GB2100854A - Solid fuel combustion appliances - Google Patents

Abstract

A solid fuel combustion appliance (1) incorporates a secondary combustion apparatus (13; 13') in addition to a primary combustion compartment (2). The secondary combustion apparatus (13; 13') comprises at least one chamber (20; 20') having an internal flue (16; 16'') of a refractory material. The or each chamber (20; 20') has an inlet (19) for receiving combustion gases from the primary combustion compartment and an outlet (17) which communicates with a main flue (11) open to atmosphere. An air supply is introduced at the level of the inlet (19) so as to support secondary combustion within the chamber(s) (20; 20'). <IMAGE>

Description

SPECIFICATION Improvements in solid fuel combustion appliances This invention relates to solid fuel combustion appliances and, in particular, to secondary combustion apparatus for use with solid fuel combustion appliances. The apparatus of the invention has particular, although not exclusive, application to solid fuel combustion appliances of the type described in our Patent Specification Nos.

1526524 and 1588433.

Oil supply shortages and high costs have led to an increased use of solid fuel for domestic heating.

While certain grades of solid fuel, for example, anthracite and carbonised fuel, burn with little or no smoke, other grades and varieties of solid fuel which are cheaper and more freely available emit smoke when burned. This contributes to an increasing problem of urban pollution. It also constitutes inefficient use of fuel, which is aggravated by carbon build-up on heat transfer surfaces.

It is an object of the present invention to provide apparatus whereby smoke and gaseous products of combustion (hereinafter referred to as combustion gases) are subjected to a secondary combustion so as to reduce the emission of combustion gases into the atmosphere when solid fuel is burned.

Accordingly, the invention provides secondary combustion apparatus for a solid fuel combustion appliance having a primary combustion compartment, said apparatus comprising a chamber having an internal flue of a refractory material for combustion gases, said chamber being spaced-apart in use from the primary combustion compartment and having an inlet for receiving the combustion gases from said primary combustion compartment and an outlet adapted to communicate with a main flue open to atmosphere, and means integral with said inlet for feeding an air supply to said inlet to support secondary combustion of said gases within said chamber.

Preferably, the apparatus includes a plurality of said chambers.

Also, preferably, at least some of the heat of secondary combustion is adapted to heat water, for example for a domestic hot water system and/or a central heating system.

The or each chamber is preferably elongated and upright in use with the inlet and the outlet, respectively, defining the lower and upper ends of said chamber(s). However, the or each chamber need not necessarily have its major axis in a vertical plane and the or each chamber may, for example, be angled in a direction either towards or away from the primary combustion compartment.

In the case of a single chamber, the chamber is preferably of rectangular section in plan while the internal flue is of irregular vertical section. Suitably the cross-section of the flue is greatest at a point approximately midway along the length of said flue. Such a variable cross-section gives rise to turbulence which improves secondary combustion due to greater mixing of combustion gases with oxygen. Furthermore, a variable cross-section slows down combustion gases so that the time for secondary combustion is extended in the secondary combustion apparatus.

It will be appreciated that the use of a flue of variable cross-section is not restricted to the case where the secondary combustion apparatus comprises a single chamber, since it is desirable to have maximum turbulence in all secondary combustion apparatus according to the invention.

Preferably, at least part of the or each chamber functions as a heat exchanger. Alternatively, the or each chamber has an associated heat exchanger whereby heat is transferred from the combustion gases to a flue before said combustion gases are discharged into the main flue.

Furthermore, the apparatus may include an additional heat exchanger to heat exchange means integral with the or each chamber when such an integral heat exchanger is present.

The or each internal flue of the or each chamber may be isolated from the surrounding boiler by means of a layer of insulating material.

Preferably, the air supply is introduced radially into the inlet of the or each chamber. Also preferably the means for feeding the air supply additionally comprises heat exchange means for pre-heating the air supply by the combustion gases.

The apparatus may be provided with a passage adjacent to, but spaced-apart from, the or each flue and wherein the air supply is admitted as a counter-flow to the flow of combustion gases in said flue(s) such that the air supply is preheated thereby.

The material of the or each flue is suitably firebrick, fireclay, cast iron, mild steel, a cast high temperature alloy or other material resistant to high temperatures.

The invention will be understood from the following description of embodiments thereof given by way of example only with reference to the accompanying drawings in which: Fig. 1 is a side elevation in section of a solid fuel combustion appliance incorporating apparatus according to the invention; Fig. 2 is a perspective view in section of portion of an alternative apparatus according to the invention.

Referring to Fig. 1 of the drawings there is illustrated a solid fuel combustion appliance, indicated generally at 1, said appliance 1 including a primary combustion compartment 2 having firebars 3 or supporting a firebed 4 of solid fuel 5.

