CN112880460B

CN112880460B - PTFE double-layer composite tube, preparation method thereof and heat exchanger

Info

Publication number: CN112880460B
Application number: CN202110023960.XA
Authority: CN
Inventors: 宋朋泽
Original assignee: Individual
Current assignee: Individual
Priority date: 2021-01-08
Filing date: 2021-01-08
Publication date: 2023-09-22
Anticipated expiration: 2041-01-08
Also published as: CN112880460A

Abstract

The invention provides a PTFE double-layer composite pipe, which comprises a PTFE inner pipe, an inorganic fiber net woven and wound on the outer surface of the PTFE inner pipe and a PTFE outer pipe composited on the surface of the inorganic fiber net; the inorganic fiber net is obtained by weaving and sintering pretreated inorganic fibers on the outer surface of the PTFE inner tube; the pretreated inorganic fiber is an inorganic fiber which is immersed in polytetrafluoroethylene solution with the mass concentration of 50-70%. According to the invention, a net-shaped mosaic structure is formed between the inner layer and the outer layer of the PTFE pipeline by adopting the inorganic fiber net subjected to special pretreatment, so that the compression resistance and the dimensional stability of the PTFE pipeline wall material are effectively enhanced, and the bursting pressure of the PTFE composite pipe is remarkably improved compared with that of a pure PTFE pipe, so that the PTFE composite pipe is more suitable for being used in high-temperature, high-pressure and certain strong corrosion environments. The invention also provides a preparation method of the PTFE double-layer composite tube and a heat exchanger.

Description

PTFE double-layer composite tube, preparation method thereof and heat exchanger

Technical Field

The invention belongs to the technical field of organic polymer materials, and particularly relates to a PTFE double-layer composite tube, a preparation method thereof and a heat exchanger.

Background

The shell and tube heat exchanger is equipment commonly used in industries such as petroleum, chemical industry, metallurgy, electric power, food, medicine and the like, has two functions of heat exchange and cooling, the dominant metal heat exchanger usually has a rapidly reduced heat exchange effect due to various corrosive media, the service life of the equipment is greatly shortened, and great hidden danger can occur in production safety, so that the novel nonmetal heat exchange equipment is researched to have great practical significance. The nonmetal heat exchangers which are widely applied in the market at present are graphite heat exchangers, silicon carbide heat exchangers and various plastic heat exchangers, wherein the fluoroplastic heat exchangers are heat exchange equipment with the most excellent comprehensive performance in the application of the plastic heat exchangers.

Fluoroplastic is a paraffin polymer with part or all of hydrogen replaced by fluorine, has very stable chemical properties, and is more accepted in a plurality of fields than the defects of large volume, fragility and the like of heat exchangers made of non-metal materials such as graphite, ceramic and the like. Among the fluoroplastic materials, polytetrafluoroethylene (PTFE), fluorinated Ethylene Propylene (FEP), soluble Polytetrafluoroethylene (PFA), and Polytrifluoroethylene (PCTFE) are widely used.

The PTFE (polytetrafluoroethylene) tube has the advantages of extremely strong chemical stability, ageing resistance, non-tackiness, incombustibility and the like, and is suitable for conveying strong acid, strong alkali and various corrosive media relative to other pipeline materials, and the use temperature range of the PTFE tube can reach-180 ℃ to 260 ℃ under normal pressure. In addition, PTFE has small friction coefficient and self-lubricating effect, and can greatly reduce the conveying resistance of pipeline media. The PTFE pipe is widely used in many fields such as petroleum, chemical industry, food, national defense industry, advanced technology, medicine and the like. However, in practical applications, PTFE tubes still have problems such as poor antiknock capability and poor dimensional stability, and particularly, for some fields requiring higher service pressure, further improvements in pressure resistance, tensile strength, and the like have been desired.

Disclosure of Invention

The invention aims to provide a PTFE double-layer composite pipe, a preparation method thereof and a heat exchanger.

