EP0117829B2 - Tubular heat exchanger - Google Patents

Abstract

A tube-type heat exchanger comprises a plurality of tubes (1) all of which are of the same length, wound in the form of concentric cylindrical coils that touch each other, the pitch of the coils being different and the various coils being either wound in the same direction, or wound in alternately opposite directions. Each coil is comprised by one or more tubes bent to the same radius, the number of tubes in each coil increasing as the diameter of the coil.

Description

The present invention relates to the field of construction of thermal machines, in particular heat pumps, and relates to a pipe heat exchanger.

The heat exchangers currently existing, of the condenser or evaporator type, are generally constituted by a coil arranged in a heat exchange chamber, in which the coil transmits the heat or cooling energy which it transports to the fluid in change of state, which surrounds it.

However, in these known exchangers, the pressure drops in the primary and secondary circuits are significant and these exchangers require significant sizing to allow good heat efficiency, this sizing having an entirely positive effect on the cost price of such exchangers.

It has been proposed to obviate these drawbacks by producing a heat exchanger constituted by a spiral winding of a sheet of contiguous pipes arranged in an annular chamber delimited by an inner tube forming the core of the winding, and by an external tube enveloping the winding.

However, such an exchanger does not allow uniform flow rates and pressure losses in the pipes constituting it due to the difference in length necessarily existing between the interior pipes and the exterior pipes, so that its thermal efficiency is also affected.

DE-A-2 736 489 also discloses a heat exchanger comprising pipes of substantially equal length, and of the same diameter, which are wound in concentric tubes around a core and in one direction of winding. The pipes of these tubes are not, however, contiguous, but are connected to an intermediate screen serving, moreover, for heat exchange.

In addition, in DE-A-2 736 489, the intervals between the pipes of the various concentric coil windings are variable, so that the circulation characteristics of the heating fluid are not identical in all points of the exchanger.

It is also known, from FR-A-1 523 810, a heat exchanger constituted by concentric windings which are not contiguous with pipe strands.

In such an exchanger, the circulation of the fluid in change of state outside the pipes takes place almost vertically due to the very arrangement of the non-contiguous concentric tubes.

However, such an embodiment does not allow an optimal thermodynamic effect to be obtained.

In addition, FR-A-1 523 810 describes an embodiment with pipes of relatively long lengths, not allowing free axial expansion in the event of accidental freezing, when the exchanger is used as an evaporator.

Furthermore, there is known from FR-A-1 321 766 a heat exchanger, constituted by coils in concentric tubes of pipes, but whose pipes are of unequal lengths, these tubes being, moreover, maintained at regular spacing by means spacers.

This exchanger has substantially the same characteristics as that object of FR-A-1523 810 and has the same drawbacks as the latter.

There is also known from the publication Scholz H., Linde Berichte aus Technik und Wissenschaft 33/1973, pages 34 to 39, a heat exchanger in accordance with the preamble of the claim.

We have, of course, sought to obtain an exchanger whose flow rates and pressure drops in the tubes are uniform, and this is why this heat exchanger has pipes of equal length and constant diameter. But the turns of the same flange are not contiguous and cannot be contiguous, any more than the concentric flanges between them, because otherwise the pressure drops around the pipes would be too great, due to the long length of the pipes which causes necessarily very low pitch turns, and therefore an obstacle to the circulation of the fluid, almost perpendicular to these turns.

On the other hand, this exchanger needs long loops in order to make the connection between the tubes and the tubular plates, this to obtain an exchanger whose length of all the pipes is equal. Finally, the diameter of the pipes is relatively large compared to the wall thickness and their length is important.

Finally, US-A-1 091 369 discloses a heater for a steam boiler having different flow rates in the different tubes. In addition, all the tubes are wound in the same direction, which increases the pressure losses in the case of a fluid in change of state.

The problem posed by the invention therefore consists in producing a compact exchanger, without this compactness having a negative influence, on the one hand, on the thermal efficiency and, on the other hand, on the pressure drops. In addition, security is sought in the event of accidental freezing.

To this end, the subject of the invention is a pipe heat exchanger, essentially constituted by pipes arranged in an annular chamber formed by a core and by an external envelope, the pipes of the annular chamber all having equal lengths, being wound in the form of concentric tubes around the core and whose turns are wound in an inverse, alternating fashion, each tube being made up of one or more pipes wound on the same generator, the name bre of pipes increasing with the winding diameter of the rod, said pipes being formed so as to have neither dead angle nor change in diameter, on the path in contact with the fluid in change of state, their ends opening into the next tube a direction parallel to this tube and to the direction of flow of the liquid at the inlet and at the outlet of the exchanger, the tubes being wound in different steps, the turns of the tubes being contiguous, or practically contiguous, and do not leaving only a minimum space for the passage of the fluid in change of state to be heated or cooled, characterized in that the concentric tubes are joined, in that the pipes have a diameter and a length allowing flow velocities fast, and a diameter less than six times their wall thickness, and their ends open, via short bends, with an inner radius of about it is equal to 1.25 diameter, outside the annular chamber in the fluid flow tube.