An ashpit 6 is situated below the firebars 3. Doors (not shown) to the primary combustion compartment 2 and the ashpit 6 are provided in a front wall 7 of the appliance 1. The appliance 1 is a downdraught or downfiring combustion appliance of the kind described in our Patent Specification No. 1 526524 and the combustion gases pass down through the firebed 4 and the firebars 3 and thence through a passage 8 located intermediate two halves 9a and 9b of a boiler 9 located at rear 10 of the primary combustion compartment 2, before exiting to atmosphere by way of a main flue 11. When upfiring is required, for example when a fire is initially lit in the combustion compartment 2, a damper 12 is opened, as indicated by dashed lines in Fig. 1, so that the combustion gases pass directly into the main flue 11.

Secondary combustion apparatus, indicated generally at 13, is provided in the passage 8 at the rear 10 of the combustion compartment 2. The secondary combustion apparatus 1 3 includes a plurality of open-ended tubular elements 14 which run parallel to the primary combustion compartment 2 from a point below the level of the firebars 3 to the main flue 11 via a smoke box 1 1a.

Each element 14 defines a flue 1 6 at its interior surface 1 5 for the combustion gases exiting from the primary combustion compartment 2 and conveys them to the main flue 11 which communicates with upper open end 1 7 of each said element 14 via the smoke box 11 a. Lower open end 18 of each element 14 defines an inlet 19 for the gases. The passage 8 has located therein a plurality of annular air chambers 20, each of which surrounds one of said tubular elements 14. An annular cover 21 is provided at each end of the air chamber 20. The covers 21 are not airtight so as to allow for expansion of the boiler and tubular element surfaces, respectively, at high temperature. The tubular elements 14 are made of firebrick.However, other refractory materials such as fireclay or cast iron would be equally suitable.

The lower end 18 of each element 14 is provided with an annular chamber 22 for feeding a pre-heated secondary supply of air into the element 14 so as to support secondary combustion therein. The secondary supply of air is drawn from a point externally of the appliance 1 by way of one or more openings 23 in the rear wall of the combustion appliance 1. Each chamber 22 extends below the inlet 1 9 of the element 14 and defines an annular opening 24 for feeding the pre-heated secondary air radially to the combustion gases as they enter the flue 1 6.

Interior surfaces of the chambers 22 may be provided with fins so as to increase the temperature of the incoming secondary air.

In use, after the usual heating up time of approximately ten minutes, the combustion gases are drawn by natural or forced draught up through the flues 1 6 in a very hot environment and are subject to secondary combustion in the presence of the secondary air supply, which supply is drawn or forced in to mix with the combustion gases at the base of the flues 16. The amount of secondary air which enters the flues 16 is regulated by the rate of combustion in the primary combustion compartment 2 and the degree of natural or forced updraught. As a result of the secondary combustion of the combustion gases in the flue 16, the output of combustion gases to atmosphere from the main flue 11 is reduced to a minimum.

The heat derived from the secondary combustion is conducted from the perimeter of the tubular elements 14 which serve as heat exchangers to the boi!er surfaces 9a and 9b via the air chambers 20, resulting in greater heat transfer than that which occurs under normal circumstances in the absence of secondary combustion.

The interior surfaces 1 5 of each flue 1 6 will remain clean because at the lower end 18 the temperature is so high that any soot and deposits are burnt off and at the upper-end 1 7 deposits of soot are unlikely to occur, and if they do not occur, they would tend to burn off during peak usage of the appliance 1.

The external surfaces of the boiler 9 will remain clean because they are not in contact with the combustion gases. Hence, heat transfer will not become less efficient with time due to a build up of deposits on said surfaces, as in the case with many conventional appliances. Furthermore, with the present apparatus one does not have the usual cooling of combustion gases as they contact the boiler surface, since the flues 1 6 are maintained spaced-apart from the boiler surfaces by the air chambers 20.

In place of the water boiler 9, one might simply have an enlarged air chamber surrounding the tubular element(s) when hot air is required for space heating only.

In order to increase the temperature within the flues 1 6 and increase the rate of secondary combustion, a layer of insulating material (not shown) may be included at the lower end of each annular chamber 20 in which case only the lower part of each element 14 will function as a heat exchanger. Indeed, the annular chamber 20 may be entirely filled with insulating material, whereupon the appliance will be provided with a separate heat exchanger for extraction of heat from the combustion gases before they enter the main flue. For example, hot gases exiting from the secondary combustion appliance could be caused to travel down a pass having a heat transfer surface and which is parallel to but spaced-apart from the flue by the layer of insulating material before entering a main flue open to atmosphere.

