US20080184734A1

US20080184734A1 - Flat Tube Single Serpentine Co2 Heat Exchanger

Info

Publication number: US20080184734A1
Application number: US11/908,754
Authority: US
Inventors: Parmesh Verma; Tobias H. Sienel; Hans-Joachim Huff; Yu Chen
Original assignee: Carrier Comercial Refrigeration Inc
Current assignee: Taylor Commercial FoodService LLC
Priority date: 2005-03-18
Filing date: 2005-12-30
Publication date: 2008-08-07
Also published as: EP1864060A4; CN101142452A; JP2008533427A; WO2006101565A1; EP1864060A1

Abstract

A refrigeration system includes 0 compressor for driving 0 refrigerant along flow path in at least a first mode of system; a first heat exchanger along the flow path downstream of the compressor in the first mode; a second heat exchanger along the flow path upstream the compressor in the first mode; and a pressure regulator or expansion device in the flow path downstream of the first heat exchanger and upstream of the second heat exchanger in the first mode, wherein at least one of the first heat exchanger and the second heat exchanger comprises a flat tube heat exchanger.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This applications claims the benefit of the filing date of earlier filed provisional application Ser. No. 60/663,957 filed Mar. 18, 2005. Further, copending application docket 05-258-WO, entitled HIGH SIDE PRESSURE REGULATION FOR TRANSCRITICAL VAPOR COMPRESSION SYSTEM and filed on even date herewith, and the aforesaid provisional application Ser. No. 60/663,962, disclose prior art and inventive cooler systems. The disclosure of said applications is incorporated by reference herein as if set forth at length.

BACKGROUND OF THE INVENTION

The invention relates to the design of the tube of a heat exchanger and, more particularly, to the design of a heat exchanger for use in applications where space for the heat exchanger is in short supply, and/or in connection with transcritical vapor compression systems.
Heat exchange efficiency is a concern in connection with heat exchangers, for example heat exchangers used in various refrigeration and other air handling applications. Various types of heat exchangers have been provided, including tubes having fins and the like. The need remains, however, for a heat exchanger configuration which provides excellent heat exchange efficiency while occupying a relatively small space.
It is the primary object of the present invention to provide a heat exchanger meeting this need.
Other objects and advantages of the present invention will appear herein.

SUMMARY OF THE INVENTION

According to the invention, the foregoing objects and advantages have been attained.
According to the invention, a refrigeration system is provided which includes a compressor for driving a refrigerant along a flow path in at least a first mode of system operation; a first heat exchanger along the flow path downstream of the compressor in the first mode; a second heat exchanger along the flow path upstream of the compressor in the first mode; and a pressure regulator or expansion device in the flow path downstream of the first heat exchanger and upstream of the second heat exchanger in the first mode, wherein at least one of the first heat exchanger and the second heat exchanger comprises a flat tube heat exchanger.
The flat tube heat exchanger is preferably a heat exchanger defined by a serpentine bending of a single flat tube heat exchanger. Further, the flat tube heat exchanger itself advantageously comprises a conduit for carrying refrigerant, wherein the conduit has a height or minor dimension, and a width or major dimension, and wherein the heat exchanger is arranged with the short dimension facing into the flow of heat exchange medium such as air.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a vapor compression system;

FIG. 2 is an illustration of a cassette with a single serpentine flat tube heat exchanger according to the invention;

FIG. 3 is a schematic illustration of CO₂vapor compression system cassette;

FIGS. 4 and 5 are schematic illustrations a flat tube single serpentine single row heat exchanger according to the invention;

FIG. 6 illustrates a flat tube two row counterflow serpentine heat exchanger according to the invention;

FIGS. 7 and 8 are illustrations of a flat tube single serpentine evaporator according to the invention; and

FIGS. 9( a) and 9(b) are schematic cross sections of embodiments of the flat tube heat exchanger of the present invention.

