in the art, the
system or process of the present invention will also work above
and below these conditions, e.g., at temperatures of 65-85.degree.
F. or 95.degree. F. and above and moisture contents of 90-130 gr/lb
or 145-180 gr/lb.
As compared to conventional techniques as discussed above, the
present invention has significant advantages over alternative techniques
for producing air at indoor air comfort zone conditions from outside
air. The most significant advantage of the invention is low energy
consumption. That is, the energy required to treat the air with
a desiccant assist in accordance with the present invention is 25-45%
less than that used in previously disclosed cooling technologies.
In accordance with an aspect of the present invention, a method
and apparatus is disclosed in which a conventional refrigerant cooling
system is combined with a rotatable desiccant wheel. The refrigerant
cooling system includes a conventional cooling coil, condensing
coil and compressor. Means are provided for drawing a supply air
stream, preferably an outdoor air stream over the cooling coil of
the refrigerant system to reduce its humidity and temperature to
a first predetermined temperature range. The thus cooled supply
air stream is then passed through a segment of the rotary desiccant
wheel to reduce its moisture content to a predetermined humidity
level and increase its temperature to a second predetermined temperature
range. Both the temperature and humidity ranges are within the comfort
zone. This air is then delivered to the enclosure.
The system of the present invention also includes means for regenerating
the desiccant wheel by passing a regeneration air stream, typically
also from an outside air supply, over the condensing coil of the
refrigerant system, thereby to increase its temperature to a third
predetermined temperature range. The thus heated regeneration air
is passed through another segment of the rotatable desiccant wheel
to regenerate the wheel.
The above, and other objects, features and advantages of the present
invention will be apparent in the following detailed description
of illustrative embodiments thereof, which is to be read in connection
with accompanying drawings, wherein:
FIG. 1 is a schematic diagram of a conventional energy recovery
ventilator (ERV) system;
FIG. 2 is a schematic diagram of a conventional cool/reheat air
conditioning system;
FIG. 3 is a schematic diagram of a conventional desiccant cooling
system;
FIG. 4 is a psychometric chart describing the cycle achieved by
the present invention;
FIG. 5 is a psychometric chart showing the cycle achieved with
a prior art system such as shown in Northrup U.S. Pat. No. 4180985;
FIG. 6 is a psychometric chart for a cool/reheat system;
FIG. 7 is a schematic diagram of the basic system of the present
invention;
FIG. 8 is a schematic diagram of another embodiment of the present
invention in which some of the regeneration air is dissipated before
entering the desiccant wheel;
FIG. 9 is a schematic diagram of another embodiment of the present
invention using an air bypass for some of the supply air;
FIG. 10 is a schematic diagram of an embodiment similar to that
of FIG. 9 but utilizing some of the enclosure return air for the
supply air stream;
FIG. 11 is a schematic diagram of yet another embodiment of the
present invention in which the system can be operated, alternatively,
as an ERV system under certain conditions;
FIG. 12 is a schematic diagram similar to FIG. 7 showing another
embodiment of the invention using an evaporative cooler; and
FIG. 13 is a psychometric chart for the system of FIG. 12.
Referring now to the drawings in detail, and initially to FIG.
1 thereof, a prior art energy recovery ventilator air conditioning
system is illustrated in which a conventional rotary passive desiccant
wheel 10 is provided, operating in the conventional manner. An outside
air supply is supplied to a portion or segment of the rotating desiccant
wheel 10 by a fan or blower 12 and it is dehumidified. The dry air
is then supplied through a duct system 14 directly to the enclosure
or space to be conditioned. Return air is drawn by another fan or
blower 16 through duct work 18 to and through another segment of
the rotating desiccant wheel 10 in order to regenerate the desiccant
in the wheel. That air is then exhausted to the atmosphere. As noted
above, this type of prior art device is effective at reducing moisture
load, but requires an exhaust air stream nearly equal in volume
to the air supply stream.
