Waste Heat Recovery
For most fuel-fired heating equipment, a large amount of the heat supplied is wasted as exhaust or flue gases. In furnaces, air and fuel are mixed and burned to generate heat, some of which is transferred to the heating device and its load. When the heat transfer reaches its practical limit, the spent combustion gases are removed from the furnace via a flue or stack. At this point, these gases still hold considerable thermal energy. In many systems, this is the greatest single heat loss. The energy efficiency can often be increased by using waste heat gas recovery systems to capture and use some of the energy in the flue gas. Exhaust gas loss or waste heat depends on flue gas temperature and its mass flow, or in practical terms, excess air resulting from combustion air supply and air leakage into the furnace. The excess air can be estimated by measuring oxygen percentage in the flue gases. Heat losses must be minimized before waste heat recovery is investigated. Waste heat recovery should generally be considered if the exhaust temperature is higher than 1,000°F, or if the flue gas mass flow is very large.
The most commonly used waste heat recovery methods are preheating combustion air, steam generation and water heating, and load preheating.
Benefits of waste heat recovery include:
- Improved heating system efficiency. Energy consumption can typically be reduced 5% to 30%
- Lower flue gas temperature in chimney. Less heat is wasted.
- Higher flame temperatures. Combustion air preheating heats furnaces better and faster.
- Faster furnace startup. Combustion air preheating heats furnaces faster.
- Increased productivity. Waste heat used for load preheating can increase throughput.
Resources
Improving Process Heating System Performance: A Sourcebook for Industry. U.S. Department of Energy (DOE) and the Industrial Heating Equipment Association (IHEA). This document can be obtained from www1.eere.energy.gov/industry/bestpractices/pdfs/process_heating_sourcebook2.pdf.Waste Heat Reduction and Recovery for Improving Furnace Efficiency. DOE and IHEA. This document can be obtained from http://www1.eere.energy.gov/industry/bestpractices/pdfs/35876.pdf.
American Society of Heating, Refrigerating and Air-Conditioning Engineers: www.ashrae.org
Waste Heat Recovery: Technology and Opportunities in U.S. Industry
