GB2149082A - Burners - Google Patents

Abstract

A gas burner for use with a second similar gas burner has a combustion air supply duct containing a heat store 19 (which uses the heat from exhaust gases) and an aspirating venturi 18 connected at one end to exhaust through a valve 21. With the valve closed air entering the venturi via pipe 20 at the throat is directed turbulently through the heat store to form a combustible mixture with gas from supply line 16. With the valve open, and the burner not ignited, the entering air sucks exhaust gases through the heat store and blows them out through the valve, the store thereby being heated. A control system for two burners senses the temperature at the heat stores and uses this to switch over the gas and main combustion air flow. The aspirating flow of air is under the control of a pressure sensor in the combustion chamber. <IMAGE>

Description

SPECIFICATION Burners FIELD OF THE INVENTION The present invention relates to fluid fuel burners especially gas burners for heating confined spaces such as kilns.

BACKGROUND OF THE INVENTION A considerable amount of heat can be wasted by exhausting waste gases at a high temperature. It has been known for many years that a considerable saving can be achieved by having the incoming combustion air and the exhaust gases flow in two passages so the exhaust gases heat up one passage whilst the other is cooled by the combustion air and by intermittently swopping the passages by dampers so the combustion air is preheated by the previously heated passage returning the heat to the fire. This technique has largely been used in very large installations with the passages constructed of heavy masonry to act as a heat store.A gas burner produced by Consultant Gas Engineers for some years has incorporated a heat absorbing mass of refractory pebbles in its exhaust path to recover some of the heat in the exhaust gases, which mass transmitted recoverd heat to the combustion air by thermal conduction. Gas burners are often used in pluralities for many reasons including having a range of heat outputs (sometimes termed the turn-down ratio). In these circumstances, it would seem that there are a plurality of air passages heated inefficiently by a plurality of exhaust passages relying on thermal conduction and that increased efficiency would result from using an unused exhaust passage for combustion air and backwashing all the captured heat to the burner as in the first-mentioned prior technique.However there are problems related to the hydraulic impedance of the mass or store which affects the power needed to supply combustion air and also the pressure in the confined space being heated tending to blow out waste gases through any imperfect seals wasting heat and creating a hazard. The mass in the existing Consultant Gas Engineers burner has a very low flow impedance, and has a negligible effect on performance. However if the impedance is too low, the capacity of the heat store might also be low and this would entail overrapid switching of the burners in and out. Moreover it can be forseen that two or more burners might be lighted and exhausting through a single burner increasing the pressure drop across the that burner and the pressure in the space.

Whilst a pressurised space is desirable in some ways, there is a tendency for heat to escape spoiling the efficiency and also presenting a safety hazard. It is possible to reduce the pressure in the space by using some induced draught. However induced draught affects the combustion air flow and complicates the control of fuel-air mixture.

Moreover induced draught can cause the pressure to be sub-atmospheric and to suck cold air into the space.

SUMMARY OF THE INVENTION The present invention provides a fluid fuel burner having a fuel supply means leading to a fuel orifice and a forced draught air supply, wherein the air supply comprises a duct containing a heat store, which duct has at one end an air orifice co-operating with the fuel orifice to provide a combustible mixture and at the other end has a closable valve and an air supply pipe for feeding the forced draught air into the duct between the store and the valved end, which supply pipe is designed to aspirate surrounding air towards the valved end.

When the burner is not lighted, the valve will be open and air from the supply pipe will such air through the store and out through the valved end from the confied space being heated. Thus it will act as an induced draught producer. Since the forced draught is always present and it is merely a matter of regulating a flow, the aspirating response can be very fast. When the burner is ignited, the valve is closed and the arrangement of the supply pipe then merely creates a desirable turbulent flow of the combustion air.

