AIRFLOW: The total amount of dry air and associated water vapor flowing through the tower, measured in cubic feet per minute at the exhaust from the tower and converted to standard air (which has a density of 0.075 lb/ft3).

AIR HORSEPOWER: The measure of useful power required to move a given air flow against a given resistance. The ratio of air horsepower to fan inlet horsepower is the measure of fan efficiency.

AIR INLET: The opening at the lower end of the tower that allows air to enter.

ALGAE: A low form of plant life, which generally requires sunlight and air for existence. It causes plugging of cooling tower distribution systems, fill, and heat exchanger tubes.

ALGAECIDE: A chemical that will retard or prevent the growth of algae and slimes. Some of the more commonly used algaecides are chlorine, copper sulfate, and phenolic compounds.

AMBIENT: The atmosphere that is adjacent to, but not affected by, the cooling tower. Generally, this means upwind of the tower.

AMBIENT WET-BULB TEMPERATURE: The wet-bulb temperature that is measured in accordance with the definition of ambient. Readings are obtained by means of a mechanically aspirated psychrometer.

ANCHOR BOLT: A threaded bolt embedded in a concrete basin or fitted to supported members, to which an anchor casting is attached.

ANCHOR CASTING: Also known as COLUMN ANCHOR. A device for attaching the tower structure to the foundation; it does not include the anchor bolt.

APPROACH: The difference between the cold water temperature in F, and the ambient or inlet wet-bulb temperature in F.

ATMOSPHERIC TOWER: A type of cooling tower in which air movement through the tower is dependent upon atmospheric conditions and not by mechanical fans.


BASIN CURB: The top level of the wall of the cold water basin-, usually the datum point from which tower elevation points are measured, including pumphead.


BAY: The area between two bents or lines of framing members.


BENT: A line of structural framework composed of columns, girts, or ties; a bent may incorporate diagonal bracing members.


BLOWDOWN: Water discharged from the system to control concentration of salts or other impurities in the circulating water.


BRAKE HORSEPOWER (BHP): The actual power output of an engine or a motor.

BRITISH THERMAL UNIT (BTU : The quantity of heat required to raise the temperature of one pound of water one *F.



CASING: An enclosing sidewall or endwall of a tower, exclusive of the louvers.

CELL: The smallest tower subdivision which can function as an independent unit with regard to air and water flow; it is bounded by exterior walls or partitions. Each cell may have one or more fans and one or more distribution systems.

CELL DIMENSIONS: WIDTH: dimension perpendicular to tower longitudinal axis-, LENGTH: dimension parallel to longitudinal axis; HEIGHT: distance from basin curb to the top of the fan deck but not including the fanstack. Nominal width and length are measured from the column centerlines.

CIRCULATING WATER RATE: The quantity of hot water entering the tower.

COIL SHED: Portion of the tower structure housing tubular heat exchangers.

COLD WATER BASIN: An open-topped structure installed under the tower which collects the cold water from the tower and direct its flow to the suction line or pump suction sump.

COLD WATER TEMPERATURE (CWT): The temperature of the water entering the cold water basin, in F before addition of makeup or removal of blowdown.

COLUMN: Framework member; a main vertical supporting member in the tower framework.



CONCRETE COOLING TOWER: Cooling tower in which the structure is constructed of concrete.


COUNTERFLOW TOWER: One in which air flows upward through the fill material as the water to be cooled flows downward.

CROSSFILOW TOWER: One in which air flows horizontally across the fill section and crosses the falling hot water at right angles.


CYCLES OF CONCENTRATION: Compares dissolved solids in makeup water with solids in the circulating water. Since chlorides are soluble in water, for example, the cycles of concentration are equal to the ratio of chlorides in circulating water to chlorides in the make-up water.


DECK STRINGER: Part that holds the splash bars of the fill deck in a fixed position with respect to the air and water flow.

DECK SUPPORT: A horizontal member supporting the fill decks.

DESIGN CONDITIONS: Defined as the hot water temperature (HWT), cold water temperature (CWT), gallons per minute (GPM) and wet-bulb temperature (WBT) in Mechanical Draft Towers. In Natural Draft Towers: HWT, CWT, GPM, WBT, plus either dry bulb temperature (DBT) or relative humidity (RH).

