Abstrict An air-conditioning process uses an internally-cooled, liquid-desiccant
absorber through which the air to be conditioned, the coolant and
the liquid desiccant are passed. The desiccant is passed into the
absorber at a flooding rate not exceeding 1.0 gpm/ft.sup.2. Preferably
cooling water (as coolant) is passed into the absorber at a flooding
rate not exceeding 1.0 gpm/ft.sup.2 and, in that case, cooling air
is passed through the absorber in intimate contact with the cooling
water to promote evaporative cooling of the cooling water.
Claims I claim:
1. An air-conditioning process using an internally-cooled, liquid-desiccant
absorber through which the air to be conditioned, the coolant and
the liquid desiccant are passed, comprising the step of passing
the desiccant into the absorber at a flooding rate of 0.03-1.0 gpm/ft
while passing the air to be conditioned into the absorber at a flow
rate not exceeding 800 fpm.
2. The process of claim 1 additionally including the steps of
passing cooling water as the coolant into the absorber at a flooding
rate of 0.1-1.0 gpm/ft.sup.2 and passing cooling air through the
absorber in intimate contact with the cooling water and at a flow
rate not exceeding 800 fpm to promote evaporative cooling of the
cooling water.
3. The process of claim 2 wherein the air to be conditioned and
the cooling water are passed in counterflow.
4. The process of claim 3 wherein the air to be conditioned flows
upwardly.
5. The process of claim 2 wherein the air to be conditioned and
the cooling water are passed in cross-flow.
6. The process of claim 1 wherein said desiccant flooding rate
is 0.1-0.3 gpm/ft.sup.2.
7. The process of claim 2 wherein said cooling water flooding rate
is 0.1-0.3 gpm/ft.sup.2.
8. An air-conditioning process using an internally-cooled, liquid-desiccant
absorber through which the air to be conditioned, the cooling water,
the cooling air and the liquid desiccant are passed, comprising
the step of passing the desiccant into the absorber at a flooding
rate of 0.03-1.0 gpm/ft.sup.2 while passing the air to be conditioned
into the absorber at a flow rate not exceeding 800 fpm, passing
the cooling water into the absorber at a flooding rate of 0.1-1.0
gpm/ft.sup.2 and passing cooling air through the absorber in intimate
contact with the cooling water and at a flow rate not exceeding
800 fpm to promote evaporative cooling of the cooling water.
9. An air-conditioning process using an internally-cooled, liquid-desiccant
absorber through which the air to be conditioned, the coolant and
the liquid desiccant are passed, comprising the step of passing
the desiccant and the air to be conditioned into the absorber at
a liquid-to-gas mass flow ratio (liquid desiccant-to-air to be conditioned)
of about 0.005-0.17.
10. The process of claim 9 wherein cooling water and cooling air
are passed into the absorber at a liquid-to-gas mass flow ratio
(cooling water-to-cooling air) of about 0.014-0.14.
Description BACKGROUND OF THE INVENTION
The present invention relates to an internally-cooled desiccant
absorber for use in a liquid-desiccant air conditioner and, more
particularly, such an air conditioner having a high thermal coefficient
of performance (COP).
The primary function of the absorber in a liquid-desiccant air
conditioner is to dehumidify the supply air to the building. This
is done most simply in a packed-bed absorber, which consists of
a porous bed of contact medium that is flooded with desiccant. As
the desiccant flows down through the bed, it comes in contact with
the water-containing process air that can be flowing either down,
up, or across the bed. The desiccant--which, by definition, has
a strong affinity for water vapor--absorbs moisture from the process
air.
During dehumidification, heat is released as the water vapor condenses
and mixes with the desiccant. This heat will equal the latent heat
of condensation for water plus the chemical heat of mixing between
the desiccant and water. At desiccant concentrations typical of
a liquid-desiccant air conditioner, the chemical heat of mixing
will be about an order of magnitude smaller than the latent heat
of condensation.
