Cooling tower glossary

A

Air horsepower

The power genuinely spent moving air through the tower against its resistance, as distinct from the power drawn at the motor. The gap between the two is lost in the drive train and in fan inefficiency, which is why fan and drive upgrades can cut energy use without changing the duty.

Air inlet

The opening low on the tower through which air enters. On a crossflow tower it is the louvered face below the fill; on a counterflow tower it is the open band between the basin curb and the fill. Anything that obstructs it (vegetation, stored material, a neighboring structure, ice) directly reduces capacity.

Airflow

The total quantity of air and the water vapor it carries passing through the tower, quoted in cubic feet per minute at the discharge and corrected to standard air. Correcting to a standard density matters because warm saturated discharge air is lighter than the air that entered, so an uncorrected figure flatters the fan.

Algae

Simple plant growth that needs light and air, and therefore colonizes exactly the parts of a tower that are both wet and daylit, open distribution basins above all. It blocks nozzles and fill passages and carries through to foul heat exchanger tubes. It is the standing argument for basin covers on crossflow towers.

Algaecide

A treatment chemical dosed to suppress algae and slime. Chlorine, copper, and phenolic compounds are all long-established. Chemical control addresses the symptom; excluding light from the water addresses the cause.

Ambient

The air surrounding the tower but not yet influenced by it; in practice, upwind and clear of the discharge plume. The qualification is not academic: measure in the wrong place and you are reading the tower's own exhaust, which will make a healthy tower look deficient.

Ambient wet-bulb temperature

The wet-bulb temperature of genuinely undisturbed air upwind of the tower, read with a mechanically aspirated psychrometer. This is the figure a tower is designed against, and the one against which its cold water temperature is judged. See also: Wet-bulb temperature (WBT), Psychrometer

Anchor bolt

The threaded bolt cast into the concrete basin or fixed to supporting steel, to which the anchor casting is bolted. In older towers these corrode where they pass through the concrete face, and they are worth exposing and checking during any structural inspection, because a sound casting on a wasted bolt is not an anchorage.

Anchor casting also: Column anchor

The fitting that ties a structural column to its foundation. The casting is the fitting itself and does not include the anchor bolt that secures it.

Approach

How close the tower brings the cold water to the ambient wet-bulb temperature: cold water temperature minus wet bulb, in °F. This is the number that measures the tower itself. A smaller approach is better performance, an approach of zero is thermodynamically unreachable, and closing the approach is what a thermal upgrade is bought to do. Approach and range are routinely confused, and they are not interchangeable. See also: Range, Wet-bulb temperature (WBT)

Atmospheric tower

A tower with no fans, relying on wind and on natural air movement through the fill. Performance therefore varies with the weather in a way a mechanical draft tower's does not. Largely superseded, but examples remain in service.

B

Basin

Used on its own, "basin" is ambiguous: a tower has a cold water basin underneath it and, if it is a crossflow tower, distribution basins on top. Worth resolving before ordering anything, because the two have nothing in common. See also: Cold water basin, Distribution basin

Basin curb

The top of the cold water basin wall. It is conventionally the datum for tower elevations and for pumping head, so a dimension quoted without its datum should be assumed to start here.

Basin sump

Another name for the sump: the deepest part of the cold water basin, where the pump suction is taken. See also: Sump

Bay

The space between two adjacent bents. Bays are the natural unit for describing where work is needed, since a structural repair is usually scoped and priced bay by bay. See also: Bent

Beam

In cooling tower framing, used interchangeably with joist for a horizontal supporting member. See also: Joists

Bent

One complete transverse line of structural framing: the columns with their girts and ties, and any diagonal bracing in that plane. A tower is effectively a row of bents, and bent numbers are how positions inside a tower get referenced on drawings and in reports.

