WO2022026327A1

WO2022026327A1 - Multilayer composite tube with flame-resistant layers

Info

Publication number: WO2022026327A1
Application number: PCT/US2021/042993
Authority: WO
Inventors: Donald Bradley Campbell; Jordan STRUNK
Original assignee: Titeflex Corporation
Priority date: 2020-07-31
Filing date: 2021-07-23
Publication date: 2022-02-03
Also published as: EP4189270A1; US20220032571A1; EP4189270A4

Abstract

One aspect of the invention provides a composite refrigeration line set including: a suction line and a return line. One or more of the lines are a composite refrigeration line set tube including: an inner plastic tube; a first adhesive layer external to the inner plastic tube; an aluminum layer surrounding the first adhesive layer and coupled to the inner plastic tube via the first adhesive layer; a second adhesive layer external to the aluminum layer; and an outer plastic layer surrounding the aluminum layer and coupled to the aluminum layer via the second adhesive layer. The outer plastic tube includes a flame-resistant compound combined with PERT. The composite refrigeration line set tube has a flame and smoke spread rating of no more than 25/50 when tested in isolation using Appendix A1.22 of the CAN/ULC-S102-10 Standard Test Method for Surface Burning Characteristics of Building Materials and Assemblies.

Description

MULTILAYER COMPOSITE TUBE WITH FLAME-RESISTANT LAYERS

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority 35 U.S.C. § 119(e) to U.S. Patent Application Serial No. 17/150,107, filed January 15, 2021, and U.S. Provisional Patent Application Serial No. 63/059,616, filed July 31, 2020. The entire content of this application is hereby incorporated by reference herein.

BACKGROUND OF THE INVENTION

Multilayer composite tubes are designed and used to convey liquids, primarily water, for applications such as in floor heating, radiator heating, and water supply.

SUMMARY OF THE INVENTION

One aspect of the invention provides a composite refrigeration line set including: a suction line and a return line. One or more of the suction line and the return line are a composite refrigeration line set tube including: an inner plastic tube; a first adhesive layer external to the inner plastic tube; an aluminum layer circumferentially surrounding the first adhesive layer and coupled to the inner plastic tube via the first adhesive layer; a second adhesive layer external to the aluminum layer; and an outer plastic layer circumferentially surrounding the aluminum layer and coupled to the aluminum layer via the second adhesive layer. The inner plastic tube is polyethylene of raised temperature. The outer plastic tube is polyethylene of raised temperature. The outer plastic tube includes a flame-resistant compound combined with the polyethylene of raised temperature. The composite refrigeration line set tube has a flame and smoke spread rating of no more than 25/50 when tested in isolation using Appendix A1.22 of the CAN/ULC- S102-10 Standard Test Method for Surface Burning Characteristics of Building Materials and Assemblies.

This aspect of the invention can have a variety of embodiments. At least the aluminum layer can remain intact after completion of the CAN/ULC-S102-10 Standard Test Method for Surface Burning Characteristics of Building Materials and Assemblies.

The at least one flame-resistant compound can include one or more selected from the group consisting of: a flame retardant, glass fibers, nanoclay, nanofibers, and a combination thereof. The at least one flame-resistant compound can be a thermal foaming agent. The at least flame-resistant compound can have a specific gravity of greater than 1.5 with respect to the outer plastic tube.

The outer plastic layer can be composed of at least 50% by weight of the flame-resistant compound. The outer plastic layer can be composed of at least 90% by weight of the flame- resistant compound.

The at least one flame-resistant compound can include magnesium hydroxide.

Another aspect of the invention provides a refrigeration system including: a compressor; an evaporator coil; the composite refrigeration line set as described herein coupled between the compressor and the evaporator coil to form a fluid circuit between the compressor and the evaporator coil; and a refrigerant received within the fluid circuit.

This aspect of the invention can have a variety of embodiments. The refrigerant can be a flammable refrigerant. The refrigerant can be selected from Table 2. The refrigerant can include difluorom ethane. The refrigerant can be R-32. The refrigerant can be R-454B. The refrigerant can be a low-global-warming-potential refrigerant. The refrigerant can be an A2L refrigerant.

