Refrigeration system and a method for reducing the charge of refrigerant there in

Abstract

In a given design of a refrigeration system, the present invention provides a method for reducing the charge of the refrigerant flowing through the refrigeration system. The refrigeration system includes a fluid conduit through which a flammable refrigerant circulates. The fluid conduit couples, in serial order, a compressor, a first heat exchanger, an expansion device, and a second heat exchanger. The conduit defines an interior diameter, an interior surface, and a given friction factor range. The method includes the steps of reducing the interior diameter of the conduit to thereby reduce the system design refrigerant charge, and coating the interior surface of the reduced diameter conduit with a coating composition to thereby reduce the surface roughness of the interior surface and provide a friction factor for the reduced diameter conduit that is substantially within the given friction factor range.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to refrigeration systems and, more particularly, to methods for reducing the total charge of refrigerant in a refrigeration system.

2. Description of the Related Art

Refrigeration systems for refrigeration, air conditioning and other cooling applications typically include a fluid circuit that operably connects the components of the system and through which the refrigerant flows through the system. The fluid circuit is typically constructed of a set of tubes connecting the components. At any given time, a substantial amount of the refrigerant flowing through the system is contained within these tubes. In some systems, the refrigerant is a combustible hydrocarbon fluid, such as propane. Unfortunately, hydrocarbon refrigerants have a low Flammability Limit. The Flammability Limit refers to the concentration level that may be sufficient to trigger an explosion in the presence of oxygen and a flame or spark. It is desirable to minimize the total change or volume of refrigerant in the system to thereby reduce the likelihood that the concentration of hydrocarbon refrigerant in the ambient air will exceed the Flammability Limit in the event of a leak. This may be particularly desirable in compact refrigeration systems wherein the volume of ambient air is minimized due to circumstances, such as ventilation and elevation of the system from floor/support surface. Therefore, a need remains for a system having a reduced refrigerant charge without reducing the performance and capacity of the system.

SUMMARY OF THE INVENTION

The present invention provides a system and method for reducing the charge of refrigerant in refrigeration system. In one form, in a given design of a refrigeration system, the present invention provides a method for reducing the charge of the refrigerant flowing through the refrigeration system. The refrigeration system includes a fluid conduit through which a flammable refrigerant circulates. The fluid conduit couples, in serial order, a compressor, a first heat exchanger, an expansion device, and a second heat exchanger. The conduit defines an interior diameter, an interior surface, and a given friction factor range. The method includes the steps of reducing the interior diameter of the conduit to thereby reduce the system design refrigerant charge, and coating the interior surface of the reduced diameter conduit with a coating composition to thereby reduce the surface roughness of the interior surface and provide a friction factor for the reduced diameter conduit that is substantially within the given friction factor range.

In a given design of a refrigeration system the present invention provides another method for reducing the charge of the refrigerant flowing through the refrigeration system. The refrigeration system comprises a fluid conduit through which a combustible refrigerant circulates, the fluid conduit coupling, in serial order, a compressor, a first heat exchanger, an expansion device, and a second heat exchanger. The conduit defines an interior diameter and an interior surface. The flow of refrigerant in the conduit has a given Reynolds number range. The method includes the steps of reducing the interior diameter of the conduit to thereby reduce the system design refrigerant charge, and coating the interior surface of the reduced diameter conduit with a coating composition to thereby reduce the surface roughness of the interior surface and provide a Reynolds number for the flow of refrigerant in the reduced diameter conduit that is substantially within the given Reynolds number range.

In still another form, the present invention provides a vapor compression system for use with a refrigerant. The system includes a fluid circuit coupling, in serial order, a compressor, a first heat exchanger, an expansion device, and a second heat exchanger. The fluid circuit comprises a refrigerant conduit through which the refrigerant flows. The flow of refrigerant defines a Reynolds number (Re) of greater than 2000. The fluid circuit defines an interior surface coated with a composition, which provides the interior surface with a wall roughness of less than about 0.0001 ft.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and objects of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of a refrigeration system in accordance with one embodiment of the present invention;

FIG. 2 is a perspective view of a section of a fluid conduit of a refrigeration system in accordance with one embodiment of the present invention; and

FIG. 3 is a cross section of the conduit of FIG. 2 taken along lines 3-3.

