WO1987001788A1

WO1987001788A1 - Pipe coupling for refrigerant fluid pipes

Info

Publication number: WO1987001788A1
Application number: PCT/SE1986/000416
Authority: WO
Inventors: Bertil SJÖSTEDT; Roland Kall
Original assignee: ELEKTRO STANDARD AB
Current assignee: ELEKTRO STANDARD AB
Priority date: 1985-09-20
Filing date: 1986-09-17
Publication date: 1987-03-26
Anticipated expiration: 1988-03-20
Also published as: SE8504374L; SE8504374D0; SE450523B; CA1286335C

Abstract

A pipe coupling for fluid-sealed, electrically insulated coupling of two metal pipes (1, 3 and 2, 4, respectively) intended for the flow of a refrigerant fluid at high pressure and high temperature. The coupling comprises a coupling sleeve (8) of polytetrafluoroethylene and an external clamping sleeve (9) of metal. The wall thickness (t) of the coupling sleeve (8) amounts to 0,5 - 2,0 mm. The distance (d) between the two metal pipe ends is 0,5 - 2,0 mm and must not exceed twice the sleeve wall thickness (t), i.e. d <= 2t. Hereby, a good sealing effect is achieved even after long use.

Description

PIPE COUPLING FOR REFRIGERANT FLUID PIPES

The invention relates to a fluid-sealed, electrically insulated coupling of two metal pipes intended for the flow of a refrigerant fluid, especially of fluorocarbon type, such as dichlorodifluoromethane (R12), monochloride fluoromethane (R22) or a mixture of monochloride fluoromethane and monochloropentaf luoroethane (R502). More particularly, the invention relates to a pipe coupling of the kind defined in the preamble of claim 1.

Refrigerant fluids of the above-mentioned types (R12, R22 and R502) are normally used in refrigerant circulation circuits, especially in heat pump systems. In this connection, conduits of copper are normally used, which are resistant to such refrigerant fluids and stand the prevailing temperatures (up to about 150°C) and pressures (up to about 30-50 bar). In certain cases, however, it has turned out to be necessary to keep certain parts of the conduit circuit electrically insulated from the other parts of the circuit, in particular in water heaters in systems where heat is transferred from an exhaust air heat exchanger to consumption hot water by means of a heat pump. Thus, in order to avoid corrosion problems, the part of the copper circuit located in the water heater must be kept electrically insulated from the container wall and also from the heat pump compressor, since the latter, as well as the container wall, is normally earthed.

Consequently, there is a need to achieve an electrically insulated pipe coupling which is resistant to the circulating refrigerant fluid.

According to the invention, this need is met with by a pipe coupling having the features stated in claim 1. Thus, the coupling sleeve, which is radially pressed between the pipe end portions and an external clamping sleeve, consists of polytetrafluoroethylene (PTFE) having a sleeve wall thickness of 0,5 - 2,0 mm, whereas the distance between the two metal pipe ends is 0,5 - 2,0 mm and does not exceed twice the thickness of the sleeve wall.

It is acknowledged that DE-A-15 25 648 discloses a pipe coupling (for hot water pipes) having a coupling sleeve of plastic material which is radially pressed between each pipe end portion and an external clamping sleeve, wherein the distance between the pipe ends is of the same magnitude as the thickness of the coupling sleeve wall. However, in the embodiments having such a relation between the pipe end distance and the sleeve wall thickness, the coupling sleeve is separated into two parts each enclosing the respective pipe end portion. Such an arrangement cannot be used for refrigerant fluids, because leakage will inevitably occur. Furthermore, there is no teaching of the use of polytetrafluoroethylene as a material for the coupling sleeve, and there is no suggestion indicating that the relation between the pipe end distance and the sleeve wall thickness would be critical.

