BURNERS AND CONTROLS

Follow the underlined links for more detailed information and descriptions of our range of burners and controls.

ATMOSPHERIC (VENTURI) BURNERS

These are burners that operate on gas only. The velocity of the gas stream flowing through an orifice entrains atmospheric air for combustion from a venturi throat. The resultant mixture burns at a specially designed tip, of which there are a wide variety, known as a flame retention head. For hot glass glory-hole, tank and pot furnaces, atmospheric burners generally use L.P. Gas at high pressure as the pressure of reticulated natural gas is usually too low to inspirate sufficient air to generate a hot, short flame with any forward velocity. Atmospheric natural gas burners are used successfully on ceramic kilns.

Kiln Burner There is a definite ratio between the burner port area and the venturi throat. Typically, depending on the kiln or furnace back pressure, gas pressure and burner head design, the venturi throat area should be approx. 40-50% of the total burner port area. Mismatching may result in a decrease in mixture velocity producing inadequate burning or at worst "flash back". Low rates can be achieved by using a preheat pilot.

This method of firing is the cheapest in terms of equipment costs but is also the least economical. Attention should be paid to obtaining as neutral (correct mixture of atmospheric air and gas) a flame as possible as either oxidising (more air then gas) or reducing (less air) flames or atmospheres are wasteful. Special process or glaze firings are of course excluded. Although some secondary air (air entrained around the burner tip) is necessary for complete combustion, try to keep it to a minimum by correctly sized burner ports and flue outlets. It is possible to check the furnace conditions by restricting the flue exit and checking for slight reduction. This will indicate the settings are close to perfect.

It is difficult to achieve sufficient temperature in higher temperature furnaces with these burners. The greatest limitation, due to the low mixture pressure produced, is the volume of combustion products that can be introduced to the furnace combustion area. Simply increasing the combustion space size will not achieve results as losses will increase proportionally. Obviously there is a fine balance. In some cases better results have been attained by using several smaller burners rather than a single, large burner. Smaller burners with smaller gas orifices and higher gas pressure would develop higher mixture pressures allowing more combustion products into the space in a shorter time. A better alternative is to increase the pressure the combustion products are forced into the space by using a forced air supply. Please contact for more advice on this option.

Flame safety systems for these burners are usually the thermoelectric type that can be used with or without a separate pilot burner. The pilot burner can serve as a low-fire setting. Mount the pilot and/or safety probe well away from the furnace back heat. It is always better and safer to purchase the burners assembled and pre-tested with the appropriate controls.

Automatic ignition and quick lockout safety systems are readily available as an option for these burners.

Sealed Burner PRE-MIX OPEN AND SEALED BURNERS

These are burners using a machined mixing set and forced air from a blower or compressor. Open burners use a suitable cast iron or steel flame retention tip and the sealed type utilise a R.I. castable tunnel or MP multiport tip (illustrated) mounted into the furnace wall. Natural gas or L. P. Gas may be used at low pressure as the forced air induces the gas and produces a blast-type flame. These burners are more efficient than atmospheric burners as greater control is available over the air and gas mix, a hotter flame is produced and the sealed burner requires no wasteful secondary air. The open burner will need secondary air for cooling purposes to prolong the tip’s life and to complete combustion. Some operators have traded tip life for lower noise and lower capacity by sealing the tip in the furnace port with fibre.

The higher mixture pressures developed by this burner style enable greater combustion volumes into the available furnace combustion space. Some careful preheating of the combustion air may be possible but is not recommended, as there is a risk of flashback or damage to the controls through heat conduction.

Premix burners generally have a shorter flame length than comparable nozzle mix burners, the air/gas ratio is easier to control and the burner overall generally easier to set up. Special flow regulators can be fitted to simplify adjustment enabling alteration of air flow only to raise or lower the temperature. This lends itself to simple, accurate temperature control. Again, care must be taken to correctly size the mixer as a definite ratio exists between the size of the mixer chambers and the burner orifice for proper operation. It is possible to construct a simple mixer from pipe pieces but extra care should be taken to ensure it is not possible for the air to flow into the gas line if the burner or feed pipe is blocked. As a minimum, a light flap safety check valve should be fitted to the gas line. If accurate mixing and turn down are required, use the correct mixer.

State gas regulations concerning forced draft (premix, nozzle mix) burners tend to differ, with some States demanding full sequence electronic flame failure while others may allow glory hole burners without safety if the burner is constantly supervised. All enclosed kilns, pot or tank furnaces should have quick lock out safety as these burners can produce large amounts of unignited mixture in a short period of time.

NOZZLE-MIX BURNERSNozzle Mix Burner

These types of burners accomplish the mixing of the air and gas after they leave the burner port. Up to the burner head the air and gas are kept separate, lower gas and air pressures (unless high velocity is required) may be used and there is no chance of flash back. They generally have greater turn down than other burner types by controlling the gas only and can use preheated air. As the preheated combustion air is kept separate there is no chance of over-heating the gas controls.

