日本ヒートアイランド学会論文集 Vol.7 (2012)
Journal of Heat Island Institute International Vol.7 (2012)

学術論文

EVAPORATIVE MIST COOLING OF AN URBANWASTE-TO-ENERGY PLANT
Craig FARNHAM*1 Akito TAKAKURA*2 Masaki NAKAO*1
1
1
Masatoshi NISHIOKA* Minako NABESHIMA**1 Osaka City University, Dept. of Urban Engineering, Osaka, Japan* Osaka City Institute of Public Health & Environmental Sciences, Osaka, Japan
2

Corresponding author: Craig FARNHAM, craig.farnham@gmail.com

ABSTRACTThe City of Osaka has several urban waste-to-energy incineration facilities that both contribute to, and are affected by, theurban heat island. Applying mist evaporative cooling to the forced-draft, air-cooled condensers at these plants can both reduce theexhaust air temperature and increase the amount of electricity produced. The condenser reaches over 70˚C and the exhaust is released
at 30m height. Power output drops by over 1%/˚C as air temperatures rise in summer. A misting system was installed over 1/8 of the
air intake at a 14.5MW plant. Water droplets with a Sauter mean diameter of 45 microns were sprayed at up to 2.88tons/hr withnear-complete mist evaporation. The exhaust air temperature dropped an average of 1.0K. If implemented full-scale, it should yield
exhaust temperature reductions of 4-5K. The cooling effect increased the power output by 1.2% for misting periods greater than 45
minutes, but uncertainty is high. Full scale use at all Osaka plants during the summer could increase power generation by 6MW,
offsetting 1700 tons of CO2 per season.
Key Words : Evaporative cooling, Urban heat island, Mist, Power plant, Waste-to-energy

1.

result in wetting. Wetting can cause scaling and corrosion. The

INTRODUCTION

use of fine mist sprays with average droplet diameters under 50
The city of Osaka incinerates 97% of its garbage (1) at 9

microns by Yamada et al. (4) and Uchiyama et al. (5) allowed for

waste-to-energy plants with a total capacity of 117MW, allefficientlocated inside the city limits. In summer, the steam cycle

O’Rourke’s(6) work provides a thorough explanation of droplet

outdoorcooling

without

significant

wetting.

forced-draft, air-cooled condenser temperature can reach overphenomena.

77˚C and the heated air is released at a height of 30m,

The primary goal of the research is to show that a mist

contributing to the urban heat island. In summer, when electricevaporation system can be installed that can generate a spraydemand peaks, higher air temperatures cause a reduction in cycle

which will largely evaporate before reaching the power plant’s

efficiency of the plant, yielding less power output.

forced-draft condenser and fans. If the spray evaporates, theApplication of mist evaporative cooling at the forced-draft

system can yield significant cooling without need of an

inlet could have two benefits. First, it can reduce the condenser

extensive refit of the condenser system. Many costs can be

exhaust temperature, countering the contribution to the heat

avoided; such as increased corrosion reducing equipment

island. Second, the reduction in inlet temperature may reduce thelifespan and effectiveness, a need to install mist eliminators, or acycle steam condensation temperature and increase power output.

need for added drainage or a water recycling system.

The city’s water resources can be used to both improve the urban
climate and increase energy production.
Spray evaporative cooling has been used as an efficient
low-energy cooling technique in dry climates for decades by(2)

The secondary goal was to measure any change in power
output of the plant and determine if the potential extra electricity
water demands.

In

Takakura et al.(7) made a study of the subject plant power

humid climates such as Japan, evaporative cooling is more

output data for 1 year from Sept. 2007 – Sept. 2008. The data

problematic. Coarse sprays do not evaporate well, which can

was correlated with weather data from the Osaka cityresearchers including Alvarez et al. , and Pearlmutter et al.

(3)

generation can outweigh the price of the system’s electricity and- 27 -

meteorological station(8) located 7km east of the plant. As typicalhappens as temperatures rise. Then the severe rainstorm occurs.

for Rankine cycles, power output drops as the condenser

During this time, power output increased by 15%.

temperature increases (9) as illustrated in Fig. 1. Fig. 2 shows

Both the 1-year temperature data set and the specificpower output expressed in terms of kW per ton of steam throughexamples of rain lead to an expectation that 1 degree of coolingthe turbine plotted against air temperature. In this case theduring hot summer conditions will boost power output by

best-fit curve shows power drops of more than 1% per degree asroughly 1%.

air temperatures pass 30˚C. At temperatures around 25˚C, thePower output per ton of steam (kW/t)150

no ability to improve the cooling of the condenser by increasing
fan speed, so power output starts dropping quite sharply.
The normalized root-mean square difference between alldata points and the best-fit curve is 2.0% of the power output(0.020P). The variation of the data with the best-fit curve isclose to that typical for normal distributions, with 73% of thedata points within +/-1RMSE, 96% within +/-2RMSE, and 99%

145

70

140

60

135

50

T = 56˚C
P = -87kPaG

130

40

Δw = +18.7kW/t (+15%)
125

30
RainfallPower output
Env. Air temperature
Turbine outlet temp.

120

115

20

10

110

0

24
2
4
6
8
10
12
14
16
18
20
22
24
2
4
6
8
10
12
14
16
18
20
22

within +/-3RMSE. The coefficient of determination (R2)80

T = 74˚C
P = -66kPaG

Temperature (˚C)
Rainfall (mm/hr)

forced-draft condenser fans start to reach full capacity. There is

between the air temperature and power output is 0.63.

