EP1722174A2

EP1722174A2 - Process for cooling CO2 in a refrigerator system and a finned battery heat exchanger for carrying out such process

Info

Publication number: EP1722174A2
Application number: EP06113215A
Authority: EP
Inventors: designation of the inventor has not yet been filed The
Original assignee: Costan SpA
Current assignee: Costan SpA
Priority date: 2005-05-11
Filing date: 2006-04-27
Publication date: 2006-11-15
Also published as: ITPD20050132A1; EP1722174A3

Abstract

A process for cooling CO₂ in a refrigerator system comprising a circulation step of CO₂ into circuits 10, 20 of at least one finned battery heat exchanger 1. Exchanger 1 is divided into a first and a second cooling section S₁ and S₂, each comprised of a group of circuits 10,20 connected in parallel. The two sections S₁ and S₂ are connected to one another in series. The exchanger is sized so that the second cooling section S₂ has a flow section comprised between 20% and 40% of the total flow section offered by all the circuits as a whole. Concurrently with the circulation step, a ventilation step is envisaged, wherein a first air flow A₁ is made to pass through the first cooling section S₁ and a second air flow A₂ is made to pass through the second cooling section S₂. Concurrently with the ventilation step, an adiabatic saturation step is envisaged, wherein the second air flow A₂ is humidified at least up to saturation before flowing through the second cooling section S₂. Concurrently with the circulation step, the process envisages a first cooling step, wherein the CO₂ is cooled starting from an initial temperature T₁ up to an intermediate temperature T₂ at the first cooling section S₁ by the effect of a thermal exchange with the first air flow A₁ in the ambient conditions of temperature and of relative humidity, and a second cooling step, wherein the CO₂ is cooled from the intermediate temperature T₂ up to a final temperature T₃ at the second cooling section S₂ by the effect of a thermal exchange with the second air flow A₂.

A further object of the present invention is a finned battery heat exchanger for carrying out such process.

