GB2052029A - Furnace with Reduced Heat Loss - Google Patents

Abstract

Heat which would otherwise be wasted in the form of hot flue gases from a combustible fuel furnace may, at least in part, be recycled by providing a heat exchange relationship between the hot flue gases and gas in which the furnace fuel is to be burnt by the furnace burner. Thus, the invention provides a combustible fuel furnace incorporating a burner, a separate flue for hot combustion products and means for passing gas in which the fuel is to be burnt by the burner into heat exchange relationship with the flue and thereafter to the burner. The principle of the invention is broad and the invention therefore also includes a method for operating a combustible fuel furnace incorporating a burner and a separate flue, which method comprises operating the burner, feeding hot combustion products from the burner through the flue and passing gas in which the fuel is to be burnt into a heat exchange relationship with the flue and thereafter to the burner. <IMAGE>

Description

SPECIFICATION Furnace With Reduced Heat Loss This invention relates to the more efficient usage of heat in furnaces and provides both a furnace structure and a method for achieving the same.

In the operation of furnaces a high proportion of heat is wasted, in the absence of any form of heat recovery, in the hot flue gases. This source of heat loss increases in significance with operating temperature. The problem is particularly severe with batch-type furnaces in which it has been estimated that an average batch forge furnace has a thermal efficiency of about 10%, with approximately 70% of the heat supplied being lost in the flue gases. Recuperative burners have been designed in an attempt to overcome this problem in which the burner chamber is surrounded by a heat exchanger which provides, in the form of a series of concentric tubes, annular combustion air and flue gas passages having heat exchange surfaces therebetween so as to preheat the combustion air using the heat from the flue gases when the burner is in use.However, such a structure suffers from a lack of flexibility and cannot conveniently be applied to any existing furnace structure in which a flue system is already built in.

According to the present invention there is provided a combustible fuel furnace incorporating a burner, a separate flue for hot combustion products and means for passing gas in which the fuel is to be burnt by the burner into heat exchange relationship with the flue and thereafter to the burner. Preferably the furnace incorporates a valve or valves enabling the amount of preheated gas in which the fuel is to be burnt (hereinafter called "combustion gas") and which is fed to the burner to be adjusted and/or such flow of preheated gas to be mixed with incoming cold gas. In this way, considerable flexibility can be achieved in the operation of a conventional burner and furnace system.It will be appreciated that the principle of the present invention may be applied to any existing furnace having a separate flue and burner simpiy by the installation of a custom-built system of flue jacket(s) and piping (and, optionally, valves if it is desired to provide a facility for the use of some cold combustion gas) in the furnace in order to provide the necessary combustion gas heat exchange facility with the hot flue gases when the furnace is in use. The invention includes any such system.The invention also includes a method for operating a combustible fuel furnace incorporating a burner and a separate flue, which method comprises operating the burner, feeding hot combustion products from the burner through the flue and passing gas in which the fuel is to be burnt (combustion gas) into a heat exchange relationship with the flue and thereafter to the burner.

Preferably the furnace of the present invention also comprises a casing enclosing a crucible to be heated, the burner being positioned on the casing for heating a space between the casing and the crucible, the flue leading away from the aforesaid space. In this type of furnace the crucible is usually made from clay or graphite.

In the furnace of the present invention, in addition to the means for establishing the aforesaid heat exchange relationship, a part of the flue may have wrapped therearound a series of chambers or a single chamber to serve as an ingot preheater (where the furnace is for melting metal). By storing ingots in the preheater prior to their introduction into the furnace body (or crucible) the dangers inherent in the presence of moisture in the ingots can be reduced since at the relatively low temperature obtained in the ingot preheater water will be removed speedily without danger.

It should be clearly appreciated that the great advantage of the present invention with its means for passing gas through a heat exchange relationship with the flue and thereafter to the burner, optionally incorporating an inlet and valve arrangement to allow mixing of preheated and cold combustion gas is flexibility. A system can be custom-built for any given furnace having a separate burner and flue. The principle can be applied to continuous or batch furnaces and to furnaces having a single flue for a single burner or one or more flues for a number of burners. In the case of a furnace having a number of burners and a single flue, conduits may be provided to convey preheated combustion gas to each or only some of the burners after passage of the gas through a heat exchange relationship with the flue.

Naturally, the combustion gas is almost invariably air or an oxygen-containing gas.

However, in principle the invention is so broad that it may be applied to the combustion of any combustible fuel in any gas in which it will burn.

