Furnaces are an integral part of many industrial processes, and their efficiency enhancement is of great importance for reducing energy costs and improving process productivity. One of the ways to improve the efficiency of these systems is by using dampers and waste heat recovery. This article will discuss the different types of dampers used in furnace operations and the benefits of waste heat recovery using waste heat boilers.
Types of Furnace Dampers
Furnace stack dampers are installed on automatic control systems and normally kept closed during normal operations. However, they should be opened in the event of an induced draft fan failure or air preheater failure. Forced draft fan dampers are used to control oxygen levels, while induced draft fan dampers are used to control box pressure.
Manually operated isolation dampers include the air preheater bypass damper. This damper is used to control the preheater flue gas outlet temperature during cold weather operation to prevent condensation corrosion. Isolation dampers are also necessary in the air ducts on either side of the preheater for maintenance when the forced draft fan is in operation. For natural draft burners, isolation dampers are required in the air feed ducts for burner maintenance with the preheater in service. For forced draft burners, isolation dampers are necessary in the feed ducts to individual burners for maintenance while the furnace is in operation and using preheated or ambient air.
Benefits of Connecting Two Furnaces to a Single Air Preheater System
When two furnaces with similar operating characteristics are located very close to one another, they can be connected to a single air preheater system. This system will require an investment that is at least 20% less than two individual systems.
Waste Heat Recovery Using Waste Heat Boilers
Steam generation, or waste heat boiler systems, are used to recover heat that would normally be lost in the high temperature furnace stack. In some furnace designs and special process applications, it is sometimes possible to use the convection section to heat up a portion of the main feed to the furnace.
For most furnaces, steam generation systems involve the installation of a ground level waste heat boiler and induced draft fan. These systems are generally more expensive and offer fewer credits than air preheater systems. Therefore, they are usually only considered when additional steam generation capacity is needed to balance demand and avoid the installation of a new boiler.
Steam generation facilities can sometimes be added to the convection section of some furnaces without major structural or stack modifications. In this case, the extent of the modification and the associated downtime must be considered to determine if this approach is more attractive than a ground level waste heat system.
Evaluating the Addition of a Waste Heat Boiler System
To evaluate the addition of a waste heat boiler system to a process furnace, operating data such as heat fired, stack temperature, and excess air must be obtained. With this data and a simple computer program or even literature charts, the potential heat recovery rate and steam credits can be estimated.
Typical Waste Heat Boiler Installation
A typical waste heat boiler installation consists of three main components: the waste heat boiler, the induced draft fan, and the flue gas ducts. The induced draft fan is necessary to compensate for the additional flue gas pressure drop and the loss in stack effect due to the lower stack temperature.
The waste heat boiler consists of an extended surface tube bank in an enlarged section of ductwork. Most of these units are double drum designs and operate via natural circulation. The mud drum on the bottom of the system serves as a tube header and settling area for solids in the system. The steam drum serves as a steam/water separating device and provides the static head for water circulation.
Design Considerations
When designing a waste heat boiler system, the outlet temperature approach (difference between flue gas and steam saturation temperature) should not be less than 40°F, as lower approach temperatures would result in excessive surface area requirements. The flue gas pressure drop is usually between 3 and 5 inches of water. The sulfur level in the fuel burnt will determine the minimum acceptable metal temperature that can be tolerated with carbon steel (the normal tube material of construction). If sulfur corrosion will be an issue when designing to a 40°F approach temperature, then the flue gas outlet temperature should be increased, which will reduce the heat recovery.
Efficiency enhancement of furnaces can be achieved by using dampers and waste heat recovery. Different types of dampers are needed for various applications, and connecting two furnaces to a single air preheater system can reduce the investment costs. Waste heat recovery using a waste heat boiler system is also a viable option and can involve the installation of a ground level waste heat boiler and induced draft fan. Design considerations must be taken into account when installing these systems to ensure maximum efficiency.
How to Improve the Efficiency and Capacity of Process Heater
Process heaters are an essential part of many industrial processes and can be used to generate heat for a variety of applications. However, their efficiency and capacity can be improved through a number of methods. Here, we look at how to improve the efficiency and capacity of process heaters by improving heat transfer characteristics, enhancing flame luminosity, installing recuperators or air-preheaters, improved controls and/or using waste heat recovery boiler systems.
Heat Transfer Characteristics
Improving the heat transfer characteristics of process heaters can help to improve their efficiency and capacity. One way to do this is to use a recuperator or air-preheater. A recuperator is a device that uses heat from the flue gases of the furnace to preheat the combustion air. This can improve the thermal efficiency of the furnace by up to 1% for every 35°F drop in the exit of the flue gas temperature. Installing an air-preheater can also help to improve the efficiency and capacity of process heaters as they can be used to preheat the combustion air.
Flame Luminosity
Enhancing the flame luminosity of process heaters is another way to improve their efficiency and capacity. This can be done by improving the mixing of fuel and air and increasing the efficiency of heat transfer. One way to do this is to use lean-premix burners, swirl burners, pulsating burners and rotary burners. These types of burners can help to improve the flame luminosity and reduce NOx emissions.
Controls
Improved controls can also help to increase the efficiency and capacity of process heaters. One way to do this is to use advanced control systems that allow the furnace to run with 1% excess oxygen instead of 3 to 4%. This can reduce energy use by 3 to 6% and also reduce NOx emissions by 10 to 25%.
Waste Heat Recovery Boilers
Using waste heat recovery boiler systems is another way to improve the efficiency and capacity of process heaters. These systems can capture the heat from the flue gases and use it to generate steam or hot water. The steam or hot water can then be used to power a turbine and generate electricity or provide process heat.
Maintenance
Regular maintenance of burners, draft control and heat exchangers is essential to maintain safe and energy efficient operation of a process heater. An energy audit of an old refinery found that reducing excess combustion and draft air would result in annual savings of almost $1.2 million per year.
Conclusion
Improving the efficiency and capacity of process heaters can help to reduce energy use and emissions while also improving safety. This can be done by improving heat transfer characteristics, enhancing flame luminosity, installing recuperators or air-preheaters, improved controls and/or using waste heat recovery boiler systems. Regular maintenance is also essential to maintain safe and energy efficient operation of process heaters.