Another aspect of the invention provides a refrigeration system including: a compressor; an evaporator coil; a first pair of compression fittings coupled to the compressor; a second pair of compression fittings coupled to the evaporator coil; a composite refrigeration line set coupled between the compressor and the evaporator coil via one of the compression fittings to form a fluid circuit between the compressor and the evaporator coil; and a refrigerant received within the fluid circuit. The composite refrigeration line includes: a suction line and a return line. Each of the compression fittings includes: a nipple; a ferrule; and a nut. The nipple includes: a distal annular channel; a distal O-ring received in the distal annular channel; a plurality of medial barbs; a proximal annular channel; a proximal O-ring received in the proximal annular channel; and a first threaded region. The ferrule is sized to fit over the nipple and compress a tubing end between the ferrule and the nipple. The nut includes: an axial bore sized to fit over the tubing end; a second threaded region complementary to the first threaded region of the nipple; and an internal geometry adapted and configured to apply compression to the ferrule when the nut is threaded proximally relative to the nipple. One or more of the suction line and the return line are a composite refrigeration line set tube including: an inner plastic tube; a first adhesive layer circumferentially surrounding the inner plastic tube; an aluminum layer circumferentially surrounding the first adhesive layer and coupled to the inner plastic tube via the first adhesive layer; a second adhesive layer circumferentially surrounding the aluminum layer; and an outer plastic layer circumferentially surrounding the aluminum layer coupled to the aluminum layer via the second adhesive layer. The inner plastic tube is polyethylene of raised temperature. The outer plastic tube is polyethylene of raised temperature. The aluminum layer is butt-welded to itself.

This aspect of the invention can have a variety of embodiments. The refrigeration system can be a heat pump.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and desired objects of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawing figures wherein like reference characters denote corresponding parts throughout the several views.

FIGS. 1 and 2 depict a plastic/gas-barrier/flame protective layer composite tube according to embodiments of the invention.

FIG. 3 depicts an air conditioning system that can implement a flame protective composite tube according to an embodiment of the invention.

FIG. 4 depicts a workflow process for fabricating a flame protective composite tube according to an embodiment of the invention.

FIGS. 5A-5C depict a compression fitting according to an embodiment of the invention.

DEFINITIONS

The instant invention is most clearly understood with reference to the following definitions:

As used herein, the singular form “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise.

Unless specifically stated or obvious from context, as used herein, the term “about” is understood as within a range of normal tolerance in the art, for example within 2 standard deviations of the mean. “About” can be understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the stated value. Unless otherwise clear from context, all numerical values provided herein are modified by the term about. As used herein, the term “alloy” refers to a homogenous mixture or metallic solid solution composed of two or more elements. Examples of alloys include austenitic nickel- chromium-based super-alloys (available, e.g ., under the INCONEL® trademark from Huntington Alloys Corporation of Huntington, West Virginia), brass, bronze, steel, low carbon steel, phosphor bronze, stainless steel, and the like.

As used in the specification and claims, the terms “comprises,” “comprising,” “containing,” “having,” and the like can have the meaning ascribed to them in U.S. patent law and can mean “includes,” “including,” and the like.

As used in the specification and claims, the term “fiberglass” refers to fiber-reinforced plastic using glass fiber. Generally speaking, “E-glass” is understood to refer to alumina- calcium-borosilicate glasses used as a general purpose reinforcement where strength and high electrical resistivity are desired, while “S-glass” is understood to refer to magnesium aluminosilicate glasses used for textile substrates or reinforcement in composite structural applications that require high strength, modulus, and durability under conditions of extreme temperature or corrosive environments.

Unless specifically stated or obvious from context, the term “or,” as used herein, is understood to be inclusive.

As used herein, the term “metal” refers to any chemical element that is a good conductor of electricity and/or heat, and alloys thereof. Examples of metals include, but are not limited to, aluminum, cadmium, niobium (also known as “columbium”), copper, gold, iron, nickel, platinum, silver, tantalum, tin, titanium, zinc, zirconium, and the like.

As used herein, the term “resin” refers to any synthetic or naturally occurring polymer.