Alternatively, the main flue may be positioned at the front of the combustion appliance and the gases exiting from the or each flue are brought through a horizontal heat exchanger located at the top of the combustion appliance before entering said main flue. For example, the gases might be cause to follow a convoluted path within a chamber located above the primary combustion compartment, said convoluted path being defined by suitably disposed baffles.

A secondary air supply for the secondary combustion apparatus could be introduced at one or more points at the top of the appliance and then conveyed to a passage surrounding the or each flue so that a counter flow of secondary air is set up in said passage relative to the flow of combustion gases in the or each flue which would result in preheating of the secondary air supply with consequent augmentation of the secondary combustion.

Referring to Fig. 2 of the drawings, there is illustrated alternative secondary combustion apparatus, indicated generally at 13', for use with the combustion appliance 1 illustrated in Fig. 1 and which comprises a chamber 20' which is rectangular in plan and which includes an internal flue 16' of variable cross-section. Combustion gases enter the flue 16' through an inlet 19' and preheated secondary air for secondary combustion is introduced via elongated slots 24' at the level of the inlet 19' from an air chamber 22'.

The secondary combustion apparatus depicted in Fig. 2 operates in essentially the same manner as the secondary combustion apparatus in Fig. 1 described above. The remaining parts which the apparatus 13' has in common with the apparatus of Fig. 1 are not indicated by reference numerals.

However, they will be readily deduced by referring to Fig. 1 and the related description above.

The flue 1 6' is of widest cross-section at a point P approximately midway along the length of said flue. This variation in cross-section along the length of the flue 1 6' increases turbulence within said flue which improves secondary combustion therein due to a more intimate mixing of combustion gases with the oxygen of the secondary air supply. Furthermore, the combustion gases are slowed down in the region of greatest cross-section, extending the effective combustion time of the combustion gases.

It is well known in the art of heating that the greater the temperature, turbulence and residence time of combustion gases within the combustion compartment the greater the efficiency of combustion.

Claims (14)

1. A secondary combustion apparatus for a solid fuel combustion appliance having a primary combustion compartment, said apparatus comprising a chamber having an internal flue of a refractory material for combustion gases, said chamber being spaced-apart in use from the primary combustion compartment and having an inlet for receiving the combustion gases from said primary combustion compartment and an outlet adapted to communicate with the main flue open to atmosphere, and means integral with said inlet for feeding an air supply to said inlet to support secondary combustion of said gases within said chamber.

2. Apparatus according to claim 1, which includes a plurality of said chambers.

3. Apparatus according to claim 1 or claim 2, wherein at least some of the heat of secondary combustion is adapted to heat water for a domestic hot water system and/or a central heating system.

4. Apparatus according to any one of claims 1-3, wherein the or each chamber is elongated and upright in use, with the inlet and outlet, respectively, defining the lower and upper ends of said chamber(s).

5. Apparatus according to any one of claims 4, wherein the major axis of the or each chamber is at an angle to the vertical.

6. Apparatus according to any one of claims 1-5, wherein the cross-section of the or each flue is variable so as to promote turbulence within said flue.

7. Apparatus according to any one of claims 3-5 when dependent on claim 1, wherein the chamber is of rectangular section in plan while the internal flue is of variable cross-section.

8. Apparatus according to claim 7, wherein the cross-section of the flue is greatest at a point approximately midway along the length of said flue so as to promote turbulence within the flue and assist the secondary combustion.

9. Apparatus according to any one of claims 1-8, wherein part of the or each chamber functions as a heat exchanger.

10. Apparatus according to any one of claims 1-8, wherein the or each chamber has an associated heat exchanger whereby heat is transferred from the combustion gases to a fluid before the combustion gases are discharged into the main flue.

1 Apparatus according to claim 10, wherein the chamber also has heat exchange means integral therewith.

12. Apparatus according to any one of claims 1-11, wherein the or each flue of the or each chamber is isolated from a surrounding boiler by means of a layer of insulating material.

1 3. Apparatus according to any one of claims 1-12, wherein the air supply is introduced radially into the inlet of the or each chamber.

14. Apparatus according to any one of claims 1-13, wherein the means for feeding the air supply additionally comprises heat exchange means for pre-heating the air supply by the combustion gases.

1 5. Apparatus according to any one of claims 1-14, wherein the apparatus is provided with a passage adjacent to but spaced-apart from the or each flue and wherein the air is admitted as a counter flow to the flow of combustion gases in said flue(s) such that the air supply is heated thereby.

1 6. Apparatus according to any one of claims 1-15, wherein the refractory material of the flue is selected from firebrick, fireclay, cast iron, mild steel or a cast high temperature alloy.

1 7. Apparatus according to claim 1, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.