GB2059568A - Burner controls - Google Patents

Abstract

Apparatus for controlling the supply of fuel gas and combustion air to a burner comprises valves 1 in the air line 3 and 2 in the gas line 4, operating in conjunction to maintain a predetermined air/gas ratio, a branch line 5 to supply gas to one part of the burner and a pressure relief valve 6 in the gas line 4 through which gas can pass to a second part of the burner. In use of the apparatus, gas passes to the second part of the burner only while the pressure at the inlet of the relief valve is high enough to hold the valve open but the gas supply to the first part of the burner is maintained without interruption. Thus the second part of the burner becomes operative or inoperative as the gas supply is varied and the transition is smooth and progressive. <IMAGE>

Description

SPECIFICATION Burner control systems This invention relates to gas burners and in particular to apparatus for controlling the supply of gas and air to such burners.

In recent years, there have been many advances in the design of gas burners and the heating devices in which they are used, particularly for industrial purposes. Improved thermal efficiency and higher flame temperatures have resulted, for example, from the use of integral recuperators to preheat the combustion air and nozzle-mixing tunnel burners have been developed which provide the high exit velocity of combustion products required for jet-driven recirculation within furnace chambers, radiant tubes and immersion tubes. Application of these developments has led to the introduction of compact combustion equipment, much of it automatically controlled and constructed as self-contained units.

Compactness, however, though an advantage and often the feature that makes some new use possible, can give rise to difficulties in providing means for safe and reliable ignition when small burners are to be used. For example, burners such as those disclosed in British Patent 1 404 578 are generally ignited by an electrode which produces a spark in the annulus around the burner nozzle where the gas and air are mixed. When space is limited, as with burners rated below about 3 therms/hour, it is not possible to locate the electrode in a position which ensures satisfactory ignition and it is necessary, therefore, to provide a pilot which lights from the spark and then lights the main flame.In providing a pilot, however, it is generally desirable to avoid the use of a separately governed gas supply, which would interfere with control of the overall air/gas ratio, and it is morn usual to take a sidestream to the pilot from the main gas supply downstream of the gas control valve. Usually, a solenoid valve, is used to divert the gas supply to the pilot during start-up. Subsequently, this valve is opened when the gas supply rate is increased to light the main flame. This simple arrangement does not interfere with the control of air/gas ratio but when used in conjunction with small burners in compact equipment it does not always give satisfactory ignition because there is often an appreciable pressure drop through the gas supply line to the pilot with such installations.Consequently, the reduction in pressure which occurs when the solenoid valve is opened can starve the pilot of gas, making ignition of the main flame unreliable or even impossible.

According to the invention there is provided apparatus for controlling the supply of fuel gas and combustion air to a burner, which apparatus comprises linked valves in supply lines to the burner which admit gas and air in predetermined proportion, a branch line downstream of the valve in the gas supply line through which gas passes to a first part of the burner and a pressure relief valve in the gas supply line downstream of the branch line such that when the pressure at the inlet of the pressure relief valve is below a preset minimum gas passes only along the branch line to the first part of the burner and when the pressure is increased above the preset minimum gas passes to a second part of the burner, the gas supply to the first part being maintained without interruption.

The invention will now be described by way of example with reference to the accompanying drawing in which Figure 1 is a diagramatic representation of a burner control system incorporating the invention and Figure 2 is a graph depicting air/gas ratio against firing rate for a typical burner showing how the limits of ignition are extended by application of the apparatus in accordance with the invention.

Referring to Figure 1, a linked pair of motorised control valves 1 and 2 control the supply of combustion air through line 3 and gas through line 4 to a burner (not shown). Back-loaded governors 8, 9, are shown working in conjunction with valves 1 and 2 to control the overall air-gas ratio but other controls could be used. A branch line 5 supplies gas to the burner pilot, and the gas supply to the main burner passes through a pressure relief valve 6 in the gas line 4. The burner is normally lit at about 20% of its rated output and the relief valve 6 is set to open at a pressure above that at its inlet at this low rate so that all the gas passing through valve 2 flows along the branch line 5 to the pilot, where it is ignited, for example by an electric spark ignitor.As valve 2 is opened to incease the gas supply to the burner, valve 1 opens in step to maintain the required air/-gas ratio, the pressure at the inlet of relief valve 6 increases and this valve opens progressively to allow gas to pass to the main burner, where it is ignited by the pilot flame. There can be no reduction in pressure at the junction of branch 5 with line 4 as the relief valve 6 opens, as there can be when some other kind of valve is used (e.g. a solenoid valve), and thus gas flows along the branch line to the pilot without interruption. Ignition of the main flame occurs reliably and smoothly.

