Condensing Gas-Fired Humidifier With Dual Heat Recovery and Low NOx
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Solution Overview
Problem
Gas-fired humidifiers have low thermal efficiency and high NOx emissions, making them less desirable compared to condensing appliances, and they are not suitable for using low-cost plastic flue venting materials due to high exiting flue temperatures.
Innovation Solution
A gas-fired humidifier design with a primary heat exchanger located within an atmospheric pressure storage tank, incorporating a secondary shell and tube heat exchanger that pre-heats combustion air and water using exhaust gases, achieving thermal efficiency greater than 90% and NOx emissions below 20 ppm, while allowing for PVC flue venting by maintaining flue temperatures below 149F.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a gas-fired humidifier uses a typical primary heat exchanger design, then it can heat water to generate steam, but the exiting flue temperatures are high (350F-400F) which limits thermal efficiency to 80-84% and prevents use of low-cost plastic flue venting
Solution Approach 1:
The patent implements a nested heat exchanger configuration where the secondary heat exchanger is positioned within or around the primary heat exchanger, allowing exhaust gases to pass through multiple heat transfer zones. This nested arrangement maximizes heat recovery from the exhaust gases before they exit the system, reducing both temperature and energy loss without requiring separate independent heat exchangers.
Solution Approach 2:
The heat exchanger system is divided into distinct segments: a primary heat exchanger for initial steam generation and a secondary heat exchanger for further heat recovery. This segmentation allows each component to perform its specific function optimally, with the primary exchanger handling the main heating load and the secondary exchanger capturing residual heat to preheat combustion air or water, thereby reducing overall exhaust temperature and improving thermal efficiency.
2Productivity
If a gas-fired humidifier operates at atmospheric pressure with typical efficiency (80-84%), then it can provide steam for humidification, but it produces high NOx emissions and cannot use PVC flue venting
Solution Approach 1:
The patent modifies operational parameters by implementing extended heat recovery that lowers exhaust gas temperature and modifies combustion conditions. The secondary heat exchanger recovers additional heat from exhaust gases, and the system is designed to operate with parameters that reduce thermal NOx formation while maintaining steam generation productivity through optimized heat transfer surfaces and combustion control.
3Loss of energy
If a gas-fired humidifier achieves condensing efficiency (>90% thermal efficiency), then it can reduce stack losses and enable PVC flue venting, but it requires complex heat exchanger design and lower exiting flue temperatures
Solution Approach 1:
The patent combines the primary and secondary heat exchanger functions into an integrated assembly where exhaust gases sequentially pass through multiple heat transfer zones. This merged design achieves condensing efficiency by maximizing heat recovery in a unified structure, reducing the need for separate complex systems while enabling PVC flue venting through sufficiently lowered exhaust temperatures.
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 design enhances thermal efficiency and reduces NOx emissions, maintaining reliability and minimizing maintenance, while enabling the use of PVC flue venting and meeting ultra-low NOx standards.
Implementation Method 1
a primary heat exchanger connected to the gas-fired burner and combustion blower assembly, the primary heat exchanger being located within the atmospheric pressure storage tank and configured to heat the volume of water to generate steam
Implementation Method 2
a secondary heat exchanger having a first heat exchange section for heating a first fluid stream and a separate second heat exchange section for heating a second fluid stream, the first and second heat exchange sections being in heat transfer communication with exhaust gases generated by the gas-fired burner and combustion blower assembly
Implementation Method 3
Water vapor in flue gasses begins to condense at about 90% thermal efficiency with natural gas, or at about 135F flue gas temperature
Data Source
AI summary
A gas-fired atmospheric pressure steam humidifier having high efficiency and ultra-low NOx(3) emissions is disclosed. In some examples, the gas-fired humidifier can have an efficiency of greater than 90 percent and a NOx(3) output of less than 20 parts per million (ppm). In one aspect, the humidifier includes a secondary heat exchanger having a first heat exchange section for pre-heating combustion air and a separate second heat exchange section for pre-heating make-up water, wherein the first and second heat exchange sections are in heat transfer communication with exhaust gases generated by the gas-fired burner and combustion blower assembly. In some examples, the first heat exchange section includes orifices for enabling flue gas recirculation.