I'll wrap up this series of columns with a look at some more ratio control systems. These are less used on ovens, dryers and low temperature processes, but you may run into them where special conditions require their unique features.

A cross-connected regulator with bleeder is used where gas supply pressures are too low to allow the system to function properly.

Cross-Connected Regulator with Bleeder

This variation on the cross-connected regulator is used where gas supply pressures are too low to allow the system to function properly (figure 1). You'll recall the cross-connected gas regulator develops an outlet pressure equal to the air loading signal it receives. If the outlet pressure required is higher than the gas pressure entering, the system won't work - pressure, like water, doesn't flow uphill.

Putting some numbers on it helps it make sense. Say the combustion air loading pressure is 10" w.c., but gas pressure into the regulator is only 7" w.c. Pressure drop through the regulator valve cuts this to 5" w.c. at the outlet. No problem, you say -- I'll compensate by opening the limiting orifice valve farther than normal. This works fine at high fire, but as the air control valve begins to drive to low, the gas pressure and flow won't change. Remember, the regulator is trying to produce an outlet pressure equal to the air loading pressure, so until it decreases to 5" w.c., the regulator stays wide open, and the fuel-air ratio goes rich.

Now you add a bleeder -- a T fitting in the loading line with one side open to atmosphere. The open connection vents, or bleeds, some of the loading signal, decreasing the signal reaching the regulator. Suppose you set the bleeder to knock the loading pressure down to 50% of its original value, so at a high fire air pressure of 10" w.c., only 5" reaches the regulator. Now, the regulator's outlet pressure matches the loading signal, and as the loading pressure decreases, the regulator responds immediately. The correct ratio is held all the way down to low fire.

Bleeders also are sometimes used where a recuperator or other resistance in the combustion air line requires combustion air supply pressures much higher than the gas pressure.

Apart from these situations, bleeders are best avoided. Their small orifices often get blocked with airborne dust and dirt, and this causes the burner gas-air ratio to change. Filters in the air loading line help, but they, too, plug up. The bottom line is that a bleeder system requires more maintenance.

A throttled bleed system is another type of excess air control system, where the combustion air flow stays constant at all firing rates while the fuel is throttled up and down.

Throttled Bleed System

This is another excess air control system where the combustion air flow stays constant at all firing rates while the fuel is throttled up and down (figure 2). It operates much like the bleeder system, except that the amount of air escaping through the bleed opening is regulated by a reverse-acting motorized valve driven by the temperature controller. When the temperature control system calls for a reduction in firing rate, the bleed valve begins to open, decreasing the loading to the regulator and causing it to partially close. All this time, the combustion air continues flowing at the high fire rate, so the burner ratio shifts to excess air.

Why go to all this cost and trouble when a simple valve in the gas line will do? If the control zone has only one burner, there's no need, but if several burners are controlled by one regulator, this system makes it easier to balance gas flows to the burners. Otherwise, manually increasing or decreasing the gas flow to one burner causes the flow to all the other burners to change in the opposite direction.

There are many types of mass flow systems, but all use flow-measuring devices such as metering orifices in the air and gas lines.

Mass Flow Ratio System

There are many types of mass flow systems, but all use flow-measuring devices such as metering orifices in the air and gas lines (figure 3). Signals proportional to pressure drop or flow are compared in the controller, which can be pneumatic, hydraulic or, most often these days, electronic. The system is on ratio when the signals match. If they differ, the controller compensates by sending out a correcting signal to the control valve on one of the lines.

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