GB2072831A

GB2072831A - Supplying secondary combustion air

Info

Publication number: GB2072831A
Application number: GB8010157A
Authority: GB
Original assignee: Towler M O
Current assignee: Towler M O
Priority date: 1980-03-26
Filing date: 1980-03-26
Publication date: 1981-10-07

Abstract

A solid fuel fired heating appliance in which secondary air and/or combustible gases given off by the fuel are introduced via jets K in a restriction in the combustion products outlet ducts. <IMAGE>

Description

SPECIFICATION Solid fuel fired equipment with improved combustion Related Field This invention may be applicable to any form of enclosed solid fuel burning equipment, stoves, boilers, air heating furnaces, and the like; although it may yield the greatest benefit when applied to equipment burning vegetable fuels such as wood, straw or peat, for which it is primarily intended.

Background Information When solid fuels are subject to heat under burning conditions they tend to break down in three stages: 1. Moisture is driven off; 2. Combustible gasses, termed volatiles, are given off; 3. The solids are burnt.

Since the volatiles represent a useful proportion of the total heat value of the fuel it is clearly desirable that they should also be burnt. For this to occur it is necessary that both ignition and the appropriate proportion of air be present simultaneously. The burning of the solids is generally termed primary burning, and the air required for that purpose primary air. The burning of the volatiles is generally termed secondary burning and the air required for that purpose secondary air.

Not only is the burning of volatiles important with regard to vegetable fuels, they also have the feature of burning particularly well on a bed of hot ash.

Whilst these facts are generally acknowledged, current practice leaves many practical problems such as regulating the flow of secondary air to match the fluctuating requirements of an appliance with a variable output, and arranging for the mixture of volatiles plus secondary air to pass an area of burning so that they may be ignited.

Appliances with grates offer convenience but lack the benefit of an ash bed and those with ash beds present problems when the bed becomes too deep, since the removal of ash cannot be done if there is much fuel on top.

The Invention This invention comprises an appliance in which the combustion gasses are led from the fire to a duct or multiplicity of ducts, the entrance to which is closely adjacent to the fire. This may be achieved by having a sloping firebed, preferabiy so inclined as to just support an ash bed of the desired depth. Towards the lower portion of the incline and possibly at an angle to it may be fitted a grate. Thus, under the action of fuel movement caused by burning, the ash bed will tend to be moved towards the grate, allowing excess ash to fall through. The entrance to each of the above mentioned ducts is restricted to form a venturi within which an opening or jet or a multiplicity of same are provided for the introduction of secondary air.As a variant of this arrangement, some or all of the jets may be fed by volatiles or a mixture of volatiles and air from the firebox. The duct may then open out into a chamber or be extended to provide an area wherein the volatiles burn with the facility to extract the useful heat generated. After the chamber or extension the spent gasses may be exhausted to the flue or chimney.

The effect of this arrangment is that very hot combustion gasses from the primary burning together with some spark and flame will flow at relatively high speed through the venturi ensuring ignition of the volatiles plus air mixture. The volatiles are either drawn through the venturi with the combustion gasses or are introduced separately as indicated above. Since the positional relationship between the primary burning area, the venturi, the mixture point of secondary air and volatiles are all fixed and do not vary with the state of fuel load within the appliance or rate of burning the circumstances necessary for secondary combustion are maintained at all times.Since the secondary air is introduced by virtue of the venturi principle, the actual amount of air injected will vary with the rate of burning and if taken as a proportion of the total air input to the appliance, which is the preferred arrangement, will form a fluctuating proportion of the primary air, such proportion increasing as the rate of burning increases. Further control of the combustion may be achieved by manual or automatic adjustment of the jets possibly regulated according to chimney depression or temperature sensors. Further control of ash movement may be achieved by agitator bars resting on or adjacent to the inclined firebed and capable of movement. These may be linked with the grate riddling mechanism or operated independently.

EXAMPLES The selected mode of employment for this invention will depend of the size of appliance. A small appliance with a short inclined firebed may not warrant agitator bars for example, whilst they could prove of great benefit in a larger appliance with a long firebed.

Figure 1 shows the invention applied to a central heat boiler, being a cross-section of an enclosed appliance surrounded by a water jacket A, a firebox B, containing fuel C which has been loaded through door D and which falls under gravity to feed the fire E supported on the inclined firebed F and the grate G. Air, primary and secondary together, is admitted through a flap which may be thermostatically controlled in the ash door H. The primary air flows via the grate G to the fire E and thence the combustion gases, together with volatiles given off by the heated fuel C pass through the venturi duct J mixing with secondary air flowing through jets K and promoting secondary combustion within chamber L. Secondary air is ducted to the jets K from the ashpan M. Agitator bars N laying on firebed F are suspended from a transverse bar P which is held in eccentric bearings, not drawn, at each end.

Rotation of the shaft in the eccentric bearings induces agitation of the bars.

Figure 2 depicts a cross-section of a stove utilising the invention in a different form. The stove is circular on plan and has a conical sloping firebed F. No agitator bars are fitted. In this example secondary air is led to the top of the firebox B through the tube R and allowed to mix with volatiles before being drawn down the outer sleeves of the duct J to the jets K. Apart from this the operation is the same as for the example in Figure 1 and the same designatory letters are used.

Claims

1. A heating appliance constructed or adapted to burn solid fuels or the type that give off combustible gasses under the action of heat characterised in that secondary air, or secondary air and combustible gasses independently, or a mixture of air and combustible gasses is introducted via a jet or multiplicity of jets in a restricted duct or multiplicity of restricted ducts carrying the gaseous products of combustion.

2. A heating appliance as claim in Claim 1 in which the firebed is inclined upwards towards the entrance to the duct or ducts.

3. A heating appliance as claimed in Claim 2 with a grate adjacent to the lower end of the inclined firebed.

4. A heating appliance as claimed in Claim 2 or Claim 3 with bars capable of agitation by an external source fitted adjacent to the surface of the inclined firebed.

5. A heating appliance as claimed in Claim 1 in which the jet or jets may be adjusted with respect to effective cross-sectional area.

6. A heating appliance as claimed in Claim 1 in which the jet or jets may be adjusted with respect to positional relationship with the duct.

7. A heating appliance as claimed in Claim 1 in which the effective duct restriction in the area of the jets is adjustable.