Furnaces – With control means responsive to a sensed condition – Feed control
Reexamination Certificate
2001-01-16
2001-12-25
Esquivel, Denise L. (Department: 3749)
Furnaces
With control means responsive to a sensed condition
Feed control
C110S185000, C110S186000, C431S012000, C431S075000, C431S090000
Reexamination Certificate
active
06332408
ABSTRACT:
BACKGROUND—FIELD OF INVENTION
This invention relates to combustion systems primarily where there is more than one burner in a controlled zone or section.
BACKGROUND—Discussion of Prior Art
At the present time, there are four basic types of combustion control systems used in multiple burner furnaces. They are; pressure balance control, linked valve control, flow balance control and mass flow control.
In the first three cases, a single flow control valve is normally used to control the combustion air flow to a group (zone) of burners. This valve is usually actuated by a valve actuator (electric control motor) through mechanical linkages. Control of the valve actuator is by an output signal from the furnace zone temperature controller. The controller sends an output signal to the valve actuator that proportionally positions the air flow control valve. Thus a 10% output signal will position the valve in the 10% open position and the 70% output signal positions the valve in a 70% open position, etc.
Since the result of a temperature control output is a specific valve position, these systems do not respond directly to system pressure changes. In the typical event of a burner being shut-off in a multiple burner zone, there is a decrease in airflow through the control valve and, as a result, there is a decreased pressure drop across the control valve. With the lower pressure drop, the net pressure downstream of the control valve will increase, thus increasing the flow to the remaining burners. Therefore, as more burners are shut-off, increasing amounts of air (and gas) will go to the remaining burners, partially defeating the purpose (less heat input) of shutting off the burners.
The mass flow control system measures the air mass flow and fuel mass flow and controls each according to a calculated ratio. Differential pressure transmitters or other accurate flow measuring devices are needed to achieve optimum ratio control. Air and fuel temperature and pressure measurements are made to correct for minor variations. A microprocessor based control unit calculates and controls the actual mass flow of both streams to suit the process requirements. North American Combustion Company's MARC® IIIE Combustion Controller is an example of this type of system. These systems are generally complex and thus expensive and are not suitably designed for a simple air valve repositioning.
SUMMARY
This invention concerns a novel method of air control where there is more that one burner or item per control zone on a firnace, heating system, cooling system or other apparatus requiring a controlled air flow to multiple devices. A pressure transducer in the air piping, located downstream of the flow control device, sends a feedback signal to a pressure control loop that is a logical cascade from the temperature control loop. The pressure control loop repositions the air flow control device to compensate for changes in both downstream and upstream conditions.
OBJECTS AND ADVANTAGES
Accordingly, several objects and advantages of our invention are:
1. The system is designed to readjust the air flow control device to correct for variations of the upstream and downstream air pressure.
2. The system is ideally suited to compensate for the pressure drop changes that occur across the zone air flow control valve when burners are started or stopped in a multiple burner zone. This provides a much higher “turndown ratio” and better control at “low fire” settings.
3. The system can be easily retrofitted to most existing burner system at reasonable cost.
4. The pressure feedback signal system can optimise air flow control valve positioning on; pressure balance, linked valve and flow balance combustion systems.
5. Our invention provides greater fuel efficiency in multiple burner systems by providing better control at low fire and an increased operating range.
6. The system provides a fast response time.
Further objects and advantages of our invention will become apparent from a consideration of the drawings and ensuing description.
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Crowle William E.
Howlett Michael
Murray Gordon Alexander
Esquivel Denise L.
Rinehart K. B.
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