DESILTING SUMP: The area in the cold water basin, usually at a low point, where silt can be flushed to a drain.

DIAGONAL: In wood towers a load bearing framework member transmitting forces at other than a right angle with reference to columns or horizontal ties. Also known as BRACE. In natural draft towers an inclined framework member, generally a column that supports the hyperbolic shell.

DIFFUSION DECK: A fill deck that is located directly under the distribution basin or nozzle bank. The purpose of this deck is to receive water from the basin or nozzle and distribute it uniformly over the fill decks.

DISCHARGE STACK: A walled enclosure extending upward above the drift eliminators to direct exhaust air vertically away from fans in a forced draft tower. See FANSTACK for operation in Induced Draft Towers.

DISTRIBUTION BASIN: A shallow pan-shape basin used to distribute hot water over the tower fill.

DISTRIBUTION BOX: Used in conjunction with the manifold and valve assembly in a crossflow Tower to disperse the hot water uniformly in all directions across the distribution basin thereby increasing the effectiveness of the distribution nozzles.


DISTRIBUTION SYSTEM: Parts of a tower, beginning with the inlet connection, which distribute the hot circulating water across the tower plan area above the fill. In a counterflow tower, this includes the main header, the lateral pipes, and the distribution nozzles. In a crossflow tower, the system includes the header or manifold, valves, distribution box, basin, and nozzles.

DOUBLE-FLOW WATER COOLING TOWER: A crossflow tower with two fill sections and one plenum chamber which is common to both.

DRIFT: Water lost from the tower as liquid droplets entrained in the exhaust air. It is independent of water lost by evaporation. Units are usually as a percentage of circulating water flow. Drift eliminators control this loss from the tower.

DRIFT ELIMINATOR: A baffle-like assembly constructed of wood, plastic, cement fiber board, steel, or other material which serves to remove entrained moisture from the discharge air.

DRIVER: Primary drive for the fan drive assembly. It may be an electric motor, gas engine, steam turbine, hydraulic motor, or other power source.

DRIVESHAFT: A device including couplings for transmitting torque from the driver to the speed reducer.

DRY BULB TEMPERATURE (DBT): The temperature of the inlet or ambient air adjacent to the cooling tower as measured by a dry-bulb thermometer, in F.



END WALL: The wall on the end of the tower structure.

ENTERING AIR: Air from the atmosphere surrounding the cooling tower which is drawn into the tower on an Induced Draft Tower or is discharged into the tower by a fan on a Forced Draft Tower.

ENTERING (INLET) WET-BULB TEMPERATURE: Average wet-bulb temperature of the entering air, in F. It includes any effects of recirculation.

EVAPORATION LOSS: Water evaporated from the circulating water into the atmosphere during the cooling process.

EXHAUST AIR: The mixture of air and its associated vapor leaving the tower. See AIR FLOW.

EXHAUST WET-BULB TEMPERATURE: Average wet-bulb temperature of the air discharged from the tower. See AIR FLOW.

EXIT DIAMETER: Diameter of the shell at the top in a hyperbolic Natural Draft Tower. In an induced draft tower, the inside diameter at the top of the fanstack.

FAN: A device for moving air in a Mechanical Draft Tower. The fan can be applied as induced draft or forced draft.

FAN DECK: The surface enclosing the top of an Induced Draft Tower. In a Counterflow Tower, the fan deck covers the top surface of the tower except for the fan area. In a Crossflow Tower, the fan deck covers only the tower plenum area, leaving the distribution system exposed. An extended fan deck encloses the distribution basins.

FAN DRIVER ASSEMBLY: Mechanical components furnishing power to the fan, usually consisting of an electric motor, drive shaft, speed reducer, and supporting members.

FAN DRIVER INPUT: Horsepower input to the driver. For 3-phase alternating current (AC) motors: HP= (Amps x Volts x 1.73 x Power Factor x Efficiency) / 746

FAN DRIVER OUTPUT: Brake horsepower output of the driver to the drive shaft. Fan driver input multiplied by motor efficiency.

