[0001] The present invention relates to a process for cooling CO
2 in a refrigerator system and a finned battery heat exchanger for carrying out such
process.
[0002] The process and the heat exchanger object of the present invention may be adopted
both in refrigerator systems operating according to a steam compression cycle of the
transcritical type and in systems operating according to a subcritical type cycle.
Refrigerator systems may be both large sized and intended for serving multiple end
utilities at the same time, such as a plurality of refrigerating rooms for example
in a commercial centre, and small sized and intended for serving a single utility,
such as a single refrigerating room or a display counter for perishable foodstuff.
[0003] CO
2 refrigerator systems are known wherein the cooling stage is obtained by gas coolers
or condensers (according to whether the operating cycle is transcritical or subcritical)
consisting of finned battery heat exchangers cooled with air taken from the environment.
These exchangers consist in a bundle of parallel tubes, connected to each other to
form a plurality of circuits, inside which CO
2 is made to flow. Externally to the bundle of tubes, preferably in countercurrent
relative to the CO
2 flow direction, a cooling air flow sucked from the environment by one or more fans
is made to flow. The CO
2 heat is transferred by thermal exchange to the air flow that flows close to the tubes.
[0004] A finned battery heat exchanger traditionally consists of a series of parallel aluminium
plates, arranged at regular pitch and of a bundle of tubes which orthogonally cross
the plates and are fixed thereto by keying. The tubes of the bundle, usually made
of copper, are arranged according to a triangular grating illustrated in Figure A.
The tubes are aligned in vertical direction along multiple parallel vertical rows,
whereas in the horizontal direction, they are staggered relative to one another along
multiple broken lines parallel to each other having a globally horizontal pattern.
The dimensions of a finned battery exchanger are defined by the tube length and diameter,
by the number of vertical rows of tubes (better known in the field as ranks) and by
the total number of tubes.
[0005] Constructively, the circuits of the exchanger are obtained by connecting the tubes
to one another at their ends by special tubular pipe fitting elements, shaped as a
curve (hereinafter called pipe fittings for simplicity). The tubes may all be connected
in a series to form a single circuit or, more frequently, they may be connected to
form multiple circuits in parallel, where each circuit consists of multiple tubes
in a series. In the latter case, the exchanger is provided with an inlet header and
an outlet header from where the different circuits in parallel respectively branch
off and lead in.
[0006] In theory, the above circuits may be made following the most varied construction
diagrams for the connection between the tubes. However, experience teaches that it
is preferable to adopt two particular construction diagrams, illustrated in Figures
B and C, which for every single circuit envisage the connection in series of the tubes
respectively belonging to a same horizontal broken row or to two horizontal contiguous
broken lines. In the case of 5 rank battery, the diagram illustrated in Figure B envisages
the implementation of a single circuit by connecting in series the 5 tubes of a same
broken row, whereas the other diagram, illustrated in Figure C, envisages the implementation
of a single circuit by alternately connecting in series the 10 tubes of two contiguous
horizontal broken rows. By adopting the second layout, that is, that in Figure C,
the number of circuits, and thus a flow section, is equal to half what can be obtained
by adopting the layout of Figure B, obtaining substantially double speed values, the
mass flow rate of CO
2 being equal.
[0007] Traditionally, once the dimensions of the finned battery have been determined based
on the rated power that the gas cooler or the condenser of the refrigerator system
must discharge, the sizing of exchanger substantially comes down to the choice of
how to connect the bundle tubes to one another, that is, to the choice of the number
and of the linear development of the circuits. This choice is made in order to find
the speed profile of CO
2 inside the circuits which allows maximising the thermodynamic COP (Coefficient of
performance) of the refrigerator system. The choice occurs by essentially reconciling
two opposite operating needs, that is, that of obtaining a high cooling of the CO
2, and that of having low load losses into the circuits.
