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The rate at which heat is transferred in a
cooling tower depends upon four factors:
- the area of the water surface in contact with
air;
- the relative velocity of air and water;
- the time of contact between air and water;
- the difference between the wet bulb
temperature of the inlet air, A, and the temperature of the returned water, R.
Item (1) depends upon the construction of the
fill; (2) can be controlled by regulating speed of the fans; (3) is a function
of (2) and the height of the tower; (4) is fixed by climate.
Wet bulb temperature can be measured with a sling
psychrometer.
Under ideal conditions, when a stream of unsaturated air passes over a wetted
surface water evaporates saturating the air and lowering the temperature of the
remaining water. When the water becomes cooler than the air, sensible heat flows
from the air to the water, eventually reaching equilibrium at the wet-bulb
temperature, where the loss of heat from the water by evaporation is equal to
the sensible heat. Thus, as water falls through a cooling tower, the latent
heat of vaporization and the sensible heat approach each other so that in an
infinitely high structure the temperature of the bulk water would be equal to
the wet-bulb temperature of the entering air. In a finite tower, however, it is
impossible to achieve zero approach (approach = Supply temperature – wet bulb
temperature) because not all the water falling through the structure can contact
fresh cool air.
One measure of the efficiency of a cooling tower is its approach, which is the
difference between the temperature of the cooled water in the basin of the tower
and wet-bulb temperature of the atmosphere.
The second measure of performance is the cooling range, which is the difference
between the supply temperature and return temperature.
The amount o heat rejected by a cooling tower can be calculated from the cooling
range and the recirculation rate. This is also known as heat duty.
1 BTU is the amount of heat required to raise the temperature of one pound of
water one degree F. Therefore:
Heat duty, BTU/hr = gpm (circulation rate) >< 8.34 lb/gal >< Delta T deg F
Another important characteristic of a cooling towers performance is L/G, the
liquid-gas mass transfer ratio: L/G = (water, kg/hr)/(air, kg/hr)
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Introduction |
Combined heat transfer process |
Heat transfer in cooling tower |
Variables affecting performance of CT heat transfer |
Heat transfer within
cooling system (heat exchanger) |
Types of heat exchanger |
Basic design
procedure and theory |
Designing a test heat exchanger |
Log Mean Temperature
difference | L.M.T.D. Correction factors |
Overall heat transfer coefficient |
Elaborated method for calculating U values |
Effect of scale formation |
Condensation of steam |
Condenser, where the hot fluid temperature varies |
Significance of pressure |
Significance of flow rate |
Methods of checking steam
condenser performance |
Common conversion factors
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