BHP

Abbreviation for brake horsepower. See also: Brake horsepower (BHP)

Blowdown

Circulating water deliberately discharged to stop dissolved solids concentrating without limit as evaporation removes pure water and leaves its impurities behind. Too little blowdown scales and corrodes the tower; too much wastes water, chemicals and the heat in it. See also: Cycles of concentration, Make-up

Brace

A diagonal member that stiffens the frame against lateral load. See diagonal. See also: Diagonal

Brake horsepower (BHP)

The power actually delivered at the shaft of a motor or engine, as opposed to the power it draws or the power stamped on its nameplate.

British thermal unit (BTU)

The heat needed to raise one pound of water by one degree Fahrenheit. Cooling tower heat loads are quoted in BTU per hour, and for water the arithmetic is convenient: BTU/hr is approximately gallons per minute × 500 × range.

C

Caissons

Deep foundation elements carrying tower loads down to competent material. Common under large field-erected towers on soft Gulf Coast ground. See also: Foundation

Capacity

In a cooling tower context this almost always means thermal capacity (flow at a stated duty) rather than a volume. See also: Thermal capacity

Casing

The sidewalls and end walls that enclose the tower and contain the airflow, excluding the louvers. Casing that has split, gone soft or blown off does more than look poor: it lets air bypass the fill, and lost capacity follows.

Cell

The smallest part of a tower that works on its own, with its own air and water path, bounded by end walls or partition walls. Each cell may carry more than one fan or distribution system. Cells are why multi-cell towers can be repaired without a full shutdown: one is isolated and worked on while the rest keep running.

Cell dimensions

Width is measured across the tower, perpendicular to its long axis; length runs parallel to that axis; height runs from the basin curb to the top of the fan deck, excluding the fan stack. Nominal width and length are taken between column centerlines rather than to outside faces. See also: Nominal tower dimensions, Overall tower dimensions

Circulating water rate

The flow of hot water arriving at the tower, in gallons per minute. With range, it fixes the heat load.

Coil shed

The part of the structure that houses tubular heat exchangers, where the tower serves a closed circuit rather than cooling the circulating water directly.

Cold water basin

The open-topped structure beneath the tower that catches the cooled water and routes it to the pump suction. It is also where silt, debris and biological growth accumulate, and on steel-basin towers it is very often the first thing to fail by corroding through from the inside.

Cold water temperature (CWT)

The temperature of water entering the cold water basin, taken before make-up is added or blowdown removed. Together with the wet bulb it gives the approach, so where it is measured matters. See also: Approach

Column

A main vertical member of the tower frame, carrying load down to the anchor casting and foundation. In timber towers, columns decay preferentially at the waterline and at bolted connections, where moisture is held against the wood.

Column anchor

Another name for the anchor casting. See also: Anchor casting

Concentration

Ambiguous on its own. In water chemistry it refers to cycles of concentration; in tower sizing it is sometimes used loosely for water loading. See also: Water loading, Cycles of concentration

Concrete cooling tower

A tower whose structure is reinforced concrete. Extremely durable and effectively immune to the rot and corrosion that end timber and steel towers, so work on them is almost entirely internals, distribution and mechanical equipment. Where the concrete itself is distressed, the causes are usually reinforcement corrosion or freeze damage.

Cooling range

See range. See also: Range

Counterflow tower

A tower in which air travels upward while water falls downward against it. Distribution is a closed pressurized system of header, laterals and spray nozzles, which keeps light off the water and largely prevents algae, at the cost of hiding developing problems until the system is opened. Counterflow generally reaches a closer approach than crossflow, which is why converting from crossflow to counterflow is a genuine upgrade route. See also: Crossflow tower

Crossflow tower

A tower in which air passes horizontally through the fill while water falls vertically through it. Water is delivered to open basins above the fill and fed through gravity nozzles, so distribution can be inspected and cleaned while the cell runs, the great practical advantage of the arrangement. The open basins admit daylight, so algae is a permanent maintenance item. See also: Counterflow tower