Description

The present invention relates to a process for cooling CO₂ in a refrigerator system and a finned battery heat exchanger for carrying out such process.
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.
CO₂ 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 CO₂ is made to flow. Externally to the bundle of tubes, preferably in countercurrent relative to the CO₂ flow direction, a cooling air flow sucked from the environment by one or more fans is made to flow. The CO₂ heat is transferred by thermal exchange to the air flow that flows close to the tubes.
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.
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.
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 CO₂ being equal.
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 CO₂ 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 CO₂, and that of having low load losses into the circuits.
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 (CO₂), 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 T₃ of CO₂ 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 T₃ it is possible to increase the thermal exchange coefficient on the CO₂ side increasing the speed of the CO₂. 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.
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 CO₂, 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.
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.
As known, one of the main limits of finned battery exchangers is the strong reduction of the thermal jump DT between air and CO₂ which is found at the end portions of the circuits.
Normally, the strong reduction of the thermal jump DT between air and CO₂ 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 CO₂ in output from the exchanger as required by the design conditions without having to intervene on the temperature of the ambient air flow.
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 T₃ of CO₂ 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.
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.
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.
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 CO₂ in a refrigerator system, which should allow improving the electrical COP of the system without requiring high water consumptions and large system investments.
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.
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:
- Figure 1 shows a simplified diagram of the finned battery heat exchanger according to a preferred embodiment;
- 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;
- Tables 1 and 2 show the pattern of the thermal-physical properties of CO₂ and of the cooling air relative to the operation of a heat exchanger manufactured according to the invention; and
- 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.
The process and the heat exchanger object of the present invention are intended for carrying out the cooling stage in a CO₂ refrigerator system using air as cooling fluid and they may be adopted in CO₂ 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.
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 (CO₂) 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 CO₂ 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 CO₂ could undergo a partial condensation (subcritical cycle), or it could remain at the gaseous state (transcritical cycle). In the following description, the CO₂ temperature at the beginning of the cooling stage will be indicated as initial temperature T₁, whereas the temperature of CO₂ at the end of this stage will be indicated as final temperature T₃.
The process for carrying out the cooling stage of a CO₂ 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 CO₂ circulating in the refrigerator system.
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.
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 CO₂. 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.
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.
As it can be seen in Figure 1, the CO₂ enters the circuits of exchanger 1 through an inlet header C_E, which relative to the flow direction of CO₂ is arranged upstream of the finned battery, and comes out through an outlet header C_u arranged downstream of the finned battery.
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 CO₂. As can be seen in Figure 1, the two cooling sections, hereinafter indicated as first cooling section S₁ and second cooling section S₂, 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 CO₂ flows through the circuits of the first cooling section S₁ and then the circuits of the second section S₂.
More in detail, the first cooling section S₁ 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 SF₁ for the CO₂.
The second cooling section S₂ 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.
According to the invention, the subdivision of the finned battery into first and second cooling section S₁ and S₂ is carried out so that the second section S₂ 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 S₂ must define a second partial flow section SF₂ for the CO₂ having an extension comprised between 20% and 40% of the total flow section SF_tot. The extension of the above first flow section SF₁ 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 SF₂.
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 m². The first cooling section S₁ of exchanger 1 comprises 33 circuits connected in parallel, whereas the second cooling section S₂ comprises the remaining 11. According to this solution, the second partial flow section SF₂ is equal to 25% of the total flow section SF_tot.
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 A₁ through the above first cooling section S₁ and at least a second air flow A₂ through the above second cooling section S₂. 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 A₁ and A₂. The total air flow m_atot generated by the fans may be varied according to the environmental and operating conditions of the system.
The surface of the air gaps in each of the two cooling sections S₁ and S₂ 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 A₂ has a rate m_air substantially comprised between 20% and 40% of the total air rate m_atot generated by the ventilation means 30.
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 A₂ a water flow rate m_H2O sufficient for humidifying such flow at least up to saturation before it flows through the second cooling section S₂.
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 S₂ relative to the moving direction of the second air flow A₂. 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.
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.
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 A₂.
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 A₁, and not by the second air flow A₂ partly humidified, so as to detect the actual ambient temperature and humidity conditions of the air.
Advantageously, heat exchanger 1 further comprises softening means 50 of the water intended to be atomised into the second air flow A₂. 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.
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.
In describing the process for carrying out the cooling stage of a CO₂ refrigerating cycle object of the present invention, the same alphanumerical references used for describing the finned battery exchanger 1 shall be used.
The process according to the invention envisages a step of CO₂ 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 CO₂ 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.
In the circulation step, the CO₂ is made to circulate continuously through the exchanger, at first through the first circuits 10 of the first cooling section S₁ and then through the second circuits 20 of the second cooling section S₂. As already mentioned above, the second circuits 20 define a partial flow section SF₂ for the CO₂ having an extension comprised between 20% and 40% of the total flow section SF_tot.
Concurrently with the above circulation step, a ventilation step is envisaged, wherein air is sucked through the finned battery, generating a first air flow A₁ through the first circuits 10 of the first cooling section S₁ and a second air flow A₂ through the second circuits 20 of the second cooling section S₂.
