FIELD OF THE INVENTION The present invention relates to a thermal isolation suitable for isolating a refrigeration compressor gas discharge tube, where the discharge tube is coaxially arranged inside an isolation tube, forming a confined space between said tubes. The confined space may be evacuated or provide air or another gas in its inside. BACKGROUND OF THE INVENTION

As is generally known, in order to increase the performance of the refrigeration compressors, it is necessary to reduce the thermal losses that mostly result from the heating of the suction gas along its path inside the compressor, from its entry into the housing to the compression cylinder. In addition to the performance issue, the reduction of the inner temperature of the components increases the compressor life, since it reduces the wear of bearings and the degradation of other non-metallic components, such as rubbers and polymers.

It has been verified that the increase in the temperature of the suction gas and the compressor inner components is caused by hot sources situated inside the compressor, one of the greatest hot sources being the compressed gas discharge tube.

An excellent alternative utilized for reducing the compressor inner temperature and avoiding the problems mentioned above has been the dis- charge tube thermal isolation by means of the concept of enclosed space. As is already known, the concept of confined space consists of placing an isolation tube coaxially to the discharge tube so that a space is formed between the two tubes where a gas (e.g., the air) remains that does not move. There also exist isolations where this space is evacuated, but this feature implies higher costs. PRIOR ART AND ITS DRAWBACKS

Thus, various types of isolation have been created utilizing this concept, the main hindrance to obtaining satisfactory results from the already known thermal isolations being the flexibility problem of said isolation, since, with a not much flexible isolation, the vibrations from the operation of the compressor disrupt said isolation. Also, a not much flexible isolation makes its insertion into the tube to be isolated difficult.

To address this problem, documents US 3.926.009 and US 4.371.319 show a thermal isolation by means of the technique of the confined space associated with corrugations, the discharge tube of document US 3.926.009 being fully inserted into a corrugated tube. However, the corrugated structure has a drawback in that the corrugations act as if they were fins, increasing the heat exchange area, and, as a consequence, not providing a satisfactory thermal isolation for the discharge tube. Another drawback is that the corrugated tubes have a higher manufacture cost than that of smooth tubes.

In addition, the corrugated tubes, in isolation curved regions, fail to structurally maintain the tube to be isolated spaced apart, and, thus, bear against said tube, which decreases the efficiency of isolation. OBJECTS AND ADVANTAGES OF THE INVENTION

It is the object of the present invention to provide a thermal isolation for the discharge tube of a refrigeration compressor that possesses optimal isolating properties, long durability and low cost.

This object is achieved by means of a thermal isolation, par- ticularly suitable for isolating the gas discharge tube of a refrigeration compressor, wherein the discharge tube is coaxially arranged inside an isolation tube, forming a confined space between said tubes, the isolation tube comprising a helical spacer, arranged around and spaced from the gas discharge tube, and a cover of flexible material hermetically enveloping the spacer, the spacer in certain points having reduced diameters and firmly enveloping the discharge tube.

Conveniently, the spacer may be a metal or plastic spring.

Thus, one of the advantages of the thermal isolation accord- ing to the present invention is the fact that the insulator outer area is smaller than that of the prior art corrugated tube, which provides for a thermal isolation with an excellent isolation capacity.

Another advantage is that the isolation flexibility may vary according to the needs of the path travelled by the discharge tube inside the com- pressor. Thus, in the event of a bend or a sinuous stretch, one can reduce or increase the distance between two points of the spring with reduced diameter, or the pitch between the spring coils, such that the isolation flexibility is also reduced or increase.

Another advantage of the invention is the fact that the cover is supported by the spring and, therefore, does not have a structural function. In this manner, said cover may present a very small thickness, which allows for the use of low cost materials, such as, for example, a plastic film. Also, a very thin wall provides more space for the optimization of the confined space. As is already known, the isolation itself is provided by the gas layer around the discharge tube, and, thus, the cover has the function of containing this gas only, so that the this latter does not move.

