Description
FIELD OF INVENTION
The present invention relates to a cooling container for a
sorption apparatus in which an operating medium cools off
therein by partial evaporation and solidifies starting from the
surface of the operating medium in the cooling container.
BACKGROUND OF INVENTION
A sorption apparatus is an apparatus wherein a liquid or solid
adsorption medium adsorbs a second medium (i.e. the operating
medium) as vapor at a higher temperature, thereby releasing
heat. During the adsorption phase, the adsorption medium adsorbs
the operating medium which is transformed into vapor as it
evaporates upon absorbing heat from the heat exchanger in a
cooling container. During the adsorption phase, the evaporating
temperatures in the cooling container are in the range of
between -40.degree. to +40.degree. C.
Sorption apparatus with solid adsorption mediums, referred to
as "adsorption" apparatuses, operate periodically, that is, an
adsorption phase is always followed by a desorption phase
wherein the operating medium (e.g. water) is again separated
from the adsorption medium (e.g. zeolite). During the desorption
phase, the operating medium cannot evaporate and can therefore
not absorb any heat in the cooling container. Also, during the
adsorption phase, the "evaporation capacity" in the cooling
container is not constant since the sorption force of the
adsorption medium depends on several factors; e.g. the
temperature of the adsorption medium, and the given
concentration of the operating medium in the adsorption medium.
In order to maintain a constant evaporation capacity, or a
constant evaporation temperature in the cooling container, an
additional control effort is necessary with adsorption apparatus
in particular.
An example of a prior art cooling container of a
periodically-operating adsorption apparatus, is disclosed in
German Patent Application DE-OS 35 21 484. In such apparatus,
during the adsorption phase, an ice bank is built up during
partial evaporation of the water operating medium in the cooling
container. In the above-referenced German Patent Application,
the adsorption apparatus includes in addition to the cooling
container, an adsorption container with a shut off means
interposed between the adsorption container and the cooling
container. During the adsorption phase, the water operating
medium contained in the cooling container, partially evaporates
and the remaining portion freezes to form ice. A heat exchanger
is also provided in the cooling container and is used for
cooling beverages, for example. In connection therewith, the
beverage to be cooled is stored in a supply container, and is
permitted to flow through the heat exchanger and transfer its
heat to the water operating medium which is present in the
cooling container. In order to control the discharge temperature
of the beverage, a control unit is provided to control the
evaporation temperature of the water by opening and closing of
the shut off means.
However, such a cooling container suffers from several
significant shortcomings and drawbacks. When the control of the
shut off means fails, the water operating liquid in the cooling
container freezes into ice, which (due to the initiated
sublimation process in the cooling container during the
adsorption phase) cools down to a temperature far below
0.degree. C. Consequently, the beverage which remains in the
heat exchanger during this condition, freezes and thus a
continuation of the beverage tapping process is interrupted. In
addition, the provided control unit and the closable shut off
means are complicated and expensive elements which renders such
an equipped adsorption apparatus, prohibitively expensive.
Consequently, such adsorption cooling apparatus is precluded
from being used in a variety of cooling applications.
Accordingly, it is a primary object of the present invention to
provide a cooling container for an adsorption apparatus, which
incorporates a simple, inexpensive and reliable way of allowing
a liquid (such as a beverage) flowing through or remaining in
the heat exchanger within the cooling container, from being
cooled down below 0.degree. C. during the adsorption phase of
operation.
These and other objects of the present invention will become
apparent hereinafter and in the claims.
SUMMARY OF THE INVENTION
In general, the cooling container of the present invention is
for use with a sorption apparatus, and is adapted to contain a
liquid operating medium, which, during the adsorption phase of
operation of the sorption apparatus, cools off by partial
evaporation and solidifies into a solidifying layer starting
from the surface of the liquid operating medium. In general, the
cooling container comprises a vessel and a heat exchanger. The
vessel is provided for containing the operating medium, which
during the adsorption phase, cools off by partial evaporation
and forms the solidifying layer which grows from the surface
downwardly into the vessel until the solidifying layer obtains a
maximum thickness. The heat exchanger on the other hand is
provided for adsorbing heat from a liquid (e.g. beverage)
passing through the heat exchanger, and is positioned in the
vessel and within the liquid operating medium so that during the
adsorption phase, the heat exchanger does not come into contact
with the downwardly growing solidifying layer of operating
medium, and also so that the liquid operating medium below the
solidifying layer transfers the heat adsorbed by the heat
exchanger, to the solidified layer of operating medium.
Thus, with the cooling container of the present invention, the
heat exchanger and liquid (e.g. beverage) passing therethrough
are prevented from ever being exposed to temperatures below
freezing. Consequently, ice formation within the heat exchanger
itself is prevented using a very simple, yet highly effective
temperature control mechanism.