Concurrently with the above ventilation step, an adiabatic saturation step is further envisaged, wherein the second air flow A₂ 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 S₂, the second air flow A₂ is cooled from the ambient temperature T_amb up to the corresponding adiabatic saturation temperature T_sat.
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 S₁ and S₂ of exchanger 1 are only cooled with ambient air.
More in detail, during the saturation step, a water flow rate m_H2O is atomised in the second air flow A₂ 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 S₂.
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 A₂ 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.
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 CO₂ in the second cooling section S₂ at a temperature of 28°C and not at a temperature of 35°C anymore.
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.
The cooling of the CO₂ from its initial temperature T₁ to the final temperature T₃, occurs in two different steps, at the same time as the above circulation step.
In a first cooling step, the CO₂ is cooled starting from its initial temperature T₁ up to an intermediate temperature T₂, higher than the final temperature T₃, at the first cooling section S₁. This cooling is due to the thermal exchange with the first air flow A₁ that enters in the finned battery with a temperature equal to the ambient one T_amb. The value of the above intermediate temperature T₂ depends on the extension of the first cooling section S₁.
With reference to the example shown in table 1, which shows the pattern of the thermal-physical properties of the CO₂ and of the air as regards the first section S₁ of the exchanger, the first air flow A₁ 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 CO₂ cools down flowing through the first section S₁ and changing from an initial temperature T₁ = 130.0°C to an intermediate temperature T₂ = 37.4°C, its density increases progressively and its speed decreases correspondingly. In fact, the speed of CO₂ 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 S₁ the total load losses are equal to about 34.5 kPa. As known, as the speed of the CO₂ decreases, the value of the thermal exchange coefficient on the CO₂ 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 CO₂. In fact, it is known that the thermal exchange coefficient is positively affected by temperature, besides other factors.
During the second cooling step, the CO₂, after having left the first circuits 10 of the first cooling section S₁, is cooled starting from the above intermediate temperature T₂ up to the final temperature T₃ at the second cooling section S₂. This cooling is due to the thermal exchange with the second air flow A₂ that has been humidified up to saturation before flowing through the second section S₂.
As can be observed in Table 2, which shows the pattern of the thermal-physical properties of CO₂ and of the air as regards the second section S₂ of the same exchanger considered in table 1, the second air flow A₂ enters at a saturation temperature of 28.0°C and heats up to an output temperature equal to 35.0°C. As the CO₂ completes its cooling, flowing through the second section S₂ and changing from the intermediate temperature T₂ = 37.4°C to the final temperature T₃ = 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 CO₂ side would be evident, given the reduced temperatures of the CO₂. However, thanks to the fact that the second partial flow section SF₂ is smaller than the first partial flow section SF₁ (and equal to about 25% of the total one), the speed of the CO₂ at the inlet of the second section S₂ is higher than the speed with which it has left the first cooling section S₁ and this allows maintaining the thermal exchange coefficient high on the CO₂ side. In fact, the speed of the CO₂ changes from an input speed of 3.91 m/s to an output speed of 3.9 m/s. In this second cooling section S₂ the total load losses are more than those of the first section S₁ and equal to about 88.4 kPa.
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.
The process according to the invention allows improving the thermal exchange between the CO₂ and the cooling air in the end cooling stages, when the thermal jump between the CO₂ and the air is very small, without having to increase the thermal exchange surface and without envisaging high water consumptions for air saturation.
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.
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 CO₂ and the cooling fluid. At the same time, the increase of the CO₂ speed resulting from the reduction of the flow section in the second cooling section S₂ allows maintaining the values of the thermal exchange coefficient on the CO₂ side and thus of the overall coefficient high, which would otherwise progressively reduce as the temperature and the speed of the CO₂ decreases.
It has been found that by sizing exchanger 1 so that the second cooling section S₂ has a partial flow section for the CO₂ 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 A₂ is still limited.
Thus, the higher cooling of the CO₂ 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.
Tables 1 and 2 show the pattern of the thermal-physical properties of CO₂ and of the cooling air in a heat exchanger manufactured according to the invention. The values shown for CO₂ relate to a single circuit and are organised into columns, each one referring to one of the 4 tubes forming the circuit itself.
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 S₁ consists of 33 circuits, whereas the second section S₂ of the remaining 11.
Table 1 shows the thermal-physical properties of the CO₂ and of the first air flow A₁ as regards the first cooling section S₁ of the exchanger. In particular, the values of the thermal-physical properties of CO₂ refer to a single circuit among the 33 forming the entire first section S₁. For completeness of description, given the high thermal jump existing between CO₂ and air at the inlet of the first circuits 10 of the first section S₁, the properties of CO₂ 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.
Table 2 shows the thermal-physical properties of the CO₂ and of the second air flow A₂ as regards the second cooling section S₂ of the exchanger. In particular, the values of the thermal-physical properties of CO₂ refer to a single circuit among the 11 forming the entire second section S₂.
In the two tables 1 and 2, "Tin, CO2" and "Tout, CO2" respectively indicate the inlet temperature and the outlet temperature of the CO₂ 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 T₁ of the CO₂, whereas in Table 2 the value of the Tin,CO2 relating to the tube of 4 is the intermediate temperature T₂ and the value of Tout,CO2 relating to the tube of rank 1 is the final temperature T₃.
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 CO₂ and of the air; "visc" is the dynamic viscosity of the CO₂ 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 CO₂ in the circuit, "m,co2/circ" indicates the mass flow rate of CO₂ 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 CO₂ 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.
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 CO₂ 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 CO₂ is equal to about 0.47 kg/s.
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.
When the refrigerator system operates with the traditional heat exchanger cooled with ambient air, the CO₂ 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.
When the refrigerator system operates with a traditional heat exchanger cooled with saturated air, the CO₂ 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.
When the refrigerator system operates using the heat exchanger according to the invention, that is, cooled with ambient air in the first cooling section S₁ (33 circuits) and with saturated air in the second section S₂ (11 circuits), the CO₂ 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.
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.
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.
The invention thus conceived thus achieves the intended purposes.
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.
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.

