RU2502931C2 - Double-pipe heat exchanger - Google Patents

Abstract

FIELD: heating.

SUBSTANCE: double-pipe heat exchanger, in the internal pipe and in inter-pipe space of which screw inserts are installed. Inner space of the internal pipe and inter-pipe space between internal and external pipes represent screw cavities formed with walls of pipes and screw inserts. Screw inserts are installed so that an internal screw insert is connected mainly by welding or soldering to inner surface of the internal pipe. Screw insert in inter-pipe space is connected in the same manner to outer surface of the internal pipe and to inner surface of the external pipe. Materials of the internal pipe, screw inserts and points of joints of screw inserts with walls of the internal pipe shall have minimum thermal resistance. Flows of liquid or gaseous media in the internal pipe and in inter-pipe space flow along helical spirals.

EFFECT: invention allows shortening the length of double-pipe heat exchangers up to ten times and more and reducing the weight and overall dimensions of a heat exchanger.

2 dwg

Description

The claimed invention relates to heat exchange equipment and can be used in various industries, agriculture and utilities.

Known heat exchangers of the "pipe in pipe" type, which are two pipes, one of which, of a smaller diameter, is located inside the other, of a larger diameter, with an annular gap called the annular space. A medium (liquid or gaseous), for example, of a higher temperature (hot) is pumped through the inner pipe, and a medium with a lower temperature (cold) is pumped through the annulus. In this case, the wall of the inner pipe heats up and transfers heat to the cold medium, in which the temperature thus rises. The direction of heat transfer may be as described above or in the opposite direction depending on the temperature ratio in the inner pipe and in the annulus.

The heat transfer efficiency, in addition, depends on the degree of turbulization of the flow and on the viscosity of the media - the efficiency increases with increasing turbulization and with a decrease in viscosity.

If for specific media their initial temperatures, and therefore viscosity, and pipe diameters are assumed to be the same, then the only way to increase the heat transfer efficiency between them will be to increase turbulence, which with smooth pipes can be achieved only by increasing the speeds of the media.

Improving the efficiency of heat transfer reduces the required heat transfer area, reduce the length of the heat exchanger, its other dimensions and weight. But increasing the speed of the media in the pipes requires an increase in the power of the pumps that pump these fluids. Given that the increase in turbulence is proportional to the speed of the medium, and the required pump power is proportional to the square of the speeds, it is obvious that the increase in the speeds of the media has a certain limit, after which a further increase in speeds becomes unprofitable.

Therefore, they seek to increase turbulization due to the installation in the inner tube and in the annular space of a different type of turbulizing elements.

For example, pipe-in-pipe heat exchangers are known in which a wire having various winding steps and configurations is wound around an inner pipe.

The disadvantage of such heat exchangers is a slight increase in turbulization with an outstripping increase in hydraulic resistance.

Also known are heat exchangers, on the inner pipe of which are installed, for example, for welding, helical ribs, the height of which is almost equal to the distance from the inner pipe to the outer. Such ribs to a greater degree increase the turbulence of the medium in the annulus compared with the winding of the wire. In addition, they increase the area of thermal contact of the wall of the inner pipe with the annulus; increase heat exchange efficiency.

The disadvantages of such heat exchangers are as follows:

- not all of the medium in the annular cavity is involved in the screw movement - part of it flows through the annular gap between the screw ribs and the outer pipe;

- the increase in the speed of the medium, its turbulization occurs by only a few percent, in extreme cases, by several tens of percent, since the angle of elevation of the helical line of the ribs is small, and with an increase in the angle of elevation, the hydraulic resistance increases much faster than turbulization and an increasing amount of medium begins to flow through annular clearance;

- heat transfer from the medium in the inner pipe to its wall remains at the same, relatively low level, and it determines the efficiency of heat transfer as a whole.

Known heat exchanger "pipe in pipe" according to patent No. SU 1222207. In this heat exchanger, a turbulizing insert is installed inside the inner pipe in the form of a strip of metal sheet swirling along a helical line with turbulent lobes along its longitudinal edges. This insert causes twisting along the helix, significantly increases the turbulization of the medium in the inner pipe and the heat transfer from the medium to the wall.

The specified heat exchanger is taken as a prototype.

However, it has the following disadvantages:

- not all the medium in the inner tube is involved in the screw movement (approximately only 20-30%), which does not allow to achieve the maximum possible turbulization of the medium;

- the contact thermal resistance of the turbulizing insert with the inner surface of the pipe is large (the turbulizing insert touches the inner surface of the pipe only at individual points, moreover, by simply pressing it due to elastic forces. And such a pressing is not completely reliable and can weaken at any moment - i.e. thermal resistance at the point of contact will increase and may become unacceptably large).

- a large contact thermal resistance deprives the turbulent insert of its essential function - to transfer heat from it to the wall of the inner pipe due to heat conduction (which is equivalent to an increase in the heat transfer surface of the inner pipe).

