WO2015000044A1

WO2015000044A1 - A cooling-fluid sub-cooler

Info

Publication number: WO2015000044A1
Application number: PCT/BR2014/000187
Authority: WO
Inventors: Edson Rocha; Paulo Roberto BITTAR
Original assignee: THERMOSAVE S/A
Current assignee: THERMOSAVE S/A
Priority date: 2013-07-01
Filing date: 2014-06-06
Publication date: 2015-01-08
Anticipated expiration: 2016-01-01
Also published as: BR102013017026A2

Abstract

The present invention relates to a cooling-fluid (10) sub-cooler (1) used in a refrigeration cycle, this sub-cooler (1) operating in the refrigeration cycle by making use of nucleate boiling. The sub-cooler is constituted by a pressure vessel (2), at least one pair of support assemblies (3, 4), respectively positioned on at least one pair of fixing assemblies (5, 6) and arranged with spacing along the inner of the pressure vessel (2), each support assemblies (3, 4) being provided with a plurality of bores (12), and a plurality of capillary tubes (7), each of these capillary tubes (7) being associated to a bore of each of the support assemblies (3, 4), these capillary tubes (7) being twisted so as to exhibit spiral streae inside them, and the inner (8) and outer (9) surfaces of each capillary tube (7) being provided with a layer having porous surface. This sub-cooler (1) enables a quite effective sub-cooling of the cooling fluid (10) used in a refrigeration cycle by virtue of the flashing of the cooling fluid (10), which represents an improvement in the coefficient of performance (COP) and, as a result, a reduction in the consumption of electric energy of the compressor motors (105), which makes the system more efficient as a whole.

Description

Specification of the Patent Application for: "A COOLING-FLUID SUB- COOLER"

The present invention relates to a sub-cooler for a cooling fluid, used in a refrigeration cycle, capable of promoting, in its internal geometry, the phenomenon of nucleate boiling, thus achieving a superior transfer of heat.

Description of the prior art

There are many types of mechanical refrigeration systems. Such systems may exhibit a wide variety of shapes, sizes, arrangement of the components and uses. However, the majority of these systems provide refrigeration through a common principle of operation, called standard compression refrigeration cycle.

In a standard compression refrigeration cycle, a specific cooling fluid travels a refrigeration circuit, where it is refrigerated. Afterwards, the low temperature cooling fluid exchanges heat with the mean that is supposed to be refrigerated. There are basically four main components that are part of the standard compression refrigeration cycle through which the cooling fluid acts: namely a compressor device, a heat exchanger used as a condenser, a heat exchanger used as an evaporator and a device that promotes expansion of this cooling fluid.

One can describe the circulation of the cooling fluid within the standard refrigeration cycle as follows:

Starting from the compressor entrance, the cooling fluid is sucked by the compressor in the form of saturated vapor. The compressor raises its pressure and temperature by a mechanical compression process.

In the condenser, the cooling fluid in the form of superheated vapor loses heat under a constant pressure, lowering its temperature by condensation.

After going through the condenser, the cooling fluid goes through the expansion device and suffers an abrupt depressurization, changing its state from liquid to gaseous and having its temperature lower again. Finally, after the expansion device, the cooling fluid goes through the evaporator, where it indirectly contacts with the mean that is supposed to be refrigerated. Therefore, said mean loses heat to the cooling fluid, while the latter evaporates again and follows to the compressor, restarting the cycle. The concept of sub-cooling after condensation of the cooling fluid, with a view to achieve greater energy efficiency in a refrigeration cycle, is also widely known in the cooling area.

Sub-cooling is the condition in which the cooling fluid is cooler than the saturation temperature, a temperature above which this fluid begins to exhibit a fraction in the gaseous state. The amount of sub-cooling in a given condition is the difference between the saturation temperature and the reai temperature of the cooling fluid.