DETAILED DESCRIPTION

The invention relates to vapor compression systems and, more particularly, to a heat exchanger tube configuration for such systems, particularly for transcritical vapor compression systems such as those operated with CO₂.
For transcritical CO₂refrigeration systems, to maintain peak efficiencies it is critical to minimize the temperature difference between the hot and cold fluid at the exit of the high-side (gas cooler) heat exchanger. Due to higher densities of CO₂in comparison with conventional HFC, for a given temperature, pressure, and mass flux, the refrigerant-side heat transfer coefficients and pressure drop for CO₂are smaller. Thus for CO₂heat exchangers it is imperative to have higher refrigerant mass fluxes which will increase not only CO₂heat transfer coefficients but also CO₂pressure drop. However, the net effect should be to increase overall heat exchanger effectiveness while limiting the CO₂pressure drop below a certain limit such that higher cycle efficiency can be attained.
For some applications, such as bottle or beverage coolers and other refrigeration applications, due to air-side fouling constraints, additional air-side surface area in the form of fins is limited thus limiting the total surface area. This necessitates a heat exchanger to have reduced air-side blockage and significantly reduced resistance on the refrigerant-side i.e. CO₂.
Additionally, in the case of an evaporator, the heat exchanger should have uniform refrigerant distribution and satisfactory condensate drainage in order to improve overall heat exchanger effectiveness and reliable operation of the compressor.
The heat exchanger combines the benefits of flat surfaces, single/multiple ports, single serpentine, multiple rows, counterflow, cross-counterflow, high heat transfer coefficients, low cost, suitable materials, corrosion resistant, high burst strength, ease of manufacturing, and reduced air blockage which helps to achieve size, efficiency, cost and reliability constraints of a CO₂bottle cooler refrigeration system.
One of the ways to increase overall heat exchanger effectiveness is to have a flat tube heat exchanger.
FIG. 1 shows a refrigeration system 10 having a compressor 12, a heat rejection heat exchanger 14 which in a normal mode of operation, as illustrated by the arrows in FIG. 1, is a downstream heat exchanger when considered with respect to compressor 12, an expansion device 16 which is positioned downstream of heat exchanger 14, and a heat absorption heat exchanger 18 which is downstream of expansion device 16. Flow in system 10 from heat exchanger 18 returns back to compressor 12.
FIG. 1 shows a first flow 20 of heat exchange medium, for example, air, which is driven across heat exchanger 14 by a fan 22. This flow serves to take the heat rejected by refrigerant passing through heat exchanger 14.
FIG. 1 also shows another flow 24 of air which is driven by a fan 26 past heat exchanger 18. Flow 24 represents a portion of air in the treated space, and this air is cooled by refrigerant passing through heat exchanger 18.
As set forth above, in accordance with the present invention, an improved heat exchanger tube configuration is provided which is particularly useful in vapor compression systems which use a transcritical refrigerant, for example, CO₂.
FIG. 2 shows a portion of a transcritical vapor compression system 28 and shows compressor 12, heat exchanger 14 and heat exchanger 18 in positions they occupy in this particular configuration.
FIG. 3 is a side schematic of a similar structure, and also shows compressor 12, heat exchanger 14 and heat exchanger 18.
As will be set forth below, the flat tube heat exchanger in accordance with the present invention provides enhanced function per space occupied by the heat exchanger tubes, and can therefore be utilized to allow the heat exchanger to take up less space, thereby freeing up such space for use in other capacities. For example, it should readily apparent from a consideration of FIGS. 2 and 3 that the flat tube heat exchanger of the present invention, when implemented as heat exchanger 14, allows for a single heat exchanger to be used as shown in FIG. 2 as compared to several different components of the heat exchanger as shown in FIG. 3.
FIGS. 4-8 illustrate various embodiments of the present invention in connection with a flat tube heat exchanger as described.
FIG. 4 shows a schematic illustration of flat tube 30 which is formed into a substantially serpentine structure as illustrated, and which is positioned to receive air flow 32 such that air flows along the long dimension of flat tube 30 and thereby improves heat exchange efficiency between the medium of air flow 32 and the refrigerant within flat tube 30.
FIG. 4 further shows fins 34 which can advantageously be positioned extending from flat tubes 30 or otherwise connected with a flat tube heat exchanger according to the invention, so as to extend further across air flow 32 and enhance heat exchange capacity between the medium and the refrigerant.