Cycles of concentration

How many times the dissolved solids in the circulating water have concentrated relative to the make-up. It is measured with a soluble tracer that does not precipitate (chlorides are conventional) by comparing its concentration in the circulating water with that in the make-up. Running higher cycles saves water and chemicals but pushes the water towards scaling, so the figure is a deliberate trade-off rather than a target to maximize. See also: Blowdown, Make-up

D

DBT

Abbreviation for dry bulb temperature. See also: Dry bulb temperature (DBT)

Deck support also: Fill support

The framing that carries the fill and, in a crossflow tower, the distribution basin above it. It spends its life continuously wet and loaded, and because it sits under the fill it is among the least visible parts of the tower, which is why deck support failure is so often found only once fill is pulled for replacement.

Design condition

The set of figures a tower was sold against, and the only proper basis for judging it. For a mechanical draft tower that is hot water temperature, cold water temperature, flow and wet-bulb temperature. A natural draft tower needs one more, either dry bulb temperature or relative humidity, because its draft depends on air density. Any claim about a tower's performance is meaningless without the design condition beside it.

Desilting sump

A low point in the cold water basin where silt collects so it can be flushed to drain rather than be drawn into the pumps.

Diagonal also: Brace

A member set at an angle within a bent to resist lateral loads from wind and, where relevant, seismic action. Diagonals carry load only when their end connections are sound, so a slack, missing or corroded-through diagonal is a structural finding and not a cosmetic one.

Diffusion tray

A tray immediately beneath the distribution basin or nozzle bank that catches the incoming water and spreads it evenly across the fill below. Its job is to convert a pattern of discrete streams into uniform coverage, and where it has collapsed the fill below it will be unevenly wetted no matter how good the nozzles are. See also: Fill, Distribution basin

Distribution basin

The shallow, normally open-topped basin above the fill of a crossflow tower, which holds hot water and feeds it down through gravity nozzles. Its water level sets the flow through every nozzle, so silt, growth or debris that disturbs that level disturbs distribution across the whole cell.

Distribution box

A box fed from the manifold and valve assembly of a crossflow tower which breaks up the incoming jet and spreads flow evenly in all directions into the distribution basin, so the nozzles see a settled water level rather than a moving stream.

Distribution nozzle

See nozzle. See also: Nozzle

Distribution system

Everything between the inlet connection and the fill that spreads hot water across the tower's plan area. In a counterflow tower that means the main header, the laterals and the spray nozzles; in a crossflow tower it means the header or manifold, the valves, the distribution box, the basin and its nozzles. Distribution is the most common cause of lost thermal performance, because fill can only cool water that is actually delivered to it. See also: Counterflow tower, Crossflow tower, Nozzle

Double-flow water cooling tower

A crossflow tower with fill on both sides of a single shared plenum, so air enters through two opposite faces. It fits more fill into a given footprint, which is why large field-erected towers are commonly built this way. Both inlet faces and both distribution basins need attention, and the less accessible side is regularly neglected.

Drift

Circulating water carried out of the tower as liquid droplets in the discharge air, normally quoted as a percentage of circulating flow. Drift is not evaporation: evaporated water leaves its dissolved solids behind, while drift carries them away with it. That is why drift, and not evaporation, is what deposits salts on surrounding plant and vehicles. See also: Drift eliminator, Evaporation loss

Drift eliminator

A baffle assembly in the leaving air path that makes the air change direction sharply so entrained droplets strike a surface and drain back into the tower. Made in PVC, timber, fiber-cement, steel and other materials. Eliminators that have collapsed, gone brittle or blown out show up immediately as visible drift and as deposits downwind.

Driver

Whatever provides the power for the fan drive: usually an electric motor, but gas engines, steam turbines and hydraulic motors are all found on cooling towers. See also: Fan driver assembly

Driveshaft

The shaft, with its couplings, that carries torque from the driver to the gear reducer. It spans a gap between two separately mounted machines in a wet, vibrating environment, so alignment and coupling condition are what determine its life. A driveshaft failure at speed is violent and takes surrounding equipment with it, which is the reason vibration switches exist.

