Process Heating
Quick Tip: Use of an infrared camera quickly reveals areas where thermal energy is being wasted.
General Description: [1]
Process heating is essential in the manufacture of many consumer and industrial products, including those made out of metal, plastic, rubber, concrete, glass, and ceramics. Process heating systems include fuel based, electric based, and steam based systems. With fuel-based systems, heat is generated by the combustion of solid, liquid, or gaseous fuel, and transferred either directly or indirectly to the material. Electric-based process heating systems use electric currents or electromagnetic fields to heat materials. The choice of the energy source depends on the availability, cost, and efficiency; and, in direct heating systems, the compatibility of the exhaust gases with the material to be heated. Hybrid systems use a combination of process heat systems by using different energy sources
Although steam is generated by using fuel or electricity in a boiler, it is a major source of energy for many industrial processes, such as fluid heating and drying. Steam has several favorable properties for process heating applications such as low toxicity, ease of transport, and high heat capacity. In addition to steam, several other secondary energy sources are used by industry. They include hot air, heat transfer by liquids, and water. The secondary sources are generated by a heating system of its own that can fall under the general category of other process heating systems.
In all process heating systems, energy is transferred to the material to be treated. Direct heating methods generate heat within the material (e.g., microwave, induction, or controlled exothermic reaction), whereas indirect methods transfer energy from a heat source to the material by conduction, convection, radiation, or a combination of these functions. In most processes, an enclosure is needed to isolate the heating process and the environment from each other
The performance of a process heating system is determined by its ability to achieve a certain product quality under constraints (for example, high throughput, and low response time). The energy efficiency of a process heating system is determined by the costs attributable to the heating system per unit produced. Efficient systems manufacture a product at the required quality level and at the lowest cost. Energy efficient systems create a product with less input energy to the process heating systems per unit produced.
Some energy sources are more expensive than others, and equipment efficiency needs to be considered. Comparatively expensive energy types tend to promote shorter payback periods for projects that improve system efficiency. In contrast, byproduct fuel sources, such as wood chips, bagasse (the residue remaining after a plant has been processed, for instance, after the juice has been removed from sugar cane), and black liquor (a byproduct of the paper production process) tend to be much less costly than conventional fuels, making the payback periods for efficiency improvement projects comparatively longer.
Potential Energy and Cost Savings Opportunities:
The below energy conservation opportunities or energy efficiency actions (EE ACTIONS) are provided as a partial list of potential savings opportunities in your plant. They are grouped as no-to-low cost, moderate cost, and long-term cost investments. Consider each for your plant and feel free to contact us for clarification or any assistance you may need in assessing specific projects.
No-to-low cost investment
EE ACTION: Check Burner Air-to-Fuel Ratios [2]
Periodic checking and resetting of air-fuel ratios is one of the simplest ways to get maximum efficiency out of fuel-fired process heating equipment such as furnaces, ovens, heaters, and boilers. Most high temperature direct-fired furnaces, radiant tubes, and boilers operate with about 10 to 20 percent excess combustion air at high fire to prevent the formation of dangerous carbon monoxide and soot deposits on heat transfer surfaces and inside radiant tubes. For the fuels most commonly used by U.S. industry, including natural gas, propane, and fuel oils, approximately one cubic foot of air is required to release about 100 British thermal units in complete combustion. Exact amount of air required for complete combustion of commonly used fuels can be obtained from the information given in one of the references. Process heating efficiency is reduced considerably if the air supply is significantly higher or lower than theoretically required.
EE ACTION: Check Heat Transfer Surfaces [2]
Industrial process heating systems use various methods to transfer heat to the load. These include direct heat transfer from the flame or heated gases to the load and indirect heat transfer from radiant tubes, muffles, or heat exchangers. Indirect heating systems that use fuel firing, steam, or hot liquids to supply heat are discussed in this tip sheet. In each case, clean heat transfer surfaces can improve system efficiency. Deposits of soot, scale or oxides, sludge, and slag on the heat transfer surfaces should be avoided. A regular maintenance schedule that includes cleaning heat transfer surfaces is recommended.