Basic cast nozzle-mix burners can handle low preheat temperatures providing some fuel savings but deterioration may result if these burners are exposed to high temperatures for a period of time. Nozzle mix burners that will provide high preheat temperatures and maximum fuel savings are constructed from stainless steel internals or in some special cases ceramic materials. Various types of flame shapes and capacities can be designed to suit the customers’ requirement including flat flame burners. This type of burner has a specially designed burner tip and refractory quarl to produce a spinning flat flame that spreads at 90 degrees to the mixture outlet. They have been used in industry where little forward flame travel is desirable and efficient radiant heat is best.

Air and gas controls and safety equipment are similar to premix burners.

RECUPERATION

Recuperation is the process of preheating the combustion air by utilising the waste flue products. Although recuperation has been used in industry in various forms for many years, its use in small production situations has only recently become viable due to high fuel costs. Fuel savings of up to 40% can be achieved through properly designed and implemented systems. A simple counter flow design is illustrated.

 A simple and effective recuperator uses a stainless steel tube on the flue outlet with a larger tube sited around it. The area between the inner and outer tubes must be adequate to allow for free circulation of the cold combustion air but sufficiently restricted to enable the heat conducted through the inner tube to heat the air to the desired preheat temperature by the time the air exits the recuperator and flows to the burner head. The passage of the hot combustion products flowing through the inner tube must similarly be restricted to allow sufficient heat penetration. Baffles may be added to the inner tube or the single inner tube exchanged for multiple tubes to increase the available surface area. It is important not to exceed the working temperature of the steel and a refractory base or longer section may need to be used to take the initial heat. Experience has shown that over a period of time a coating of products produced by the glass making process tend to accumulate on the inner tube surface. Provision must be made for periodic cleaning of the inside tube and it is a good idea to have a small reservoir underneath the flue outlet to gather these products rather than allow them to block the furnace outlet. Refractory recuperators can be made to increase the preheated air temperature to the burner and prolong life.

Air piping from the recuperator to the burner head should have a large cross sectional area to cope with the expansion of the cold air as it is heated. This can be up to 40% at typical temperatures.

FIRING LOW TEMPERATURE OVENS

There are many functional ovens in use including some with well-designed features such as adjustable combustion spaces for varied throughput’s and automatic doors for hands free loading. An efficient oven will have many capabilities including even heat distribution, the ability to maintain a set temperature and fire down over a period of time at an accurate rate if required.

Gas ovens work best using a down draft design (the flue outlet near the base of the oven) and burners that have a short, clean flame with good turndown characteristics (high flame to low flame). The burners usually fire through the base, on either side of the loading area, however higher gas pressure burners can fire horizontally along a base channel. Better ovens have been constructed using high pressure burners firing around the top of the space, creating a circular swirl, for even temperature gradients.  

Insulation materials are a matter for personal preference with many people maintaining that R.I. brick ovens can virtually cool down (with all openings closed) at the required rate without any added heat input. Against this it must be remembered that it takes extra energy to heat a brick oven than a light-weight fibre type.

Low gas pressure burners suitable for these ovens are atmospheric and can be pipe-type burners with either a row of drilled holes, slots or newer designs that incorporate a mixer with many fine slots in the burner casing. The newer types are generally cheaper, more efficient and have a better turndown. Safety controls are usually thermoelectric.

As temperature control is such an important consideration, programmable units are available to accurately maintain and control the temperature gradient. Accurate electronic digital units are available up to 8 or 12 stages and control the burners using solenoid valves according to demand. Electric kilns are easily controlled with programmers, the contactor coil or relay substituted for the solenoid valve.

HIGHER TEMPERATURE FURNACES

These furnaces fire to higher temperatures (more than 700oC) but can employ similar burners. Downdraft designs are effective for general work but it is also possible to fire high pressure smaller burners across the product. This method relies not only on convection heat input but also, to a degree, on radiation from the flame. It is important if using this method that sufficient draw is available from the fluing system particularly in the early stages of a firing. It may be necessary to preheat the flue but in any case pressure drops across the flue pipe and exit port must be minimised. Sealed burner tips are better.

Larger, more industrial kilns should use higher pressure forced draft burners firing across, under or through the load. These use an air fan/blower with the appropriate mixing mechanism to deliver sufficient velocity for even heat distribution.

Automatic temperature programmers are important and can be effectively connected with any gas burner system. Multiple tips commonly utilise a ladder type pilot arrangement on either side, modulating the main burners to relight from the pilot for accurate temperature control. Flame safety is simply fitted to the ladder pilot with manual or automatic spark ignition an option.

