Water Chemistry and Biological Terms
Acid    any substance that donates a proton (H+) when dissolved in a solution. In water treatment, it usually means circulating water with a pH of less than 7.0. Sulfuric acid is the most common acid used to control cooling water pH.
Adsorption    assimilation of molecules or other substances onto the physical structure of a liquid or solid without chemical reaction.
Aeration    blowing or mixing of air through water to sweep out other dissolved gases and to equilibrate the water with primarily nitrogen, oxygen and carbon dioxide.
Aerobe    an organism that grows only in the presence of free (molecular) oxygen.
Algae    a low form of plant life containing chlorophyll that generally requires sunlight and air for existence. Many are microscopic but under favorable conditions can grow sufficiently dense to plug cooling tower distributors on the decks and to interfere with water splashing in the fill. Large masses often slough off the tower and plug heat exchangers or deposit in piping.
Algaecide    a toxic material that will kill algae. Some of the more commonly used algaecides are chlorine, copper sulfate and phenolic compounds.
Alkalinity    an expression of the total basic anions (hydroxyl groups) that is present in a solution. It also represents, particularly in water analysis, the bicarbonate, carbonate, hydroxyl and occasionally the borate, silicate, and phosphate salts which will react with water to produce acid neutralizable anions.
Ammonia    a water-soluble, colorless, pungent gas with the formula NH3.
Ammonium    positive ion with the formula NH4+ that forms when ammonia dissolves in water. It adds non-natural alkalinity to the water.
Anaerobe (strict)    an organism that grows only in the absence of free oxygen (e.g., sulfate reducing bacteria).
Anaerobe (facultative)    an organism that can grow under either aerobic or anaerobic conditions.
Anion    a negatively charged ion.
Anode    a positively charged electrode that, during electrolysis, attracts anions.
Anodic area    area on a metal surface where electrons are given up and metal dissolves (corrosion).
Antifoam    an agent that prevents foam from forming or reduces its presence.
Atomic number    a number given to each element representing the number of protons in one atom of that element.
Atomic weight    weight (in grams) of 6.02x1024 atoms of a particular element. The weight of an atom of any element compared with that of oxygen that is considered to be 16.
Autotrophic    an organism that is capable of producing its own food through the use of chlorophyll and light.
Bacteria    Unicellular microorganisms which reproduce by fission or spores, identified by their shapes: coccus, round; bacillus, rod shaped; and spirillum, curved.
Bicarbonate alkalinity    in a water solution, the presence of ions resulting from the hydrolysis of carbonates when these salts react with water. A strong base and a weak acid are produced and the solution is alkaline. Its formula is HCO3-, but its concentration is usually denoted as ppm as CaCO3.
Biocide    a chemical that is designed to control the population of troublesome microbes by killing them.
Biological deposit    water-formed deposits of organisms or their waste products (example: slimes, barnacles, etc.).
Biostat    a chemical that is designed to control the population of troublesome microbes by inhibiting their reproduction and subsequent increase in population.
Blowdown    water discharged from the system to control concentration of salts or other impurities in the circulating water.
Carbonate hardness    hardness in water caused by bicarbonates of calcium and magnesium. If alkalinity exceeds total hardness, all hardness is carbonate hardness; if hardness exceeds alkalinity, the carbonate hardness equals the alkalinity.
Cathode    a negatively charged electrode (which attracts cations in electrolysis).
Cation    a positively charged ion resulting from dissociation of molecules in solution.
Chemical dosage    the amount of chemical added to a system, usually expressed as ppm, or pounds of chemical per million pounds of water.
Chloramine    a compound formed when chlorine reacts with ammonia or a chemical containing the ammonium ion. The result is mono-, di-, tri-, and organochloramines. The resulting chloramines are weak oxidants or nonoxidants. They are a component of total chlorine. These are very weak biocides.
Chlorination    adding chlorine or a chlorine derivative to water to prevent the growth of various organisms that cause biofouling.
Chlorination, dechlorination    the act of removing chlorine from water, usually via a reducing agent or strong aeration.