Time (24h)2008/7/29 - 2008/7/30

Fig. 3. Change in power output during a period of heavy rain.
1.1

Mist Evaporation
Mist droplets in non-saturated air evaporate, exchanging

latent heat with sensible heat from the air, causing the air to cool
while becoming more humid. Mist droplet evaporation istypically a quasi-steady state in which the diffusion of vapor
away from the droplet is balanced by the flow of heat into thedroplet, while the droplet shrinks. Vapor diffusion is expressedin terms of a loss of droplet mass, md which is proportional tothe difference between the ambient vapor pressure, ρv,∞ and theFig. 1. Rankine cycle with superheat. Increase in turbine output(+W) as temperature of the condenser decreases.

vapor pressure at the droplet surface, ρv,s which is the saturation
vapor pressure, as shown in Eq (1).
Flow of heat, q into the droplet is proportional to the150
RMSE = 2.0%

145

-1.5%

140

-1.2%

135

-0.9%

Best-fit curve

130

-0.6%

Power output% change in Power/degC

125

-0.3%

120

surface temperature, Ts as shown in Eq (2). The vapor diffusion% Change in power output per C

Power output per ton of steam (kW/t)difference between the ambient temperature, T∞ and the droplet-1.8%

P = 146.7 - 0.6e0.107T

and heat transfer are related by the latent heat of evaporation, L
in Eq (3).

10

20

30

Eq (1)q  4πrkT  Ts 

Eq (2)dmd
dt

Eq (3)q  L

0.0%

0

dmd
 4πrDρv,  ρv,s dt

40

Environment Air Temperature (°C)

Fig. 2. Power output decreases at higher air temperature.

1.2

Significance of the Wet Bulb Depression
Pruppacher and Klett found that the surface temperature of

To show that the power output could quickly change inwater droplets evaporating in typical atmospheric air conditions

response to sudden cooling, cases of heavy rainfall wereis within 0.5K of the wet bulb temperature (10) as in Eq (4). Thus,

examined in detail. A case of over 30mm/hr of rainfall with air

the temperature difference driving Eq (2) can be estimated as thetemperature dropping 10K in 1 hour is shown in Fig. 3. From

wet bulb depression, the difference between the ambient dry

6:00 to 13:00, the typical summertime power output drop

bulb temperature and the wet bulb temperature, the right-side- 28 -

Δx  xsat  x0

term in Eq (5). This error is relatively large at low wet bulb

Eq (9)

depression values (high relative humidity), but the mist

The moisture for this change in humidity comes from theevaporation system is not used during periods of high humidity.
Ts  TWB  0.5K
Eq (4)

evaporated mist. Thus, the maximum amount of mist that canT  Ts  T  TWB

evaporate into the air is equal to this Δx. Adapting this to aforced draft of air with volume flow Va and dry air density ρda ,Eq (5)

By using the ideal gas law and substituting Eqs. (2)~(5) into
Eq.(1), an approximation for the evaporation rate of a single

the maximum evaporable mist spray rate mmax is,
mmax  Va ρda Δx

Eq (10)droplet, the change in droplet mass dm over time, can be

Both the change in air temperature ΔTa and the change in

expressed in terms of the wet bulb depression, here termed

absolute humidity Δx due to evaporation are directly

ΔTWB.

proportional to the amount of mist evaporated, as shown in Eq
dmd
4πrk ΔTWBdt
L(8) and Eq (10), respectively. Therefore in the case of complete

Eq (6)

evaporation, the ratio of the change in air temperature to theSingle droplet evaporation is proportional to ΔTWB. For mist
(11)

sprays, which are collections of droplets, Farnham et al.

found

ΔTWB to be a useful single parameter for predicting the mist

maximum possible temperature change (ΔTWB) is proportional to
the ratio of the mist sprayed to the maximum evaporable mist

sprayed that does not collect on a horizontal surface below thespray rate.
ΔTa
m
 m
ΔTWB mmax

misting nozzle. Further, as the coolest temperature that can be

Applying Eq (11) to an example, if the forced-draft inlet air

produced by evaporative cooling is the wet bulb temperature,

conditions are such that ΔTWB is 10K, and 10% of the maximum

ΔTWB is a useful reference as the maximum possible temperatureevaporable spray rate as determined by Eq (10) is sprayed into

reduction.

the air in an otherwise adiabatic process, the average

evaporation rate as a whole, defined as the fraction of mist

Whenexperimentinginvariousconditions

of

air

temperature and humidity, using ΔTWB as a single parameter

Eq (11)temperature drop in the area where the mist has completely
evaporated should be 1K.

allows for easy evaluations of evaporative cooling potential.
Higher ΔTWB yields faster and more complete evaporation with

1.4

greater temperature reductions.

Single Droplet Evaporation Time
Using the single droplet evaporation rate as given in Eq (6),The reduction in air temperature depends on the ratio of thethe time needed for the droplet to completely evaporate can bemasses of evaporating mist and the air into which it evaporates.

calculated. Assuming the mass of a single droplet moving

In the case of a mist spray of mass flow mm evaporating, the net

through air is too small to change the condition of theheat transfer with the mist, qm is,q m  LmmY

surrounding environment, the wet bulb depression is a constant.
Eq (7)

Droplets falling in air with diameters under 280μm can beassumed as spheres.(12) With all mist droplets as spheres with thewhere Y is the fraction of the mist spray that has evaporated.
The change in temperature of air, ΔTa with volume flow Va
and density ρa into which the mist evaporates is,qm
Lmm Y
ΔTa  Va ρa C P
Va ρa C P

density of water, the evaporation equation can be put in terms ofa change in droplet radius.

dmd dmd  dr  dtdr  dt 

Eq (8)

if the mist is uniformly-mixed throughout the affected air.
However, mist will likely not be uniformly distributed through4πr 2 ρw

the air. Local temperature drops will vary according to the local
ratio of mist to air and the evaporation until that point.
1.3

r

dr
4πrk
 ΔTWB
dt
L

ΔT k
dr
  WBdt
Lρ w

Eq (13)
Eq (14)

The radius and time variables can be separated and the resulting

Maximum Evaporable Mist Spray Rate
In saturated air, water mist will not evaporate. If a large

enough amount of mist is sprayed into unsaturated air, the air

equation integrated to yield the equation for droplet radius as afunction of time, rt.