These concepts can be verified in old patents, such as GB 482,21 1 , EP 38,442, among others, wherein all of these mention directly sub- cooling equipment used in refrigeration cycles. An ordinary sub-cooler consists of a heat exchanger positioned at the outlet of a 1 -cycle cooling condenser. In a sub-cooler the cooling fluid used in a refrigeration system will undergo loss of heat, assuming the sub-cooled liquid state, which is suitable for its subsequent expansion. This sub-cooling process usually has, as a result, a reduction in the discharge pressure of the compressor, which finally represents an increase in the energy efficiency of the system as a whole.

From the literature, one can observe that a disadvantage referring to the sub-cooling conventionally applied in refrigeration cycles is characterized by achieving a reduction of the temperature of the fluid of about 5°C below the saturation temperature of the fluid. This represents a gain in efficiency that is little significant when evaluated in terms of energy efficiency.

In a sub-cooler, the heat transfer rate also represents an important aspect for achieving greater energy efficiency, since higher heat transfer results in lower temperatures achieved at the outlet of the sub- cooler. Thus, the heat transfer rate should be as high as possible.

There are a number of phenomena known from the literature, which are capable of providing a significant improvement in heat transfer between two different parts, chiefly as far as transfer of heat by convection is concerned.

In convection, these phenomena are known for being related to the pouring characteristics (which are directly linked to the non-dimensional Reynolds number) . In this case, it is known from any basic heat-transfer literature that the presence of turbulence (associated to a high Reynolds number) is responsible for an increase in heat transfer in a large number of geometries and flow patterns.

Besides, other phenomena that influence directly the behavior of heat transfer by convection between two parts are associated to the characteristics of the fluid employed (linked to the Prandtl number) to the flow dimensions (diameters, areas, perimeters and lengths related to the flow), to the temperature of the parts involved in the transfer of heat and to the possible changes in phase of the liquid in question.

One of these phenomena consists in achieving nucleate boiling of a liquid from the nucleation points, forming bubbles. The heat transfer coefficients obtained with this practice are high, but the correlations used to foresee the transfer of heat carried out by this phenomenon are still imprecise.

This phenomenon can be found in detail in documents such as patent US 4,663,243, "Flame-sprayed Ferrous Alloy Enhanced Boiling Surface", in which one lists concepts of nucleate boiling and surface porosity, achieved specifically with the technique known as "Flame Spray", capable of producing surfaces where nucleate boiling takes place.

The prior art does not show any devices used as sub-coolers in refrigeration cycles using nucleate boiling inside them. However, this can be observed in other pieces of equipment used in the refrigeration cycle, as shown in US 7,093,647 "Ebulition Cooling Device for Heat Generating Component", which shows an evaporative unit in which surface irregularities are present, with a view to achieve nucleate boiling of the cooling liquid and, as a result, an improvement in the heat exchange of the device. It is relevant to mention also that a heat exchanger may be used in other positions in the refrigeration cycle for the same purpose of promoting an increase in energy efficiency of the system. Document PI 0604893- 7,"Coversor Termico de Alta Performance para Sistemas Termodinamicos" (High-performance Thermal Converter for Thermodynamic Systems) presents an example of a heat exchanger used for this purpose.

In this example, the cooling fluid of a refrigeration cycle passes through a heat exchanger upon coming out of a compressor under high pressure. This heat exchanger make use of the cooling fluid itself to reduce the pressure and lower the temperature of the fluid discharged from the compressor, so that it is expected that, at the end of the process, the discharge pressure of the compressor will diminish. However, it is possible to observe that this system promotes a change in the physical state of the cooling fluid from vapor to liquid, thus failing to achieve any advantage from the nucleate boiling promoted by a plurality of porous capillary tubes, since what one seeks is condensation of the cooling fluid, not the boiling thereof. Moreover, this document does not specify, in its specification, the way in which one achieves said performance.

Thus, there is still no solution, in the prior art, capable of providing a sub-cooling that makes use of nucleate boiling or manages to guarantee an increase in performance of a refrigeration cycle on the order of 20%.