[0008] More in detail, with reference to the pressure (P) - specific enthalpy (h) diagram
of Figure D, which illustrates a traditional refrigerating cycle with steam compression
with cooler R744 (CO
2), the thermodynamic COP of a refrigerating cycle, that is, the ratio between the
refrigerating capacity available at the evaporator and the power consumed at the compressor
for the heat transfer is given by the ratio between the (specific) cooling enthalpic
jump Δh
refr and the (specific) compression work Δh
comp. The enthalpic jump of refrigeration Δh
refr available at the evaporator (points 3 and 4) increases as the final temperature T
3 of CO
2 in output from the exchanger increases (point 2). As known, once the temperature
of the cooling fluid and the thermal exchange surface (dimensions of the exchanger)
have been determined, in order to decrease the above final temperature T
3 it is possible to increase the thermal exchange coefficient on the CO
2 side increasing the speed of the CO
2. In fact, the thermal exchange coefficient increases as the speed increases, even
though not in a proportional manner. On the other hand, as speed increases, also the
load losses ΔP into the circuits unavoidably increase. Considering that for operating
requirements pressure P
B at the evaporator must remain substantially constant, the additional load losses
ΔP must necessarily be compensated by a corresponding increase of pressure P
A in the gas cooler (points 1 and 2), which translates into an increase of the compression
work Δh
comp. Thus, based on the design conditions, there exists an interval of speed values
that can be obtained with a suitable sizing of the exchanger circuits, which may be
considered optimum as they allow maximising the thermodynamic COP of the refrigerator
system.
[0009] In recent years, the need of improving the overall energy performance of refrigerator
systems has become stronger in the field of refrigeration, by adopting technical solutions
which should be sustainable also from an environmental point of view. A useful parameter
for assessing the overall performance is the so-called electrical COP, which is given
by the ratio between the refrigerating power available at the evaporator (m Δh
refr), where m denotes the mass flow rate of CO
2, and all the electrical power used for the system operation, that is, in particular,
the rated (plate) power absorbed by the compressor and by the fans, as well as by
any other auxiliary devices.
[0010] From this point of view, one of the issues at hand is the improvement of the thermal
exchange efficiency in the gas coolers that translates into a higher attention to
the sizing and the management of heat exchangers, and in particular of finned battery
exchangers.
[0011] As known, one of the main limits of finned battery exchangers is the strong reduction
of the thermal jump DT between air and CO
2 which is found at the end portions of the circuits.
[0012] Normally, the strong reduction of the thermal jump DT between air and CO
2 in the end portions of the circuits of the exchanger is compensated, in the design
step, by properly increasing the thermal exchange surface or by increasing the development
of the circuits at the above end portions. In this way, thanks to a larger thermal
exchange surface available, it is possible to impose the last few cooling degrees
to the CO
2 in output from the exchanger as required by the design conditions without having
to intervene on the temperature of the ambient air flow.
[0013] Recently, finned battery exchangers have been proposed on the market, provided with
a water atomisation system suitable for humidifying all the cooling air flow up to
saturation before it flows through the exchanger. In this way, the cooling air flow
is made available for thermal exchange at the saturation temperature T
sat that is, at a lower temperature than the ambient one. Other conditions being equal,
with the passage from a cooling with ambient air to one with saturated air, it is
possible to decrease the final temperature T
3 of CO
2 without increasing the load losses. This leads to a considerable improvement of both
the thermodynamic COP and the electrical COP as compared to the solution that does
not envisage the saturation of the cooling air.
[0014] The improvement of the electrical COP however is not as strong as that of the thermodynamic
COP due to the negative weight of the electrical powers absorbed by the pumping devices
of the water used in the atomisation system and, in particular, of the water demineralisation
devices. In fact, it is known that for temperatures higher than 45°C (certainly reached
in a gas cooler), calcium carbonate becomes insoluble and originates calcareous deposits.
Water must therefore be subject to a softening treatment before it contacts the exchanger.
The water flow rates required to saturate all the air used for cooling are considerable
and impose the adoption of expensive reverse osmosis softening plants, which absorb
considerable electrical powers. For example, for a finned battery exchanger capable
of discharging a power of about 100 kW, the water consumption required for saturating
all the cooling air rate may be estimated about 125 1/h.
[0015] Even though the solution of finned battery heat exchanger which envisages the saturation
of all the cooling air allows considerably increasing the thermodynamic COP, and to
a smaller extent also the electrical COP, it exhibits the non-unimportant disadvantage
of requiring considerable plant investments (for reverse osmosis plants) and above
all, considerable water consumptions, not always acceptable from the point of view
of environmental sustainability.
[0016] In this situation, therefore, the object of the present invention is to overcome
the disadvantages of the mentioned prior art, by providing a process for cooling CO
2 in a refrigerator system, which should allow improving the electrical COP of the
system without requiring high water consumptions and large system investments.
[0017] A further object of the present invention is to provide a finned battery heat exchanger
for carrying out the process object of the present invention which should be economically
inexpensive and operatively fully reliable.