Chlorine    a poisonous yellow gas with chemical symbol Cl2 used for water treatment. It is soluble in water but can be removed by reducing aeration and reaction with sunlight.
Chlorine demand    the relation of the amount of chlorine to be added to a system to react with chlorine-oxidizable material until a free residual in a given system is achieved.
Chlorine dioxide    a compound with the symbol ClO2, used mainly for bleaching wood pulp for paper. It is also used for the taste and odor control of phenolic compounds in water and can be used as a disinfectant in cooling towers.
Chlorine, free    the total amount of hypochlorous ion and hypochlorous acid in the system.
Chlorine, residual    the amount of available chlorine present in water at any given time subsequent to the addition of chlorine.
Circulating water rate    quantity of water pumped from the tower basin to the equipment to be cooled, usually expressed as gallons per minute (gpm).
Combined Chlorine (new)    the amount of chloramine or chloro-organic compounds present in water.
Compounds    substances formed from the chemical bonding of two or more atoms.
Concentration    the process of increasing solids per unit volume of solution, usually by evaporation of the liquid; also, the amount of material dissolved in a unit volume of solution. This occurs in a cooling tower due to evaporation that cools the water. It is normally expressed directly as ppm or indirectly as mhos conductivity.
Conductivity    the ability of water to conduct electricity. When measured with a standard apparatus, it is called specific conductivity and is a function of the total ionic dissolved solids. As a rule of thumb, TDS = 2/3 specific conductance measured as micromhos.
Cooling water    water circulated through a cooling system to remove heat from certain areas.
Copper alloys    metals made from a mixture of copper and another metal (Example brass = copper + zinc).
Corrosion    to be dissolved away, usually by oxidation or acidification. This usually refers to metal loss in cooling systems, often recognized by the local reaction of the dissolved metal with oxygen, carbon dioxide, acids or galvanic action. May result in general (widespread) or pitting loss of the metal.
Corrosion coupon    a small piece of metal that is inserted into a circulating system that can be analyzed to find the corrosive nature of the system's water.
Corrosion fatigue    the weakening of a pipe or similar piece of metal due to corrosion acting on it.
Corrosion inhibitor    chemical used in a system to prevent corrosion.
Cycles of Concentration    compares dissolved solids in makeup water with solids concentrated through evaporation 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 makeup water.
Deflocculation    the ability of some materials such as polyphosphides to peptize and disperse suspensions of colloidal particles.
Delignification    the dissolving of the lignin portion of cooling water wood usually by strong alkaline and/or oxidizing agents.
Denitrification    the removal of nitrogen from a system. This may be done chemically or biologically.
Disinfecting    to remove microorganisms from a particular surface or liquid.
Dispersant    a chemical that causes particulates in a water system to remain in or be placed into suspension.
Dissolved gases    total gases dissolved in a liquid.
Dissolved solids    total solids that have been dissolved into a liquid. They may be ionic and/or polar in nature.
Drift    water lost from a cooling tower as liquid droplets entrained in the exhaust air. It is independent of water lost by evaporation. Units may be in lbs./hr. or percentage of circulating water flow. Drift eliminators control this loss from the tower.
Electrode    conductor that carries an electrical charge.
Electrolyte    a chemical compound that dissociates or ionizes in water to produce a solution that will conduct an electric current; an acid, base, or salt.
Elements    substances that cannot be divided into smaller units without nuclear destruction.
Equivalent weight    molecular or atomic weight divided by the valence of the substance being considered.
Evaporation loss    water evaporated from the circulating water into the atmosphere by the cooling process.
Evaporation rate    the rate at which a liquid turns into gas and leaves a system.
Fatigue, corrosion    The weakening of a piece of metal or pipe due to corrosion.
Ferrobacillus    a type of iron-reducing bacteria.
Filamentous bacteria    bacteria with a long, threadlike shape.
Float valve    a valve that is actuated by a float, generally used to control makeup water supply to a cooling system.
