GB2067225A - Quenching metal - Google Patents

Abstract

Moving metal products are cooled continuously by means of a conical jet of liquid injected into a jet of gas projected at low pressure and high speed on to the products. The injection is carried out in such a way that particles of liquid are distributed throughout the jet forming a mist which is projected on to the products. Variable high coefficients of heat exchange are obtained. In the preferred embodiment, air at high speed and low pressure emerges from orifices 3 in tubes 2a or boxes 2b and conical jets of water are generated by injectors 4. <IMAGE>

Description

SPECIFICATION Cooling of metal The present invention relates to a method and device for cooling for continuous treatment of metals especially metallic articles or treated strip especially sheet metal.

In numerous methods of treatment of metals there is used after an appropriate heating cycle a cooling of which the speed determines the final metallurgical structure of the product. This cooling should be adjusted as a function of the dimensions of the articles or strip treated as well as the rate of production of the installation in order to obtain a constant cooling curve.

There have been suggested various methods intended to answer these requirements. Among the known techniques there may be mentioned blowing of air, blowing of a sprayed liquid suspension in air, contact with a jet of liquid, etc.

Each of these methods has faults: - the blowing of air on its own, even in a large quantity, is unsuitable for achieving high coefficients of heat exchange; - the methods in which there is carried out blowing with a suspension of a liquid sprayed in air, even though flexible and effective, from the point of view of heat exchange achieved, generally requires large pressures for the carrying gas and they are therefore not economic; - the systems using contact with a jet of liquid are very effective with regard to cooling but they cannot operate over a large range of exchange coefficients.

The method according to the invention is intended to assure the characteristics of cooling which are required, that is high exchange coefficients and the possibility of adjustment over a large range of these coefficients.

The method according to the invention is characterised in that it consists of introducing a conical jet of liquid at a position which is carefully chosen in a jet of gas at lower pressure projected at high speed on the products to be cooled, the injection being carried out in such a manner that the particles of liquid are distributed in the whole volume of the gaseous jet forming therein a mist projected onto the products to be cooled.

According to an embodiment of this invention the pressure of the gas, which may be air, is less than 0.15 bars which has the advantage of allowing obtaining of a gaseous jet from a centrifugal fan.

In the method according to this invention, the conical jet of liquid, formed by droplets generally having a diameter of the order of 0.8 to 2 mm injected in the gaseous jet, is taken in charge by the latter and under the effect of friction is broken into droplets which are much finer of the order of 0.05 to 0.2 mm diameter. As indicated above the geometry of the injection is chosen such that the particles of liquid distributed in the gaseous jet form a mist inside the latter. This mist projected on the products to be cooled puts in contact the fine particles of liquid with the very hot surfaces of the product to be cooled, of which the temperature may reach 1 000C. The evaporation which results absorbs the heat and shows itself by very intense heat exchange.

According to the invention there may be used high rates of feed of gas and relatively low rates of feed of liquid. Preferably the gas is charged with liquid in a proportion generally equal to or less than 0.25 kg of liquid per 1 Nm3 of gas. Within these limits and by modifying the rate of feed of liquid, there are obtained coefficients of heat exchange which vary in a ratio of 1:10. Owing to the small content of liquid and by choosing suitably the rates of feed of gas, it is possible to ensure total evaporation of the drops.

Variation in intensity of cooling may be obtained by modifications of the rate of feed or liquid or rate of feed of gas or the two simultaneously.

The invention also relates to a device for carrying out the method detailed above. This device which may be inserted in a cooling zone of a treatment installation operating continuously on metallurgical products comprises, on both sides of the surfaces of the products to be cooled which are moving continuously, lines either of tubes or of blowing boxes for gas comprising discharge openings, discharging the gas in the form of conical jets, each opening being provided with a liquid injector, the injection of liquid in the gaseous jet being carried out so as to obtain an optimal breaking up of the droplets issued from the liquid jet.

The height of an injector with respect to the opening above which it is placed may be equal to the effective diameter of this opening.

The discharge openings may be arranged in lines perpendicular to the direction of displacement of the product to be cooled and the openings of the different lines may be separated one from the others.

The gas may be air and the liquid may be water.

Embodiments of this invention will now be described by way of example with reference to the accompanying drawing. In the drawing: Figure 1 shows schematically in perspective a device according to the invention applied to cooling of sheet metal passing continuously, and Figure 2 shows schematically an example of the positioning of an injector relative to its orifice.