In the preferred embodiment, the cross-sectional dimension of
the vessel in the region of the solidified operating medium, is
smaller than the cross-sectional dimension of the vessel in the
region of the heat exchanger. Notably, this geometrical
condition placed on the cooling container vessel results in a
substantially higher vapor pressure drop within the cooling
container vessel, and consequentially this higher pressure drop
enables the use of a thinner ice layer. Thus, on the way through
the thin ice layer, there is a sufficient vapor pressure drop to
prevent growth of the ice layer beyond a predetermined distance
within the cooling container, as desired.
In the cooling container of the present invention, the volume
of liquid operating medium above the heat exchanger is smaller
than the maximum volume of the solidifying layer of operating
medium during the adsorption phase, and the heat adsorbed by the
liquid operating medium from the heat exchanger forms vapor
operating medium below the solidifying layer, which flows toward
the solidifying layer. Also, in the present invention, using the
vapor pressure drop caused by the ice layer formation and the
reduced cross-sectional dimension of the cooling container, the
maximum thickness of the solidifying layer of operating medium
is determined to be sufficient to increase the vapor pressure
drop of the vapor operating medium so as to prevent further
growth of the solidifying layer beyond the maximum thickness.
This restriction of the solidifying (e.g. ice) layer, in turn,
prevents the solidifying layer of operating medium from coming
into contact with the heat exchanger, and thereby prevents the
liquid (e.g. beverage) in the heat exchanger from freezing.
BRIEF DESCRIPTION OF THE DRAWINGS
The subject matter of the present invention will be explained
in greater detail by referring to the drawings, in which:
FIG. 1 is a schematic representation of conventional,
periodically-operating sorption apparatus; and
FIG. 2 is a cross-sectional view of a cooling container
of the present invention for use in connection with the sorption
apparatus of the type shown, for example, in FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 illustrates a conventional periodically-operating
sorption cooling apparatus comprising an adsorption container 1,
a cooling container 2, and shut off means 3. The adsorption
container 1 contains the adsorption medium 4 and the cooling
container 2 contains the operating medium. While use of a
variety of adsorption medium/operating medium pairs are possible
with the present invention, in the preferred embodiment hereof,
the zeolite/water pair is employed with excellent results.
During the desorption phase, the shut-off means 3 is opened,
and the adsorption medium 4 is heated in the adsorption
container 1. As a result, the preadsorbed operating medium is
desorbed from adsorption medium 4. The desorbed operating
medium, in vapor form, flows through the shut off means 3 to the
cooling container 2 and condenses therein by releasing heat of
condensation. After separating the operating medium from the
adsorption medium 4, the shut off means 3 is closed. Thereafter,
the adsorption medium 4 is allowed to cool down to the ambient
temperature without adsorbing the vapor-like and/or liquid
operating medium in cooling container 2.
The shut off means 3 is opened during the adsorption phase.
Liquid operating medium can now be evaporated in (i.e. suctioned
off from) the cooling container 2 and adsorbed by the adsorption
medium 4. During the evaporation of the operating medium in
cooling container 2, heat is released from the operating medium
and as its temperature drops, it becomes very cold, and
initially freezes rapidly to form a layer 9 of ice. This cooling
effect can be used in different ways, depending on the use of
the sorption apparatus. On the other hand, the heat which is
released in the adsorption medium 4 during the adsorption phase,
is discharged to the ambient environment through adsorption
container 1.
Referring now to FIG. 2, there is schematically illustrated a
cooling container 5 constructed in accordance with the
principles of the present invention. In the preferred
embodiment, the cooling container 5 replaces the cooling
container 2 of the sorption apparatus shown in FIG. 1.
As shown in FIG. 2, a vapor pipe 6 discharges into the upper
part of cooling container 5 with which the cooling container 5
in the preferred embodiment is connected to an adsorption
container of the type shown in FIG. 1. A schematically
illustrated heat exchanger 7 shown in cross-sectional view, is
provided in the lower portion of cooling container 5. As
illustrated, this lower portion is flooded with an operating
medium 8, such as water, when used for example with zeolite as
an adsorption medium 4 in the adsorption container 1. As shown,
the water 8 surrounds the heat exchanger 7 in a heat exchanging
relationship. An ice layer 9 which forms during the adsorption
phase of operation, is shown on top of the water layer 8, and
does not reach the heat exchanger 7 as required by the present
invention.
Notably, FIG. 2 illustrates the condition of the cooling
container 5 shown at a point in time, where the thickness of the
ice layer 9 above the water operating medium 8, is maximum.