DETAILED DESCRIPTION

The embodiments hereinafter disclosed are not intended to be exhaustive or limit the invention to the precise forms disclosed in the following description. Rather the embodiments are chosen and described so that others skilled in the art may utilize its teachings.

Referring first to FIG. 1, refrigeration system 10 of the present invention includes fluid circuit 12 through which a refrigerant circulates. The refrigerant may be any conventional refrigerant used in refrigeration systems, including flammable refrigerants, such as propane and ammonia. Fluid circuit 12 operably couples, in serial order, compressor 14, first heat exchanger or condenser 16, expansion device 18 and second heat exchanger or evaporator 20. As illustrated in FIG. 1, refrigeration system 10 may also include accumulator 22 coupled to fluid circuit 12 between evaporator 20 and compressor 14. Suction line heat exchanger 24 may also be operably coupled to fluid circuit 12. Suction line heat exchanger 24 includes first and second portions 24a and 24b, which are in a heat exchange relationship with one another. First portion 24a is operably coupled to fluid circuit 12 between first heat exchanger 16 and expansion device 18. Second portion 24b is operably coupled to fluid circuit 12 between accumulator 22 and compressor 14. Applications of such refrigeration systems include refrigeration, air conditioning, chillers and other cooling applications.

Compressor 14 may be any known compressor suitable for compressing a refrigerant fluid, such as a combustible hydrocarbon. Such suitable compressors may include single or multi-stage compressors having one or more rotary vane mechanisms, reciprocating piston mechanisms, orbiting scroll mechanisms, centrifugal mechanisms and/or other conventional compressor mechanisms. First and second heat exchangers 16, 20 may be of any conventional condenser/evaporator design. Accumulator 22, expansion device 18 and suction line heat exchanger 24 may also be of conventional types.

In general operation, the refrigerant circulates through system 10 via conduit 12 in the direction of the arrows shown in FIG. 1. More specifically, the refrigerant is drawn by suction pressure into compressor 14 where it is compressed to a relatively high discharge pressure. The resulting high pressure refrigerant is discharged from compressor 14 and flows via conduit 12 to condenser 16 wherein thermal energy is removed from the refrigerant, thereby condensing the refrigerant. The condensed refrigerant then flows through conduit 12 to first portion 24a of suction line heat exchanger 24, wherein thermal energy is transferred from the refrigerant in first portion 24a to the refrigerant in second portion 24b. The refrigerant then flows from first portion 24a of suction line heat exchanger 22 to expansion device 18, which reduces the pressure of the refrigerant and meters the refrigerant to evaporator 20. In evaporator 20 thermal energy is transferred from the medium being cooled to the refrigerant flowing in evaporator 20, thereby cooling the ambient air and evaporating the refrigerant. The refrigerant then flows from evaporator 20 through circuit 12 to accumulator 22. Accumulator 22 collects and stores any liquid refrigerant remaining in the refrigerant exiting evaporator 20 and releases the refrigerant at a controlled rate to compressor 14. The vapor refrigerant exiting evaporator 20 flows through accumulator 22 and is discharged into fluid circuit 12. From accumulator 22, the refrigerant flows, via fluid circuit 12, to second portion 24b of suction line heat exchanger 22. In second portion 24b the refrigerant receives thermal energy from the refrigerant flowing through first portion 24a, thereby warming the refrigerant in second portion 24b. The warmed refrigerant then flows via conduit 12 to compressor 14 and the cycle is repeated.

Fluid circuit 12 is constructed of a fluid conduit or tube. In conventional refrigeration systems the fluid conduit typically has a diameter of up to about 0.500 inches (1.275 cm). In such systems the percentage of refrigerant residing in the fluid conduit is typically between 18% and 29% of the total refrigerant in the system (not including the refrigerant in the compressor). For example, Table 1 shows how the refrigerant charge is distributed among components of a particular compact refrigeration system. This particular system has a microchannel type condenser and evaporator. The compressor is a reciprocating compressor, which has low pressure refrigerant gas in the housing and is lubricated with mineral oil.