The invention provides a PTFE double-layer composite pipe, which comprises a PTFE inner pipe, an inorganic fiber net woven and wound on the outer surface of the PTFE inner pipe and a PTFE outer pipe composited on the surface of the inorganic fiber net;

the inorganic fiber net is obtained by weaving and sintering pretreated inorganic fibers on the outer surface of the PTFE inner tube;

the inorganic fiber is one or more of glass fiber, special glass fiber, basalt fiber, perlite fiber, high silica fiber and high silica modified fiber; the pretreated inorganic fiber is an inorganic fiber which is immersed in polytetrafluoroethylene solution with the mass concentration of 50-70%;

the diameter of the inorganic fiber is 3.5-6 mu m.

Preferably, the inorganic fiber web has a weave density of 1 to 30 pieces/cm.

Preferably, the inorganic fiber web has a thickness of 0.1 to 0.3mm.

Preferably, the total thickness of the PTFE double-layer composite pipe is 0.4-2.0 mm.

The invention provides a preparation method of a PTFE double-layer composite pipe, which comprises the following steps:

a) Dipping inorganic fibers in polytetrafluoroethylene solution with the mass concentration of 50-70% to obtain pretreated inorganic fibers;

b) Sealing one end of the PTFE inner tube, inputting compressed air into the other end of the PTFE inner tube, enabling the PTFE inner tube to be always in a full expansion state, and winding and braiding inorganic fibers on the outer surface of the PTFE inner tube to obtain the PTFE inner tube covered with the inorganic fiber net;

the inorganic fiber is one or more of glass fiber, special glass fiber, basalt fiber, perlite fiber, high silica fiber and high silica modified fiber;

the diameter of the inorganic fiber is 3.5-6 mu m;

c) And (3) nesting and compositing the PTFE outer tube and the PTFE inner tube covered with the inorganic fiber net, degreasing and sintering to form the PTFE double-layer composite tube.

Preferably, the speed of impregnation in step A) is 2-5m/min.

Preferably, the compressed air pressure in said step B) is 0.35 to 0.85kPa.

Preferably, the pretreated inorganic fiber is woven on the surface of the PTFE inner tube after being subjected to presintered treatment;

the temperature of the presintered is 300-600 ℃;

the presintering time is 5-20 min.

Preferably, the draw ratio in step C) is 2 to 10.

The invention provides a heat exchanger, which comprises a heat exchange tube, wherein the heat exchange tube is the PTFE double-layer composite tube.

The invention provides a PTFE double-layer composite pipe, which comprises a PTFE inner pipe, an inorganic fiber net woven and wound on the outer surface of the PTFE inner pipe and a PTFE outer pipe composited on the surface of the inorganic fiber net; the inorganic fiber net is obtained by weaving and sintering pretreated inorganic fibers on the outer surface of the PTFE inner tube; the inorganic fiber is one or more of glass fiber, special glass fiber, basalt fiber, perlite fiber, high silica fiber and high silica modified fiber; the pretreated inorganic fiber is an inorganic fiber which is immersed in polytetrafluoroethylene solution with the mass concentration of 50-70%; the diameter of the inorganic fiber is 3.5-6 mu m. According to the invention, a net-shaped mosaic structure is formed between the inner layer and the outer layer of the PTFE pipeline by adopting the inorganic fiber net subjected to special pretreatment, the fusion performance with the PTFE pipeline is better, the bonding fastness is high, the compression resistance and the dimensional stability of the PTFE pipeline wall material are effectively enhanced, and the bursting pressure of the PTFE composite pipe is obviously improved compared with that of a pure PTFE pipe, so that the composite pipe is more suitable for being used in high-temperature, high-pressure and certain strong corrosion environments.

