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Identifying and Reducing Your Waste Heat Losses

Source: www.loc.gov

Some amount of heat loss is unavoidable in the operation of fuel-fired equipment. However, boilers, furnaces, and ovens used in industrial heat processing use a tremendous amount of thermal energy. Heat loss in these applications can have a significant impact on the bottom line. So, all you need to do is to reduce heat loss to reap the benefits of lower energy costs and increased profits. Sounds simple, right? Unfortunately, heat loss in fuel-fired equipment can be very complex. In order to optimize energy efficiency, it is important to understand the different ways that thermal energy is lost in the heat-processing environment. This knowledge will help you to specify improvements that may enhance productivity, reduce waste, and increase energy efficiency.

Types of Heat Loss

In the heat-processing environment, fuel and air are mixed, creating a combustion that maintains a certain temperature in the heating chamber. Thermal losses associated with this process reduce efficiency and increase operating costs. Sources of heat loss include the following: waste heat in flue gases, heat conducted through walls or doors, heat transported out by material handling devices or by cooling media, and heat radiated to cooler outside surfaces.

Waste gas losses. Waste gas loss, also known as flue gas or stack loss, is made up of the heat that cannot be removed from the combustion gases inside the heat processing equipment. This occurs when there is no difference between the heat source and the load. For example, if a furnace is heating a product at 1,500°F, the combustion gases cannot be cooled below this temperature. After they reach the same temperature as the furnace and load, they must be removed. The process temperature and the fuel-to-air ratio can have a significant effect on the amount of waste gas loss.

Wall or structure losses. Wall, or transmission, losses occur through the conduction of heat through the walls, roof, and floor of the heating device. After the heat reaches the outer shell of the device and radiates to the surrounding area or is carried away by air currents, it must be replaced by an equal amount taken from the combustion gases. This process continues as long as the device is at an elevated temperature.

Material handling losses. Many heat processing devices have material handling equipment to convey work into and out of the heating chamber. These pieces of equipment can lead to heat losses. Conveyor belts or product hangers that enter the heating chamber cold and leave it at higher temperatures drain energy from the combustion gases. This lost thermal energy must be replaced to maintain the temperature in the heating chamber.

Losses from cooling media. Water-cooling protects rolls, bearings, and doors in heat-processing environments, but at the cost of lost energy. These cooling media components and their cooling water become the conduit for additional heat losses. Maintaining an adequate flow of cooling media is essential, but proper insulation may protect the heating device and load from heat loss.

Radiation losses. Heat processing devices operating at temperatures above 1,000°F may signify radiation heat loss. Hot surfaces radiate energy to any cooler surface nearby and the rate of heat transfer increases with the surface temperature. Anyone who has ever stood in front of the open door of a high-temperature furnace can vouch for the amount of thermal energy radiated into the room.

The U.S. Department of Energy's Process Heating Assessment and Survey Tool (PHAST) is a free, downloadable software program that you can use to assess the performance of your heat processing equipment, spot potential areas of heat loss, and identify energy-saving opportunities.

Source: www.doe.gov

Methods to Increase Energy Efficiency

After you have identified the most likely sources of heat loss, it is time to consider methods to reduce these losses. Heat-loss reduction not only helps to lower energy costs, but it can improve heat-processing productivity and lower emissions of carbon monoxide and nitrogen oxides. Reducing waste-heat losses also may contribute to more consistent product quality and better equipment reliability.

• Optimize fuel-to-air ratio. Excess air is the result of the air-to-fuel ratio going into the boiler. This is an important factor governing heat processing efficiency and affects both combustion and heat loss. To minimize heat loss, reduce excess air used for fuel combustion in burners.
• Ensure proper insulation. For heat loss through heating equipment walls or outside surfaces, use the proper type and thickness of insulation for furnace/oven walls. Undertake a review of all process heating requirements and select appropriate materials. Install and maintain them according to manufacturer's recommendations.
• Material handling equipment. Minimize the weight of fixtures, trays, and baskets used for material handling. Make sure that conveyor belt fixtures are returned as hot as possible.
• Minimize air leakage. Reduce openings, cracks, holes in the heating equipment walls. Minimize door openings during charging and discharging the load or charge material.
• Control make-up air. Control and minimize the amount of make-up air in ovens and dryers, while making sure to follow all safety guidelines.
• Optimize equipment run-time. Minimize equipment idling time. Continuous use will reduce cooling of heat process equipment walls, thus reducing heat loss. Operate equipment at or close to the design capacity.
• Increase oxygen content of combustion air. The total volume of exhaust gases could be reduced by increasing the oxygen content of the combustion air, either by mixing extra oxygen into the ambient air or by using 100% oxygen. The reduced exhaust gases would result in substantial fuel savings. The exact amount of energy savings depends on the amount of oxygen in the combustion air and the flue gas temperature. In considering this step, it is important to compare the cost of the enriched oxygen with the potential savings from the improvement in energy efficiency.

Waste Heat Recovery

Reducing heat loss should always be the first step in a well-planned energy conservation program for a heat-processing environment. After that goal has been met, consider the next level—waste-heat recovery. Waste-heat recovery elevates heat processing efficiency to higher levels because it extracts energy from the exhaust gases and recycles it to the process. The most commonly used waste-heat recovery methods are preheating combustion air, steam generation and water heating, and load preheating. For more information, see Waste Heat Recovery in Industry.