| Term | Definition |
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| Damper | A device for introducing a variable pressure drop in a system used for regulating the volumetric flow of a gas, such as air.
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| Davit | The structure on large firetube boilers from which the front and rear doors are suspended when opened.
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| Deaeration | Removal of air and gases from boiler feed water prior to its introduction to a boiler.
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| 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.
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| Delayed Combustion | A continuation of combustion beyond the furnace. (See also Secondary Combustion.)
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| Design Load | The load for which a steam generating unit is designed, considered the maximum load to be carried.
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| 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.
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| Design Steam Temperature | The temperature of steam for which a boiler is designed.
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| Dew Point | The temperature at which condensation starts.
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| Disengaging Surface | The surface of the boiler water from which steam is released.
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| Dissociation | The process by which a chemical compound breaks down into simpler constituents, as do CO2 and H2O at high temperature.
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| Dissolved Solid | Those solids in water which are in solution.
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| Distillate Fuels | Liquid fuels distilled usually from crude petroleum.
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| 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.
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| Distilled Water | Water produced by vaporization and condensation with a resulting higher purity.
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| Downcomer | A tube or pipe in a boiler or waterwall circulating system through which fluid flows downward.
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| Downtime | Amount of time a piece of equipment is not operational.
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| Draft | The difference between atmospheric pressure and some lower pressure existing in the furnace stack or gas passages of a steam generating unit.
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| Draft Differential | The difference in static pressure between two points in a system.
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| Draft Gauge | A device for measuring draft, usually in inches of water.
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| Drain | A valved connection at the lowest point for the removal of all water from the pressure parts.
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| Drum | A cylindrical shell closed at both ends designed to withstand internal pressure.
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| 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.
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| Dry Gas | Gas containing no water vapor.
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| Dry Steam | Steam containing no moisture. Commercially dry steam containing not more than one half of one percent moisture.
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| 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.
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| Dryback Boiler | Firetube boiler with a refractory lined back door. Door opens to allow maintenance and/or inspection.
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| Duct | A passage for air or gas flow.
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