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 - A device used to regulate to flow of air.

Deaeration - Act of separating air from fluids.

Decibel - Unit used for measuring relative loudness of sounds.

Defrost Cycle - Refrigerating cycle in which evaporator frost and is melted.

Defrost Timer – A device which shuts unit off the compressor long enough to permit ice and frost accumulation on evaporator to melt.

Defrosting - See defrost

Degree Day - The number of degrees that the mean temperature for that day is below 65° F.

Dehumidify - the process of removing moisture from the air.

Dehumidifier - Device used to remove moisture from air.

Dehydrator –Receiver – A liquid refrigerant receiver that also contains a desiccant to remove moisture.

Deice 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 wired in the delta configuration, this is also forms a triangle.

Demand Meter -  A instrument that measures the kilowatt-hour usage of a circuit.

Density - The mass per unit volume.

Design Pressure - Highest or most severe pressure expected during operation.

Desiccant - A substance used to remove moisture from refrigerant.

Desuperheater - A heat exchanger designed to remove excess superheat from a system. Normally used as a heater for pools or domestic water.

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

Dew Point -: Temperature at which begins to condense .

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

Diaphragm – Now get your mind out of the gutter J  now that your awake it’s A flexible material usually made of thin metal, rubber, or plastic.

Die Casting - Process of molding low-melting-temperature metals in ac­curately shaped metal molds.

Dielectric Fluid - Fluid with high electrical resistance.

Dilution Air - is air that is mixed with the products of combustion prior to them exiting the vent.

Diode- a electrical component that will only allow current to pass in one direction.

Differential - The difference between the cut-in and cut-out of a control.

Direct Current - Current that travels only in one direction. 

Direct Digital Control (DDC) - Use of a digital computer to perform required automatic control operations.

Direct Expansion  -  When the refrigerant is the system is either condensed or evaporated directly by the medium being heated or cooled.

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

Distilling Apparatus - Fluid-reclaiming device used to reclaim used refrigerants.

Distribution Controls - Systems which 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.

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.

Draft - A heating term used to ensure the escape of flue gases and aid combustion.

Draft Gauge - An Instrument used to measure the draft in an apalince.

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

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

Dry Capacitor Condenser - Electrical device made of dry metal and dry insulation; used to store an electrical charge.

Dry Cell Battery - A battery, having no liquid in the cell.

Dry Ice -Refrigerating substance made of solid carbon dioxide which changes directly from a solid to a gas (sublimates).

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

Dynamometer - A device for measuring power input or output of a mechanism.