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Acceptance Testing - Uniform standards to determine the water cooling capability of cooling towers. Instrumentation and measurements are performed in accordance with methods outlined in the Cooling Tower Institute (CTI) Bulletin Acceptance Test Procedure (ATP) 105. Testing normally is done by third party - not the Owner or Contractor. A confidential report is submitted to the person paying for testing and the Owner. 

Access Tunnel - Opening in Natural Draft type cooling tower used for access to the cold water basin. It can be large enough for a man only, or large enough for mechanical equipment.

Air Flow - Total amount of dry air and associated water vapor flowing through the tower, measured at exhaust from the tower and converted to standard air which has a density of 0.075 lb. per cu. ft.

Air Horsepower - The measure of useful power required to move a given air rate against a given resistance. The ratio of air horsepower to fan input horsepower is the measure of fan efficiency.

Air lnlet - Area of cooling tower where air is initially introduced into unit by action of airflow generator. 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. Usually measured vertically on a Counterflow tower and horizontally on a Crossflow tower.

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

Algae - A low form of plant life, which generally requires sunlight and air for existence. Algae can cause plugging of the heat exchanger tubes and cooling tower distribution systems.

Algaecide - A toxic substance which will retard or prevent the growth of algae.

Alignment - The placement of two or more machines, such that when complete, the shaft centerlines of the machines are collinear.

Ambient Temperature - External outdoor temperature as reported by periodic readings, also known as the dry bulb temperature, measured in the regular manner with conventional instruments.

Ambient Wet-Bulb Temperature - Wet bulb temperature of air measured windward of the tower and free from the influence of the tower. Generally, measured upwind of a tower in a number of locations sufficient to account for all extraneous sources of heat.

Approach - Temperature difference in degrees between the cold water leaving the tower and the wet bulb
temperature of the air entering the cooling tower.

Anchor Bolt - A threaded bolt imbedded in a concrete basin or fitted to supporting members, to which an anchor casting is attached.

Anchor Casting or Bracket - A device or clip for attaching the tower structure to the foundation; it does not include the anchor bolt.

Atmospheric - Refers to the movement of air through a cooling tower purely by natural means, or by the aspirating effect of water flow.

Atmospheric Tower - One in which air movement is dependent upon atmospheric conditions, not on mechanical fans, to force or induce air through the tower. The hot moist air rises in the chimney pulling in colder outside air through the intake louvers.

Automatic Variable Pitch Fan - A propeller type fan whose hub incorporates a mechanism that enables the fan blades to be re-pitched simultaneously and automatically. Sensors and pneumatic controllers change fan blade angles to utilize minimum HP depending upon the varying heat rejection load required at any given time. Much like a Variable Frequency Drive (VFD), they are used on cooling towers and air-cooled heat exchangers to trim capacity and/or conserve energy.

Axial Fan - Fans characterized by flow through the impeller, which is generally parallel to the shaft axis.

Axial Flow - Air movement parallel to spinning axis generated by rotating fan blades.



Basin - Bottom collection area for the cold water returning to the suction line. Or on a Crossflow type cooling tower an additional uppermost hot water distribution basin and in some few cases, an intermediate hot water distribution basin is located halfway between the uppermost basin and the bottom collection basin.

Basin Curb – Top level of the cold-water basin retaining wall; usually the datum point from which tower elevation points and pumping head is measured.

Bay - The area between adjacent transverse and longitudinal framing bents.

Belt - An elastomeric band which is attached to two or more machines, used to transmit power by rotation.

Belt Drive - A power transfer mechanism consisting of belts and sheaves, or pulleys.

Belt Guard - A component designed to cover a belt drive mechanism in order to prevent human injury.

Bent or Bent Line - A transverse or longitudinal line of structural framework composed of columns, horizontal ties, and diagonal bracing members.

Bevel Washer - A metal fitting used to accommodate through- bolts to angular position of a diagonal member, usually connecting to a column or other framework members.

Blowdown (Purge) - Water discharged from the system to control concentrations of salts or other impurities in the circulating water due to intensification of solids by water evaporation loss.

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

Blow out - Water that is blown or pulled out of the air inlet by wind.

Board Foot or Board Measure - A unit of measurement of lumber represented by a board 1” thick by 12” long by 12” wide, or its cubic equivalent. In practice, the board foot calculation for lumber 1 in. or more in thickness is based on its nominal thickness and width and the actual length. Lumber with a nominal thickness of less than 1 in. is calculated as 1 in.