Referring to Figure 1 it will be seen that the sheet 1 passing in the direction indicated by the arrow traverses a cooling unit formed by a certain number of lines of tubes such as 2a or boxes such as 2b in which is blown a gas, especially air, at low pressure, using for example a centrifugal fan which is not shown. It will be understood that the cooling systems formed by the tubes and the boxes represent two variants which will not generally exist simultaneously in the same installation. These boxes or tubes are arranged on both sides of the sheet 1. The tubes 2a and the boxes 2b are provided with a plurality of openings 3 through which the gas is discharged in the form of conical jets. As may be seen in the drawing, each opening 3 is provided with an injector 4 for mechanical spraying of liquid, feed of these injectors being carried out through collectors 5.

The openings 3 are arranged in planes parallel to the belt in lines perpendicular to the direction of displacement of the sheet to be cooled. The openings on these lines are separated one with respect to the others as is clearly visible in Figure 1,so as to give improved homogeneity of cooling over the width of the product to be treated, formed here by the sheet 1.

The mist obtained as indicated above by projection of the liquid particles in the whole volume of the gaseous jet is projected on the sheet to be cooled.

The geometry of injection of the liquid in the gaseous jet is determined in such a manner as to obtain an optimum fractionation of the droplets issued from the liquid jet. Referring to Figure 2 it is seen that the position of the injector 4 is defined by the distance h separating it from the opening 3 above which it is placed. This distance is, in this embodiment, substantially equal to the effective diameter of the opening 3, the ratio d/D between this diameter and the real diameter of the opening 3 depending on the profile of this opening.

The effective diameterdis a function of the angle of spray of the injector 4, this angle being generally close to 300.

By way of non-limiting example, the other parameters of this installation are as follows: - Diameter D of openings: 30 to 100 mm - Pressure of glass blown (air) 200 to 1200 da Pa (2000-12000 N/m2) liquid pressure (water): 1 to 7.105 N/m2 - Rate of feed of liquid (water) by the injector: 15 to 200 I/h.

Operating within such limits an installation according to the invention allows obtaining of average coefficients of heat exchange situated between 100 and 2000 W/m2.0C, these coefficients being adjustable in the ratio of 1:10 for a given installation.

Among examples of application of this invention there may be mentioned cooling of strips or heavy plates or slabs or billets.

Claims (11)

1. A method of cooling continuously moving metal objects comprising injecting a conical liquid jet at a chosen position into a gaseous jet of low pressure projected at high speed onto the objects to be cooled, the injections being carried out in such a manner than the liquid particles are distributed in the whole volume of the gaseous jet, forming in the latter a mist which is projected on the object to be cooled.

2. A method according to Claim 1, characterised in which the pressure of the gas into which is injected the liquid jet is less than 1 5000 Newton/m2 and is preferably from 2000 to 12000 Newton/m2.

3. A method according to Claim 1 or 2, in which the gas is charged with liquid in a proportion equal to or less than 0.25 kg of liquid for 1 Nm3 of gas.

4. A method according to any one of Claims 1 to 3, in which the gas is air and the injected liquid is water.

5. A continuous method of cooling metal objects, substantially as hereinbefore described with reference to the drawings.

6. Device for carrying out a method according to any one of Claims 1 to 5, in a cooling zone of an installation for continuous treatment of metallurgical products, comprising on both sides of the objects to be cooled as they move, lines of tubes or blowing boxes for gas comprising openings discharging the gas in the form of conical jets, each opening being provided with a liquid injector, the injection of liquid in the gaseous jet being carried out so as to obtain an optimal breaking up of the drops issuing from the liquid jet.

7. Device according to Claim 6, in which the height h of an injector with respect to the opening above which it is placed is equal to the effective diameter d of said opening.

8. Device according to one of Claims 6 and 7, in which the openings are arranged in a plane parailel to the belt in lines perpendicular to the direction of displacement of the objects to be cooled.

9. Device according to Claim 8, in which the openings of different lines are spaced relative to each other.

10. A device according to any one of Claims 6 to 9, in which the injectors are of the mechanical type.

11. A device for continuously cooling metal objects, substantially as hereinbefore described with reference to the accompanying drawings.