Cooling Tower Definitions



The actual volumetric flow rate of air-vapor mixture.

Air Horsepower  – 

The power output developed by a fan in moving a given air rate against a given resistance.

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.

Air travel – 

Distance which air travels in its passage through the fill. Measured vertically on counter flow towers and horizontally on cross flow towers.

Air velocity – 

Velocity of air-vapor mixture through a specific region of the tower (i.e. the fan).

Ambient wet-bulb temperature – 

The wet bulb temperature 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 extraneous sources of heat.

Approach – 

Difference between the cold water temperature and either the ambient or entering wet-bulb temperature.

Atmospheric – 

Refers to the movement of air through a cooling tower purely by natural means, or by the aspirating 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 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 longitudinal framing bents.

Bent – 

A transverse or longitudinal line of structural framework composed of columns, grid, ties, and diagonal bracing members.

Blowdown – 

Water discharged from the system to control concentrations of salts and other impurities in the circulating water.

Blower – 

A squirrel-cage (centrifugal) type fan; usually applied for operation at higher than- normal static pressures.

Brake Horsepower – 

The actual power output of a motor, turbine, or engine.

Btu (British thermal unit) – 

The amount of heat gain (or loss) required to raise (or lower) the temperature of one pound of water 1oF.

Capacity – 

The amount of water (gpm) that a cooling tower will cool through a specified range, at a specified approach and wet-bulb temperature.

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 distribution systems.

Circulating water rate –

Quantity of hot water entering the cooling tower.

Cold water temperature – 

Temperature of the water leaving the collection basin, exclusive of any temperature effects incurred by the addition of make-up and/or the removal blowdown.

Collection basin – 

Vessel below and integral with the tower where water is transiently collected and directed to the sump or pump suction line.

Counter flow – 

Air flow direction through the fill is counter-current to that of the falling water.

Cross flow – 

Air flow direction through the fill is essentially perpendicular to that of the falling water.

Cycles of concentration (C.O.C) – 

The ratio of dissolved solids in circulating water to the dissolved solids in make up 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 cross flow 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 opposed fill banks are served by a common air plenum.

Drift – 

Circulating water lost from the tower as liquid droplets entrained in the exhausted air stream.

Drift eliminator – 

An assembly of baffles or labyrinth passage 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 device for the fan drive assembly. Although 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.

Entering wet-bulb temperature – 

The wet-bulb temperature of the air actually entering the tower, including any effects of recirculation. In testing, the average of multiple readings taken at the air inlets to establish a true entering wert-bulb temperature.

Evaluation – 

A determination of the total cost of owning a cooling tower for a specific period of time. Includes first cost of tower and attendant devices, cost of operation, cost of maintenance, cost of financing, etc., all normalized to a specific point in time.

Evaporation loss – 

Water evaporated from the circulating water into the air stream in the cooling process.

Fan cylinder – 

Cylindrical or venturi-shaped structure in which a propeller fan operates.

Fan deck – 

Surface enclosing the top structure of an induced draft cooling tower, exclusive of the distribution basins on a crossflow tower.

Fan pitch – 

The angle which the blades of a propeller fan make with the plane of rotation, measured at a prescribed point on each blade.

Fan scroll – 

Convolute housing in which a centrifugal (blower) fan operates.

Fill – 

That portion of a cooling tower which constitutes its primary heat transfer surface.

Fill cube – 

(1) Counterflow: the amount of fill required in a volume one bay long by

one bay wide by an air travel high. 

(2) Crossflow: The amount of fill required in a volume one bay long by an air travel wide by one story high.

Fill deck – 

One of a succession of horizontal layers of splash bars utilized in a splash fill cooling tower. The number of fill decks constituting overall fill height, as well as the number of splash bars incorporated within each fill deck, establishes the effective primary heat transfer surface.

Fill sheet – 

One of a succession of vertically-arranged, closely-spaced panels over which flowing water spreads to offer maximum surface exposure to the air in a film-fill cooling tower. 

Sheets may be flat, requiring spacers for consistent separation; or they may be formed into corrugated, chevron, and other patterns whose protrusions provide proper spacing, and whose convolutions provide increased heat transfer capability.

Float valve – 

A valve which is mechanically actuated by a float. Utilized on many cooling towers to control make-up water supply.

Flow-control valves – 

Manually controlled valves which are used to balance flow of incoming water to all sections of the tower.

Flume – 

A through which may be either totally enclosed, or open at the top. Flumes are sometimes used in cooling towers for primary supply of water to various sections of the distribution system. Flumes are also used to conduct water from the cold water basins of multiple towers to a common pumping area or pump pit.

Fogging – 

A reference to the visibility and path of the effluent air stream after having exited the cooling tower. If visible and close to the ground, it is referred to as “fog”. If elevated, it is normally called the “plume”.

Forced draft – 

Refers to the movement of air under pressure through a cooling tower. Fans of forced draft towers are located at the air inlets to “force” air through the tower.

Heat load – 

Total heat to be removed from the circulating water by cooling tower per unit time.

Height – 

On cooling towers erected over a concrete basin, height is measured from the elevation of the basin curb. “Nominal” heights are usually measured to the fan deck elevation, not including the height of the fan cylinder.

Hot water temperature – 

Temperature of circulating water entering the cooling tower by means of an induced partial vacuum. Fans of induced draft towers are located at the air discharges to “draw” air through the tower.

Interference – 

The thermal contamination of a tower’s inlet air by an external heat source (i.e. the discharge plume of another cooling tower).