FIGS. 5 and 6 further illustrate a flat tube heat exchanger in accordance with present invention. FIG. 5 shows a single row flat tube heat exchanger configured into a substantially serpentine configuration or structure 36 such that the single flat tube defining the structure is bent in a serpentine manner to define a plurality of portions 37 or flow paths, which are substantially parallel to each other. In the embodiment shown in FIG. 5, as in the embodiment of FIG. 4, there is a single refrigerant inlet 38 and a single refrigerant outlet 40 to handle flow of refrigerant through structure 36. Structure 36 is advantageously positioned to interact with a flow 39 of air as shown, preferably with the narrow edge of flat tube 30 facing into flow 39, and with the longer dimension or width of flat tube 30 being substantially parallel to flow 39.
As shown, the flat tube heat exchanger in accordance with the present invention is defined by a refrigerant conduit which has a substantially rectangular outer shape and which has an internally-defined flow passage for carrying refrigerant.
The outside shape of flat tube 46, also referring to FIGS. 9 a and b, is substantially rectangular. This substantially rectangular outer shape has a relatively shorter side 47 and a relatively longer side 49, and the substantially shorter side 47 has a length which is preferably between about 0.45 and about 4 mm. The relatively longer side 49 or dimension of the rectangular out shape preferably has a length or dimension which is between about 12.7 and about 101.6 mm.
With reference also to FIGS. 7 and 8, it should be readily apparent that the substantially serpentine structure 36 of flat tube heat exchanger in accordance with the present invention defines a series of substantially parallel flow paths. In accordance with the present invention, it has been found that a tube pitch 50, or distance between the tubes of the substantially parallel flow paths, as measured from transverse center to center, is preferably between about 5 and about 50 mm. As shown in FIGS. 4 and 7, it is preferred to position fins, where possible, to further enhance heat exchange as desired. When possible to include fins, it is preferred that such fins be positioned in a fin density of up to about 20 fins/inch.
From a consideration of FIG. 6, it should also be readily apparent that serpentine substantially flat tube heat exchanger structures 36 can be provided in a plurality of rows 42, 44 so as to treat a flow 39 of air through a first row 42 and then through second row 44. In this regard, it is preferable in accordance with the present invention to position such rows with a row pitch 52, or distance from the center of each row to the center of a next row, of between about 12.7 and about 50 mm. In accordance with the present invention, heat exchangers can be provided with up to at least about 20 rows, and will provide excellent results in accordance with the present invention.
As set forth above, the internal flow path for refrigerant defined within a flat tube heat exchanger can have a variety of different shapes. FIGS. 9 a and 9 b illustrate two embodiments. In the embodiment of FIG. 9 a, a cross section through a flat tube 46 in accordance with the present invention is shown wherein the entire internal space of flat tube 46 is provided for flow of refrigerant.
FIG. 9 b, on the other hand, shows flat tube 46 having flow passages defined as a series of substantially circular flow paths 48, in this case five (5) circular flow paths, which extend in substantially parallel relationship along the length of flat tube 46.
Such a heat exchanger, as described above, provides higher overall heat transfer coefficients owing to higher refrigerant mass fluxes (hence higher CO₂heat transfer coefficient) and lower air-side pressure drop due to reduced air-side blockage by a flat tube compared to a conventional round tube. Based on the flow cross-sectional area of the flat tube, the overall length of the flat tube could be designed such that the CO₂pressure drop is below an acceptable limit enabling higher cycle efficiencies for the range of operating conditions.
Additionally, for operation as an evaporator, the use of a single circuit (one inlet and one outlet) could eliminate CO₂maldistribution, which is inherently present in the case of a heat exchanger (conventional or flat tube) with multiple inlets and outlets. Moreover the heat exchanger orientation should be such that the tubes lie in the vertical plane as shown in FIG. 4 and the fins can be provided with slots/louvers. This would ensure satisfactory condensate drainage from flat tube and fin surfaces, which would otherwise be a limiting factor for use of a flat-tube heat exchanger as an evaporator.
The flat tube heat exchanger could have one or multiple ports. Multiple ports help to withstand high operating pressures, an inherent characteristic of a transcritical CO₂vapor compression refrigeration system, and reduces CO₂pressure drop which in turn helps to improve thermal performance.
