HOT TIPS PAGE
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= 1.055 kJ
= 105.5 MJ
= 746.0 W
= 3.600 MJ
= 4.187 kJ
= 0.037 MJ/m3
Natural Gas S.G. = 0.6
H.V. 38.7 MJ/m3/1050 BTU/ft3
L.P.Gas S.G. = 1.55
H.V. 95.5 MJ/m3/2500/BTU/ft3
50.4 MJ/kg/21,500 BTU/lb
RULE OF THUMB FORMULAS
Rule of thumb formulas are invaluable for quick calculations.
Optimum burner input = port loading x port area
|Natural Gas||CV 1050 BTU ft3||SG - 0.65||Port loading 30,000 BTU/Hr/in2|
|Towns Gas||CV 500 BTU ft3||SG - 0.55||Port loading 40,000 BTU/Hr/in2|
|L.P.G. (Propane)||CV 2500 BTU ft3||SG - 1.52||Port loading 20,000 BTU/Hr/in2|
Oven heating, approx. 15 -30 MJ/Hour/m3 (using atmospheric non recirculating) for low temp. ovens to 200 - 400oC.
Inspirators: Low gas pressure type throat area approx. 45% of total burner port area. Higher gas pressures the ratio is closer to 75-95%.
Atmospheric low pressure burners: No rule of thumb heat release figures available, depends on draft and other factors. At 1-4 kPa, approx. 30-50% primary air through inspirator, rest secondary air.
Higher pressure premix burners: Approx. .3 - .9 MJ/mm2 of burner port area. The heat release depends on the co-efficient of discharge (the shape and length of the port) and the ability to utilise high mixture pressures. At 7-70 kPa, approx. 40-70% primary air, 70 kPa and above approx. 65-100%.
For complete descriptions of an Atmospheric burner, Premix burner, Nozzle mixing burner, Inspirator, Aspirator check "Gas Burners and Controls" data.
Flame retention is the ability to "sit" on the tip. Higher mixture pressure burners generally have longer main ports and angled retention ports around the main port to relight the flame.
The port area is the total area of the burner exit port/s. This includes the retention ports and dictates the maximum heat release depending on the mixture pressure, process back pressure and combustion space size.
The venturi throat area regulates the primary air. Secondary air is required to complete combustion. Possible to elevate gas pressure and eliminate secondary air.
The mixture pressure is the resulting pressure of the air and the gas. Typically a low pressure (2 kPa gas pressure) burner will develop only 125 Pa of mixture pressure restricting heat inputs to the combustion space. A premix burner may develop approx. 2 kPa or more allowing high heat inputs.
Premix burners are possibly the most common industrial burner type in use and have several important advantages:
- Higher mixture pressures enable higher combustion chamber and process heat inputs
- A wide variety of flame shapes are possible to suit all applications
- Easy adjustment of the air/gas ratio and flame configuration
- Simplified burner trains
A basic premix burner train will consist of the burner tip (flame holder), air/gas mixing chamber, air and gas control valves and frequently a gas proportionator valve (also known as a zero governor).
THE MIXING UNIT
This is the heart of a successful system. Premix burners require the air and gas to be mixed in precise proportions to enable accurate adjustment of the flame. A well engineered mixer will also ensure there is an auto proportioning effect over the entire range. The air jet and venturi sections are machined according to the total burner port area, mixture pressures, air and gas delivery pressures and likely manifold back pressure. A mixer will only cover a certain range of burner capacities i.e. a small burner requires a small mixer. An incorrectly sized mixer or a simple tee arrangement can not only restrict adjustment but more importantly can cause unsafe conditions to occur. It is important to check for a high negative pressure (suction) on the gas inlet to the mixer with air on maximum.
Premix burner tips can vary from simpler open type cast iron tips to multiport line burners. The common principle with all these burner ends is the requirement for flame retention. This will ensure the flame stays on the tip/s and can be pilot flame strips or holes or even a particular tunnel shape.
A safety light flap check valve is required by the safety authorities, as a minimum, to be fitted into the gas line (unless a gas proportionator is fitted) to stop the ingress of air in the event of a blockage up stream. Full automatic ignition and flame safety is frequently fitted to automate the process and provide quick lockout on flame out or process stop. The safety module controls solenoid valves fitted to the gas train and high temperature ignition and sensing probes mounted to the tip supply the high tension spark and complete the flame sensing rectification circuit. We can also provide flame out alarms if preferred. More technical information is available on any of the above concepts.
MULTIPLE BURNER SYSTEMS
It is common on particular processes to have separate burner tips fitted to a common manifold to convey the mixed air and gas to the burner tip/s. It is imperative these are correctly sized relative to the burner port area and mixture pressure. A badly engineered manifold system will cause flash back (detonation in the pipe), back flow and inadequate mixing. Hoses must be used that have minimal pressure drop and there must not be valving in the mixture line from the mixer to the burner tip.
TROUBLE SHOOTING PREMIX BURNERS
In addition to operational faults detailed in our "Fault Finding" data sheet, there are particular conditions that may be associated with these burners. Inadequate turn down and lightback are the most common.
