Now that you understand the ways combustion air-fuel ratios can be controlled, I'll describe the three most common systems used to achieve that control: linked valves, cross-connected regulators, and fixed air systems.

In my last few columns, I looked at the different ways combustion air-fuel ratios can be controlled. Basically, there are three: On-Ratio Control, where the burner air-to-gas ratio is held close to correct, or stoichiometric, ratio at all firing rates from high fire to low fire; Excess Air Control, where the burner operates at correct ratio at high fire but shifts to increasing amounts of excess air as it's throttled to low fire. This is usually done by fixing the combustion air flow and controlling only the fuel flow; and Variable Ratio Control, where both the air and the fuel are varied over the firing range of the burner, but the fuel flow is decreased at a faster rate as the system drives to low fire. This changes the ratio from nearly correct at high fire to excess air at low, but the amount of excess air is not as much as with the fixed air excess air system.

But how is it done? Most ratio control systems are one of six basic types. Here are the three most popular ones.

In a linked valve system, also called a position-proportioning system, the air and fuel valves are connected to each other so that when one moves, the other moves with it.

Linked Valve System

One of the oldest and most popular control schemes, this is also called position-proportioning (figure 1). The air and fuel valves are connected to each other so that when one moves, the other moves with it. If the goal is to control the burner on-ratio, the valves and their linkage are set to produce equal or proportional air and fuel pressures upstream of the fixed air and gas orifices at all firing rates. The control system also can be set up to make the fuel valve travel over a greater range than the air valve, giving you variable ratio control.

With a cross-connected regulator system, the fuel flow is controlled by a regulator called a ratio or proportioning regulator. If the air valve moves one way or another, the loading signal changes, and the regulator internals adjust to maintain the downstream gas pressure at the same value.

Cross-Connected Regulator System

The original intent of this system was to eliminate the setting and adjustment of valve-to-valve linkage, which is sometimes tedious and time-consuming (figure 2). The fuel flow is controlled by a regulator called, among other things, a ratio regulator or proportioning regulator. The vent on the regulator's diaphragm housing is connected via a tube or pipe to the combustion air line downstream of the control valve. The air pressure transmitted by this tube pushes against the regulator's main diaphragm, providing the force to open it. In addition, it's a positioning signal. A pressure feedback tube or opening in the regulator's downstream connection transmits the controlled gas pressure to the opposite side of the main diaphragm. When these pressures are equal, the regulator is stationary. If the air valve moves one way or the other, the loading signal changes, and the regulator internals adjust to maintain the downstream gas pressure at the same value. This is the origin of the system's other name -- pressure-proportioning.

Downstream of the regulator, a restriction is placed in the gas line. It acts as a dam, building a back pressure the regulator can sense. This restriction is sized so that the gas flow across it will match the airflow when the controlled air and gas pressures are equal. The restriction takes many forms -- a drilled passage in the burner's gas inlet or nozzle, a fixed orifice plate inserted in the line, or an adjustable valve that can be field set for the proper opening. These valves are known as limiting orifice valves, adjusting orifices or gas adjusters.

Cross-connected systems usually are thought of as strictly on-ratio systems -- and many are -- but they can operate as variable ratio systems. Backing off the regulator spring reduces some of the support for the regulator's internals, and their weight causes the valve to close slightly. This, in effect, subtracts from the air loading pressure and causes the air-gas ratio to go lean at low fire.

With a fixed air (also called excess air) system, the only control valve is on the fuel line.

Fixed Air, Excess Air System

  This is the simplest of all the ratio control systems and the easiest to set up (figure 3). The only control valve is on the fuel line. Combustion airflow is locked in at a fixed rate with an air shutter on the combustion air fan inlet or a lockable manual valve in the air line.

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