The aim of the present invention is to more significantly increase the heat transfer coefficient - not by tens of percent, but several times, which, in turn, will allow to reduce the length of the heat exchanger by the same amount and, therefore, also reduce its dimensions and weight by several times, although slightly less degree than length reduction.

The pipe-in-pipe heat exchanger proposed by the present invention differs from the prototype in that the inner space of the inner pipe and the annular space between the inner and outer pipes are screw cavities formed by pipe walls and screw inserts installed inside the inner pipe and inside the annular space in this way that the inner screw insert is connected, mainly by welding or soldering, to the inner surface of the inner pipe, and the screw insert in the interior The foot space is connected in the same way with the outer surface of the inner pipe and the inner surface of the outer pipe, moreover, the materials of the inner pipe, screw inserts and the joints of the screw inserts with the walls of the inner pipe must have minimal thermal resistance.

The device of the proposed heat exchanger is schematically shown in figure 1 and figure 2.

Figure 1 shows a longitudinal section of a heat exchanger, figure 2 - its cross section.

Figure 1: 1 - inner tube; 2 - an external pipe; 3 - screw insert in the inner pipe; 4 - screw insert in the annulus; 5 - a helical cavity in the inner pipe; 6 - a helical cavity in the annulus; B — medium inlet to the inner pipe; In - the output of the medium from the inner pipe; G is the input of the medium into the annulus; D is the output of the medium from the annulus.

Figure 2: E - helical movement of the medium in the inner pipe; F - helical motion of the medium in the annulus.

The heat exchanger operates as follows: a hot medium enters the screw cavity of the pipe pos. 1 and immediately acquires a helical movement, for example, clockwise. During its movement, the medium washes the surface of the screw insert pos.3 and the inner surface of the pipe pos.1 and transfers heat to them. At the same time, heat to the inner surface of the pipe, pos. 1, is transferred by thermal conductivity through a screw insert, pos. 3.

The efficiency of heat transfer from the medium in the pipe, item 1 to its wall, is to a first approximation proportional to the Reynolds criterion (Re), and that, in turn, is proportional to the speed of the medium relative to the wall, all other things being equal. If a screw insert is not installed in the inner pipe, then the speed of the medium inside it will be equal to that at the entrance to the pipe (i.e., along arrow B, Fig. 1) and the path of the medium will be equal to the length of the taken pipe section.

When the insert is installed and with the step of its helical surface equal, for example, to the inner diameter of the pipe, item 1, the path of the medium relative to the wall increases 3.14 times. But, so that the entire medium entering the screw cavity of the pipe pos. 1 has time to pass this segment of the pipe, its speed should also increase by 3.14 times. In proportion to this, the Reynolds criterion increases and, consequently, the heat transfer coefficient is also proportional.

Thus, the coefficient of heat transfer from the liquid to the wall increases at least 3.14 times. In fact, the increase will be greater, since the assessment did not take into account two significant factors contributing to an increase in the heat transfer coefficient:

a) turbulization of the boundary layer at the inner wall of the pipe;

b) heat transfer to the inner wall due to the thermal conductivity of the screw insert.

It is definitely not possible to assess the degree of influence of these factors on increasing the heat transfer coefficient, but approximately this is 40-80%.

But, even without taking these two factors into account, the increase in the heat transfer coefficient is very impressive. Moreover, it is possible to increase it several times more. To do this, only reduce the pitch of the screw insert in the inner pipe. For example, if this step is reduced by a factor of three, the velocity of the medium relative to the inner wall of the inner pipe, the Reynolds criterion, and the heat transfer coefficient as a whole will increase by approximately the same amount.

A similar picture is observed in the annulus. Those. with a screw insert installed, depending on the inner diameter of the outer pipe and the pitch of the screw insert in it, the heat transfer coefficient from the wall of the inner pipe to the annular medium increases about the same amount as from the medium in the inner pipe to its wall.

The use of the invention allows to intensify heat transfer both by improving the hydrodynamic structure of the flow and increasing the portable properties of the medium, and due to the development factor of the heat exchange surface. This entails a reduction in the required length of the pipe-in-pipe heat exchangers up to ten or more times and a corresponding reduction in the mass and overall dimensions of the heat exchangers.

Claims (1)

A tube-type heat exchanger in a pipe, in which there are screw inserts installed in the inner pipe and in the annular space, characterized in that the inner space of the inner pipe and the annular space between the inner and outer pipes are screw cavities formed by the pipe walls and screw inserts thus installed that the inner screw insert is connected, mainly by welding or soldering, to the inner surface of the inner pipe, the screw insert in the annulus e is connected in the same way with the outer surface of the inner pipe and with the inner surface of the outer pipe, the materials of the inner pipe, screw inserts and the joints of the screw inserts with the walls of the inner pipe must have minimal thermal resistance, flow of media (liquid or gaseous) in the inner pipe and in the annulus flow along helical spirals.

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