GB2077903A

GB2077903A - Producing a gas-air mixture

Info

Publication number: GB2077903A
Application number: GB8116885A
Authority: GB
Original assignee: Otto Junker GmbH
Current assignee: Otto Junker GmbH
Priority date: 1980-06-13
Filing date: 1981-06-02
Publication date: 1981-12-23
Also published as: DE3022181C2; GB2077903B; IT1142528B; DE3022181A1; FR2484610B3; IT8148568A0; FR2484610A1

Abstract

This invention provides a method of and a gas furnace 1 for producing a gas-air mixture, wherein the combustion air is preheated by means of the hot exhaust gases of the furnace in a heat exchanger 9. The preheated combustion air and the fuel gas are fed to a mixer 20 in which a gas-air mixture is produced which is then supplied to the gas burners 2-5 of the furnace 1. The ratio of gas to fuel in the mixture is continuously adjusted in response to a thermostat 39 sensing the temperature of the combustion air. The thermostat being connected to a shut off valve 35 for the fuel supply and a throttle valve 40 for the exhaust gases. <IMAGE>

Description

SPECIFICATION Method of and gas furnace for producing a gasair mixture This invention relates to a method of and a gas furnace for producing a gas-air mixture suited for gas burners of industrial furnaces, in particular gas quick-heating fumaces; wherein the combustion air is initially preheated by means of the hot exhaust gases of the furnace in a heat exchanger and then is fed to the gas burners.

Gas is an ideal fuel, because the heatconsuming gasification process required for combustion is dispensed with. An optimum operation is possible if the firing is able to be carried out approximately with the air theoretically required, which is in particular possible with a satisfactory mixing of the fuel gas with the combustion air. It is a disadvantage that for various gas combustion processes the exhaust gases obtained upon combustion of the gas-air mixture are more or less unused expelled into atmosphere.

The thermal content of the exhaust gases of gas-heated furnaces mainly depends on the exhaust gas temperature. Furnaces with high exhaust gas temperatures have accordingly high exhaust gas losses and low thermal efficiencies.

For improving economies, the thermal content of the exhaust gases is utilized for preheating fuel gas or air, in part. Generally that fluid is preheated which has the greater thermal content for equal preheating temperature. This as a rule is the combustion air. Preheating of the combustion air in addition thereto brings about the advantage that the heat exchanger need not be absolutely tight, contrary to a possible gas preheating.

It is known for preheating combustion air (German Auslegeschrift 1 9 05 489) to use stone or steel recuperators in-order to preheat the combustion air to about 1 ,0000C by means of the exhaust gases of the furnace. By means of this preheating of the combustion air, the efficiency of the furnace is able to be substantially increased.

If, however, the furnace is equipped with a plurality of burners, the design is very complex when preheating the air and combining the two fluid streams only in the burner. In particular, it is difficult to produce in all burners the same mixture ratio between preheated combustion air and fuel gas so that different ignition values are to be expected.

It is, therefore, the object of the present invention to obtain an optimum gas-air ratio for all burners of gas furnaces equipped with a plurality of burners both during permanent operation and during start-up operation, and also to provide an accordingly controllable gas furnace for performing the method.

According to one aspect of the present invention a method of producing a gas-air mixture suited for gas burners of industrial furnaces comprises the steps of initially preheating the combustion air by means of the hot exhaust gases of an industrial furnace in a heat exchanger; intermixing the fuel gas and the preheated combustion air already prior to supplying them to the gas burners, the mixing ratio being continuously adjusted according to the preheating temperature of the combustion air, and feeding the- intermixed combustion air and fuel gas to the gas burners.

An advantage of the invention is that all gas burners of such a gas furnace are supplied with the same fuel, i.e. with the same gas-air mixture, so that hence also a uniform heating effect or calorific value is able to be ensured. This also is true for the start-up phase or in the event of trouble, since the mixing ratio is variable and is able to be selected respectively according to the preheating temperature of the combustion air.

It is convenient to currently supervise the temperature of the combustion air stream and to adjust the fuel gas quantity and the flow velocity of the exhaust gas accordingly. In this way, a quick conforming to optimum or any desired correction is respectively permitted.

According to another aspect of the present invention a gas furnace for producing a gas-air mixture suited for its gas burners comprises an exhaust gas line; a heat exchanger mounted in the exhaust gas line and arranged to preheat combustion air supplied to the heat exchanger by a fan; a mixer; a combustion air line for leading the combustion air from the heat exchanger to the mixer; a fuel gas line for feeding fuel gas to the mixer so as to be intermixed with the combustion air in order to produce a gas-air mixture, and a distribution system for the gas-air mixture, said distribution system being connected to the mixer and comprising a main feed conduit followed in the direction of flow of the gas-air mixture by a plurality of distribution tubes each carrying one of the gas burners at its free end.The separate supplying of combustion air and fuel gas to the mixer permits to respectively check the temperature of the combustion air and to accordingly vary the fuel gas addition. Since the gas-air mixture leaving the mixer is passed to the individual gas burners practically in one stream, it is ensured that each gas burner is actually supplied with the same fuel gas.