In addition, the invention also provides a preparation method of the PTFE double-layer composite pipe, compressed air is filled in the PTFE inner pipe, so that the pipe is kept in a full expansion state, the problems of loose fiber winding, unstable combination with the pipe and deformation of the PTFE pipe caused by tight fiber winding are effectively solved, and the surface fiber winding of the pipe is uniform and the tension is consistent.

Detailed Description

the diameter of the inorganic fiber is 3.5-6 mu m.

The PTFE double-layer composite pipe consists of a three-layer nested composite structure, and sequentially comprises a PTFE inner pipe, an inorganic fiber net and a PTFE outer pipe, wherein the inorganic fiber net is a reinforced core layer of the composite pipe.

In the present invention, the material of the PTFE inner tube is polytetrafluoroethylene, and the thickness of the PTFE inner tube is preferably 0.2 to 0.5mm, and specifically, in the embodiment of the present invention, may be 0.25mm, 0.3mm or 0.4mm. The invention is not particularly limited to the outer diameter of the tube material of the PTFE inner tube.

In the present invention, the material of the PTFE outer tube is polytetrafluoroethylene, and the thickness of the PTFE outer tube is preferably 0.2 to 0.5mm, and specifically, in the embodiment of the present invention, may be 0.25mm, 0.3mm or 0.4mm. Preferably, the PTFE outer tube is of uniform thickness as the PTFE inner tube. The inner diameter and the outer diameter of the PTFE outer tube are not particularly limited, and the PTFE outer tube can be matched with the inner tube and the inorganic fiber net to form a good nested composite structure.

In the invention, the inorganic fiber net is wound and woven on the outer surface of the PTFE inner pipe and is compounded with the inner wall of the PTFE outer pipe to form a three-layer nested compound structure.

The inorganic fiber net is obtained by weaving and sintering pretreated inorganic fibers on the outer surface of the PTFE, and the inorganic fibers are preferably one or more of glass fibers, special glass fibers, basalt fibers, perlite fibers, high silica fibers and high silica modified fibers; the pretreatment of the inorganic fibers refers to soaking the inorganic fibers in a polytetrafluoroethylene solution, wherein the polytetrafluoroethylene solution is emulsion prepared by polytetrafluoroethylene and water, and the mass concentration of the polytetrafluoroethylene solution is preferably 50-70%, more preferably 55-65%, and most preferably 60%; the diameter of the fiber is preferably 3.5-6 mu m, namely, the fiber is preferably woven by monofilament fine denier fiber, and the fiber is preferably woven by cross weaving, crisscross grid weaving or spiral winding weaving along the pipe wall, and more preferably woven by crisscross grid weaving; the density of the weave is preferably 1 to 30, more preferably 5 to 25, and in particular, may be 6, 10 or 20 in embodiments of the invention.

The thickness of the inorganic fiber web is preferably 0.1 to 3mm, more preferably 0.1 to 2mm, most preferably 0.2 to 1mm, and in particular, may be 0.1mm or 0.2mm in embodiments of the present invention.

In the present invention, the total thickness of the PTFE double-layer composite tube is preferably 0.4 to 2.0mm, more preferably 0.6 to 1.0mm, and in particular, in the embodiment of the present invention, may be 0.6mm, 0.8mm, or 1.0mm.

The invention also provides a preparation method of the PTFE double-layer composite pipe, which comprises the following steps:

the diameter of the inorganic fiber is 3.5-6 mu m;

In the present invention, the type, amount and source of the polytetrafluoroethylene solution are the same as those of polytetrafluoroethylene described above, and the present invention is not repeated here.

In the present invention, the impregnation speed of the inorganic fiber impregnation treatment is preferably 2 to 5m/min, more preferably 3 to 4m/min.

The invention preferably carries out sintering treatment on the pretreated inorganic fibers, and aims to remove impregnating compound residues on the surfaces of the fibers and ensure better and firmer combination of the inorganic fiber woven layer and the PTFE tube. The temperature of the pre-sintering is preferably 300-600 ℃, more preferably 300-450 ℃, and in particular, in the embodiment of the invention, the temperature can be 300 ℃, 400 ℃ or 450 ℃; the time of the pre-sintering is preferably 8 to 12 minutes.