In a large number of tests it has turned out that a refrigerant fluid of the type R22 reacts with and diffuses through most of the plastic materials which endure the temperatures and pressures prevailing in heat pump circuits of said kind. Polytetrafluoroethylene appears to be chemically resistant but, nevertheless, simple sleeve couplings turned out to be unsatisfactory because of a certain diffusion and accompanying leakage. Actually, the leakage flow was minor but still unacceptable. Thereafter, an external clamping sleeve of metal, extending axially over the entire region of the pipe coupling, was tested in order to achieve the necessary support and sufficient tightness. However, in a strange way, after being in use for a rather long time, the coupling sleeve was damaged, and leakage could not be avoided.

Surprisingly, after several unsuccessful experiments, a perfect tightness could be obtained only upon reducing the distance between the free ends of the pipes to a value of about the same magnitude as the wall thickness of the coupling sleeve or at most twice said thickness.

Afterwards, by comparative studies of the couplings, this phenomenon could be explained after long use thereof. Thus, when using a large axial distance between the free ends of the pipes, the surrounding coupling sleeve tended to collaps radially inwardly. By a slow leakage radially through the coupling sleeve, a pressure of the same magnitude as the work pressure inside the conduit was built up between the outside of the coupling sleeve and the external clamping sleeve. As long as the work pressure was maintained inside the conduit, a balance was maintained between the inside and the outside of the coupling sleeve, but as soon as the work pressure was suddenly reduced when the heat pump compressor no longer needed to work, a force directly radially inwards arose on the wall of the coupling sleeve due to the remaining pressure in the space between the outside of the coupling sleeve and the inside of the clamping sleeve. This inwardly directed compressive force could not be absorbed by the PTFE material, unless the portion of the coupling sleeve extending axially between the pipe ends was made sufficiently short in relation to the wall thickness.

Moreover, a further improvement of the sealing effect can be achieved, if the end portions of the metal pipes, the coupling sleeve and the external clamping sleeve are made slightly conical so that the diameter increases towards the ends of the metal pipes and the central portions of the clamping sleeves, respectively. The invention will now be explained more fully with reference to the appended drawings illustrating two preferred embodiments.

Fig. 1 shows an axial section through a pipe coupling according to a first embodiment of the invention; and

Fig. 2 shows a corresponding section through a second embodiment.

In the pipe coupling according to Fig. 1, two copper pipes 1,2 are at their free ends permanently joined to relatively short steel tubes 3 and 4, respectively, which are internally coated with copper. The joints between the copper pipes and the steel tubes consist of sleeve portions 5 and

6, respectively, made in one piece with the respective steel tube, each sleeve portion 5,6 having a relatively large diameter and into which each copper pipe end has been inserted and secured by solding in such way that the respective steel tube 3, 4 internally adjoins the corresponding copper tube 1,2.

The steel tubes 3,4 are externally provided with circumf erentially extending rims 7 intended to engage into a

surrounding coupling sleeve 8 of polytetrafluoroethylene. This coupling sleeve 8 extends axially along the major parts of the two steel tubes 3,4 and also encloses the space between the free ends of the steel tubes. An external clamping sleeve 9 with internal, circumf erentially extending rims 10 encloses the coupling sleeve 8 along the major part of the length thereof. When making the pipe coupling, the clamping sleeve 9 is radially compressed, namely along its entire length, so that the coupling sleeve 8 is pressed tightly between the steel tubes 3,4 and the clamping sleeve 9, wherein the rims 7 and 10 contribute to an effective sealing and locking against axial displacement. According to the invention, it is essential that the distance d between the free ends of the pipes is not too large. Naturally, it must exceed the shortest distance necessary for electrical insulation of the tubes 3,4 and, consequently, of the copper pipes 1,2. The distance d can possibly be 0,5 -2,0 mm but must not exceed twice the wall thickness t of the coupling sleeve 8. This wall thickness may amount to 0,5 - 2,0 mm.

