Sheet 4343-6 Page 2
CONTROL
FURNACE FLUE (clean flue gases)
This is a general description of furnace pressure control. There are a number of schemes available.
TwinBed II dœs not require that all of the combustion products it generates be brought back through its regenerators in order to achieve optimum air preheat. The heat carrying capacity of the combustion air is approximately 20% less than that of the POC. Thus combustion air can be preheated with 80% of the POC exhausted through the regenerator. The remaining 20% exhausts through the furnace flue direct from the furnace chamber which reduces exhaust system power requirement.
Flow through the regenerators is controlled by the temperature of the exhaust gases leaving the regenerator. If temperature is too low, flow is increased. Furnace pressure is maintained by modulation of the flue damper. If the damper is in the full closed position and furnace pressure is below setpoint, the exhaust tem- perature setpoint is reduced to lower exhaust gas flow through the burner regenerators.
Size furnace flue for 20% of total POC. If you want TwinBeds to Direct Fire (see page 2), size flue for 100% POC.
DIRECT FIRE SYSTEM FOR FOULING APPLICATIONS
Furnace pressure control
Exhaust cycle valves closed
Air jet damper
Furnace pressure transmitter
Exhaust temp. control
EPIC ® II
Combustion air
Hard damper
From combustion air
Reference
Pressure tap
Exhaust control valve closed
All POC exits through furnace flue
POC = products of combustion
In order to accommodate the wide range in POC flow rates from direct fire to regenerative cycling, systems with direct fire capability use a two stage flue damper (air jet damper and hard damper) and a split range cascade control loop.
When used in processes such as aluminum melting furnaces or glass melting tanks where routine fouling of the regenerators is expected, the TwinBed system is usually installed with a furnace flue capable of accepting the full POC flow of all burners in direct fire. In addition to firing with cold combustion air, the system al- lows cleaning a fouled regenerator while the other burners fire.
Detailed information on this system is available from North American's High Temperature Systems Group.
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