Dry bulb temperature (DBT)

Air temperature as an ordinary thermometer reads it, in °F, taken next to the tower. It is what most people mean by "temperature", but a mechanical draft tower's performance is governed by the wet bulb, not this. See also: Wet-bulb temperature (WBT)

E

Effective volume

See net effective volume. See also: Net effective volume

End wall

The wall closing off the end of the tower structure.

Entering (inlet) wet-bulb temperature

The average wet-bulb temperature of the air actually entering the tower, in °F. Unlike the ambient figure this one includes any recirculation of the tower's own discharge, so it is the higher of the two whenever recirculation is occurring, and the difference between them is how recirculation is quantified. See also: Ambient wet-bulb temperature, Recirculation

Entering air

Air drawn into the tower through the air inlet on an induced draft tower, or pushed in by the fan on a forced draft tower.

Evaporation loss

Water lost to the atmosphere as vapor, which is the mechanism by which the tower cools. It is unavoidable and roughly proportional to the heat rejected. Because evaporation removes pure water and leaves dissolved solids behind, it is also the reason blowdown is necessary. See also: Drift, Make-up

Exhaust air

The warm, effectively saturated mixture of air and water vapor leaving the tower. See also: Airflow

Exhaust wet-bulb temperature

The average wet-bulb temperature of the air discharged from the tower.

Exit diameter

On a hyperbolic natural draft tower, the diameter of the shell at the top. On an induced draft tower, the inside diameter at the top of the fan stack.

F

Fan

The device that moves air through a mechanical draft tower, arranged either as induced draft at the discharge or forced draft at the inlet. Cooling tower fans are large, slow and lightly built for their diameter, and they run in a corrosive wet atmosphere, so blade condition, hub condition and pitch uniformity all matter more than they would in a dry duty. See also: Fan pitch, Mechanical draft cooling tower

Fan deck

The surface closing the top of an induced draft tower. On a counterflow tower it covers the whole top except the fan openings; on a crossflow tower it covers only the plenum, leaving the distribution basins open, unless an extended fan deck is fitted to enclose them. It is also the working platform for all mechanical maintenance, which makes its structural condition a safety matter and not just a structural one.

Fan driver assembly

The complete power train to the fan: typically motor, driveshaft, gear reducer and the steel supporting them. It is treated as one system because misalignment anywhere in it appears as vibration and wear everywhere in it. See also: Driver, Driveshaft, Gear reducer

Fan driver input

The power drawn by the driver. For a three-phase AC motor it is obtained from measured amps and volts together with the power factor and efficiency, using the √3 factor for three-phase power.

Fan driver output

The brake horsepower the driver actually delivers into the driveshaft: the input power reduced by the driver's efficiency.

Fan guard

A protective screen over the fan, at the inlet of a forced draft fan or at the discharge of an induced draft one.

Fan pitch

The angle of the fan blades from horizontal, which sets how much air the fan moves and how much power it absorbs. Pitch is adjustable on most cooling tower fans, and getting it wrong is expensive in both directions: too flat and the tower is short of air, too steep and the motor overloads. Blades must be set uniformly, since a single blade out of pitch shows up as vibration.

Fan stack

The cylinder surrounding the fan, in fiberglass, concrete or timber. It is not merely a shroud; it controls the air path through and above the fan, so a stack that is distorted, split or has lost its seal to the fan deck costs airflow. The clearance between blade tips and stack is a real performance parameter. See also: Velocity recovery fan stack

Fan stack height

The distance from the top of the fan deck to the top of the fan stack.