TABLE I
                        Refrigerant
Component               Charge, grams
Dis Line                0.292
Cond sup head           0.602
Cond Superheat          0.210
Cond 2ph                5.678
Cond 2ph head           0.939
Cond Subcooled          2.615
Cond sub head           0.939
Liq Line                4.002
Evap 2ph head           0.245
Evaporator              8.277
Evap sup head           0.031
Suction Line            0.129
Charge in compressor    30.000
Total out of compressor 23.960
Total in lines          4.423
Total in lines, %       18.460

As illustrated in Table I, the fluid conduit including the discharge line, the liquid line and the suction line, of a refrigeration system may hold over 18% of the total refrigerant (not including the refrigerant in the compressor) at any given time.

In another example, the total refrigerant charge in the system has been reduced by plugging every other microchannel in the heat exchanger, thereby reducing the amount of refrigerant in the heat exchanger. Table II shows the distribution of refrigerant among the components of such a system. By reducing the refrigerant charge in the heat exchanger, the percentage of refrigerant residing in connecting lines is increased from 18% to 29%.

TABLE II
Dis Line                0.28816
Cond sup head           0.59306
Cond Superheat          0.10841
Cond 2ph                3.04814
Cond 2ph head           0.98704
Cond Subcooled          0.00288
Cond sub head           0.98704
Liq Line                4.01242
Evap 2ph head           0.12291
Evaporator              4.82839
Evap sup head           0.01626
Suction Line            0.12650
Charge in oil (g)       30.00000
Total out of compressor 15.12119
Total in lines          4.42707
Total lines, %          29.27729

These figures demonstrate that, at any given time, a substantial amount of refrigerant charge may reside in the fluid conduit.

Referring now to FIGS. 2 and 3, to reduce the total charge of refrigerant in system 10, the conduit 12 of the refrigeration system of the present invention has a reduced diameter “D” relative to the diameter of conduits in conventional systems. More specifically, diameter D is preferably reduced to between 0.375 inches (0.9525 cm) and 0.0625 inches (0.15875 cm). As a result, the volume of the conduit is reduced and the total charge of refrigerant in the system is thereby reduced. In addition, the amount of space occupied by conduit 12 is reduced as a result of the reduced diameter conduit, thereby enabling the production of a more compact system.

However, the reduction of the diameter of conduit 12 is not a simple means for reducing the total refrigerant charge of the system. A reduction in the diameter of conduit 12 results in an increase in pressure loss or head loss (h_L) due to friction. The head loss (h_L) due to a flow of refrigerant through a segment of a pipe or conduit, such as conduit 12, may be calculated using the well-known Darcy-Weisbach formula below. $h_L=f\frac{L{V}^2}{\mathrm{D2g}}$

In the Darcy-Weisbach equation, “h_L” equals head loss; “f” equals the friction factor; “L” equals the length of the conduit; “V” equals average fluid velocity; “D” equals the diameter of the conduit; and “g” equals the acceleration of gravity. As is demonstrated by the Darcy-Weisbach equation, head loss increases directly as the diameter decreases. This result is exacerbated by the friction factor, which also increases as the diameter decreases. The friction factor (f) is a function of the velocity, density and viscosity of the fluid, as well as the diameter and internal roughness of the conduit. The Darcy-Weisbach friction factor, “f”, may be calculated using the equation below. $f=F\left(\mathrm{Re},\frac{\varepsilon }{D}\right)$

In this equation, “F” represents a functional relationship that can be developed; “Re” equals the Reynolds number; “ε” equals the index of the internal conduit roughness; and “D” equals the conduit diameter. The Darcy-Weisbach friction factor equation demonstrates that the friction factor is increased when the diameter is decreased. An increase in the friction factor results in a further increase in heat loss.

The Reynolds number is an expression of the fluid velocity, density, viscosity and conduit size. The Reynolds number equation is found below. $\mathrm{Re}=\frac{4Q}{\pi vD}$

In the Reynolds number equation “Q” equals the conduit line flow rate; “ν” equals the viscosity of the fluid; and “D” equals the diameter of the conduit. This equation illustrates that the Reynolds number also increases with a decrease in diameter. An increased Reynolds number yields an increased friction factor which, in turn, further increases the head loss.