The flat tube (single or multi ports) could be easily made out of Copper or Aluminum or other suitable material that can withstand high burst pressures of transcritical CO₂refrigeration system and could be bent and/or brazed at one or both ends to form one continuous serpentine heat exchanger as shown in the drawings. The fins could be connected mechanically or brazed to the flat surface of the tubes. Moreover the tube and/or fin material may be treated (coating, heat etc.) to increase corrosion resistance of such heat exchangers.
In another design, multiple rows of these flat tube single serpentine heat exchangers could be interconnected while maintaining single circuiting and with flow going from one row to another such that it closely resembles counter-flow arrangement (between air and CO₂) which is well known for high efficiency. Counter-flow arrangement is very critical for CO₂gas coolers for which the temperature gradient between the hot and cold fluids must be a minimum to maintain peak cycle efficiency.
Such a heat exchanger would be very useful for CO₂bottle cooler applications wherein the design of the heat exchanger is highly constrained by space and cost limitations and existing round-tube plate fin heat exchangers cannot provide a feasible solution.
This invention is especially beneficial for compact commercial refrigeration systems such as bottle coolers etc.
Existing heat exchangers for vapor compression systems are typically round-tube plate-fin heat exchangers with tube diameters of 1-7 mm or larger. For transcritical CO₂vapor compression systems such tubes provide low efficiency due to higher density of CO₂compared with conventional HFC refrigerants like R134a, R404a etc. Flat tube (multiple ports) heat exchangers with flow cross-sectional area much smaller than typical round tube heat exchangers are well known to reduce air-side flow resistance and improve heat transfer coefficients on the refrigerant side. However, use of such heat exchangers is limited by major technical challenges like maldistribution of refrigerant, poor condensate drainage, reduced burst strength, cross-flow arrangement, and high cost and complexity of fabrication including, but not limited to, expensive brazing of multiple tubes connected to manifolds at either ends, brazing of fins to tube surfaces, and low thermal conductivity material of the tubes for ease of brazing etc.
Such a heat exchanger is not found to exist for transcritical CO₂bottle cooler systems wherein minimization of the temperature gradient between hot and cold fluids at the exit of the gas cooler (high-side heat exchanger) is highly critical to maintain peak-efficiency.
For a bottle cooler evaporator, the single serpentine vertical tube configuration eliminates major technical challenges like maldistribution and condensate drainage which otherwise would limit use of such heat exchangers as evaporators, while still maintaining high CO₂heat transfer coefficients and reduced CO₂pressure drop.
The heat exchanger combines benefits of flat surfaces, single or multiple ports, single serpentine, multiple rows, high heat transfer coefficients, low cost, suitable materials, corrosion resistance, high burst strength, ease of manufacturing, and reduced air blockage which helps to achieve size, efficiency, cost and reliability constraints of a bottle cooler refrigeration system.
The compact characteristic of the flat surface heat exchanger can allow changes to the physical location of the heat exchanger inside the beverage cooler such that the entire footprint of the beverage cooler can be reduced while maintaining or increasing the system efficiency. For example, the high-side heat exchanger can be moved to other locations, thereby creating additional space which can be utilized for other purposes such as air management for the hot or cold surfaces etc.
The compact characteristics of such a heat exchanger would also result in reduction in the amount of refrigerant or charge within the refrigeration system and therefore reduce cost which is highly constrained for bottle cooler applications.
As set forth above, range of different specifications of such flat tube heat exchangers could be, tube width or major dimension from 12.7 mm to 101.6 mm; tube height or minor dimension from 0.5 mm to 4 mm; single or multi-port, circular or non-circular ports, flow hydraulic diameter from 0.1 mm to 3 mm; tube pitch or transverse center to center distance between tubes from 5 mm to 50 mm; row pitch or longitudinal center to center distance between tubes from 12.7 mm to 50 mm; fin density from 0 to 20 fins per inch, single or multiple rows up to 20.
A preferred embodiment proposed for the high-side heat exchanger or gas cooler for bottle or beverage cooler applications would have, 25.4 mm tube width, 2.0 mm tube height, 12 circular ports, 11.0 mm port hydraulic diameter, 12.7 mm tube pitch, 4 fins per inch, single row heat exchanger.
One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, when implemented as a remanufacturing of an existing system or reengineering of an existing system configuration, details of the existing configuration may influence details of the implementation. Accordingly, other embodiments are within the scope of the following claims.

