If the voltages entering and leaving the resistance are constant, the voltage drop across the resistance also is constant. How can you regulate the flow of current? You can't -- unless you install a variable resistance in the line.

In the last column[links at bottom of page], I looked at the relationship between the flow of air and gases and their pressure drop through a system resistance. That relationship is governed by the Square Root Law, which says the flow through a resistance (valve, orifice meter or burner internals, for example) is proportional to the square root of the pressure drop across that resistance. Conversely, the pressure drop is proportional to the square of the flow.

In most practical electrical circuits, the supply voltage is constant -- it's coming from a battery, AC outlet or some other source designed to deliver a reliably steady voltage. In the simple electrical circuit I'm using as an example, the voltage downstream of the resistance also is constant -- zero -- because the system goes to ground. Now, if the voltages entering and leaving the resistance are constant, the voltage drop across the resistance also is constant. How can you regulate the flow of current?

Figure 1. If you install a variable resistance in the line, you can regulate the flow of current.
You can't -- unless you install a variable resistance in the line. By changing the value of the variable resistance, you can raise or lower the voltage entering the fixed resistance. When you do that, the voltage drop across the fixed resistance changes, and along with it, the current flow. Now you have a rudimentary current flow controller. It takes two components -- a variable resistance to regulate the voltage into the fixed resistance, and a fixed resistance to take the voltage drop that controls the current flow.

Flow control systems operate on the same principle. In most of these systems, the pressure source is constant -- a pressure regulator in a gas line, for example, or a combustion air blower designed to deliver a constant outlet pressure regardless of the flow rate. These systems ultimately discharge their flows to the atmosphere, so the final pressure, like the electrical ground, is zero. If you want to vary the flow across a burner, valve or orifice meter, you have to vary the pressure drop available across it. A variable resistance -- a valve -- does the trick.

Figure 2. Valves, gas/air ratio regulators and other devices control pressure, not flow.
Most people commonly think of valves and gas/air ratio regulators as flow controllers. They're not -- they're actually pressure controllers. Together with a fixed resistance -- a burner, limiting orifice valve or orifice meter -- they comprise a flow control system, and both are essential if you're to regulate air and gas flows through burner systems. Look at any combustion control system, and somewhere, you'll find a fixed orifice and a variable orifice in the air and gas systems. They can't work any other way.

I'll wrap up with a simple example using the air system of a burner. The burner functions as the fixed air orifice (most of them do, by the way). I'll say this one has a flow capacity of 10,000 scfh at a pressure drop of 12" w.c. A blower supplies combustion air at a discharge pressure of 15" w.c. The excess pressure is used to overcome the pressure resistance of the air piping and to take a drop across the control valve for good controllability.

Set the control valve's high fire position to take that small pressure drop, leaving you with 12" w.c. at the burner inlet. The burner discharges to atmospheric (zero) pressure, so the high fire drop across the burner is (12 - 0) = 12" w.c.

The burner airflow has to be turned down to 2,500 scfh at low fire. The flow ratio, 2,500/10,000 = 1/4, so the ratio of pressure drops across the burner is 1/4 squared, or 1/16. Divide 12" w.c. by 16 and you get the low fire pressure drop needed, 0.75" w.c. So the valve has to be set to deliver that pressure to the burner at low fire.

Now you know how the air and gas flows are controlled. Next time, you'll see how the two flows are held in the correct ratio to each other.