A high operating reliability is provided for in a convenient manner if a thermostat is arranged downstream of the heat exchanger, said thermostat being associated with the combustion air line and allowing both a throttle valve arranged in the exhaust gas stream and a shut-off valve arranged in a branch running parallel to a section of the fuel gas line to be controlled. Upon variation of the temperature of the combustion air, in such an arrangement the optimum gas-air mixture is able to be varied by varying the fuel gas supply, and then at the same time by influencing the exhaust gas flow velocity in the heat exchanger a correction of the combustion air temperature and thus a reduction of the fuel gas quantity supply variation is able to be effected. This provides for an advantageous operation of the furnace in cost aspects.

An exact controlling of the fuel gas supply is permitted in the event that restrictors are respectively arranged in the section of the fuel gas line and in the branch running parallel thereto.

These restrictors are conveniently adjustable manually, since by virtue of the method and the control arrangement proposed by the invention a current correction of the restrictors is not required.

For ensuring a uniform mixing of both fluid streams in the mixer it is advisable to provide a gas balance pressure valve upstream of the restrictors and the shut-off valve in the fuel gas line, said gas balance pressure valve being in communication with the combustion air line via a feedback. The gas pressure of the fuel gas is currently conformed to circumstances in the combustion air line as the result of an according control. Otherwise, the central fuel gas line is also already controlled on pressure and is accordingly adjusted. To achieve this end, for the fuel gas line a pressure control valve may be arranged upstream of the gas balance pressure valve.By means of this central arrangement, a uniform output pressure is provided for even when interconnecting a plurality of such gas furnaces, so that the subsequent control is permitted to be much simpler and also more accurate.

It has been found that with such predetermined and precisely maintained fluid streams the mixer conveniently is a pipe socket. The two fluid streams are combined uniformly and aimed in such a pipe socket so that a sufficient intermixing of both fluids occurs.

An embodiment of the invention will now be described by way of example and with reference to the accompanying drawing, in which the only Figure is a schematic cross section of a gas quickheating furnace according to the present invention.

The drawing shows a gas quick-heating furnace which is indicated generally by the reference numeral 1. The gas quick-heating furnace T is provided with a plurality of gas burners of which only gas burners 2, 3,4 and 5 are shown, said gas burners being interconnected via a distribution system 6 for the gas-air mixture. The burners 2, 3, 4 and 5 are fed with a gas-air mixture uniformly via the distribution system 6 which includes a main feed conduit followed in the direction of flow by a plurality of distribution tubes each of which carries one of the gas burners at its free end.

A heat exchanger 9 is arranged in the exhaust gas line 8 of the gas quick-heating furnace 1 conducting the exhaust gas stream 7. Sufficient quantities of air are passed through the heat exchanger 9 by means of a fan 10. The air cleaned by means of a filter 11 in the fan 10 leaves the heat exchanger 9 for instance with a temperature of 3O00C. Such a heating up of the air from 20 to about 300 C is obtained for instance with an exhaust gas temperature of about 550 to 9500C.

The combustion air fed to the heat exchanger 9 via a pipe 12 leaves the heat exchanger 9 via a combustion air line 1 5 in which a throttle valve 1 6 is arranged able to be adjusted by means of a motor 1 7. When the throttle valve 1 6 is open, the combustion air then passes via a restrictor 1 8 to a mixer 20. There the combustion air is intermixed with the fuel gas.

The fuel gas flows via a shut-off valve 22, a fuel gas line 23 and a pressure control valve 24 arranged in the fuel gas line 23 to a gas balance pressure valve 30, a section 28 of the fuel gas line 23 being able to be shut-off upon requirement via a shut-off valve 26 equipped with a motor 27. A uniform pressure within the fuel gas line 23 is currently controlled by means of a pressure sensor 25 which is associated with the pressure control valve 24.

The fuel gas is passed from the gas balance pressure valve 30 via a restrictor 31 in a section 34 of the fuel gas line 23 to the mixer 20 which for instance consists of a pipe socket. In the mixer 20, the fuel gas stream and the combustion air stream are intermixed in such a way that an optimum gas-air ratio is able to be fed to the gas burners 2, 3, 4 and 5 via the distribution system 6.

