TYPES OF COOLING TOWERS
This section describes the two main types of cooling towers: the natural draft and mechanical draft cooling towers.
Natural draft cooling tower
· Cross flow natural draft cooling tower
· Counter flow natural draft cooling tower
Mechanical draft cooling tower
BASIC TERMS
Air flow
Total quantity of air
including the associated water vapor flowing through the tower.
Re-circulation
That proportion of the
outlet air which re-enters the tower.
Drift loss
Water loss caused by
liquid drops carried away by the outlet air stream.
Purge
Water deliberately discharged
from the system in order to reduce the concentration of salts and other
impurities in the circulating water.
Make-up
Water added to the
circulating water system to replace leakage, evaporation, drift loss and purge.
Concentration ratio
Ratio of the total
mass of impurities in the circulating water to the corresponding total mass in
the make-up water.
Cooling range
Difference between the hot
water temperature and the re-cooled water temperature.
Approach
Difference
between re-cooled water temperature and the inlet air wet bulb temperature.
Capacity Control Dampers-
Airfoil blades placed at the discharge if a
centrifugal fan that change position so as to regulate airflow.
Cavitation- The phenomenon that occurs in a water pump when the pressure becomes sufficiently low to allow vaporization of the fluid followed by a sudden collapse of the vapor ‘bubble’ as it passes to the high pressure area of the pump.
Casing- That part of a
cooling tower, which encloses the wet deck fill.
Cold Water Basin- The
collection point near the bottom of a cooling tower for the collection of
cooled water.
Composite- Construction
material utilizing high strength glass materials in a precise order so as to
maximize strength held in place by cured epoxy resins.
Counter flow- A cooling
tower configuration where the air and water flow in opposite directions.
Cross flow- A cooling tower
configuration where the air and water flow at right angles to one another.
Conductivity Monitor- A
device that measures the ease with which electricity passes through cooling
system water. Conductivity is directionally proportional to the amount of
dissolved solids in the water and is used to initiate bleeding, feeding
chemicals, etc..
Cycles of Concentration- The
number of times the solids content of water has been increased. Two fold = 2
cycles; Three fold = 3 cycles, etc..
Discharge Hood- A discharge
duct with sides that gently taper reducing the cross sectional area thereby
accelerating the discharge air. Used to ‘blast’ discharge air from an enclosure
thereby reducing recirculation potential. Note: Suitable for centrifugal fan
towers only.
Dry Bulb- Temperature of air
measured with a conventional thermometer with a dry bulb.
Eliminator- A device placed
in the discharge airstream of a cooling tower that attempts to ‘eliminate’
entrained water droplets. It works by rapidly changing the direction of airflow
causing the heavier water particles to collide with the eliminator surface and
fall back inside the tower.
Equalizer Line- A pipe
connected between the cold water basins of multiple cooling towers. It’s
purpose is to force the ‘equalization’ of water levels.
Fan Deck- The upper
horizontal surface surrounding the fan stacks of a draw-thru, propeller fan
cooling tower.
Fill- Material added to a
cooling tower to enhance evaporation.
Hot Water Basin- Water
collection area at the top of a crossflow cooling tower the bottom of which is
perforated to distribute water over the wet deck fill.
Latent Heat- Heat which
changes the properties of a material without changing its temperature.
Louvers- Horizontal blades
placed at the air inlet of some cooling towers to prevent water splash out.
Psychrometric Chart- A
graphical representation of the physical characteristics of air
Range- A cooling towers
inlet water temperature minus its outlet water temperature.
Sensible Heat- Heat which
increases the temperature of a body to which it is added.
Ton- The rate of heat
transfer represented by 2,000# of ice melting in a 24 hour period.
(12,000Btu/Hr.)
Turn Down- The allowable
percent reduction of inlet water flow to a cooling tower.
Wet Bulb- The temperature
read from the wetted bulb of a thermometer placed in a moving airstream.
Velocity Recovery Stack- A
hyperbolically shaped discharge plenum at the top of a draw through thru, prop
fan cooling tower. The shape increases the efficiency of the fan by converting
some of the velocity pressure to static pressure for increased air flow.
Absolute humidity:
The ratio of mass of vapor to mass of gas is
the absolute humidity.
Relative humidity:
It is defined as the ratio of
partial pressure of vapor to the vapor pressure of the liquid at the gas
temperature. It is usually expressed as percentage.
Percentage humidity:
It is the ratio of actual
humidity to the saturation humidity
Cooling towers are evaporative coolers used for cooling water or other working medium to near the ambient wet-bulb air temperature
Cooling towers use evaporation of water to reject heat from processes such as cooling the circulating water used in oil refineries and power plants, building cooling, or chemical reactions.
