Baird, R. D.

Myers, D. M.

Shan, A.

English

NASA Technical Reports Server

IV-C- 107
THE THERMAL PERFORMANCE CHARACTERISTICS
OF LARGE SPRAY COOLING PONDS
R. D. Baird & D. M. Myers
Ford, Bacon & Davis Utah Inc.
Salt Lake City, Utah
A. Shah
Spray Engineering Company
Burlington, Massachusetts
II
ABSTRACT
In recent years, there has been a renewed interest in deter-
mining the performance characteristics of large spray cooling
ponds, since the verification of the adequacy of pond design
is difficult because of the limited amounts of performance
data for large ponds. The results of previous well instrumented
tests on a large pond at a nuclear power installation have been
published and analyzed over the limited ranges of data for
critical parameters. The findings of these tests are briefly
discussed.
Testing has recently been performed on a large conventional
spray cooling pond presently operating in southern Florida.
This spray pond is similar in design to ponds used in nuclear
power plants, and its performance characteristics are rep-
resentative of those ponds. The paper describes the facility
and testing procedures, and reports the results of preliminary
testing. The new data reported broadens the data base that
can be used by engineers to design large fixed piping spray
pond systems.
INTRODUCTION
Spray cooling ponds have been used for many years as an effi-
cient and economical means of dissipating waste heat. Past
experience has shown that an 8 to 11° C (15 to 20° F) approach
to wet bulb temperature for large ponds can be achieved with
a straightforward system design which compares quite favorably
to alternative waste heat rejection systems when operating
costs and maintenance are considered. In applications where
a closer approach is required, design modifications result in
acceptible system performance, albeit at greater capital and
operating costs.
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In recent years, spray cooling ponds have been used in the
nuclear power industry in safety related cooling water systems
(Ultimate Heat Sinks) because of their reliability and adequate
performance. However, considerable difficulties have plagued
the design effort for spray cooling ponds for two reasons.
First, the performance of spray cooling ponds is complex and
strongly influenced by numerous variables whose effect on per-
formance can only be described over the full range of variables
by computer analyses. Secondly, a limited data base for the
performance of large spray ponds makes very difficult the
verification of design adequacy, through any means but actual
pond performance testing.
It is precisely this lack of adequate performance data for
large ponds which promted the investigation reported herein.
Tests have been performed on a large spray cooling pond, and
the data taken compares well with results of tests performed
earlier at Rancho Seco. [1] The objectives of this paper are to
briefly discuss factors significant in spray pond operation,
to describe the facility tested and the test procedures used,
to report test results, to compare the results to those of
earlier tests, and to evaluate the needs for further testing.
GENERAL CONSIDERATIONS
There are many design parameters for spray cooling ponds which
have significant effects on pond performance. Most important
among these are the following; meteorology (including, but not
limited to wet bulb temperature and wind velocity), hot water
temperature, pond geometry (size, spray unit spacing, and
nozzle elevation) and nozzle operating pressure.
Meteorology
Two uncontrollable variables which are dominant in determining
spray cooling pond performance are the wet bulb temperature
and the wind velocity.
Wet Bulb Temperature
The wet bulb temperature represents the minimum temperature
to which a droplet could be cooled if it were allowed to come
to equilibrium with its surroundings. However, in a spray
coolin'g pond, the droplet flight time is such that equilib-
rium is not achieved and the actual temperature of the droplet
as it enters the pond is significantly greater than local wet
bulb temperature. A frequent practice is to relate the cooling
achieved to the ambient wet bulb temperature as an expres-
sion of the pond performance.
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Wind Velocity
Ambient wind speeds and directions markedly influence pond per-
formance. At zero wind speed, the draft induced by the in-
creased specific volume in and above the spray region over the
ambient specific volume results in a supply of fresh air to
interior zones of the spray region. As the winds increase,
the plume is shifted downwind and the supply of fresh air to
upwind portions of the spray region is enhanced. In general,
the greater the wind speed, the greater is the availability of
fresh air to all zones of the spray region, and the greater is
the pond heat dissipation capability.
The wind direction has an important influence on pond perform-
ance if the spray region presentation changes with wind direction.
For example, if a pond is designed so that the spray region is
91 m. (300 ft) long by 40 m. (130 ft) deep, pond performance will
change with wind direction. The cooling will be greatest when
the wind is perpendicular to the pond major axis and reduced
when the wind is parallel to the major axis. In any case, how-
ever, the degree of cooling achieved will not be less than that
for low wind speeds.
Hot Water Temperature