This will be apparent as an inability to control from a high to a low flame on ratio and is usually caused by lack of adequate negative pressure on the gas inlet to the mixer. Other factors are:
- Deterioration in the air supply possibly caused by a buildup of dirt or diminution of the air supply or blower motor, bearings or belts if fitted. A pressure check should be performed on the air line just prior to the air orifice with the burner or air rate on maximum. The mixture pressure should also be measured and compared with the air pressure. Although other factors can have an effect on this ratio an air pressure of 3.5 kPa should provide an approx. mixture pressure of 1.3 kPa.
- Deterioration of the burner port or the mixer air orifice. This could involve wearing from the constant flow of dirt in the air or a buildup on these ports and will cause the fine ratio mentioned above to not be maintained. The burner must be disassembled and the mixer and burner port accurately measured.
- Deterioration of the gas proportionator if fitted. Again this should be carefully disassembled by an approved person with the valve and valve seat checked and the diaphragms and the spring tension checked. Check for sticking on the seat and loss of spring tension.
- Similar conditions to "Burner lightback" detailed below.
Reducing the air orifice size by a small amount will improve the negative pressure providing the other factors have been checked first.
This occurs when the burning velocity of the mixture is less than the flame speed of the gas and can be caused by the following:
- A misalignment of the air jet to venturi in the mixer. It is important these are concentric, do not have sharp edges and have a smooth surface.
- An unsuitable air jet/venturi relationship. This can be caused by wear or incorrect initial or post machining.
- The gap between the air jet and the venturi is too large. This should only be approx. 2 mm or less.
- The burner port area is too large and/or the burner manifold area is out of ratio with the total burner port area.
- The downstream components provide excessive restriction i.e. too many bends or the mixture pipe is too small.
- Inadequate burner tip design for the mixture pressure developed particularly the flame retention.
The converse where the flame "lifts off" the burner tip can also occur and will be due to a high air velocity or blocked retention holes (inadequate flame retaining ability).GENERAL FAULT FINDING INFORMATION
The basis for trouble free operation can only be ensured by the correct combined effect of the three factors: electricity, gas flow and combustion air. Faulty flue or combustion chamber design also will have an effect on burner performance but basically these three factors should be checked first if troubles arise. It has been proven that many problems have rather simple causes. Before calling the serviceman the following checks should be made:
1. Are the gas cocks of the installation open?
2. Are the fuses in order and the power switched on and available at the burner?
3. Are the controls (room thermostats, boiler thermostats, interlocks etc..) correctly adjusted?
4. Is the gas pressure at the burner sufficient?
5. Is the safety burner relay ready to start and not locked out?
6. Is the air supply to the burner sufficient and the air passage unrestricted?
The following are the most frequent faults in a forced draft type burner installation and probable remedies.
The Burner Does Not Start
|No gas.||Check all gas cocks are open and the regulator is reset if required.|
|No power.||Check fuses, thermostats, and electrical connections.|
|The fan motor fails to start.||Safety unit is locked out, air pressure switch is faulty or incorrectly adjusted, fan motor or safety unit defective.|
The Burner Motor Is Running But There Is No Ignition After The Pre-Purge Time Has Elapsed
|No power on the solenoid terminals.||Check the flame safety relay and replace.|
|The ignition or sensing electrodes are touching, the porcelain is cracked, they are touching earth or incorrectly positioned.||Replace and readjust as necessary, check the spark gap is approx. 3-5 mm and the sense electrode is in a position on the edge of the flame. Disconnecting the sense wire may assist with diagnosis.|
|The ignition and sensing (ionisation) cables are damaged or the connections are corroded.||Check and replace as necessary.|
|There is no secondary voltage on the ignition side of the transformer/safety unit.||Replace transformer/safety unit.|
|The ignition cable and the sensing electrode cables are interchanged.||Change.|
No Flame Establishment In Spite Of A Trouble Free Start.
|Gas solenoid valve defective.||Replace.|
|The gas solenoid valve does not open in spite of it obtaining voltage.||Check and replace if necessary coil or valve.|
|No voltage to the solenoid coil.||Check appropriate contact on the safety unit.|
|No power through the air pressure switch.||Check the adjustment and the correct operation.|
|The start air and gas settings, are incorrect for reliable ignition.||Reduce or increase the gas supply, reduce the air supply.|
The Burner Locks Out After The Safety Time Has Elapsed In Spite Of Flame Establishment.
