Heat Exchanger

Abstract

A heat exchanger from which is absent a dividing wall between an axial flow of hot fluid, preferably a flame, and an encircling spiral flow of fluid to be heated which is maintained separate by the centrifugal force due to its cyclonic flow and is contained by a refractory-lined wall which absorbs radiant heat from the hot fluid.

Description

The present invention concerns an apparatus in which two fluids flow and exchange heat.

In heat exchangers of known type, the two fluids flow on either side of a fluidtight wall, which is at the same time the seat of the heat exchange, this wall being that of tubes, flues, etc. The wall has to resist thermal and mechanical stresses as well as chemical attacks resulting from the elevated temperatures to which it is subjected. Its price is therefore high, its reliability is problematical and the exchange temperatures are limited to values compatible with its resistance. Finally, if one or the other of the wall faces is exposed to a corrosive fluid or a fluid charged with impurities, the deposits formed are dangerous for the safety of the heat exchanger and reduces to its efficiency.

The present invention relates to a heat exchanger which does not have the above-mentioned disadvantages because of the absence of separation walls between the two heat-exchanging fluids.

An apparatus according to the invention comprises an enclosure of revolution, equipped with an axial admission and discharge for the passage of the hot fluid, a tangential admission and discharge for the fluid to be heated and means for ensuring rotation of the fluid to be heated at a velocity sufficient for the centrifugal force to maintain it in the vicinity of the wall of the enclosure between the tangential admission and discharge.

The internal wall of the enclosure is preferably lined with a refractory material insensitive to attack at the high temperature of the fluid to be heated with which it is in contact, having an absorbent power as close as possible to that of black body absorption, and having a surface as extended as possible, arranged for promoting convection but for limiting turbulence in contact with the fluid to be heated. For example, the refractory material lining, which may be of steel, ceramic, etc., may comprise sections of helical fins.

The hot fluid passing axially through the enclosure may result from combustion controlled to produce a flame of large surface which radiates strongly.

In one embodiment of the exchanger according to the invention one or more longitudinal slits are provided in the wall of the enclosure for recovering the solid particles which may be taken up by the fluid to be heated, the apparatus then acting simultaneously as a separating cyclone.

In an exchanger according to the invention, the axial hot fluid heats by radiation the internal surface of the enclosure, which restores this heat by convection to the fluid to be heated having a helical movement. The hot fluid may result, for example, from the combustion of oil fuel, while the fluid to be heated may be air, for example. The differences in the angular velocities, densities and viscosities are practically opposed, as shown by experience, to mixing of the combustion gases and air to be heated, such that the latter is practically uncontaminated if its removal at the outlet is suitably limited by a closure device. The combustion gases which leave at a temperature which is still high, may be sent to a boiler or a conventional heat exchanger, in which the fuel and combustion air are preheated before their introduction into the burner with which the apparatus according to the invention is equipped.

This type of heater according to the invention, in which exchanges at the highest temperatures occur without an intermediate wall, does not have most of the disadvantages of conventional exchangers. Thus, the convection surfaces, which may be cut out as desired, are practically not subjected to either thermal or mechanical stresses; their expansion takes place freely and fluidtightness is not necessary; they are protected from contact with the combustion gases and are consequently insensitive to their attack; they have simply to resist the chemical attack of the fluid to be heated which, in most cases, is a clean and homogeneous, possibly neutral, gas. Finally if deposits are produced, their effect is not to destroy but to protect the walls against radiation; at the most they may modify their coefficient of absorption.

The efficiency of the exchanger according to the invention, defined as the ratio of the quantity of heat exchanged by radiation to the calorific value of the fuel, is higher, the higher is the temperature of the flame itself, that is to say, the higher the temperature to which the combustion air has been previously heated. When the fluid to be heated is not air, the burner receives its combustion air separately. In the case where the fluid to be heated is air, if the heat exchanged by radiation represents for example 50 percent of the calorific value of the fuel, and if the combustion air is admitted at a temperature of 400.degree. C., it is sufficient to take a small fraction (about one-fourth) of the total air to ensure combustion, the principal part of the air being heated to 800.degree. or 900.degree. C. by contact with the wall.