The slope of the water vapor pressure versus temperature curve
increases monotonically with increasing temperature. Therefore,
the potential for evaporation, with a given difference between
hot water and wet bulb temperatures, is markedly greater when
the hot water temperature is higher. Since the cooling resulting
from spray cooling systems is dominated by that resulting from
evaporation, performance in spray cooling ponds is more effective
at higher hot water temperatures. Also, the higher hot water
temperature and evaporation rate lead to increased induced air
flow under low wind conditions.
Pond Geometry
Geometry factors which influence pond performance include pond
size, spray unit spacing and nozzle elevation.
Pond Size
As the size of the spray region increases, the ratio of spray
region perimeter to spray region area decreases. Since avail-
ability of fresh air is determined largely by the perimeter,
the cooling which can be achieved per unit of sprayed mass
decreases as the pond size increases. The practical effect of
increased pond size is that droplets from the spray units lo-
cated in the interior zones of the spray region are exposed to
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a degraded environment (with increased moisture content). The
cooling of the droplets in interior zones is less than the
cooling of droplets from spray units in exterior zones of the
spray region where the moisture content of the air is nearer
the ambient moisture content. The overall effect is that the
average cooling achieved per unit mass sprayed decreases with
pond size, even though the total cooling increases.
Spray Unit Spacing
As the spray unit spacing is increased, the availability of
fresh air to each spray unit is increased such that each spray
unit behaves more nearly like an isolated spray unit with an
unlimited availability of fresh air. Thus, pond performance
improves with increased spray unit spacing.
Nozzle Elevation
Increasing the nozzle elevation above the water level increases
the droplet flight time and therefore increases the cooling of
the droplets, though this effect is less significant than the
previously mentioned geometric factors.
Nozzle Pressure
The pressure at which the nozzle operates influences droplet
cooling because of two effects; the shift of droplet size dis-
tribution and a change of droplet flight time.
Droplet Size Distribution
L •
As the nozzle pressure is increased, the droplet size distrib-
ution is generally shifted to smaller sizes. The energy in a
droplet is proportional to its volume. However, heat transfer
is governed by droplet surface area and since the ratio of area
to volume increases as the droplet decreases in size, the cooling
potential is greater for smaller droplets.
Droplet Flight Time
As the nozzle pressure is increased, the droplet exit velocities
increase resulting in longer flight times andf as previously
discussed, produces greater droplet cooling.
TESTING PROGRAM FOR THIS INVESTIGATION
This section will discuss the facility at which these tests were
performed, the variables which were monitored, the testing pro-
cedure and results.
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Test Facility
The facility at which these tests were performed is located in
southern Florida where wet bulb temperatures are generally high.
However, during the time when the testing was performed, unusu-
ally cool temperatures existed so performance data at higher
wet bulb temperatures were not obtained.
The spray region is 101 m. (330 ft) long by 49 m. (160 ft) wide
with the pond major axis oriented perpendicular to prevailing
winds. The spray units consisted of 4 nozzles on 1.5 m. (5 ft)
spray arms and a fifth nozzle on the junction box, except for
spray units on the two exterior rows, which lacked the fifth
nozzle. The spray units were spaced 4.0 m. (13 ft) apart on
the rows which were separated by 7.6 m. (25 ft).
The spray flow rate varied from 2,780 I/sec (44,000 gpm) to
3,030 I/sec (48,000 gpm) at pressures of 69,000 Pa (10.0 psi)
and 82,000 Pa (11.9 psi), respectively. The heat load was
approximately 73.3 x 106 watts (250 x 106 BTU/hr).
The pond is oriented so that prevailing winds are perpendicular
to the pond major axis.
Variables Monitored
In order to establish the performance characteristics of the
spray cooling pond the following ambient variables were moni-
tored; wind speed, wind direction, wet bulb temperature, and
dry bulb temperature. In addition, the hot water temperature,
the cooled water temperature, and the pond water temperature
were recorded at appropriate times.
Testing Procedures
In the present tests, batch samples of cooled water were col-
lected just as the droplets were about to enter the pond water,
at several points within the spray region. The batches were
collected in covered insulated containers which were located
appropriately before the cover was removed for the collection
time. The ambient conditions of wind speed and direction and
dry bulb and wet bulb temperatures were recorded during the
collection time and the batch temperatures, hot water temper-
ature and pond water temperatures were measured and recorded
immediately after each batch was collected. The hot water
temperature changed slowly enough that no significant error
was induced by not measuring this temperature simultaneously
with batch collection.
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During much of the testing, rather high winds dominated, which