SAFETY CONTROLS

Safety controls for gas appliances vary from over-temperature controls to sophisticated automatic start-up and flame failure controllers. Flame safety controls are recommended for all types of burners where there is the risk of a build-up of unignited gas should the flame be extinguished. Flame rectification or UV units are required on all forced air burners and gas will not be connected unless they are fitted. The regulations cover natural gas and L.P. Gas. In some instances the authorities may also insist on over temperature protection or explosion relief. Explosion relief is simply a panel fitted to the furnace of the correct dimensions of a material with less resistance to an explosion than the furnace itself. Non-return check valves may also be required in the gas main to prevent air flowing back to the meter. A regulator is always required on either natural gas or L.P. gas installations to monitor the gas flow and provide the correct pressure outlet. All valves and fittings must be approved and pipe fittings particularly must be suitable for gas. Safety valves should be listed in the gas association bulletin and the burners and controls installed by a licensed person. 

We can advise on any of the above safety considerations and recommend suitable components.

THERMOELECTRIC SAFETY

This is the simplest form of flame safety and is permitted on atmospheric burners with a capacity under 500 MJ/hour. These operate on an electromagnetic principle and require no power. A small current is generated when the tip of a thermocouple probe is heated by the flame. This current excites an electro-magnet located in a safety valve and attracts a plate allowing gas to flow. Shut off time, should the probe cool, can be up to 20 seconds.

ELECTRONIC QUICK-LOCKOUT

These units require power and are usually fitted to forced air burners or safer atmospheric burners. They shut down on a flame failure in approx. one second by closing a solenoid valve fitted into the gas line. Two main types are available; the flame rectification type and the ultra-violet type. Flame rectification relies on the ability of ionised gases in the flame to rectify on AC current from the control unit. A flame sensing rod made from special high temperature material is used and must be situated near the edge of the main flame. A micro-amp meter may be used in series with the rod to check the best position and minimise nuisance shutdowns. The wire must be sturdy enough to resist drooping or deterioration at high temperatures and the porcelain insulators must be kept clean or replaced if cracks develop. For reliable operation the earthing point on the burner must have at least four times the area of the rod in the flame.

UV monitors are sensitive to the ultra violet radiation produced by flames. They also sense the arc of a spark so must be sited away from any automatic spark igniters fitted to the burner. Their main advantage in high temperature situations compared to flame rods is that they can be mounted away from the heat zone. They are protected and see through a protective cover such as ultra violet transmitting fused quartz glass. In some situations cooling air should be blown across the UV cell face to remove dust and protect from excessive back-heat.

NOTES ON FIRING HIGH TEMPERATURE FURNACES

Nozzle mixing and premix furnace burners can be oversized to reduce the ambient noise level providing control is not compromised. Ideally furnaces should be commissioned using combustion analysers, however in the absence of these tools, a slightly reducing gas/air mix with the air rate kept at a minimum will achieve the temperature required. The open type premix burners can have the primary air reduced further as a percentage is entrained as secondary air. The ideal open flame has slight yellow tips; sealed burners should be blue without hard, defined cones. Furnace pressures should be slightly positive to ensure the flue is not drawing excessively and there is no possibility of wasteful ingress of secondary air. This can be tested in a basic fashion by using a hollow tube fitted through a gap in the door. Seal the rest of the door opening completely with ceramic fibre and fit a balloon on the other end of the tube. A slight inflation of the balloon indicates slight positive pressure.

Furnace pressure is controlled by the flue and flue exit size assuming the gas and air flows are at the most efficient settings. If burners have been undersized, more capacity is possible by increasing the flue height and the draw to allow greater furnace gas inputs. Furnaces take longer to reach operating temperatures during the initial heat up phase. Allow this time to slowly heat castables or furnace parts that may not have been fired past critical temperatures previously. Check the heat up rate recommended by the manufacturer to be safe and use a pilot or small torch as a low rate if necessary.

Refer to specific instructions on setting up and adjusting burners. Nozzle mixing burners usually require special proportionators in the gas line for accurate high to low control of the flame although some furnaces require models that are able to modulate the gas only for better turn down and even temperatures. Premix burners have a gas adjusting valve under the aluminium cap located on the mixing chamber. Loosen the locknut and turn the brass screw anti-clockwise for more gas. If a zero type regulator is used on the gas line for single (air) valve control of the burner output, the adjustment of the gas valve will alter the air/gas ratio only. i.e. if more gas is supplied there will be a gas rich flame over the entire range. If manual control is preferred, this screw may be set for maximum gas only and the manual air and gas valves adjusted to alter the flow.

It is possible to reduce the noise on an open type premix burner by sealing the burner in the open port (cutting the secondary air) with ceramic fibre. The trade off will be a shortened life for the cast iron tip as the cooling secondary air is not available. The tip/s should be cleaned frequently with particular attention given to the small retention ports around the main port. They should be replaced if there is excessive corrosion or the outer casting has burnt away. Of course, a better option is to use the MP ceramic tip.

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