[0018] The technical features of the invention, according to the above objects, are clearly
found in the contents of the annexed claims and the advantages of the same will appear
more clearly from the following detailed description, made with reference to the annexed
tables and drawings, which show a purely exemplifying and nonlimiting embodiment thereof,
wherein:
[0019] - Figure 1 shows a simplified diagram of the finned battery heat exchanger according
to a preferred embodiment;
[0020] - Figure 2 shows in a psychrometric diagram of wet air a step of saturation of an
air flow envisaged in the process of the present invention;
[0021] - Tables 1 and 2 show the pattern of the thermal-physical properties of CO
2 and of the cooling air relative to the operation of a heat exchanger manufactured
according to the invention; and
[0022] - Table 3 shows the comparison between three pairs of values of the thermodynamic
COP and of the electrical COP relating to the operation of a refrigerator system which
uses as gas cooler respectively a finned battery heat exchanger of traditional type
cooled with ambient air, a finned battery heat exchanger of traditional type cooled
with saturated air and a finned battery heat exchanger according to the invention.
[0023] The process and the heat exchanger object of the present invention are intended for
carrying out the cooling stage in a CO
2 refrigerator system using air as cooling fluid and they may be adopted in CO
2 refrigerator systems operating according to a steam compression cycle of both transcritical
type and of subcritical type. Refrigerator systems may be large sized and intended
for serving multiple end utilities at the same time, as is the case for example in
a commercial centre, and small sized and intended for serving a single utility, such
as a single refrigerating room or a display counter for perishable foodstuff.
[0024] Steam compression refrigerating cycle herein means a traditional cycle intended for
transferring heat from a cold source to a hot source continuously treating a refrigerating
fluid (CO
2) through an evaporation stage, a compression stage, a cooling stage (or condensation,
if the cycle is subcritical rather than transcritical) and finally, a lamination stage.
Such cycle is carried out in a closed circuit provided with an evaporator, a compressor,
a gas cooler or a condenser, and with lamination means, connected to one another in
a series. Thus, cooling stage is understood to be the stage of the refrigerating cycle
carried out in the gas cooler (or condenser), wherein CO
2 at the gaseous state, after the compression stage is cooled by thermal exchange with
a cooling fluid before it undergoes the lamination stage. During this cooling stage,
the gaseous CO
2 could undergo a partial condensation (subcritical cycle), or it could remain at the
gaseous state (transcritical cycle). In the following description, the CO
2 temperature at the beginning of the cooling stage will be indicated as initial temperature
T
1, whereas the temperature of CO
2 at the end of this stage will be indicated as final temperature T
3.
[0025] The process for carrying out the cooling stage of a CO
2 refrigerating cycle object of the present invention uses air as cooling fluid and
envisages the use of at least one finned battery heat exchanger for carrying out the
heat exchange between the cooling air and the CO
2 circulating in the refrigerator system.
[0026] The finned battery exchanger intended for carrying out such process is a further
object of the present invention and is globally indicated with reference numeral 1
in the figures of the annexed drawings. For simplicity, the heat exchanger 1 will
be described first, and then the process.
[0027] To obtain exchanger 1 according to the invention it is possible to use a finned battery
of the traditional type, consisting of a bundle of parallel tubes and of a plurality
of parallel plates arranged to support the tubes. The bundle tubes are connected to
one another to form a plurality of circuits 10, 20 for the circulation of CO
2. Such circuits 10, 20 as a whole define a total flow section SF
tot which is equal to the product between the inner section of a single tube and the
number of circuits.
[0028] Each circuit 10, 20 is defined by multiple tubes connected to one another in series
by suitable tubular pipe fittings, shaped as a curve (not illustrated). Preferably,
circuits 10, 20 of heat exchanger 1 are identical to each other, that is, they are
comprised of the same number of tubes. The circuits may be made, for example, following
the construction layout illustrated in Figure B, or as an alternative, the construction
layout illustrated in Figure C.
[0029] As it can be seen in Figure 1, the CO
2 enters the circuits of exchanger 1 through an inlet header C
E, which relative to the flow direction of CO
2 is arranged upstream of the finned battery, and comes out through an outlet header
C
u arranged downstream of the finned battery.