Claims

1. A refrigeration system comprising:

a compressor for driving a refrigerant along a flow path in at least a first mode of system operation;

a first heat exchanger along the flow path downstream of the compressor in the first mode;

a second heat exchanger along the flow path upstream of the compressor in the first mode; and

a pressure regulator or expansion device in the flow path downstream of the first heat exchanger and upstream of the second heat exchanger in the first mode, wherein at least one of the first heat exchanger and the second heat exchanger comprises a flat tube heat exchanger.

2. The system of claim 1 wherein the flat tube heat exchanger comprises a conduit having a substantially rectangular outside shape and at least one internal flow port for carrying refrigerant flow.

3. The system of claim 2 wherein the rectangular outside shape has a short side and a long side, and wherein the flow paths are arranged with the short side facing a flow of heat exchange fluid.

4. The system of claim 3, wherein the short side has a length of between about 0.5 and about 4.0 mm, and the long side has a length of between about 12.7 and about 101.6 mm.

5. The system of claim 2, wherein the conduit has a flow hydraulic diameter of between about 0.1 and about 3.0 mm.

6. The system of claim 1 wherein the flat tube heat exchanger comprises a flat tube in a serpentine configuration defining a plurality of substantially parallel flow paths.

7. The system of claim 6 wherein the flat tube heat exchanger is positioned having the substantially parallel flow paths arranged in a substantially vertical plane.

8. The system of claim 7, wherein the fins are present in a fin density of up to about 20 fins per inch.

9. The system of claim 6, wherein the serpentine configuration has a tube pitch of between about 5 and about 50 mm.

10. The system of claim 1, wherein the first heat exchanger comprises a plurality of rows of flat tube heat exchanger, and wherein the rows have a row pitch of between about 12.7 and about 50 mm.

11. The system of claim 1 further comprising fins extending from the flat tube heat exchanger across a flow path for heat exchange medium.

12. The system of claim 1 further comprising fins extending from the flat tube heat exchanger and having slots or louvers to allow drainage of condensation on the heat exchanger and fins.

13. The system of claim 1 wherein the flat tube heat exchanger has a single inlet and a single refrigerant outlet.

14. The system of claim 1 wherein the flat tube heat exchanger comprises flat tubes made of copper or aluminum.

15. The system of claim 1 wherein the flat tube heat exchanger comprises multiple rows of flat tube heat exchanger components arranged along a flow path of heat exchange fluid so as to provide counter flow heat exchange between refrigerant in the flat tube heat exchanger and heat exchange fluid.

16. The system of claim ˜1 wherein:

the refrigerant comprises, in major mass part, CO₂; and

the first and second heat exchangers are refrigerant-air heat exchangers.

17. The system of claim 1, wherein the system is adapted to operate under a transcritical vapor compression mode.

18. A beverage cooling device comprising the system of claim 1.