After the sintered inorganic fibers are obtained, one end of the PTFE inner tube is closed, compressed air is input into the other end of the PTFE inner tube, so that the PTFE inner tube always keeps a full expansion state, and then the inorganic fibers are wound and woven on the outer surface of the PTFE inner tube, so that the PTFE inner tube covered with the inorganic fiber net is obtained.

In the present invention, the pressure of the compressed air to be introduced into the PTFE inner tube is preferably 0.35 to 0.85kPa, more preferably 0.5 to 0.7kPa, and most preferably 0.5 to 0.6kPa.

The PTFE inner tube is preferably prepared according to the following steps,

and mixing polytetrafluoroethylene resin and an auxiliary agent, standing, and sequentially prepressing, pushing, pressing and sintering to obtain the PTFE inner tube.

In the invention, the auxiliary agent preferably comprises aviation kerosene and a silane coupling agent, wherein the aviation kerosene is preferably high-flash aviation kerosene, and the mass ratio of the aviation kerosene to the polytetrafluoroethylene resin is preferably (15-35): 100, more preferably (20 to 30): 100, most preferably (25 to 30): 100;

the silane coupling agent is preferably one or more of vinyl trimethoxy silane, vinyl triethoxy silane, gamma-glycidol ether oxygen propyl trimethoxy silane and gamma-aminopropyl triethoxy silane; the mass ratio of the silane coupling agent to the polytetrafluoroethylene resin is preferably (0.1-1.5): 100, more preferably (0.5 to 1): 100.

in the present invention, the addition of the auxiliary agent is more advantageous in promoting the bonding between the PTFE inner and outer tubes and the inorganic fiber web.

According to the invention, the polytetrafluoroethylene resin and the auxiliary agent are mixed, and the uniform distribution of the polytetrafluoroethylene resin and the auxiliary agent is realized by the double functions of the stirrer and the container rotation, so that the subsequent processing is facilitated;

after the mixing is completed, the obtained mixture is subjected to standing and material awakening so as to promote the fusion of the polytetrafluoroethylene raw material and the auxiliary agent. The temperature of the standing is preferably a constant temperature of 22-26 ℃, more preferably 25+ -1 ℃; the time for the standing is preferably 8 to 16 hours, more preferably 10 to 15 hours.

After the static is finished, the mixture after static is pre-pressed and formed, wherein the pressure of the pre-pressing and forming is preferably 0.35-0.40 MPa, and the pre-pressing and forming method is preferably carried out by adopting a pre-pressing machine.

After the prepressing is finished, the prepressing blank is extruded by pushing and pressing to prepare a PTFE inner tube with uniform structure, wherein the pressure of pushing and pressing is preferably 0.40-0.45 MPa;

after pushing and pressing, the formed PTFE inner tube is sintered and shaped to obtain the PTFE inner tube, wherein the sintering and shaping temperature is preferably 325-355 ℃, more preferably 330-340 ℃; the sintering and shaping time is preferably 10-20 min, specifically, in the embodiment of the invention, may be 10min, 15min or 20min.

The preparation method of the PTFE outer tube is consistent with that of the PTFE inner tube, and different extrusion tube diameters are adjusted, so that the invention is not repeated here.

After the PTFE inner tube and the PTFE outer tube are obtained, the inorganic fiber is wound and woven on the outer surface of the PTFE inner tube, and then the PTFE inner tube and the PTFE outer tube are subjected to nested compounding, degreasing and sintering molding to obtain the PTFE double-layer composite tube.

In the present invention, the kind of the inorganic fiber, the weaving method of the inorganic fiber and the weaving density are identical to those described above, and are not described here again.

The nested composite process in the invention is to combine the inner tube and the outer tube together through the operation of sucking vacuum, and the vacuum degree is preferably-0.01 to-0.04 MPa.