Parallel to the section 34 of the fuel gas line 23 with the restrictor 31 a branch 38 is provided which has associated therewith a shut-off valve 35 and a restrictor 36. While the shut-off valve 35 is connected to a thermostat 39 via a control line 42, the gas balance pressure valve 30 has a feedback 37 to the combustion air line 1 5 directly downstream of the throttle valve 1 6.

The thermostat 39 furnishes the required measurement data which are required for a variation of the adjustment of the shut-off valve 35 and at the same time for a variation of the adjustment of a throttle valve 40 via a motor 41.

Upon start-up of the gas quick-heating furnace 1, initially air is fed by the fan 10 through the heat exchanger 9, via the opened throttle valve 1 6 and the opened restrictor 1 8 to the mixer 20. At the same time, fuel gas flows through the opened shut-off valve 22, the fuel gas line 23, the pressure control valve 24, the opened shut-off valve 26, the opened gas balance pressure valve 30, the opened controllable shut-off valve 35 and through the restrictors 31 and 36 to the mixer 20.

There, the two fluids are intensively intermixed, and then the gas-air mixture is able to be fed to the gas burners 2, 3, 4 and 5. The pressure of the combustion air in the combustion air line 1 5 and the pressure at the outlet of the gas balance pressure valve 30 are equal. The combustion air quantity to be supplied to the mixer 20 is adjusted via the throttle valve 1 6 and the restrictor 1 8. The fuel gas quantity on the other hand is determined by means of the restrictors 31 and 36.

During the start-up phase, the combustion air is gradually heated up by the exhaust gases in the heat exchanger 9, thereby the specific weight of the combustion air varying. So that the most satisfactory ratio of fuel gas and combustion air is able to be maintained, the shut-off valve 35 is gradually closed via the thermostat 39. When the temperature preset at the thermostat 39 has been reached, the shut-off valve 35 will also be closed.

The fuel gas is now exclusively fed to the mixer 20 via the section 34 of the fuel gas line 23, the parallel branch 38 and the restrictor 31.

Claims

1. A method of producing a gas-air mixture suited for gas burners of industrial furnaces comprises the steps of (a) initially preheating the combustion air by means of the hot exhaust gases of an industrial furnace in a heat exchanger; (b) intermixing the fuel gas and the preheated combustion air already prior to supplying them to the gas burners, the mixing ratio being continuously adjusted according to-the preheating temperature of the combustion air; and (c) feeding the intermixed combustion air and fuel gas to the gas burners.

2. A method as claimed in claim 1, wherein thetemperature of the combustion air stream is currently controlled and the fuel gas quantity and the flow velocity of the exhaust gas are accordingly adjusted.

3. A gas furnace for producing a gas-air mixture suited for its gas burners, comprising (a) an exhaust gas line; (b) a heat exchanger mounted in the exhaust gas line and arranged to preheat combustion air supplied to the heat exchanger by a fan; (c) a mixer; (d) a combustion air line for leading the combustion air from the heat exchanger to the mixer; (e) a fuel gas line for feeding fuel gas to the mixer so as to be intermixed with the combustion air in order to produce a gas-air mixture; and (f) a distribution system for the gas-air mixture, said distribution system being connected to the mixer and comprising a main feed conduit followed in the direction of flow of the gas-air mixture by a plurality of distribution tubes each carrying one of the gas burners at its free end.

4. A gas furnace as claimed in claim 3, wherein a thermostat is arranged downstream of the heat exchanger, said thermostat being associated with the combustion air line and allowing both a throttle valve arranged in the exhaust gas stream and a shut-off valve arranged in a branch running parallel to a section of the fuel gas line to be controlled.

5. A gas furnace as claimed in claim 4, wherein restrictors are respectively arranged in the section of the fuel gas line and in the branch running parallel thereto.

6. A gas furnace as claimed in claim 5, wherein the restrictors are adjustable manually.

7. A gas furnace as claimed in claim 3, 4, and 5, wherein a gas balance pressure valve is arranged upstream of the restrictors and the shut-off valve in the fuel gas line, said gas balance pressure valve being in communication with the combustion air line via a feedback.

8. A gas furnace as claimed in claim 5, wherein a pressure control valve for the fuel gas line is arranged upstream of the gas balance pressure valve.

9. A gas furnace as claimed in claim 3, wherein the mixer is a pipe socket.

10. A method of producing a gas-air mixture suited for gas burners of industrial furnaces, the method being substantially as herein particularly described with reference to the accompanying drawing.

11. A gas furnace for producing a gas-air mixture suited for its gas burners, the gas furnace being constructed and arranged substantially as herein particularly described with reference to the accompanying drawing.

1 2. Any novel feature or combination of features described herein.