Fill

The medium the hot water is spread over, and where most of the cooling in a tower actually happens. It works by holding water as a thin film (film fill) or breaking it into droplets (splash fill) to expose the greatest possible surface to the air. Made in PVC, timber, fiber-cement and steel. Fill is the single largest lever on thermal performance, and replacing degraded splash fill with modern film fill is the most common upgrade there is, but film fill is far less tolerant of dirty water, so the choice depends on the water as much as on the duty. See also: Fill bars, Splash bar, Drift eliminator

Fill bars

The assembly of splash bars that makes up splash-type fill. Each bar interrupts the falling water, breaking it into smaller droplets and presenting a wetted surface for contact with the air. Bars that have been washed out of position leave a channel through which water falls uncooled. See also: Splash bar, Fill

Fill hanger

The support system holding fill in position. Hangers are individually light, permanently wet and easy to overlook, and sagging fill is usually a hanger problem rather than a fill problem.

Fill support

See deck support. See also: Deck support

Fill tray

The splash bar assembly forming the fill in a crossflow tower. See fill bars for how it works. See also: Fill bars

Float valve

A float-operated valve, normally controlling make-up water into the cold water basin. When it sticks the consequences run both ways: shut, and the basin runs down towards losing pump suction; open, and treated water runs to overflow continuously. See also: Make-up

Flow control valve

A manually set valve, generally in the hot water supply line, used to balance flow: between cells of a multi-cell tower, or between the two sides of a double-flow tower.

Flume

A trough, open or closed, used in some designs to carry hot water to the lateral pipes instead of a pressurized pipe.

Fogging

The visible cloud formed when warm saturated air leaving the tower mixes with colder ambient air, becomes supersaturated and condenses part of its vapor into droplets. It is a cold-weather phenomenon and not a sign of malfunction, but where the plume drifts across a road or a boundary it becomes a real constraint and is the reason wet-dry towers exist. See also: Plume, Wet-dry tower

Forced draft cooling tower

A mechanical draft tower with the fans at the air inlet, blowing air through. It puts the mechanical equipment at ground level in dry air, which makes maintenance markedly easier: the main reason it is chosen. The penalties are a greater tendency to draw its own saturated discharge back into the inlet, and vulnerability to icing at the inlet in freezing weather. See also: Induced draft cooling tower

Foundation

Whatever the tower is built on: prepared ground, or a structural assembly in concrete, steel or timber. Differential settlement shows up as distortion in the frame and as distribution basins that no longer sit level, so a tower that will not hold its water level evenly is worth checking at the foundation before the basin. See also: Caissons, Pedestals

Framework members

The structural members carrying the tower's live and dead loads: columns, horizontal ties, diagonals, joists and beams. See also: Column, Horizontal tie, Diagonal, Joists

G

Gear reducer also: Geareducer

The right-angle speed-reducing gearbox between the driveshaft and the fan, bringing motor speed down to the much lower speed a large fan needs. It sits in the wettest, least accessible part of the tower and is the most expensive single mechanical component, which is why oil condition and alignment records repay the effort, and why a reducer can very often be rebuilt in place instead of replaced. See also: Fan driver assembly

GPM

Gallons per minute, the conventional unit for circulating water flow.

H

Handrail

Rail along a walkway, platform edge or stairway. On cooling towers handrail is structural safety equipment in a corrosive environment, and it is assessed against the relevant OSHA requirements rather than by appearance. Handrail that cannot take a load is worse than no handrail, because it will be leaned on.

Header

The main hot water pipe. In a counterflow tower it feeds the lateral pipes that carry water to the spray nozzles; in a crossflow tower it carries water to each cell's distribution basin. See also: Lateral pipe, Manifold

Heat exchanger

Any device that transfers heat between two media. A cooling tower is a direct-contact exchanger, where air and water meet; a shell and tube condenser is indirect, with the two separated by a wall.

Heat load

The heat the tower removes from the circulating water, in BTU per hour. For water it follows from flow and range: approximately gallons per minute × 500 × (hot water temperature − cold water temperature). Heat load is set by the process, not by the tower. See also: Range, Circulating water rate

Horizontal tie also: Girt

A load-bearing horizontal member in the tower frame, tying columns together within a bent.