FIG. 2 illustrates a cool-reheat prior art device in which a conventional
refrigerant air conditioning system 20 is utilized. These systems,
which are well known in the art, include a cooling coil 22 a condensing
coil 24 a fan 26 and a compressor 28. In this system outside air
is drawn by fan 26 over the condenser coil 24 to cool the refrigerant
returning from cooling coil 22 to condenser coil 24. That refrigerant
is then supplied to the cooling coil 22. A supply air stream is
drawn by a fan or blower 30 from the atmosphere through a duct system
32 and passed over the cooling coil 22 to reduce the air supply
stream temperature and moisture content. A heater, for example a
natural gas heater, 34 is then used to increase the temperature
of the cooled supply air to the desired temperature for the enclosure.
The supply air system is then supplied through duct 36 to the enclosure.
These systems are relatively expensive and inefficient.
FIG. 3 illustrates a known form of desiccant cooling system. In
these prior art systems, an air stream from the atmosphere outside
(or return air from the interior space) is drawn by a blower 40
or the like through a duct system to the rotating desiccant wheel
10. The wheel dries the outside air which is then passed to a heat
exchanger where its temperature is increased. Finally, the air passes
through an evaporative cooler 44 which functions to reduce the dried
air's temperature further to the desired internal space temperature.
From there the air and is supplied through the duct work 46 to the
space or enclosure.
In the system such as shown in FIG. 3 the desiccant wheel 10 is
regenerated by air from the atmosphere (or by return air from the
space or enclosure) which is drawn by blower 46 through the other
side of the evaporative cooler 44 and heat exchanger 42 in order
to collect heat given up in them by the air supply stream (i.e.,
the process air) and cause the regeneration air stream's temperature
to rise. If necessary, the temperature is further increased by a
natural gas heater 48 or the like before it enters the regeneration
sector of the desiccant wheel 10 to regenerate the desiccant. This
air is then exhausted to the atmosphere.
In accordance with the present invention, as illustrated in FIG.
7 a simplified air conditioning system utilizing a conventional
refrigerant cooling system and a desiccant wheel is provided. In
this system, supply air from the atmosphere is drawn by a blower
50 over the cooling coil 52 of a refrigerant system where its temperature
is lowered and it is slightly dehumidified. From there, the air
passes through a sector 54 of the rotating desiccant wheel 10 where
its temperature is increased and it is further dehumidified. That
air is then provided to the enclosure or space.
Desiccant wheel 10 is regenerated by utilizing outside air drawn
by a blower 56 over the condenser coil 58 of the air conditioning
system. This outside air stream is heated as it passes over the
condenser coil and is then supplied to another sector 60 of the
rotating desiccant wheel to regenerate the desiccant. It is then
exhausted to the atmosphere by the blower 56.
The advantages of the present invention are illustrated by the
psychometric charts of FIGS. 4-6. FIG. 4 illustrates the charts
for the system of FIG. 7. As seen therein, the outside air entering
system at point A, which in the illustrated chart has a temperature
of 90.degree. and a humidity ratio of about 140 gr/lb, initially
is cooled from the atmospheric temperature condition as it passes
over cooling coil 52 to its saturation line at point B and then
further cooled to about 60.degree.. As a result, the supply air
stream's moisture content also is reduced to point C as it leaves
the coil. This cooled and saturated air is then passed through desiccant
wheel 10 where its humidity is reduced further to about 60 gr/lb,
while its temperature is increased to about 74.degree. (point D).
The path the air takes on the psychometric chart will nearly follow
a line of constant enthalpy from point C to point D with a small
amount of temperature rise due to the heat carry-over of the wheel
from the regeneration sector. The distance that the air will travel
along the line of constant enthalpy is determined by the condition
of the regeneration air stream. As it is desired to achieve a leaving
condition from the desiccant wheel of approximately 50% relative
humidity (rh), only an approximate 17 gr/lb moisture depression
is required of the desiccant wheel to achieve point D from point
C. This depression is very small and does not require a large amount
of desiccant material nor a high regeneration temperature to regenerate
the wheel.
In order to achieve this moisture depression, the regeneration
air must be of the appropriate temperature and humidity. Typically,
when a desiccant wheel operates with two air streams that are not
far apart on the psychometric chart, the wheel will act as a relative
humidity (rh) exchanger. The process air, as described above, will
move down a line of constant enthalpy, i.e., from point C to point
D, while the regeneration air will move up a line of constant enthalpy.