The composite pipe after nested and compounded is introduced into an oil removing system for removing oil, and the oil removing process is an oil removing technology known to those skilled in the art and is not repeated here.

The composite pipe is sintered and shaped after oil removal, wherein the temperature of the sintering and shaping is preferably 325-355 ℃, more preferably 330-340 ℃, and in particular, in the embodiment of the invention, the temperature can be 330 ℃, 340 ℃ or 355 ℃; the sintering and shaping time is preferably 5 to 30min, more preferably 9 to 12min.

The invention also provides a heat exchanger, which comprises a heat exchange tube, wherein the heat exchange tube is the PTFE double-layer composite tube.

In order to further illustrate the present invention, the following examples are provided to describe a PTFE double-layer composite tube, a method for preparing the same, and a heat exchanger in detail, but the scope of the present invention is not limited thereto.

Example 1

1000g of polytetrafluoroethylene resin, 150g of aviation kerosene and 1g of vinyl trimethoxy silane are uniformly distributed through the dual actions of a stirrer and a container, so that the subsequent processing is facilitated. The evenly mixed raw materials are kept stand for 12 hours at a constant temperature of 25+/-1 ℃ to promote the fusion of the polytetrafluoroethylene raw materials and the auxiliary agent.

After standing and awakening, the premix is put into a preformer for pressing, and the pressure value is preferably 0.35MPa, so as to prepare a blank.

The pre-pressed blank is introduced into a push press and extruded under the pressure of 0.40MPa to form the inner PTFE pipe with uniform structure and 0.25mm wall thickness.

And (3) conveying the inner PTFE pipe into a sintering machine, and sintering and shaping at 330+/-2 ℃ for 20min.

The fine denier special glass fiber is immersed in a 60% polytetrafluoroethylene solution for 2m/min, and then presintered at 300 ℃ to remove the impregnating compound residues on the fiber surface. The fine denier special glass fiber is woven and wound on the surface of the inner PTFE pipe formed by sintering by a warp knitting machine in a crisscross manner, the weaving density is 10 pieces/cm, the winding thickness is 0.1mm, one end of the inner PTFE pipe is closed, and compressed air with the air pressure of 0.5kPa is input into the other end of the inner PTFE pipe in the weaving process, so that the inner PTFE pipe always keeps a full expansion state.

The outer layer PTFE tube was subjected to the same manufacturing process as the inner layer PTFE tube in order to obtain an outer layer PTFE tube having a wall thickness of 0.25 mm. The inner layer PTFE tube and the outer layer PTFE tube which are woven and wound with fine denier special glass fiber on the surface are nested and compounded, the PTFE composite tube after the compounding processing is introduced into an oil removing system to remove auxiliary oil, and the PTFE composite tube enters a sintering machine after oil removal, and is sintered and shaped at the temperature of 330+/-2 ℃. The inner layer PTFE tube, the fine denier special glass fiber winding braid and the outer layer PTFE tube are tightly combined together through nested composite processing and sintering shaping, and the high-strength special glass fiber/PTFE composite tube with the wall thickness of 0.6mm is manufactured. And finally, rolling, checking, packaging and warehousing.

The pressure resistance of the PTFE composite tube obtained in example 1 is shown in table 1,

TABLE 1 pressure resistance of PTFE composite pipe obtained in example 1

Example 2

1000g of polytetrafluoroethylene resin, 280g of aviation kerosene and 6g of vinyl triethoxysilane are uniformly distributed through double functions of a stirrer and a container, so that subsequent processing is facilitated. The evenly mixed raw materials are stood for 16 hours at a constant temperature of 22+/-1 ℃ to promote the fusion of the polytetrafluoroethylene raw materials and the auxiliary agent.

After standing and awakening, the premix is put into a preformer to be pressed into a blank under the pressure of 0.38 MPa.

