Dba - "A" weighted average of the sound pressure levels over the entire frequency band. Intended to be a more accurate representation of how a human hears sound.
Damper - A device for introducing a variable resistance of regulating the volumetric flow of gas or air.
Decibel - Unit of sound pressure or power. Abbreviation is "dB". 1 Watt of sound power is equal to 120 dB.
Deaerator - A type of feedwater heater operating with water and steam in direct contact. It is designed to heat the water and drive off oxygen.
Deflector - A device used to change the direction of a stream of air or of a mixture of pulverized fuel and air.
Design pressure - The pressure used in the design of a boiler for the purpose of determining the minimum permissible thickness or physical characteristics of the different parts of a boiler.
Differential - (of a control) the difference between cut in and cut out points.
Diffusion (raw gas) flame - Combustion state controlled by mixing phenomena. Fuel and air diffuse into one another until a flammable mixture ratio is achieved.
Downcomer - A tube or pipe in a boiler or waterwall circulating system through which fluid flows downward between headers.
Draft - The difference between atmospheric pressure and some lower pressure existing in the furnace or gas passages of the steam-generating unit.
Draft control, barometric - A device that controls draft by means of a balanced damper which bleeds air into the breeching on changes of pressure to maintain steady draft.
Draft differential - The difference in static pressure between two points in a system.
Drip leg - The container placed at a low point in a system of piping to collect condensate and from which it may be removed.
Drum - A cylindrical shell closed at both ends, designed to withstand internal pressure.
Dry back - The baffle provided in a firetube boiler joining the furnace to the second pass to direct the products of combustion, that is so constructed to be separate from the pressure vessel and constructed of heat resistant material. (Generally refractory and insulating material)
Dry steam - Steam containing no moisture. Commercially dry steam containing not more than 0.5 percent moisture.
Duct - A passage for air or gas flow.
Economizer - A series of tubes located in the path of flue gases. Feedwater is pumped through these tubes on its way to the boiler in order to absorb waste heat from the flue gas.
Efficiency - Of boiler operation: Output in heat units divided by input in heat units. The number of Btus contained in all steam evaporated is useful output. The number of Btus contained in all fuel supplied to the boiler is input.
Ejector - A device which utilizes the kinetic energy in a jet of water or other fluid to remove a fluid or fluent material from tanks or hoppers.
Electric boiler - A boiler in which electric heating means serve as the source of energy.
Embrittlement - An intercrystalline corrosion of boilerplate occurring in highly stressed zones. Cracking may result.
Emissivity - The efficiency with which a material radiates thermal energy, expressed as a fraction between 0 and 1.
Enthalpy - A thermal property of a fluid which is a function of state and is defined as the sum of stored mechanical potential energy and internal energy. It is generally expressed in Btu per pound of fluid (joules per kilogram).
Entrainment - The conveying of particles of water or solids from the boiler water by the steam.
Evaporation rate - The number of pounds of water evaporated in a unit of time.
Evaporator - A pressure vessel used to evaporate raw water by means of a steam coil. The steam is condensed by means of cooling water coils, and this distilled water is used as make-up for boiler feed.
Excess air - The amount of air needed by a burner which is in excess of the amount required for perfect or stoichiometric combustion. Some amount of excess air, depending on the available fuel/air mixing energy, is required to assure through mixing of the fuel and air for complete combustion.
Explosion door - A door in a furnace or boiler setting designed to be opened by a predetermined gas pressure.