In a perfectly efficient system the rh of the process air leaving
the wheel will be nearly equal to the rh of the regeneration air
entering the wheel. The same will be true for the regeneration air
whose rh will approach, but not exceed the rh of the process air.
Accordingly, the theoretical minimum temperature required for regeneration
can easily be calculated. In a perfectly efficient system, outside
air (point E on the chart and in FIG. 7) need only be heated to
a temperature necessary to achieve a 50%rh condition. At a typical
140 gr/lb design condition this relates only to a regeneration temperature
of about 100.degree. F. However, no mechanical systems are 100%
efficient. Thus, a 10-20%rh approach between the leaving and entering
air streams on one side of the wheel is typical of the desiccant
wheel cycle when operating in this range. Given the same outside
humidity conditions (using the same source of regeneration air and
supply air), this translates into a maximum required regeneration
temperature of 115.degree. F., i.e., point F on the chart. That
temperature is well below any stated temperature used for regeneration
of desiccant material that is doing useful work, and is easily achieved
by passing the regeneration air over the condenser coils of the
refrigerant system. Thus, by passing the regeneration air over the
condensing coils, that air is used to regenerate the desiccant and
achieve the desired performance of the refrigerant cooling system
on the delivered supply air quality, without the addition of external
heat.
With the understanding that the desiccant wheel as used in the
present invention acts as a relative humidity exchanger, the large
efficiency differences between this invention and, for example,
the system shown in the Northrup patent discussed above, are clearly
demonstrated by reference to FIG. 5. In the Northrup type system
as shown in FIG. 5 ambient air entering the system at point A is
first passed through the desiccant rotor which results in its temperature
increasing and its rh decreasing to point B. Where ambient air at
outdoor conditions is to be dried from 140 br/lb to 60 gr/lb, as
illustrated in the chart, the temperature rise occurring while the
air moves down the line of constant enthalpy will be a minimum 50.degree.
F. Given this minimum outlet temperature of 140.degree. F. in the
illustration, the rh of this air will be less than 8%rh. In order
to achieve this result with even a perfect humidity exchange device,
a reactivation rh of less than 8%rh will be required. Even utilizing
an ideal exchanger, this translates to a minimum regeneration temperature
of 180.degree. F. (point D) as compared to 115.degree. F. in the
present invention. This large minimum regeneration temperature is
well beyond the capabilities of typical refrigerant condensing systems.
Factoring real work inefficiencies, the required regeneration temperature
will be in excess of 200.degree. F., clearly indicating that the
cycle cannot have the same capacity or efficiency as the present
invention.
Another feature of the present invention is that the pre-cooling
and desiccant moisture reducing capacities of the system are balanced
in order to exclude the need for additional cooling after the desiccant
device. In all of the prior art discussed above, higher regeneration
temperatures are utilized to achieve the desired desiccant humidity
depression. Due to these temperatures, the temperature of the air
leaving the desiccant wheel is higher than can be tolerated to be
delivered to the space. Thus, in all these prior art systems, some
form of post-cooling, as illustrated in FIG. 3 is usually provided
and accomplished via an air-to-air heat exchanger in order to reduce
the supply air temperature from point B in FIG. 5 to an acceptable
limit at point N.
In comparing the current invention to conventional cool/reheat
devices such as shown in FIG. 2 with reference to the psychometric
chart for that device (i.e., FIG. 6), the efficiency of the present
invention as compared thereto can also be clearly seen. In such
a system, in order to achieve a similar delivered air quality to
that provided by the present invention (i.e., the conditions at
point D on the chart), the supply air (condition A) must be first
cooled to between 53-58.degree. F. (compared to the 60-65.degree.
F. of the present invention). This amounts to a more than 20% increase
in the cooling needed to achieve the necessary humidity condition.
That results in a decrease in compressor efficiency within the refrigeration
system, due to the need to operate at a lower evaporator temperature.