For the simple packed-bed absorber just described, the heat released
during dehumidification will raise the temperature of the air and
desiccant as they flow through the absorber. If the desiccant that
flows off the bottom of the bed is not cooled before it is recirculated
to the top spray nozzles, the air will leave the absorber at close
to the same enthalpy as it entered. For example, air enters the
absorber at 80.degree. F. [27.degree. C.], 50% R.H. (31.3 BTU/lb
enthalpy) and leaves at 97.degree. F. [36.degree. C.], 20% R.H.
(31.5 BTU/lb enthalpy). In this configuration, the absorber is a
dehumidifier.
The preceding dehumidifier may be incorporated into an air-cooling
system by cooling the desiccant before it is sprayed onto the absorber.
This can be done by cooling the desiccant with externally chilled
cooling water or other refrigerant in a heat exchanger. If the desiccant
is cooled before it is sprayed onto the absorber bed, the air will
leave the absorber at a lower enthalpy than it entered, i.e.., net
cooling. An indirect evaporative cooler can further cool the process
air without increasing its absolute humidity.
The preceding cooling/dehumidifying system can be made more compact
by integrating the desiccant cooler into the absorber bed to provide
an internally cooled absorber for a liquid-desiccant air conditioner.
This internal desiccant cooler will most commonly be configured
as a heat exchanger with liquid desiccant and process air flowing
through one set of channels and a coolant flowing through the other
set of channels. The coolant can be (a) chilled water, (b) a chilled
refrigerant such as a water/glycol mixture, or (c) a boiling refrigerant,
such as R12 or R22 or their HFC and HCFC replacements, that is
supplied from a mechanical refrigeration system. The coolant can
also be a film of flowing water in contact with an air flow that
evaporatively cools the water. One embodiment of this last approach
to an internally cooled absorber is the "three way" absorber
described by Lowenstein, A., J. Marsala, M. Spatz, S. Feldman, and
J. Tandler (1988) "Integrated gas-fired desiccant dehumidification
vapor compression cooling system for residential application,"
Phase I Final Report, GRI-88/0326. This report available from the
National Technical Information Service as Report No. PB89140842
at a nominal cost. In this report, the nominal operating conditions
of the absorber are presented as 500 cfm for the process side and
750 cfm for the cooling side (page 72 first full paragraph). The
absorber is described (page 69 third full paragraph) as being composed
of two air-to-air heat exchangers that are each 30 cm.times.30 cm.times.60
cm. FIG. 4.21 (page 68) shows how the two heat exchangers are stacked
within the absorber's cabinet. The discussion (page 69 first two
full paragraphs) explains that the process air flows upward and
the cooling air flows crosswise. In this configuration, the face
area for the process flow is 30 cm.times.60 cm, and for the cooling
flow, 60 cm.times.60 cm. Since only half the face area is available
for air flow, the actual flow areas are 0.97 ft.sup.2 and 1.94 ft.sup.2
for the process and cooling air, respectively. The air velocities
are, therefore, calculated to be about 500 fpm and 400 fpm for the
process end cooling air. These values are the lower limits of the
gas flow rates.
In the "three-way" absorbers, the internally-cooled absorber
is constructed as a cross-flow parallel-plate heat exchanger. Process
air flows through one set of passageways and cooling air through
the other. On the process side, desiccant is sprayed onto the plates;
on the cooling side, water is sprayed onto the plates. As the process
air is dried in the "three-way" absorber, the heat that
is released is immediately transferred to the cooling air. Since
the cooling air is maintained at close to its wet-bulb temperature,
the temperature rise for the desiccant as it flows through the absorber
is very low. This increases the dehumidification capacity of the
absorber.
An internally-cooled absorber is more difficult to fabricate than
an absorber that separates the dehumidification and cooling processes--i.e.,
an absorber that uses simple contact media, coupled with either
an air cooler or desiccant cooler. The most significant problem
will be leakage between the desiccant and cooling water. However,
the performance benefits provided by this absorber typically justify
its use.
The relative flow rates of air and desiccant will have an important
effect on the dehumidification performance of the absorber.
