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Glossary of Technical Terms - D

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Damper: Valve for controlling airflow.

Dead Band: A range of temperature in a heating/cooling system in which no heating or cooling is supplied.

Decibel: Unit used for measuring relative loudness of sounds.

Defrost Control: A control system on a heat pump used to detect buildup of frost on the outside coil during the heating cycle and cause system reversal for hot gas defrost of the coil.

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.

Degree-Day: Unit that represents one degree of difference from given point in average outdoor temperature of one day.

Dehydrate: To remove water

Delta Connection: The connection in a three-phase system in which terminal connections are triangular similar to Greek letter delta.

Demand: The size of any load general averaged over a specified interval of time.

Demand (billing): The demand upon which billing to a customer is based.

Demand Meter: An instrument used to measure peak kilowatt hour consumption.

Density: Weight per unit volume.

Desiccant: Substance used to collect and hold moisture.

Design Load: The amount of heating or cooling required maintaining inside conditions when the outdoor conditions are at design temperature.

Dew Point: Temperature at which vapor (at 100 percent humidity) begins to condense

Diaphragm: Flexible membrane

Dielectric: An Insulator-non conductor.

Dies (threads): tool used to cut external threads.

Differential: As applied to refrigeration and heating: Difference between “cut-in” and “cut-out” temperature and pressure.

Diffuser: Air distribution outlet designed to direct airflow into a room

Dilution Air: Air which enters a draft hood and mixes with the flue gases.

Diode: A device that will carry current in one direction but not the reverse direction.

Direct Current: Abbreviated dc. Electric current that flows only in one direction

Direct Expansion Evaporator: An evaporator containing liquid and vapor refrigerant.

Direct Return: A two pipe system in which the first terminal unit takes off the supply main is the first unit connected to the return main.

Disconnecting switch: A knife switch that opens a circuit

Domestic Hot water: The heated water used for cooking, washing. Etc.

Double-Pole Switch: Simultaneously opens and closes tow wires of a circuit.

Double Thickness Flare: Tubing end which has been formed into two wall thickness

Dowel Pin: Pin pressed through two assembled parts to ensure accurate alignment.

Downdraft: Downward flow of the flue gas

Draft Gauge: Instrument used to measure air pressure

Drier: A substance or device used to remove moisture from a refrigeration system

Drift: Entrained water carried from a cooling tower by wind

Drilled Port Burner: A burner in which the ports have been formed by drilled holes

Drip Pan: Pan-Shaped panel or through used to collect condensate form evaporator coil

Dry Bulb Temperature: Air temperature as indicated by ordinary thermometer

Dry Ice: Solid Carbon dioxide

Dynamometer: Device for measuring power.


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