Flocculation    the process of agglomerating coagulated particles into settleable flocs, usually of a gelatinous nature.
Flow control valve    a manually controlled valve generally located in the hot water supply line used to increase or decrease the flow of a liquid in a system.
Fungi    simple, plant-like life forms that lack true roots, stems, leaves, and chlorophyll. They are filamentous in structure (e.g., mushrooms, mildews, molds, and yeasts).
Galvanic attack    corrosion behavior caused by the current generated when one metal type is in contact with a different metal type.
Galvanic corrosion    corrosion formed from galvanic attack.
Galvanic couple    the contact of one metal type with another as seen with a steel bolt in an aluminum sheet or a copper joint on an iron pipe, which allows a differential current to be generated.
Groundwater    water pumped from underground rivers, wells and lakes.
Half life    in water treatment, the time it takes to lose " of a slug fed product from the system through blowdown, drift and windage when taking into account its continuous concentration by evaporation and dilution by fresh makeup.
Hardness, calcium    the calcium compounds dissolved in water, usually expressed as calcium carbonate.
Hardness, carbonate    the calcium and magnesium carbonate and bicarbonate dissolved in water, expressed as calcium carbonate. Other metallic cations such as ferrous iron, barium, zinc and manganous ions are also included.
Hardness, magnesium    magnesium compounds dissolved in water, expressed as calcium carbonate.
Hardness, noncarbonate    the difference between the total hardness and the total alkalinity of a water.
Hardness, permanent    the hardness that cannot be removed from water by precipitation reactions. Essentially, the same thing as the non-carbonate hardness.
Hardness, temporary    the hardness that can be removed from water by precipitation. Essentially, the same as the carbonate hardness.
Hardness, total    the sum of the calcium and the magnesium hardness. Also the sum of the permanent hardness and temporary hardness. The U.S. Geological Survey hardness criteria for potable water are: soft water 0-60 ppm, moderately hard water 61-120 ppm, hard water 121-180 ppm, very hard water >180 ppm.
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. Can also be the tube or fin tubed bundles in a wet/dry tower.
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.=gpmx500x(HWT - CWT).
Impurities    any substance that contaminates another.
Iron bacteria    oxidative bacteria that cause taste, staining, and odor problems in water. The precipitation of iron by the bacteria can build up in piping and heat exchangers to restrict water flow. Although they are not a direct cause of corrosion, differential aeration cells will form under deposits and localized corrosion will appear.
Langelier's Index    a formula for predicting whether water will tend to dissolve or precipitate calcium carbonate.
Makeup    water added to the circulating water system to replace water lost from the system by evaporation, drift, blowdown, and leakage.
Microorganism    an organism that is too small to be studied without the aid of a microscope.
Molluscicide    a material that will kill molluscs.
Nonoxidizing biocide    a biocide whose effectiveness depends on some property other than its ability to oxidize organic material (i.e., systematic poisons and surface activity).
pH    the logarithm of the reciprocal of the hydrogen ion concentration. Water with a pH of 7 is neutral at room temperature. A pH greater than 7 indicates alkaline water. A pH less than 7 indicates acidic water.
Phosphates    inorganic ions comprised of phosphorous and oxygen.
Photosynthesis    the process by which plants use sunlight to produce energy by consuming CO2 and releasing O2.
Residual chlorine    (see chlorine, free) the amount of available chlorine present in water at any specified period, subsequent to the addition of chlorine.
Ryznar Index    an empirical method, often used in combination with the Langelier's Index, to predict the scaling tendencies of water. It is based on a study of operating results with water of various saturation indices.
Scale    the deposition on heat transfer surfaces of material normally in solution, as opposed to fouling, which is deposition of material normally in suspension.
Slime    a deposit build-up directly related to the excessive growth of microorganisms that secrete or form thick, sticky material. These slimes cause tube pluggage, reduce heat transfer, entrap migratory suspended solids and can result in underdeposit corrosion.
Solubility    the ability of one material (the solute) to dissolve in another (the solvent).