The invention will be better understood thanks to the description below, which relates to a preferred embodiment, given by way of nonlimiting example, and explained with reference to the appended schematic drawing, the single figure of which is a partially view torn off, and partially in section of an exchanger according to the invention.

The figure of the appended drawing shows, by way of example, the heat exchanger according to the invention, which essentially consists of pipes 1 arranged in an annular chamber 2 formed by a core 3 and by an outer casing 4. The pipes of this exchanger all have identical lengths and are wound around the core 3 in the form of contiguous concentric strands which have different pitches, and which are wound in alternating reverse fashion. These tubes are each made up of one or more pipes 1 wound on the same generator, and the number of pipes 1 depends on the winding diameter of the pipe, and increases with said diameter.

The turns of the strands thus wound are joined, or practically contiguous, so as to leave only a minimum space for the passage of the fluid in change of state to be heated or cooled, while providing a maximum exchange surface. .

The pipes 1 are advantageously in large number and have a relatively small diameter and length, allowing, even at low flow rates, relatively rapid flow rates resulting in good heat exchange. The passage of the liquid in pipes 1 of small diameter allows an increase in the exchange surface for a given flow rate, so that the thermal efficiency of the exchanger is improved compared to an exchanger comprising a small number of large pipes.

The provision of pipes 1 all having an identical length also makes it possible to standardize the flow rates and pressure drops in said pipes.

The embodiment according to the invention therefore ensures an identical change in temperature of the liquid for the pipes, which is very important from the thermodynamic point of view.

Due to the concentric and contiguous arrangement of the tubes 1 and the joined winding of the latter on each tube, the free space 5 between the tubes 5 in the annular chamber 2 is reduced to a minimum, which is sufficient for absorb the relatively low mass flow rates of the fluid in change of state in the evaporator or the condenser, these flow rates being much lower than those of the liquid circulating in the pipes 1 and with which the heat exchange takes place.

In addition, when the exchanger according to the invention is used in the form of an evaporator, the preferred position of which is the vertical position, the boiling liquid is in the lower part and the vapors are discharged in the part superior. In refrigeration circuits, the fluid subjected to the change of state generally entails a part of the lubricating oil of the compressor, which must therefore be constantly brought back to the latter. However, in known evaporators, the oil-refrigerant mixture is subjected to a distillation as a result of which the non-volatile oil tends to remain and accumulate in the lower part of the evaporator.

The invention makes it possible to avoid this accumulation thanks to the reduction in the available volume of the annular chamber 2, which has the effect of causing a forced vertical routing of the oil in the form of droplets or mist entrained by the vapors, which evolve at relatively high speeds.

According to another characteristic of the invention, the pipes 1 have a diameter less than six times their wall thickness, and are formed so as to have neither dead angle nor change in diameter, on the path in contact with the fluid in change of state, and their ends open, by means of short elbows 7 with an internal radius approximately equal to 1.25 diameter, outside the annular chamber 2 in the fluid flow tube 6, in a direction parallel to this tube and to the direction of flow of the liquid at the inlet and at the outlet of the exchanger.

Such an embodiment makes it possible to avoid, in the event of accidental freezing of the liquid in the pipes 1, destruction of the latter, the pipes 1 resisting the pressure and the frozen liquid bead expanding axially.

The exchanger according to the invention is more particularly intended for use in machines refrigerators in the form of an evaporator or condenser, in which the heat exchange takes place between a liquid circulating in the pipes 1 and a fluid in change of state passing through the annular space 3.

Claims (1)

1. A tubular heat exchanger, essentially consisting of tubes (1) disposed in an annular chamber (2) formed by a core (3) and by an outer casing (4), the tubes (1) of the annular chamber (2) all being of equal length and being coiled round the core (3) in the form of concentric coils and of which the turns are coiled alternately in an opposite manner, each coil consisting of one or more tubes (1) coiled over one and the same generatrix, the number of tubes (1) increasing with the coiling diameter of the coil, the said tubes being constituted so as not to have any dead angles nor changes in diameter over t he length h in contact with theflu- id changing state, their ends opening in the pipe (6) in a direction parallel to this pipe and to the direction of flow of the liquid at the inlet and at the outlet of the exchanger, the coils being coiled with different pitches, the turns of the coils being contiguous, or substantially contiguous, and only leaving a minimum space for the passage of the fluid changing state which is to be heated or cooled, characterised in that the concentric coils are contiguous, in that the tubes (1) have a diameter and a length permitting rapid rates of flow and a diameter less than six times the thickness of their walls and their ends open, by means of short bends (7), having an internal radius equal to about 1.25 diameter, outside the annular chamber (2) into the pipe (6) for the fluid.

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