The present invention concerns an apparatus for producing fire-fighting foam, a foam generator.

The apparatus has an air inlet and consists of a fan which is operated by a reaction jet motor and a foam net which can be made of a perforated plate. The reaction jet motor has nozzles and is connected to a liquid under pressure. The liquid is usually water to which a foaming agent has been added.

When the liquid is sprayed from the nozzles the reaction forces will operate the fan. At the same time the nozzles will atomize the liquid and spray it against the foam net. The air from the fan blows the liquid through the foam net thus gener¬ ating foam.

From Norwegian patent no. 120 715 there is known a similar apparatus which is operated by a reaction jet motor, for producing fire-fighting foam.

In US 3 999 612 there is disclosed a foam generator where a tilted baffle plate is located outside the nozzles. The baffle plate will deflect the jets of liquid, thus distributing the liquid evenly over the foam net. However, the jets of liquid are not atomized by the baffle plate.

In a foam generator it is desirable to produce as much foam as possible with a high expansion ration in the shortest possible time.

The expansion ratio is usually expressed as how many times a quantity of liquid increases in volume when it is converted into foam. An expansion ratio of 1 000 is achieved when 1 litre of water is converted into 1 000 litres of foam. The fan's rotation speed is crucial for the velocity of the air flow and the volume of air which forces the liquid through the foam net.

An increase in fan speed can be achieved by using nozzles with short moment arms and/or by employing a higher water pressure.

If the water pressure is too great, however, the disadvantage arises that the water jet strikes the foam net with such force that it breaks up its own foam and the foam production is reduced. Known foam generators are encumbered with this

defect, and they therefore normally do not operate with a water pressure higher than 5 bar.

Another drawback with known foam generators is that the water has to be atomized by the nozzles before it strikes the foam net. If the water is not suffi- ciently atomized the generator does not produce light foam, i.e. foam with a high expansion ratio.

It is known in the prior art to employ nozzles which are designed in such a manner that they atomize water. In addition the nozzles are also constructed in such a way that they disperse the water in a fan shape, so that the water covers the entire foam net. A further flaw in some designs of known foam generators is that the foam net is located in such a manner that the nozzles have to be tilted in relation to the shaft in order for the water jet to be able to cover the entire foam net.

Thus the known foam generators do not take maximum advantage of the available water energy. Atomizing nozzles which also disperse the water in a fan shape, and nozzles which are tilted in relation to the shaft will substantially reduce the thrust. In the case of nozzles tilted at an angle of 45 , e.g., the thrust is reduced by approximately 50%, thereby reducing the fan's speed and volume of air. This results in a low foam production.

The object of the present invention is to avoid the above-mentioned drawbacks.

This problem is solved according to the invention by means of an apparatus which effectively achieves maximum utilization of the available water energy and which is characterized by the features in the claims presented.

The invention will now be described in more detail with reference to the drawings which illustrates an embodiment of the apparatus, only the principles of the invention being illustrated.

Fig. 1 is a sectional elevation of a foam generator. Fig. 2 is a section taken after line A-A in fig. 1.

In fig. 1 the foam generator is indicated by 1. It has an axial fan 2 mounted in a fan casing 3 with an air intake. The air supply is indicated by arrows 4. The fan

2 is mounted on the same shaft as a reaction jet motor 6 which consists of a number of nozzles 5 mounted on arms. The nozzles 5 can be located in several planes.

Through the shaft on which the fan 2 and the reaction jet motor 6 are mounted, liquid is supplied to the nozzles 5. The liquid inlet 7 is connected to liquid under pressure from a tank which is not shown in the drawing. The liquid is usually water to which a foaming agent has been added.

A foam net 8 can be constructed as a perforated cylindrical metal wall. Between the nozzles 5 and the foam net 8 there is installed a circular grid 9 mounted parallel to the foam net and at a certain distance from it.

During operation of the foam generator 1 liquid will be sprayed from the nozzles 5 and the reaction forces will operate the fan 2. The nozzles 5 are designed in such a manner that they do not atomize the liquid or disperse it in a fan shape, but spray it out in a straight jet which is cohesive, compact and parallel. Maximum thrust is thereby achieved in the water jet and full use is made of the water energy. This is one of the advantages of the invention.

Nozzles 5 which provide a compact jet are smoothly bored and designed to give a cohesive jet.

The liquid from the nozzles 5 will be dispersed with compact and cohesive liquid jets at high pressure. However, it is important for the liquid to be atomized before it reaches the foam net. This is achieved when the liquid, after having taken maximum advantage of the thrust, strikes the grid 9 and is atomized and dispersed in such a manner that the liquid covers the entire foam net 8. This is one of the advantages of the invention.

The circular grid 9 can be constructed from laths 10 which can be located axially at a certain distance from one another. Other designs of the grid are also possible. It can be constructed, e.g., from a fine-meshed netting or a plate with stamped-out holes, where the holes can have any shape and dimensions, but preferably rectangular and measuring between 10 mm and 100 mm.

When lathes are used they can be made of wires with a circular, oval, triangular or polygonal cross section. Successful tests have been performed, e.g., with a

grid 9 constructed from laths 10 made of 2.5 mm diameter round wires made of steel.

The spacing of the laths in the grid 9 will be important for the atomization of the liquid mixture. The spacing will be dependent on the water pressure and the volume of water and the capacity of the foam generator. Appropriate spacings between the laths 10 in the grid 9 can be from 10 mm to 60 mm, preferably from 20 mm to 40 mm.

The radial distance of the laths or the grid from a centre line or from the nozzle outlets is determined by the water pressure and the relevant dimensions in the foam generator.

By means of the grid 9 an even atomization of the liquid is obtained when it is sprayed against the foam net 8. The air from the fan 2 blows the atomized liquid mixture through the foam net 8, thus achieving light foam, i.e. foam with a high expansion ratio.

In addition the water jets from the nozzles 5 will lose most of their energy when they strike the grid 9. This makes it possible to use a much higher water pres¬ sure than that employed by previously known foam generators, e.g. 10 bar.

With even atomization of the liquid mixture and with higher velocity of air flow and a greater volume of air the expansion ratio can be increased. A high expansion ratio has many technical and financial advantages. A smaller number of foam generators is required in an installation, and this also reduces the amount of piping required. Foam with a high expansion ratio is generated with a smaller volume of water, thus enabling the tank capacity to be reduced. These advantages are achieved with the present invention.