Hot water temperature (HWT)

The temperature in °F of circulating water arriving at the tower distribution system.

Hydrogen ion concentration also: pH

The acidity or alkalinity of the circulating or make-up water on the pH scale, where 7.0 is neutral, below 7.0 acidic and above 7.0 alkaline. It drives material selection throughout the tower: water well away from neutral in either direction rules out some metals and fixes what fill, hardware and fasteners can be used.

Hyperbolic tower

The tall waisted concrete shell associated with power stations, relying entirely on natural draft. With no fans there is no mechanical equipment to maintain, so work on these towers is fill, distribution, eliminators and concrete. See also: Natural draft cooling tower, Throat diameter

I

Induced draft cooling tower

A mechanical draft tower with the fans in the leaving air at the top, drawing air through. This is the great majority of industrial towers. It places the gear reducer, driveshaft, motor and fan up on the fan deck in warm saturated air, which is precisely why access, walkways and grating condition matter so much on these towers. See also: Forced draft cooling tower

Inlet connection

The fitting where the circulating water supply piping, usually a riser, joins the tower distribution system. See also: Riser

Inlet wet-bulb temperature

The average of wet-bulb readings taken at several evenly spaced stations close to the tower's air inlets, including both the windward and leeward sides. Averaging around the tower is what makes the figure representative, since recirculation and wind affect the faces unequally. See also: Entering (inlet) wet-bulb temperature

J

Joists

Horizontal framing members acting as supports.

L

Ladder

Fixed vertical access from grade to the fan deck. Cage, clearance and rung condition are all inspection items, and a ladder is frequently the finding that stops a crew working before anything on the tower itself does.

Landing

A platform giving the offset clearance a stairway or ladder run requires.

Lateral pipe

A pipe or flume distributing water from the header out to the nozzles or other discharge points above the fill. See also: Header, Nozzle

Lintel

In a natural draft tower, the structural member where the diagonals meet the shell, generally forming the top of the air inlet opening.

Longitudinal

The tower dimension measured parallel to the air intakes, and so perpendicular to the direction of airflow through the fill.

M

Make-up

Water added to replace everything the system loses: evaporation, drift, blowdown and leakage. Make-up quality sets the chemistry of the whole circuit, because whatever it brings in will concentrate. See also: Evaporation loss, Blowdown, Float valve

Manifold

The main header pipe of a crossflow tower. See also: Header

Mechanical draft cooling tower

Any tower in which fans move the air, as opposed to natural or atmospheric draft. There are two arrangements: forced draft, with the fans at the inlet, and induced draft, with the fans in the leaving air. See also: Induced draft cooling tower, Forced draft cooling tower

Mechanical equipment support

The framework carrying the fan drive assembly. Its stiffness is part of the machine: a support that has softened through corrosion or timber decay will not hold alignment, and the vibration that follows is regularly misdiagnosed as a gearbox or fan fault. See also: Fan driver assembly

Motor rated horsepower also: Nameplate horsepower

The horsepower stamped on the nameplate of the fan motor.

N

Natural draft cooling tower

A tower where airflow is driven by the density difference between the warm, moist, lighter air inside and the cooler ambient air outside. As water gives up heat, the warmed air rises and pulls fresh air in at the base. No fans, and therefore no fan power and no mechanical equipment. See also: Hyperbolic tower

Net effective volume also: Effective volume

The part of the tower's internal volume where circulating water and airflow are genuinely in intimate contact. It is the volume that does the cooling, which is smaller than the volume the structure encloses, and shrinks further wherever fill is missing, water is maldistributed or air is bypassing.

