Gas burners are the heat source for most ovens, dryers and process heaters. Their reliable operation is essential to maintaining production rates and product quality, but relatively few people really understand how they work. This column begins a short series on the theory and operation of burners and their gas and air control systems.
First, though, you have to understand how gas and air flows are controlled and how those flows can be matched to create predictable, repeatable combustion ratios. The best starting point, surprisingly, is electricity, because its control seems to be understood by more people.
A gas or air piping system is like an electrical line with current flowing through it. In the electrical circuit below, I represents current flow in amps, R is the resistance of the circuit in ohms, and E1 and E2 are voltages into and out of the resistance.
Sample Electrical Circuit
The mathematical expression tying all this together is Ohm's Law:
In essence, Ohm's Law says that if you want to change the current flow through the resistance, you have to change the voltage drop across that resistance. You can do this by changing the voltage into the resistance (E1), changing the voltage out of the resistance (E2), or changing both E1 and E2 to create a different voltage differential (ΔE).
The ratio of the current flows before and after the change is:
Ratio of Current Flows Before and After the Change
In other words, current flow through a fixed resistance is directly proportional to the voltage drop across that resistance. To double the current flow, you have to double the voltage drop. To triple the current flow, triple the voltage drop, and so on.
You can set up a similar system with air or gas flowing through a pipe (below).
Air or Gas Flowing Through a Pipe
In fluid flow terminology, Q represents the air or gas flow in cubic feet per minute, cubic meters per hour, or whatever. It's the equivalent of the electrical current. R represents a restriction to flow, akin to the resistance in the electrical circuit. It can be an orifice, valve or the internal passages of a burner. Unlike electricity, you don't have a unit of measure such as ohms to describe the value of the restriction, but that's OK -- here you're only interested in the relationship between pressure drop and flow. As long as the characteristics of the restriction don't change, its behavior is neutral, and you can ignore it. P1 and P2 are the air or gas pressures above and below the resistance, in ounces per square inch, inches water column or whatever. Pressure equates to voltage.
If you want to change flow through a given restriction, you have to adjust either P1, P2 or both. However, the relationship between Q, R and P is a little bit different. The ratio of the flow rates before and after the pressure drop change looks like this:
Ratio of Flow Rates Before and After Pressure Drop Change
This is known as the Square Root Law of flow and pressure drop, and it says the fluid flow is proportional to the square root of the pressure drop across a fixed resistance. If you're more interested in the effect of a change in flow on the pressure drop needed to make that change, the equation can be rearranged to look like this:
In other words, pressure drop is proportional to the square of the flow through a fixed resistance.
What all this means is that to double the flow rate through a fixed resistance, you have to increase the pressure drop by four times (the square of two). To triple the flow, you need nine times the pressure drop (three squared), and so on. Every added increment of flow requires a larger and larger increment of pressure drop. In pre-calculator days, all this meant resorting to a slide rule or the now-lost art of calculating square roots.
Enough for one session. Next time [see links at bottom of page], I'll look at how pressure drops can be manipulated to control flow rates of air and gas.