improved droplet cooling. Although it had been an objective to
examine the degradation of pond performance as a function of
depth into the spray region parallel to both the major and minor
axes, these higher winds made edge effects much more significant,
i.e., cooling around the perimeter perpendicular to the wind
direction was markedly enhanced by the winds. Therefore, the
degradation of cooling performance was examined only through
the center of the spray region parallel to the pond minor axis.
The degradation data obtained are presented graphically in Fig-
ure 1. The edge effects are clearly visible from curves A and
B. Intuition suggests that as the edge of the spray region is
approached from interior zones, the rate of change of cooling
achieved decreases due to a reduced rate of moisture addition
to ambient air.
As winds increase, improved performance is observed deeper into
the spray region from the upwind edge, and then cooling falls
off rapidly. As the wind speed decreases to low values and the
induced draft becomes the dominant air flow mechanism, the co-
oling at the downwind edge of the spray region increases rela-
tive to the cooling in the interior zones. This effect is shown
in curves B and E of Figure 1.
As a matter of interest, several temperatures have been shown
in Figure 2. An approach of approximately 10° C (18° F) was
achieved, with a cooling range of about 7° C (13° F).
RESULTS OF PREVIOUS TESTING
Numerous spray pond tests have been performed to determine
design performance. However, most of those tests were not
instrumented well enough to provide data that can be used to
gain significant insight into the effect of various parameters
on performance. The only well instrumented testing that has
been performed is that at the Rancho Seco Nuclear Power Plant
of Sacramento Municipal Utility District in California. The
results of that testing were presented in a report by the
University of California, Berkeley, California. [U
The tests done at Rancho Seco were instrumented comprehensively
but were limited to a maximum hot water temperature of about
38° C (100° F). The results of the Rancho Seco tests UJ are
presented in Figure 1 showing spray efficiency as a function
of position in the spray region. The dependence of spray ef-
ficiency on wet bulb temperature is not indicated explicitly
in the results, but the results do indicate the magnitude of
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efficiencies that can be expected at various depths into the
spray region.
Although the spacing of the spray units was the same in both
designs, the Florida pond had nearly twice the sprayed area,
and almost twice the depth of spray region in the direction of
the wind. The increased depth of spray region in the direction
of the wind provides a better base for degradation data. Spray
efficiency in the Florida testing was measured at six locations
through the spray volume to quantify degradation, while the
Rancho Seco tests apparently included three measurement points
to describe degradation.
The results of the Florida testing indicate that a large spray
pond may be expected to have spray efficiencies varying between
60% and 35% depending on position in the spray region and on
the wet bulb temperatures and wind conditions.
CONCLUSIONS
The Florida tests significantly expanded the data base by pro-
viding data at "higher nozzle pressure and for a higher water
loading than those characterizing Rancho Seco. Even though
these tests did not significantly expand the range of perform-
ance data with respect to hot water and wet bulb temperatures,
the results of these tests reinforce the validity of the Rancho
Seco tests. However, further testing needs to be performed with
hot water temperatures in the range of 49° C (120° F) to 54° C
(130° F) and with wet bulb temperatures in the range of 21° C
(70° F) to 27° C (80° F) in order to provide actual operating
data in design ranges similar to those encountered in conven-
tional and nuclear Ultimate Heat Sink applications.
Since numerous mathematical models have been developed to pre-
dict spray pond performance outside the ranges where data exist,
additional testing should be performed in order to demonstrate
the credibility of these models.
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REFERENCES
1. Schrock, Virgil E. and Trezek, George J., Rancho Seco
Nuclear Service Spray Ponds Performance Evaluation,
University of California, July 1, 1973.
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TABLE 1. COMPARISON OF VARIABLES
CHARACTERIZING TEST FACILITIES
Florida Rancho Seco
Spray Flow Rate
Sprayed Area
Nozzle Pressure
Nozzle Elevation
Nozzles per Spray Unit
*See sub-section "Test Facility"
(I/sec)
(gpm)
(m2)
(ft2)
(Pa)
(psi)
(m)
(ft)
2,780
44,000 ,
1
4,910
52,800
69,000
10
2.1
7
1,010
16,000
2, 920
31, 400
48,300
7
1.5
5
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SO •
REFERENCE 1
FLORIDA
FLORIDA
RANCHO SECO
RANCHO SECO
RANCHO SECO1
DEPTH INTO SPRAY REGION (M)
FIGURE 1 DEGRADATION OF PERFORMANCE WITH
DEPTH INTO SPRAY REGION
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tt
3
< 25
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U
0.I
9 10 11 12 13 14 ts 16 17 18
TIME OF DAY (O'CLOCK)
FIGURE 2 TEMPERATURES AS A FUNCTION
OF TIME OF DAY
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The thermal performance characteristics of large spray cooling ponds

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