This type of heater will find applications whenever it is a matter of heating a fluid in which a slight entrainment of combustion gas is not a serious disadvantage. It may be applied for example to the feeding of two gas turbines operating simultaneously: in one of them the combustion gases are expanded after their passage through a conventional exchanger (boiler, tubular air heater), which reduces their temperature to a value acceptable for the gas turbine (for example 630.degree. C. if the fuel is unrefined heavy fuel); the other gas turbine receives the gases which are uncontaminated and are heated to high temperature (for example 800.degree. to 900.degree. C.), and the weight of which is several times that of the combustion gases. Despite the use of unrefined heavy fuel, the second gas turbine, which is much larger, thus operates on a thermodynamic cycle of high efficiency without danger of corrosion.

The following additional description, with reference to the accompanying drawings, given mainly as an example, is intended to illustrate the invention.

In these drawings:

FIG. 1 is a diagrammatic axial section of a radiation gas heater according to the invention;

FIG. 2 shows diagrammatically an exchanger according to the invention used for the feeding of gas turbines, and

FIG. 3 is a diagram showing the application of the invention to heating the superheaters of a boiler.

A fluid heater according to the invention, such as is shown in FIG. 1, comprises an enclosure of revolution 1, for example cylindrical, conical etc., provided with an axially arranged inlet 2 and outlet 3. The inlet 2 may receive a hot combustion-supporting gas associated with a fuel inlet conduit 4, or a combustible mixture, intended to burn in the interior of the enclosure, the outlet 3 serving for the outflow of the hot gas formed of the combustion gases in the example shown. The enclosure also comprises an inlet 5 and a tangential outlet 5a for the fluid to be heated. For this purpose, a volute, vanes, etc., may be provided. The cold fluid set in rotation, for example by the vanes 7, ought to have a sufficient velocity for it to be kept by the centrifugal force in the vicinity of the wall 1 along the entire length of the latter. The inlet for the fluid to be heated is generally so arranged that this fluid and the heating fluid are in parallel currents; it may, however, also be such that the flows are in countercurrent. The first possibility is used in the embodiment of FIG. 1, where the tangential inlet 5 is close to the inlet 2, while the tangential outlet 5a is close to the axial outlet 3. The intake of the heated fluid is regulated by varying the relative vacuum in the outlet conduit 6.

In addition, the wall of the enclosure 1 is preferably provided with helical convection vanes 8.

Finally, in the embodiment shown, there is provided some preheating of the air, which entering at 9 flows along the enclosure 1, owing to a jacket 10 concentric with this wall, before being admitted at 5 through the vanes 7.

Such an apparatus is of simple construction and does not necessitate the provision of material resistant to very high temperatures, the wall of the enclosure 1 being continuously cooled by the fluid to be heated. The risk of corrosion of the walls is thus obviated.

The exchanger may be used for heating any gaseous or liquid fluid. In the latter case, it may be used, for example, for the gasification of any liquid (including a fuel), the latter being preferably finely sprayed in the gas already produced and carried in closed circuit in contact with the wall of the exchanger.

If the fluid to be heated is air, the latter may be used for domestic purposes; indeed, the apparatus may be regulated such that the air is practically free from combustion gases.

FIG. 2 shows the application of an exchanger according to the invention to the feeding of a mixed installation generating power from two gas turbines. In this case, the furnace is at the pressure prevailing upstream of the turbines; the turbine 11, by far the more powerful, has passing through it the clean gas 14, heated to an elevated temperature, for example 850.degree. C.; the turbine 12 receives only the combustion gases after the latter have been cooled in an exchanger 15 to a temperature compatible with the use of gases containing flue dust and corrosive gases, for example 630.degree. C. As indicated in the foregoing, the exchanger 15 may serve either to heat the clean gas (for example air) and combustion air before their introduction 16 into the furnace 13, or to produce or superheat steam, or to heat boiler feed-water. Finally, the clean gas may flow in a closed circuit and under high pressure, passing successively through the furnace 13, turbine 11 and cooler-exchangers before being reintroduced into the inlet of the compressors. The combustion gases, on the contrary, are discharged to the outside after passing through the turbine 12 and the outlet exchangers or economizers. However, these combustion gases may be neutralized, purified and filtered wholly or in part, so as to form the gas of the actual circuit, and compensate any losses of the latter.

Another application of the invention is that of heating boiler superheaters (or any other high-temperature exchangers) according to the diagram of FIG. 3.