Accordingly, it is an object of the present invention to provide
an air conditioning process wherein the internally-cooled absorber
is operated at the optimum flow rates of liquid desiccant and cooling
water in order to maximize the coefficient of performance (COP)
of the air conditioner.
SUMMARY OF THE INVENTION
It has now been found that the above and related objects of the
present invention are obtained in an air-conditioning process using
an internally-cooled, liquid-desiccant absorber through which the
air to be conditioned, the cool ant and the liquid desiccant are
passed. The process comprises the step of passing the desiccant
into the absorber at a flooding rate not exceeding 1.0 gpm/ft.sup.2
(gallons per minute per cross-sectional area of the channels through
which the desiccant and process air flows). Preferably cooling water
(as the coolant) is passed into the absorber at a flooding rate
not exceeding 1.0 gpm/ft.sup.2 and, in that case, cooling air is
passed through the absorber in intimate contact with the cooling
water to promote evaporative cooling of the cooling water.
In a preferred embodiment, the desiccant flooding rate is 0.1-0.3
gpm/ft.sup.2 and at least 0.03 gpm/ft.sup.2 and the cooling water
flooding rate is 0.1-0.3 gpm/ft.sup.2 and at least 0.03 gpm/ft.sup.2.
BRIEF DESCRIPTION OF THE DRAWING
The above and related objects, features and advantages of the present
invention will be more fully understood by reference to the following
detailed description of the presently preferred, albeit illustrative,
embodiments of the present invention when taken in conjunction with
the accompanying drawing wherein:
FIG. 1 is a schematic view of a liquid-desiccant air conditioner
with a regenerator according to the present invention; and
FIG. 2 is a schematic view of the absorber thereof according to
the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawing, and in particular to FIG. 1 thereof,
therein illustrated schematically is a liquid-desiccant air conditioner
according to the present invention, generally designated by the
reference numeral 10 comprising an absorber generally designated
12 and a regenerator generally designated 14 (illustrated as a gas-fed
burner boiler). Process air (for example, from a building) is fed
into the absorber 12 at return air duct 20 and leaves the absorber
12 through a supply air duct 22 after dehumidification (for example,
for return to the building). The cool weak desiccant discharged
by the absorber 12 passes through a heat interchanger 24 where it
is preheated by the hot strong desiccant discharged by the regenerator
14. The preheated weak desiccant is regenerated in the regenerator
14 (e.g., a boiler) with strong desiccant being returned to the
desiccant sprayer 26 (see FIG. 2) of absorber 12 (via interchanger
24) and liquid water or water vapor being discharged or recycled
through outlet duct 30.
Referring now to FIG. 2 therein illustrated schematically in greater
detail is the counterflow absorber 12. A cooling water spray 40
(as coolant), a strong desiccant spray 42 and cooling air 44 are
passed downwardly while the process air 46 is passed upwardly. The
water spray 40 is optionally collected in a water reservoir 50 for
subsequent recalculation, and the weak desiccant spray 42 is optionally
collected in a desiccant reservoir 52 for subsequent return to the
boiler regenerator 14 (via the interchanger 24).
While the description herein of the internally-cooled absorber
is based primarily on a model using evaporative cooling within the
absorber (that is, a stream of cooling air flowing over the film
of cooling water within the absorber), it is also possible to have
an internally cooled absorber that does not utilize evaporative
cooling within the absorber. Thus, cooling water or another refrigerant
may be chilled before it enters the absorber, the chilled coolant
flowing through tubes within the absorber or between plates of a
plate-type heat exchanger (without any cooling air flow within the
absorber in either case). Alternatively, chilled coolant could be
replaced with boiling refrigerant, such as R12 R22 or one of their
HFC or HCFC replacements, that is delivered from a mechanical refrigeration
system. Thus, it will be appreciated that the benefits arising from
the use of low desiccant flow rates, as described hereinbelow, apply
to all internally cooled absorbers, whether with or without internal
evaporative cooling.