Claims

A process for cooling CO₂ in a refrigerator system comprising the following operating steps:
- a circulation step of CO₂ into the circuits (10, 20) of at least one finned battery heat exchanger (1) which define as a whole a total flow section (SF_tot) for said CO₂, in said circulation step said CO₂ being made to circulate at first through a first cooling section (S₁) of said exchanger (1) consisting of a first group of said circuits (10), which are connected to one another in parallel and as a whole define a first partial flow section (SF₁) for said CO₂, and afterwards through a second cooling section (S₂) consisting of a second group of said circuits (20) corresponding to the remaining part of the circuits themselves, which are also connected to one another in parallel and as a whole define a second partial flow section (SF₂) having an extension comprised between 20% and 40% of said total flow section (SF_tot);

- a ventilation step, concurrent to said circulation step, wherein a first air flow (A₁) is made to pass through said first cooling section (S₁) for impinging the first group of said circuits (10) and wherein a second air flow (A₂) is made to pass through said second cooling section (S₂) for impinging the second group of said circuits (20) ;

- an adiabatic saturation step, concurrent to said ventilation step, wherein said second air flow (A₂), starting from ambient temperature (T_amb) and ambient relative humidity (RH_amb) conditions, is humidified at least up to adiabatic saturation conditions and then cooled up to the corresponding adiabatic saturation temperature (T_sat) before passing through said second cooling section (S₂);

- a first cooling step, concurrent to said circulation step, wherein said CO₂ is cooled starting from an initial temperature (T₁) up to an intermediate temperature (T₂) at said first cooling section (S₁) by the effect of a thermal exchange with said first air flow (A₁), said first air flow (A₁) being in the conditions of ambient temperature (T_amb) and of ambient relative humidity (RH_amb) before flowing through said first section (S₁);

- a second cooling step, concurrent to said circulation step, wherein said CO₂ is cooled from said intermediate temperature (T₂) up to a final temperature (T₃) at said second cooling section (S₂) by the effect of a thermal exchange with said second air flow (A₂).
A process according to claim 1, wherein said saturation step is carried out when said ambient temperature T_amb is higher than a threshold temperature comprised in the range between 15°C and 20°C.
A process according to claim 1 or 2, wherein during said saturation step, a water flow rate (m_H2O) is atomised in said second air flow (A₂) through atomisation means (40) in countercurrent relative to the latter, in order to humidify said second air flow (A₂) up to saturation before it flows through said second cooling section (S₂).
A process according to claim 3 comprising a regulation step of said water flow rate (m_H2O), preceding said saturation step, wherein once the air flow rate (m_air) of said second air flow rate (A₂) has been determined, the value of said water flow rate (m_H2O) is adjusted based on the values of said ambient temperature (T_amb) and of said ambient relative humidity (RH_amb) in order to deliver to said atomisation means (40) the water flow rate sufficient for saturating all said air flow rate (m_air).
A process according to claim 3 comprising a softening step of said water flow rate (m_H2O)_' preceding said saturation step, wherein said water flow rate (m_H2O) is subject to a softening treatment for reducing the contents of calcium carbonate dissolved therein.
A finned battery heat exchanger for carrying out the process according to any one of the previous claims, comprising:
- a plurality of circuits (10, 20) that form a first cooling section (S₁) and a second cooling section (S₂), defining as a whole a total flow section (SF_tot) for said CO₂;

- ventilation means (30) capable of sucking air from the environment for generating at least a first air flow (A₁) through said first cooling section (S₁) and at least a second air flow (A₂) through said second cooling section (S₂);