Nominal tower dimensions

Width and length taken between column centerlines or walls, with height from the top of the basin curb to the top of the fan deck on a counterflow tower, or to the top of the distribution basin on a crossflow tower. Nominal and overall dimensions are not the same, and confusing them is a common source of error when planning access or lifts. See also: Overall tower dimensions, Cell dimensions

Nozzle also: Sprayer

The fitting that meters water onto the fill. Counterflow nozzles work under pressure and produce a spray pattern; crossflow nozzles work by gravity from the distribution basin and produce a solid stream, usually with a splash plate. Nozzles are cheap, numerous and the single most common cause of lost performance, because a blocked or missing one leaves fill dry while the water it should have carried goes somewhere it is not needed. See also: Distribution system, Lateral pipe

O

Obstruction lights

Aviation warning lights required by FAA regulation at the top of the shell of a natural draft tower.

Overall tower dimensions

Width across the tower perpendicular to its long axis, length along that axis, and height from the top of the basin curb to the top of the fan stack or the tower's exit plane, all to outside faces rather than centerlines. See also: Nominal tower dimensions

P

Partition wall

An internal wall dividing a tower into cells or separating fan plenums. Partition walls are what make cell isolation possible, so a partition that has holed will let air short-circuit from a running cell into one that has been taken out of service, and will cost performance on the cell that is running. See also: Cell

Pedestals

Concrete components carrying the load from the inclined shell support columns of a natural draft tower down into the foundation. See also: Foundation

pH

See hydrogen ion concentration. See also: Hydrogen ion concentration

Pier

In a timber tower, a support raising column footings clear of the basin floor or of foundation grade, keeping the column base out of standing water, which is where timber decays fastest.

Pitot tube

An instrument that reads flow from the difference between total and static pressure. On cooling towers its main use is accurate measurement of circulating water flow, which is required for any credible thermal performance test; a test is only as good as its flow measurement.

Plenum

The enclosed air space inside the tower: between the drift eliminators and the fan stack on an induced draft tower, or between the fan and the fill on a forced draft one. It is also a confined space, and entry is permitted and monitored accordingly.

Plume

The visible cloud of condensed water vapor above a tower. It is condensed pure water, not contamination, and it is most visible in cold weather. See also: Fogging

Power factor

The ratio of real power in watts to apparent power, the product of volts and amps. It is needed to turn an ammeter reading into actual motor power.

Psychrometer

The instrument used to measure wet-bulb temperature, by reading a thermometer whose bulb is covered by a wetted wick in a moving air stream. A mechanically aspirated type is acceptable for testing provided it is shielded from radiant heat and the air velocity over the wick is held to roughly 1,000 ft/min. With a dry wick the same instrument simply reads dry bulb, which is a classic source of bad test data. See also: Wet-bulb temperature (WBT), Dry bulb temperature (DBT)

R

Range also: Cooling range

The temperature drop the tower produces: hot water temperature minus cold water temperature. Range is fixed by the process heat load and the circulating flow, not by the tower, which is the crucial difference between range and approach: change the tower and the approach changes, while the range stays where the process puts it. These two figures are the ones most often mixed up in discussions of tower performance. See also: Approach, Heat load

Rated horsepower

Nameplate horsepower for fan drivers other than electric motors.

Recirculation

The condition where part of the tower's own warm, saturated discharge is drawn back into its air inlets along with ambient air. How much occurs depends on the tower's design, where it has been sited and the weather. Its effect is measured as the rise in entering wet-bulb temperature above ambient, and because performance is governed by the entering wet bulb, recirculation makes a sound tower underperform without anything being wrong with it. It is often a siting problem rather than a tower problem. See also: Entering (inlet) wet-bulb temperature, Forced draft cooling tower

Redistribution basin

An intermediate elevated basin between the hot water basins of a tall crossflow tower, catching water partway down and redistributing it so coverage stays even over the full height of the fill. See also: Distribution basin

Riser

The vertical pipe carrying circulating water from the supply line at the base of the tower up to the inlet connection. See also: Inlet connection, Supply header

S

Shell height

On a natural draft tower, the height from the top of the basin curb to the top of the shell.

