Glossary of Terms

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

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 and/or repair, cost of land use, 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. Units: % of circulating water rate or gpm.

Exhaust (Exit) Wet-Bulb Temperature — See "Leaving Wet-Bulb Temperature".

Fan Cylinder — Cylindrical or venturi-shaped structure in which a propeller fan operates. Sometimes referred to as a fan "stack" on larger towers.

Fan Deck — Surface enclosing the top structure of an induced draft cooling tower, exclusive of the distri-bution 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. Unit: degrees.

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. Sometimes referred to as "packing".

Fill Cube —

  1. Counterflow: The amount of fill required in a volume one bay long by one bay wide by an air travel high. Unit: cu ft.
  2. Crossflow: The amount of fill required in a volume one bay long by an air travel wide by one story high. Unit: cu ft.


Fill Deck — One of a succession of horizontal layers of splash bars utilized in a splash-filled 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-filled 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.

Film-Filled — Descriptive of a cooling tower in which film-type fill is utilized for the primary heat-transfer surface.

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 trough 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.

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.

Geareducer® — See "Speed Reducer".

Heat Load — Total heat to be removed from the circulating water by the cooling tower per unit time. Units: Btu per min. or Btu per hr.

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. Heights for towers on which a wood, steel, or plastic basin is included within the manufacturer's scope of supply are generally measured from the lowermost point of the basin, and are usually overall of the tower. Unit: ft.

Hot Water Temperature — Temperature of circulating water entering the cooling tower's distribution sys-tem. Unit: °F. Symbol: HW.

Hydrogen Ion Concentration — See "pH".

Induced Draft — Refers to the movement of air through a 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.

Inlet Wet-Bulb Temperature — See "Entering Wet-Bulb Temperature".

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 discharged from a cooling tower. Unit: °F. Symbol: LWB.

Length — For crossflow towers, length is always per-pendicular to the direction of air flow through the fill (air travel), or from casing to casing. For counterflow 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. Unit: ft.

Liquid-to-Gas Ratio — A ratio of the total mass flows of water and dry air in a cooling tower. (See Total Air Rate & Total Water Rate) Unit: Ib per Ib. Symbol: L/G.

Longitudinal — Pertaining to occurrances 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 splashout and/or promote uniform air flow through the fill. In the case of film-type crossflow fill, they may be integrally molded to the fill sheets.

Make-Up — Water added to the circulating water system to replace water lost by evaporation, drift, windage, blowdown, and leakage. Units: % of circulating water rate or gpm.

Mechanical Draft — Refers to the movement of air through a cooling tower by means of a fan or other mechanical device.

Module — A preassembled portion or section of a cooling tower cell. On larger factory-assembled towers, two or more shipped 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 the total struc-tural volume within which the circulating water is in intimate contact with the flowing air. Unit: cu ft.

Nozzle — A device used for controlled distribution of water in a cooling tower. Nozzles are designed to de-liver water in a spray pattern either by pressure or by gravity flow.

Packing — See "Fill".

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.

Performance — See "Capacity".

pH — A scale for expressing acidity or alkalinity of the circulating or make-up water. A pH below 7.0 indi-cates acidity and above 7.0 indicates alkalinity. A pH of 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 the 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) discharged from a cooling tower.

Psychrometer — An instrument incorporating both a dry-bulb and a wet-bulb thermometer, by which si-multaneous dry-bulb and wet-bulb temperature read-ings can be taken.

Pump Head — See "Tower Pumping Head".

Range — Difference between the hot water temperature and the cold water temperature (HW - CW) Unit: °F.

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 induce air flow through a natural draft tower. Sometimes referred to as a "stack" or "veil".

Slowdown — Water discharged from the system to control concentrations of salts or other impurities in the circulating water. Units % of circulating water rate or gpm.

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 for the fan. The use of geared reduction units predominates in the cooling tower industry, although smaller towers will utilize differen-tail pulleys and V-belts for the transmission of relatively low power.

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 bars may be flat, or may be formed into a shaped cross-section for improved structural rigidity and/or im-proved heat transfer capability. When flat, they are sometimes referred to as "slats" or "lath".

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

Spray-Filled — Descriptive of a cooling tower 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 pur-pose is to achieve elevation of the discharge plume. Also see "Fan Cylinder" and "Shell".

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 Ib per cu ft. Essentially equivalent to 70°F 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 characteristics of the framework material used. Unit: ft.

Sump — A depressed chamber either below or alongside (but contiguous to) the collection basin, into which the water flows to facilitate pump suction. Sumps may also be 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. Unit: Ib per hr. Symbol: G.

Total Water Rate — Total mass flow of water per hour through the tower. Unit: Ib per hr. Symbol: L

Tower Pumping Head — The static lift from the elevation of the basin curb to the centerline 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. Unit: ft of water.

Transverse — Pertaining to occurrances 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. Unit: gpm per sq ft.

Water Rate — Mass flow of water per square foot of fill plan area of the cooling tower per hour. Unit: Ib per sq ft per hr. Symbol: L'.

Wet-Bulb Temperature — The temperature of the entering or ambient air adjacent to the cooling tower as measured with a wet-bulb thermometer. Unit: °F. Symbol: WB.

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. Sometimes called "blowout".

Wind Load — The load imposed upon a structure by a wind blowing against its surface. Unit: lb per sq ft.

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