FAN GUARD: A protective screen installed either at the inlet of a forced draft fan or at the exit of an induced draft fan.

FAN PITCH: The angle a fan blade makes in degrees from horizontal to effect the flow of air through the tower.

FANSTACK: Also known as a SHROUD. Cylindrical structure in which the fan rotates. Fanstacks are used on induced draft and forced draft towers and may be constructed of fiberglass, concrete or wood.

FANSTACK HEIGHT: Distance from the top of the fan deck to the top of the fan stack.

FILL: The medium installed below the nozzles upon which the warm water is sprayed. The fill area is where the greatest portion of cooling in the tower occurs. Fill provides the required surface area on which the water flows as a film (film fill) or as droplets (splash fill) for the cooling service. Fill may be manufactured of PVC, wood, fiber-cement, steel, or other materials.

FILL BARS: The assembly of splash bars comprising the tower fill. Fill bars intercept the downward fall of water at regular intervals, forming splash surfaces causing water drops to break into smaller droplets, and provide the wetted surfaces for air-water contact.

FILL DECK: The assembly of splash bars comprising the tower fill in a Crossflow Tower. See FILL BARS for description of operation.

FILL HANGER: Support system in a tower to hold the fill in place.


FLOAT VALVE: A valve that is actuated by a float, generally used to control make-up water supply into the water collection basin.

FLOW CONTROL VALVE: A manually controlled valve generally located in the hot water supply line.

FLUME: A trough that may be totally enclosed or open at the top. Flumes are used in some cooling tower designs to distribute the hot water to the lateral pipes.

FOGGING: A fog condition created when the exhaust air from a cooling tower, which is essentially a saturated air-water vapor mixture warmer than the ambient air, becomes supersaturated so that part of the water vapor condenses into visible liquid droplets.

FORCED DRAFT COOLING TOWER: A type of Mechanical Draft Tower in which one or more fans are located at the air inlet to force air into the tower.

FOUNDATION: The support beneath the tower. This may be prepared earth or a steel, wood or concrete structural assembly.

FRAMEWORK MEMBERS: The structural members designed to support all live and dead loads. They consist of columns, horizontal ties, diagonals, and joists and beams.

GEAR REDUCER: Also known as SPEED REDUCER. A speed reducing transmission, used on fan drives to reduce the driver speed to fan speed requirements.


GPM (abbr.): Gallons per minute.

HANDRAIL: A horizontal or sloping rail placed along an access way or at the edge of a plafform or long a stairway.

HEADER: In a Counterflow Tower, the main pipe carrying hot water to a series of lateral pipes for distribution over the fill material. In a Crossflow Tower, it is the main pipe carrying hot water to the distribution basin of each cell.

HEAT EXCHANGER: A device for transferring heat from one substance to another. Heat transfer can be by direct contact, as in a cooling tower, or indirect, as in a shell and tube condenser.

HEAT LOAD: Heat removed from the circulating water within the tower. It may be calculated from the range and the circulating water flow. Unit: Btu/hr. = G.P.M. x 500 x (H.W.T. - C.W.T.).

HORIZONTAL TIE: Framework members; a load-bearing horizontal connection member in the tower framework. Also known as a GIRT.

HOT WATER TEMPERATURE (HWT): Temperature in OF. of circulating water entering the distribution system.

HYDROGEN ION CONCENTRATION: A scale for expressing acidity or alkalinity of the circulation or make-up water. A pH below 7.0 indicates acidity and above 7.0 indicates alkalinity. A pH of 7.0 is neutral.

HYPERBOLIC TOWER: A cooling tower of hyperbolic shape that depends on natural draft for air movement through the tower.

INDUCED DRAFT COOLING TOWER: A type of Mechanical Draft Tower in which the fans are located in the exhaust air to induce airflow through the air inlet.


INLET CONNECTION: Also known as INLET FLANGE. The fitting to which the circulating water supply piping, usually a riser, is connected to serve the tower distribution system.