Solutions    liquid mixtures that are uniform throughout.
Sulfate-reducing bacteria    group of anaerobic bacteria capable of reducing sulfates to sulfides, especially hydrogen sulfide, a very corrosive gas.
Surfactant    in water, any molecule that modifies the interfacial tension of a liquid and the surface it contacts (surfaces can be air, metal, biomass, etc.).
Suspended solids    insoluble particles suspended in water
Synergism    the condition existing where the effect of two or more materials added to water is greater than the sum of their individual effects
Thermophilic    a type of bacteria that thrives in very high temperatures.
Total Chlorine (new)    the total amount of all types of chlorine compounds present. This includes Cl2, hypochlorous acid, hypochlorite ion, and all the chloramine compounds. Free Chlorine + Combined Chlorine = Total Chlorine.
Total Dissolved Solids    the sum of the organic and inorganic materials dissolved in water.
Tuberculation    localized attack typified by the formation of inverted pliable conical structures. In water systems, tuberculation is more commonly associated with localized corrosion of iron and the conical structures are predominately made up of several oxidation states of iron (Fe2O3, Fe3O4, etc.). Pitting is normal result.
Turbidity    the interference to light transmission caused by suspended solids usually colloidal in nature.
Water-formed deposit    any accumulation of insoluble material derived from water or formed by the reaction of water upon surfaces, including scale, sludge, foulants, sediments, corrosion products or biological deposits.


Access tunnel    Opening in natural draft 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 in cubic feet per minute at the 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 inlet    Opening in cooling tower through which air enters a tower. On induced draft towers, the air inlet is commonly called the louvered face.
Algae    A low form of plant life which generally requires sunlight and air for existence. Causes plugging of heat exchanger tubes and cooling tower distribution systems, and fill.
Algaecide    A toxic material which 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 which is adjacent to but not affected by the cooling tower. Generally, this means upwind of the tower, and the other areas where other heat producing equipment is located that supplies extraneous sources of heat to the air coming to the tower.
Ambient dew point    The ambient temperature in oF when dew begins to be deposited.
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    A device for attaching the tower structure to the foundation; it does not include the anchor bolt. Also known as Column Anchor.
Approach    The difference between the cold water temperature in oF and the ambient or inlet wet-bulb temperature in oF.
Atmospheric tower    One in which air movement through the tower is dependent upon atmospheric conditions, not induced by mechanical fans.
Basin    (see Cold Water Basin and Distribution Basin).
Basin curb    The top level of the retaining wall of the cold water basin; usually the datum point from which tower elevation points are measured.
Basin sump    (see Sump).
Bay    The volume between adjacent columns.
Beam    (see Joist).
Bent    A line of structural framework composed of columns, girts or ties; a bent may incorporate diagonal bracing members.
Bevel washer    A metal fitting used to accommodate through-bolts to angular position of a diagonal member, usually connecting to columns or other framework members.
BHP    (abbr) (see Brake Horsepower).
Blow out    Water that is blown or pulled out of the air inlet by wind.
Blowdown    Water discharged from the system to control concentration of salts or other impurities in the circulating water.
Blower    A squirrel-cage type of air moving device usually applied for operation at higher than normal static pressures or for sound control reasons.
Brace    (see Diagonal).
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 lb. of water 1oF.
Caissons    (see Foundation).
Canopy (Natural draft tower)    Connects the hyperbolic shell to the cooling section, acting as an air conduit and air seal between the two.
Capacity    (see Thermal Capacity).
Casing    A vertical enclosing sidewall or endwall of a tower, exclusive of the louvers.
Casing panel    A pre-assembled or pre-cut unit of the casing.
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 or stacks and one or more distribution systems.
Cell dimensions    (a) Width: dimensions perpendicular to tower longitudinal axis and usually at right angles to the louver area; (b) Length: dimension parallel to longitudinal axis and the plane where louvers are usually placed; (c) 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 center lines.
Circulating water rate    Quantity of hot water entering the tower.