Ranges provided herein are understood to be shorthand for all of the values within the range. For example, a range of 1 to 50 is understood to include any number, combination of numbers, or sub-range from the group consisting 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,

16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, or 50 (as well as fractions thereof unless the context clearly dictates otherwise). DETAILED DESCRIPTION OF THE INVENTION Multilayer composite tubes with flame protection layers are described herein. A layer of the composite tube can be a flame protective layer. In some cases, the flame protective layer can be an extruded polymer layer loaded with flame retardants, glass fibers, nanoclays, nanofibers, and the like. The flame protective layer can form a complete or substantially complete layer of char when exposed to flame, thereby preventing the flame from penetrating other layers of the composite tube.

Multilayer composite tubes can be fabricated from multiple layers of material including various plastics, adhesives and, in some cases metal layers. Exemplary constructions include are summarized below.

A variety of multilayer composite tubes and applications for the same are described in U.S. Patent Application Publication No. 2020/0400251.

Referring to FIG. 1, these constructions can include an inner layer of a type of plastic 102, a layer of adhesive 104, a gas ( e.g ., oxygen) barrier ( e.g ., a layer of aluminum) 106, a layer of adhesive 108, and a flame protective layer 110.

The inner plastic layer 102 can be selected from a variety of materials such as thermoplastics, thermoplastic elastomers, polyethylene, polyethylene, polypropylene, polyvinyl chloride (PVC), polyamide, fluoropolymers, polyvinylidene fluoride (PVDF), fluorinated ethylene propylene (FEP), perfluroalkoxy alkane (PFA), and the like.

The gas barrier 106 can be a metallic composition. For example, the gas barrier 106 can be aluminum, steel, copper, and the like. Aluminum may include beneficial properties for the gas barrier 106, such as reduced weight, anti-corrosiveness, manufacturing cost, and the like.

The flame protective layer 110 can include flame resistant material within the layer’s composition. For example, the flame protective layer 110 can include flame retardants (e.g., magnesium hydroxide, aluminum trihydrate, and/or halogenated fire retardants), glass fibers, nanoclays, nanofibers, and the like. Further, the flame protective layer 110 can also include a plastic component, similar to the plastic layer 102 discussed above. For example, the plastic component can include one or a combination of thermoplastics, thermoplastic elastomers, polyethylene, polyethylene, polypropylene, PVC, polyamide, fluoropolymers, PVDF, FEP, PFA, and the like. For example, the flame protective layer 110 can be solution or suspension in which a polymer is the dispersion medium and the flame retardant is dissolved or dispersed within the polymer or adhered to the polymer.

The flame protective material of the layer 110 can be highly loaded. For example, the filler loading for the flame protective material can be 50 - 90% by weight of the layer 110. In some cases, the specific gravity for the flame protective material can be greater than 1.5. In some cases, thermal foaming agents can also be a component of the layer 110, which can increase the char volume of the layer 110 when exposed to flame.

In some cases, the composite tube can undergo extrusion procedures for formation. For example, the flame protective layer 110 can be extruded to form the shape necessary for the tube. In some cases, the flame protective materials and the plastic components of the layer 110 can be extruded together, thereby forming the layer 110. In some cases, the layer 110 can undergo tandem extrusion or co-extrusion with other layers of the composite tube. For example, the layer 110 can be co-extruded with the adhesive 108, which can bond the layer 110 and the adhesive 108. This can facilitate a reduction in thickness of the layer 110 required to adequately form a char layer in case of exposure to flame.

In some cases, an intermediate plastic layer can be located between the flame protective layer and the gas barrier. FIG. 2 depicts a composite tube according to embodiments of the claimed invention. The composite tube can include an inner plastic layer 202, an adhesive layer 204, a gas barrier 206, an adhesive layer 208, an intermediate plastic layer 210, an optional adhesive layer 212, and a flame protective layer 214. The inner plastic layer 202, adhesive layers 204, 208, and 212, gas barrier 206, and a flame protective layer 214 can be examples of inner plastic layer 102, adhesive layers 104 and 108, gas barrier 106, and flame protective layer 110 as described with reference to FIG. 1. The tube in FIG. 2 can also include the intermediate plastic layer 210, which can be made of materials similar to those which the inner plastic layer 102 of FIG. 1 is composed. The intermediate plastic layer 210 can be located between the flame protective layer 214 and the gas barrier 206. The intermediate plastic layer 210 can further reduce thickness and tensile requirements of the flame protective layer 214. For example, the flame protective layer 214 may reduce the amount of plastic components within the layer 214 due to the location of the intermediate plastic layer 210.

