Cooling Tower Terminology

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

Bleed Off – is the circulating water in the tower which is discharged to waste to help keep the dissolved solids concentration of the water below a maximum allowable limit. As a result of evaporation, dissolved solids concentration will continually increase unless reduced by bleed off.

Blowdown – water intentionally discharged from the cooling system to control concentrations of salts and other impurities in the circulating water

British Thermal Unit (BTU) – the heat energy required to raise the temperature of one pound of water one degree Fahrenheit in the range from 32° F to 212° F

Cell – smallest tower subdivision which can function independently

Cold water temperature – temperature of the circulating water leaving the cooling tower

Counterflow tower – design in which air flows upward through the fill section and interfaces counter currently with the down coming hot water.

Crossflow tower – design in which air flows horizontally across the fill section and interfaces perpendicularly with the falling hot water

CTI – Cooling Tower Institute.  Organization for testing and certification of cooling towers

Cycles of Concentration – compares dissolved solids in makeup with solids in the circulating water

Drift – water lost from the tower as liquid droplets entrained in the exhaust air stream

Drift eliminators – baffling to control the drift loss in towers

Heat load – heat removed from the circulating water within the tower

Makeup – water added to the circulating water to replace water lost from the system by evaporation, drift, blowdown and leakage

Mechanical draft tower – tower in which a fan moves the air through the fill

Natural draft tower – hyperbolic design in which air moves through the fill because of pressure difference between top and bottom of tower, creating a chimney effect

pH – number between 0 and 14 indicating degree of acidity (below 7) and alkalinity (above 7)

Pumping head – minimum pressure required to lift water from basin level to tower top and force it into the water distribution system

Recirculation air – portion of the exhaust air which reenters the tower measured on the basis of the increase in entering wet-bulb temperature compared to the ambient

TDS – total dissolved solids contained in solution in cooling system water

Ton – an evaporative cooling ton is 15,000 Btu / hr

Water loading – circulating water flow expressed in gal / min per square foot of effective horizontal wetted area of the tower

Wet-bulb temperature – temperature indicated by a psychrometer

This is the formula used to calculate the heating surface of a power plant boiler:

An HRT boiler is 5 ft in diameter and 16 ft long. It contains 60 tubes of 3 inch outside diameter and 2.732 inch inside diameter. Find the boiler-heating surface. Take the inner surface of tubes, half of the shell surface, and two-thirds of the tube plate area, less the area of the tube holes.

Circumference of shell = 5 * 3.14 = 15.700 ft

Half circumference of shell = 15,700 * ½  = 7.850 ft

Area of half of shell = 7.850 * 16 = 125.60 sq. ft

Inner circumference of fire tube = 2.732 * 3.14 = 8.578 in or         8.578 / 12 = 0.7148ft

Surface of one tube = 0.7148 * 16 = 11.437 sq. ft

Surface of 60 tubes = 60 * 11.437 = 686.2 sq. ft

Total area of one tube sheet = 0.785 * 5 * 5 = 19.625

Two-thirds area one tube sheet = 2/3 * 19.625 = 13.083 sq. ft

Area of one tube hole = 0.785 * 3 * 3 = 7.065 sq. in

Area of 60 tube holes = 60 * 7.065 = 423.9 sq. in or 423.9 / 144 = 2.94 sq. ft

Net heating surface of one tube sheet = 13.08 – 2.94 = 10.14 sq. ft

Net surface of two tube sheets = 10.14 * 2 = 20.28 sq. ft

Total net area = 125.60 + 686.2 + 20.28 = 832.08 sq. ft

832 sq. ft