Objectives of the Invention

A first objective of the present invention is to provide a device that enables quite effective sub-cooling of the cooling fluid used in a refrigeration cycle.

Thus, the present invention aims at making use of natural phenomena that enable high coefficients of heat transfer by convection, such as nucleate boiling and flow of the cooling fluid by means of a turbulent flow, so as to obtain a high heat exchange.

A second objective of the present invention, resulting from the first objective, consists in diminishing the work carried out by the compressor used in the cooling cycle in question, resulting from the flashing of the cooling fluid, which causes a reduction of the discharge pressure of the compressor of up to 20%, subsequent to the cooling of the cooling fluid effected by the present invention.

This decrease results in an improvement of the coefficient of performance (COP) of the system, which represents a reduction in the consumption of electric energy of the compressor motors and makes the system more efficient as a whole.

Brief description of the invention

The objectives of present invention are achieved by means of a sub-cooler of a cooling fluid used in a refrigeration cycle (as for instance, refrigerators, air-conditioners, direct or indirect coolers and similar equipment), this sub-cooler operating in a refrigeration cycle that makes use of nucleate boiling.

This sub-cooler exhibits an internal geometry and materials capable of promoting nucleate boiling of the cooling fluid, since it is built in, in a preferred embodiment, by a pressure vessel and by a plurality of capillary tubes, each capillary tube having a surface composed surface, so as to have porosity capable of promoting the improvement of nucleate boiling of the cooling fluid and, later, the formation of cooling fluid in the form of flash vapor resulting from the passage of the cooling fluid through the end of these capillary tubes.

Brief description of the drawings

The present invention will now be described in greater detail with reference to an example of embodiment represented in the drawings. The figures show:

Figure 1 is a sectional view representing the internal structure and the components of the sub-cooler;

Figure 2 is a detail view representing the set of capillary tubes inside the structure of the sub-cooler;

Figure 3 is a detail view representing a top view of the perforated metallic disc inserted into the support disc; Figure 4 is a detail view representing the connections existing between the perforated metallic disc, the support disc and the capillary tubes;

Figure 5 is a scheme representing the spiral streae existing inside one of the capillary tubes and the turbulent flow of cooling fluid inside it;

Figure 6 is a representation of a refrigeration cycle using R134a as a cooling fluid, in which the sub-cooler has been inserted; and

Figure 7 is a representation of a refrigeration cycle using ammonia as a cooling fluid, in which the sub-cooler has been inserted.

Detailed description of the figures

Figure 1 shows a preferred, but not restrictive embodiment of the complete assembly of devices mounted inside the pressure vessel 2, thus composing the sub-cooler 1.

The cooling fluid 10 (represented in figure 5) goes into the sub- cooler 1 from the top inlet. Then it is forced to pass through a set of capillary tubes 7, which are correspondingly mounted on a frame formed by at least one pair of support assemblies 3 and 4 (that is, one end of a capillary tube 7 is mounted in a bore in the support assembly 3 and its other end is mounted in a corresponding bore in the perforated support assembly 4). Each of these perforated support assemblies 3 and 4, in turn, is positioned on a fixing assembly 5 and 6, respectively, and then the support assemblies are arranged at spacing close to each of the opposite ends of the pressure vase 2.

In a possible preferred embodiment, the support assemblies 3 and 4 have a geometry in the form of a disc, since they have the function of associating to the pressure vessel 2, the shape of which is usually cylindrical, fixing the capillary tubes 7 in the adequate position, as well as sealing the compartments through which the cooling fluid 10 passes with the aid of the fixing assemblies 5 and 6, which also exhibit, in this embodiment, a geometry in the form of a disc.