The pre-pressed blank is introduced into a push press and extruded under the pressure of 0.43MPa to form the inner PTFE pipe with uniform structure and 0.3mm wall thickness.

And (3) conveying the inner PTFE pipe into a sintering machine, and sintering and shaping at 340+/-2 ℃ for 15min.

Fine denier basalt fiber is immersed in 60% PTFE solution for 3m/min, and then presintered at 400 ℃ to remove impregnating compound residues on the fiber surface. The fine denier basalt fiber is woven and wound on the surface of the inner layer PTFE tube formed by sintering through a warp knitting machine, the weaving density is 6 pieces/cm, the thickness is 0.2mm, one end of the inner layer PTFE tube is closed, and compressed air with the air pressure of 0.6kPa is input into the other end of the inner layer PTFE tube in the weaving process, so that the PTFE inner tube always keeps a full expansion state.

The outer layer PTFE tube is sequentially subjected to the same manufacturing procedure as the inner layer PTFE tube to prepare the outer layer PTFE tube with the wall thickness of 0.3mm. The inner PTFE tube and the outer PTFE tube which are woven and wound with fine denier basalt fibers on the surfaces are nested and compounded, the PTFE composite tube after compounding processing is introduced into an oil removing system to remove auxiliary oil, and the PTFE composite tube enters a sintering machine after oil removal and is sintered and shaped at the temperature of 340+/-2 ℃. The inner layer PTFE tube, the fine denier basalt fiber winding braid and the outer layer PTFE tube are tightly combined together through nested composite processing and sintering shaping, and the high-strength basalt fiber/PTFE composite tube with the wall thickness of 0.8mm is manufactured. And finally, rolling, checking, packaging and warehousing.

The pressure resistance of the PTFE composite tube obtained in example 2 is shown in table 2,

TABLE 2 pressure resistance of PTFE composite pipe obtained in example 2

Example 3

1000g of polytetrafluoroethylene resin, 350g of aviation kerosene and 10g of gamma-glycidol ether oxypropyl trimethoxy silane are uniformly distributed through the dual actions of a stirrer and a container rotation, so that the subsequent processing is facilitated. The evenly mixed raw materials are kept stand for 8 hours at a constant temperature of 26+/-1 ℃ to promote the fusion of the polytetrafluoroethylene raw materials and the auxiliary agent.

After standing and awakening, the premix is put into a preformer to be pressed into a blank under the pressure of 0.4 MPa.

The pre-pressed blank is introduced into a push press and extruded under the pressure of 0.45MPa to form the inner PTFE pipe with uniform structure and 0.4mm wall thickness.

And (3) conveying the inner PTFE pipe into a sintering machine, and sintering and shaping at 355+/-2 ℃ for 10min.

The fine denier high silica (modified) fiber is soaked in a 60% polytetrafluoroethylene solution for 4m/min, and then presintered at 450 ℃ to remove the sizing agent residues on the fiber surface. The surface of the inner layer PTFE tube formed by sintering is vertically and horizontally interwoven and wound with fine denier high silica (modified) fibers by a warp knitting machine, the weaving density is 20 pieces/cm, the thickness is 0.2mm, one end of the inner layer PTFE tube is closed, and compressed air with the air pressure of 0.55kPa is input into the other end of the inner layer PTFE tube in the weaving process, so that the PTFE inner tube always keeps a full expansion state.

The outer layer PTFE tube is sequentially subjected to the same manufacturing procedure as the inner layer PTFE tube to prepare the outer layer PTFE tube with the wall thickness of 0.4mm. The inner layer PTFE tube and the outer layer PTFE tube which are woven and wound with fine denier high silica (modified) fiber on the surface are nested and compounded, the PTFE composite tube after the compounding processing is introduced into an oil removing system to remove auxiliary oil, and the PTFE composite tube enters a sintering machine after oil removal, and is sintered and shaped at 355+/-2 ℃. The inner layer PTFE tube, the fine denier high silica (modified) fiber winding braid and the outer layer PTFE tube are tightly combined together through nested composite processing and sintering shaping, and the high strength high silica (modified) fiber/PTFE composite tube with the wall thickness of 1mm is manufactured. And finally, rolling, checking, packaging and warehousing.

