Access tunnel - opening in natural draught tower used for access to the cold water basin. It can be large enough for a man only, or large enough for mechanical equipment.

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. Sulphuric 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.

Air flow - total amount of dry air and associated water vapour flowing through the tower.

Air inlet - opening in cooling tower through which air enters a tower. On induced draught towers, the air inlet is commonly called the louvered face.

Algae - a low form of plant life containing chlorophyll that generally requires sunlight and air for existence. Many are microscopic but under favourable 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 sulphate and phenolic compounds.

Alkalinity - an expression of the total basic anions (hydroxyl groups) that are 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 neutralisable anions.

Ambient - the atmosphere that 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 when dew begins to be deposited.

Ambient wet bulb temperature - the air temperature in consideration of the quantity of water present (humidity). Readings are obtained by means of a mechanically aspirated psychrometer.

Anaerobe (strict) - an organism that grows only in the absence of free oxygen (e.g., sulphate reducing bacteria).

Anaerobe (facultative) - an organism that can grow under either aerobic or anaerobic conditions.

Anchor bolt - threaded bolt embedded in a concrete basin or fitted to supported members, to which an anchor casting is attached.

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.

Approach - the difference between the cold water temperature and the inlet wet-bulb temperature.

Atmospheric tower - one in which air movement through the tower is dependent upon atmospheric conditions, not induced by mechanical fans.

Autotrophic - an organism that is capable of producing its own food through the use of chlorophyll and light.

Bacteria - Unicellular micro organisms which reproduce by fission or spores, identified by their shapes: coccus, round; bacillus, rod shaped; and spirillum, curved.

Basin kerb - the top level of the retaining wall of the cold water basin; usually the datum point from which tower elevation points are measured.

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 (or Bleed) - water discharged from the system to control concentration of salts or other impurities in the circulating water.

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

British thermal unit (BTU) - the quantity of heat required to raise the temperature of one lb. of water 1°F.

Carbonate hardness - that 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.

Cell - the smallest tower sub-division that 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.

Chemical dosage - the amount of chemical added to a system, usually expressed as ppm, or pounds of chemical per million pounds of water.

Chlorination - adding chlorine or a chlorine derivative to water to prevent the growth of various organisms that cause biofouling.

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-oxidisable 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 odour 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 hot water entering the tower.

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.

Combined Chlorine - 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 mg/l.

Concrete cooling tower - cooling tower in which the structure is made of concrete.

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 micro-siemens.

Cooling range - (see Range).

Cooling tower casing or shell - the structure enclosing the heat transfer components.

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 analysed 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.

Counterflow tower - one in which air, drawn in through the louvers (induced draught) or forced in (forced draught) at the base by the fan, flows upward through the fill material and interfaces counter currently with the falling hot water.

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 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.

DBT - (abbr.) - (see Dry Bulb Temperature).

Dechlorination - the act of removing chlorine from water, usually via a reducing agent or strong aeration.

Design conditions - defined as the hot water temperature (HWT) cold water temperature (CWT), water flow and wet bulb temperature (WBT) in mechanical draught towers.

Disinfecting - to remove micro organisms 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.

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 Distribution system).

Distribution system - those parts of a tower, beginning with the inlet connection, that distribute the hot circulating water within the tower to the points where it contact 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. Normally expressed as a percentage of circulating water flow. Drift eliminators control this loss from the tower.

Drift eliminators - an assembly, normally constructed of plastic which serves to remove entrained moisture from the discharged air.

Driver - primary drive for the fan drive assembly, normally an electric motor.

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).

Electrode - conductor that carries an electrical charge.

Electrolyte - a chemical compound that dissociates or ionises 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.

Eliminator - (see Drift Eliminator).

Entering air - air from the atmosphere surrounding the cooling tower that enters through the louvers on an induced draught tower or is discharged into the tower by a fan on a forced draught tower.

Entering wet bulb temperature (air inlet wet bulb temperature) - average wet-bulb temperature of the entering air. Includes any effects of recirculation.

Equivalent weight - molecular or atomic weight divided by the valency of the substance being considered.

Evaporation loss - water evaporated from the circulating water into the atmosphere by the cooling process. The change of state of water from a liquid to vapour, when hot water comes in contact with air. For practical purposes evaporation can be calculated as 1.8% of the water flow rate through the cooling tower, per 10 ° C cooling range.

Exhaust air - the mixture of air and its associated vapour 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 draught tower. The fan design may be either an axial flow propeller or centrifugal blower. The fan can be applied as induced draught or forced draught.

Fan deck - the surface enclosing the top of an induced draught 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 guard - a protective screen installed either at the inlet of a forced draught fan or at the exit of an induced draught fan.

Fan pitch - the angle that a fan blade makes with the plane of rotation.

Fan stack - cylindrical or modified cylindrical structure in which the fan operates. Fan stacks are used on both induced draft and forced draught axial flow propeller fans. Also known as Cylinder.

Fan stack height - distance from top of fan deck to top of fan stack.