will become saturated as some mist evaporates, then ther0

remaining mist will not evaporate. The absolute humidity of thert

air would change from the level at the initial state x0 to the levelat the saturated state on a constant-enthalpy line xsat as it is a0

r dr  

ΔTWBk

 Lρ dt

Eq (15)

2ΔTWBktLρEq (16)r02  rt 2 constant-enthalpy process. This change in absolute humidity canbe expressed as,

Eq (12)tThis is sometimes known as the D2 law (diameter square law) of- 29 -

droplet evaporation. (13) Complete evaporation occurs when the

diagonally, with steam entering from a header at the apex of theradius reaches zero. Substituting zero for rt yields the equation

2 fin tube banks splitting to both sides, and condensate flowing

for time to complete evaporation, tev.

out the bottoms and routed to a condensate tank. The fin tubes

tev 

are adjoining and stacked 4 layers thick.

Lρr02
2ΔTWBkEq (17)The misting system consists of 156 hydraulic nozzles thatspray droplets at 5MPa with a SMD (Sauter mean diameter) of

Plotting this for several values of wet bulb depression yields a

45μm. The droplet diameter distribution (in 2μm bins) and CVF

quick reference chart, Fig. 4. This shows what should be the(Cumulative Volume Fraction) as measured by a PDPA (Phased

best-case (fastest) droplet evaporation time. As mist evaporates,

Doppler Particle Analyzer) at a point 50cm from a nozzle at thethe wet bulb depression will decrease as the air becomes cooler

centerline are shown in Fig. 6. The CVF expresses the portion ofand more humid, increasing evaporation times.

the total volume of the entire spray accounted for by droplets of

Evaporation time (s)the indicated size or lower. Here, the CVF is 0.9 at 95μm, thus
10

10% of the spray volume is relatively large droplets over 95μm.

9In preliminary experiments, these nozzles showed mist

8

evaporation rates of 85-100% (complete evaporation) for freefall

7

heights of 15m-25m, depending on air conditions.

6
5
4
3
2
1
0
0

20

40

60

80

100

120

Droplet diameter (μm)

Fig. 4. Time to complete evaporation for single droplets of waterin unchanging air conditions.

2.
2.1

METHODS AND EQUIPMENT
Power Plant, Misting Equipment and Method
The Nishiyodogawa waste incineration plant can generate

14.5MW of electricity. The plant condensers handle an averageFig. 5. Layout of the condensers, air inlet section, and installed
misting system.

of 70 tons/hr of steam from the turbine. The turbine inlet
conditions are held at 271˚C +/-2˚C and 2100kPag +/-10kPag,
15%

with the steam mass flow varying from 33 to 90ton/h, with an

1

12%

0.8

typically vary over a temperature range from 43˚C to 77˚C and

0.7

Droplet fraction

pressures from -95kPag to -64kPag.
The forced-draft for the plant’s two main condenser units isgenerated by 16 fans of 6m diameter with a capacity of
3

9%

0.6
0.5

6%

0.4
0.3

8770m /min each. The air intake is a walled-off section on the3%north side with an open roof cross-sectional area of 400m2 as

0.2

Cumulative Volume Fraction

0.9

average at about 70ton/h. In summer, the turbine exit conditions

0.1

shown in Fig. 5. The cross-sectional area of the outlet above the

140

120

Droplet diameter (µm)

100

80

60

40

0

0

condensers is 700m2. Air velocity at the fans running at capacity

200%

was measured with a hot-wire anemometer at about 3m/s withhigh spatial variability due the many obstructions (support
columns, beams, catwalks, etc,). In summer temperatures overFig. 6. Nozzle droplet diameter distribution (in 2 μm bins) andcumulative volume fraction.

25˚C, the fans always run at full speed, which should yield an
average upward air velocity of 3.3m/s across the outlet plane.
The condensers are large banks of aluminum fin tubes. The
fins are 5cm in diameter. The tubes are about 10m long, oriented

Three high-pressure water pumps drawing 4kWe eachsupplied a total 2.88 tons/hr of mist spray. This is about 14-23%of the maximum evaporable mist spray rate as determined by Eq(10) for the range of inlet air conditions. The pumps can be- 30 -

turned on and off individually, to allow spraying of about 1/3 orshown in Fig. 8, there should be a 1.8K temperature drop.

2/3 of the maximum with some variability. The total flow rate of

According to the power output and weather correlations aswater was measured for each pump on/off combination at theshown in Fig. 2 and Fig. 3, this is expected to yield roughly awater main with a mechanical flow meter.

1% increase in power output.

Experiment trials were done by spraying at a constant flowrate for periods of 30, 45, 60 or 120 minutes. All 3 pumps werestarted simultaneously and stopped simultaneously at the end of
the period. It took about 15 seconds from activation until thespray became steady. In 1 case (noted in Results) the total
amount sprayed was lower than desired, due to trouble with thewater supply.
The mist nozzles were mounted in a 50cm grid covering a
6m x 8m section of the air intake at a height of 15m. Nozzles
were oriented straight down. The installation was temporary,with scaffolding left in place to allow access to the nozzles. Thisis shown in Photo 1.