Bolt-Bound - A problem that arises when the piece of equipment you wish to move cannot be adjusted any further horizontally because the foot is hitting the hold down bolt.

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 one degree F.


Canopy - On a Crossflow type natural draft cooling tower the canopy connects the hyperbolic shell to the cooling section, acting as an air conduit and air seal between the two.

Capacity - The amount of water (GPM) that a cooling tower will cool through a specified range, at a specified approach and wet-bulb temperature. Expressed in terms of gallons per minute (GPM)  

Casing - Exterior enclosing wall for the longitudinal walls and the endwall of a Counterflow type tower and/or the endwall of a Crossflow type cooling tower exclusive of the air inlet louvers.

Cell - One complete unit of a cooling tower designed to operate independently of its neighbors. 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.

Cell Dimensions - (1) Width: dimension perpendicular to tower longitudinal axis and usually at right angles to the louver area; (2) Length: dimension parallel to longitudinal axis and the plane where louvers are usually placed; (3) Height: distance from basin curb to top of fan deck but not including fan stack. Nominal width and length are measured from and to the column centerlines. (See Nominal Tower Dimensions).

Cellular Fill - The most efficient fill packing developed. Unlike splash bars the circulating water is spread into a thin film throughout the cells of the fill thereby cooling a larger surface area for the same energy

Centrifugal Fan - Forced draft or blower wheel (sometimes called a “squirrel cage”) causing air movement at right angles to rotating impeller axis.

Characteristic – KaY/L (Crossflow) or KaV/L (Counterflow) is a measurement of the order of difficulty for the cooling tower heat rejection requirements exclusive of the fill. The characteristic is located on the vertical or “Y” scale, the ordinate. The value is expressed as a dimensionless number and is usually obtained by locating the Liquid to Gas ratio and intersection of the approach curve. Integration of the Tchebrycheff Formula will also locate the value on the vertical trigonometric scale.

Cellulose - The carbohydrate that is the principle constituent of wood and forms the framework of the wood cells.

Check - A lengthwise separation of the wood that usually extends across the rings of annual growth.

Circulating Water Rate - Quantity of hot water entering the cooling tower expressed in terms of gallons per minute (GPM).

Coil Shed or Coil Section - Portion of the tower structure housing atmospheric (tubular) heat exchangers.

Cold Water Temperature (CWT) – The temperature of the water entering the cold water collection basin before the addition of make-up or the removal of blowdown or the temperature of the water leaving the collection basin, exclusive of any temperature effects incurred by the addition of make-up and/or the removal of blowdown.

Cold Water Collection Basin - Vessel below and integral with the tower where water is briefly collected and directed to the sump or pump suction line.

Column - Framework member; a main vertical supporting member in the tower framework.

Convection - Transference of heat by the circulation of the heated parts of gas or liquid whereby heated portions are lighter and rise vertically.

Cooling Factor - L/G (L over G)—Ratio of the pounds of water being circulated per unit time to the pounds of dry air per unit time, which are cooling the water.

Cooling Pond - Large reservoir on inexpensive real estate whereby warm water is cooled as air contacts pond’s relatively large surface area and colder water is either reused or discharged into public waterway. 

Cooling Tower - A device used for the evaporative cooling of water by contact with air. This is achieved partially by an exchange of latent heat resulting from the evaporation of some of the circulating water, and partially by transfer of sensible heat to the air. 

Counterflow - A type of cooling tower where air movement and hot water mix at 180 degrees, or counter, to each other with the air moving vertically through the fill packing and water. The airflow direction through the fill is countercurrent to that of the falling water. 

Crossflow - A type of cooling tower where air movement and hot water mix at 90 degrees, or cross, to each other with the air moving horizontally through the fill packing and water. The airflow direction through the fill is essentially perpendicular to that of the falling water. 

Cross Over Piping (See Header) 

Cross Struts - On a natural draft cooling tower the cross struts are the framework that holds up the hyperbolic shell, the number of which will vary depending on shell size. 

Cycles of Concentration - Compares dissolved solids in make-up with solids in the circulating water. Since chlorides are soluble, for example, cycle of concentration is equal to ratio of chlorides in circulating water to chlorides in make-up.