Also, the cooling container 5 of the preferred embodiment has a
cross-section in the area of the ice layer 9, which is smaller
than in the area of the heat exchanger 7. The purpose of such a
geometrical condition on the cooling container 5 of the present
invention is to provide a larger vapor pressure drop on the way
through the ice layer 9, so that the growth of the ice layer is
stopped by a sufficiently high vapor pressure drop present
within the cooling container 5. This feature of the present
invention will be discussed in greater detail hereinafter.
In accordance with the present invention, the heat exchanger 7
is arranged in the cooling container vessel 5 so that the heat
exchanger 7 is not reachable by the downwardly expanding ice
layer 9 formed during the adsorption phase of operation. In
order to assure this critical condition in the cooling container
5, the water level above the heat exchanger 7 in the cooling
container 5 must be present in a sufficient amount at the start
of the adsorption phase. This condition can be satisfied for the
following reasons. Foremost, since the amount of the adsorption
medium 4 in the adsorption container is determined for any given
application, the maximum amount of ice generatable in the
cooling container 5 per adsorption phase is also pre-determined.
Also, provided that the amount of water in the cooling container
5 above the heat exchanger 7 prior to the desorption phase, is
larger than the maximum generatable amount of ice during the
adsorption phase, then the heat exchanger 7 cannot come into
contact with the downwardly growing ice layer 9 formed during
the adsorption phase. Thus, it has been discovered that
satisfying these two considerations, it is possible to determine
(i) the amount of water operating medium for a particular
cooling container 5 of selected geometry, to be used in
connection with a particular adsorption container having a
predetermined amount of adsorption medium; and (ii) what the
water level above the heat exchanger 7 must be in order to carry
out the present invention. These determinations can be made in a
straight forward manner, employing for example, the parameters
(i) and (ii) above, and the equilibrium condition specifying
that the ice layer completely ceases to grow at a point of
equilibrium where the rate of sublimination above the ice layer
9 equals the rate of crystallization (i.e. freezing) of water 8A
below the ice layer 9.
In order to generate ice in the cooling container, a portion of
the water operating medium in the cooling container must
evaporate. During this evaporation process, heat is absorbed
initially from the water in the cooling container 5 lowering the
temperature thereof, until a portion of the water freezes,
thereby producing a "cooling capacity" within the cooling
container 5. As is well known, the icing process always starts
from the surface of the water, and propagates downwardly. In
order not to interrupt the icing process, water 8A below the ice
layer 9 continuously evaporates at the water/ice layer boundary,
and ice 9 above the water 8A continuously transforms into vapor
12 by sublimation. As a result, an ice layer 9 of only a few
centimeters causes a considerable vapor pressure drop at the ice
layer, that is, with respect to the total vapor pressure within
the cooling container 5. Consequently, this vapor pressure drop
causes the ice layer 9 to grow more slowly as the ice layer 9
grows thicker, until eventually the ice layer 9 completely stops
growing at a point of equilibrium where the rate of sublimation
above the ice layer 9 equals the rate of crystallization (i.e.
freezing) of water 8A below the ice layer 9.
In FIG. 2, a state of the process hereof is schematically
illustrated where the volume of ice .DELTA.V.sub.ice is maximum
and consequentially, the volume of water above the heat
exchanger .DELTA.V.sub.AHE is minimum; notably, however, at this
state, the ice layer 9 does not reach the heat exchanger 7, as
required by the temperature control mechanism of the present
invention. More or less a theoretical value, .DELTA.V.sub.solid
is the maximum volume of ice which can be produced during the
entire adsorption phase, that is, if (i) no heat enters the
cooling container 5 via the heat exchanger 7, and (ii) if no
additional vapor pressure drops occur. As .DELTA.V.sub.solid is
a maximum value, .DELTA.V.sub.AHE is less than
.DELTA.V.sub.solid during the adsorption phase of practical
cooling applications.
Depending on the geometry of the sorption apparatus, the ice
layer 9 can cease growing after reaching, for example, 5-20
centimeters in thickness. In connection therewith, the geometry
of the cooling container 5 is a critical design parameter, since
according to the present invention, modification in the geometry
of the cooling container can be used to create vapor pressure
drops therewithin, and the maximum volume of ice produced
depends on the vapor pressure drops within the cooling container
5. Thus, the following considerations are important with respect
to ice layer formation and its limits of growth: the geometry of
the cooling container 5; the ambient heat flow through thermal
insulation provided to the cooling container 5; additional vapor
pressure drops along the path of vapor to the adsorption medium
4; and the instantaneous adsorption rate of the adsorption
medium 4 (i.e. mass of operating medium adsorbed per second).
Notably, while in the preferred embodiment, thermal insulation
is provided to the exterior surfaces of the cooling container 5
where ambient heat exchange is not required or desired, such
thermal insulation is not necessary to carry out the present
invention.