The performance of such a liquid-desiccant air conditioner system
is studied herein at ARI indoor conditions with an internally-evaporatively-cooled
absorber using mathematical models. Furthermore, flow rates for
desiccants first, and then for cooling water, are varied over a
wide range to determine their effect on system COP. Counterflow
and cross-flow configurations are then compared. Heat transfer coefficients
are calculated from an empirical expression that applies to channels
bounded by flat plates with no liquid-gas interactions (i.e., waviness
on the surface of the liquid films). The Lewis number is assumed
to be equal to one.
All absorbers are studied as part of the complete liquid-desiccant
air conditioner characterized by
lithium chloride (LiCl, a liquid desiccant) at a spray concentration
that provides, in most cases, cooling at 400 cfm/ton (53.6L/(s.multidot.kW)),
a single-effect boiler that operates subatmospherically to produce
a 120.degree. F. steam saturation temperature (Lowenstein, A., and
M.H. Dean (1992) "The effect of regenerator performance on
a liquid-desiccant air conditioner," ASHRAE Transactions 98(1)),
a 67% effective interchanger,
an 80% gas utilization efficiency in the boiler,
absorber composed of flat plates spaced 0.157 in. (4.0 mm) apart,
height and width 3 ft (0.91 m),
velocity of the process and cooling air is 800 fpm (4.1 m/s),
cooling water flow rate is 7.0 gpm/ft.sup.2 (4.75L/(s.multidot.m.sup.2))
unless otherwise stated, and
operation processing inlet air at ARI indoor conditions (80.degree.
F. [27.degree. C.], 50% RH).
In a liquid-desiccant air conditioner using an adiabatic absorber,
heat is rejected to ambient in either a desiccant cooler, an air
cooler, or both. These coolers will be major components that will
have cost, size, and electrical power requirements that are comparable
to the absorber. A more compact alternative is to design the absorber
so that dehumidification and heat rejection occur simultaneously
in a single component. As already noted, this is the approach that
has been taken in an internally-cooled absorber.
In an adiabatic absorber, the flow rate of desiccant must be sufficiently
high to prevent appreciable temperature rises in the desiccant due
to the heat released in the drying process. In a typical adiabatic
absorber one-half to two gallons per minute of desiccant are flowed
per 100 cfm of gas. However, an internally-cooled absorber differs
from an adiabatic unit in that, since the desiccant is continually
cooled, the desiccant flooding rate no longer must be sufficient
to prevent a significant temperature rise in the desiccant as it
flows down through the bed. Thus the internally-cooled absorber
will have a good dehumidification effectiveness at L/G (liquid/gas)
ratios that are more than an order of magnitude less than that for
the adiabatic unit.
Nonetheless in conventional internally-cooled absorbers, desiccant
flow rates are of the same order of magnitude as in the comparable
adiabatic absorbers, even though there is no longer the requirement
to prevent a temperature rise in the desiccant (since the desiccant
is cooled as it flows through the absorber). It is theorized that
conventional desiccant flow rates are kept high partly as a carryover
from the experience on adiabatic absorbers, and partly to maintain
good wetting of the contact surface. Indeed, in current practice,
the flow rates are so high that the concentration of desiccant will
change very little as it flows through the absorber. Thus, the concentration
of the weak desiccant that is sent to the regenerator is approximately
the same as the concentration that is sprayed into the absorber.
In a practical desiccant system, there is a requirement to keep
both (1) the desiccant spray concentration in the absorber high,
so that the process air can be adequately dried, and (2) the desiccant
concentration in the regenerator low, so that the regenerator operates
at a lower temperature and the regeneration process is more efficient.
Since the desiccant concentration in the regenerator must be higher
than in the absorber, this requirement tends to minimize the concentration
difference between the absorber and the regenerator.
However, the exchange flow of desiccant between the regenerator
and the absorber will be approximately inversely proportional to
the difference between the absorber and regenerator concentrations.
Since large exchange flows penalize the efficiency of the system,
a practical lower limit is placed on difference between the absorber
and regenerator concentrations.