Damper - A mechanical device that regulates the volume of air passing through a fan.

Decay - The decomposition of wood substance by fungi.


Advanced (Typical) Decay - The older stage of decay in which the destruction is readily recognized because the wood has become punky, soft and spongy, stringy, ring shaked, pitted, or crumbly. Decided discoloration or bleaching of the rotted wood is often apparent.


Brown Rot - In wood, any decay in which the attack concentrates on the cellulose and associated carbohydrates rather than the lignin, producing a light to dark brown friable residue – hence loosely termed “dry rot.” An advanced stage where the wood splits along rectangular planes, in shrinking, is termed “cubical rot.”


Dry Rot - A term loosely applied to any dry, crumbly rot but especially to that which, when in an advanced stage, permits the wood to be crushed easily to a dry powder. The term is actually a misnomer for any decay, since all lumber destroying fungi require considerable moisture for growth.


Incipient Decay - The early stage of decay that has not proceeded far enough to soften or otherwise perceptibly impair the hardness of the wood. It is usually accompanied by a slight discoloration or bleaching.


Pocket Rot - Advanced decay that appears in the form of a hole or pocket, usually surrounded by apparently sound wood.


Delignification (Soft Rot) - A special type of decay developing under very wet conditions (as in cooling

towers and boat timbers) in the outer wood layers, caused by cellulose-destroying micro-fungi that attack the secondary cell walls and not the intercellular layer. The loosening of surface fibers of softer lignite’s resulting in a weakening of timbers or splash bars, reducing the members in cross-section and strength.


White Rot - In wood, any decay or rot attacking both the cellulose and the lignin, producing a generally whitish residue that may be spongy or stringy rot, or occur as pocket rot.


Delaminating - The separation of layers in laminated wood or plywood because of failure of the adhesive,

either within the adhesive itself or at the interface between the adhesive and the adherent.





Decibel - Unit of sound level measurement, gauged by converting the pressure energy of noise to electrical impulses and transferring them to a voltmeter calibrated in decibels.

Deck Stringer (Fill) - Holds the splash bars of the fill deck in a fixed position with respect to air and water flow.

Design Conditions - The thermal parameters for which the cooling tower is purchased and/or rebuilt. Expressed as cooling a given GPM flow of water entering the tower at a specified temperature, cooling through a given range leaving the tower at the required temperature and having a designated approach to a stated wet bulb all the while consuming no more than a given amount of energy (horsepower)

De-Silting Sump - An area in the cold water basin, usually a low point, where any silt can be flushed to a drain.

Dew Point - The temperature at which a given mixture of air and water will have a relative humidity of 100% saturation.

Diagonal (Brace) - Framework member; any load-bearing member transmitting forces at other than a right angle with reference to columns or horizontal ties.

Diffusion Deck or Pad - A fill deck, grid or pad which is located directly under the distribution basin or nozzles. The purpose of this deck or pad is to receive water from the basin or nozzles and distribute it uniformly over the fill decks below.

Disc Coupling - A coupling which uses thin steel discs, bolts, and elastic bushings to connect two coupling hubs. Slight misalignment is compensated by slight bending of the discs and by elasticity of the bushings.

Discharge Stack or Hood - On a forced draft type cooling tower a walled enclosure extending upward above the Drift Eliminators to direct exhaust air vertically away from fans.  

Distribution Basin or Pan - On a Crossflow type cooling tower an elevated shallow pan-type basin used to distribute and/or re-distribute hot water over the cooling tower fill by means of orifices or nozzles in the basin floor.

Distribution Box - On a Crossflow type cooling tower a Distribution Box is used in conjunction with a manifold and valve assembly to disperse the hot water uniformly in all directions across the distribution basin thereby increasing the effectiveness of the distribution nozzles.

Distribution System - Those parts of a tower beginning with the inlet connection which distribute the hot circulating water uniformly within the tower to the points where it contacts the air. A low pressure spray system is normally used in a Counterflow tower, and a gravity system is mainly utilized in Crossflow towers. The system may include headers, laterals, branch arms, nozzles, distribution basins, and flow-regulating devices.

Doubleflow - A Crossflow type cooling tower where two opposing fill sections are served by one common air plenum chamber. 

Downspout - A short vertically placed pipe or nozzle used in a gravity distribution system to divert water from a flume or lateral to a splasher or splash plate located below.