This mounting can be observed more clearly in figures 2 and 3, which show images of the internal frame of the pressure vessel 2 exhibiting the capillary tubes 7, of the disc-shaped support assemblies 3 and 4 and of the fixing assemblies 5 and 6 seen from the side (figure 2) and from the front (figure 3), also exhibiting in greater detail the support assemblies 3 and 4. Then, in figure 4, one presents a side detail view, highlighting the details of the fixation of the support assemblies 3 and 4 on the fixing assemblies 5 and 6. As can be seen in figure 5, the cooling fluid 10 that goes through the capillary tubes 7 do not have direct contact with the cooling fluid 10' that cools it, by only exchanging heat. The heat exchange obtained within the capillary tubes 7 is extraordinarily high as a result of the large area of the inner surfaces 8 and outer surfaces 9 of the capillary tubes 7. Additionally, upon leaving the capillary tubes 7, the cooling fluid 10 expands and undergoes the effect known as flashing. This flashing consists in the instantaneous evaporation of the cooling fluid as a result of the abrupt drop in pressure, thus forming cooling fluid 10 in the form of flash vapor. This cooling fluid 10, upon leaving the sub-cooler 1 , can reach temperatures of about 40°C below the saturation temperature of the cooling fluid 10. This surface area is obtained by the process described hereinafter.

The capillary tubes 7 may be manufactured from a plurality of metallic materials (namely, any type of steel, aluminum, copper, tungsten, silver or alloys of other metallic materials), being preferably manufactured from 316 stainless steel for applications that use ammonia, or else from copper for applications that use chlorofluorocarbons, so as to guarantee the desired, but not restrictive efficiency. The lengths of these capillary tubes 7 may range from 10 mm to 3000 mm, preferably from 400 mm to 1000 mm, preferably being of 800 mm for vapor sub-coolers and of 400 mm for liquid sub-coolers, and their inner diameters may range from 0.2 to 10 mm, preferably from 0.5 to 0.8 mm, being preferably of 0.7 mm.

Then, these capillary tubes 7 are preferably covered by a metal- alloy bath composed of nickel and aluminum, coating the inner 8 and outer 9 surfaces of the capillary tubes 7, wherein the outer surface 9 is subjected to a glass-sphere jetting, which makes this surface extremely porous. After the jetting, both surfaces are subjected to a chemical treatment using alkaline solutions, such as sodium hydroxide, which increases the surface porosity and, consequently, the surface area of heat exchange of the capillary tubes 7. As a result, one obtains surfaces with an exceptionally high surface area and with a large number of bubble nucleating points. This renders the emergence of nucleate boiling on both surfaces of the capillary tubes 7, imparting to the assembly a high heat-transfer capacity, which is from 4 to 5 times as high as that achieved in the prior art.

One can see in figure 5 that the capillary tubes 7 are also twisted for the purpose of exhibiting streae inside them. Such streae, coupled to the roughness of the inner surface 8, promote a turbulent flow in the cooling fluid 10, which improves the heat exchange. It is also possible to use non-twisted capillary tubes, but with reduced efficiency.

In the case of large systems, like those that use ammonia, the cooling fluid 10 received from the condenser 101 that exchanges heat in the sub-cooler 1 comes from the separator 103. In smaller systems, which use CFC's, this fluid is previously cooled by using duly calibrated expansion valves 104.

Due to the existence of a wide diversity of embodiments applied to refrigeration systems on the market, since such systems are developed to meet the specific needs of each company, the embodiments of the sub- coolers change in order to conform to each specific system. The changes do not affect the sub-coolers in their structure or construction, but rather how and where they will be installed, as well as the manner in which the pieces of equipment will be interconnected.

In figure 7, one can see the representation of the application of more than one sub-cooler 1 in a system using ammonia. In this non- restrictive example, a first sub-cooler 1 receives liquid ammonia from a reservoir 106, as well as the chilled ammonia supplied by the separator 103. This chilled ammonia in liquid form exchanges heat with the ammonia that is at room temperature and comes from the reservoir 106, cooling it down to a temperature 15° C lower than that at the inlet. Once the ammonia is cooled, it no longer has vapor mixed with the liquid, which implies a larger mass of ammonia per volume unit. After the ammonia is cooled, it passes through the capillary tubes and, upon leaving them, it undergoes a large expansion, volatilizing because of the flashing and causing the temperature to drop by up to 30°C. Strongly cooled, the ammonia leaves the sub-cooler 1 and is led to the separator 103.

