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Table of contents Power Plant Dictionary


DALTON'S LAW OF PARTIAL PRESSURE - Each constituent of a mixture of gases behaves thermodynamically as if it alone occupied the space. The sum of the individual pressures of the constituents equals the total pressure of the mixture.

DAMPER - A device used to vary the volume of air passing through an air outlet, air inlet or duct.

DASHPOT - A damping device, usually consisting of a cylinder and a piston in which relative motion of either displaces a fluid such as air or oil, resulting in friction.

DEAD BAND - In HVAC, a temperature range in which neither heating nor cooling is turned on; in load management, a kilowatt range in which loads are neither shed nor restored.

DE-AERATING HEATERS - Mechanical device using steam to strip dissolve gases from the boiler feedwater and heating the feedwater.

DE-AERATION - Act of separating air from substances.

DE-AERATOR - An apparatus or device which is used to remove dissolved air or oxygen from water.

DE-ALKALIZATION - The removal of alkalinity from a water supply by neutralization or ion exchange.

DE-ALKALIZER - An apparatus or device used to remove the alkaline carbonate and bicarbonate ions from a water supply.

DE-ALLOYING - This is a corrosion process whereby one constituent of a metal alloy is preferentially removed from the alloy, leaving an altered residual microstructure.

DEASHING - The removal from a solution of inorganic salts by means of adsorption by ion-exchange resins of both the cations and the anions that comprise the salts. See deionization.

DE-CARBONATION - Refers to the removal of carbon dioxide from the boiler feedwater.

DECIBEL (dB) - A decibel is a division of a logarithmic scale for expressing the ratio of two quantities proportional to power or energy. The number of decibels denoting such a ratio is ten times the logarithm of the

DECONCENTRATOR - This is a cylindrical tank connected before the boiler to receive the boiler feedwater before entering the boiler. It is designed to promote settling of suspended solids, which then could be removed via its own blowdown device. Was used for operation with very high suspended solids.

DE-FLOCCULANT - An electrolyte adsorbed on colloidal particles in suspension that charges the particles to create repulsion forces which maintain the particles in a dispersed state, thus reducing the viscosity of the suspension.

DEFROST CYCLE - Refrigerating cycle in which evaporator frost and ice accumulation is melted.

DEFROST TIMER - Device connected into electrical circuit which shuts unit off long enough to permit ice and frost accumulation on evaporator to melt.

DEFROSTING - Process of removing frost accumulation from evaporators.

DEFROSTING CONTROL - Device to automatically defrost evaporator. It may operate by means of a clock, door cycling mechanism or during "off" portion of refrigerating cycle.

DEFROSTING TYPE EVAPORATOR - Evaporator operating at such temperatures that ice and frost on surface melts during off part of operating cycle.

DEGREE DAY - A unit, based upon temperature difference and time, used in estimating fuel consumption and specifying nominal heating load of a building in winter. For any one day, when the mean temperature is less than 65F there exist as many degree days as there are Fahrenheit degrees difference in temperature between the mean temperature for the day and 65F.

DEGREES OF SUPERHEAT - The amount by which the temperature of a superheated vapor exceeds the temperature of the saturated vapor at the same pressure.

DEHUMIDIFICATION - The condensation of water vapor from air by cooling below the dewpoint or removal of water vapor from air by chemical or physical methods.

DEHUMIDIFIER -(1) An air cooler or washer used for lowering the moisture content of the air passing through it; (2) An absorption or adsorption device for removing moisture from air.


DEHUMIDIFYING - Removal of moisture from the air.

DEHYDRATION - The removal of water vapor from air, stored goods or refrigerants.

Deionization - Deionization, a more general term than deashing, embraces the removal of all charged constituents or ionizable salts (both inorganic and organic) from solution.

DE-IONIZER - An apparatus or device used to remove the ions of dissolved salts from water.

DEMAND - The probable maximum rate of water flow as determined by the number of water supply fixture units.

DEMAND CHARGE - That part of an electric bill based on kW demand and the demand interval, expressed in dollars per kilowatt. Demand charges offset construction and maintenance of a utility's need for a large generating capacity.

DEMAND CONTROL - A device which controls the kW demand level by shedding loads when the kW demand exceeds a predetermined set point.