Dressed Size - The dimensions of lumber after being surfaced with a plainer. The dressed size is usually ½ to ¾ in. less than the nominal or rough size. A 2-by-4 in. stud, for example, actually measures about 1 ½ by 3 ½ in.

Drift - Circulating water lost from the tower as liquid droplets entrained in the leaving air stream.

Drift Eliminators - An assembly of baffles, curves or cellular labyrinth passages through which the leaving air passes prior to its exit from the tower, for the purpose of stripping the entrained water droplets from the leaving air stream.

Driver - Primary drive for the fan drive assembly. Although electric motors predominate, it may also be a gas engine, steam turbine, hydraulic motor, water turbine or other power source.

Drive Shaft - A device including a coupling (or couplings) for transmitting torque from the driver to the speed reducer.

Dry-Bulb Temperature - The ambient temperature of air measured in the regular manner with conventional instruments, i.e., a dry-bulb thermometer.

Early Wood - The portion of the growth ring that is formed during the early part of the growing season. It is usually less dense and weaker mechanically than latewood. Also known as Springwood.

Elastomeric Coupling - A coupling which uses an elastomeric material, such as polymer, rubber, or neoprene, as the flexible element. Slight misalignment is compensated by the elasticity of flexible element.

Eliminator Board or Baffle - The smallest component in a wood Drift Eliminator assembly, which is usually installed in a fixed position at an angle to the direction of air flow. Sometimes referred to as a Baffle

End Wall - The wall on the end of the tower structure. 

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

Enthalpy - The total heat content, which is the sum of the sensible heat of the air and water vapor plus the latent heat of vaporization of the water. 

Entry Loss - A pressure drop caused by mechanical energy losses as air decelerates at the entrance of a duct, pipe, or fan opening. This loss can be minimized by providing a smooth rounded orifice at the entrance.

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

Evaporative Cooling - Heat transfer where a liquid condenses into its gaseous state thereby giving up latent heat.

Evasé - An expansion transition located directly on the fan discharge. It is used to convert some of the kinetic energy (velocity pressure) of the air into potential energy (static pressure).

Exhaust Air - The mixture of air and its associated vapor leaving the tower.

Exhaust (Exit) Wet-Bulb Temperature - Average wet bulb temperature of the air discharged from the tower.

Exit Basin Temperature - Temperature of the circulating water leaving the cold water basin. If blowdown is removed from or make-up water is added to the basin, the temperature will be affected accordingly.

Exit Diameter - The diameter of a fan stack at the top or on a Natural Draft type tower the diameter of the hyperbolic shell at the top.



Fan - An airfoil rotating to move (induce or force) air through a cooling tower.

Fan Cylinder or Fan Stack - Cylindrical or modified cylindrical, hyperbolic, or venturi-shaped structure in which an axial fan operates. Sometimes referred to as a “Fan Stack".  

Fan Deck - Surface enclosing the top of an induced draft cooling tower. In a Counterflow type cooling tower, the fan deck covers the entire top surface of the tower except the deck opening for fan diameter. In a Crossflow type cooling tower, the fan deck covers only the tower plenum area, exclusive of the distribution basins. An extended fan deck encloses the distribution system and covers the entire top surface of the tower, except of course, the deck opening for fan diameter.

Fan Drive Assembly - Mechanical components furnishing power to the fan, usually consisting of driver, drive shaft, speed reducer, and supporting members.

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, which the blades of a propeller fan make with the plane of rotation, degrees from horizontal, measured at a certain point on each blade.

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

Fill - That portion of a cooling tower that constitutes its primary heat transfer surface. The Fill can be a splash type or film type. Splash bars of wood, metal, plastic, or film pack (cellular fill) made of thin section. The splash type cools water droplets bouncing down in the vertical or horizontal air stream, while film packing turns the droplets into thin films for highly efficient cooling by producing a larger surface area for the air movement to cool. Sometimes referred to as "packing".

Fill Bars - The assembly of splash bars comprising the tower filling or packing. The Fill Bars intercept the downward fall of water at regular intervals, forming splash surfaces, which cause water drops to break into smaller droplets, and provide wetted surfaces for air-water contact.

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 Hanger - The Fill Hangers are part of the support system in a Crossflow type Cooling Tower supporting and holding the individual Fill Bars in place.

