DAMPER - A device for introducing a variable pressure drop in a system used for regulating the volumetric flow of a gas, such as air.
DAVIT – The structure on large firetube boilers from which the front and rear doors are suspended when opened.
DEAERATION – Removal of air and gases from boiler feed water prior to its introduction to a boiler.
DEGASIFICATION – Removal of gases from samples of steam taken for purity test. Removal of CO2 from water as in the ion exchange method of softening.
DELAYED COMBUSTION - A continuation of combustion beyond the furnace. (See also Secondary Combustion.)
DESIGN LOAD – The load for which a steam generating unit is designed, considered the maximum load to be carried.
DESIGN PRESSURE – The pressure used in the design of a boiler for the purpose of calculating the minimum permissible thickness or physical characteristics of the different parts of the boiler.
DESIGN STEAM TEMPERATURE – The temperature of steam for which a boiler is designed.
DEW POINT - The temperature at which condensation starts.
DISENGAGING SURFACE – The surface of the boiler water from which steam is released.
DISSOCIATION - The process by which a chemical compound breaks down into simpler constituents, as do CO2 and H2O at high temperature.
DISSOLVED SOLID – Those solids in water which are in solution.
DISTILLATE FUELS – Liquid fuels distilled usually from crude petroleum.
DISTILLATION - Vaporization of a substance with subsequent recovery of the vapor by condensation. Often used in less precise sense to refer to vaporization of volatile constituents of a fuel without subsequent condensation.
DISTILLED WATER - Water produced by vaporization and condensation with a resulting higher purity.
DOWNCOMER – A tube or pipe in a boiler or waterwall circulating system through which fluid flows downward.
DOWNTIME – Amount of time a piece of equipment is not operational.
DRAFT – The difference between atmospheric pressure and some lower pressure existing in the furnace stack or gas passages of a steam generating unit.
DRAFT DIFFERENTIAL – The difference in static pressure between two points in a system.
DRAFT GAUGE -A device for measuring draft, usually in inches of water.
DRAIN – A valved connection at the lowest point for the removal of all water from the pressure parts.
DRUM - A cylindrical shell closed at both ends designed to withstand internal pressure.
DRY AIR - Air with which no water vapor is mixed. This term is used comparatively, since in nature there is always some water vapor included in air, and such water vapor, being a gas, is dry.
DRYBACK BOILER - Firetube boiler with a refractory lined back door. Door opens to allow maintenance and/or inspection.
DRY GAS – Gas containing no water vapor.
DRY-GAS LOSS – The loss representing the difference between the heat content of the dry exhaust gases and their heat content at the temperature of ambient air.
DRY STEAM – Steam containing no moisture. Commercially dry steam containing not more than one half of one percent moisture.
DUCT – A passage for air or gas flow.
ECONOMIZER - Utilizes waste heat by transferring heat from flue gases to warm incoming feedwater.
EDR – Equivalent direct radiation is the rate of heat transfer from a radiator or convector. It is equivalent to the square feet of surface area necessary to transfer heat at the same rate at which it is produced by a generator. A single boiler horsepower equals 140 ft2 EDR.
EFFICIENCY - The ratio of output to input. See also Combustion, Fuel-to-Steam and Thermal Efficiency.
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 energy is used as the source of heat.
ELECTROSTATIC PRECIPITATOR – A device for collecting dust, mist or fume from a gas stream, by placing an electrical charge on the particle and removing that particle onto a collecting electrode.
ENTRAINMENT – The conveying of particles of water or solids from the boiler water by the steam.
EQUALIZER – Connections between parts of a boiler to equalize pressures.
EQUIVALENT EVAPORATION - Evaporation expressed in pounds of water evaporated from a temperature of 212 °F to dry saturated steam at 212 °F.
EVAPORATION – The change of state from a liquid to a vapor.
EVAPORATION RATE – The number of pounds of water that is evaporated in a unit of time.
EXCESS AIR - Air supplied for combustion in excess of that theoretically required for complete oxidation.
EXPANSION JOINT - The joint to permit movement due to expansion without undue stress.
EXPLOSION DOOR – A door in a furnace or boiler setting that is designed to be opened by a pre-determined gas pressure.
EXTERNAL TREATMENT – Treatment of boiler feed water prior to its introduction into the boiler.