VACUUM - Pressure lower than atmospheric pressure.

VACUUM BREAKER - A device to prevent a suction in a water pipe.

VACUUM PUMP - Special high efficiency device used for creating high vacuums for testing or drying purposes.

VALVE, MODULATING - A valve which can be positioned anywhere between fully on and fully off to proportion the rate of flow in response to a modulating controller (see modulating control).

VALVE, NEEDLE - A form of globe valve that contains a sharp pointed, needle like plug that is driven into the and out of a cone shaped seat to accurately control a relatively small rate of flow of a fluid.

VALVE, POP - A spring loaded safety valve that opens automatically when pressure exceeds the limits for which the valve is set. It is used a safety device on pressurized vessels and other equipment to prevent damage from excessive pressure, also called relief valve or a safety valve.

VALVE, POPPET - A device that controls the rate of flow of fluid in a line or opens or shuts of the flow of fluid completely. When open, the sealing surface of the valve is moved away from a seat. When closed, the sealing surface contacts the seat to shut of the flow. Poppet valves are used extensively as pneumatic controls and as intake and exhaust valves in most internal combustion engines.

VALVE, PRESSURE RELIEF - A valve designed to minimize the possibility of explosion when air temperature surrounding a refrigeration system may rise to a point where the pressure of the refrigerant gas to increase to a danger point.

VALVE, RELIEF - Also called pressure relief valve.

VALVE, TWO-POSITION - A valve which is either fully on or fully off with no positions between. Also called an "on-off valve".

VANE - That portion of an impeller which throws the water toward the volute case.

VAPOR - A gas, particularly one near to equilibrium with the liquid phase of the substance and which does not follow the gas laws. Usually used instead of gas for a refrigerant, and, in general, for any gas below the critical temperature.

VAPOR BARRIER - A moisture-impervious layer applied to the surfaces enclosing a humid space to prevent moisture travel to a point where it may condense due to lower temperature.

VAPOR LOCK - A condition where liquid flow is impeded by vapor trapped in a liquid line.

VAPOR PHASE INHIBITORS - A system using an organic nitrite compound, a powder which vaporizes slowly to protect ferrous metal from contact with oxygen.

VAPOR PRESSURE - Vapor pressure denotes the lowest absolute pressure that a given liquid at a given temperature will remain liquid before evaporating into its gaseous form or state.

VAPOR, SATURATED - Vapor in equilibrium with its liquid; i.e., when the numbers per unit time of molecules passing in two directions through the surface dividing the two phases are equal.

VAPOR, SUPERHEATED - Vapor at a temperature which is higher than the saturation temperature (i.e., boiling point) at the existing pressure.

VAPOR, WATER - Water used commonly in air conditioning parlance to refer to steam in the atmosphere.

VAPOROUS CARRYOVER - Referring to impurities carried over with the steam and then forming a deposit on turbine bladings. This type of carryover is difficult to prevent.

VELOCITY - A vector quantity which denotes, at once, the time rate and the direction of a linear motion.

VELOCITY COMPOUNDING -

VELOCITY HEAD - The vertical distance a liquid must fall to acquire the velocity with which it flows through the piping system. For a given quantity of flow, the velocity head will vary indirectly as the pipe diameter varies.

VELOCITY, TERMINAL - The highest sustained air stream velocity existing in the mixed air path at the end of the throw.

VENT - An opening in a vessel or other enclosed space for the removal of gas or vapor.

VENTILATION - The process of supplying or removing air by natural or mechanical means, to or from a space; such air may or may not have been conditioned.

VENTURI TUBE METER - A flow meter used to determine the rate of flow and employing a venturi tube as the primary element for creating differential pressure in flowing gases or liquids.

VISCOSITY - That property of semi-fluids, fluids, and gases by virtue of which they resist an instantaneous change of shape or arrangement of parts. It is the cause of fluid friction whenever adjacent layers of fluid move with relation to each other.

VISCOSITY INDEX - A commonly used measure of the change in viscosity of a fluid with temperature. The higher the viscosity index, the smaller the relative change in viscosity with temperature.

VITAL HEAT - The heat generated by fruits and vegetables in storage; caused by ripening.

VOLATILE SOLIDS - Those solids in water or other liquids that are lost on ignition of dry solids at 550°F.

VOLATILE TREATMENT - Based on the use of hydrazine and neutralizing amines or ammonia. Leaves no solids in the boiler.

VOLATILITY - Volatility, surface tension and capillary action of a fluid are incidental to environmental systems. Volatility is the rapidity with which liquids evaporates extremely rapidly and therefore is highly volatile.

VOLT - The unit of potential difference or electromotive force in the meter-kilogram-second system, equal to the potential difference between two points for which 1 coulomb of electricity will do 1 joule of work in going from one point to another.

VOLTAGE (E) - The electromotive force in an electrical circuit. The difference in potential between two unlike charges in an electrical circuit is its voltage measured in "volts" (V).

VOLTAGE DROP - The voltage drop around a circuit including wiring and loads must equal the supply volt age.

VOLTAIC CELL - A storage device that converts chemical to electrical energy.

VOLUME, SPECIFIC - The volume of a substance per unit mass; the reciprocal of density.

VOLUTE - The spiral-shaped casing surrounding a pump impeller that collects the liquid discharged by the impeller.