Intermediate plastic layer 210 and flame protective layer 214 can be co-extruded such that the intermediate plastic layer 210 provides tensile strength and elasticity ( e.g ., with regard to bending) while the flame protective layer 214 is bonded to and protects the intermediate plastic layer 214. Even if the flame protective layer 214 cracks (e.g., at a bend) the exposed intermediate plastic layer will be relative small and foaming and charring of the flame protective layer 214 can shield and/or limit combustion of the intermediate plastic layer 210.

Embodiments of the invention can include multiple flame retardant layers, either adjacent or separate from each other. For example, a flame retardant layer can be included between layers 102 and 104, 104 and 106, 202 and 204, 204 and 206, and the like. In some cases, the flame protective material can also be included in layers 102 and 202.

Metal Deactivators

In some cases, the flame protective composite tube can include metal deactivators. For example, the adhesive layers (e.g., layers 104 and/or 108 of FIG. 1) can include the metal deactivators. The metal deactivators can enable use of high-magnesium content alloys in other layers of the composite tube (e.g., gas barrier 106 of FIG. 1), which can include increased mechanical performance compared to other alloy layers. For example, the metal deactivators can help reduce potential oxidation in the alloys (e.g, alloys having high magnesium content) and subsequent delamination. Examples of metal deactivators can include, but are not limited to, N,N'-bis(3,5-di-t-butyl-4-hydroxyphenylpropionyl)hydrazine, N,N'-diphenyloxamide, N- salicylal-N'-salicyloyl-hydrazine, N,N'-bis(salicyloyl)hydrazine, 3 -salicyloylamino- 1,2,4- triazole, bis(benzylidene)oxallyl dihydrazide, oxanilide, isophthaloyl dihydrazide, sebacoyl bisphenylhydrazide, N,N'-diacetyladipoyl dihydrazide, N,N'-bis(salicyloyl)oxallyl dihydrazide, N,N'-bis(salicyloyl)thiopropionyl dihydrazide, is also advantageously used in the composition. N,N'-bis(3,5-di-t-butyl-4-hydroxyphenylpropionyl)hydrazine (EVERNOX™ MD 1024), and the like. Applications

The flame protective composite tubes described above can be used for a variety of applications. For example, the flame protective composite tubes can be used for common water conveyance applications. However, there are many other applications for which this type of tube can be used. These other applications could include the conveyance of other types of liquids and gases such as refrigerants, natural gas, propane, and process and medical gases such as argon, helium, nitrogen, and the like.

FIG. 3 depicts a refrigeration system, specifically an air conditioning system. The refrigeration system can be configured to act as a heat pump that extracts heat from air surrounding the condenser coil and transfers that heat to the evaporator coil to heat a structure. Notably, operation as a heat pump generates higher refrigerant temperatures that soften plastic layers 102 and 110 and place increased tensile loads on gas barrier 106.

The refrigeration system can include a suction line and a return line. Either or both of the suction line and the return line can include the flame protective composite tubes described above.

In one embodiment, multilayer composite tubes can be utilized as line sets for a refrigeration or air conditioning system carrying a flammable ( e.g ., slightly flammable or highly flammable) refrigerant.

Refrigerants are listed by the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) in ASHRAE Standard 34 (2019). The ASHRAE 34 Standard Committee determines toxicity and flammability classification. Class A refrigerants have lower toxicity. Class B refrigerants have higher toxicity. Flammability classifications are summarized in Table 6-1 of ASHRAE Standard 34. Embodiments of the invention can be utilized with Al, A2L, A2, A3, Bl, B2L, B2, or B3 refrigerants.