|No ionisation current from the sensing electrode or the electrode is in the wrong position.||Adjust the electrode, check cables.|
|Flame sensing section of safety relay defective.||Replace.|
|Main voltage lower than 185 volts.||Contact the electricity authorities.|
|The ignition electrodes are disturbing the ionisation current.||Adjust ignition electrode and repole the ignition unit if necessary.|
|Bad earthing.||Clean earth contact, ensure there is a large enough earthing area and good connections.|
|Phase and neutral interchanged.||Check wiring diagram.|
|Gas pressure too low, gas train or fitting line undersized.||Check and rectify.|
Pulsations At Start
|The ignition and sensing electrodes wrongly adjusted.||Readjust.|
|Gas pressure too high.||Check with pressure testing device and reset.|
|The flue is blocked.||Check the chimney/combustion products exit area.|
Pulsations During Operation
|The burner is not correctly adjusted.||Readjust gas and air settings, these should be set with a flue gas analyser..|
|The burner is dirty.||Clean the burner.|
|Defective chimney.||Check and change the dimensions if necessary.|
|Air or gas pressures not stable.||Check line pressures with manometer.|
|The burner flame shape is incorrectly matched to the combustion chamber.||Change burner or flame characteristics.|
The Burner Is Operating Correctly But Locking Out Now And Then
|The ionisation current is too low.||Check. This must be at least 2 uA but should be 8-20 uA of stable current. A micro ammeter is fitted into the sense cable line in series to check.|
|The sensing electrode is in the wrong position.||Readjust.|
|Voltage drop at certain times.||Monitor and contact relevant authorities.|
|Spark over on electrodes.||Check ceramic and replace.|
|The ambient temperature of the safety relay is too high.||Relocate or insulate from the heat.|
|The ignition spark is too weak.||Check the safety unit/transformer.|
|Bad draft conditions.||Check the chimney.|
|The flue gas temperature is too high.||The appliance is over loaded, reduce the quantity of gas.|
|The CO2 content is too low.||Check the appliance does not have air leaks. Restrict the draft.|
The Co Content Is Too High (This Should Be Checked In Conjunction With The Co2 And O2)
|Excess air.||Restrict the air flow.|
|Air shortage.||Open air supply. Check the flue damper.|
|The gas burner tip is dirty/clogged.||Clean.|
|The fresh air intake (if applicable) is too small.||Check and enlarge.|
|The flame is not burning straight because the burner tip is out of position.||Check the tip and adjust.|
Condensation In The Appliance And Chimney
|The flue gas temperature is too low or the quantity of gas is not sufficient.||Increase the flue gas temperature by increasing the gas supply. Insulate the chimney.|
These are general instructions only, the start up refers to a fully automatic burner unit. The appropriate local safety regulations must be followed. Commissioning only by licensed persons. Check the appropriate procedure for more relevant details if the unit has manual start.
1. General Inspection
- Ensure the recommended burner output is suitable for the appliance.
- The standard burner train should be fully gas approved, check that non-standard additions have not been added to the gas train.
- The available gas pressure should be checked and overpressure protection fitted if required. Gas line should be purged of air prior to a start.
- Check the electrical supply. Check the safety interlocks are correctly set and wired according to the relevant code. Check all wiring connections are sound and the spark and safety connections are not damaged.
- Ensure there is adequate ventilation available. The recommended minimum is 160mm2/MJ/HOUR.
- Check the fluing for the appliance is of the correct dimension, exhausts all products of combustion and has an approved terminal. Make sure there is the minimum clearance around the appliance and the flue exit to combustible materials.
- If an open burner tip is fitted, ensure it is located outside the burner port to allow for adequate secondary air and a safe distance from the refractory if fitted with electrodes.
2. Pre-start Check (Dry run)
- Isolate all fuels.
- Close burner isolating valves.
- Check all valves on the gas train for gas tightness with a manometer. Check the setting on all gas train components i.e. air/gas pressure switches, gas regulators, temperature controllers etc..
- Switch on power supply and check the fan operation. Check the air switch contacts change and prove the correct programmer sequence with the manometer.
- Check the burner sequence up to the lockout with no gas present. Check the purge period is according to calculations.
3. Starting the burner
- Open the main gas valve and the pilot (low rate start) valve, close the main gas isolating valve.
- Turn on the power. If the lockout indicator on the safety control is illuminated press and hold until reset. The burner will now commence the purge cycle. After purge the first (start) safety shut off solenoid will open and the ignition transformer will supply a spark for a set period. If the start rate does not ignite within approx. 3 seconds, the burner will lockout. (Refer to "Fault Finding" document for more information)
- Check the low rate start and set to approx. 10% of the main capacity. Open the main gas isolating valve. Check there is a smooth transition to main flame, ensure the main flame capacity is as recommended, adjust by the main gas regulator.
- A flue gas sample should be taken to check the CO2, O2 and CO are within specifications. Generally the CO2 should be approx. 7-9% and the CO less than 500 parts per million. Particular applications such as direct air heating have special requirements, check with the manufacturer or the local gas authority. A CO reading of less than 100 PPM is usually required. It is usually preferable to wait until the appliance is at operating temperature prior to these checks. The main air damper should be set and locked in the correct position.
- Make sure the burner is operating at the recommended capacity and is not over firing. This can be evidenced, in some cases, by excess flame at the flue exit or back flame at the burner port. A combustion space can only take a certain input, carefully check the original calculations if problems persist.
- Adjust the air pressure switch until lockout occurs and reset the adjustment slightly above to prevent nuisance shut downs.
- If temperature control is fitted or excess temperature interlocks, ensure correct operation. The appliance efficiency can be checked by measuring the flue temperature.
- Ensure the customer is given the relevant documents and a servicing schedule is in place. ACS can undertake installation, commissioning and servicing if required.
- Check the appliance data plate details and record any particular plant procedures.
- Instruct operator and hand over to customer.
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