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Glossary: D

Dalton's Law
Vapor pressure created in a container by a mixture of gases is equal to sum of individual vapor pressures of the gases contained in mixture.

Damper
Device for controlling airflow.

Dasher
Stirring mechanism in a dispensing freezer.

Deaeration
Act of separating air from a substance.

Decibel (dB)
Unit used for measuring relative loudness of sounds.

Deck (coil deck)
Insulated horizontal partition between refrigerated space and evaporator space.

Defrost cycle
Refrigerating cycle in which evaporator frost and ice accumulation is melted.

Defrost timer
Device, connected into electrical circuit, that shuts unit off long enough to permit ice and frost accumulation on evaporator to melt.

Defrosting
Process of removing frost accumulation from evaporators.

Defrosting evaporator
Evaporator operating at such temperatures that ice and frost on surface melts during the Off cycle.

Degreasing
Removal of oil or grease from refrigerator parts with a solution or solvent.

Degree-day
Unit that represents on degree of difference from inside temperature and the average outdoor temperature; often used in estimating fuel requirements for a building.

Dehumidifier
Device used to remove moisture from air.

Dehydrated oil
Lubricant that has had most of its water content removed (dry oil).

Dehydrator
See Drier.

Dehydrator-receiver
Small tank that serves as liquid refrigerant reservoir and also contains a desiccant to remove moisture. Used on most automobile air conditioning installations.

De-ice control
Device for operating a refrigerating system in such a way as to provide melting of the accumulated ice and frost.

Delta transformer
Three-phase electrical transformer that has ends of each of three windings electrically connected to form a triangle.

Demand meter
Instrument that measures the kilowatt-hour usage of a circuit or group of circuits.

Density
Closeness of particles within a given substance. The weight per unit volume.

Deodorizer
Device that absorbs or adsorbs various odors, usually by principle of absorption. Activated charcoal is commonly used.

Department of Transportation (DOT)
A governmental unit that regulates the transportation of refrigerants from one location to another.

Desert bag
A bag used to keep water cool in the desert. The fabric is not waterproof, so water leaks through and evaporates, cooling the water inside the bag.

Desiccant
Substance used to collect and hold moisture. A drying agent. Common desiccants are activated alumina and silica gel.

Design pressure
Highest pressure expected during operation. Sometimes calculated as operating pressure plus a safety allowance.

Detector, leak
Device used to detect and locate refrigerant leaks.

Dew
Condensed atmospheric moisture deposited in small drops on cool surfaces.

Dew point
Temperature at which vapor (at 100% humidity) begins to condense and deposit as a liquid.

Diac
A two-lead alternating current semiconductor that allows current to flow in both directions at a preset voltage.

Diagnostics
The process of identifying or determining the nature and circumstances of an existing condition.

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

Dichlorodifluoromethane
Refrigerant commonly known as R-12.

Die casting (dc)
Process of molding low-meltingtemperature metals in accurately shaped metal molds.

Dielectric fluid
Fluid with high electrical resistance.

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

Diffuser
Attachments for duct openings that distribute the air in wide flow patterns.

Diode
two-element electrontube that will allow more electron flow in one direction in a circuit than in the other direction; tube that serves as a rectifier.

Direct current (dc)
Electron flow that moves continuously in one direction in a circuit.

Direct Digital Control (DDC)
Use of digital computer to perform required automatic control operations in a total energy management system.

Direct expansion evaporator
Evaporator using either an automatic expansion valve (AEV) or a thermostatic expansion valve (TEV) refrigerant control.

Direct-spark ignition
A furnace control in which a spark is used to ignite the gas-air mixture. There is no constantly-burning pilot light.

Dispensing freezers
A freezer with built-in dispensing equipment, used for serving ice cream and frozen drinks.

Displacement
Volume obtained by multiplying the area of the cylinder bore by the length of the piston stroke.

Distilling apparatus
Fluid-reclaiming device used to reclaim used refrigerants. Reclaiming is usually done by vaporizing and then condensing refrigerant.

Distribution controls
Systems that help evenly and efficiently transfer the heating or cooling medium to the area where it is needed.

District heating and cooling
Use of a central utility system designed to provide heating and cooling to large residential and industrial areas.

Dome-hat
Sealed metal container for the motor compressor of a refrigerating unit.

Door heater
A heater located around the door opening of a freezer, used to prevent ice buildup from freezing the door closed.

Double-duty case
Commercial refrigerator in which a part of space is for refrigerated storage and part is equipped with glass windows for display purposes.

Double-thickness flare
Copper, aluminum, or steel tubing end that has been formed into two-wall thickness, 37 to 45 bell mouth or flare.

Dowel pin
Accurately dimensioned pin pressed into one assembly part and slipped into another assembly part to ensure accurate alignment.

Downflow furnace
A furnace in which return air enters through the top and is pulled down through the heat exchanger. Also called counterflow furnace.

Draft gauge
Instrument used to measure air movement by measuring air pressure differences.

Draft indicator
Instrument used to indicate or measure chimney draft or combustion gas movement. Draft is measured in units of 1" of water column.

Draft regulator
Device that maintains a desired draft in a combustion-heated appliance by automatically controlling the chimney draft to the desired value.

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

Drip pan
Pan-shaped panel or trough used to collect condensate from evaporator.

Dry bulb
An instrument with a sensitive element to measure ambient air temperature.

Dry bulb temperature
Air temperature as indicated by an ordinary thermometer.

Dry capacitor
See Electrolytic capacitor.

Dry cell battery
Electrical device used to provide dc electricity, having no liquid in the cells.

Dry ice
Refrigerating substance made of solid carbondioxide, which changes directly form a solid to a gas (sublimates). its subliming temperature is -109F (-78C).

Dry system
Refrigeration system that has the evaporator liquid refrigerant mainly in the atomized or droplet condition.

Dual-pressure regulator
A combination of a high-pressure and low-pressure regulator.

Duct
Tube or channel through which air is conveyed or moved.

Duct sweeper
A tool used to remove dirt and debris from ducts.

Dynamometer
Device for measuring power output or power input of a mechanism.

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