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Glossary of Technical Terms - A

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Absolute humidity: The amount of moisture actually in a given volume of air.

Absolute Pressure: Gauge pressure plus atmospheric pressure.

Absolute Temperature: Temperature measured from absolute zero.

Absolute Zero: Temperature at which all molecular motion ceases (-460 F and -273 C)

Absorbent: Substance which has the ability to take up or absorb another substance.

Absorber: A device containing liquid for absorbing refrigerant vapor or other vapors.

Accumulator: Storage tank which receives liquid refrigerant from evaporator and prevents it from flowing into suction line.

Activated Carbon: Specially processed carbon commonly used to clean air.

Adiabatic Compression: Compressing refrigerant gas without removing or adding heat.

Adsorbent: Substance which has property to hold molecules of fluids without causing a chemical or physical change.

Air Binding: A condition in which a bubble or other pocket of air is present in a pipeline that prevents the desired flow in the pipeline

Air Changes: The amount of air leakage through a building in terms of the number of building volumes exchanged.

Air Conditioner: Device used to control temperature, humidity, cleanliness, and movement of air in a conditioned space.

Air Cushion Tank: A closed tank that allows for water expansion without creating excessive pressure.

Air Shutter: An adjustable shutter on the primary air openings of a burner, which is used to control the amount of combustion air

Air Vent: A valve installed at the high points in a hot water system to eliminate air from the system.

Aldehyde: A class of compounds, which can be produced during incomplete combustion of a fuel gas.

Allen-Type Screw: Screw with recessed head designed to be turned with hex shaped wrench.

Alternating Current: Current that reverses polarity or direction periodically. It rises from zero to maximum strength and a return to zero in one direction then goes to similar variation in the opposite direction. This is a cycle which is repeated at a fixed frequency. It can be single phase, two phase, three phase, and poly phase. Its advantages over direct or uni-directional current are that its voltage can be stepped up by transformers to the high values which reduce transmission costs.

Alternator: A machine which converts mechanical energy into alternation current.

Ambient Temperature: Temperature of fluid (usually air) which surrounds an object.

American Wire Gauge: AWG. A system of numbers which designate crows-sectional area of wire; as the diameter gets smaller, the number gets larger, e.g., AWG #14=0.0641 in., AWG #12 = 0.0808 in.

Ammeter: An electric meter used to measure current.

Ampere-Turn: Abbreviated AT or NI. Unit of magnetizing force produced by a current flow of one ampere through one turn of wire in a coil

Amplitude: The maximum Instantaneous value of alternating current or voltage. It can be in either a positive or negative direction.

Anemometer: Instrument for measuring the rate of flow of air.

Angle of Lag or Lead: Phase angle difference between two sinusoidal wave forms having the same frequency.

Annealing: Process of heat treating metal to obtain desired properties of softness and ductility

Anticipator: A heater used to adjust thermostat operation to produce a closer temperature differential that the mechanical capability of the control.

Armature: The moving or rotating component of a motor, generator, relay or other electromagnetic device.

ASME: American Society of Mechanical Engineers.

Aspect Ratio: Ratio of Length to width of rectangular duct.

Aspirating Psychrometer: A device which draws sample of air through it for humidity measurement purposes

Atom: Smallest particle of element

Atomic Weight: The number of protons in an atom of a material is classified as its atomic weight.

Atomize: Process of changing a liquid to a fine spray

Attenuate: Decrease or lessen in intensity

Automatic Defrost: System of removing ice and frost from evaporators automatically.

Automatic Expansion Valve (AEV): Pressure controlled valve used as metering device.

Autotransformer: A transformer in which both primary and secondary coils have turns in common. Step-up or step-down of voltage is accomplished by taps on common winding.

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