A typical difference between the absorber and regenerator concentrations
in a LiCl liquid-desiccant system would be 4 percentage points.
At this value, the energy required to heat the weak desiccant from
its temperature in the absorber to the operating temperature of
the regenerator will be approximately the same as the energy required
to remove water vapor from the desiccant in the regenerator. Since
a heat exchanger is used to transfer energy from the hot, strong
desiccant leaving the regenerator to the weak desiccant, this energy
for preheating, which must be provided from an outside source, is
reduced. For a 67% effective heat exchanger, the additional energy
that is needed to preheat the weak desiccant will degrade the system's
COP by about 20%.
More particularly, at all desiccant flooding rates above 0.1 gpm/ft.sup.2
(0.068 L/(s.multidot.m.sup.2)) the COP of a hypothetical liquid-desiccant
air conditioner increases as the concentration differential between
the absorber spray and boiler increases. This increase is caused
by the lower exchange rate of desiccant between the absorber and
boiler that is needed to meet the water load on the system.
At a concentration differential of 4 percentage points and a desiccant
flooding rate of 5 gpm/ft.sup.2 (3.4 L/(s.multidot.m.sup.2)), approximately
2% of the absorber flow must be diverted to the boiler for regeneration.
This weak stream must be heated from 80.degree. F. (27.degree. C.)
to approximately 180.degree. F. (82.degree. C.). Although the interchanger
provides much of this preheating requirement, a 67% interchanger
leaves a preheating load that is 20% of the total thermal requirements
for regeneration--i.e., a 20% reduction in the system's COP. This
preheating requirement will decrease at higher concentration differentials
because less desiccant must be exchanged between the boiler and
absorber to meet a given water load.
Although this improvement in COP would continue beyond the mentioned
4 percentage concentration differential, the desiccant's concentration
in the boiler becomes unacceptably high. A concentration limit is
roughly set by the need to avoid crystallization at the lowest temperature
reached by the strong desiccant as it returns to the absorber. At
a 6 percentage point differential, the concentration in the boiler
is more than 44%, which corresponds to a saturated salt solution
at about 70.degree. F. (21.degree. C.). Designing the regenerator
to operate at this concentration would leave little safety margin
to avoid crystallization.
LOW DESICCANT FLOW RATE
A second approach to decreasing the penalty imposed by preheating
the weak desiccant is to operate the absorber so that a very weak
stream of desiccant is produced and there is appreciable dilution
of the desiccant as it flows through the absorber.
There is a steady increase in COP as the desiccant flooding rate
decreases from 5.0 to 0.05 gpm/ft.sup.2 (3.4 to 0.034 L/(s.multidot.m.sup.2)).
At 0.05 gpm/ft.sup.2 (0.034 L/(s.multidot.m.sup.2)), the desiccant
weakens by between 7 and 8 percentage points as it flows down the
bed (compared to less than 0.1 percentage point for a 5.0 gpm/ft.sup.2
(3.4 L/(s.multidot.m.sup.2)) flooding rate). At the 0.05 gpm/ft.sup.2
(0.034 L/(s.multidot.m.sup.2)) desiccant flooding rate, the air
conditioner's COP is approximately 0.70 for all concentration differentials.
This is a 26% improvement over the more typical operating condition
of 5.0 gpm/ft.sup.2 (3.4 L/(s.multidot.m.sup.2)) and 4 percentage
point concentration differential. Since the flow to the regenerator
is drawn from this very weak desiccant, its flow rate, and hence
preheating requirements, will be very low. This accounts for the
high system COP at low flooding rates.
Dilution that significantly degrades the performance of the system
will occur at a desiccant flooding rate of about 0.03 gpm/ft.sup.2
(0.02 L/(s.multidot.m.sup.2)). One would expect that at desiccant
flooding rates below 0.03 gpm/ft.sup.2 (0.02 L/(s.multidot.m.sup.2)),
the COP would decrease as the absorber's latent effectiveness is
degraded by the dilution of the desiccant.