[0030] According to an important aspect of the present invention, circuits 10, 20 of heat
exchanger 1 are connected to one another in parallel for forming two different groups,
each forming a different cooling section for the CO
2. As can be seen in Figure 1, the two cooling sections, hereinafter indicated as first
cooling section S
1 and second cooling section S
2, are connected to one another in series through a first intermediate header C
I1 and a second intermediate header C
I2. Operatively, at first the CO
2 flows through the circuits of the first cooling section S
1 and then the circuits of the second section S
2.
[0031] More in detail, the first cooling section S
1 is comprised of a first group of circuits of the exchanger, hereinafter indicated
as first circuits 10. These first circuits 10 branch off from the above inlet header
C
E to lead into the first intermediate header C
I1. These first circuits globally define a first partial flow section SF
1 for the CO
2.
[0032] The second cooling section S
2 is comprised of a second group of circuits of the exchanger corresponding to the
remaining part of circuits. These circuits, hereinafter indicated as second circuits
20, branch off from the second intermediate header C
I2, which is connected in series to the above first intermediate header C
I1, to lead into the outlet header C
U.
[0033] According to the invention, the subdivision of the finned battery into first and
second cooling section S
1 and S
2 is carried out so that the second section S
2 has a number of circuits comprised between 20% and 40 % of the total number of circuits.
In other words, the circuits attributed to the second section S
2 must define a second partial flow section SF
2 for the CO
2 having an extension comprised between 20% and 40% of the total flow section SF
tot. The extension of the above first flow section SF
1 is therefore equal to the remaining fraction of the total flow section SF
tot, that is, it can vary correspondingly between 80% and 60% of the total section SF
tot based on the extension of the second partial flow section SF
2.
[0034] In the example relating to tables 1 and 2, heat exchanger 1 according to the invention
is obtained using a finned battery consisting of 176 tubes having an inside diameter
D
i = 5.52 mm and a length L = 4.8 m, organised on 4 ranks. The tubes are connected to
one another to form 44 single circuits identical to one another, according to the
diagram illustrated in Figure B. The circuits define a total flow section SF
tot = (D
i)
2 x II/4 x 44 equal to about 1.05 x 10
-3 m
2. The first cooling section S
1 of exchanger 1 comprises 33 circuits connected in parallel, whereas the second cooling
section S
2 comprises the remaining 11. According to this solution, the second partial flow section
SF
2 is equal to 25% of the total flow section SF
tot.
[0035] The finned battery exchanger 1 comprises ventilation means 30 capable of sucking
air from the environment to force it through the circuits of the finned battery, through
gaps present between one tube and the other. Such means 30 allow generating at least
a first air flow A
1 through the above first cooling section S
1 and at least a second air flow A
2 through the above second cooling section S
2. Preferably, the ventilation means 30 comprise one or more fans installed downstream
of the finned battery relative to the moving direction of the two cooling air flows
A
1 and A
2. The total air flow m
atot generated by the fans may be varied according to the environmental and operating
conditions of the system.
[0036] The surface of the air gaps in each of the two cooling sections S
1 and S
2 is proportional to the number of circuits belonging to each of the two sections.
Thus, ignoring the difference of density existing between ambient air and saturated
air, it is possible to estimate that the second air flow A
2 has a rate m
air substantially comprised between 20% and 40% of the total air rate m
atot generated by the ventilation means 30.
[0037] According to another important aspect of the present invention, the finned battery
exchanger 1 comprises humidification means 40 capable of atomising into the second
air flow A
2 a water flow rate m
H2O sufficient for humidifying such flow at least up to saturation before it flows through
the second cooling section S
2.
[0038] Advantageously, as can be observed in Figure 1, the above humidification means 40
comprise one or more atomiser nozzles 41 arranged upstream of the second cooling section
S
2 relative to the moving direction of the second air flow A
2. Such humidification means 40 further comprise a water feeding circuit 42 for the
atomiser nozzles 41 and regulation means 43 of the above water flow rate m
H2O.
[0039] As described hereinafter, considering the low water flow rates m
H2O envisaged in the process according to the invention, the feeding circuit 42 does
not need dedicated pumping means but it can be connected directly to the waterworks.
[0040] More in detail, the regulation means 43 preferably consist of a regulation valve
inserted in the above feeding circuit 50 upstream of nozzles 41. Such regulation means
43 allow varying the water flow rate m
H2O and to this end they are actuated by a controller 44 capable of determining the value
of the water flow rate m
H2O sufficient to saturate all the air flow rate m
air of the second flow A
2.