The pressure resistance of the PTFE composite tube obtained in example 3 is shown in table 3,

TABLE 3 pressure resistance of PTFE composite pipe obtained in example 3

Comparative example 1

The PTFE composite tube was prepared according to the preparation method in example 1, except that only polytetrafluoroethylene resin was used as the raw material for the PTFE inner tube and the PTFE outer tube in the present comparative example, and aviation kerosene and a silane coupling agent auxiliary agent were not used.

The pressure resistance of the obtained PTFE composite pipe is shown in Table 4.

TABLE 4 pressure resistance of PTFE composite pipe obtained in comparative example 1

Comparative example 2

A PTFE composite tube was prepared according to the preparation method in example 1, except that the inorganic fibers in this comparative example were directly used for weaving an inorganic fiber web without performing the impregnation pretreatment.

The pressure resistance of the obtained PTFE composite pipe is shown in Table 5.

TABLE 5 pressure resistance of PTFE composite pipe obtained in comparative example 2

TABLE 6 comparison of the Performance of PTFE composite tubes and pure PTFE tubes in example 3 and comparative examples 1-2

Performance of	Unit (B)	Pure PTFE	Comparative example 1	Comparative example 2	Example 3
						Specific gravity	—	2.16	2.19	2.18	2.18
Shrinkage rate of molding	％	4.91	4.12	4.09	3.55
						Tensile Strength	MPa	17.55	20.44	21.73	35.50
Elongation at break	％	352	228	235	186
						Flexural modulus	MPa	5977	6342	6860	7870
Notched impact Strength	kgf·cm/cm ²	16.32	18.31	19.22	23.22
						Melting point temperature	℃	327	327	327	327
Long-term heat resistance temperature	℃	260	260	260	260
						Heat distortion temperature (4.6 kg/cm) ² )	℃	121	133	135	180
Dielectric constant	-	2.11	2.13	2.27	2.06

The foregoing is merely a preferred embodiment of the present invention and it should be noted that modifications and adaptations to those skilled in the art may be made without departing from the principles of the present invention, which are intended to be comprehended within the scope of the present invention.

Claims

1. The preparation method of the PTFE double-layer composite pipe comprises a PTFE inner pipe, an inorganic fiber net woven and wound on the outer surface of the PTFE inner pipe and a PTFE outer pipe composited on the surface of the inorganic fiber net;

the PTFE double-layer composite tube is prepared by the following steps:

a) Dipping inorganic fibers in polytetrafluoroethylene solution with the mass concentration of 50-70% to obtain pretreated inorganic fibers, and then presintering the pretreated inorganic fibers; the speed of the impregnation in the step A) is 2-5m/min;

the diameter of the inorganic fiber is 3.5-6 mu m; the air pressure of compressed air in the step B) is 0.35-0.85 kPa;

2. The method of claim 1, wherein the inorganic fiber web has a weave density of 1 to 30 pieces/cm.

3. The method of claim 1, wherein the inorganic fiber web has a thickness of 0.1 to 0.3mm.

4. The method of claim 3, wherein the PTFE double-layer composite tube has an overall thickness of 0.4 to 2.0mm.

5. The method according to claim 1, wherein the pretreated inorganic fiber is subjected to pre-sintering treatment and then woven on the surface of the PTFE inner tube;

the temperature of the pre-sintering is 300-600 ℃;

the presintering time is 5-20 min.

6. The method according to claim 1, wherein the stretching ratio in the step C) is 2 to 10.

7. A heat exchanger comprising a heat exchange tube which is a PTFE bilayer composite tube according to any one of claims 1 to 6.