In the same way, a second sub-cooler 1 , identical (in constructive characteristics, but not necessarily in dimensions) to the first sub-cooler 1 , may be installed in the refrigeration system to receive the vapor coming from the reservoir 106. The separator 103 also supplies chilled liquid ammonia to this sub-cooier 1 , cooiing this ammonia vapor, making it liquid. Even if the phenomenon of nucleate boiling is not present in this application, ammonia in liquid state undergoes expansion when it passes through the capillary tubes, due to the loss of charge, causing the temperature of the liquid to drop even more and leading it to the separator 103.

The tubing that comes out of the condenser 101 brings the CFC as far as the sub-cooler 1 , causing it to pass through the cooling processes described before. As a result, at the outlet of the sub-cooler 1 , the liquid CFC is led to the evaporator 102. Residual amounts of CFC in the form of vapor are sent to the compressor 05 in the suction line.

Figure 6 does not show the tubing in which the CFC in the form of vapor is extracted from the outlet of the evaporator 102 and used for cooling the liquid CFC coming from the condenser 101. The other components of this figure comprise the vales 104 used for pre-cooling the fluid and for controlling the flow, and the other elements of a standard refrigeration cycle (compressor 105, evaporator 102, and condenser 101 ) that will not be described in greater detail in this document.

When making a comparison between the structure described in the present invention and the evaporative unit that exhibit nucleate boiling of document US 7,093,647, one can note that the two structures presented have irregularities on their surfaces, built for the purpose of achieving greater efficiency with regard to transfer of heat. However, it is important to point out that the present invention makes use of completely different geometry and process for obtaining porosity.

Besides, the sub-cooler 1 of the present invention is capable of achieving a significantly different effect by virtue of this transfer of heat, reducing by up to 20% the discharge pressure of the compressor 105 used in the refrigeration cycle in question and, as a result of this lower pressure, one achieves a less work performed by the compressor 105, which brings about an increase in the efficiency of the cycle as a whole.

In a preferred embodiment, a refrigeration cycle that operates initially with a discharge pressure of the compressor 105, preferably ranging from 8 to 15 kgf/cm2, and more usually 10 kgf/cm2.

At the same time, it is possible to state that the presence of nucleate boiling achieved through the surface porosity applied to a plurality of capillary tubes 7 also represents a significant difference between the present invention and the heat exchanger referred-to as "Conversor Termico" (Thermal Converter) in document PI0604892-7, since, even if the geometry of the two models is similar at the first sight, the association of this geometry with the achievement of nucleate boiling in a sub-cooling process using capillary tubes enables one to achieve a superior transfer of heat, the mechanisms of which are clearly defined in this patent.

It is also possible to state that the application of a "Conversor Termico" (Thermal Converter) in the position determined for the sub-cooler (1) would not be capable of achieving the objectives of this document, just as the contrary is true.

One should understand that the sub-cooler 1 described above is nothing more than a preferred embodiment of the present invention, the comprehension of which is defined by the accompanying claims.

Claims

1. A cooling-fluid sub-cooler (10) used in a refrigeration cycle, characterized in that it operates in the refrigeration cycle by making use of nucleate boiling.

2. The sub-cooler according to claim 1 , characterized in that it is provided with an internal geometry comprising a porous surface, this surface providing the emergence of nucleate boiling in the cooling fluid (10) that passes through the sub-cooler (1 ).

3. The sub-cooler according to claim 1 , characterized in that the porous inner surface of the sub-cooler (1) is composed by a conjunction of two or more materials, moided so as to exhibit this porosity.

4. The sub-cooler according to claim 1 , characterized by being arranged between a condenser (101 ) and an evaporator (102) of the refrigeration cycle.