[0041] Operatively, as described hereinafter, if the value of the air flow rate m
air is known, the value of the water flow rate m
H2O is defined by the values of ambient temperature T
amb and ambient relative humidity RH
amb. To this end, as can be observed in Figure 1, controller 44 is provided with a temperature
sensor 45 and with a relative humidity sensor 46. These two sensors 45 and 46 are
arranged upstream of the finned battery so that they can be impinged only by the first
air flow A
1, and not by the second air flow A
2 partly humidified, so as to detect the actual ambient temperature and humidity conditions
of the air.
[0042] Advantageously, heat exchanger 1 further comprises softening means 50 of the water
intended to be atomised into the second air flow A
2. Such softening means 50 are inserted in the above feeding circuit 42 upstream of
the atomiser nozzles 41 and allow reducing the contents of calcium carbonate present
in the water flow rate m
H2O so as to prevent the onset of scale deposits on the exchange surfaces of the tubes
and of the plates of the finned battery.
[0043] In consideration of the reduced water flows m
H2O envisaged in the process according to the invention, the softening means 50 could
be comprised of simple softening cartridge filters with ion exchange resins, of the
type also used in the household field.
[0044] In describing the process for carrying out the cooling stage of a CO
2 refrigerating cycle object of the present invention, the same alphanumerical references
used for describing the finned battery exchanger 1 shall be used.
[0045] The process according to the invention envisages a step of CO
2 circulation into circuits 10, 20 of at least one finned battery heat exchanger 1.
Advantageously, the process may also be used in refrigerator systems wherein the cooling
of CO
2 is carried out in multiple heat exchangers connected in parallel, such as an exchanger
of the V type, without departing from the scope of protection of the present invention.
[0046] In the circulation step, the CO
2 is made to circulate continuously through the exchanger, at first through the first
circuits 10 of the first cooling section S
1 and then through the second circuits 20 of the second cooling section S
2. As already mentioned above, the second circuits 20 define a partial flow section
SF
2 for the CO
2 having an extension comprised between 20% and 40% of the total flow section SF
tot.
[0047] Concurrently with the above circulation step, a ventilation step is envisaged, wherein
air is sucked through the finned battery, generating a first air flow A
1 through the first circuits 10 of the first cooling section S
1 and a second air flow A
2 through the second circuits 20 of the second cooling section S
2.
[0048] Concurrently with the above ventilation step, an adiabatic saturation step is further
envisaged, wherein the second air flow A
2 is humidified at least up to adiabatic saturation conditions starting from ambient
temperature T
amb and ambient relative humidity RH
amb conditions. During this saturation step, before flowing through the above second
cooling section S
2, the second air flow A
2 is cooled from the ambient temperature T
amb up to the corresponding adiabatic saturation temperature T
sat.
[0049] Advantageously, the saturation step is carried out only when the ambient temperature
T
amb is higher than a threshold temperature which is comprised in the range between 15°C
and 20°C. For ambient temperatures below the above threshold temperature, the saturation
step is not carried out and both cooling sections S
1 and S
2 of exchanger 1 are only cooled with ambient air.
[0050] More in detail, during the saturation step, a water flow rate m
H2O is atomised in the second air flow A
2 through the above atomisation means 40 in countercurrent relative to the same flow,
in order to humidify it up to saturation before it flows through the second cooling
section S
2.
[0051] Advantageously, in order to avoid useless waste of water, a step of regulation of
the above water flow rate m
H2O is envisaged before the saturation step. In this regulation step, once the air flow
rate m
air of the second air flow A
2 has been determined, the value of the water flow rate m
H2O is regulated based on the values of ambient temperature T
amb and of ambient relative humidity RH
amb of the air in order to deliver the water flow rate to the atomisation means 40 sufficient
to saturate all the air flow rate m
air.
[0052] In fact, as can be observed in the psychrometric diagram of Figure 2, which has the
specific enthalpy h shown on the ordinate and the air mass x required for a given
air humidification process on the abscissa, the initial air conditions being known
(T
amb, RH
amb; point 1), assuming an adiabatic saturation process (substantially isoenthalpic,
if the work is null), it is possible to calculate the water mass Δx that must be atomised
to bring one unit of air mass to saturation conditions (T
sat, RH=100%; point 2). For example, starting from available ambient air at a T
amb = 35°C and a RH
amb = 60%, it is necessary to atomise a water mass Δx of about 0.0025 kg to bring an
air mass of 1 kg to the corresponding saturation conditions, that is T
sat = 28°C and RH = 100%. When the air flow rate to be saturated is known, it is therefore
possible to calculate the water flow rate required for saturation. In the example
considered herein, after the saturation, cooling air is available for the heat exchange
with the CO
2 in the second cooling section S
2 at a temperature of 28°C and not at a temperature of 35°C anymore.
