North American 4452 Gas Burner Control System & Installation Instructions
Sheet 4452-2
1122 Butterfly Valve
Motorized Air Valve for zone capacity control
Air
1807 Limiting Orifice Valve
Gas
Figure 1
1821 Ball Valve
8697 Metering Orifice
7216 Regulator
Ignition Transformer
INPUT/RATIO CONTROL The 4452 is a nozzle-mix burner with relatively simple control requirements. The burner stability range is wide and the gas pressure requirement for the burner is much lower than the air pressure so the fuel/air ratio can be controlled with a conven- tional cross-connected ratio regulator. Turndown can be accom- plished with a conventional motorized air valve or StepFire TM (pulse) control. Figure 1 shows an on-ratio, air primary control system with a 7216 cross-connected ratio regulator on the fuel. The high fire air pressure can be set from 12-20 osi depending on capacity requirements, and low fire set from 0.2-2.0 osi depending on desired turndown. Fuel/air ratio can be set to 10-15% excess air at high fire and 20-40% excess air at low fire. If more turndown is needed, the low fire excess air can be set to 100% excess air. Additional thermal turndown can be used at high or low fire rates, but CO emissions will increase as the burner fuel/air ratio exceeds 75% excess air at high fire. The 4452 is not designed to run with excess fuel. EMISSIONS CONTROL The 4452 is designed to have a fairly wide operating range where both low NOx and low CO can be achieved, but there is an optimum range of operation. Use engineering data from Sheet 4452-3 to help determine the optimum turndown and ratio control settings. NOX CONTROL: Air staging and internal flue gas recirculation (FGR) are used in the 4452 to reduce NOx emissions. Air staging works by adding air to the fuel in controlled "stages" which delays some of the combustion. The addition of FGR lowers the peak flame temper- ature by adding thermal ballast and by reducing the concentra- tion of oxygen in the flame.
The advantage of using internal self induced FGR is that there are no added costs for external flue gas piping, but the level of NOx reduction is dependent on the quality and temperature of the FGR being induced. The lower the oxygen content in the entrained FGR the lower the NOx emissions will be from the burner. It is important to limit the amount of air that infiltrates into the furnace if there is a desire to minimize NOx. As the furnace or oven temperature increases, the temperature of the FGR and the burner flame also increase, which will in turn increase the NOx emissions. NOx concentration may also increase as the burner is turned to low fire. CO CONTROL: Good ratio control and correct burner set-up is an important part of ensuring that CO emissions are kept to a minimum. Excessive CO emissions can also be caused by flame quench- ing from excess combustion air or cold recirculated flue gases impinging directly on the flame. The 4452 has a firing tube which shields the flame from relatively cool recirculating flue gas. As the firing tube's temperature increases it becomes more effective at controlling CO. When a burner is first lit under cold conditions the CO emissions will be higher for a few minutes until the firing tube is heated. CO will be minimized at high fire if the burner is operated with 10-50% excess air. At 3:1 turndown and lower, the burner should be run at slightly higher excess air rates, 20-75%, to minimize CO. As the furnace heats up, the flue gas drawn into the 4452 will heat the combustion air, which will reduce the burner's air capacity by 3-5 percent. So the final fuel/air ratio setting should be checked when the furnace is at its maximum temperature.
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