Fig. 7. Cross-section of plant condenser area and nozzle position.
Arrows indicate flow of air and mist.
2.2

Sensors
The effect of the mist on the air was measured with bothT-type thermocouples and platinum-resistance temperature /capacitive-chip humidity sensors. Sensors were placed 3m above
the misting nozzles, 1m above and below the condenser unit in
the path of the mist, and at the condenser region farthest fromthe mist. Thermocouples placed above the condenser wererigged as 4 in parallel and the average reading taken to counter
Photo 1. Nozzle array during spraying. Photo taken from below.
Note scaffolds at left and right, and structural beams in center
which capture some of the mist

any spatial variation in temperature between the individual fintubes. Sensor locations are shown in Fig. 8 and Fig. 9 with an
explanation of the chart abbreviations and locations in Table 1.
An example of sensor mounting onto the condenser is shown in

The flow of mist is directed by the flow of air through the

Photo 2.

condenser area as shown in Fig. 7. Inlet air (straight blue arrow)from above passes the nozzles and is drawn (curved blue arrow)

2.3

Mist Evaporation Rate

through the open space (9m height) into the fans. Warmed outlet

Observations of misting test runs showed that the sprayedair (red arrows) is blown upward, with some short circuitingwater would be in one of the following states after the end of(curved red arrow). The amount of short circuiting, likely

spraying.

influenced by changes in the ambient wind speed and direction,- Collected at ground level, some draining away and some

may account for the high variability in air inlet and outlet

evaporating away.

temperatures, and even the power output.

- Adhering to an obstruction (scaffold, beams, screens, fans)

The mist array directly covers about 1/4 of the inlet area forwith some portion dripping to the ground and the rest

1 condenser unit, but spreads to enter 6 of the 8 fans, shown in

eventually evaporating.

Fig. 8 as “Mist area”. If all the mist evaporates it should yield- Completely evaporated without adhering to any object.

1960kW of cooling as per Eq (3). The air flow through each fan

Observation of the space between the fans and the condenser fin

requires 177kW of cooling to reduce the temperature 1K. Thus,

tubes including the underside (windward side) of the fin tubes

if the mist spreads evenly to all 8 fans with complete

while in operation was not permitted for safety reasons. No mist

evaporation at maximum spray, the average cooling should be

was visible passing through the condenser on the outlet side. No

1.4K as per Eq (8). If the mist spread evenly to only 6 fans aswetting was seen on the outer, visible side of the fin tubes nor on- 31 -

the sensors placed there.
It is possible that there were some drops passing through thecondenser, but their number too small to be noticed visually. If
droplets survived to pass the fin tubes, they would evaporate
quickly in the 60˚C-70˚C air. For example, according to droplet
evaporation times in Fig. 4 for the typical observed outlet air
conditions with a ΔTWB of about 30K, even a 100μm droplet
would likely evaporate in 4 seconds. As the average wind speed
through the condenser is 3m/s, this would mean evaporation
about 12m above the condenser.

Fig. 8. Layout of the sensors (Top view, locations not to scale)

Table 1. Sensor layout: Types and measurements.
SensorLoc.
Measurement target
code (Qty)
Environment above nozzles,
1
S,A,N(4)Possible short circuit air flow effect
Condenser outlet in misting zone,
2
S,N,N x 4
Side near mist
Condenser outlet in misting zone,
2’
N,N x 4
Side far from mist
Condenser inlet below fans in misting
3
S,A,F,Nzone, Near mist
Condenser inlet below fans in misting
3’
A,Nzone, Far from mist
4
N(2)Floor level below nozzles
Environment conditions 20m away
4aS,Afrom experiment area
Floor level below fans, 30cm and5N(2)150cm height
Condenser outlet in no-mist zone,
6
S,N,N x 4
Side near mist
Condenser outlet in no-mist zone,
6’
N,N x 4
Side far from mist
Condenser inlet below fans in no-mist
7
Nzone, near mist
Condenser inlet below fans in no-mist
7’
Nzone, far from mist
Sensor CodeDescription
A
Rain/wetness sensor
F

Anemometer (ultrasound, 3D)N

T-type thermocouple

Nx4

4 T-type thermocouples in parallelS

Temperature/humidity sensor in
non-ventilated shelter (wind speed atmeasurement locations is about 3m/s)

Fig. 9. Layout of the sensors (Side view, locations not to scale)
The effect of evaporation depends on where and when it
occurs. Evaporation before or in contact with the condenser cancontribute to reducing the condenser temperature and possibly
increasing power output.
Evaporation after passing through the condenser would still
have value in reducing the sensible heat load from the condenser
as a heat island countermeasure. Further, it may still have some

Photo 2. Sensors at Location 2. Thermocouples (TCs) in parallel
to average out local differences among fin tube temperatures.
Location 6 visible in background at left.

effect on power output, as there is some short-circuiting of theoutlet air back into the inlet.
During misting and after misting is stopped, evaporationfrom wetted surfaces will provide a delayed cooling effect.
Thus, it can be thought that the only “waste” is when mist
water collects in large enough amounts to make its way to floor
drains and exit the experiment area before evaporating.

Here, the “mist evaporation rate” is meant to measure theamount of water that would not collect in significant amounts atground level to drain way as “wasted” mist. It was measured bycollecting the unevaporated spray which fell to ground level,
either directly from the nozzles or after dripping from
obstructions. Twenty 0.4m2 plastic trays were regularly spaced
across the spray area to collect the fallen spray for a set period.

- 32 -

The trays were covered within 1 minute of the end of the mistingthat reached the floor or dripped to the floor from obstructions.

period to reduce evaporation of the collected water. This is aThis mist “wasted” to the floor could drain away and its

trade off, as it prevents capturing of delayed dripping from

evaporative cooling effect lost. The inner area immediately

obstructions. However, the amount of this dripping after misting

beneath the misting nozzle headers tended to become quite wet

stopped was not large enough to be significant except under theas mist hit the temporary scaffold and nozzle piping and dripped

rig scaffolding (including safety nets) which tended to capture

below. If the system were permanently installed, there would be

the mist and become soaking wet. Thus, the captured mist data is

no scaffold. In the outer area beyond the scaffold, under thedivided into two types (in Table 2 in the Results section) to

condensers, less mist collected. In this case, much of theaccount for this difference.

collected water was drops falling from obstructions in the path

The unevaporated water per unit area was interpolated overof air flow. These results are separated in Table 2. “Mist waste to

the visible spray area and compared to the amount of waterfloor - In” is the water captured under the scaffold. “Mist waste

sprayed. Evaporation rate measurements were labor-intensive

to floor - Out” is the water captured outside the scaffold area,

and could not be performed for all misting cases.

under the condenser fans. Both are included in calculations of
the net evaporative cooling.