[0053] Advantageously, alternative regulation systems may be envisaged, with closed loop
rather than open loop, or less complex with only the detection of the ambient temperature.
[0054] The cooling of the CO
2 from its initial temperature T
1 to the final temperature T
3, occurs in two different steps, at the same time as the above circulation step.
[0055] In a first cooling step, the CO
2 is cooled starting from its initial temperature T
1 up to an intermediate temperature T
2, higher than the final temperature T
3, at the first cooling section S
1. This cooling is due to the thermal exchange with the first air flow A
1 that enters in the finned battery with a temperature equal to the ambient one T
amb. The value of the above intermediate temperature T
2 depends on the extension of the first cooling section S
1.
[0056] With reference to the example shown in table 1, which shows the pattern of the thermal-physical
properties of the CO
2 and of the air as regards the first section S
1 of the exchanger, the first air flow A
1 enters at an ambient temperature T
amb equal to 35.0°C and heats up to an outlet temperature equal to 63.4°C. As the CO
2 cools down flowing through the first section S
1 and changing from an initial temperature T
1 = 130.0°C to an intermediate temperature T
2 = 37.4°C, its density increases progressively and its speed decreases correspondingly.
In fact, the speed of CO
2 switches from a value of 3.18 m/s in input to a value of 1.1 m/s in output. In this
first cooling section S
1 the total load losses are equal to about 34.5 kPa. As known, as the speed of the
CO
2 decreases, the value of the thermal exchange coefficient on the CO
2 side (in the Table indicated as "alfa int co2") should decrease correspondingly.
Actually, from Table 1, this is not so as in this first cooling step, the speed decrease
is still considerably compensated by the high temperature values of CO
2. In fact, it is known that the thermal exchange coefficient is positively affected
by temperature, besides other factors.
[0057] During the second cooling step, the CO
2, after having left the first circuits 10 of the first cooling section S
1, is cooled starting from the above intermediate temperature T
2 up to the final temperature T
3 at the second cooling section S
2. This cooling is due to the thermal exchange with the second air flow A
2 that has been humidified up to saturation before flowing through the second section
S
2.
[0058] As can be observed in Table 2, which shows the pattern of the thermal-physical properties
of CO
2 and of the air as regards the second section S
2 of the same exchanger considered in table 1, the second air flow A
2 enters at a saturation temperature of 28.0°C and heats up to an output temperature
equal to 35.0°C. As the CO
2 completes its cooling, flowing through the second section S
2 and changing from the intermediate temperature T
2 = 37.4°C to the final temperature T
3 = 34.9°C, its density continues to increase progressively and its speed decreases
correspondingly. In this second cooling step, as compared to the first step, the decrease
of the thermal exchange coefficient on the CO
2 side would be evident, given the reduced temperatures of the CO
2. However, thanks to the fact that the second partial flow section SF
2 is smaller than the first partial flow section SF
1 (and equal to about 25% of the total one), the speed of the CO
2 at the inlet of the second section S
2 is higher than the speed with which it has left the first cooling section S
1 and this allows maintaining the thermal exchange coefficient high on the CO
2 side. In fact, the speed of the CO
2 changes from an input speed of 3.91 m/s to an output speed of 3.9 m/s. In this second
cooling section S
2 the total load losses are more than those of the first section S
1 and equal to about 88.4 kPa.
[0059] Advantageously, the process according to the invention further comprises a softening
step of the water flow rate m
H2O, before the atomisation step, wherein the water flow rate m
H2O is subject to a softening treatment for reducing the contents of calcium carbonate
dissolved therein.
[0060] The process according to the invention allows improving the thermal exchange between
the CO
2 and the cooling air in the end cooling stages, when the thermal jump between the
CO
2 and the air is very small, without having to increase the thermal exchange surface
and without envisaging high water consumptions for air saturation.
[0061] As known, the heat Q exchanged in the unit of time between two fluids in thermal
contact is given by the product between the thermal exchange surface S
st, the overall thermal exchange coefficient K
tot and the temperature difference ΔT between the two fluids, that is Q = K
tot S
st AT.
