Process heating system efficiency

Reduce Air Infiltration in Furnaces
Fuel-fired furnaces discharge combustion products through a stack or a chimney. Hot furnace gases are less dense and more buoyant than ambient air, so they rise, creating a differential pressure between the top and the bottom of the furnace. This differential, known as thermal head, is the source of a natural draft or negative pressure in furnaces and boilers.
A well-designed furnace (or boiler) is built to avoid air leakage into the furnace or leakage of flue gases from the furnace to the ambient. However, with time, most furnaces develop cracks or openings around doors, joints, and hearth seals. These openings (leaks) usually appear small compared with the overall dimensions of the furnace, so they are often ignored. The negative pressure created by the natural draft (or use of an induced-draft fan) in a furnace draws cold air through the openings (leaks) and into the furnace. The cold air becomes heated to the furnace exhaust gas temperature and then exits through the flue system, wasting valuable fuel. It might also cause excessive oxidation of metals or other materials in the furnaces.
The heat loss due to cold air leakage resulting from the natural draft can be estimated if you know four major parameters:
• The furnace or flue gas temperature
• The vertical distance H between the opening (leak) and the point where the exhaust gases leave the furnace and its flue system (if the leak is along a vertical surface, H will be an average value)
• The area of the leak, in square inches
• The amount of operating time the furnace spends at negative pressure.
Secondary parameters that affect the amount of air leakage include these:
• The furnace firing rate
• The flue gas velocity through the stack or the stack cross-section area
• The burner operating conditions (e.g., excess air, combustion air temperature,
and so on).
For furnaces or boilers using an induced-draft (ID) fan, the furnace negative pressure depends on the fan performance and frictional losses between the fan inlet and the point of air leakage. In most cases, it would be necessary to measure or estimate negative pressure at the opening.
Suggested Actions
Taking the following actions can
reduce air leakage in a furnace:
1. Repair the air leakage area by replacing or repairing insulation or seals.
2. Close furnace doors properly to maintain a tight seal and avoid opening.
3. Install a pressure control system that maintains balanced, slightly positive (in hundredths of an inch) pressure,
at the point of major air leakage.
4. Install a damper in the stack that can be adjusted manually if an automated furnace pressure control cannot be used or justified.
5. Install or use a “draft gage” to monitor furnace pressure at the level of air leakage if it cannot be sealed properly, and adjust the manual damper to maintain balanced, slightly positive (in hundredths of an inch) pressure, at the point of major air leakage.
Note: Actions 3-5 work only in forced and balanced draft furnaces.
Resources
See also Improving Process Heating System Performance: A Sourcebook for Industry. Washington, D.C.: U.S. Department of Energy and Industrial Heating Equipment Association, 2004.
U.S. Department of Energy—
For additional information on process heating system efficiency, to obtain DOE’s publications and Process Heating Assessment and Survey Tool (PHAST) software, or to learn more about training, visit the BestPractices Web site at www.eere.energy.gov/industry/bestpractices.

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