The following terms are commonly used in cooling tower science, many of which are unique to the
cooling tower industry:

Acfm — The actual volumetric flow rate of air-vapor mixture. Unit: cu ft per min.
Air Horsepower — The power output developed by a fan in moving a given air rate against a given resistance. Unit: hp. Symbol: ahp.
Air Inlet — Opening in a cooling tower through which air enters. Sometimes referred to as the louvered face on induced draft towers.
Air Rate — Mass flow of dry air per square foot of cross-sectional area in the tower's heat transfer region per hour. Unit: Ib per sq ft per hr. Symbol: G'. (See Total Air Rate).
Air Travel — Distance which air travels in its passage through the fill. Measured vertically on counterflow towers and horizontally on crossflow towers. Unit: ft.
Air Velocity — Velocity of air-vapor mixture through a specific region of the tower (i.e. the fan). Unit: ft per min. Symbol: V.
Ambient Wet-Bulb Temperature — The wet-bulb temper-ature of the air encompassing a cooling tower, not including any temperature contribution by the tower itself. Generally measured upwind of a tower, in a number of locations sufficient to account for all ex-traneous sources of heat. Unit: °F. Symbol: AWB.
Approach — Difference between the cold water temper-ature and either the ambient or entering wet-bulb temperature. Unit: °F.
Atmospheric — Refers to the movement of air through a cooling tower purely by natural means, or by the as-pirating effect of water flow.
Automatic Variable-Pitch Fan — A propeller type fan whose hub incorporates a mechanism which enables the fan blades to be re-pitched simultaneously and automatically. They are used on cooling towers and air-cooled heat exchangers to trim capacity and/or conserve energy.
Basin — See "Collection Basin" and "Distribution Basin".
Basin Curb — Top level of the cold water basin retaining wall; usually the datum from which pumping head and various elevations of the tower are measured.
Bay — The area between adjacent transverse and longi-tudinal framing bents.
Bent — A transverse or longitudinal line of structural framework composed of columns, girts, ties, and di-agonal bracing members.
Bleed-Off — See "Slowdown".
Blower — A squirrel-cage (centrifugal) type fan; usually applied for operation at higher-than-normal static pressures.
Blowout — See "Windage".
Brake Horsepower — The actual power output of a motor, turbine, or engine. Unit: hp. Symbol: bhp.
Btu (British Thermal Unit) — The amount of heat gain (or loss) required to raise (or lower) the temperature of one pound of water 1°F.
Capacity — The amount of water (gpm) that a cooling tower will cool through a specified range, at a specified approach and wet-bulb temperature. Unit: gpm

Casing — Exterior enclosing wall of a tower, exclusive of the louvers.
Cell — Smallest tower subdivision which can function as an independent unit with regard to air and water flow; it is bounded by either exterior walls or partition walls. Each cell may have one or more fans and one or more distribution systems.
Chimney — See "Shell".
Circulating Water Rate — Quantity of hot water entering the cooling tower. Unit: gpm.
Cold Water Temperature — Temperature of the water leaving the collection basin, exclusive of any temper-ature effects incurred by the addition of make-up and/or the removal of blowdown. Unit: °F. Symbol: CW.
Collection Basin — Vessel below and integral with the tower where water is transiently collected and directed to the sump or pump suction line.
Counterflow — Air flow direction through the fill is countercurrent to that of the falling water.
Crossflow — Air flow direction through the fill is essen-tially perpendicular to that of the falling water.
Distribution Basin — Shallow pan-type elevated basin used to distribute hot water over the tower fill by means of orifices in the basin floor. Application is normally limited to crossflow towers.
Distribution System — Those parts of a tower, beginning with the inlet connection, which distribute the hot circulating water within the tower to the points where it contacts the air for effective cooling. May include headers, laterals, branch arms, nozzles, distribution basins, and flow-regulating devices.
Double-Flow — A crossflow cooling tower where two op-posed fill banks are served by a common air plenum.
Drift — Circulating water lost from the tower as liquid droplets entrained in the exhaust air stream. Units: % of circulating water rate or gpm. (For more precise work, an UG parameter is used, and drift becomes pounds of water per million pounds of exhaust air. Unit: ppm.)
Drift Eliminators — An assembly of baffles or labyrinth passages through which the air passes prior to its exit from the tower, for the purpose of removing entrained water droplets from the exhaust air.
Driver — Primary drive for the fan drive assembly. Al-though electric motors predominate, it may also be a gas engine, steam turbine, hydraulic motor or other power source.
Dry-Bulb Temperature — The temperature of the entering or ambient air adjacent to the cooling tower as measured with a dry-bulb thermometer. Unit: °F. Symbol: DB.
Entering Wet-Bulb Temperature — The wet-bulb tem-perature of the air actually entering the tower, in-eluding any effects of recirculation. In testing, the average of multiple readings taken at the air inlets to establish a true entering wet-bulb temperature. Unit °F. Symbol: EWB.

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