[0062] According to the invention, the use of saturated air in the above second cooling
step allows having a cooling fluid available at lower temperature and thus increasing
the thermal jump ΔT existing between the CO
2 and the cooling fluid. At the same time, the increase of the CO
2 speed resulting from the reduction of the flow section in the second cooling section
S
2 allows maintaining the values of the thermal exchange coefficient on the CO
2 side and thus of the overall coefficient high, which would otherwise progressively
reduce as the temperature and the speed of the CO
2 decreases.
[0063] It has been found that by sizing exchanger 1 so that the second cooling section S
2 has a partial flow section for the CO
2 comprised between 20% and 40% of the total one offered by the exchanger, the increase
of the load losses is still acceptable and the water consumption for the saturation
of the second air flow A
2 is still limited.
[0064] Thus, the higher cooling of the CO
2 that is obtained as compared to the traditional solution with ambient air cooling
is accompanied by an improvement of the thermal-dynamic COP and of the electrical
COP and requires a limited water consumption.
[0065] Tables 1 and 2 show the pattern of the thermal-physical properties of CO
2 and of the cooling air in a heat exchanger manufactured according to the invention.
The values shown for CO
2 relate to a single circuit and are organised into columns, each one referring to
one of the 4 tubes forming the circuit itself.
[0066] As mentioned before, in the example of tables 1 and 2, exchanger 1 according to the
invention is obtained using a finned battery consisting of 176 tubes having an inside
diameter D
i = 5.52 mm and a length L = 4.8 m, organised on 4 ranks. The tubes are connected to
one another to form 44 single circuits identical to one another, according to the
diagram illustrated in Figure B. The first cooling section S
1 consists of 33 circuits, whereas the second section S
2 of the remaining 11.
[0067] Table 1 shows the thermal-physical properties of the CO
2 and of the first air flow A
1 as regards the first cooling section S
1 of the exchanger. In particular, the values of the thermal-physical properties of
CO
2 refer to a single circuit among the 33 forming the entire first section S
1. For completeness of description, given the high thermal jump existing between CO
2 and air at the inlet of the first circuits 10 of the first section S
1, the properties of CO
2 and of the air as regards the first tube of the circuit, that is, that of rank 4,
have been measured with reference to three different sections of such first tube.
[0068] Table 2 shows the thermal-physical properties of the CO
2 and of the second air flow A
2 as regards the second cooling section S
2 of the exchanger. In particular, the values of the thermal-physical properties of
CO
2 refer to a single circuit among the 11 forming the entire second section S
2.
[0069] In the two tables 1 and 2, "Tin, CO2" and "Tout, CO2" respectively indicate the inlet
temperature and the outlet temperature of the CO
2 relative to the tube of the circuit the column refers to. In particular, in Table
1, the value of the Tin,CO2 relating to the first section of the tube of rank 4 is
the initial temperature T
1 of the CO
2, whereas in Table 2 the value of the Tin,CO2 relating to the tube of 4 is the intermediate
temperature T
2 and the value of Tout,CO2 relating to the tube of rank 1 is the final temperature
T
3.
[0070] Similarly: "Tin,a" and "Tout,a" refer to the cooling air inlet and outlet temperatures
relative to the gap existing between two contiguous tubes of the same circuit; "Tprop"
is the arithmetical average between the temperature of the CO
2 and of the air; "visc" is the dynamic viscosity of the CO
2 or of the air, "cp" is the specific heat at constant pressure of the two fluids,
and "k" is the thermal conductivity of the two fluids; "Re" is the number of Reynolds,
"Pr" is the number of Prandtl, "Nu" is the number of Nusselt and "f" is the friction
coefficient; "vel/circ" indicates the speed of CO
2 in the circuit, "m,co2/circ" indicates the mass flow rate of CO
2 by circuit, whereas "m,a" is the mass flow rate of air by gap between the tubes;
"alfa int CO2" indicates the thermal exchange coefficient on the CO
2 side, "alfa ext aria" is the thermal exchange coefficient on the air side and "K
tot" is the total thermal exchange coefficient (in the calculation of which, the thermal
exchange resistances of the tube walls have been ignored); "DP" indicates the load
losses in the single tube of the circuit, whereas "Dptot" indicates the overall load
losses of a cooling section; "Sext,tube" indicates the outer surface of a tube; "DTML"
indicates the value of the mean logarithmic thermal jump and "Dt,a" indicates the
difference of temperature between "Tin,a" and "Tout,a"; "Qexchanged" indicates the
heat exchanged in the unit of time in a single tube, "Qtot/circ" the heat exchanged
in the unit of time in a single circuit and "Wsection" the heat exchanged in the unit
of time in all the circuits of the cooling section.