And, once the air is thus cooled, it must be reheated (as shown
in FIG. 2) from point C to point D to achieve acceptable air temperature
limits. This, of course, utilizes further energy. While it may be
argued that in such cool/reheat devices reheating need not be utilized
if cooling in the space is required, that will not be the case at
off-peak conditions, i.e., morning or evening, and it also leads
to the delivery of saturated air to the duct system.
One problem which has been encountered with desiccant cooling systems
that utilize lower temperature regeneration is that the desiccant
wheels tend to give off strong odors under certain operating parameters.
Typically this problem has been avoided by utilizing higher regeneration
temperatures, or by avoiding the passage of two nearly saturated
air streams through the rotor simultaneously. In accordance with
the present invention, this problem is overcome by utilizing a rotor
that does not have the capability to pick up odors (for example,
in the form of volatile organic compounds, "VOCs") or
which contains ingredients that will contain those odor molecules
even under the worst operating conditions. This is accomplished,
for example, by utilizing a desiccant wheel which either contains
a small pore desiccant, typically a molecular sieve that is not
capable of absorbing VOC molecules, or by utilizing a silica gel
desiccant that has incorporated in it an appropriate amount of odor
collecting particles, such as activated carbon. Such components
exist in the art of desiccant wheel technology, but have not been
applied in low regeneration temperature conditions such as are present
in the desiccant cooling system of the present invention.
Turning again to FIG. 7 in the illustrative embodiment, the supply
air stream at a temperature of about 90.degree. F. and a humidity
of 140 gr/lb is drawn through or over the cooling coil 52 where
its temperature is reduced to between 45-68.degree. F., or preferably
between 60-65.degree. F. The air then passes through the desiccant
rotor sector 54 where its moisture is reduced and temperature increased
to achieve a temperature and humidity level within or just below
(in terms of temperature or humidity) the ASHRAE comfort zone. This
air is then delivered to the space with the fan or with the fan
of an accompanying air conditioning unit.
The regeneration air stream at conditions E is first heated with
the condensing coil 58 of the refrigerant cooling system and then
passed through the desiccant rotor. In the preferred embodiment
of the invention the air is heated to a temperature of between 105-135.degree.
F. The amount of air used for regeneration ideally should be varied
in a manner that its temperature upon leaving the condenser, i.e.,
its regeneration, is held within that desired range.
The amount of regeneration air typically required to regenerate
a desiccant is 0.5 to 1.5 times the air quantity to be supplied
to a building or enclosed space. Airflow above this amount will
do little to improve the performance of the desiccant, but quantities
of air above this amount are often needed to provide the proper
condensing energy for the refrigerant system. In accordance with
a second embodiment of the invention, as illustrated in FIG. 8
a secondary fan 70 may be provided to draw a quantity of air only
through the condensing coil in order to provide the proper condensing
energy for the system. This air is then exhausted to the atmosphere
without entering the wheel. In this manner, fan pressure drop across
the desiccant wheel is minimized as the need to pull this additional
air through the relatively high pressure drop desiccant wheel is
avoided. Preferably, fan 70 is controlled using a conventional controller
system in response to the condensing head pressure of fluid in the
condensing coil. When that pressure exceeds a desired limit, typically
250-350 psi, the control system turns on the fan and the additional
cooling air is provided to the condensing coil, thereby reducing
the compressor head pressure. With the control set in this fashion
there is an independent control of the regeneration temperature
via regeneration airflow control and the compressor head pressure
via the condenser fan. Alternatively the fan can be controlled in
response to the temperature of the refrigerant in the refrigerant
system or to the temperature of the air leaving the condenser. In
another embodiment, the condenser coil can be formed in two sections,
with one section receiving only the portion of the airflow drawn
by fan 70 and the other being exposed only to the portion of the
ambient air to be supplied to the desiccant wheel by blower 56.
By this construction of the present invention, the ratio of latent
(dehumidification) work to sensible (cooling) work, can be easily
changed in a number of ways. For example, if additional cooling
is needed and less dehumidification is required, the regeneration
temperature of the air exiting the condenser coil can be reduced
by increasing the airflow across the condensing coil to one or both
of the fans which move the air across that coil. Additionally, the
rotary speed of the desiccant wheel may be reduced in order to lessen
the dehumidification capacity and increase the cooling capacity
to a maximum ratio wherein the wheel is stopped.