DEMAND INTERVAL - The period of time during which kW demand is monitored by a utility service, usually 15 or 30 minutes long.

DEMAND LOAD - The actual amount of load on a circuit at any time. The sum of all the loads which are ON. Equal to the connected load minus the loads that are OFF.

DEMAND READING - Highest or maximum demand for electricity an individual customer registers in a given interval, example, 15 minute interval. The metered demand reading sets the demand charge for the month.

DEMINERALIZER - A process to remove dissolved matter from boiler pretreated water by contacting the water with ion-exchange resins.

DENSITY - The ratio of the mass of a specimen of a substance to the volume of the specimen. The mass of a unit volume of a substance. When weight can be used without confusion, as synonymous with mass, density is the weight per unit volume.

DENSITY, ABSOLUTE - Mass per unit volume of a solid material, expressed usually in kg/m3.

DESALINATION - The removal of inorganic dissolved solids from water.

DESICCANT, LIQUID - A hygroscopic liquid, such as glycol, used to remove water from other fluids.

DESICCANT - Any absorbent or adsorbent, liquid or solid, that will remove water or water vapor from a material. In a refrigeration circuit, the desiccant should be insoluble in the refrigerant.

DESIGN PRESSURE - Highest or most severe pressure expected during operation. Sometimes used as the calculated operating pressure plus an allowance for safety.

DESIGN WORKING PRESSURE - The maximum allowable working pressure for which a specific part of a system is designed.

DESSERT BAG - A canvas bag which permits seepage of its liquid. The liquid will evaporate and obtains the to evaporate partly from the content of the bag and thus cooling its content.

DESSERT BAG - A canvas bag which permits seepage of its liquid. The liquid will evaporate and obtains the to evaporate partly from the content of the bag and thus cooling its content.

DETERGENT ADDITIVE - In lubrication technology, a surface active additive that helps to keep solid particles suspended in an oil

DETERGENT CLEANING - A boiler cleaning process using an alkaline solution, primarily to remove oil and grease.

DETERGENT OIL - A heavy duty oil containing a detergent additive. These oils are mainly used in combustion engines.

DETERGENT-DISPERSANT - A compound mixture of cleaning agents that have both surface-active properties and suspending properties.

DEW POINT - Temperature at which vapor (at 100 percent humidity) begins to condense and deposit as liquid.

DEW POINT DEPRESSION - The difference between dry bulb and dew point temperatures.

DEW POINT TEMPERATURE - The temperature at which condensation begins, if air is cooled at constant pressure.

DIAPHRAGM - Flexible material usually made of thin metal, rubber or plastic.

DIATOMACEOUS EARTH FILTRATION - Is a process in which a filter cake or precoat of diatomaceous earth is used as a filter medium.

DIELECTRIC - A nonconductor of electricity.

DIELECTRIC FITTING - A non conductive substance such as plastic that is placed between two dissimilar metals to prevent galvanic current flow.

DIELECTRIC STRENGTH - A measure of the ability of a dielectric (insulator) to withstand a potential difference across it without electric discharge.

DIFFERENTIAL - The temperature or pressure difference between cut-in and cut-out temperature or pressure of a control.

DIFFERENTIAL AERATION CELL - An electrolytic cell, the electromagnetic force of which is due to a difference in air (oxygen) concentration at one electrode as compared with that at another electrode of the same material. (see concentration cell)

DIFFERENTIAL SOLUTE CONCENTRATION - A potential difference between an anode and cathode on metal, because of a concentration cell due to dissolved metals.

DIFFUSER - A circular, square, or rectangular air distribution outlet, generally located in the ceiling and comprised of deflecting members discharging supply air in various directions and planes, and arranged to promote mixing of primary air with secondary room air.

DIRECT ACTING - Instruments that increase control pressure as the controlled variable (such as temperature or pressure) increases; while reverse acting instruments increase control pressure as the controlled variable decreases.

DIRECT CURRENT - A source of power for an electrical circuit which does not reverse the polarity of its charge.

DIRECT-EXPANSION EVAPORATOR - One that contains only enough liquid to continue boiling as heat is absorbed by it.

DISPERSANT - A chemical which causes particulates in a water system to remain in suspension.