2.4

This data shows that on average about 94% of the mist did

Power output data
Power output data was provided in the form of separate

coarse, color graphic printouts with 1 pixel representing

not collect on the floor as wasted mist, and implies that if there
were no scaffold, the rate would be over 98%.

0.06MW of electric generation and 0.3 ton/hr of cycle steam.

At first glance, the mist waste amounts seem to have no

The printouts were scanned and converted into numerical dataclear correlation to experiment parameters. This could beusing the software WinDIG created by Lovy(14). Assuming the

explained by two competing influences.

software yields accuracy to within 1 pixel of the coarse printout,
the resulting error in power output per ton of steam is 1.4kW/ton,which is about 1% of the total. This poses a problem, as the
expected power increase per ton of steam is about 1%.
Evaluating power output by comparing several periods withmisting and several periods without misting from data scannedand converted in the same way should reduce the influence ofany bias due to data scanning.
Further, plant staff neglected to print out the power output
logs after some experiments and the data was lost. This meantthat some evaporation rate measurements could not be correlatedwith power output data.

3.

RESULTS

3.1

Mist Evaporation Rate

Table 2. Unevaporated mist collection rate and expectedevaporative cooling.
Mist
Mist “waste” Evap.
Date Time
ΔTWB Misttime
to floor (%) Cooling
Min
K
t/hr
In
Out
kW
8/24 13:33 120
8.5
2.89 10.4
2.1
1720
8/23 13:43
60
6.3
2.89
9.2
1.4
1760
8/24 10:58
60
6.5
2.89
4.3
0.6
1870
8/18 13:41
60
10.2 2.30
0.2
2.2
1530
7/27 11:42
60
9.7
2.89
4.5
1.4
1850
7/27 16:10
60
9.4
1.84
1.6
1.0
1220
7/28 10:03
60
5.7
1.84
3.5
1.4
1190
8/13 11:12
60
6.2
1.83
0.9
0.9
1220
8/25 13:59
30
7.4
2.89
1.0
0.2
1940
8/25 15:1030
10.0 2.89
0.4
0.1
1960
8/13 13:35
30
6.8
1.81
1.5
2.9
1180
Time-weighted avg.
4.4
1.2
1620
The percentage of mist collected should tend to increase as

The fraction of the mist that evaporated before reaching themisting time increases. More spray will yield more wetting offans, or the area between the fans and condenser was unknown.

surfaces, and more dripping into the collection trays. Water

Due to evaporation form surfaces and dripping, and the fact thatpooling in the trays or on the floor will evaporate much more

the wind was blowing upward, there was no way to reliably

slowly than mist in droplet form, due to the much lower ratio of

measure the amount of mist reaching the fan area. Plant

surface area to mass. Conversely, a very short spray period could

operations (including fan speed) could not be adjusted or

yield almost no surface wetting, or wetting with quick

stopped for the purpose of the experiments, neither for initial

evaporation, and no water collected in the trays.
The percentage of mist collected should also tend to increase

installation of equipment nor during the trials.
During trials, the fan unit motor (motors are below the fans,with a higher evaporable mist ratio mm/mmax due to increasing

thus upwind and exposed to the mist) and much of the accesssaturation of the air. More droplets will survive to adhere to the

catwalk for the 1 fan closest to the mist nozzles became

floor and other surfaces. Water on wetted surfaces will evaporate

noticeably wet with some dripping water in most experiments,

more slowly, with greater tendency to collect and drip to the

except when ΔTWB was 10K or above.

floor.

It was possible to capture and measure the amount of mist

- 33 -

Collected mist rates should be proportional to the ratiomm/mmax and to total spray time tspray. We can combine these two

1620kW of cooling, nearing a maximum of 2MW. If scaled up

into a single parameter, which we term B, with units of time (seethis could yield 16MW of cooling, a significant fraction of theTable 3). This would be the amount of time it would take toheat of condensation for the typical 70tons/hr of steam in thespray the same amount of mist if the mist flow rate werecycle, which is about 45MW. The 1620kW of cooling should

increased to the maximum evaporable mist spray rate mmax.
m
B  m t spray
Eq (18)mmax

yield an average temperature drop at the outlet of 1.5K if spread
across 6 fans.

Plotting B against the values of collected mist fraction, a

3.2

Mist Cooling Temperature Drops

good correlation, with a coefficient of determination of 0.75, isDuring misting, the sensors below the condenser at theapparent (see Fig. 10). The best-fit line has a y-intercept at about

forced-draft fan intake on the side closest to the mist (Location

4 minutes. This matches the expectation that short bursts of3) tended to become wet. An example case of the temperature

spray could yield no collected mist. This suggests that if total

and relative humidity data from the sheltered temperature

mist flow and spray time is limited to keep parameter B under 4/humidity sensors for Location 3 and for the outlet area above(for example, spraying for 20 minutes at mm/mmax = 1/5) in athe condenser (Location 2) are shown in Fig. 11. The wet bulb

single spraying period, mist might not collect on the floor. It istemperature calculated at the inlet (Location 1) sensor data ispossible no water would be lost down the drain.

included. The Loc. 3 sensor temperature dropped to around thewet bulb temperature for most of the 60 minute misting trial.