Coil shed    Portion of the tower structure housing atmospheric (tubular) heat exchangers.
Cold water basin    A device underlying the tower to receive the cold water from the tower and direct its flow to the suction line or sump.
Cold water temperature (CWT)    Temperature of the water entering the cold water basin before addition of make-up.
Column    Framework member; a main vertical supporting member in the tower framework.
Column anchor    (see Anchor Casting).
Concentration    (see Water Loading).
Concrete cooling tower    Cooling tower in which the structure is made of concrete.
Cooling range    (see Range).
Counterflow tower    One in which air, drawn in through the louvers (induced draft) or forced in (forced draft) at the base by the fan, flows upward through the fill material and interfaces counter currently with the falling hot water.
Cross struts    Framework that holds up the hyperbolic shell of a natural draft tower. The number of cross struts varies according to shell size.
Crossflow tower    One in which air, drawn or forced in through the air intakes by the fan, flows horizontally across the fill section and interfaces perpendicularly with the falling hot water.
CWT    (abbr.) (see Cold Water Temperature).
Cycles of concentration    Compares dissolved solids in make-up 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.
Cylinder    (see Fan Stack).
DBT    (abbr.) (see Dry-Bulb Temperature).
Deck Batt    (see Splash Bar).
Deck Stringer    Holds the splash bars of the fill deck in a fixed position with respect to 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), water flow 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). Allowable noise level also applies to both.
De-silting sump    The area in the cold water basin, usually at a low point, where silt can be flushed to a drain.
Diagonal    (see also Cross Struts for Natural Draft Tower) A framework member; a load bearing member transmitting forces at other than a right angle with reference to columns or horizontal ties. Also known as Brace.
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    (see also Fan stack) A walled enclosure extending upward above the eliminators to direct exhaust air vertically away from fans in a forced draft tower. See Fan Stack 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 thereby increasing the effectiveness of the distribution nozzles.
Distribution header    Pipe or flume delivering water from inlet connection to lateral headers, troughs, flumes, or distribution basins.
Distribution nozzle    (see Nozzle and next entry).
Distribution system    Those parts of a tower, beginnings with the inlet connection, which distribute the hot circulating, water within the tower to the points where it contacts the air. In a counterflow tower, this includes the header, laterals, and distribution nozzles. In a crossflow tower, the system includes the header or manifold, valves, distribution box, basin pan, and nozzles.
Double-flow water-cooling tower    A crossflow tower with two fill sections and one plenum chamber, which is common to both.
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.
Drift    Water lost from the tower as liquid droplets entrained in the exhaust air. It is independent of water lost by evaporation. Units may be in lbs./hr. Or percentage of circulating water flow. Drift eliminators control this loss from the tower.
Drift eliminators    An assembly constructed of wood, plastic, cement board, or other material that serves to remove entrained moisture from the discharged 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.
Drive Shaft    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.
Effective volume    (see Net Effective Volume).
Eliminator    (see Drift Eliminator).
Eliminator baffle    (see Eliminator Board).
Eliminator blade    (see Eliminator Board).
Eliminator board     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. Also known as Eliminator Baffle (Blade).
End wall    The wall on the end of the tower structure.
Entering air    Air from the atmosphere surrounding the cooling tower which enters through the louvers on an Induced draft tower or is discharged into the tower by a fan on a Forced Draft Tower.
Entering wet-bulb temperature    Average wet-bulb temperature of the entering air. Includes any effects of recirculation.
Evaporation loss    Water evaporated from the circulating water into the atmosphere by 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.
Exit basin temperature    Temperature of the circulating water leaving the cold water basin if blowdown or make-up is added to the basin, the temperature will be affected accordingly.
Exit diameter    Diameter of the shell at the top in a hyperbolic natural draft tower or at the top of a fan stack.
Fan    A device for moving air in a mechanical draft tower. The fan design may be either an axial flow propeller or centrifugal blower. 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 entire top surface of the tower. In a crossflow tower, the open fan deck covers only the tower plenum area, leaving the distribution system exposed. An extended fan deck encloses the distribution system and covers the entire top surface of the tower.