Single-flow water cooling tower

A crossflow tower with fill on one side of the plenum only, the other side being closed. Chosen where a tower must sit against a building, or where prevailing wind or recirculation makes a single orientation preferable. See also: Double-flow water cooling tower

Splash bar

The individual horizontal element of a fill tray that provides the splash surface. See also: Fill bars, Fill

Spray-filled cooling tower

A tower with no fill at all, where air and water contact depends entirely on breaking the water up through pressure spray nozzles. Thermally inefficient by modern standards, which makes these towers strong candidates for a fill retrofit.

Standard air

Dry air at a density of 0.075 lb/ft³, corresponding to 70 °F and 29.92 inches of mercury. Airflow figures are corrected to this reference so that measurements taken in different conditions can be compared. See also: Airflow

Static pressure

The pressure of a gas or fluid measured without reference to its motion: total pressure less velocity pressure. In cooling tower work it is quoted in inches of water or mercury and is how the resistance the fan works against is characterized.

Sump also: Basin sump

The lowest part of the cold water basin, where the cooled water collects and the pump suction is normally taken. See also: Cold water basin, Desilting sump

Supply header

The part of the supply system from which the riser or inlet connection draws the circulating water flow. See also: Riser

T

Thermal capacity also: Capacity

The flow in gallons per minute a tower can cool at a stated design condition. A capacity figure quoted without its design condition means nothing, and thermal performance testing is what establishes capacity as a percentage of design. See also: Design condition

Throat diameter

The diameter of a natural draft tower's shell at its narrowest point, the waist of the hyperbolic profile.

Total pumping head

The total head, measured above the basin curb, needed to push the circulating water through the distribution system. It is the figure the pumps have to be selected against, and one reason converting a distribution system changes the pumping duty as well as the thermal performance. See also: Basin curb

U

Underflow water cooling tower

An unusual crossflow arrangement with a forced draft fan lying horizontally at the bottom of the plenum. Air travels out horizontally through the fill sections and discharges vertically at each end of the tower.

V

Velocity recovery fan stack

A venturi-shaped fan stack. Its flared profile recovers velocity pressure from the leaving air, reducing the total head the fan works against. The result can be taken either as more airflow at the same power or the same airflow at less power, so fitting one is a recognized way to buy capacity or save energy without touching the fill. See also: Fan stack

Vibration cut-out switch

A safety switch that de-energizes the fan driver if the mechanical assembly vibrates beyond a set limit. It is the only protection against a fan or driveshaft failure escalating into a destroyed gearbox, a wrecked fan deck or worse. A switch that has tripped is reporting a fault, and resetting it to get the cell back is the single most costly mistake made on cooling towers. See also: Fan driver assembly

W

Water cooling tower

An enclosed steady-flow device that cools water by evaporating a small part of it into air brought into direct contact with the rest.

Water loading

Circulating flow per unit of effective horizontal wetted area, in gallons per minute per square foot. It is how heavily a tower is being worked, and comparing it with what the fill was designed for quickly shows whether a tower is overloaded or has been derated by lost fill area.

Wet-bulb temperature (WBT)

The temperature read by a thermometer with a wetted wick in moving air, formally the temperature of adiabatic saturation. It is the lowest temperature evaporative cooling can theoretically reach, and therefore the limit every cooling tower works against. This, not the dry bulb, is the ambient figure that governs how a tower performs, which is why a tower can struggle on a humid 85 °F day and cope easily on a dry 95 °F one. See also: Psychrometer, Approach, Dry bulb temperature (DBT)

Wet-dry tower

An evaporative tower combined with a dry, non-evaporative heat exchanger section, arranged to reduce or eliminate the visible plume in cold weather. Chosen where fogging would otherwise be unacceptable: across a public road, a boundary or a flight path. See also: Fogging, Plume

Wind load

The load wind applies to the structure, in lb/ft². Cooling towers present a large area for their weight, so wind governs much of their bracing, and on the Gulf Coast the design wind speed is the reason structural upgrades are so often specified after a named storm. See also: Diagonal