Many of such refrigerants have a low global warming potential (GWP), e.g., a GWP of 500 or lower. Exemplary refrigerants are listed in Table 2 below.

Accordingly, embodiments of the invention can include both a system including one or more AC/refrigeration components ( e.g ., a compressor and/or an evaporator coil), a multilayer- composite-tube line set, and refrigerant ( e.g ., in the assembled system or in a container for charging the system after assembly) as well as a system including a multilayer-composite-tube line set and a container of refrigerant for charging an AC/refrigeration system after installation of the multilayer-composite-tube line set between the evaporator coil and the compressor.

The composite multi-layer tube can be configured to meet one or more applicable standards. For example, the composite multi-layer tube can be configured to have a flame and smoke spread rating of no more than 25/50 when tested in isolation (e.g., a pair consisting of a suction line and a return line as would be used in the field) using the ASTM E84 - 20 Standard Test Method for Surface Burning Characteristics of Building Materials and/or Appendix A1.22 of the CAN/ULC-S102-10 Standard Test Method for Surface Burning Characteristics of Building Materials and Assemblies. The composite multi-layer tube can be configured such that the aluminum layer remains intact after completion of one of these tests.

Reinforcement Lavers

Depending on the application of the use for the flame protective composite tube, greater performance standards may be required, making it necessary to further enhance the standard multilayer product design to ensure higher pressure and temperature limits. This enhancement can be accomplished by adding yet another layer of material to the overall construction, thereby creating a reinforcement layer. Additionally or alternatively, the reinforcement can be added within the one of the layers described above.

The reinforcement can be constructed in several forms. For example, the reinforcement can be spirally (e.g, helically) wrapped, longitudinal, braided, and the like under, over, or within any of the layers. For example, a reinforcement layer can be around or within the inner layer of plastic 102, around or within the outer layer of plastic 110, around the gas (e.g, oxygen) barrier (e.g, metal) layer 106, or around or within the adhesive layers 104, 108. The reinforcement layer can completely cover or partially cover the surface of a tube layer 102, 104, 106, 108, 110.

The reinforcement material can include one or more individual material spirals wrapped around the tube (e.g, one material spirally wound with axial pitch of 0.25" or four spirals with individual pitch of 1" or .25" collectively). Tube capacity (e.g, in terms of burst strength) can be adjusted based on pitch, material selection, and the like. For example, the tubing can have a burst pressure in excess of 1,900 psi at 70° F and 1,500 psi at 200° F. The reinforcement can include one or more materials such as metal foils ( e.g ., aluminum or copper), plastic films, metal wire, plastic wire, fiberglass cords or fabric (e.g., AR-glass, C- glass, D-glass, E-glass, E-CR-glass, R-glass, S-glass, and the like), any type of filament material, aramids, para-aramids, poly-aramid synthetic fibers, aromatic polyester strands, and the like.

The reinforcing materials can be coated (e.g, with a binder or primer), machined (e.g, roughened), etched, or otherwise treated to bond to or be embedded within the adhesive layers.

In some cases, the reinforcement can be coated with a flame retardant so that the reinforcement layer provides flame protection. In some embodiments, a particular adhesive layer (e.g, a tie resin, a solvent-based adhesive, a hot-melt adhesive, and the like) is utilized to bond particular reinforcements.

In some embodiments, the reinforcement is applied after the product is extruded (e.g, a spiral wrap applied with a wrapping machine). In other embodiments, a spiral wrap is formed with a rotating extrusion crosshead such that the spiral material is extruded within a layer of polymer or adhesive (e.g, wire inside polymer). In still another embodiment, a spiral wrap is formed with a rotating extrusion crosshead (e.g, polyester cord extruded in a helix around an underlying tube). In still another embodiment, a longitudinal wrap can be added to any layer of the tube.

Method of Fabricating Flame Protective Composite Tube

FIG. 4 depicts a process for fabricating a flame protective composite tube, according to an embodiment of the claimed invention.

At Step 405, a first plastic layer can be provided. In some cases, the first plastic layer can be a resin layer formed via extrusion.

At Step 410, a gas barrier layer can be applied to the exterior of the first plastic layer.