INLET WET-BULB TEMPERATURE: The average of the wet-bulb temperatures obtained from several stations located close to, and uniformly spaced with respect to, the air inlet areas of the tower. Both the windward and leeward sides of the tower would be measured in this case.

JOISTS: Supports for the fan decking.

LADDER: A vertical ladder from grade to the fan deck used for emergency tower access or egress.

LANDING: A platform designed to provide offset clearances for stairways, ladders, or other access arrangements.

LATERAL PIPE: A pipe or flume distributing water from the distribution header to nozzles, or from other points of discharge to the fill area.

LONGITUDINAL: Always measured parallel to air intakes and perpendicular to airflow.

LINTEL: A structural member in a natural draft tower where the diagonals connect to the shell. Generally, the top of the air inlet opening.

MAKE-UP: Water added to the circulating water system to replace water lost from the system by evaporation, drift, blow-down, and leakage.

MANIFOLD: The main header pipe in a Crossflow Tower.

MECHANICAL EQUIPMENT SUPPORT: Structural framework members that comprise the primary support for the fan drive assembly.

MECHANICAL DRAFT COOLING TOWER: A tower through which air movement is effected by fans. There are two main types: Forced draft with fans located in the air inlet; Induced draft with the fans located in the warm air exhaust.

MOTOR RATED HORSEPOWER: Horsepower rating inscribed on nameplate of the motor driving the fan. Also called "Nameplate" horsepower.

NATURAL DRAFT COOLING TOWER: One in which air movement is dependent upon the difference in density between the entering air and the warmed, less dense internal air. As the heat of the water is transferred to the air passing through the tower, the warmed air tends to rise and draw in fresh air at the base of the tower.

NET EFFECTIVE VOLUME: That portion of the total structural volume within which the circulating water is in intimate contact with the airflow through the tower.

NOMINAL TOWER DIMENSIONS: Width and length measured from column to column centerline or walls; height measured from the top of the basin curb to the top of the fan deck (counterflow towers) or to top of distribution basin (crossflow towers).

NOZZLE: A device for controlled distribution of water in a cooling tower. Nozzles are designed to deliver water in either a spray pattern by pressure or a solid stream by gravity flow. Also referred to as a "sprayer."

OBSTRUCTION LIGHTS: Warning lights required by FAA regulations, placed on the outside of the hyperbolic shell of a Natural Draft Tower.

OVERALL TOWER DIMENSIONS: (A) Width: overall dimensions perpendicular to the towers longitudinal axis; (B) Length: overall dimension along the longitudinal axis; (C) Height: distance from the top of the basin curb to the top of the fan stack.


PARTITION WALL: An interior wall sub-dividing the tower into cells or into separate fan plenum areas.

PEDESTALS: Structural concrete components used as a transition from the shell support diagonal columns to the foundation of a Natural Draft Tower.

PH: See HYDROGEN ION CONCENTRATION. PIER: In a wood tower, a column support used to elevate column footings above the basin floor or foundation grade.


PITOT TUBE: An instrument that operates on the principle of differential pressures in a flow of fluid (Total Pressure = Static Pressure + Velocity Pressure). The primary use on cooling towers is the accurate measurement of circulating water rate.

PLENUM: The enclosed space between the drift eliminators and the fan stack in induced draft towers or the enclosed space between the fan and the fill in forced draft towers.

PLUME: Visible exhaust from a cooling tower. See FOGGING.

POWER FACTOR: The ratio of true power (Wafts) to the apparent power, as indicated by the product of amps x volts.

PSYCHROMETER: An instrument used primarily to measure the wet bulb temperatures. A mechanically aspirated type of psychrometer is acceptable provided the instrument is properly shielded from radiation and the air across the wick is limited to approximately 1,000 ft./min. (See WET-BULB TEMPERATURE). Note: a drywick instrument will read Dry-Bulb Temperature.

RANGE: The difference between the hot water temperature and the cold water temperature. Unit: F.

RATED HORSEPOWER: Nameplate horsepower of fan drivers other than electric motors.

