Cooling Tower Glossary

DRB Industries Cooling Tower Glossary

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.


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.


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.


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.

Anchor bolt

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


casting A device for attaching the tower structure to the foundation; it does not include the anchor bolt. Also known as Column Anchor.

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.

Blow out

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


Water discharged from the system to control concentration of salts or other impurities in the circulating water.

Canopy (Natural draft tower)

Connects the hyperbolic shell to the cooling section, acting as an air conduit and air seal between the two.

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.

Concrete cooling tower

Cooling tower in which the structure is made of concrete.

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.

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.

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 header

Pipe or flume delivering water from inlet connection to lateral headers, troughs, flumes, or distribution basins.

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.

Fan driver output

Brake horsepower output of the driver to the drive shaft. Fan driver input x motor efficiency.

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.


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.

Gear reducer

A reduction gear, commonly used on fan drivers to reduce driver speed to fan speed requirements. Also known as Speed Reducer.


(abbr.) Gallons per minute


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.)


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 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).

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.)


Supports for fan decking, or for film fill modules.


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.


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.


The main header pipe in a crossflow tower. (See Header.)


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.)

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.

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.


Used as a transition from the cross struts to the foundation of a natural draft tower or fan mount in force draft towers.


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.

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.

Safety handrail

Railing around top of tower, platforms, and stairways; usually composed of top handrail, kneerail, and toeboard. (See Handrail.)

Single-flow water-cooling tower

A crossflow tower having a fill section on one side of the plenum chamber only.

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.


– 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


– 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.


– Always measured perpendicular to air intakes.

Abbreviations and Letter Symbols

A – approach
acfm – actual cubic feet per minute
alt – altitude
amb – ampere
amp – atmosphere
atm – azimuth
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