The operation of the absorber at very low desiccant flooding rates
can improve the performance of the liquid-desiccant air conditioner
in other ways. It should be possible to stage the regeneration process,
partially regenerating the very weak desiccant in a first stage
that operates at a lower temperature, and then completing the regeneration
in a second high-temperature stage. This approach could improve
the efficiency of regeneration by between 5% and 15% depending on
whether a simple boiler, a multiple-effect boiler, or a vapor-compression
distillation regenerator is used.
The operation of the absorber at desiccant flooding rates as low
as 0.05 gpm/ft.sup.2 (0.34 L/(s.multidot.m.sup.2)) requires the
use of methods for both distributing the desiccant over the top
of the bed and completely wetting the surface of the contact media.
For example, a wicking material may be applied to the contact surface
in the absorber to distribute the low flow evenly over the surface.
LOW DESICCANT AND WATER FLOW RATES
When the above-mentioned low desiccant flow rates are used within
an internally-evaporatively-cooled absorber, the air conditioner
system COP is improved even further by the use of low water flow
rates within the absorber as well. This involves the passing of
cooling air through the absorber in intimate contact with the cooling
water to promote evaporative cooling of the cooling water, thereby
to maintain it at a more-or-less constant temperature. Chilling
of the cooling water before it enters the absorber does not provide
sufficient internal cooling of the absorber in view of the low cooling
water flow rate.
At very low flow rates of cooling water, scale may accumulate on
heat transfer surfaces of the absorber. This phenomenon will depend
on the water quality as well as flow rate. If very pure water is
available--such as condensate from the desiccant regenerator--scaling
will not occur.
Consider absorber operation for high and low cooling water flow
rates (7.0 gpm/ft.sup.2 [4.75 L/(s.multidot.m.sup.2)] vs. 0.3 gpm/ft.sup.2
[0.2 L/(s.multidot.m.sup.2)]), the cooling water flowing through
a separate set of channels than the desiccant and process air. For
both flow rates, the process air flows downward and the desiccant
flow is 0.3 gpm/ft.sup.2 (0.2 L/(s.multidot.m.sup.2)). (The lowest
possible cooling water flow rate for the mentioned operating conditions
will be about 0.1 gpm/ft.sup.2 (0.068 L/(s.multidot.m.sup.2)). At
this value almost all the water evaporates before reaching the end
of the absorber. If the need to avoid scaling imposes a requirement
that no more than 10% of the cooling water evaporates, the limit
on water flow rates would be about 1.0 gpm/ft.sup.2 (0.68 L/(s.multidot.m.sup.2)).)
For the high water flow rate, the COP of the liquid-desiccant air
conditioner will be 0.63. For the low water flow rate, the COP will
be 0.75. This is almost a 20% improvement in efficiency.
As noted earlier in regard to very low desiccant flow rates, the
use of very low cooling water flow rates requires the use of techniques
for uniformly distributing the liquid over the internal surfaces
of the absorber, but these appear to be warranted by the potential
performance improvements that could be achieved if absorbers were
designed to operate at these low flow conditions. For example, a
wicking material may be applied to the contact surface in the absorber
to distribute the flow evenly over the surface.
COUNTERFLOW VS. CROSS-FLOW DESIGN
The internally-cooled absorbers that were mathematically modeled
and studied hereinabove were counterflow designs wherein cooling
air and process air flowed in different directions through the absorber,
with the desiccant and cooling water flowing down. Although this
configuration provides the highest latent and sensible effectivenesses,
it can lead to an absorber that is difficult to manufacture and
an air conditioner that either is too large or has inconvenient
requirements regarding the location of cooling and process air ducts.
Since a cross-flow absorber can overcome some of these limitations,
it is important to know the performance compromises imposed by this
design. Accordingly, the internally-evaporatively-cooled absorber
was also modeled as a cross-flow unit. In this case, process air,
desiccant, and cooling water all flowed down, and cooling air flowed
crosswise.