DISPERSANT OIL - A heavy duty oil containing a dispersant additive.

DISPLACEMENT PUMP - Pumps in which energy is added to the water periodically and the water is contained in a set volume.

DISPLACEMENT VOLUME - The volume displaced by the piston between top dead center and bottom dead center.

Dissociation - Ionization.

DISSOLVED GASES - Gases soluble in water.


DISSOLVED SOLIDS (TDS) - The measure of the total amount of dissolved matter.

DISTILLATION - Involves boiling water and condensing the vapor.

DMA - Direct memory access. A process where block of data can be transferred between main memory and secondary memory without processor intervention.

DOMESTIC HOT WATER - Potable hot water as distinguished from hot water used for house heating.

DOWEL PINS - Metal pins that guarantee exact alignment of a flange joint on a horizontally split casing as found in turbine construction

Down flow - Conventional direction of solutions to be processed in ion-exchange column operation, that is, in at the top, out at the bottom of the column.

DOWNSTREAM - The outlet side of an instrument, a pump, valve, etc..

DRAFT GAUGE - Instrument used to measure air movement by measuring air pressure differences.

DRIER - Substance or device used to remove moisture from a refrigeration system.

DRIERITE - Desiccant which operates by chemical action.

DRIFT - Entrained water in the stack discharge of a cooling tower.

DRIFT - Term used to describe the difference between the set point and the actual operating or control point.

DROOP - Terms used to describe the difference between the set point and the actual operating or control point.

DRUM WATER LEVEL LINE - The water level in the drum during the normal operating mode.

DRY BULB - An instrument with a sensitive element to measure ambient air temperature.


DRY BULB TEMPERATURE - The temperature registered by an ordinary thermometer. The dry bulb temperature represents the measure of sensible heat, or the intensity of heat.

DRY COMPRESSION - The compression of vapor, in a vapor-liquid vapor-compression refrigeration cycle.

DRY ICE - Refrigerating substance made of solid carbon dioxide which changes directly from a solid to a gas (sublimates). Its subliming temperature is -78C.

DRY PIPE - A perforated or slotted pipe or box inside the drum and connected to the steam outlet.

DRY STANDBY - A method of sealing al water and steam connections and placing a desiccant in the unit and applying an airtight seal.

DRY SYSTEM - Refrigeration system which has the evaporator liquid refrigerant mainly in the atomized or droplet condition.

DRYNESS FRACTION OR QUALITY - Weight fraction of the vapor in a vapor-liquid mixture.

DUAL SHAFT GAS TURBINE - a gas turbine which has one turbine on one shaft driving the compressor and when the gas discharged from this turbine is directed to another turbine on a separate shaft to drive a load.

DUAL-TEMPERATURE RE-CIRCULATING WATER SYSTEM - A closed re-circulating water system that uses water either for cooling, by circulating it through a chiller, or for heating, by circulating it through a boiler or heat-exchanger depending upon need.

DUCTILE GOUGING - Referring to irregular wasting of the tube metal beneath a porous deposit The micro structure of the metal does not change with this process and the ductility remains, but the thinning leads to rapture.

DUCTILITY - The ability of a material to deform plastically without fracturing.

DUMMY PISTON -  The axial thrust in reaction turbines can be nearly eliminated by the use of balance or dummy pistons. With the correct size of a dummy piston exposed to two different bleed point pressures, the thrust is nearly equalized. There is a small leakage across the labyrinth seal of the dummy piston as steam leaks from the high to the lower bleed point

DUST - An air suspension (aerosol) or particles of any solid material, usually with particle size less than 100 microns.

DYNAMIC DISCHARGE HEAD - Static discharge head plus friction head plus velocity head.

DYNAMIC LOAD - An imposed force that is in motion, that is, one that may vary in magnitude, sense, and direction.

DYNAMIC PUMPS - Pumps in which energy is added to the water continuously and the water is not contained in a set volume.

DYNAMIC SUCTION HEAD - Positive static suction head minus friction head and minus velocity head.

DYNAMIC SUCTION LIFT - The sum of suction lift and velocity head at the pump suction when the source is below pump centerline.

DYNAMIC SYSTEM - An ion-exchange operation, wherein a flow of the solution to be treated is involved.

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Table of contents Power Plant Dictionary