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.

Flexible Coupling - Generic term for a non-rigid coupling, which compensates for slight misalignment by its ability to flex, bend, slide, and/or compress that transmits the power in the fan driver to the gear reducer through the drive shaft.

Float Valve - A valve that is mechanically actuated by a float. Float valves are utilized on many cooling towers to control make-up water supply.

Flow-Control Valves - Manually controlled valves, which are used to balance the 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 at various sections of the distribution system to distribute the hot water over the Fill.

Fog or Fogging - A reference to the visibility and path of the effluent air stream after having exited the cooling tower. Saturated air being discharged from the cooling tower under atmospheric conditions whereby the ambient atmospheric temperature cannot absorb all of the moisture as vapor, and excess fog condenses. The intensity of the fog plume is a function of the heat rise of air passing through the tower and the temperature and humidity of the ambient air. Fog plumes are normally permissible since there are no droplets of water raining out of the discharge area. However, on large industrial sites this could cause icing of the roads in the winter and restricted visibility, which could necessitate corrective action. 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.

Framing or Frame Work Members - The structural members designed to support all live and dead loads imposed. They consist of columns, horizontal ties, diagonals, and joists and beams.




Gear Reducer - Power transmission unit on cooling tower, which provides a right angle turn from motor shaft and couplings to gear down motor speed thereby generating power of air movement from the fan. A power transmission unit, which reduces the RPM from the power source through a system of gears to the design speed for the fan to produce optimum airflow. Sometimes referred to as a Gear Box.

Girt - (See Horizontal Tie)

Grout - Material such as concrete or epoxy that serves as filler between the bottom of the machine base and the top of the concrete foundation or metal equipment skid.



Handrail - A horizontal or sloping rail placed along the peripheral edge of a platform or access way such as a cooling tower fan deck, distribution basin and/or stairway. The handrail system must be in accordance with OSHA standards and usually includes a knee rail or mid-rail and a toe board.

Header or Header Pipe - In a Counterflow type cooling tower, the main pipe carrying hot water to a series of laterals for distribution over the Fill material. In a Crossflow type cooling tower, the main pipe carrying hot water to each cell hot water distribution basin longitudinally. A Crossover pipe carries the hot water laterally within the cell.

Heartwood - The wood extending from the pith to the sapwood, the cells of which no longer participate in the life processes of the tree. Heartwood may contain phenolic compounds, gums, resins, and other materials that usually make it darker and more decay resistant than sap wood.

Heat Exchange - 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 condensor.

Heat Load - The total amount of heat removed from the circulating water in a cooling tower measured in BTU’s; is equal to the weight of water circulated per unit time multiplied by the cooling range. BTU per hr. = (GPM) (500) (HWT - CWT).

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. 

Horizontal Tie - Framework member; a load-bearing horizontal connecting member in the tower framework.

Hot Water Temperature - Temperature of circulating water entering the cooling tower's distribution system.

Humidity, Absolute - The amount of water vapor contained in the air at a given condition usually expressed in
pounds of water per pounds of dry air.

Humidity, Relative - The ratio of water vapor pressure in the air to the saturated vapor pressure at the same temperature usually expressed as a percentage.

Hyperbolic - A cooling tower having a hyperbolic shape or shell that depends on natural draft for air movement through the tower. Counterflow and Crossflow cooling towers can be used in conjunction with the hyperbolic shell.  




lnduced 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, exhaust vents, etc.



Joist - One of a series of parallel beams used to support floor and ceiling loads and supported in turn by larger beams, girders, or bearing walls. In a cooling tower Fan Decking, Drift Eliminator or Fill Module supports are sometimes referred to as Joists.



Ladder - A typical rung-and-rail device providing access to the outside or inside of the cooling tower. All ladders should be installed and maintained in strict accordance with OSHA standards.  

Ladder Cage - A fall protection safety cage installed parallel with and connecting to a vertical access ladder.

Landing - A horizontal floor, designed to provide offset clearances for stairways, ladders, or other access arrangements.

Latewood - The portion of the growth ring that is formed after the early wood formation has ceased. It is usually denser and stronger than early wood. (Also known as summer wood.)

Leaving Wet-Bulb Temperature - Wet-bulb temperature of the air being discharged from a cooling tower.

