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CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
COOLING TOWER AND CONDENSER
WATER CONTROL
COOLING TOWER PERFORMANCE
CHARACTERISTICS
The cooling tower dissipates the heat collected from the
building by the chiller and chilled water system by cooling the
condenser water. Evaporatively cooled condenser water will
cool refrigerant to within 3.5 kelvins of the outdoor air wet-
bulb temperature. Air cooled condensers will cool refrigerant
to within 11 kelvins of the outdoor air dry-bulb temperature. A
cooling tower normally provides a refrigerant pressure about
17 kelvins lower than an air cooled condenser. This means an
evaporative cooling tower provides a significantly lower cooling
cost than an air cooled condenser.
Figure 25 shows water-air counterflow in a cooling tower. The
fill increases the time that the water and air are in contact making
the cooling tower more efficient. Fill is generally one of two types,
either splash or film type. A splash type fill is a series of slats over
which the water cascades. Film type fill causes the water to flow
down thin closely spaced vertical sheets of plastic.
HEATED
AIR OUT
SPRAY
DISTRIBUTION
FILL
WATER
AIR IN
Fig. 25. Cooling Tower, Showing Water-Air Counterflow.
The range (inlet water temperature minus outlet water
temperature) at design load of a cooling tower is determined
by the amount of heat to be rejected and the water flow used to
carry this heat to the tower. The cooling capability is then
expressed as the design approach (approach specifies how close
to the OA WB a cooling tower can cool water) at design range.
Since most operation is at less than design load and/or design
outdoor air temperatures, partial load operating characteristics
have a strong influence on operating costs. Partial load operating
characteristics are also used to establish the cooling capacity
and capability for free cooling cycles at low outdoor air
temperatures. Partial load characteristics for a tower at design
flow rate are shown in Figure 26.
DRIFT
ELIMINATORS
HOT WATER
FROM
CONDENSER
AIR
AIR IN
TO CONDENSER
WATER PUMP
C2694
316
TEMPERATURE RANGE (KELVINS)
35
30
25
14
15
12
13
10
20
11
8
9
6
7
15
4
5
2
10
3
5
0
0
5
10
15
OA WET BULB TEMPERATURE, ˚C
Reprinted by permission from the ASHRAE
Handbook–1996 Systems and Equipment
Fig. 26. Typical Cooling Tower
Partial Load Characteristics.
In Figure 26 summer design conditions are approximately
27.5 C cold water leaving the tower, 32.5 C water entering,
and OA wet-bulb of 24 C. This is 3.5 kelvins approach at 5K
range. Notice that the point is plotted on the 5K range line. The
same tower used for free cooling at an OA wet-bulb of 10 C
would provide 18 C leaving water (8 kelvins approach) at full
load (5K range) or 14.5 C water (4.5K approach) at 40 percent
load (2K range).
COOLING TOWER CAPACITY CONTROL
Fan control is the usual method of reducing tower capacity
to maintain condenser water supply temperature in mild
weather. A tower bypass valve is used to further limit cooling
when the fans are off (Fig. 27). On-off fan control is very
difficult at light loads because at light load the OA WB is usually
well below design, which increases the tower capacity,
producing short fan "on" periods. Controlling the air volume
with dampers, blade pitch, or fan speed provides much closer
and more stable control. A variable speed fan is more efficient
than a two-speed fan when varying tower capacity.
Modulating tower water bypass for capacity control at low
ambient temperatures can cause freeze-up in some tower designs.
Since, use of a tower bypass mixing valve in the tower leaving
water can lower the pump suction pressure below the pumps
minimum pressure, a diverting valve in the tower entering water
is generally used.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
SUMMER DESIGN
CONDITIONS:
ENTERING WATER, 32.5˚C
LEAVING WATER, 27.5˚C
OA WET BULB, 24˚C
FREE COOLING
EXAMPLES
20
25
C4087
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