EE ACTION: Reduce Air Infiltration in Furnaces [2]
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 or the chimney effect, is the source of a natural draft or negative pressure in furnaces and boilers. In turn, the internal negative pressure can draw unconditioned air into the furnace which dilutes the heat transfer process. Therefore, infiltration openings should be minimized. In addition, furnace pressure controllers can be used to reduce the negative pressure.
Moderate investment
EE ACTION: Preheated Combustion Air [2]
For fuel-fired industrial heating processes, one of the most potent ways to improve efficiency and productivity is to preheat the combustion air going to the burners. The source of this heat energy is often the exhaust gas stream, which leaves the process at elevated temperatures. A heat exchanger, placed in the exhaust stack or ductwork, can extract a large portion of the thermal energy in the flue gases and transfer it to the incoming combustion air. Recycling heat this way will reduce the amount of the purchased fuel needed by the furnace.
EE ACTION: Oxygen-Enriched Combustion [2]
When a fuel is burned, oxygen from the combustion air chemically combines with the hydrogen and carbon in the fuel to form water and carbon dioxide, releasing heat in the process. Air is made up of 21% oxygen, 78% nitrogen, and 1% other gases. During air–fuel combustion, the chemically inert nitrogen in the air dilutes the reactive oxygen and carries away some of the energy in the hot combustion exhaust gas. An increase in oxygen in the combustion air can reduce the energy loss in the exhaust gases and increase heating system efficiency.
EE ACTION: Reduce Radiation Losses from Heating Equipment [2]
Heating equipment, such as furnaces and ovens, can experience significant radiation losses when operating at temperatures above 1,000°F. Hot surfaces radiate energy to colder surfaces in their line of sight, and the rate of heat transfer increases with the fourth power of the surface’s absolute temperature.
EE ACTION: Install Waste Heat Recovery Systems for Fuel-Fired Furnaces [2]
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.
Long-term investment
EE ACTION: Use exhaust flue gases to preheat load [2]
The thermal efficiency of a heating system can be improved significantly by using heat contained in furnace flue gases to preheat the furnace load (material coming into the furnace). If exhaust gases leaving a fuel-fired furnace can be brought into contact with a relatively cool incoming load, heat will be transferred directly to the product. Since there is no intermediate step, like air or gas preheating, in the heat recovery process, this can be a successful approach to capturing waste heat. Load preheating is best suited for continuous processes, but it can sometimes also be used with intermittently operated or batch furnaces.
EE ACTION: Using Waste Heat for External Processes [2]
The temperature of exhaust gases from fuel-fired industrial processes depends mainly on the process temperature and the waste heat recovery method. Energy from gases exhausted from higher temperature processes (primary processes) can be recovered and used for lower temperature processes (secondary processes). One example is to generate low-pressure steam using waste heat boilers. In addition, many companies install heat exchangers on the exhaust stacks of furnaces and ovens to produce hot water or to generate hot air for space heating.
EE ACTION: Use Lower Flammable Limit Monitoring Equipment to Improve Process Oven Efficiency [2]
Process heating applications involving flammable solvent removal use large amounts of energy to maintain safe lower flammable limits (LFL) in the exhaust air. National Fire Protection Association (NFPA) guidelines require the removal of significant amounts of exhaust air to maintain a safe, low-vapor solvent concentration. If LFL monitoring equipment is used to ensure proper vapor concentrations, these guidelines allow for less exhaust air removal. LFL monitoring equipment can improve the efficiency of the solvent removal process and significantly lower process energy requirements.
References
[1] exerpts from Improving Process Heating System Performance: A Sourcebook for Industry, Industrial Technologies Program (U.S.), 2004, United States Department of Energy, Washington, D. C., pp. 78 (cited 9/13/10)
[2] Energy Tips – Process Heating, Industrial Technologies Program (U.S.), United States Department of Energy, Washington, D. C., (cited 9/13/10). To find more tip sheets go to EERE Publication and Product Library.