Length - In Crossflow type cooling towers, the length is always perpendicular to the direction of airflow through the Fill (Air Travel), or from endwall to endwall. In Counterflow type cooling towers, the 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.

Lignin - The second most abundant constituent of wood, located principally in the secondary wall and the middle lamella, which is the thin cementing layer between wood cells. Lignin is commonly referred to as the glue that binds the wood fibers together.

Liquid-to-Gas Ratio - A ratio of the total mass flows of water and dry air in a cooling tower. Symbol: L/G.

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 airflow 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.

Mechanical Draft - Refers to the movement of air through a cooling tower by means of a fan or other mechanical device, either induced by the fans on top or forced draft by impellers located at bottom of the tower.

Mechanical Equipment - Refers to air moving devices only. Induced draft normally consists of a fan assembly mounted on a vertical shaft of a gear reduction unit box with a flexible coupling fixed to the horizontal (or input shaft of gear reducer) and transmission shaft with another flexible coupling joining shaft to motor shaft. Forced draft towers normally are belt driven from motor to fan sheave, which is keyed to Rotor Fan (or squirrel cage) shaft. Some fan designs are direct coupled to motor.

Mechanical Equipment Support – Heavy timbers, usually steel or concrete support members, which comprise the primary supports for the motor, gear reducer, fan drive assembly.  

Mechanical Packing - Generally flexible material used to seal fluids, such as oil, grease, or water, between segments of a machine.

Mechanical Seal - A mechanical device used to seal fluids, such as oil, grease, or water, between segments of a machine. More rigid and susceptible to damage than packing.

Merkel’s Theory - The total heat theory, which has been almost universally adapted to calculations of tower performance. All heat transfer taking place at any position in the cooling tower is proportional to the difference between the total heat of the air at that point in the tower and a total of air saturated at the temperature of the water at that point in the tower.

Misalignment - A measurement of the degree of non-collinearity between two or more rotating shafts.

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.

Motor Rated Horsepower – The horsepower rating inscribed on nameplate of the motor driving the fan.


Natural Draft - Is a reference to the movement of air through a cooling tower purely by natural means. Typically, by the driving force of a density differential.

Natural Draft Cooling Tower - A cooling tower in which air movement is dependent upon the difference in density between the entering air and 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 spaces the circulating water is in intimate contact with the air flowing through the tower.  

Nominal Tower Dimensions - Width and length measured from and to column centerlines; height measured from top of basin curb to top of fan deck (Counterflow design) or to top of distribution basin (Crossflow design).

Nozzle - A component of a cooling tower distribution system. A nozzle is a device used for controlled distribution of water in a cooling tower. Nozzles are designed to deliver water in a spray pattern either by pressure or by gravity flow.


Overall Tower Dimensions - (1) Width: overall dimensions perpendicular to the tower’s longitudinal axis; (2) Length: overall dimension parallel to the air inlet louvers and the longitudinal axis; (3) Total Height: Distance from basin curb to top of fan stack.


Packing - (See Fill)

Partition or Partition Wall - An interior vertical wall subdividing the cooling tower into cells or into separate fan plenum chambers. Partitions may also be selectively installed to reduce windage and subsequent water loss. However, such walls are often referred to as “Windwalls”

Performance Curves - Performance of a cooling tower is rated in terms of water temperatures, approach, wet bulb temperature, range, static pressure, and air movement. The ratio of a cooling tower is established by developing a series of charts that relate to these variables.

PH of Water - A scale for measuring acidity, or alkalinity of water. 7.0 is selected as the neutral area where both the hydrogen and hydroxyl ions are present in exactly the same proportions so that the water is neutral. A pH below 7.0 indicates acidity and a pH above 7.0 indicates alkalinity. A pH of 7.0 indicates neutral water on a scale of 0 to 14. 

Pier - In a wood, steel or fiberglass framed cooling tower erected on a concrete basin the Pier is an interior column support used to elevate column footings above the basic basin floor or foundation grade. Also known as an Internal Pier.

Pilaster - In a wood, steel or fiberglass framed cooling tower erected on a concrete basin the Pilaster is an exterior column support, usually an integral part of the basin wall. Also known as an External Pier.

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.

Plan Area - The cross section of the working area of the cooling tower, measured in feet of length and width.

Plenum or Plenum Chamber - The enclosed space between the Drift Eliminators and the Fan in an induced draft cooling tower, or the enclosed space between the Fan and the Fill in a forced draft cooling tower.