The burner lends itself to ease and accuracy of control. The fuel flow can be regulated in response to a temperature senser in a confined space or a programme controller and then branched to various burners through valved lines; if as is usual only two burners are to be ganged, the valved lines can be merely on or off. The fuel flow or the control signal therefor can be used to regulate air flow through one branch of a forced air supply leading to an ignited burner and a pressure senser in the confined space can be used to regulate air flow to unignited burners. A timer or a temperature senser behind each store to sense when the store was fully charged with heat would close the valved end of its duct and open the valved end of the duct of the other burner and switch over all the supplies of air and gas.A burner to be ignitied could be ignited by a signal derived from the switch-over signal by means of a suitable igniter or by means of a permanently lit pilot flame. Thus the installation would continue to operate as before with the sole change being that another burner is burning.

In a co-pending application, there is described a kiln with an improved seal arrangement allowing kilns to be constructed less expensively whilst at the same time permitting the kiln to operate at a higher presure thus improving heat transfer between the wares and the gas; it also permits the kiln atmos phere to be regulated be between an oxidising and a carburising atmosphere. The present burner is especially suitable for use with such a kiln.

Since the kiln pressure drives the exhaust gases through the heat store, the induced draught caused by the aspirating effect merely has to cope with swings of kiln pressure.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic section through a burner embodying the present invention, and Figure 2 is a schematic circuit diagram of a control system for operating two burners as illustrated in Fig. 1 alternately.

BEST MODE OF PERFORMING THE INVEN TION An embodiment of the invention will now be described by way of example.

In Fig. 1 there is a burner having the usual body 11 in the form of a cylinder sealed at one end and containing at its other end a refractory venturi-shaped quarl 1 2. A gas supply pipe 1 4 enters this body transversely on the outside of a mounting flange 1 5 and terminates in an orifice 1 6 directing the gas axially down the quarl. The quarl with the gas orifice defines a combustion air orifice 1 7 directing the air down the quarl and mixing it with the gas. The air supply comprises a duct consisting of the back end of the body and a venturi-shaped tube 1 8 entering the body transversely.The body behind the orifices contains a permeable heat store 1 9. A forced draught air supply pipe 20 enters the tube 1 8 at the throat of the venturi and directs the air upwards towards the end of the tube where there is a flap valve 21 with drive means 22 for fully opening and closing the flap. A flue (not shown) can be fitted at this valved end of the tube and this can be provided with means for causing an induced draught. To enable automatic ignition to be achieved the gas supply pipe is of an electrically conductive refractory material. When the burner is in use and lighted, the flap valve 21 will be closed and the arrangement of the pipe 20 will merely cause desirable turbulence of the air flow which will be directed through the heat store to the orifices.However when the burner is not ignited, the flap valve will be open and air will suck a flow in the reverse direction.

The heat store is made of a mass of refractory particles mixed with a refractory cement.

Whilst the cement is still wet, air is blown through it to adjust its flow impedance and then the cement is dried and fired to form a unitary structure. This structure is dropped into the body and sealed in place. If it becomes clogged in use, the structure can be fired to burn it clean.

Fig. 2 shows a control arrangement for a kiln 25 which is preferably of the type described in our copending Application. That type of kiln uses ware decks for supporting wares to be fired and has kiln walls vertically displaceable relative to the ware decks so the decks can be positioned relative to the kiln and then the the walls seated on the ware decks to make a seal. To avoid excessive lifting of the walls or the ware decks, the ware enters the kiln through doors which in use seal on the edges of the decks by virtue of a closing motion which applies sealing pressure.

The kiln uses two or more burners as described in relation to Fig. 1 but these are usually ganged in pairs so the control arrangement will deal with only two burners.

There is a common gas supply 26 with a valve 50 regulated by a programme controller 27 which receives an input from a temperature senser 28 within the kiln. This common supply is then branched through lines 29 containing on-off valves 30. A forced draught combustion air supply 31 is branched through lines 51 containing regulatable valves 32.