The gas barrier layer can be applied to the exterior of the first plastic layer by a variety of techniques. In some cases, the gas barrier layer can be a resin layer applied to the tubing by extrusion. The gas barrier layer can be a foil, laminated foil, tape or wire layer or the like and can be wound onto the tubing. A foil, laminated foil, tape or wire can be wrapped around the first plastic layer through a number of methods, including helically and radially wrapping.

At Step 415, a flame protective layer can be applied to the exterior of the gas barrier layer and the first plastic layer. The flame protective layer can be a resin layer applied to the first plastic layer by extrusion. In this case, the components of the flame protective layer (e.g., plastic components and flame resistant materials) can be components of the resin as well.

In some cases, an adhesive (e.g., a conductive adhesive, a non-conductive adhesive, etc.) can be used to secure the gas barrier to the first plastic layer. Further, in some cases, an adhesive (e.g, a conductive or non-conductive adhesive) can be used to secure the flame protective layer to the gas barrier layer.

In some cases in which the first plastic layer and the flame protective layer are both resin layers, Steps 410 and 420 can be carried out simultaneously through co-extrusion.

Compression Fitting

Referring now to FIG. 5A-5C, another aspect of the invention provides a compression fitting 500. Compression fitting 500 advantageously seals ends of the composite tubes described herein and resists leaks and blow-off when subject to pressures commonly found in refrigeration systems (e.g, between about 200 psi and about 300 psi, between about 300 psi and about 400 psi, and the like).

Proximal end 502 of compression fitting is designed to be coupled a refrigeration system component (e.g, compressor or evaporator coil). Proximal end 502 can include threads to couple with complementary threads on the refrigeration system component, which can be standardized across equipment. In some embodiments, the proximal end 502 can be coupled to the refrigeration system component using brazing or another non-threaded coupling technique. The proximal end 502 can be coupled to the refrigeration system component before the composite tubing is attached to the distal end 504.

The distal end 504 can include a nipple 506 including a distal annular channel 508, one or more medial barbs 510 (e.g, annular barbs), a proximal annular channel 512, and first threaded region 514. The nipple 506 can be sized to allow an inner diameter of a corresponding composite tube to slide snugly over the nipple 506 (e.g, without the need for tools and without kinking of the composite tube during pushing).

Compression fitting 500 can also include wrench flats 516 that can be used to attach the compression fitting 500 to the refrigeration system component and/or hold back when tightening other components relative to the first threaded region 514.

Referring now to FIG. 5B, the compression fitting can further include a ferrule 516 adapted and configured to fit over an outer diameter of the composite tubing and the nipple 506. The ferrule 516 can include one or more internal annular grooves 518. The grooves 518 can be positioned to align with one or more O-rings received within distal annular channel 508 and/or a proximal annular channel 512 and/or medial barbs 510.

Referring now to FIG. 5C, a nut 520 can be placed over an end of composite tube via axial bore 522 (followed by ferrule 516 before an end of the composite tube is slid over the nipple 506). A second threaded region 524 on nut 520 can then engage with the first threaded region 514. An internal geometry ( e.g ., sloped shoulder 526) of the nut 520 can be configured to apply compression (e.g., axial and/or radial) to the ferrule 516 when the nut 520 is threaded proximally relative to the nipple 506. Fitting 500 can be fabricated from a material such as a metal (e.g, brass) through various techniques including machining, casting, and the like.

EQUIVALENTS

Although preferred embodiments of the invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.

INCORPORATION BY REFERENCE

The entire contents of all patents, published patent applications, and other references cited herein are hereby expressly incorporated herein in their entireties by reference.

Claims

1 A composite refrigeration line set comprising: a suction line; and a return line; characterized in that one or more of the suction line and the return line are a composite refrigeration line set tube comprising: an inner plastic tube; a first adhesive layer external to the inner plastic tube; an aluminum layer circumferentially surrounding the first adhesive layer and coupled to the inner plastic tube via the first adhesive layer; a second adhesive layer external to the aluminum layer; and an outer plastic layer circumferentially surrounding the aluminum layer and coupled to the aluminum layer via the second adhesive layer; wherein: the inner plastic tube is polyethylene of raised temperature; the outer plastic tube is polyethylene of raised temperature; and the outer plastic tube comprises a flame-resistant compound combined with the polyethylene of raised temperature; and wherein the composite refrigeration line set tube has a flame and smoke spread rating of no more than 25/50 when tested in isolation using Appendix A1.22 of the CAN/ULC-S102-10 Standard Test Method for Surface Burning Characteristics of Building Materials and Assemblies.

