Capturing heat from the furnace
Heat recovery options
Natural gas-fired process furnaces are widely used in industry, particularly in applications involving metallurgy, heat-treating, glass and ceramics. Even at today’s attractive rates for natural gas as an industrial fuel, it makes complete sense to recover as much of the waste heat from the furnace as possible and use it for beneficial purposes.
Sources of Heat Loss
In any furnace operation, heat is lost from the furnace skin, from heated product leaving the treatment area, from conveyer or pusher equipment, from excessive draft and from open doors or other access points. But easily the largest and most concentrated type of heat loss is from the furnace exhaust, some with temperatures of 1000° F or higher. Fortunately, this is usually also the most practical heat to recover and reuse.
Two types of heat recovery systems that are commonly used with industrial furnaces are recuperators and regenerators. According to the U.S. Department of Energy, the recuperator is the most widely used heat recovery device. A recuperator is a gas-to-gas heat exchanger that is installed on the furnace exhaust that preheats incoming combustion air.
With relatively clean natural gas combustion exhaust, these heat exchange surfaces may even be finned or dimpled to capture the maximum amount of heat. Although the combustion exhaust itself is clean, the heating process may be producing corrosive or particulate byproducts that could damage or clog a high efficiency heat exchanger such as one with fins. Be sure your recuperator design takes into account the specific characteristics of your furnace operation.
Technology is Available
John Sultzbaugh is the Director of Engineering for Hauck Manufacturing Company of Lebanon, Pennsylvania. Hauck both manufactures furnace combustion equipment, including recuperative burners, and offers custom engineering services to industrial furnace users. Sultzbaugh points out that the current attractive pricing of natural gas from domestic sources encourages the use of gas-fired furnaces, but in some cases may give recuperation retrofits a longer payback. However, he also points out that for operators with site emission permits written on a lb/MMBtu basis, recuperation allows the use of more furnace capacity, hence increased productivity.
Hauck offers a range of burner types for industrial furnaces, including its Ecomax direct-fired self-recuperative burner for high temperature furnaces in the North American market. The company also provides assistance to customers looking to add recuperation to an existing burner system.
He notes that items to consider include existing burner construction to determine exposed metal parts and insulation that may have to be upgraded to accommodate higher temperature combustion air and flame temperatures. Air supply piping may need to be increased in size or redesigned to allow higher air pressure. Sultzbaugh adds, “We also need to look at the air-fuel ratio, which will require adjustment.”
Asked whether existing burner controls can be adapted to the addition of recuperation, Sultzbaugh explains, “It depends on the type of control method being used. Older type technology using relay logic is not readily adaptable. If an electronic control is used, such as one using mass flow control, it can more readily be adapted via programmable logic controller (PLC) programming changes.“
Firebridge, Inc is a combustion engineering firm headquartered in Burlington, Ontario with wide experience in industrial furnace engineering. Russ Chapman from that firm points out that flue gas recuperators are generally restricted to about 1800° F flue gas temperatures. His company is working on designs that might allow systems to operate at temperatures as high as 2200° F, but for the time being the limitation still stands.
Eclipse, Inc. is a long-time leader in industrial furnace burner technology. According to Jim Roberts from Eclipse, in recent years an important improvement in furnace heat recovery is development of self-recuperative burners, such as Eclipse’s ThermaJet design. He says, “This is the culmination of 20 years of development in the marketplace to get burners that not only eject a hot stream of gases, but bring the exhaust gases back through the burner to recover heat normally lost to the flue.”