It has been discovered that in suitably formed cooling
containers, for example, the reduction in the cross-sectional 10
dimension of the cooling container 5 above the heat exchanger 7,
can be a small fraction of the maximum possible amount of ice
.DELTA.V.sub.solid. The reason for using such a configured
cooling container 5, is that use of a small cross-section 10
means a higher vapor pressure drop over such a region of the
cooling container 5, and a higher pressure drop thereabout means
a reduction in the thickness of the ice layer 9, necessary to
cause ice layer growth to completely stop before reaching the
heat exchanger 7 disposed therebelow.
During the adsorption of vapor by the adsorption medium 4, heat
of adsorption is released. This heat must be discharged from the
adsorption medium 4 so that the adsorption rate of the
adsorption medium 4, does not prematurely decrease. As is well
known, only so much "cold or cooling power" can be generated in
the cooling container 5 as heat is discharged from the
absorption container 1. As a result, it is possible to transfer
this heat in a controlled manner to the ambient environment, so
as to limit or throttle the "cooling power" produced in the
cooling container 5. Thus by using this control effect on the
"cooling power", together with the increased pressure drop
caused by the ice layer formation in the cooling container 5
hereof, it is possible to construct a sorption cooling apparatus
which "adjusts" the thickness of ice layer 9 growth using a
small column of water over which a relatively thin ice layer can
form, thereby protecting the heat exchanger 4 from icing during
use.
Caused by the density anomaly of water, water always has a
temperature of +4.degree. C. below an ice layer such as ice
layer 9 in FIG. 2. Therefore, in the cooling container 5 of the
present invention, the liquid to be cooled in the heat exchanger
7 can become no colder than +4.degree. C., as desired. Thus,
freezing of liquid (e.g. beverage) in the heat exchanger 7 is
not possible when using a cooling container of the present
invention. Accordingly, if the beverage flows into the heat
exchanger 7 with a higher temperature than +4.degree. C., the
heat exchanger 7 transfers heat to the surrounding water layer
8. In turn, the vapor pressure of this water layer increases
until the vapor pressure is so high that the water evaporates
and condenses on the superimposed ice layer 9. Notably, this
process of evaporating and condensing is equal to the heat
transfer in an overflooded condenser.
If the hydrostatic pressure above the heated water layer is
larger than the evaporation pressure, there is no evaporation at
first, but a slow rise or ascent of the heated water layer
occurs, thereby decreasing the hydrostatic pressure thereof
until the evaporation pressure suffices the hydrostatic pressure
to form steam bubbles. The same then rapidly rises, caused by
the enormous difference in density, and also condenses on the
superimposed ice layer 9. In this manner, a very effective heat
transfer mechanism is provided from the heat exchanger 7 to the
ice layer 9 lying above the water layer 8, all without cooling
the heat exchanger 7 below +4.degree. C. as desired.
This additional advantage of highly effective heat transfer
from the heat exchanger 7 to the ice layer 9, can also be used
in conventional ice storage buffers if the conventional ice
storage buffers are placed under a vacuum, so that the
evaporation and condensation processes can occur at such low
temperatures.
In beverage cooling applications, beverage tapping temperatures
of +4.degree. C. to +8.degree. C. are achievable using the
cooling container of the present invention. This beverage
tapping temperature range is possible with the cooling container
hereof without any further control efforts. Using the present
invention, thermostatically controlled valves and control of
heat transfer away from the adsorption medium 4, are not
required.
In an adsorption apparatus constructed in accordance with the
principles of the present invention, the shut off means 3 can
remain open during the total adsorption phase of operation, and
the operating medium (e.g. water) can be continuously adsorbed.
The excess "cooling power" which is generated in the cooling
container 5, is stored in form of the ice layer or block 9.
During the total time of the adsorption phase of operation, the
adsorption heat released from the adsorption medium 4, can be
transferred to the ambient environment, for example. Also, since
the time of adsorption phase operation is substantially longer
than the actual beverage tapping time (e.g. the sum of the times
in which the beverage to be cooled runs through the heat
exchanger 7), the adsorption container 1 used with the cooling
container 5 of the present invention, can be equipped with a
smaller and therefore more cost effective heat exchanging
surface.
The maximum adsorption capacity of the adsorption medium 4
depends on the lowest temperature that the adsorption medium 4
finally attains prior to the adsorption phase. The smaller this
end temperature is, the more vapor can be adsorbed in the
absorption medium 4, and consequently, much more cooling power
can be made available in the cooling container 5. Thus, by using
the cooling container of the present invention, adsorption
medium process can be used more advantageously at the same time.
While the particular embodiment shown and described above has
proven to be useful in many applications involving the
adsorption arts, further modifications herein disclosed will
occur to persons skilled in the art to which the present
invention pertains and also such modifications are deemed to be
within the scope and spirit of the present invention defined by
the appended claims.