RECIRCULATION: This term describes a condition in which a portion of the discharge air enters the tower along with the fresh air. The amount of recirculation is determined by tower design, tower placement, and atmospheric conditions. The effect is generally evaluated on the basis of the increase in the entering wet bulb temperature compared to the ambient.

REDISTRIBUTION BASIN: An elevated basin installed between the hot water basins in a Crossfflow Tower to maintain correct water distribution throughout the entire height of the fill.

RELATIVE HUMIDITY: The amount of water vapor in the air at a given temperature and atmospheric pressure as compared with the greatest amount of water vapor that air can hold at that temperature and pressure. The ratio is generally expressed in pressure.

RISER: Piping which connects the circulating water supply line from the level of the base of the tower to the supply header of the tower inlet connection.

SAFETY HANDRAIL: Railing around top of tower, platform, and stairways; usually composed of top handrail, kneerail, and toeboard. SHELL DIAMETER: Diameter of the shell in a Natural Draft Tower measured at the top of the curb.

SHELL HEIGHT: Dimension from the top of the curb to the top of the hyperbolic shell in a Natural Draft Tower.


SINGLE-FLOW WATER COOLING: A Crossflow Tower having a fill section on one side of the plenum chamber only.


SPLASH BAR: Horizontal component of a fill deck that constitutes the principal splash surface.

SPRAY-FILLED COOLING TOWER: A tower that has no fill, and water-to-air contact depends entirely on the break-up of the water by means of pressure spray nozzles.

SPRAY NOZZLE: Device used in a distribution system to break up the flow of the circulating water into droplets and effect uniform spreading of the water over the wetted areas of the tower.

STAIRWAY: Typical tread-and-riser structure providing access to and from the top of the tower.

STANDARD AIR: Dry air having a density of 0.075 Ibs/ft3 at 70F and 29.92 inches Hg.

STATIC PRESSURE: The pressure of a gas or fluid in a system as referred to a state of rest or lack of motion. Static pressure is equal to total pressure minus velocity pressure. Unit: inches Hg.

SUMP: Also known as BASIN SUMP. Lowest portion of the basin to which cold circulating water flows, usually the point of suction connection.

SUPPLY HEADER: Portion of the water supply system from which the riser or inlet connection receives the circulating water flow.

THERMAL CAPACITY: Also known as CAPACITY. The number of gallons per minute (GPM) a cooling tower will handle for a specified range, wet-bulb temperature, and approach.

THROAT DIAMETER: Diameter of the shell in a Natural Draft Tower at its narrowest point.

TOTAL PUMPING HEAD: The total head of water, measured above the basin curb, required to deliver the circulating water through the distribution system.


TRANSVERSE: Always measured perpendicular to air intakes.

UNDERIFLOW WATER COOLING TOWER: A Crossflow Tower with a forced draft fan located in a horizontal plane at the bottom of the plenum chamber. Air from the plenum travels horizontally through fill sections and discharges vertically at each end of the tower.

VELOCITY RECOVERY FANSTACK: A venturi-shaped fanstack. The increase in performance obtained on this type of stack is due to the reduction in total head above the fan. The attendant decrease in total head differential across the fan results in an increase in air rate at constant horsepower or a decrease in horsepower at constant air rate.

VIBRATION CUT-OUT SWITCH: Safety switch used to de-energize the driver circuit should the mechanical equipment assembly vibrate excessively.

WATER COOLING TOWER: An enclosed, steady-flow device for cooling water by evaporation through direct contact with air.

WATER LOADING: Circulating water flow expressed in gallons per minute (GPM) per square foot of effective horizontal wetted area of the cooling tower.

WET-BULB TEMPERATURE (WBT): Temperature indicated by a psychrometer. Also known as the thermodynamic wet-bulb temperature or the temperature of adiabatic saturation.

WET-DRY TOWER: A wet (evaporative) cooling tower in combination with a dry (nonevaporative) heat exchanger section, used to reduce or abate the visible exhaust plume or fogging that occurs during cold weather by reducing the moisture in the cooling tower exhaust air

WIND LOAD: The load applied to a structure by a wind blowing against its surface. Wind load is expressed in lb/ft2

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