Modulating Furnace Firing Control to Prevent Flame Extinction
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Solution Overview
Problem
Furnaces with modulating firing rates often experience flame extinction at lower firing rates due to safety features or blockages, leading to inefficient heating and potential occupant discomfort or pipe freezing, as they cycle between ignition and reduced firing rates without maintaining sufficient heat.
Innovation Solution
A furnace with a controller that sets the firing rate above the minimum after a call for heat, modulates downward, and if the flame is lost, resets to a higher firing rate for ignition and maintains that rate until the heat demand is met, ensuring continuous heating.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If the firing rate is reduced to improve fuel efficiency and occupant comfort, then energy consumption decreases, but the flame may be extinguished due to safety features or blockages
Solution Approach 1:
The system performs preliminary action by establishing a minimum firing rate threshold before allowing modulation downward. When the flame is detected to be lost, the system has already predetermined that the firing rate must be maintained above the extinction point, preventing the harmful effect of flame extinction while enabling energy-efficient operation at moderate loads.
Solution Approach 2:
The system employs feedback by continuously monitoring flame presence and using this information to adjust the firing rate modulation behavior. When flame loss is detected, the feedback loop triggers maintenance of a higher firing rate, creating a closed-loop control system that adapts to actual combustion conditions and prevents repeated cycling.
2Use of energy by moving object
If the furnace cycles between ignition and reduced firing rates, then fuel efficiency improves, but occupant comfort deteriorates due to insufficient heating
Solution Approach 1:
The system applies dynamics by enabling continuous modulation of the firing rate between minimum and maximum levels rather than operating in discrete on/off cycles. The controller dynamically adjusts the firing rate based on the call for heat intensity and flame stability, allowing smooth transitions that maintain occupant comfort while optimizing fuel efficiency across varying load conditions.
Solution Approach 2:
The system changes the operating parameter of firing rate continuously rather than in discrete steps. By modifying the firing rate parameter dynamically and maintaining it above the extinction point when flame loss occurs, the system achieves efficient operation across a range of conditions without the discomfort of repeated cycling and insufficient heating.
3Adaptability or versatility
If the firing rate is modulated downward to meet varying heat demands, then adaptability improves, but the risk of flame extinction increases
Solution Approach 1:
The system establishes predetermined minimum firing rate thresholds and modulation rules before operation begins. These preliminary settings create safety margins that prevent the firing rate from dropping to levels where flame extinction is likely, while still allowing sufficient downward modulation to meet varying heat demands adaptively.
Solution Approach 2:
The system applies beforehand cushioning by maintaining the firing rate above the extinction point as a protective buffer. This cushioning effect absorbs the variability and instability that would otherwise cause flame extinction during modulation, ensuring reliable operation while preserving adaptability to load changes.
4Reliability
If safety features extinguish the flame at reduced firing rates, then safety reliability improves, but heating effectiveness deteriorates
Solution Approach 1:
The system uses feedback from safety controllers and flame detection to inform firing rate modulation decisions. When safety features indicate potential flame instability, the feedback loop adjusts the firing rate downward more conservatively or maintains it above critical thresholds, reconciling safety requirements with heating effectiveness by avoiding unnecessary flame extinction.
Solution Approach 2:
The system modifies the firing rate parameter based on safety controller inputs and flame detection feedback. By dynamically changing the firing rate parameter within safe operating boundaries, the system maintains heating effectiveness while respecting safety constraints, preventing the binary on/off behavior that would reduce productivity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution prevents repeated cycling and maintains sufficient heat, enhancing occupant comfort and preventing freezing issues by maintaining a firing rate above the extinction point until the heat demand is satisfied.
Implementation Method 1
a furnace employs a burner that burns a fuel such as natural gas, propane, oil or the like, and provides heated combustion gases to the interior of a heat exchanger
Implementation Method 2
a circulating air blower typically forces return air from the building, and in some cases ventilation air from outside of the building, over or through the heat exchanger, thereby heating the air
Implementation Method 3
The combustion gases typically proceed through the heat exchanger, are collected by a collector box
Data Source
AI summary
A furnace is disclosed that includes a burner with a firing rate that is variable between a minimum and a maximum firing rate. After a call for heat is received, the firing rate is set to an initial level above the minimum firing rate, and the burner is ignited. The firing rate is then modulated downward toward the minimum firing rate. If the flame is lost during or after modulation, the burner is reignited and the firing rate is maintained above the firing rate at which the flame was lost until the current call for heat is satisfied. In some cases, the firing rate is maintained until one or more subsequent calls for heat are satisfied.