In another embodiment of the present invention, latent (dehumidification)
work capacity of the system can be reduced under appropriate conditions
by bypassing some of the supply air from the condenser coil 52 around
the wheel to avoid dehumidification of some of the supply air. This
can be done by appropriate duct work, vents or air valves and controls,
as would be apparent to those skilled in the art.
Another embodiment of the invention is illustrated in FIGS. 10
and 11. In this embodiment, when exhaust air is available from the
enclosed space, that air can be added to the supply air stream,
as illustrated in FIG. 10 and provided to the cooling coil, with
or without a bypass of the wheel.
In this embodiment, by providing appropriate ducting, air valves
and controls, the exhaust air from the room can be used for regeneration
by the desiccant wheel, as illustrated in FIG. 11 enabling the
system to be switched between an active moisture processing unit
(FIG. 10) and a typical ERV device (FIGS. 1 11).
When dehumidification is needed, the system airflow is arranged
as shown in FIG. 10 the refrigerant system operates, exhaust air
from the room is supplied to the cooling coil and then to the desiccant
wheel as described above. In this condition the wheel will spin
at a slow rate of 6-20 rph and act as an active desiccant wheel.
However, when conditions require no dehumidification, the refrigerant
system is shut down and airflow is arranged so that the room return
air flows over the condenser coil to the atmosphere and the wheel
spins at a rate of 10-30 rpm taking on the characteristics of an
enthalpy wheel, similar to that shown in FIG. 1. In this manner,
the summer moisture and cooling load and the winter heating and
humidification loads on the system are minimized as is typical of
an ERV installation. However, the system in accordance with the
invention has the added benefit of active dehumidification capacity
when needed.
It is noted that the system of the present invention need not be
designed in such a manner that all of the cooled air travels through
the desiccant wheel. In environments where latent heat ratios are
smaller, or when the unit is used in a recirculating mode, only
part of the treated air may need to travel through the wheel, as
shown in the examples of FIGS. 9 and 10. Also, the desiccant wheel
may be retrofitted into a standard cooling unit, utilizing the existing
fans and coils for the primary air moving device, with additional
plenums, ducts or fans for directing the condenser heat through
the regeneration side of the rotor.
In yet another embodiment of the present invention shown in FIG.
12 an evaporative cooler 80 may be used to selectively cool the
ambient air prior to entering the condenser coil to increase the
efficiency of the coil in lieu of the fan 70 used in the embodiment
of FIG. 8. In this embodiment the evaporative cooler (which is of
conventional construction) is operated when the regeneration temperature
of air leaving the condenser exceeds the air temperature required
for regeneration of the desiccant wheel or when the compressor head
pressure reaches a predetermined pressure. As seen in FIG. 8 condenser
water collected at the cooling coil 52 is pumped by condensate pumps
82 through a supply line 84 to the water distribution device 86
located conventionally above the corrugated layers of the evaporative
cooler body 88. Water discharged from the bottom of that body to
sump 90 is supplied by line 92 to the condensate sump 94.
While this system is counterintuitive since it adds moisture to
the desiccant wheel air regeneration or drying air stream, it has
significant advantages in the systems of the present invention,
as demonstrated by the psychometric chart of FIG. 13. As seen therein
atmospheric air supplied to the cooling coil passes through the
temperature and humidity conditions A, B, C, and D before being
supplied to the space or enclosure in the same manner as described
above with respect to the embodiment of FIG. 7. On the regeneration
side, however, when the temperature of air leaving the condenser
coil 58 exceeds a desired level, or if the compressor head pressure
exceeds a predetermined pressure as described above, pump 82 is
activated to activate the evaporative cooler which lowers the temperature
of cooling air entering the condenser to point E thereby improving
compressor efficiency in the refrigeration system with only a slight
increase in moisture content in the regeneration air stream.
Although illustrative embodiments of the present invention have
been described herein with reference to the accompanying drawings,
it is to be understood that the invention is not limited to those
precise embodiments, but that various changes and modifications
can be effected therein by those skilled in the art without departing
from the scope or spirit of this invention.
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