It is certainly possible using the present invention to reduce, at least for some batch furnaces, the temperature of gases leaving the flue from approximately 8000C or more to 3500 or less, the heat saved thereby being used to achieve more efficient combustion in the burner.

The invention will now be further described and illustrated with reference to the accompanying drawings, in which: Figure 1 shows a cross-sectional view of a furnace in accordance with the present invention; and Figure 2 shows a view of the furnace in Figure 1 with a lid provided for the crucible and looking down in the direction of the arrow A in Figure 1.

The furnace in Figure 1 has an outside casing 1 which may be designed to be easily removable for repairs to internal furnace linings. Within the casing 1 a crucible 1 3 is provided standing on a clay stand 14. Crucible 1 3 may be provided with a lid 12 (Figure 2) which can be formed from an outer mild steel casing containing ceramic fibre as an insulator. The lid may be large enough to cover not just the mouth of crucible 1 3 but an adjacent bale out area also. Crucible 13 may be, for example, a standard 400 Ib (say 200 kg) capacity clay/graphite crucible standing on a 4 inch (say 10 cm) clay stand (14). Around crucible 13 is a space 1 5 to be heated by the operation of a burner 5.Space 15 is bounded by a segmented refractory lining 8 around the outside of part of which is a layer of ceramic fibre 9 providing a backup hot face lining in case of failure to the lining 8 and serving as a first stage insulator. The use of ceramic fibre 9 enables the weight and outside diameter of the furnace to be kept to a minimum. Around the outside of ceramic fibre 9 is a mineral wool layer 10 which is, of course, of low cost and has high insulation properties. The use of this latter material in combination with ceramic fibre 9 and the refractory lining 8 enables very low outside casing temperatures to be achieved. The base of furnace 1 comprises a castable insulation 11 1 which stands on feet 16.

A flue 3 leads hot combustion gases away from space 1 5 when the furnace is in use. Flue 3 is, of course, open at the top and is wrapped by a heat exchanger 4 which in turn is wrapped by further ceramic fibre 9. Positioned beneath heat exchanger 4 (and also around flue 3) are ingot preheating chambers 2 in which ingots may be preheated prior to introduction into crucible 13. In addition to this preheating facility for ingots, the ingot preheating chambers 2 enable storage of ingots without cluttering the bale out area of the furnace around the mouth of crucible 13.

Conduit 1 7 leads from heat exchanger 4 to burner 5 and inlets 6 and 7 are provided for incoming combustion gas (say air) and combustible fuel (say oil), respectively. An inlet conduit 18 (see Figure 2) is provided to lead combustion gas to heat exchanger 4 from a source of such gas. Conduit 1 7 is provided with a valve system 1 9 and a by-pass facility 20. The use of the by-pass facility 20 and valve system 19 enable the amount of preheated combustion gas being fed to burner 5 when the furnace is in use to be adjusted appropriately for any given application. Naturally, inlets 6 and 7 may be provided with appropriate valves if desired.

The structure of burner 5 is not particularly significant and may be selected depending upon availability of burners and the desired application of the present invention. Conventional controls for such a burner include an on/off switch, a high/low switch (for controlling the nature of operation thereof) and a start button. If desired, the adjustments to the burner may be made inside casing 1 before installation and operation of the furnace so as to fix fuel consumption. Since burner 5 has its burner port totally enclosed the furnace is relatively quiet in operation, especially on low fire conditions.

The furnace shown in the accompanying drawings preferably has many of its component parts assembled together in a manner which permits removal and replacement and/or cleaning thereof. Thus, for example, casing 1 may be easily removable for repairs to the lining and the flue assembly and lid may be made removable.

Refractory lining 8 is also shown in segmented form but it should be appreciated that this is merely for convenience and is by no means essential. The use of a segmented construction for the refractory lining 8 is convenient for speed and ease of assembly of the overall furnace.

Suitable refractory units may be pre-cast to a desired quality and shape and positioned together speedily. Joints between such refractory units may be stepped to prevent heat passing through the joints and to inhibit cracking of the refractory lining 8 when assembled. After assembly, individual refractory units making up refractory lining 8 may be steel banded to prevent any undesired movement.