As is demonstrated by these equations, a decrease in the diameter of the conduit directly increases the head loss in the Darcy-Weisbach equation and further increases the head loss through the friction factor and Reynolds number in the Darcy-Weisbach equation. An increase in head loss may reduce the efficiency and efficacy of the refrigeration system. Consequently, simply reducing the diameter of conduit 12 is not an efficient, effective means for reducing the total refrigerant charge of the system 10.

To compensate, at least in part, for the effect the reduction in diameter has on the head loss, the conduit 12 of the present system also has a reduced interior surface roughness. Conventional conduits typically are constructed of steel and/or copper and have an interior surface of about 0.003 ft. Referring to FIGS. 2 and 3, conduit 12 has an interior surface 26, which is coated with a coating 28. Coating 28 provides conduit 12 with a reduced surface roughness of less than 0.0001 ft., and preferably between about 5×10⁻⁵ ft. (1.524×10⁻³ cm) 4×10⁻⁶ ft. (1.219×10⁻⁴ cm). Coating 28 preferably has a thickness of less than 0.200 inches (0.508 cm). Coating 28 may be any composition capable of reducing the interior surface roughness of conduit 12 and capable of withstanding the temperatures and chemical properties of the refrigerant. Such compositions may include one or more of the following compounds: PVC(polyvinylchloride); two component epoxy COPON EP2306H.F.; polyester reinforced polyethylene; polyvinylchlor polyethylene; polypropylene; polybutylene; aluminous cement mortar; polyurethane-acrylic blends; silicone rubber; poly(di-methyl siloxane); molybdenum disulphide; teflon and teflon mixtures (e.g. HiFlon); graphite; ANL-NFC (Argonne Laboratory developed amorphous carbon coating); ceramic/polymer mixtures (e.g. Series 100 Infused Matrix Endura coating); octadecyltrichlorosilane; 1H,1H,2H,2H-perflourodecyltrichlorosilane; unsaturated hydrosable resin (e.g. Fluiglide); poly(vinylpyrrolidone); polyester acrylic co-polymer (e.g. Polyglass HA); vinyl ester/acrylic copolymer (e.g. Polyglass VE); bisphenol ‘A’ polyester resin; polyester/polyurethane polymer alloy (e.g. HN4); polyurethane resin; and carbon based PVD coatings (e.g. Genocoa). Coating 28 may also thermally insulate conduit 12, thereby minimizing heat transfer in conduit 12. The reduction of interior surface roughness compensates for the reduction in diameter such that the Reynolds number and flow are not significantly affected. The Reynolds number preferably remains between 2300 and 50000.

The present invention reduces the total charge of refrigerant by reducing the volume of the conduit, while minimizing head loss due to the reduction in diameter by reducing the surface roughness. As a result, the present invention reduces the total charge of refrigerant in the system, thereby minimizing the risks associated with flammable refrigerants. The reduced total refrigerant charge permits the system to be used in smaller areas with reduced ambient circumstances such as ventilation, elevation from the floor, and ambient air/space surrounding the system.

While this invention has been described as having an exemplary design, the present invention may be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains.

Claims

1. In a given design of a refrigeration system, a method for reducing the charge of the refrigerant flowing through the refrigeration system, the refrigeration system comprising a fluid conduit through which a flammable refrigerant circulates, the fluid conduit coupling, in serial order, a compressor, a first heat exchanger, an expansion device, and a second heat exchanger, the conduit defining an interior diameter and an interior surface, the conduit defining a given friction factor range, the method comprising the steps of: reducing the interior diameter of the conduit to thereby reduce the system design refrigerant charge, and coating the interior surface of the reduced diameter conduit with a coating composition to thereby reduce the surface roughness of the interior surface and provide a friction factor for the reduced diameter conduit that is substantially within the given friction factor range.

2. The method of claim 1 wherein the coating composition reduces the surface roughness to an equivalent sand grain roughness value of less than 0.0001 ft.

3. The method of claim 2 wherein the coating composition reduces the surface roughness to an equivalent sand grain roughness value of between about 0.00005 and 0.000004 ft.

4. The method of claim 1 wherein the reduced interior diameter is less than about 0.375 inches.

5. The method of claim 4 wherein the reduced diameter is greater than about 0.0625 inches.

6. The method of claim 1 wherein the coating composition coats the interior surface at a thickness of less than 0.200 inches.