Leaving wet-bulb temperature – 

Wet-bulb temperature of the air discharge from a cooling tower.

Length – 

For cross flow towers, length is always perpendicular to the direction of air flow through the fill (air travel), or from casing to casing. 

For counter flow towers, length is always parallel to the long dimension of a multi-cell tower, and parallel to the intended direction of cellular extension on single-cell towers.

Liquid-to-gas ratio – 

A ratio of total mass flows of water and dry air in a cooling tower.

Longitudinal – 

Pertaining to occurrences in the direction of tower length.

Louvers – 

Blade or passage type assemblies installed at the air inlet face of a cooling tower to control water splash out and/or promote uniform air flow through the fill. In the case of film-type cross flow fill, they may integrally molded to the fill sheets.

Make-up – 

Water added to the circulating water system to replace water lost by evaporation, drift, windage, blow down, and leakage.

Mechanical draft – 

Refers to the movement of air through cooling tower by means of a fan or other mechanical devices.

Module – 

A pre assembled portion or section of a cooling tower cell. On larger factory assembled towers, two or more shipping modules may require joining to make a cell.

Natural draft – 

Refers to the movement of air through a cooling tower purely by natural means. Typically, by the driving force of a density differential.

Net effective volume – 

That portion of total structural volume within which the circulating water is in intimate contact with the flowing air.

Nozzle – 

A device used for controlled distribution of water in cooling tower. 

Nozzles are designed to deliver water in a spray pattern either by pressure or gravity flow.

Partition – 

An interior wall subdividing the tower into cells or into separate fan plenum chambers. Partitions may also be selectively installed to reduce windage water loss.

pH – 

A scale for expressing acidity or alkalinity of the circulating or make-up water. 

A pH below 7.0 indicates acidity and above 7.0 indicates alkalinity. A pH 7.0 indicates neutral water.

Pitot tube – 

An instrument that operates on the principle of differential pressures. Its primary use on a cooling tower is in measurement of circulating water flow.

Plenum chamber – 

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

Plume – 

The effluent mixture of heated air and water vapor (usually visible) discharge from a cooling tower.

Psychrometer – 

An instrument incorporating both a dry-bulb and a wet-bulb thermometer, by which simultaneous dry-bulb and wet-bulb temperature readings can be taken.

Range – 

Difference between the hot water temperature and the cold water temperature (HW-CW).

Recirculation – 

Describes a condition in which a portion of the tower’s discharge air re-enters the air inlets along with the fresh air. Its effect is an elevation of the average entering wet-bulb temperature compared to the ambient.

Riser – 

Piping which connects the circulating water supply line, from the level of the base of the tower or the supply header, to the tower’s distribution system.

Shell – 

The chimney-like structure, usually hyperbolic in cross-section, utilized to induced air flow through a natural draft tower.

Speed reducer – 

A mechanical device, incorporated between the driver and the fan of a mechanical draft tower, designed to reduce the speed of the driver to an optimum speed of the fan.

Splash bar – 

One of a succession of equally-spaced horizontal bars comprising the splash surface of a fill deck in a splash-filled cooling tower. Splash bar may be flat, or may be formed into shaped cross-section for improved structural rigidity and/or improved heat transfer capability.

Splash fill – 

Descriptive of a cooling tower in which splash type fill is used for the primary heat transfer surface.

Spray fill – 

Descriptive of a cooling tower in which has no fill, with water-to-air contact depending entirely upon the water break-up and pattern afforded by pressure spray nozzles.

Stack – 

An extended fan cylinder whose primary purpose is to achieve elevation of the discharge plume.

Stack effect – 

Descriptive of the capability of a tower shell or extended fan cylinder to induce air (or aid in its induction) through a cooling tower.

Standard air – 

Air having a density of 0.075 lb/cuft. Essentially equivalent to 70oF dry air at 29.92 in Hg barometric pressure.

Story – 

The vertical dimension between successive levels of horizontal framework ties, girts, joists, or beams. Story dimensions vary depending upon the size and strength characteristic of the framework material used.

Sump – 

A depressed chamber either below or along-side (but contiguous to) the collection basin, into which the water flows to facilitate pump suction. Sump may also designed as collection points for silt and sludge to aid in cleaning.

Total air rate – 

Total mass flow of dry air per hour through the tower.

Total water rate – 

Total mass flow of water per hour through the tower.

Tower pumping head – 

The static lift from the elevation of the basin curb to the center line elevation of the distribution system inlet; plus the total pressure (converted to ft of water) necessary at that point to effect proper distribution of the water to its point of contact with the air.

Transverse – 

Pertaining to occurrences in the direction of tower width. 

Velocity recovery fan cylinder – 

A fan cylinder on which the discharge portion is extended in height and outwardly flared. Its effect is to decrease the total head differential across the fan, resulting in either an increase in air rate at constant horsepower, or a decrease in horsepower at constant air rate.

Water loading – 

Circulating water rate per horizontal square foot of fill plan area of the cooling tower.

Water rate – 

Mass flow of water per square foot of fill plan area of the cooling tower per hour.

Wet-bulb temperature – 

The temperature of entering or ambient air adjacent to the cooling tower as measured with a wet-bulb thermometer.

Wet-bulb thermometer – 

A thermometer whose bulb is encased within a wetted wick.

Windage – 

Water lost from the tower because of the effects of wind.

Wind load – 

The load imposed upon a structure by a wind blowing against its surface.

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Naitik Patel
Industrial Guide

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