Furthermore, the distance d between the pipe ends should preferably be selected so as to correspond to 10-20% of the external diameter of the pipe end portions 3,4, which external diameter normally may amount to 7-10 mm. Thus, the distance d may suitably be 0,7 - 2,0 mm. Then, the wall thickness t should be 0,8 - 1,2 mm.

When making the pipe coupling, the pipe end portions may be held at a well-defined, desired mutual distance d by securing the sleeves 5 and 6 in a fixture.

In fig. 1, the tubes 3,4, the coupling sleeve 8 and the clamping sleeve 9 are cylindrical with a uniform diameter along the respective length thereof. In Fig. 2 an alternative embodiment is shown, wherein these portions are made somewhat conical. Thus, each tube 3',4' has an external diameter which increases towards the central portion of the pipe coupling. The coupling sleeve 8' and the clamping sleeve 9' are likewise conical (having a larger diameter centrally), but the thickness is substantially constant. The conicity (half the angle of the top of the cone) may amount to 1° - 2° and is preferably 1,25°. Such a conical structure of the pipe coupling gives a still better sealing effect, especiallly at relatively high internal pressures. The co-operating conical surfaces prevent the pipe ends from moving apart from each other and, additionally, the clamping effect of the coupling sleeve 8' is increased under the influence of an internal pressure.

Claims

C L A I M S

1. A pipe coupling for a fluid-sealed, electrically insulated coupling of two metal pipes (1,3 and 2,4, respectively) intended for the flow of a refrigerant fluid at high pressure and high temperature, comprising a coupling sleeve (8), which consists of an electrically insulating plastic material being resistant to said refrigerant fluid and which is dimensioned to externally sealingly enclose two axially separated ends (3,4) of said metal pipes, and an external clamping sleeve (9) which consists of metal and is dimensioned to enclose and clamp the coupling sleeve in the region of the separated metal pipe ends, c h a r a c t e r i z e d in that the coupling sleeve (8) also encloses the space between the metal pipe ends (3,4) and is made of polytetrafluoroethylene, the coupling sleeve wall thickness (t) amounting to 0,5 - 2,0 mm, wherein the distance (d) between the metal pipe ends is 0,5-2,0 mm and does not exceed twice the wall thickness (t).

2. A pipe coupling as defined in claim 1, c h a r a c t e r i z e d in that the metal pipes (3', 4'), the coupling sleeve (8') and the clamping sleeve (9") are slightly conical and have the largest diameter in the central part of the pipe coupling.

3. A pipe coupling as defined in claims 1 or 2, c h a r a ct e r i z e d in that said distance (d) between the metal pipe ends amounts to 10-20% of the external diameter of the pipe end portions.

4. A pipe coupling as defined in claim 3, c h a r a c t e r i z e d in that the external diameter of the metal pipe end portions is 7-10 mm.

5. A pipe coupling as defined in claim 4, c h a r a c t e r i z e d in that said sleeve wall thickness (t) is 0,8-1,2 mm and that said distance (d) between the metal pipe ends is 0,7-2,0 mm.

6. A pipe coupling as defined in anyone of the preceding claims, c h a r a c t e r i z e d in that said metal pipe end portions consist of steel tubes (3,4), which are internally coated with copper, and that these tubes are axially connected to copper pipes (1,2).

7. A pipe coupling as defined in claim 6, c h a r a c t e r i z e d in that the respective tube (3,4) is connected to the respective copper pipe (1,2) by means of a sleeve portion (5,6) having a larger diameter than the tube.

8. A pipe coupling as defined in claims 6 or 7, c h a r a ct e r i z e d in that said external clamping sleeve (9) also consists of steel.

9. A pipe coupling as defined in anyone of the preceding claims, c h a r a c t e r i z e d in that said metal pipe end portions (3,4) are provided with external rims (7) extending circumf erentially.

10. A pipe coupling as defined in anyone of the preceding claims, c h a r a c t e r i z e d in that said clamping sleeve (9) is provided with internal rims (10) extending circumferentially.