7. The method of claim 1 wherein the coating composition comprises at least one of polyvinylchloride, polyvinylchlor polyethylene, polypropylene, and polybutylene.

8. The method of claim 1 wherein the coating composition comprises at least one of polyester acrylic copolymer, vinyl ester acrylic copolymer, bisphenol A polyester resin, polyester polyurethane polymer alloy, polyurethane resin, polyurethane acrylic blends, polyester reinforced polyethylene, and poly(vinylpyrrolidone).

9. The method of claim 1 wherein the composition comprises teflon.

10. The method of claim 1 wherein the composition comprises silicone rubber.

11. The vapor compression system of claim 1 wherein the refrigerant is propane.

12. The vapor compression system of claim 1 wherein the refrigerant is ammonia.

13. In a given design of a refrigeration system, a method for reducing the charge of the refrigerant flowing through the refrigeration system, the refrigeration system comprising a fluid conduit through which a combustible refrigerant circulates, the fluid conduit coupling, in serial order, a compressor, a first heat exchanger, an expansion device, and a second heat exchanger, the conduit defining an interior diameter and an interior surface, the flow of refrigerant in the conduit having a given Reynolds number range, the method comprising the steps of: reducing the interior diameter of the conduit to thereby reduce the system design refrigerant charge, and coating the interior surface of the reduced diameter conduit with a coating composition to thereby reduce the surface roughness of the interior surface and provide a Reynolds number for the flow of refrigerant in the reduced diameter conduit that is substantially within the given Reynolds number range.

14. The method of claim 13 wherein the refrigerant is a combustible hydrocarbon.

15. The method of claim 13 wherein the step of reducing the volume of the fluid conduit includes reducing the interior diameter to less than about 0.375 inches.

16. The method of claim 15 wherein the reduced diameter is greater than about 0.0625 inches.

17. The method of claim 13 wherein the composition reduces the surface roughness to less than about 0.0001 ft.

18. The method of claim 13 wherein the coating composition includes at least one of polyvinylchloride, polyvinylchlor polyethylene, polypropylene, polybutylene, polyester acrylic copolymer, vinyl ester acrylic copolymer, bisphenol A polyester resin, polyester polyurethane polymer alloy, polyurethane resin, polyurethane acrylic blends, polyester reinforced polyethylene, and poly(vinylpyrrolidone).

19. The method of claim 13 wherein the coating composition includes teflon.

20. The method of claim 13 wherein the step of reducing the friction factor includes coating the interior surface of the conduit at a thickness of less than 0.200 inches.

21. A vapor compression system for use with a refrigerant, the system comprising: a fluid circuit coupling, in serial order, a compressor, a first heat exchanger, an expansion device, and a second heat exchanger, said fluid circuit comprising a refrigerant conduit through which the refrigerant flows, the flow of refrigerant defining a Reynolds number (Re) of greater than 2000, the fluid circuit defining an interior surface, said interior surface coated with a composition, said composition providing said interior surface with a wall roughness of less than about 0.0001 ft.

22. The vapor compression system of claim 21 wherein said conduit defines an interior diameter less than about 0.375 inches.

23. The vapor compression system of claim 21 wherein said Reynolds number is between 2300 and 50000.

24. The vapor compression system of claim 21 wherein said composition coats said interior surface at a thickness of less than 0.200 inches.

25. The vapor compression system of claim 22 wherein said interior diameter is greater than 0.0625 inches.

26. The vapor compression system of claim 21 wherein said coating composition comprises at least one of polyvinylchloride, polyvinylchlor polyethylene, polypropylene, and polybutylene.

27. The vapor compression system of claim 21 wherein said composition comprises at least one of polyester acrylic copolymer, vinyl ester acrylic copolymer, bisphenol A polyester resin, polyester polyurethane polymer alloy, polyurethane resin, polyurethane acrylic blends, polyester reinforced polyethylene, and poly(vinylpyrrolidone).

28. The vapor compression system of claim 21 wherein said composition comprises silicone rubber.

29. The vapor compression system of claim 21 wherein the refrigerant is flammable and comprises hydrocarbons.

30. The vapor compression system of claim 21 wherein the refrigerant is ammonia.