The heater furnace according to the invention and here shown at 18, produces a high-temperature neutral gas 19, which passes through the conventional tubular exchanger 20 (superheater, resuperheater, complete boiler, or any heater), and flows in a closed circuit owing to a pickup blower 21 which makes good the pressure losses. The furnace 18 acts as a cyclone and eliminates any solid particles in suspension. These particles are discharged from time to time from the furnace, the internal wall of which comprises at least one longitudinal slit acting as a dust trap, and some orifices, through which this dust is intermittently driven. The gas in circulation is clean, and the attacks by erosion or corrosion at elevated temperature are reduced.

By varying the speed of the blower 21, the level of the exchanges in 20 may be regulated; for example, the superheat temperature may be regulated if 20 is a superheater.

This application is particularly interesting in the case of an installation with a mixed gas-steam cycle, with a boiler through which the gases pass under pressure, where the furnace 18 is under pressure and 20 is the high-temperature part of the boiler, while the evaporator tube nest is situated at the outlet of the combustion gases from the furnace 18. The use of a clean gas in 20 eliminates the serious risks of rapid fouling and permits the use of compact tube nests. In addition, the limitation to 800.degree. or 850.degree. C. of the temperature of the gas passing through 20 considerably reduces the risk of bursting or rupturing of the tubes by overheating in case of difficulty in the circulation or abrupt variation in load. The gas turbine is then fed by an offtake at 22 of clean and hot gas upstream of the exchanger 20, the replacement gas arriving at 23 upstream of 21.

Another application is that of heating without oxidation before forging, rolling or wire-drawing, or the heat treatment in controlled atmosphere of various metallurgical products (tubes or parts of high-alloy steel, or of nonferrous metals, for example). In this case, it is merely necessary to suitably select the gas passing through the container 20 in which the parts to be heated are placed.

An exchanger constructed in accordance with the invention may also be used in drying installations. It is in fact known that drying installations necessitate the production of a considerable flow of fluid at elevated temperature, and also a mechanical device for conveying the materials to be dried. These two conditions may be met by an exchanger according to the invention; for this purpose, the exchanger is selected to have a sufficient working pressure for the combustion gases to feed a turbine serving for the mechanical conveying of the materials to be dried; the hot fluid, consisting of air, is thus at a high pressure and is therefore expelled from the apparatus at a high velocity. The range of regulation of the apparatus will be so much greater if in this case it is possible to accept a certain proportion of combustion gases in the hot air.

Yet another application is that of heating living rooms, the clean gas being then simply air which, heated directly in the furnace, leaves the latter almost free from any trace of combustion gases.

It is obvious that the embodiments described have been given mainly as examples and that they may be given numerous modifications without going beyond the scope of the present invention. The application examples are also not restrictive, the apparatus according to the invention being utilizable in all cases where a very simple heater is desired and where very slight pollution by combustion products is not embarrassing.

Claims

I claim:

1. A heat exchanger comprising an outer hollow casing, and an inner hollow casing supported internally of said outer casing and annularly spaced from the latter, said outer casing including an inlet conduit axially extending into said inner casing and annularly spaced therefrom, said inlet conduit being adapted for supplying a preheated gas into said inner casing, said inner casing including a preheated gas outlet coaxially opposite said inlet conduit for expelling said preheated gas, said inner casing being axially spaced from said inlet conduit, the annular space between said inner and outer casings constituting an inlet passageway for supplying a gas to be heated into said inner casing through the axial space between said inlet conduit and said inner casing, said inner casing including an inner wall and helical vanes helically extending along said inner wall between said inlet conduit and said preheated gas outlet, said helical vanes being directly exposed to said preheated gas and adapted for passing said gas to be heated helically through said inner casing in exposing relation to said preheated gas, said inner casing including inlet vanes for directing said fluid to be heated tangentially thereinto to engage said helical vanes, said inner casing including a further outlet for expelling said gas to be heated, said further outlet including a portion annularly surrounding said preheated gas outlet and disposed adjacent said inner wall of said inner casing.

2. Apparatus as claimed in claim 1 wherein the wall of the inner casing has a lining of refractory material absorbing radiation from said hot fluid.

3. Apparatus as claimed in claim 1 including means for maintaining said inner casing under pressure.

4. Apparatus as claimed in claim 1 including a conventional heat exchanger connected to receive heated fluid from said further outlet.