5. The sub-cooler according to claim 1 , characterized by being arranged between a condenser (101 ) and a separator (103) of the refrigeration cycle.

6. The sub-cooler according to claim 1 , characterized by comprising:

a pressure vessel (2);

at least one pair of support assemblies (3,4), respectively positioned on at least one pair of fixing assemblies (5, 6) and arranged with spacing along the inside of the pressure vessel (2); each support assembly (3, 4) being provided with a plurality of bores (12);

- a plurality of capillary tubes (7), each of the capillary tubes (7) being associated to a bore an each of the support assemblies (3,4), and the inner (8) and outer (9) surfaces of each capillary tube (7) being provided with a layer having a porous surface of a nicke!-and-aluminum alloy.

7. The sub-cooler according to claim 6, characterized in that the capillary tubes (7) are twisted so as to exhibit spiral streae inside them.

8. The sub-cooler according to claim 6, characterized in that the pressure vessel (2) comprises at least one pair of compartments delimited by the support assemblies (3, 4) and by the fixing assemblies (5, 6), filled by the cooling fluid (10).

9. The sub-cooler according to claim 6, characterized in that the support assemblies (3, 4) and the fixing assemblies (5, 6) are disc-shaped.

10. The sub-cooler according to claim 6, characterized in that the inner (8) and outer (9) surfaces of each of the capillary tubes (7) are subjected to a chemical treatment using alkaline solutions.

11. The sub-cooler according to claim 4, characterized in that the outer surface (9) of each of the capillary tubes (7) is jetted with glass spheres during its manufacture.

12. The sub-cooler according to claim 1 , characterized in that the cooling fluid (10) used in the refrigeration cycle is ammonia.

13. The sub-cooler according to claims 5 and 12, characterized in that the sub-cooling of the cooling fluid (10) is carried out by the cooling fluid itself ( 0) coming from the liquid separator (103) of one refrigeration cycle.

14. The sub-cooler according to claim 6, characterized in that the support assemblies (3, 4), the fixing assemblies (5, 6) and the capillary tubes (7) are constituted by a metallic material.

15. The sub-cooler according to claim 14, characterized in that the metallic material is either 316 stainless steel or copper.

16. The sub-cooler according to claim 1 , characterized in that the cooling fluid (10) used in the refrigeration cycle belongs to the category of chlorofluorocarbons.

17. The sub-cooler according to claims 4 and 16, characterized in that the sub-cooling of the cooling fluid (10) is carried out by the cooling-fluid

(10) itself, cooled by expansion valves (104).

18. The sub-cooler according to claim 6, characterized in that the length of the capillary tubes (7) ranges from 10 mm to 3000 mm, and the inner diameter of the capillary tubes (7) ranges from 0.2 mm to 10 mm.

19. The sub-cooler according to claim 1 , characterized by promoting a reduction of up to 20% in the discharge pressure of a compressor (105).

20. The sub-cooler according to claim 19, characterized in that the discharge pressure of the compressor (105) is substantially of 10 kgf/cm2.

21. The sub-cooler according to claim 1 or 6, characterized in that the cooling fluid (10) undergoes flashing at the outlet of the capillary tubes (7).

22. A cooling-fluid (10) sub-cooler used in a refrigeration cycle, characterized by comprising a pressure vessel (2), provided with internal geometry that comprises a plurality of capillary tubes (7), each capillary tube (7) exhibiting a surface composed so as to exhibit porosity capable of bringing about the emergence of nucleate boiling of the cooling fluid (10) and, later, the formation of cooling fluid (10) ion the form of vapor resulting from the passage of the cooling fluid (10) through these capillary tubes (7).

23. The sub-cooler according to claim 22, characterized in that the length of the capillary tubes (7) ranges from 10 mm to 300 mm and the inner diameter of the capillary tubes (7) ranges from 0.2 mm to 10 mm.

24. The sub-cooler according to claim 22, characterized in that the cooling fluid (10) undergoes flashing at the outlet of the capillary tubes (7).