2. The composite refrigeration line set tube of claim 1, wherein at least the aluminum layer remains intact after completion of the CAN/ULC-S102-10 Standard Test Method for Surface Burning Characteristics of Building Materials and Assemblies.

3. The composite refrigeration line set tube of claim 1, wherein the at least one flame- resistant compound comprises one or more selected from the group consisting of: a flame retardant, glass fibers, nanoclay, nanofibers, and a combination thereof.

4. The composite refrigeration line set tube of claim 1, wherein the at least one flame- resistant compound is a thermal foaming agent.

5. The composite refrigeration line set tube of claim 1, wherein the at least flame-resistant compound has a specific gravity of greater than 1.5 with respect to the outer plastic tube.

6. The composite refrigeration line set tube of claim 1, wherein the outer plastic layer is composed of at least 50% by weight of the flame-resistant compound.

7. The composite refrigeration line set tube of claim 1, wherein the outer plastic layer is composed of at least 90% by weight of the flame-resistant compound.

8. The composite refrigeration line set tube of claim 1, wherein the at least one flame- resistant compound comprises magnesium hydroxide.

9. A refrigeration system comprising: a compressor; an evaporator coil; the composite refrigeration line set according to claim 1 coupled between the compressor and the evaporator coil to form a fluid circuit between the compressor and the evaporator coil; and a refrigerant received within the fluid circuit.

10. The refrigeration system of claim 9, wherein the refrigerant is a flammable refrigerant.

11. The refrigeration system of claim 9, wherein the refrigerant is selected from Table 2.

12. The refrigeration system of claim 9, wherein the refrigerant comprises difluoromethane.

13. The refrigeration system of claim 9, wherein the refrigerant is R-32.

14. The refrigeration system of claim 9, wherein the refrigerant is R-454B.

15. The refrigeration system of claim 9, wherein the refrigerant is a low-global -warming- potential refrigerant.

16. The refrigeration system of claim 9, wherein the refrigerant is an A2L refrigerant.

17. A refrigeration system comprising: a compressor; an evaporator coil; a first pair of compression fittings coupled to the compressor; a second pair of compression fittings coupled to the evaporator coil; a composite refrigeration line set coupled between the compressor and the evaporator coil via one of the compression fittings to form a fluid circuit between the compressor and the evaporator coil, the composite refrigeration line comprising: a suction line; and a return line; and a refrigerant received within the fluid circuit; wherein each of the compression fittings comprises: a nipple comprising: a distal annular channel; a distal O-ring received in the distal annular channel; a plurality of medial barbs; a proximal annular channel; a proximal O-ring received in the proximal annular channel; and a first threaded region; a ferrule sized to fit over the nipple and compress a tubing end between the ferrule and the nipple; and a nut comprising: an axial bore sized to fit over the tubing end; a second threaded region complementary to the first threaded region of the nipple; and an internal geometry adapted and configured to apply compression to the ferrule when the nut is threaded proximally relative to the nipple; and wherein one or more of the suction line and the return line are a composite refrigeration line set tube comprising: an inner plastic tube; a first adhesive layer circumferentially surrounding the inner plastic tube; an aluminum layer circumferentially surrounding the first adhesive layer and coupled to the inner plastic tube via the first adhesive layer; a second adhesive layer circumferentially surrounding the aluminum layer; and an outer plastic layer circumferentially surrounding the aluminum layer coupled to the aluminum layer via the second adhesive layer; wherein: the inner plastic tube is polyethylene of raised temperature; the outer plastic tube is polyethylene of raised temperature; and the aluminum layer is butt-welded to itself.

18. The refrigeration system of claim 17, wherein the refrigeration system is a heat pump.