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Ignition of Fuel/Air Mixtures If smoke is flammable, why doesn’t it always ignite and burn? Ignition is dependent on having sufficient fuel and oxygen as well as an adequate ignition source. Ignition of a mixture of pre-mixed air and fuel requires an ignition source with sufficient strength. The minimum amount of energy required to initiate combustion is the minimum ignition energy. Factors that affect the minimum ignition energy include: Type of fuel Mixture of fuel and air Temperature Total energy supplied Rate at which energy is supplied (energy per unit time) Area over which energy is delivered
INTRODUCTION Oil & gas burners are designed to work in a similar fashion. They both are designed to mix air & fuel in the desired proportions before ignition.
GAS BURNERS Gas burners can be either atmospheric or high-pressure type depending on the pressure at which gas is admitted. high-pressure types use gas at pressures from ½ to 40 psi & create high rates of heat release. The atmospheric pressure types are supplied at pressures of 2 to 12 in. water gage.
FLAMES Gas flames may have a range of colors & shapes. But they fall in to 2 main categories: The post-aerated /non-aerated /luminousflame or burner design is where gas & air are not mixed prior to combustion. In the pre-aerated flame air is drawn in by natural draught or fan to mix it with gas prior to combustion. This is primary air & makes up only 40-50% air required. Secondary air is needed for complete combustion is obtained form the air surrounding the flame.
FLAME LIFT When lifting flames occur, part of the flame lifts or "dances" on the port. Lifting flames may occur on a few or on all of the ports of a burner. If flames lift from a number of ports, they may create a distinct flame noise. Lifting burner flames result when the flow velocity of the gas-in-air mixture from a port exceeds the flame velocity. The flame cannot stabilize on or just slightly above the burner port, as in normal operation.
FLAME LIFT Lifting flames which create a roaring noise in an appliance can lead to customer complaint. Of a more serious nature, products of incomplete burning may escape the flames if the flame cones break. Unburned gas also may escape, reducing appliance efficiency. The simplest way to stop burner flames from lifting is to reduce primary air.
FLASHBACK When flashback occurs in a burner, the gas-air mixture ignites inside the burner to burn near the orifice. This burning in the mixing tube usually creates a roaring noise. The burning action inside the mixing tube does not get enough air. Combustion is incomplete and produces carbon monoxide or free carbon (soot) which clogs the inside of the burner. Prolonged burning inside the burner can cause damage.
FLASHBACK Flashback on ignition or during burner operation usually can be eliminated by reducing primary air to the burner. The orifice size may be enlarged, or gas pressure increased
FLAMES The yellow color in a luminous flame is basically from hot particles of soot in the flame. In a blue flame, there are no particles of soot to give that incandescent radiation. The difference between a yellow and blue flame depend on the amount of oxygen that is mixed in with the fuel. More oxygen means that the combustion reaction is more efficient and more energy is released. Remember that the more blue the visible light appears, the more energy that light will have.
FLAMES The color of a flame is primarily dependent on the air-fuel ratio. This air-fuel mixture is often referred to as lean or rich. The non-luminous fire exist when there is a complete combustion or complete burning process. It happens when there is more than the enough oxygen in the surroundings or in the place where the process will happen. The temperature of a non-luminous flame is hotter than the temperature of a luminous flame
Orifice The orifice is the very last component before the burner, and has a very precise hole manufactured into it. The orifice and the pressure of the gas passing through it set the size of the burner flame. If one or the other is incorrect, then the flame will not be correct. Usually, if there is a problem, the orifice is dirty. Microscopic debris has accumulated around the hole making it smaller and therefore not the correct size. A dirty orifice can reduce primary air intake & cause yellow luminous flame.
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