Cooling Tower Fundamentals - SPX Cooling Technologies

SECTION I
20
In point of fact, design air conditions for the
example problem would not have been given to
a cooling tower designer as “78°F at a 50% relative
humidity”. Rather, instructions would have
been to design for a “65°F wet-bulb temperature”.
Only if there was a requirement to know the
exact amount of evaporation, or if the selection
were for something other than a normal mechanical
draft cooling tower, would there have been
a need to know the design dry-bulb temperature
of the air, or its relative humidity.
Wet-bulb temperatures are measured by
causing air to move across a thermometer
whose bulb (properly shielded) is encased in a
wetted muslin “sock”. As the air moves across
the wetted bulb, moisture is evaporated and
sensible heat is transferred to the wick, cooling
the mercury and causing equilibrium to be
reached at the wet-bulb temperature. For most
acceptable and consistent results, the velocity
of the air across the wick must be approximately
1000 fpm, and the water used to wet the wick
should be as close as possible to the wet-bulb
temperature. Distilled water is normally recommended
for wetting of the wick.
When a wet-bulb thermometer and a dry-bulb
thermometer are combined in a common device,
simultaneous coincident readings can be taken,
and the device is called a “psychrometer”.
Figure 20 — Sling psychrometer.
Although the mechanically aspirated
psychrometer (Fig. 19) is generally used for purposes
of testing and scientific study, the sling
psychrometer (Fig. 20), used as indicated in Figure
21, can give satisfactory results.
Selection of the design wet-bulb temperature
must be made on the basis of conditions existing
at the site proposed for a cooling tower, and
should be that which will result in the optimum
cold water temperature at, or near, the time of
peak load demand. Performance analyses have
shown that most industrial installations based
upon wet-bulb temperatures which are exceeded
in no more than 5% of the total hours during
a normal summer have given satisfactory results.
The hours in which peak wet-bulb temperatures
exceed the upper 5% level are seldom consecutive
hours, and usually occur in periods of
relatively short duration. The “flywheel” effect of
the total water system inventory is usually sufficient
to carry through the above-average periods
without detrimental results.
Figure 21 — Sling psychrometer as used to determine wet
bulb temperature.
There are some applications, however, where
a comprehensive study should be made of
the daily (Fig. 22) wet-bulb temperature cycle
through critical months and, in some instances,
the entire year. (Fig. 23) High-load power generating
stations, applications of the cooling
tower to off-season free cooling (Sect. V-K), and
certain critical processes fall into this category.
Wet-bulb duration curves (Fig. 24) may be established
from which it is possible to evaluate
and compare equipment installed costs, plant
operating costs, efficiencies and capabilities
at various operating wet-bulb conditions. From
such a study, which would consider both seasonal
loads and the annual wet-bulb pattern, it is
possible to select the optimum cooling tower for
the installation. In many cases, the study would
result in reduced capital expenditure, while still
providing the desired ultimate operating characteristics.
Air temperatures, wet-bulb as well as coincident
dry-bulb, are routinely measured and
recorded by the United States Weather Bureau,
worldwide U.S. military installations, airports, and
various other organizations to whom anticipated
weather patterns, and specific air conditions, are
of vital concern. Compilations of this data exist
which are invaluable to both users and designers
of cooling towers. One such publication is
entitled “Engineering Weather Data”, compiled
by the Departments of the Army, Navy and Air
Force, and available at www.wbdg.org. Excerpts