Sample Pages: Understanding Burners
Chapter 2.
Gas Burner Functions
Mixing Air With Fuel
Regardless of whatever else it may accomplish, the process of mixing combustion air with the fuel must be achieved in the burner. The ability of the burner to achieve air-fuel mixing is the measure of its performance. A good burner mixes well and liberates a maximum of heat from the fuel; a poor burner does not accomplish this task well at all. It is too bad the burner manufacturers do not have a rating scale so that we could know what we are getting.
Air Mixing Process
The process of mixing fuel and air is really quite tricky and requires some finesse. We have included some of the information from Gas Kiln Firing for these fundamental descriptions. The chart of burner types is still the best basic reference. At the top of the chart is found the simplest burner type, which is also the cheapest and least efficient. As we proceed down the chart, the burners become more complex and do their job better. Overall, these characteristics are inherent. For example, when the gas is injected before the blower in the type next to the bottom of the chart, a safety device must insure that gas does not flow if the blower is not working. In addition, the gas-air mixture is explosive and must be conveyed to the burner without leaks.
Safety Considerations For Gas Burners
What happens when the air supply is interrupted and the flame goes out? Air must be present or gas cannot burn. The gas-air mixture is explosive. An explosion is really a simultaneous burn. The magnitude of an explosion is related to how much gas has accumulated beforehand.
There are protective devices for gas shutoff in case of power loss, or flameout, or loss of air flow. Experience shows that many kiln users bypass these devices the first time they shut off the gas. What should happen is that the protective system should be worked over until it functions properly.
In addition to a protective system, continuously monitor the kiln during a firing. Arrange your work area so that you can see and monitor the kiln?s performance the entire time it is firing. This also improves the economy of firing because the gas pressure is increased in a timely manner when it is needed.
Summary
A burner causes the fuel to burn where it may do the most good. It also requires something or someone to monitor its performance.
Gas Flow Through An Orifice
Anyone would think that the most simple and foolproof possibilities would exist with gas passing through a simple hole. Oh, what a mistake! That little hole is fraught with misconceptions and ignorance about what really happens. It is probably the greatest cause of energy waste and firing problems in an entire burning system.
First, an orifice is a metering port through which a known quantity of gas will flow in a given time. The orifice is sized for the maximum pressure at which it will be used, and a unique set of relationships exist between its shape, the thickness of the metal through which the gas flows, and a given gas viscosity. Change any of these factors and you have no idea how much gas goes through that hole. Some of the possibilities are shown in the illustration.
To confuse the issue even more, gas flow does not increase linearly with a change of pressure. It means that heat will not increase proportionately with pressure. That is also shown on the chart.
The ratio of the size of the orifice to the thickness of the metal it passes through can change the gas volume as much as 90%. This means that a given sized drill hole in the side of a pipe, or pipe cap, (home brew burner making.) is a fable. Whoever drilled that hole has no idea how much gas will go through it.
Orifice Gas Flow Relationships. From Gas Kiln Firing.
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