At flooding rates greater than 4.0 gpm/ft.sup.2 (2.7 L/(s.multidot.m.sup.2)),
the cross-flow absorber produces slightly higher system COPs than
the counterflow absorbers. This occurs because the change to a cross-flow
design is degrading dehumidification more than evaporative cooling
and, hence, produces a higher sensible heat ratio for the absorber
and a higher thermal COP for the air conditioner. At 0.3 gpm/ft.sup.2
(0.2 L/(s.multidot.m.sup.2)), the COP of the cross-flow absorber
is about 5% lower than that for a counter-flow absorber operating
at the same flow rates.
To summarize, cooling and dehumidification can be achieved in a
single device--the internally-evaporatively-cooled or "three-way"
absorber; Lowenstein, A., and R.S. Gabruck (1992) "The effect
of absorber design on the performance of a liquid-desiccant air
conditioner," ASHRAE Transactions 98(1). The present invention
involves maximizing system COP by reducing the desiccant flow rate
or the desiccant/cooling water flow rates in the internally-evaporatively-cooled
absorber to values that are much lower than common practice. Values
as low as 1% of what are now common may be used. According to the
present invention, system COP is maximized by using desiccant and
water flooding rates in the absorber which do not exceed 1.0 gpm/ft.sup.2
are preferably greater than 0.03 gpm/ft.sup.2 and are optimally
0.1-0.3 gpm/ft.sup.2. A sufficient quantity of the cooling air is
passed through the absorber in intimate contact with the cooling
water to promote evaporative cooling.
At the low desiccant flow rate, the concentration of the desiccant
will decrease significantly as it flows down through the absorber.
For a LiCl absorber dehumidifying the return air from a building,
a typical change in desiccant concentration would be 5 to 7 percentage
points. If this very weak desiccant is sent to the regenerator,
the flow of desiccant to the regenerator could be greatly reduced,
while still maintaining a relatively low concentration in the regenerator.
This lower flow rate would decrease the energy required for preheating
the desiccant up to the regenerator temperature, and, therefore,
improve the system's COP.
In a counterflow parallel-plate configuration, this internally-evaporatively-cooled
absorber produces a 0.75 system COP if the desiccant and cooling
water flooding rates are kept very low (below 0.3 gpm/ft.sup.2 [0.2
L/(s.multidot.m.sup.2)]) and the process air flows concurrent to
the desiccant. These conditions increase the sensible heat ratio
of the absorber, and so produce a higher thermal COP. Also, the
low desiccant flooding rate produces a very weak desiccant stream
at the exit of the absorber. This reduces the flow of desiccant
to the regenerator needed to meet the water load and, in turn, lowers
the system's preheating requirements for the desiccant entering
the regenerator.
The long term practicality of operating at very low desiccant and
water flooding rates remains to be demonstrated. If high water flooding
rates--3.0 gpm/ft.sup.2 (2.0 L/(s.multidot.m.sup.2)) or higher--are
used to avoid scaling and achieve uniform coverage, system COPs
will degrade by about 15%. An additional 5% loss in COP will occur
if high desiccant flooding rates are also used.
Cross-flow configurations for the absorber, which are easier to
integrate into a packaged air conditioner, have comparable performance
to the counterflow units. The internally-evaporatively-cooled cross-flow
absorber will produce a system COP that is comparable to or slightly
higher than that of the counterflow absorber at all operating conditions
except very low water and desiccant flooding rates. If both the
desiccant and water flooding rates are reduced to 0.3 gpm/ft.sup.2
(0.2 L/(s.multidot.m.sup.2)), the COP at ARI conditions for a liquid-desiccant
air conditioner using the cross-flow absorber will be only 5% lower
than the best counterflow design.
Now that the preferred embodiments of the present invention have
been shown and described in detail, various modifications and improvements
thereon will become readily apparent to those skilled in the art.
Accordingly, the spirit and scope of the present invention is to
be construed broadly and limited only by the appended claims, and
not by the foregoing specification. |