Table 1 shows typical gas supply rates to the pilot and main burner for a burner rated at 1.5 therms/hour when the relief valve is set to open at a pressure just above 5 inches water gauge.

TABLE 1 Total gas rate, Pressure at Pilot gas rate, Main gas rate therms/hour pilot, ins20 therms/hour therms/hour 0.3 4 0.3 Nil 0.4 5 0.4 Nil 1.0 6 0.5 0.5 1.5 9 0.6 0.9 Figure 2 shows the limits of firing rate and air/gas ratio within which a burner rated at 2 therms/hour can be ignited by a pilot with and without application of apparatus in accordance with the invention. The burner can be ignited over a range of firing rates extending from 0.35 to 0.6 therms/hour, represented in Figure 2 by the dashed lines a-d and b-c.In known arrangements, when the pilot and main burner are simultaneously supplied with gas ignition is possible only within the narrow band or air/gas ratios close to the stoichiometric ratio represented by the shaded area, whereas control apparatus in accordance with the invention, as prevously described, gives reliable ignition within the much larger area, a b c d, i.e. over a much broader band of air/gas ratios. This improvement enables small burners to be used in a greater variety of combustion equipment where ignition difficulties would otherwise discourage or prohibit their use.

Although apparatus in accordance with the invention has been described in relation to burner ignition, it may also be used to improve turn-down of a burner from its maximum to low firing rates. The gas supplied to a burner is often distributed to several ports in a nozzle and, with some types of burner, it can be arranged that one group of ports is supplied through the gas branch line 5 and a second group through the gas line 4 and relief valve 6. In this way, said second group of ports would progressively become inoperative as the firing rate of the burner was reduced. Depending on burner design, local conditions (e.g. velocity, mixing, flow pattem etc.) for the ports which remained operative would continue to support a flame beyond the reduced firing rate at which flame instability would occur if all the ports were in use. On increasing the firing rate again, the gas supply to the inoperative ports would be smoothly and progressively restored as the relief value opened.

It is within the scope of the invention that a second branch line and a second relief valve (not shown) could be introduced into the gas line 4 downstream of the first relief valve. Then, with the second relief valve set to open at a higher pressure than the first, it would be possible to make a third group of burner ports operative or inoperative as the firing rate was varied. This arrangement could then be applied to improve both burner ignition and burner turn-down simultaneously, or to a further improvement.of turn-down.

Claims (4)

CLAIMS:

1. Apparatus for controlling the supply of fuel gas and combustion air to a burner, which apparatus comprises linked valves in supply lines to the burner which admit gas and air in predetermined proportion, a branch line downstream of the valve in the gas supply line through which gas passes to a first part of the burner and a pressure relief valve in the gas supply line downstream of the branch line such that when the pressure at the inlet of the pressure relief valve is below a preset minimum gas passes only along the branch line to the first part of the burner and when the pressure is increased above the preset minimum gas passes to a second part of the burner, the gas supply to the first part being maintained without interruption.

2. Apparatus as claimed in Claim 1 wherein a second branch line and a second relief valve are introduced downstream of the first relief valve such that, with the second relief valve set to open at a higher pressure than the first, a third part of the burner can be supplied with gas in like manner to the second part.

3. Apparatus as claimed in Claim 1 or Claim 2, wherein the first part of the burner which is supplied with gas through the branch line is an ignition pilot.

4. Apparatus as claimed in Claim 1 or Claim 2, wherein the gas supply to said parts of the burner is controlled as aforesaid whereby, in use, to improve the turn-down ratio of the burner.