Fan drive assembly    mechanical components furnishing power to the fan, usually consisting of driver, 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 3 x Power Factor x Efficiency) / 746
Fan driver output    Brake horsepower output of the driver to the drive shaft. Fan driver input x 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 that a fan blade makes with the plane of rotation.
Fan ring    (see Fan Stack).
Fan stack    Cylindrical or modified cylindrical structure in which the fan operates. Fan stacks are used on both induced draft and forced draft axial flow propeller fans. Also known as Cylinder.
Fan stack height    Distance from top of fan deck to top of fan stack.
Fan support    (see Mechanical Equipment Support).
Fill bars    The assembly of splash bars comprising the tower filling. 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    The assembly of splash bars comprising the tower filling. (See Fill Bars for description of operation in a crossflow tower.)
Fill hanger    Support system in a crossflow tower for fill bars which hold fill in place.
Fill support    (see Deck Support).
Filling    That part of a crossflow, counterflow, or natural draft tower consisting of splash bars, vertical sheets of various configurations, or honeycomb assemblies, tile or other materials, which are placed within the tower to effect heat and mass transfer between the circulating water and the air flowing through the tower.
Firewall    A double wall between cells used to isolate a fire portion system and designed according to NFPA 214.
Flexible shaft    (see Drive Shaft).
Float valve    A valve that is actuated by a float, generally used to control make-up water supply.
Flow control valve    A manually controlled valve generally located in the hot water supply line.
Flume    A trough, which may be totally enclosed or open at the top. Flumes are used in counterflow cooling towers for the manifold pipe laterals to distribute the hot water over the fill.
Fogging    A fog condition created when the exhaust air or plume from a cooling tower becomes supersaturated so that part of the water vapor condenses into visible liquid droplets.
Forced draft water cooling tower    Type of mechanical draft tower in which one or more fans are located at the air inlet to force air into the tower.
Foundation    Support material beneath the tower.
Framework members    The structural members designed to support all live and dead loads. They consist of columns, horizontal ties, diagonals and joists and beams. Can also include shear walls if designed accordingly as in a concrete tower.
Gear reducer    A reduction gear, commonly used on fan drivers to reduce driver speed to fan speed requirements. Also known as Speed Reducer.
Girt    (see Horizontal Tie).
GPM    (abbr.) Gallons per minute
Handrail    A horizontal or sloping rail placed along an access way or at the edge of a platform. Usually at 3'-6" above the walkway or floor. (See Safety Handrail. See OSHA for design criteria.)
Header    In a counterflow tower, the main pipe carrying hot water to a series of laterals 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. (See Manifold.)
Heat exchanger    A device for transferring heat from one substance to another. Heat transfer can be by direct contract, as in a cooling tower, or indirect, as in a shell and tube condenser. Also known as filling. Can also be the tube or fin tubed bundles in a wet/dry tower.
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.= gpm x 500 x (HWT - CWT).
Horizontal tie    A horizontal connection member in the tower framework. Also known as Girt.
Hot water temperature (HWT)    Temperature of circulating water entering the distribution system.
Hydrogen ion concentration (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 of 7.0 is neutral.
Hyperbolic tower    A cooling tower of hyperbolic shape, which depends on natural draft for air movement through the tower. Can be either crossflow or counterflow tower. (See Natural Draft Tower.)
Induced draft water cooling tower    Type of mechanical draft tower in which one or more fans are located in the air outlet to induce air flow through the air inlets.
Inlet air    (see Entering Air).
Inlet connection    Fitting to which the circulating water supply piping is connected to serve the tower distribution system. Also known as Inlet Flange.
Inlet wet-bulb temperature    The average of the wet-bulb temperature obtained from several stations located on both the windward and leeward sides of the tower.
Joist    Supports for fan decking, or for film fill modules.
Ladder    Typical rung-and-rail device providing access on outside or inside of the tower.