Table 3. Correlation of total wasted mist with parameter B
Mist
Total mist
Date Time
ΔTWB MistB
time“waste”, F
Min
K
t/hr
%
min
8/24 13:33 120
8.5
2.89
12.5
13.8
8/23 13:43 60
6.3
2.89
10.6
20.9
8/24 10:58 60
6.5
2.89
4.9
13.1
8/18 13:41 60
10.2 2.30
2.4
5.9
7/27 11:42 60
9.7
2.89
5.9
4.3
7/27 16:10 60
9.4
1.84
2.6
6.7
7/28 10:03 60
5.7
1.84
4.9
9.0
8/13 11:12 60
6.2
1.83
1.8
5.9
8/25 13:59 30
7.4
2.89
1.2
9.6
8/25 15:10 30
10.0 2.89
0.5
8.9
8/13 13:35 30
6.8
1.81
4.4
4.0

The relative humidity reading neared 90%. During this trial andafter, the sensor shelter was visually confirmed as wet. Using
this sensor data as representative of the entire misted area would
yield temperature drops much larger than the expected 1.0K –1.8K drop, thus they are not used to evaluate the temperaturedrops here.
On the other hand, the sensors at Location 2 did not visuallyappear wet, and the sensor data seems to indicate they did not
become wet. Relative humidity only slightly exceeded 15%
during the misting period. Further, the wet bulb temperature of
the outlet air as measured before the misting period is typically
about 35˚C, whereas the measured outlet temperature neverdropped below 50˚C in any of the measured cases. However, it is25

possible that the sensors were being slightly wetted by droplets
Parameter B (min)20

which evaporated too quickly to accumulate to a level thatB = 115F + 4
R² = 0.75

would be visible, nor to yield a wet bulb type effect. The

15

temperature readings above the condenser are used to evaluate
10

the cooling effect here.
During misting, readings of the sheltered temperature/5

humidity sensor above the condensers showed a difference arises
between temperatures of the misted area (Location 2) and the no

0
0%

2%

4%

6%

8%

10%

12%

14%

Fraction of mist collected, F

mist area (Location 6), such as in the example case shown in Fig.

Fig. 10. Trend of amount of mist collected in proportion toparameter “B”

12. The average temperature differences for all trials are given in
Table 3, with all cases showing some temperature reduction. The
average of all cases is 1.0K, but is based on the assumption that

With further investigation into drying times, and theinfluence of the thermal mass of the obstructions and floor on
flash evaporation of the initial portion of droplets contacting

the mist cooling did not affect the sensors farthest from the mist.
If the cooling effect spread to those sensors as well, the actual
temperature drop should be even larger.

them, this type of parameter could have value in determining
limits on misting time and spray amounts based on the air
conditions to reduce wetting of surfaces and wasting mist sprayto drainage by using intermittent spraying.

Fig. 13 shows the trend of this temperature difference overtime for the 30 minute misting trials (longer trials show the same
trend). The cooling effect reaches a maximum about 10 minutes
after spraying begins, and returns to zero about 10-15 minutes

The latent heat of the evaporated mist yielded an average ofafter it stops. This delay may be due to the large mass of the- 34 -

condenser unit, estimated at 90 tons. As the cooling effect of the

In the previous study, there was no clear correlation of thesemist ends about 10-15 minutes after misting is stopped, infactors with power output beyond the general temperature trend

calculations of relative power output change (Section 3.3), it isin Fig. 2. It is assumed the RMSE of 2.0% with the best-fit curve

assumed that the power output at 15 minutes after misting stops

is due to all the other factors, and a similar variability was

represents what the power output would be if there had been no

expected in results for power output changes due to mist

misting.

cooling.

The measured temperature drop is lower than the expectedvalue as determined by the evaporation rate. This may be due tothe sensor location. Sensors were not installed at multiple
locations across the condenser to get a complete spatial
distribution of temperature.

Misted Outlet Temp

Loc. 3 RH

Misted Outlet RH

Inlet Wet Bulb Temp

70

90

60

75

50

60

40

45

30

30

20

15

10

Rel. Humidity (%)

Temperature (ºC)

8/24/2010, 10:58 start, 60min misting, 2.88t/hr
Loc. 3 Temp

0
-30

-15

0

15
30
45
Mist Elapsed Time (min)

60

75

90

30 minute misting trials

Temperature difference
misted vs. non-misted (K)Fig. 11. Temperature and Rel. Humidity in the misted area at theoutlet (Loc. 7) and below the condenser fans (Loc. 3) where
sensor wetting clearly occurred.

Table 4. Difference in temperature of misted and non-mistedareas above the condenser.
Expected
Meas.
Date &ΔTWB
Misting
ΔT, 6 fanΔT
Time
spread(2010)Kmin
K
K
8/24 13:33
9.1
120
-1.4
-1.6
8/24 10:58
6.2
60
-1.2
-1.8
8/23 13:43
6.7
60
-1.5
-1.7
8/18 13:41
10.5
60
-0.3
-1.1
8/25 15:10
6.7
30
-1.1
-1.5
8/25 13:59
6.4
30
-1.3
-1.8
7/28 14:08
11.1
60
-0.9
8/18 15:01
11.5
45
-0.4
8/18 16:06
12.0
30
-0.4
8/23 15:24
7.0
30
-0.9
No evap.
rate data
8/4 13:29
9.4
30
-1.0
7/28 16:05
6.7
30
-1.0
8/4 14:25
8.8
30
-1.2
8/4 10:50
9.9
30
-0.5
Average of cases
-0.9
-1.6
Time-weighted average
-1.0
-1.6

8/24/2010, 10:58start, 60min misting, 2.88t/hr
7065

Misted Outlet Temp
Misted Outlet Abs. Hum.

22.0
21.0

Temperature (ºC)

60

20.0

Absolute humidity (g/kg')

Intake Temp
No Mist Outlet Temp
No Mist Outlet Abs. Hum.

