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The gas burner and flame
The gas burner must inject the fuel gas and mix air with the fuel to prepare it to be burned. there a number of methods to perform this job.
Above is a cast iron burner that was used for many years in gas furnaces. It is called a ribbon type burner. The gas flame is spread across a large area so it has more contact with the metal of the heat exchanger. The gas is fed into the left side thru a venturi. As the gas moves into the burner, it draws with it air that is mixed with the gas. This is called primary air. At this point, the mixture of air and gas does not have enough enough air for the gas to burn. As the gas comes out of the slots on top of the burner, it mixes with surrounding air. This air is called secondary air. This completes the burn. The video below shows several different types of burners and how they work.
The gas flame was used for many years as a light source. This flame had no primary air. As the gas came out of the orifice, it burned with a bright yellow flame. In most cases, this flame did not produce carbon monoxide as there was enough air to complete the burn. The flame appeared to be somewhat lazy and long. This flame was not the best to transfer heat.
When primary air is added to the gas, the flame shortens and burns hotter. This is a better flame for transferring heat thru a heat exchanger. For a look at what the flame looks like with and without primary air, check the video below.
When primary air is added to the gas, the flame shortens and burns hotter. This is a better flame for transferring heat thru a heat exchanger. For a look at what the flame looks like with and without primary air, check the video below.
Primary air adjustment
Virtually all of the ribbon type burners have some sort of primary air adjustment. The flame from a burner that has the primary air closed off, will be yellow in color. For a furnace, this is not an efficient flame. It appears lazy and tall. The burn may not be complete. If the burn is not complete, not all the heat will be released and an incomplete burn will produce carbon monoxide which is a poisonous gas. If there is too much primary air, the flame does burn efficiently, but however, the excessive air will cool the flame. The cooler the flame, the lower the temperature of the flame which reduces the ability of the heat to transfer to the air in the structure. When the primary air is at the right amount, there is a balance between complete burning of the fuel and high temperature of the flame. Below you can see how the adjustment is made.
Most of the burners above are atmospheric. That means they draw primary air into the burner by the pressure of the gas moving into the burner and by the heat of the flame rising and drawing secondary air around the burner to complete the burn. These burners and and the heat exchangers they burn in are designed to move the heated gases thru the heat exchanger slow enough to allow time to transfer the heat to the heat exchanger and thus to the circulating air to heat the structure. However, they also must keep enough heat in the gasses to allow the gasses to rise and exit the structure thru the vent pipe.
So, how do these burners accomplish this?
As the gas burns, the gasses expand. this causes a slight positive pressure in the heat exchanger. However, because the gasses are hot, the are lighter than air so they move up. The gasses are losing their heat to the heat exchanger as they move up and their volume is decreasing. At about the center of the heat exchanger, the pressure drops to zero compared to the atmospheric pressure. Because the gasses are still hotter than the surrounding air they continue to rise. The gasses are now under negative pressure. At the outlet of the heat exchanger, there is a unique device made especially for this type of furnace.
Below we will be discussing this device.
So, how do these burners accomplish this?
As the gas burns, the gasses expand. this causes a slight positive pressure in the heat exchanger. However, because the gasses are hot, the are lighter than air so they move up. The gasses are losing their heat to the heat exchanger as they move up and their volume is decreasing. At about the center of the heat exchanger, the pressure drops to zero compared to the atmospheric pressure. Because the gasses are still hotter than the surrounding air they continue to rise. The gasses are now under negative pressure. At the outlet of the heat exchanger, there is a unique device made especially for this type of furnace.
Below we will be discussing this device.
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The draft hood or draft diverter
Above are 2 types of draft hood or diverter. On the left is a diverter commonly used on gas conversion burners. The vent gasses from the the appliance enter at the bottom. The hood portion of the device is open to ambient air. As the gasses rise, ambient air is drawn in. The device on the right is a gas designed furnace equipped with a draft hood.
The draft hood provides a break between the heat exchanger and the vent pipe. The reason for this device is to eliminate varying draft pressures from affecting the gas flame. Remember, the pressures are very low. On the level of .02 to .04 in wc. If wind conditions either increase or decrease, and the draft hood was not there, either the flame would be lifted off of the burner (increased draft) or roll out of the front of the furnace (lowered draft). With the draft hood, the draft will spill out of the opening or pull more air into it. The flame will not be affected. Even if the vent is blocked, the flame will be normal. The exhaust gasses will spill out of the hood.
The second function of this hood is to dilute the exhaust gasses to reduce their temperature in the venting.
The video below describes the operation of the draft hood.
The draft hood provides a break between the heat exchanger and the vent pipe. The reason for this device is to eliminate varying draft pressures from affecting the gas flame. Remember, the pressures are very low. On the level of .02 to .04 in wc. If wind conditions either increase or decrease, and the draft hood was not there, either the flame would be lifted off of the burner (increased draft) or roll out of the front of the furnace (lowered draft). With the draft hood, the draft will spill out of the opening or pull more air into it. The flame will not be affected. Even if the vent is blocked, the flame will be normal. The exhaust gasses will spill out of the hood.
The second function of this hood is to dilute the exhaust gasses to reduce their temperature in the venting.
The video below describes the operation of the draft hood.
The draft hood equipped furnace was the standard for many years in the industry. It was simple, dependable and cheap to build. So why replace it?
In the late 1980s, furnace manufacturers began increasing the thermal efficiency of their furnaces.
One of the issues was the draft hood. The draft hood decreased efficiency because it drew heated air from the structure. Another was the variable and poorly controlled combustion air.
This led to an entirely different design. Rather than relying on the heat of the exhaust gasses to remove them from the structure, they would now be mechanically removed by an electric blower. At this point, we will only deal with the burner design that works best with this new design.
In the late 1980s, furnace manufacturers began increasing the thermal efficiency of their furnaces.
One of the issues was the draft hood. The draft hood decreased efficiency because it drew heated air from the structure. Another was the variable and poorly controlled combustion air.
This led to an entirely different design. Rather than relying on the heat of the exhaust gasses to remove them from the structure, they would now be mechanically removed by an electric blower. At this point, we will only deal with the burner design that works best with this new design.
The inshot burner
As the manufacturers found the need to make more efficient furnaces, burners were designed to have better control of both primary air and secondary air. This resulted in the inshot burner. On the first higher efficiency furnaces, the burners continued to be the ribbon type. This type of burner was used both in atmospheric and fan induced furnaces. After a few years, all the manufacturers changed to the inshot type. The inshot type burner was designed to be used only with a fan induced furnace. Below is a video specifically about the inshot burner.