Modulating gas furnace and associated method of control
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Solution Overview
Problem
Existing gas furnace control systems struggle to maintain a consistent air-to-fuel ratio due to variations in fuel composition, oxygen content, and altitude, leading to unstable combustion and high carbon monoxide emissions, requiring manual adjustments and compromising on heating capacity and efficiency.
Innovation Solution
A modulating gas furnace with a control system that correlates air-to-fuel ratio with the square root of combustion system pressure and uses feedback from temperature sensors to adjust the gas valve position, accounting for variations in fuel components, air density, and oxygen concentration, ensuring a stable air-to-fuel ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If pressure feedback control is used to maintain air-to-fuel ratio, then combustion stability is improved, but accuracy deteriorates due to parameter variations
Solution Approach 1:
The system uses feedback from oxygen sensors and temperature sensors to continuously monitor combustion conditions and adjust the gas valve position. The oxygen sensor measures residual oxygen in exhaust gases, and the temperature sensor monitors combustion chamber temperature, with both signals fed back to the controller to dynamically adjust fuel flow and maintain optimal air-to-fuel ratio despite variations in fuel composition and altitude.
Solution Approach 2:
The controller dynamically adjusts the gas valve position based on measured oxygen content and temperature parameters. When oxygen content or temperature deviates from optimal ranges, the system changes the fuel flow parameter to compensate, thereby maintaining accurate air-to-fuel ratio control despite variations in fuel composition, air density, and oxygen concentration.
2Adaptability or versatility
If manual adjustments are made to accommodate fuel variations, then adaptability is improved, but ease of operation deteriorates
Solution Approach 1:
The system performs self-adjustment through automated feedback control. The oxygen sensors and temperature sensors continuously monitor combustion conditions, and the controller automatically adjusts the gas valve position to maintain optimal air-to-fuel ratio. This eliminates the need for manual adjustments by installers when fuel composition varies, as the system adapts automatically to different fuel types, altitudes, and operating conditions.
3Reliability
If excess air is used to prevent unstable combustion, then reliability is improved, but efficiency deteriorates
Solution Approach 1:
The oxygen sensor provides feedback on residual oxygen levels in the exhaust, allowing the controller to precisely control the air-to-fuel ratio. Instead of using excessive air to ensure stable combustion, the system maintains the optimal ratio by dynamically adjusting fuel flow based on real-time oxygen measurements, thereby improving heating efficiency while ensuring reliable stable combustion.
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
The solution maintains a consistent air-to-fuel ratio, reducing unstable combustion and carbon monoxide emissions while optimizing heating capacity and efficiency, eliminating the need for manual adjustments and accommodating changes in fuel composition and altitude.
Implementation Method 1
a pressure sensor configured to measure combustion system pressure
Implementation Method 2
feedback from a temperature sensor or a difference between two temperature sensor readings
Implementation Method 3
a burner assembly; a variable-speed draft inducer blower configured to move air through the combustion system
Data Source
AI summary
A method is provided for controlling combustion in a modulating gas furnace. The method includes receiving an indication of a firing rate setpoint for a burner assembly, and applying the firing rate setpoint to first and second continuous functions that map the firing rate setpoint to air-to-fuel ratio and combustion system pressure setpoints. A variable-speed draft inducer blower is set to drive to a combustion system pressure setpoint, and the modulating gas valve is controlled during combustion in the combustion system. In this regard, a combustion system pressure measurement is obtained and applied to an inverse of the first continuous function that outputs an adjusted firing rate for the combustion system pressure measurement. The adjusted firing rate is applied to a third continuous function that maps the firing rate to gas valve position, and outputs a gas valve position to which the modulating gas valve is set.