8/25 13:59 30min

1.5

8/25 15:10 30min
8/04 14:25 30min

1.0

8/23 15:24 30min

0.5

7/28 16:05 30min

0.0

8/18 16:06 30min-0.5

8/04 10:50 30min
8/04 13:29 30min

-1.0

55

19.0

50

18.0

45

17.0

40

16.0

35

15.0

30

14.0

A plot of power output (see Fig. 14) smoothed by a 6-minute

13.0

time averaging (which smoothes the periodic variation due to the

25
-30

-15

0

15
30
45
Mist Elapsed Time (min)

60

75

-10

90

0

5 10 15 20 25 30
Misting elapsed time (min)

35

40

45

garbage stoking frequency) still shows the output varies by
about 3% during a 60 –minute misting trial.
A method was developed to evaluate the change in power
output by creating a hypothetical non-misting power output line.
The average power output at the start of misting cannot have

Change in Power Output
Evaluating power output changes was problematic, as plant

operating conditions could not be held constant for the purposeof the experiment. Steam conditions depended on the amount ofgarbage being burned and many other operational factors.
During some misting trials, cycle steam was vented directly
below the condensers. Further, weather conditions (temperature,
relative humidity, wind direction, wind speed, sunlight, clouds,etc.) change during the misting period.

-5

Fig. 13. Temperature differences arising between misted andnon-misted areas at outlet during misting.

Fig. 12. Example case of temperature and abs. humidity above
condenser in the misted area and no-mist area during a
60-minute trial.
3.3

2.0

been affected by mist. Further, temperature and humidity datatypically returned to initial values about 10-15 minutes after
misting was stopped. Therefore, it is assumed that the power
output data 15 minutes after misting is also not affected by mist.
Connecting these points yields a hypothetical non-misting line.
Power output is expected to vary above and below this line in
the typical manner if there had been no misting. The difference
between the recorded power output and this hypothetical line istaken as the effect of the misting on power output. Integrating- 35 -

the difference yields the total power output change. Integration

mist start time and the mist end time, rather than 15 minutes later,

was only done over the misting period.

yielded the same trend of increased power output, with the same

A single trial showing a net power increase could be just

3 cases of decreased output, shown in Table 7. The time-

random chance, but a trend to positive results over many trials

weighted average increase became +0.7% for the longer-period

could indicate that misting was effective. If there is no effectmisting. A lower increase is expected, as delayed cooling effects

from the mist, the average of all trials should be near zero.

remaining after misting was stopped are not compensated for.

The method was also applied to random selections of datafrom non-misting periods to test for bias, with an example of thecurve integration yielding almost no net change given in Fig. 15.
The average of the non-misting cases was a 0.09% increase, witha standard deviation (SD) of 1.0%. These bias check results are
in Table 5.

76

135

74

134

72

133

70

132

68

+2.31kWh/t
at 69.1t/hr

131

66

+160.1kWh (+1.8%)

130

64

400s Avg power output
Hypothetical no-mist line 0 min to 75 min
400s Avg steam flow (t/hr)

129
128
-20

0

20

40

Steam flow (t/hr)

Power output per ton of steam (kW/t)

7/28/2010 14:08 start, 60 min. misting, 2.88t/hr
136

60

80

62
60
100

Elapsed misting time (min)Fig. 14. Example of difference between time-averaged actual
power output (solid line) and hypothetical non-misting (dottedline). Area under the curve represents the total power increase.

Power output per ton of steam (kW/t)

8/23/2010 12:00 start, No misting
133
400s Avg power output
Hypothetical no-mist line 0 min to 75 min
132-4.1kWh (-0.04%)
131

130

129

128
0

20

40

60

Table 5. Test for bias in integration method by applying to
non-misting periods
PeriodChange in output
Date & Time
min
kWh
%
8/4 15:30
30
+15
+0.3%
8/4 12:10
30
-26
-0.6%
8/23 12:00
30
+5
+0.1%
8/23 12:10
30
+105
+2.2%
8/23 12:20
30
-27
-0.6%
8/18 12:05
30
-28
-0.5%
8/4 15:30
60
+109
+1.2%
8/4 12:10
60
+141
+1.6%
8/23 12:00
60-44-0.04%
8/23 12:10
60
+18
+0.2%
8/23 12:20
60
-125
-1.3%
8/18 12:05
60
-61
-1.2%
Average+13.4+0.11%
Time-weighted average+11.0+0.09%

80

Hypothetical elapsed misting time (min)Fig. 15. Example of check for bias using the integration method
for a period without misting.

Table 6. Inlet air conditions and changes in power output
calculated by the integration method (from time zero to 15
minutes after misting).
Inlet
Mist Change in output
Date & Time
RH
Temp
(2010)
˚C
%
min
kWh
%
8/24 13:33
34.1
48
120
+337
+1.7
7/28 14:08
36.5
41
60
+160
+1.8
8/24 10:58
31.8
61
60
+123
+1.3
8/23 13:43
32.8
59
60
+107
+1.2
8/18 13:41
35.5
43
60
+4
+0.1
8/18 15:01
36.2
39
45
+12
+0.2
7/28 16:05
32.4
59
30
+57
+1.2
8/4 14:25
35.0
50
30
+42
+1.0
8/4 10:50
34.9
45
30
+38
+0.8
8/25 15:10
32.6
59
30
+28
+0.6
8/18 16:06
37.0
38
30
+17
+0.4
8/23 15:24
33.2
58
30
-12
-0.3
8/4 13:29
34.6
47
30
-29
-0.7
8/25 13:59
32.1
60
30
-103
-2.0
Time-weighted avg +102.0+0.8
Misting >= 45 min Time-wtd. avg +159.6+1.2
Standard Dev. mist
0.94
Standard Dev. Mist>= 45 min
0.65
Avg. No-mist bias check+0.09
Standard Dev. No-mist bias check
1.04

Table 6 shows the results for all trials. 11 cases show someincrease in power, and 3 cases show a decrease. All cases ofThough the result agrees with the expectations that there

longer spray periods (45 minutes or more) show power increases,

would be about a 1% increase in power output, the uncertainties

with the longest spray period (120 min) being the second best.

are too large. The integral method result’s SD is also about 1%

The SD of percent power increase is larger than the averageof power output, while the variance in the relationship between

increase, giving low confidence in the result. However, for cases

power output and air temperature itself has a RMSE of about 2%of longer-period misting, the average (+1.2%) is nearly 2 SD(see Fig. 2). This experiment is not definitive proof that misting(0.65%) above the non-misting average (+0.09%).

increased power output in these cases. But the average of all

Setting the endpoints of the hypothetical non-mist line to thecases indicates a positive trend, in agreement with theory.