Ladder cage    Protective cage parallel with and connecting to a vertical access ladder.
Landing    A horizontal flooring, designed to provide offset clearances for stairways, ladders, or other access arrangements.
Lateral header    A pipe or flume distributing water from the distribution header to nozzles, or from other points of discharge to the filling area.
Longitudinal    Always measured parallel to air intakes and perpendicular to air flow.
Louvers    Members installed horizontally in a tower wall to provide openings through which the air enters the tower while also containing the falling water within the tower. Usually installed at an angle to the direction of air flow to the tower.
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. (See Header.)
Mastic    A compound usually made up of asphalt and asbestos fiber with suitable solvents and fillers which can be used to seal joints and cracks against leakage of water.
Mechanical draft water-cooling tower    A tower through which air movement is effected by one or more fans. There are two main types: Forced draft with fans located at the air inlet; Induced draft with fans located at the air exhaust.
Mechanical equipment support    Members, which comprise the primary support for the fan, drive assembly. Normally steel or concrete.
Motor rated horsepower    Horsepower rating inscribed on name-plate of the motor driving the fan. (See Rated Horsepower.) Unit: hp.
Natural draft water-cooling tower    One 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. (See Hyperbolic Tower.)
Net effective volume    That portion of the total structural volume with in which the circulating water is in intimate contact with the air flow through the tower. Unit: ft.3.
Nominal tower dimensions    Width and length measured from and to column centerline or walls; height measured from basin curb to top of fan deck (counterflow design) or to top of distribution basin (crossflow design). Unit: ft.
Nozzle    A device for controlled distribution of water in a cooling tower. Nozzles are designed to deliver water in a spray pattern by pressure or by gravity flow.
Obstruction lights or Aviation warning 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 tower's longitudinal axis; (b) length: overall dimension parallel to the air inlet louvers and the longitudinal axis; (c) total height: distance from basin curb to top of fan stack. Dimensions measured in feet.
Packing    (see Filling).
Partition    An interior wall subdividing the tower into cells or into separate fan plenum areas.
Pedestals    Used as a transition from the cross struts to the foundation of a natural draft tower or fan mount in force draft towers..
pH    (see Hydrogen Ion Concentration).
Pier    In a wood tower an exterior or interior column support used to elevate column footings above the basic floor or foundation grade. Also known as Internal Pier.
Pilaster    An exterior column support, usually an integral part of the basin wall. Also known as External Pier. Used as a central point to establish the elevation of framework members.
Piles    (see Foundation).
Pitot tube    An instrument that operates on the principal of differential pressures. The primary use on cooling towers is the measurement of circulating water rate.
Plenum    The enclosed space between the eliminators and the fan stack in induced draft towers or the enclosed space between the fan and the filling in forced draft towers.
Plume    Visible exhaust from a cooling tower. (See Fogging.)
Power Factor    The ratio of true power (watts) to the apparent power, as indicated by the product of amps x volts.
Psychrometer    An instrument used primarily to measure the wet-bulb temperatures. Either a sling or 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.
Range    Difference between the hot water temperature and the cold water temperature. Also known as Cooling Range.
Rated horsepower    Nameplate horsepower of fan drivers. Unit: hp.
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 and cold water basins in a crossflow tower to maintain correct water distribution through out the entire height of the fill.
Relative Humidity    the ratio of the mole fraction of water vapor present in the air to the mole fraction of water vapor present in saturated air at the same temperature and barometric pressure.
Ring Beam    Footing that carries the loads from the shell of a Natural Draft Tower.
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, platforms, and stairways; usually composed of top handrail, kneerail, and toeboard. (See Handrail.)
Shell diameter    Diameter of the shell in a natural draft tower at the top of the curb, measured from inside of cross struts to inside of cross struts.
Shell height    Dimension from top of curb to top of the hyperbolic shell in a natural draft tower.
Single-flow water-cooling tower    A crossflow tower having a fill section on one side of the plenum chamber only.