One of these valves 32 is under the direct or indirect control of a flow senser 33 in the gas supply but normally a ratio controller 34 receiving an input from the senser 33 will be used. The other of the valves 32 is under the direct or indirect control of a pressure senser 35 in the kiln. The signals for the various valves are passed through a change-over switch 36 which operated either by a timer 37 or by a temperature senser (not shown) behind each store. A timer is preferred; using solely a temperature sensing device to change over might lead to one burner keeping alight a long time and might lead to a poorer heat distribution than two burners operating alternately.Whilst the main advantage of this control arrangement is that the operating levels are established for a newly ignited burner and thus the only real change is in the particular burner that is lit, it is preferred to have a short dwell in each change-over to ensure amongst other things that the flap valves are fully operated. This short dwell can be easily arranged and it is not shown in Fig.

2. When the burner is to be ignited, an ignition unit 38 can apply a spark to an ignition electrode 39 (Fig. 1).

Referring back to Fig. 1, the electrode 39 can be arranged to ignite the gas directly but it is preferred to use a belt-and-braces arrangement wherein the electrode co-operates with a pilot flame jet 40. This pilot jet 40 can be left permanently lit; it is necessary to have a stable flame capable of withstanding the exhaust draught but it can be screened to assist in this. If the pilot flame goes out when the burner is not lit, the pilot flame will be reignited on the change-over. The control system individual to a burner comprises a main solenoid operated valve 41, a back-up solenoid operated valve 42, a pilot flame solenoid operated valve 43. The gas flows through valves 41 and 42 in series to the fuel orifice 1 6 and a bleed is taken off from between the valves 41 and 42 through the valve 43 to the jet 40.A flame failure senser 44 supplies a signal to a flame failure relay 45 which receives a signal also from the valve 43 to check whether the flame should be lit and if the senser 44 detects there has been a failure and the valve 43 is open, a signal is fed from the relay to trip the main valve cutting off gas to the orifice 1 6. A further signal is fed from the relay to operate the igniter 39 or to shut off the valve 43. It is thought safer to close down the gas supplies and to rely on manual intervention. The main valve 41 has an electrical control switch 45. Preferably an Ultraviolet senser 46 observes the main orifice gas flame.

It is impossible to give hard-and-fast dimensions since these depend on the application.

However it is possible to calculate the maximum flow impedance of the heat store since this flow impedance multiplied by the swing in flow rates through the store should be less than the maximum aspirating effect creatable by the air supply pipe, if the kiln pressure is to be fully stabilsed. From this maximum impedance, it is possible to calculate heat store and particle size. Whilst larger particles tend to absorb heat less rapidly, it is to be noted that because of the larger void volume a given amount of gas flowing through a store will stay in the store longer.

Claims (5)

1. A fluid fuel burner having a fuel supply means leading to a fuel orifice and a forced draught air supply, wherein the air supply comprises a duct containing a heat store, which duct has at one end an air orifice cooperating with the fuel orifice to provide a combustible mixture and at the other end has a closable valve and an air supply pipe for feedig the forced draught air into the duct between the store and the valved end, which supply pipe is arranged to aspirate surrounding air towards the valved end.

2. A burner according to claim 1 wherein the valved end of the duct is in the form of a venturi and the air supply pipe is arranged to deliver the air in the form of a jet within the throat of the venturi.

3. A control arrangement for a kiln using two burners according to claim 1 or claim 2 and which are to operate alternately, comprising a fuel supply regulatable to the demand which fuel supply is branched to each burner through a branch containing an on-off valve, a forced draught air supply branched to each burner through a branch containing a regulatable air supply, a change-over device under the control of a timer and/or temperature sensers behind the heat stores, said changeover device putting the fuel to one burner and putting the control of the air flow to that one burner under the control of the fuel flow to that burner and the air flow to the other burner under the control of a kiln pressure senser and the changeover device also opening the burner duct valve of the other burner and closing that of the said one burner.

4. A burner substantially as herein described with reference to Fig. 1 of the accompanying drawings.

5. A control arrangement substantially as herein described with reference to Fig. 2 of the accompanying drawings.