This invention relates to fuel fired burners, which find wide use in domestic and industrial- applications for boilers and other heating devices.

For a burner to function, there must be a supply of fuel, which is usually gas, and oxygen, the oxygen usually being supplied from atmospheric air. For effective combustion to take place, the quantity of air supplied must be matched to the quantity of gas supplied to provide an efficiently combustible mixture. An efficiently combustible mixture is one in which as much up to all of the gas is combusted with minimal of no creation of toxic wastes such as the oxides of nitrogen (referred to as NOX) . Although it is desirable to have no NOX produced in the combistion of liquid or gaseous fuels in a burner, in practise it is almost impossible to eliminate the creation of NOX. However, some burner constructions and modes of operation are more efficient than others.

Thus, in what is known as fully pre-mixed burners, all of the air is mixed with a fuel to produce a combustible mixture before combustion takes place, whereas in partially pre-mixed burners, some of the air which is required is mixed with the fuel to create a combustible mixture, but in addition secondary air is caused to be induced into the flame to complete the combustion process. Fully pre-mixed burners are more efficient in terms of toxic emissions, but they do suffer from other problems such as resonant noise and flame lift from the flame strip, due to the higher velocities of flame and combustion mixture, (which exist in fully pre-mixed burners) .

The present invention seeks to provide a burner construction

whereby efficient combustion may be achieved but the structure is such as to reduce burner flame velocity, despite the fact that the burner operates as a fully pre-mix burner arrangement. The burner can therefore operate as a fully premixed burner but does not necessarily need a fan for the provision of combustion air. The air being provided by the process of natural entrainment.

In accordance with the present invention, a fuel fired burner comprises a burner bar from which burner fuel issues, said burner bar extending along one side of a plenum chamber, to the other side of which is a flame strip on which the burner flame is created, said one side of the plenum chamber being sufficiently open to cause inducement of combustion air thereinto to flow across the plenum chamber with the fuel to the flame strip, and in a direction across the chamber, the chamber has a divergent cross section towards the flame strip providing enhanced inducement of the air into the plenum chamber, and velocity control of the mixture across the plenum chamber to the flame strip.

Preferably, the burner bar is provided with a plurality of fuel injection nozzles along its length, and the nozzles are directed into the open side of the plenum chamber, which open side is defined by walls which are convergent towards the burner bar whereby a throat is created in the region of the nozzle outlet for the inducement of the combustion air into the plenum chamber.

Therefore, in cross section, the plenum chamber has a convergent divergent shape, this shape being defined by plenum chamber walls which typically will be of metal sheet.

The flame strip having the burner ports therein may be of any suitable construction. For example it may be constructed as

set forth in our UK Patent Application No. 9402939.4 from which the present application is divided, or it may simply be a metal section forming an integral part of the sheet metal which also defines the plenum chamber walls.

Instead of the burner bar being provided with fuel injection nozzles, it may simply be provided with a plurality of apertures placed therealong, and from which the fuel issues in use. With the aforementioned system primary aeration levels, typical between 120% and 140%, are achieved.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, wherein:-

Fig. 1 is an exploded perspective view of the salient parts of a burner according to an embodiment of the invention; and

Fig. 2 is a sectional elevation of the burner shown in Fig. i;

Referring to the drawings, the burner shown in Fig. 1 comprises a pair of end plates 10, 12 between which extend a pair of side plates 14, 16 in order to define the plenum chamber 18 therebetween. The plates 14 and 16 as shown in Fig. 2 are of curved configuration, and extend in use generally upright, the plates 10 and 12 also being upright.

The curving of the plates 14 and 16 dictates that the plenum chamber 18 in cross sectional view will be of convergent divergent form from the lower end to the upper end, thereby to define a plenum throat 20. The top of the plenum chamber 20 is closed by means of a flame strip 22 provided in this instance with a plurality of deep burner slots 24 which are in parallel array as shown in Fig. 1. The flame of the

burner sits on top of the flame strip 22, and to this end the flame strip 22 preferably is a low resistance ceramic block, or is made up of a plurality of ceramic elements for example as described in our co-pending Patent Application No 9402939.4, from which the present application is divided.

The combustible fuel/air mixture which is supplied to the flame strip 22 is provided by supplying the fuel, typically a gas, through a burner bar 26 having a gas inlet end 28. The burner bar 26 is provided with a plurality of equally spaced nozzles 30 from which the gas issues in streams, into the convergent lower end of the plenum chamber as shown in Fig. 2. In this connection it is to be mentioned that in Fig. 1 the burner bar 26 is shown as being separated from the plenum chamber, but as will be appreciated in use, it is hung from the side plates 10 and 12 by the hanger plates 32 and 34 which have slots 36 for receiving bolts 38 which thread into apertures 40 in plate sides 10 and 12 in order to accurately position the burner bar as shown in Fig. 2.

The issuing jets of gas are indicated by arrow 42 in Fig. 2, and by virtue of the issuance of the jets of gas into the throat area 20, so atmospheric air is drawn into the throat 20 through the lower end of the plenum chamber as indicated by arrows 44 to provide the mixture of air and gas in the upper portion of the plenum chamber 18, and it is this mixture which passes through the slots 24 and into the flame.

The designing of the plenum chamber so as to have a divergent upper portion provides for velocity control of the mixture, to avoid a number of the disadvantages normally associated with a fully pre-mixed gas burner. The design of the plenum chamber to make the upper portion divergent can be other than that illustrated, and the flame strip 22 need not be of the construction described. For example it could be a strip of

metal which is integral with the sides 14 and 16, or is formed as a separate insert, clenched to the upper edges of the sides 14 and 16.

Preferably metallic sheets will be used for the plates 10, 12, 14 and 16, and the burner can be constructed in any appropriate size.

The burner could of course be used in other dispositions than the vertically upward shown in Fig. 2. It could be used at an angle or downwardly firing (ie inverted compared to Fig.

2).