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

Pressure Drop - Static pressure or amount of resistance of air movement through the tower caused by obstructions such as air louver inlets, Fill configuration, tower supports, spray system piping, drift eliminators, static and mechanical equipment components, fan cylinder resistance and construction.  

Psychrometer - an instrument incorporating both a dry-bulb and a wet-bulb thermometer, which can take simultaneous dry-bulb and wet-bulb temperature readings.

Pulley - A wheel on an axle that is designed to support movement of a cable or belt along its circumference. Most often a pulley denotes mechanical power transmission by use of a flat belt. Often used to define a sheave.

Pumping Head - Energy required to raise the water to the distribution system elevation and overcome friction losses through pipe, valves, fittings and nozzles. It is expressed in feet of liquid the pump must move and equals the totals of friction loss, static head, plus the pressure drop through the distribution system itself.





Raised Grain - A roughened condition of the surface of dressed lumber in which the hard latewood is raised above the softer early wood but not torn loose from it.

Range - Difference between the hot water temperature and the cold water temperature (HW - CW = R)

Recirculation - A phenomenon during which a portion of the tower's discharge air re-enters the air inlets along with the fresh air. This raises the wet bulb temperature of the entering air above that of the ambient air and greatly reduces the thermal performance of the tower.  

Re-Distribution Basin - An elevated basin installed between the hot and cold water basins in a Crossflow type
cooling tower to maintain uniform water distribution throughout the entire height of the Fill.

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.


Sap Wood - The wood of pale color near the outside of the log. Under most conditions sapwood is more susceptible to decay than heartwood. 

Sheave - A wheel on an axle that is designed to support movement of a cable or belt along its circumference. Most often a sheave denotes mechanical power transmission by use of a V-belt. Often used to define a pulley. 

Shell - The chimney-like structure, usually hyperbolic in cross-section, utilized to induce airflow through a natural draft cooling tower. Sometimes referred to as a "stack" or “chimney”.

Singleflow - A Crossflow type cooling tower having a Fill section on one side of the plenum chamber only.  

Soft Foot - Condition where uneven contact between equipment foot and base distorts the equipment. Distortion can lead to clearance issues (bearings, seals, impellers, motor air gap, etc.) and non-repeatable shaft movement.

Speed Reducer - A device to change speed of the driver to the speed desired for the fan. A mechanical device positioned 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. While the use of geared reduction units predominates, many smaller towers will utilize differential pulleys and V-belts for the transmission of relatively low power. 

Splash Bar - One of a succession of equally spaced horizontal bars comprising the principal 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 improved heat transfer capability. When flat, they are sometimes referred to as "slats" or "lath".

Splasher or Splash Plate - Used in a gravity distribution system to receive water from a downspout and effect
uniform spreading of the water over the wetted area of the tower.

Splash-Filled - A cooling tower in which splash-type fill is used for the primary heat transfer surface.

Spray-Filled - A cooling tower which has no Fill. Water-to-air contact depends entirely upon the water break-up and pattern afforded by pressure spray nozzles.

Spray Nozzle - Device used in a distribution system to break up the flow of the circulating water into droplets and uniformly spread the water over the wetted area of the tower.

Static Efficiency - The ratio of the static air power to the fan input power. This can be calculated by multiplying the fan total efficiency by the ratio of fan static pressure to fan total pressure.

Static Pressure - The energy carried in the air or gas used to overcome frictional resistance and acts at right angles to a surface. It exists by virtue of the degree of compression only.

Static Pressure Drop - Reduction of air movement through the tower due to the resistance of internal components such as the air intake Louvers, Fill Packing, Distribution System, internal framing, Drift Eliminators, Fan Stack configuration, etc.

Standard Air - Air having a density of 0.075 lb per cu ft. essentially equivalent to 70 (F) dry air at 29.92 in HG
barometric pressure.

Structural Members or Timbers - Pieces of wood of relatively large size, the strength or stiffness of which is the controlling element in their selection and use. 

Suction Screen or Sump Screen - Screen over the water sump or pump suction pit on a cooling tower cold water collection basin that prevents debris from entering the pump and piping system. 

Sump - A depressed area or section of the cold water collecting basin where the cooled water flows to facilitate pump suction and its return to the heat source, be it the condensers, compressors, or process water equipment. Sumps can also be designed to aid in the collection of silt and sludge for periodic cleaning.

