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This article focuses on the atmospheric or natural draft burner. Let’s start by reviewing some of the properties of natural gas.
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Properties Of Gas
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Natural gas is colorless, odorless and non-toxic, but highly combustible. The gas smell that is characteristic is the result of a perfume (Mercaptan, for example) put into the gas so we can smell it.
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Lighter Than Air
Propane Is Heavy
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Natural gas has a specific gravity of about 0.6. Air has a specific gravity of 1.0, so natural gas is lighter than air. This means that gas tends to self-dilute in the atmosphere if it leaks. Since natural gas wants to rise, it mixes with the air and is quickly diluted. Propane, on the other hand, has a specific gravity of 1.5. A propane leak accumulates in low spots and creates a more dangerous situation.
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Mostly Methane
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Natural gas is about 85 to 90% methane (CH 4 ). The rest is made up of ethane, propane, butane, nitrogen and carbon dioxide.
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1,000 BTUs Per Cubic Foot
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One cubic foot of natural gas contains about 1,000 BTUs of energy. By comparison, one cubic foot of propane contains about 2,500 BTUs. This makes sense since the specific gravity of propane is about two and a half times that of natural gas. The amount of energy stored in a cubic foot of propane is also about two and a half times that of gas.
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Ignition At 1200° – Flame Temperature Is 3550°
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The flame temperature of natural gas is about 3,550°F. This is a maximum under ideal circumstances. The ignition temperature of natural gas is about 1,200°F.
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Explosive Limits
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The lower explosive limit is 4% and the upper explosive limit is 14%. This means that a mixture of gas and air that is less than 4% gas will not ignite and a mixture of gas and air that is more than 14% gas will not ignite, at least not until it’s diluted.
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Mixing Gas And Air
Primary Air
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In atmospheric gas burners, the air is mixed with the gas in two different stages. The gas and air are partially mixed before igniting. The venturi effect created by the pressurized gas moving through the gas burner draws air into the unburned gas before it is exposed to the pilot. About 30 to 50 percent of the air needed for combustion is provided this way. This is called the primary air.
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Secondary Air
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The remainder of the air, called the secondary air, is provided by the air around the flame. It makes up the remaining 50 to 70 percent of the air needed for com-plete combustion. Primary and Secondary air are both combustion air. Neither one is the dilution air.
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Shutters Control Air
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The primary air is typically controlled with shutters at the ends of burners. The velocity of the gas is provided by the pressure of the gas in the piping (about 3 1/2 inches, WC). Gas is pushed through the pipe by the gas company and the pressure regulator reduces the pressure down to the desired level. The velocity of the gas is a result of the pressure, and is controlled by adjusting the diameter of the orifice in the burner.
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Mono-port Burner
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There are two basic types of gas burners. One is the mono-port. This is commonly used on conversion burners, where an oil heating system has been con-verted into a gas system. These often use a fan assist, and are actually forced draft rather than atmospheric burners. They are common in high efficiency furnaces, and are popular because they work equally well in any orientation. Upflow, down-flow and horizontal furnaces can use the same mono-port burners. They can be either in-shot burners or up-shot burners.
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Multi-port Burner
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The second type of gas burner is the one found in most conventional gas furnaces. This is a multi-port burner and includes ribbon, drilled and slotted burners. The burners themselves may be made of steel or cast iron.
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Crossover Igniter
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Most furnaces have more than one burner. One pilot flame usually ignites all the burners. A crossover igniter is used to bridge the flame from one burner to the next.
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Delayed Ignition
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One of the common problems with multi-burner furnaces is delayed ignition of the burners remote from the pilot. This is usually a problem with the cross-over igniter being out of alignment or obstructed in some way.
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