Insulated Concentric Duct for Wood-Burning Heater
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Solution Overview
Problem
Wood-burning heating appliances face challenges with the formation of bistre in central tubes, leading to clogging and disrupted combustion air supply due to high flue gas temperatures, which existing duct solutions fail to address effectively, especially with complex installation requirements and wind-induced pressure differences.
Innovation Solution
A heating installation with a central tube coated in insulating material and a peripheral tube forming a combustion air supply chamber, where the insulating layer extends over the entire height, reducing condensation and heat exchange, and a single roof outlet for air and fumes, with air inlets around the perimeter to minimize wind effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a central tube without insulation is used for evacuation, then the structure is simple, but bistre formation and clogging occur due to condensation
Solution Approach 1:
An insulating material layer is introduced as an intermediary between the central tube and the external environment. This layer prevents condensation by thermal insulation, thereby eliminating bistre formation and clogging while maintaining system reliability
Solution Approach 2:
The thermal insulation parameter is modified by adding an insulating material layer around the central tube. This changes the thermal parameters of the system, preventing condensation and bistre formation without compromising structural simplicity
2Device complexity
If no insulation is provided, then the duct structure is simple, but heat exchange between combustion products and air causes temperature rise and disrupted combustion air supply
Solution Approach 1:
The insulating material acts as a thermal barrier intermediary between the hot combustion products in the central tube and the combustion air in the peripheral chamber, preventing unwanted heat exchange and maintaining correct combustion air temperature
Solution Approach 2:
Thermal insulation is applied locally at the critical interface between the central tube and peripheral chamber, providing targeted heat protection where it is most needed without increasing overall system complexity
3Ease of operation
If an off-centre independent duct is used for air supply, then combustion air can be supplied, but installation becomes complex and wind-induced pressure differences occur
Solution Approach 1:
The flue and combustion air supply functions are merged into a single concentric duct structure. The central tube handles flue evacuation while the peripheral chamber supplies combustion air, eliminating the need for separate off-centre ducts and simplifying installation
Solution Approach 2:
The central tube is nested within the peripheral tube, creating a concentric dual-function duct. This nested configuration integrates both flue evacuation and combustion air supply in one structure, reducing installation complexity and eliminating wind-induced pressure differences
4Volume of moving object
If concentric tubes are used without insulation, then the structure is compact, but condensation and bistre formation limit fume evacuation
Solution Approach 1:
An insulating material intermediary is added between the concentric tubes to prevent condensation. This maintains the compact concentric structure while ensuring reliable fume evacuation by eliminating bistre formation that would otherwise clog the system
Data Source
Figure 1~2
Figure 3~6
Figure 7~8
AI summary
The installation has a building outlet (3) with a sidewall strip (36) associated to a closure plate (37) with a central opening (38) allowing an end of a central tube (7) of a connecting conduit to pass through. An outer surface (8) of the tube is coated with an insulating material layer (13) e.g. rock wool. An inner face of the strip includes a connecting unit (40) to connect a peripheral tube (10) of the conduit. An upper end (41) of the peripheral tube is extended remotely from a bottom face of the plate. An opening (42) in the strip assures new air inlet into volume delimited by the strip.