Soffitt    The underside of the lintel beam in a natural draft tower forming the bottom part of the hyperbolic shell. Generally it is thick at the base and is the first portion of the shell above the diagonals or columns, which carry the shell.
Speed reducer    (see Gear Reducer).
Splash bar    Horizontal component of a fill deck, which constitutes the principal splash surface.
Splasher or Splash plate    Used in a gravity distribution system to receive water from a down spout and effect uniform spreading of the water over the wetted area of the tower.
Spray-filled water-cooling tower    A tower, which 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 area of the tower.
Stairway    typical tread-and-riser device providing angular access to and from the top of the structure.
Standard air    Dry air having a density of 0.075 lbs./ft.3 at 70oF and 20.92" 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: lbs./in.2
Sump    Lowest portion of the basin to which cold circulating water flows: usually the point of circulating pumps suction connection. Also known as Basin Sump.
Supply Header    Portion of the water supply system, which contains the valves and distribution boxes in a crossflow tower or the lateral pipes in a counterflow tower.
Thermal capacity    The number of gallons per minute (GPM) a cooling tower will handle for a specified range, wet-bulb temperature, and approach. Also simply known as Capacity.
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. (See Tower Pumping Head.) Units: ft.
Tower dimension     (see Nominal or Overall Tower Dimensions).
Tower pumping head    that part of the "total pumping head" for which the design of the tower and the piping furnished with it are responsible. It is expressed as the head of water above the basin curb measured at the center of the inlet connecting the tower distribution system with the riser, and consists of the total pressure at the centerline of the inlet plus the vertical distance between the inlet centerline and the basin curb. Unit: ft.
Transverse    Always measured perpendicular to air intakes.

Abbreviations and Letter Symbols

A    approach*
acfm    actual cubic feet per minute*
alt    altitude*
amb    ambient*
amp    ampere
atm    atmosphere
az    azimuth
bar.    barometer
bhp    brake horsepower
Btu    British thermal unit
CWT    cold water temperature*
cu ft    cubic foot
cu in    cubic inch
cfm    cubic feet per minute (see acfm, scfm)
dB    decibel
F    degree Fahrenheit
diam    diameter
DBT    dry bulb temperature*
el    elevation
fbm    feet board measure (board feet)
fpm    feet per minute
fps    feet per second
ft    foot
gpm    gallons per minute (U.S. liquid measure)
hp    horsepower
HWT    hot water temperature*
hr    hour
in    inch
ID    inside diameter
kw    kilowatt
kwhr    kilowatt hour
lin ft    linear foot
mph    miles per hour
OD    outside diameter
ppm    parts per million
lb    pound
lb per cu ft    pounds per cubic foot
lb per hr    pounds per hr*
psia    pounds per square inch absolute
psig    pounds per square inch gage*
R    range (cooling range)*
Rc    recirculation percent*
RH    relative humidity*
rpm    revolutions per minute
sec    second
shp    shaft horsepower
sp gr    specific gravity
sq ft    square foot
std    standard
scfm    standard cubic feet per minute*
temp    temperature
vel    velocity*
WBT    wet bulb temperature*

Letter symbols - for use in equations with appropriate subscripts
(Other units of volume or rate may be indicated):

A    area: sq ft
a    area of transfer surface per unit of tower volume; sq ft per cu ft
c    specific heat
d    prefix indicating differential
D    diameter
G    mass air flow; lb dry air per hr per sq ft
g    gravitational acceleration; ft per sec per sec
h    enthalpy of air-water vapor mixture; Btu per lb dry air
K    over-all enthalpy transfer coefficient. Usually expressed as lb per hr per sq ft per lb water per lb dry air.
L    mass water flow; lb per hr per sq ft
L/G    liquid-to-gas ration; lb water per lb dry air
t    air temperature; F
T    water temperature; F
V    effective cooling tower volume; cu ft per sq ft
v    specific volume; cu ft per lb
W    humidity ratio (specific humidity); grains moisture per lb dry air (or lbs vapor per lb dry air if so indicated)
Ka V/L    tower characteristic