Supply Header – Is the portion of the cooling tower water supply system from which the riser pipe or inlet connection receives the circulating hot water flow.


TEAO - “Totally Enclosed Air Over” motor enclosure.

TEFC - “Totally Enclosed Fan Cooled” motor enclosure.

Thermal Capacity - The number of gallons per minute (GPM) a cooling tower will cool for a specified cooling range, wet bulb temperature, and approach.

Thermal Growth - The amount equipment centerlines change, both vertically and horizontally, due to temperature changes.

Throat Diameter – On a Natural Draft type Cooling Tower or in regards to a Fan Stack or a Cylinder the throat would be the diameter of the shell at its narrowest point. 

Thrust Bearing - A type of rotary bearing designed for high axial loads.

Tip Speed - The peripheral speed of a fan impeller.

Total Efficiency - The ratio of the total air power to the fan input power. This value is usually expressed as a percentage. Power losses can be due to turbulence, leakage, and friction.

Total Pressure - The sum of the static pressure and velocity pressure at any given point in a system.

Tower Dimensions - (1) Width: Overall dimension perpendicular to the tower longitudinal axis; (2) Length: Overall dimension parallel to the air inlet louvers and the longitudinal axis; (3) Total height: Distance from top of basin curb to top of fan stack.

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 necessary at that point to effect proper distribution of the water through the distribution system and spray nozzles.  

Transverse - Normally measured perpendicular to the cooling tower air inlet or length of the tower. 



V Belt - A belt used in power transmission. The V shape of the belt serves to wedge the belt into the sheaves to improve gripping power, minimize slippage, and improve efficiency.

Vibration Cut-Out Switch - A safety switch designed to interrupt power to the fan motor and/or provide an alarm in the event of excessive mechanical equipment vibration.

Velocity Pressure - The pressure necessary to maintain the movement of air (kinetic energy). Fan velocity pressure corresponds to the average velocity at the fan outlet.

Velocity Recovery or Velocity Regain Fan Cylinder or Stack - A venturi shaped fan cylinder. The increase in performance obtained on this type of cylinder is due to the reduction in total head above the fan. The attendant decrease in the total head differential across the fan results in either an increase in air rate at constant horsepower, or a decrease in horsepower at constant air rate.


Warp - Any variation from a true or plane surface. Warp includes bow, crook, cup, and twist, or any combination thereof.

Water Cooling Tower - An enclosed, steady flow device for cooling water by evaporation and sensible heat exchange through direct contact with air.

Water Loading - The circulation rate of water flow expressed in Gallons per Minute (GPM) per square foot of effective horizontal wetted area or plan area of the tower.

Weathering - The mechanical or chemical disintegration and discoloration of the surface of wood caused by
exposure to light, the action of dust and sand carried by winds, and the alternate shrinking and swelling of the surface fibers with the continual variation in moisture content brought by changes in the weather. Weathering does not include decay. 

Wet-Bulb Temperature -The temperature of the entering or ambient air adjacent to the cooling tower as measured with a wet-bulb thermometer. The temperature of saturated air. At 100% humidity the wet bulb temperature equals the dry bulb reading. The lower the humidity, the drier the air and the greater the difference in reading. The lower the wet bulb, the more work a cooling tower can do. A cooling tower cannot cool the water below the wet bulb temperature of the inlet air. 

Wet-Bulb Thermometer - A thermometer whose bulb is encased within a wetted wick. Temperature indicated by a psychrometer. Also known as the thermodynamic wet bulb temperature. 

Wet / Dry Cooling Tower - A hybrid Cooling Tower, A wet (evaporative) cooling tower in combination with a dry (non-evaporative) heat exchanger system, used to reduce or abate cooling towers fog plume during cold weather by modifying the cooling towers exhaust air condition. Usually used near highways where unmanaged exhaust plumes might diminish driving conditions.

Windage or Wind Loss - Is the water lost from the cooling tower due to the effects of wind. .

Wind Load - The load imposed upon a structure by a wind blowing against its surfaces. Wind load is expressed in pounds per square foot.Wind Wall or Windwall - An interior vertical partition wall selectively installed to reduce windage through, and subsequent water loss from, the cooling tower. Also known as Blow out.



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