- humidification means (40) capable of atomising a water flow rate (m_H2O) into said second air flow (A₂) sufficient for humidifying it at least up to saturation conditions before it flows through said second cooling section (S₂); said first cooling section (S₁) being defined by a first group of said circuits (10) which are connected to one another in parallel and define a first partial flow section (SF₁) for said CO₂; said second cooling section (S₂) being defined by a second group of said circuits (20), corresponding to the remaining part of said circuits, which, connected in parallel to one another and in series to the circuits (10) of said first group, define a second partial flow section (SF₂) for said CO₂ having an extension comprised between 20% and 40% of said total flow section (SF_tot).
A heat exchanger according to claim 6, comprising:
- an inlet header (C_E) arranged upstream of said plurality of circuits (10, 20) relative to the flow direction of said CO₂;

- an outlet header (C_U) arranged downstream of said plurality of circuits (10, 20);
the circuits (10) of said first group branching off from said inlet header (C_E) for leading into a first intermediate header (C_I1) and the circuits (20) of said second group branching off from a second intermediate header (C_I2), connected in series to said first intermediate header (C_I1), for leading into said outlet header (C_U).
A heat exchanger according to claim 6 or 7, wherein said humidification means (40) comprise:
- one or more atomiser nozzles (41) which are arranged upstream of said second cooling section (S₂) relative to the moving direction of said second air flow (A₂) and are suitable for atomising said water flow rate (m_H2O) in said second air flow (A₂);

- at least one water feeding circuit (42) connected to said atomiser nozzles (41);

- regulation means (43) of said water flow rate (m_H2O) which are inserted in said feeding circuit (50) upstream of said nozzles (41) for varying said water flow rate (m_H2O) and are actuated by a controller (44), which determines the value of said water flow rate (m_H2O) sufficient for saturating all the air flow rate (m_air) of said second flow (A₂) based on the values of said ambient temperature (T_amb) and of said ambient relative humidity (RH_amb).
A heat exchanger according to claim 8 comprising softening means (50), which are inserted in said feeding circuit (42) upstream of said atomiser nozzles (41) for reducing the contents of calcium carbonate present in said water flow rate (m_H2O).
A heat exchanger according to claim 9, wherein said softening means (50) comprise at least one cartridge softening filter with ion exchange resins.
A process for cooling CO₂ in a refrigerator system comprising the following operating steps:
- a circulation step of CO₂ into the circuits (10, 20) of at least one finned battery heat exchanger (1) which define as a whole a total flow section (SF_tot) for said CO₂, in said circulation step said CO₂ being made to circulate at first through a first cooling section (S₁) of said exchanger (1) consisting of a first group of said circuits (10), which are connected to one another in parallel and as a whole define a first partial flow section (SF₁) for said CO₂, and afterwards through a second cooling section (S₂) consisting of a second group of said circuits (20) corresponding to the remaining part of the circuits themselves, which are also connected to one another in parallel and as a whole define a second partial flow section (SF₂) having an extension comprised between 20% and 40% of said total flow section (SF_tot);

- a ventilation step, concurrent to said circulation step, wherein a first air flow (A₁) is made to pass through said first cooling section (S₁) for impinging the first group of said circuits (10) and wherein a second air flow (A₂) is made to pass through said second cooling section (S₂) for impinging the second group of said circuits (20) ;

- an adiabatic saturation step, concurrent to said ventilation step, wherein said second air flow (A₂), starting from ambient temperature (T_amb) and ambient relative humidity (RH_amb) conditions, is humidified at least up to adiabatic saturation conditions and then cooled up to the corresponding adiabatic saturation temperature (T_sat) before passing through said second cooling section (S₂) ;

- a first cooling step, concurrent to said circulation step, wherein said CO₂ is cooled starting from an initial temperature (T₁) up to an intermediate temperature (T₂) at said first cooling section (S₁) by the effect of a thermal exchange with said first air flow (A₁), said first air flow (A₁) being in the conditions of ambient temperature (T_amb) and of ambient relative humidity (RH_amb) before flowing through said first section (S₁);

- a second cooling step, concurrent to said circulation step, wherein said CO₂ is cooled from said intermediate temperature (T₂) up to a final temperature (T₃) at said second cooling section (S₂) by the effect of a thermal exchange with said second air flow (A₂), said second air flow (A₂) being in saturation conditions, that is, at said adiabatic saturation temperature (T_sat), before flowing through said second section (S₂).