In operation, combustion gas, e.g. compressed air, is supplied through inlet 6 and fuel, e.g. 35 second oil, is fed through inlet 7. As burner 5 is thus operated hot combustion products circulate through space 1 5 heating crucible 1 3 and thereafter passing out to the atmosphere through flue 3. A further source of compressed air may be fed through conduit 1 8 into heat exchanger 4 where a heat exchange relationship is established with the hot flue gases passing through flue 3. It is convenient and preferred to provide heat exchanger 4 with baffles or fins to promote efficiency in circulation of incoming cold combustion gas. Thus, heat exchanger 4 may be provided with a number of vertical fins which ensure satisfactory circulation of incoming compressed air and therefore efficient heat exchange.Heated compressed air passes from heat exchanger 4 down through conduit 1 7 and by-pass 20 into burner 5. If it is desired that a large amount of heated combustion gas be fed to burner 5, valve system 19 may be opened.

Naturally, it is not essential to provide by-pass 20.

Any suitable valving system may be employed to ensure any desired mixing ratio of heated and cold combustion gas. After a flow of heated combustion gas has been established to burner 5 through conduit 1 7 the incoming flow of cold combustion gas passing through inlet 6 may be terminated. Again, such termination totaily depends upon the desired application of the invention and the type of burner employed and the manner in which it is desired to operate the burner. All of these operating considerations will be clearly apparent to the skilled man having regard to any particular application of the furnace.

Furthermore, it will also be apparent that the combustible fuel need not be oil but could, for example, be a combustible gas.

Operating a typical furnace in accordance with the accompanying drawings having a diameter of approximately 1050 mm, a main body furnace height of approximately 860 mm, a flue height of approximately 1180 mm and a flue heat exchanger height of approximately 900 mm, very economic results can be obtained. Thus, during a typical production run using the furnace as a lead smelter, a melting rate of 700 Ibs (say 350 kg) during a 7 hour day was obtained, consuming only 7.4 gallons (say 37 litres) of oil as the combustible fuel. Oil consumption of 1 gallon (say 5 litres) per hour per 100 Ibs (say 50 kg) of molten metal is a very efficient use of fuel. In the same operation, the expected temperature of the flue at the top thereof of 750 to 800 C (in the absence of the heat exchange system of the present invention) was found to be reduced to approximately 300 C. In addition, the case temperature at no point exceeded 1 300C. The operation thus provided satisfactory results with very economic performance and increased safety of operation.

Naturally, the above description of a preferred embodiment of the invention is in no way intended to limit the scope of the present invention. The skilled reader will have no difficulty in envisaging alternative structures for himself which employ the principle of the present invention.

Claims (14)

Claims

1. A combustible fuel furnace incorporating a burner, a separate flue for hot combustion products and means for passing gas in which the fuel is to be burnt by the burner into heat exchange relationship with the flue and thereafter to the burner.

2. A furnace as claimed in claim 1 also comprising an inlet for unheated gas in which the fuel is to be burnt by the burner together with a valve arrangement adapted to adjust the proportions fed to the burner of said unheated gas and gas in which the fuel is to be burnt and which has been passed through said heat exchange relationship.

3. A furnace as claimed in claim 1 or claim 2 including a valve for adjusting the flow to the burner of gas in which the fuel is to be burnt which has passed through said heat exchange relationship.

4. A furnace as claimed in any one of claims 1 to 3 also comprising a casing enclosing a crucible to be heated, the burner being positioned on the casing for heating a space between the casing and the crucible, the flue leading away from said space.

5. A furnace as claimed in claim 4, wherein the crucible is made from clay or graphite.

6. A furnace as claimed in any one of claims 1 to 5, wherein part of the flue has wrapped therearound one or more chambers adapted to serve as ingot pre-heaters.

7. A furnace as claimed in any one of claims 1 to 6, wherein a plurality of burners are provided and a single fiue, conduits being provided to convey gas in which the fuel is to be burnt from said heat exchange relationship to each or some of the burners.

8. A furnace as claimed in any one of claims 1 to 7 which is provided with a refractory lining and a ceramic fibre layer outside said retractory lining to provide a back-up lining and to serve as a first stage insulator.

9. A furnace as claimed in any one of claims 1 to 8 which is provided with a segmented refractory lining.

10. A furnace substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

1 A method for operating a combustible fuel furnace incorporating a burner and a separate flue, which method comprises operating the burner, feeding hot combustion products from the burner through the flue and passing gas in which the fuel is to be burnt into a heat exchange relationship with the flue and thereafter to the burner.

12. A method as claimed in claim 1 wherein the gas in which the fuel is to be burnt is air or an oxygen-containing gas.

13. A method as claimed in claim 11 or claim 12, wherein said heat exchange relationship is established to an extent sufficient to result in the temperature of gases leaving the flue during operation being 3500C or less.

14. A method for operating a combustible fuel furnace incorporating a burner and a separate flue substantially as hereinbefore described with reference to the accompanying drawings.