The firing system can be consists of some components such as burner, combustor, grate, or bed. Design furnace dimension both for shape dimension and area is mainly determined by the dimension of flames and firing devices. Flame impingement should be prevented through provide required clearances burners, that may result in damage and overheating in pressure parts.
The furnace height is decided by firing device type and residence time necessity of specific fuel or volume of furnace. The following table presents a information about furnace residence time for some fuels. Residence time is needed with regard to a particle of fuel to achieve the middle point of furnace exit opening out of the firing device (Centre of heat input, should be accurate), supposing the entire furnace for being at isothermal of Furnace exit gas temperature. The furnace length is determined by flame while the furnace cross section is determined by burners; these considerations are very suitable for design furnace dimension of package boiler which has configuration of gas flow horizontally.
The furnace encounters the maximum temperature of gas inside of boiler. To get the high quantities of heat needed, evaporator surfaces usually are utilized in furnaces due to the excessive rates of heat transfer in boiling process. Tube metal and gas cooling are the best useful solutions.
Carbon steel tubes can be applied for structure in which DNB (departure from nucleate boiling) isn’t a probability and also temperature of tube metal seldom go over 50°C above saturation. Considering that no evaporation happens in supercritical boiler, cooling seriously isn’t efficient and also temperature of water closes to crucial stage namely 374°C, therefore needing low alloy material for furnace tubes.
Table 1: Variety of Furnace Residence Times pertaining to Several Fuels (in Seconds)
(Source: Book-Boiler for Power and Process-Kumar Rayaprolu)