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.

Fill (or Pack) - that part of a crossflow, counterflow, or natural draught 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.

Film flow pack - pack that comprises of a honeycomb material, where a thin film of water is created on its surface. Evaporation takes place from the surface of this film. The design of film flow pack maximises the surface area of the water in contact with the air and hence the cooling effect.

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.

Float valve - a valve that is actuated by a float, generally used to control make-up water supply.

Fogging - a fog condition created when the exhaust air or plume from a cooling tower becomes supersaturated so that part of the water vapour condenses into visible liquid droplets.

Forced draught water cooling tower - type of mechanical draught 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.

Fungi - simple, plant-like life forms that lack true roots, stems, leaves, and chlorophyll. They are filamentous in structure (e.g., mushrooms, mildews, moulds, and yeasts).

Galvanic attack - corrosion behaviour caused by the current generated when one metal type is in contact with a different metal type.

Galvanic corrosion - corrosion formed from galvanic attack.

GPM - (abbr.) - gallons per minute.

Groundwater - water pumped from underground rivers, wells and lakes.

GRP - glass reinforce plastic, a material used in the construction of cooling tower casings. Completely non-corrodible and has the advantage over galvanised or even stainless steel, of being completely inert to attack by any type of sterilising chemical or water treatment regime employed.

Half life - in water treatment, the time it takes to lose ½ 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.

Handrail - a horizontal or sloping rail placed along an access way or at the edge of a platform.

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, non-carbonate - 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. 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.

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. 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. Usually expressed in kW, kcal/hr or BTU/hr.

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 draught for air movement through the tower. Can be either crossflow or counterflow tower . (See Natural Draught Tower.)

Impurities - any substance that contaminates another.

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.

Iron bacteria - oxidative bacteria that cause taste, staining, and odour 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.

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.

Langelier's Index - a formula for predicting whether water will tend to dissolve or precipitate calcium carbonate.

Louvres - 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 with suitable solvents and fillers which can be used to seal joints and cracks against leakage of water.

Mechanical draught water cooling tower - a tower through which air movement is effected by one or more fans. There are two main types: Forced draught with fans located at the air inlet; Induced draught with fans located at the air exhaust.

Micro organism - an organism which is too small to be studied without the aid of a microscope.

Natural draught 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.

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).

Non-oxidizing biocide - a biocide whose effectiveness depends on some property other than its ability to oxidize organic material (i.e., systematic poisons and surface activity).

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.

Osmosis - spontaneous transfer of a liquid solvent through a semipermeable membrane that does not allow dissolved solids to pass. Transfer is from the area of higher concentration to the area of lower concentration.

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.

Pack - (see Fill).

Partition - an interior wall subdividing the tower into cells or into separate fan plenum areas.

Pedestal - fan mount in force draught towers.

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.

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. The saturated air leaving the cooling tower condensing. Generally more visible in lower ambient temperatures (See Fogging).

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 direct sunlight.

Purge (also referred to as blow down or bleed) - water deliberately discharged from the system in order to control the concentration of dissolved solids and other impurities.

Range - difference between the hot water temperature and the cold water temperature. Also known as Cooling Range.

Rated power - nameplate power of fan drivers.

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.

Relative Humidity - the ratio of the mole fraction of water vapour present in the air to the mole fraction of water vapour present in saturated air at the same temperature and barometric pressure.

Residual chlorine - (see Chlorine, free) the amount of available chlorine present in water at any specified period, subsequent to the addition of chlorine.

Riser - piping that connects the circulating water supply line from the level of the base of the tower to the supply header of the tower inlet connection.

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.

Safety handrail - railing around top of tower, platforms, and stairways; usually composed of top handrail, kneerail, and kick plate. (See Handrail.)

Scale - the deposition on heat transfer surfaces of material normally in solution, as opposed to fouling, which is deposition of material normally in suspension.

Single flow water cooling tower - a crossflow tower having a fill section on one side of the plenum chamber only.

Slime - a deposit build-up directly related to the excessive growth of micro-organisms that secrete or form thick, sticky material. These slimes cause tube pluggage, reduce heat transfer, entrap migratory suspended solids and can result in under deposit corrosion.

Solubility - the ability of one material (the solute) to dissolve in another (the solvent).

Solutions - liquid mixtures that are uniform throughout.

Splash Pack - 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.

Splash bar - horizontal component of a fill deck that 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 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.

Static pressure - the pressure of a gas or fluid in a system as referred to a state of rest or lack of motion.

Sulphate-reducing bacteria - group of anaerobic bacteria capable of reducing sulphates to sulphides, especially hydrogen sulphide, a very corrosive gas.

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.

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.

Throat diameter - diameter of the shell in a natural draught tower at its narrowest point.

Total Chlorine - 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.

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.

Water loading - the water flow rate expressed in relation to the plan area of the cooling tower, e.g. m³/hr per m².

Wet bulb temperature - the air temperature in consideration of the quantity of water present (humidity).