What Determines Waste-Gas Losses?
March 1, 2009
The purpose of a heating process is to introduce thermal energy into a product, raising it to a certain temperature to prepare it for additional processing, or possibly to change its properties. To do this, product is heated in an oven or furnace, which results in energy losses in different areas or forms.
In addition to the obvious heat loss of stored heat leaking out during the cycling from cold to hot and back again, there are additional areas of loss, including wall, material-handling, cooling media and radiation (opening) losses. All the means of losses compete with the workload for the energy released by the burning fuel-air mixture. However, these losses could be dwarfed by the most significant source of all, which is waste-gas loss.
Waste-gas loss, also known as flue gas or stack loss, is made up the heat that cannot be removed from the combustion gases inside the furnace. The reason? Heat flows from the higher temperature source to the lower temperature heat receiver.
In effect, the heat stream has hit bottom. If, for example, a furnace heats products to 1,500°F (816°C), the combustion gases cannot be cooled below this temperature without using design or equipment that can recover heat from the combustion gases. Once the combustion products reach the same temperature as the furnace and load, they cannot give up any more energy to the load or furnace, so they have to be discarded. At 1,500°F temperature, the combustion products still contain about half the thermal energy put into them, so the waste-gas loss is close to 50 percent. The other half, which remains in the furnace, is called available heat. The load receives heat that is available after storage in furnace walls, and losses from furnace walls, load conveyors, cooling media and radiation have occurred.
This makes it obvious that the temperature of a process, or more correctly, of its exhaust gases, is a major factor in its energy efficiency. The higher that temperature, the lower the efficiency.
Another factor that has a powerful effect is the fuel-air ratio of the burner system. For every fuel, there is a chemically correct, or stoichiometric, amount of air required to burn it. One cubic foot of natural gas, for example, requires about 10 cubic feet of combustion air. Stoichiometric, or on-ratio combustion, will produce the highest flame temperatures and thermal efficiencies.
However, combustion systems can be operated at other ratios. Sometimes this is done deliberately to obtain certain operating benefits, but often it happens simply because the burner system is out of adjustment. The ratio can go either rich (excess fuel or insufficient air) or lean (excess air). Either way, it wastes fuel.
So, the bottom line is that to get the best possible energy efficiency from furnaces and ovens, reduce the amount of energy carried out by exhaust and lost to heat storage, wall conduction, conveying and cooling systems and radiation.