- 36 -

If the trend is true, the average power increase of 102kW isover 8 times the power demand of the water pumps. If sold at 10

and a drop of 4-5K. This would counter about 30% of theexhaust heat from cycle steam condensation at this plant.

yen/kWh, with the cost of water at 100 yen/m3, the net gain is

In these trials, power output was increased by the expected

600 yen per hour of operation, or 540,000 yen/season if operated

1%, though uncertainties in the data are large. The average

10 hours/day for 3 months at the 1/8 size trial scale.

power increase from longer-term misting over 45 minutes isnearly 2 standard deviations above the null case. If scaled up, theTable 7. Inlet air conditions and changes in power output
calculated by the integration method (from time zero to mist
stop)
Inlet
Change in
Date &
Mist
outputTime
RH
Temp(2010)
˚C
%
min
kWh
%
8/24 13:33
34.1
48
120
+198
+1.0
7/28 14:08
36.5
41
60
+107
+1.2
8/24 10:58
31.8
61
60
+12
+0.1
8/23 13:43
32.8
59
60
+1
+0.0
8/18 13:41
35.5
43
60
+52
+0.1
8/18 15:01
36.2
39
45
+12
+1.0
7/28 16:05
32.4
59
30
+37
+0.8
8/4 14:25
35.0
50
30
+38
+0.9
8/4 10:50
34.9
45
30
+44
+1.0
8/25 15:10
32.6
59
30
+59
+1.4
8/18 16:06
37.0
38
30
+2
+0.1
8/23 15:24
33.2
58
30
-84
-1.8
8/4 13:29
34.6
47
30
-20
-0.5
8/25 13:59
32.1
60
30
-145
-3.2
Time-weighted avg +50.4+0.4
Misting >= 45 min Time-wtd. avg +85.5+0.7Standard Dev. mist
1.22
Standard Dev. Mist>= 45 min
0.46
Avg. No-mist bias check +0.09
Standard Dev. No-mist bias check
1.04

4.

power increases could exceed 5%. If so, full-scale use at all suchplants in Osaka could yield 6MW of additional power, and offset
1700tons of CO2 from other plants per season. Further trials
would be better served by more modern plant monitoring andlogging equipment, and the ability to control plant operations forthe purposes of the experiment.

5.

NOMENCLATURE
Symbol
B
CP
D
F
k
L
md
mm
mmax
q
qm
r
r0
rt
t
T∞
tev

DISCUSSION AND CONCLUSIONS
An evaporative mist cooling system spraying up to 2.88

tons/h of droplets with a SMD of 45μm was installed at the inlet

ts
Ts
TWB
Va
xo

of an air-cooled condenser of a steam turbine power plant toevaluate the effects. The mist spray flow was about 14-23% of
the maximum possible spray flow that could evaporate in therange of air conditions. The system covered 1/4 of the inlet areaxsat

of 1 of the 2 plant condensers, but the mist diffused to coverabout 3/4 of the inlet area at the intake fans. Spray periods

Y
ΔTa

ranged from 30 – 120 minutes.
Although the mist appeared to have completely evaporated

the sprayed mass. If the nozzles were placed at a greater height,

ΔTWB
Δx
ρa
ρda
ρv,∞

and temporary scaffolds removed, better evaporation should

ρv,sat the condenser outlet with no signs of wetting nor visible
droplets, there was some wetting of the fans, floor, and surfaces
below the condenser. Wetting of the floor averaged about 6% ofresult. Correlation of wetting with duration and amount of mist

ρw

sprayed indicates that intermittent spraying could also reduce or
eliminate water “wasted” to the floor and drains. The
evaporative cooling at 1/8 scale can yield up to about 2MW of

6.

cooling and an average temperature drop of 1.0K over 1
condenser unit. If scaled up, this could yield 16MW of cooling

Meaning
parameter B, see Eq. (18)Specific heat of air
Mass diffusivity
Fraction of mist collected at floor
Thermal conductivity of air
Latent heat of evaporation of water
Droplet mass
Mass flow rate of mist spray
Maximum mist mass flow rate thatcan evaporate completely
Rate of heat transfer
Rate of heat transfer with mist
Droplet radius
Initial droplet radius
Droplet radius at time t
Time
Temperature of environmentTimetocompletedropletevaporation
Mist spraying duration
Temperature of droplet surface
Wet bulb temperature
Volume flow rate of air
Absolute humidity of air at initialstate
Absolute humidity of saturated air
for x0 constant-enthalpy line
Evaporation fraction of mist spray
Air temperature drop from mist
cooling
Wet bulb depression
Change in absolute humidity
Density of air
Density of dry air
Density of water vapor ofenvironment
Density of water vapor at droplet
surfaceDensity of water

Units
min
kJ/kg K
m2/s
J/ m s K
kJ/kg
kg
kg/s
kg/s
W
W
m
m
m
s
˚C
s
min
˚C
˚C
m3/s
kg/kg'
kg/kg'
K
K
kg/kg'
kg/m3
kg/m3
kg/m3
kg/m3
kg/m3

ACKNOWLEDGEMENTS
This research was funded by the Osaka City Water Bureau.

Nozzles and equipment were provided by Ikeuchi Co., Ltd. The- 37 -

main author is grateful for financial aid from the Japan Student(2009), pp.105-115.

Services Organization during the trial period.

(12) H.Pruppacher and J.Klett, Ibid. p .397.
(13) A. Lefebvre. Atomization and Sprays. Hemisphere

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