[0071] Table 3 compares the energy performance of the same refrigerator system if a finned
battery heat exchanger of traditional type cooled with ambient air, a finned battery
heat exchanger of traditional type cooled with saturated air and a finned battery
heat exchanger according to the invention are respectively used as gas cooler. The
heat exchanger according to the invention is the same as that in tables 1 and 2. The
three exchangers are obtained using a finned battery consisting of 176 tubes having
an inside diameter D
i = 5.52 mm and a length L = 4.8 m, organised on 4 ranks. The tubes are connected to
one another to form 44 single circuits identical to one another, according to the
diagram illustrated in Figure B. The performance is expressed as values of the thermal-dynamic
COP and of the electrical COP. It is envisaged that the ambient air is available at
a temperature of 35°C and at a relative humidity of 60%, whereas the CO
2 is at a temperature of 130°C. The mass flow rate of air generated by the fans is
equal to about 8.7 kg/s, whereas the mass flow rate of CO
2 is equal to about 0.47 kg/s.
[0072] In the considerations below, the calculation of the percentage variations of the
electrical and thermal-dynamic COP was carried out referring to the traditional solution
with exchanger cooled with ambient air.
[0073] When the refrigerator system operates with the traditional heat exchanger cooled
with ambient air, the CO
2 is cooled up to a temperature of 38°C with total load losses equal to about 0.2 bar
(that is, 20 kPa). The refrigerating power available at the evaporator is equal to
120 kW. The thermal-dynamic COP is equal to 1.37, whereas the electrical COP is equal
to 1.23. The water consumption is null since in no case the saturation of the cooling
air is envisaged.
[0074] When the refrigerator system operates with a traditional heat exchanger cooled with
saturated air, the CO
2 is cooled up to a temperature of 30,1°C with unchanged load losses. The refrigerating
power available at the evaporator is equal to 135 kW. The thermal-dynamic COP is equal
to 1.65 with an improvement of about 20%. The electrical COP is equal to 1.37 with
an improvement of about 11%, that is, about half that observed for the thermal-dynamic
COP. As compared to the thermal-dynamic COP, the electrical one has a lower improvement
due to the negative contribution due to the electrical power absorbed by the water
circulation pumps and by the reverse osmosis demineralisers. A considerable water
consumption is envisaged for saturating all the cooling air, equal to about 125.3
l/h.
[0075] When the refrigerator system operates using the heat exchanger according to the invention,
that is, cooled with ambient air in the first cooling section S
1 (33 circuits) and with saturated air in the second section S
2 (11 circuits), the CO
2 is cooled up to a temperature of 34.9°C with load losses equal to about 1.2 bar (that
is, 120 kPa). The refrigerating power available at the evaporator is equal to 128
kW. The thermal-dynamic COP is equal to 1.49 with an improvement of about 9%. The
electrical COP is equal to 1.34, with a percentage improvement substantially equivalent
to that of the thermal-dynamic COP, and of the same order of quantity of that obtained
with the traditional solution completely cooled with saturated air. Advantageously,
the water consumption is six times less and equal to about 19.6 1/h.
[0076] If the heat exchanger according to the invention is sized with the second cooling
section having a flow section equal to about 40% (18 circuits on a total of 44), the
increase of the electrical COP and of the thermal-dynamic COP changes from about 9%
to 10% and the water consumption from 19.6 l/h to about 35 l/h.
[0077] The process according to the invention therefore envisages water consumptions considerably
lower than those envisaged in the traditional solution with saturation of the cooling
air and does not require the use of expensive reverse osmosis demineralisation plants.
In fact, the water low rates have such values as to be treated with simple cartridge
filters with ion exchange resin of the type used in the household field. This allows
keeping the cost of the system substantially in line with that of the traditional
solution without air saturation.
[0078] The invention thus conceived thus achieves the intended purposes.
[0079] Of course, in the practical embodiment thereof, it may take shapes and configurations
differing from that illustrated above without departing from the present scope of
protection.
[0080] Moreover, all the parts may be replaced by technically equivalent ones and the sizes,
shapes and materials used may be whatever according to the requirements.