A further advantage of the invention is that, by the friction effect between the discharge tube and the reduced diameter regions, energy dissipation may happen that helps the reduction of the noise and vibration levels at high frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will thereafter be described in more detail, by way of example, based on the appended figures: Figure 1 - an inner view of a refrigeration compressor, where the gas discharge tube is isolated with the thermal isolation according to a preferred embodiment of the invention;

Figure 2 - inner side view of the discharge tube enveloped by the thermal isolation according to a preferred embodiment of the invention;

Figure 3 - perspective front view of the discharge tube enveloped by the thermal isolation, shown in Figure 2;

Figure 4 - perspective view of the spacer (spring) arranged around the discharge tube, shown in Figure 2. DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows the inner view of a refrigeration compressor 1 , where an electric motor 2 is seen, with a stator 3 and an armature 4. The motor axle 5 drives a piston 6, arranged inside a cylinder 7 having a set of valves 8 and a head 9. In the compressor bottom is a lubricant oil reservoir. Thus, with the compressor operation, the gas is sucked by suction line 10, expands itself in space 11, and, after cooling motor 2, is sucked into cylinder 7 by means of an inlet passage 12. Next, the gas is compressed by piston 6 and discharged by discharge tube 13, which is isolated by a thermal isolation 14 in accordance with a preferred embodiment of the invention. As is generally known, discharge tube 13 provides a long path inside the compressor, starting from cylinder 7 and extending to the housing upper portion, so that the vibrations caused by the motor and the compression process are dampened. Otherwise, if the discharge tube length between the cylinder and the compressor housing wall were short, said tube would rapidly break due to the extremely high fatigue stresses. Figure 2 shows discharge tube 13 arranged inside thermal isolation tube 14 according to the invention. Isolation tube 14 comprises a spacer 15, that may consist, for example, of a spring made of metal or other material, such as, for example a polymer. Spring 15 is arranged around discharge tube 13 and, in certain points 16, provides a reduced diameter, so as to firmly envelop discharge tube 13. Around spring 15 there is a cover of flexible material 17 hermetically enveloping the spring, forming a confined space 18 that is filled with air or another gas. As already mentioned previously, the structural function is only performed by the spring, and, therefore, cover 17 may have a very small thickness. The purpose of the cover is only to retain the gas (e.g. air) in the clearance created so that it does not move, thereby forming an isolation layer around discharge tube 13. Also, considering that the thermal isolation tube 14 flexibility is mainly provided by spring 15, one may also utilize materials not so flexible in cover 17, provided that they are firmly anchored onto spring 15. In this manner, cover 17 may be of a very thin material, with a satisfactory resistance to breakage, such as, for example, a plastic film. Cover 17 may be placed before or after mounting spring 15, and, in case of a cylinder-shaped cover, it may be pulled over the spring as if it were a piece of clothing. Cover 17 may also be of a thermo-retractable material, which, after being placed around the spring, is heated so as to contract and adjust itself to the outer surface of said spring. This contraction substantially improves the junction between the spring and the cover, and, as a consequence, provides greater flexibility for isolation 14 and a better isolation capacity, since the heat transfer area is reduced. The lengths between points 16 can be increased or decreased so as to, respectively, also increase or decrease the spring flexibility, such that the spring can follow all of the variations in the discharge tube, such as curves, sinuous stretches and changes in diameter, keeping the flexibility and the capacity for thermal isolation unchanged. As an alternative to the variation in the length between points 16, one may also increase or decrease the spring pitch, so as to change the flexibility of isolation tube 14.

Figure 3 shows a perspective front view of discharge tube 13 and isolation tube 14 shown in Figure 2, where it can be seen in more detail con- fined space 18 formed between the outer surface of tube 13 and the inner surface of cover 17. The ends of isolation tube 14 may remain opened, or be closed by means of e.g. a finger, a metal ring or a plastic ring.

Figure 4 shows in more detail spring 15 arranged around dis- charge tube 13 and a reduced diameter point 16, where the spring firmly envelops said tube 13. In case of a metal spring e.g. of a carbon steel, in order to avoid the heat transfer by conduction between discharge tube 13 and spring 15, a plug of insulating material, such as plastic or rubber, may be placed in reduced diameter point 16, between said spring 15 and discharge tube 13. The plug may be fixed in the discharge tube or the spring. Stainless steel or plastic springs are other alternatives for reducing the heat transfer by conduction between discharge tube 13 and spring 15.

In addition to the embodiment presented previously, the same inventive concept may be applied to other alternatives or possibilities of using the invention. For example, confined space 18 may be evacuated or the isolation may be utilized to isolate vapor nets.

As such, it will be understood that the present invention is to be construed broadly, its scope determined by the terms of the appended claims.