Water Spray For Pre-Cooling Of Inlet Air For Natural Draft Dry
Cooling
Here in this paper an experimental study for the cooling of inlet air of natural draft dry cooling tower by
water spray has been carried out to obtain the optimal solution for the development of cooling system
of air. The experiments are performed on an open circuit wind tunnel with 1x1 m2 cross section. The
length of the wind tunnel is kept 10 m (with 5.2 m test section). The authors have used 9 high
pressure hollow cone nozzles which are which are commercially available in the market and can
serve the purpose in cooling tower.
Major Components used with the specifications
Test Section was kept with 1x1 m2 cross section and 5.2 m length. Air was drawn into the section
with a variable speed centrifugal fan blower run by 75kW motor. The air passed through the 4 diffuser
plates, a honeycomb and 4 woven nylon screens to get the uniform flow without eddies at the test
section. Nozzle was kept at the centre of the section with 0.6 m height. PDPA system and high speed
photography system was used to take readings. The water system consisted of three main water
tanks, high pressure water pump, a nozzle and the flow rates were controlled by a bypass valve.
Nozzles with widest cone angle and smallest droplet size were used. Various high accuracy
temperature sensors, humidity sensors, velocity sensors, pressure gauges were used to take the
reading of inlet and outlet conditions.
Experimental Procedure
In all 29 experiments were carried out with different combination of nozzles, flow parameters and
physical conditions. Water and air flow rates were adjusted so as to attain full air saturation
theoretically. The experimental procedures were as follows: (1) Adjust the air speed to the required
velocity; (2) use the heater to set the required inlet air temperature; (3) Adjust the nozzle pressure to
set then required water flow using the pressure water flow calibration chart; (4) Allow 10 min for
stabilization; (5) Take air temperature and humidity measurements; (6) Concurrently characterize the
spray using the PDPA system.
Experimental Results
The experiments were carried out with 3 different air velocities of 1 m/s, 2 m/s and 3 m/s. At the same
time 3 water pressures of 0.6, 1.4, 4.6 MP a were used. The droplet size distribution and droplet
speed at the break up length was obtained by using high speed photography and PDPA. The analysis
for the same is shown in the figure 3.The cooling efficiency of a spray is defined by ASHRAE as the
ratio of the actual air temperature drop to the maximum possible temperature drop. Consequently, it
can be expressed as:[image: ]Where Tdb,i, Tdb,o and Twb are the dry-bulb temperatures of inlet and
outlet, and wet-bulb temperature of the air, respectively. Earlier to consider the effect of coverage
area readings were plotted for full cross section of the tunnel and the graph for the same is mention in
the figure 4. It was observed that half the area remains unaffected with spray so to measure the
actual cooling efficiency the portion(area) where 5% change in temperature occurred was taken into
consideration and the graphs were plotted again which is shown in the figure 5. This shows that
mainly half area of the test section is affected by nozzle spray. Also it is observed that larger cooling
area is obtained at 1m/s velocity compared to the other two. The reason behind this is for the lower
velocity the residence time of the droplets is higher in the cooling area as compared with the higher
velocities which results in better evaporation better humidification and better cooling. The below
mention figure 7 shows the relationship between the drop let size, spray cone angle and coverage
efficiency. It is observed that for the small droplet formations better coverage area is obtained
whereas the effect of spray cone angle is mostly uncertain on the coverage area. It should be noted
that in practice we can use extra nozzle to increase the coverage area of the air washer. High
pressure water jet results in larger no. of small droplets. Air velocity and Droplet size formation are the
majority drivers of water spray cooling. Effect of the coverage area on cooling efficiency is greatly
considerable and provision for extra spray nozzles should be made to increase coverage area as well
as cooling efficiency. Increase in air velocity results in results in decreased cooling efficiency as well
as coverage area. Smaller droplet size results in better cooling efficiency. For the droplet size of less
than 60 micrometer the coverage area ratio is independent of air velocity which shows that the
maximum coverage area has been reached. For the most efficient cooling low air velocities and small
droplet size distributions are required.
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