how does a cooling tower works in rainy season when air is in saturated state?

Answer 1

A cooling tower primarily uses latent heat of vaporization (evaporation) to cool process water. Minor additional cooling is provided by the air because of its temperature increase.

Cooling tower selection and performance is based on water flow rate, water inlet temperature, water outlet temperature and ambient wet bulb temperature.

Ambient wet bulb temperature and its affect on performance is the subject of this article.

Ambient wet bulb temperature is a condition measured by a device called a psychrometer. A psychrometer places a thin film of water on the bulb of a thermometer that is twirled in the air. After about a minute, the thermometer will show a reduced temperature. The low point when no additional twirling reduces the temperature is called the wet bulb temperature.

The measured wet bulb temperature is a function of relative humidity and ambient air temperature. Wet bulb temperature essentially measures how much water vapor the atmosphere can hold at current weather conditions. A lower wet bulb temperature means the air is drier and can hold more water vapor than it can at a higher wet bulb temperature.

For example:


Dry Bulb Temperature

--------------------------------------------------------------------------------
50°F
60°F
70°F
85°F
90°F

Relative Humidity

--------------------------------------------------------------------------------
40%
50%
35%
55%
60%
Resultant
Wet Bulb
Temperature
--------------------------------------------------------------------------------
40°F
50°F
55°F
73°F
78°F
When selecting a cooling tower cell, the highest or the design wet bulb temperature your geographical area will encounter must be used. Highest wet bulb temperatures occur during the summer, when air temperatures and humidity are highest.
Again by example, in Indianapolis, Indiana the design wet bulb temperature is 78°F. Historically Indianapolis can expect less than 1 hour per year that the conditions exceed a 78°F wet bulb. Typically 6,000 hours a year will have a wet bulb of 60?F or lower meaning that a cooling tower cell designed for a 78°F wet bulb will be able to make 65° - 67°F water for 6,000 hours per year ... nearly 70% of the year.

Most cooling towers are capacity rated at a "standard" wet bulb temperature of 78°F. That means on the days when the wet bulb temperature is 78°F, the tower will produce its stated capacity. In other words, a tower rated to produce 135 tons of cooling will produce 135 tons of cooling at a 78°F wet bulb temperature. At a higher wet bulb temperature, the tower cell capacity decreases.

Every location has a unique design (worst case) wet bulb temperature that is published by organizations such as ASHRAE and can be obtained easily. You can view Wet Bulb Temperatures for a listing of sample cities (opens new window : PDF file : 20K).

So, what does it mean when your cooling tower water temperature is higher than the normal 5° - 7°F above the current wet bulb temperature?

Your cooling tower may have lost efficiency
• Due to scale build up on the tower
heat exchange surfaces.
• Due to loss of air flow across the
heat exchange surfaces.
• Due to improper water flow

What can you do to improve your tower performance?
• Add tower cell capacity.
• Check for the efficiency losses described above.
• Replace the heat exchange surfaces with new
clean fill.
• Check for proper air flow.
• Adjust the water flow.

Cooling tower performance is related to ambient wet bulb conditions. Higher wet bulb temperatures occur in the summer when higher ambient and relative humidity occurs. Initial system design and proper system maintenance is critical to be certain your cooling tower cell is providing proper cooling.
Cooling Tower Design:
Although KaV/L can be calculated, designers typically use charts found in the Cooling Tower Institute Blue Book to estimate KaV/L for given design conditions. It is important to recall three key points in cooling tower design:
1. A change in wet bulb temperature (due to atmospheric conditions) will not change the tower characteristic (KaV/L)
2. A change in the cooling range will not change KaV/L
3. Only a change in the L/G ratio will change KaV/L

where,
KaV/L = tower characteristic
L/G = liquid to gas mass flow ratio (lb/lb or kg/kg)

Answer 2

For the cooling tower to perform effectively there must be unsatuated air avaialble. However on a rainy day as the air is almost close to 100% saturation, it will have problem. Because evaporated water can not be absorbed by the air stream. However if the inelt water temperature is really high, then I think because of close contact the air itself may get hetaed first, thus creating a difference between the dry and wet bulb temperatures. May be this is how cooling towers in power plants work!

Answer 3

Wow, you got some detailed answers. The short answer is that it doesn't work as well. A properly sized cooling tower will lower the water temperature to within about 5 degrees of the wet-bulb temperature. If the air is saturated the wet bulb and dry bulb temperatures will be the same. Most places in the U.S. the wet-bulb temperature is rarely over 78 degrees F. under normal conditions. In like Las Vegas the wet-buld can be in the 60 degree F range even on a 90 degree plus day (dry desert conditions.)

I've seen refrigeration systems shut down because of a shower on a hot day, elevated the wet-bulb temperature to a point where the tower (used as an evaporative condenser) could no longer keep the head pressure (condensing temperature) low enough for the system to operate.

Answer 4

................what has rain 2 do do with cooling tower.........the water is thrown up 2 cool..................the cooler the air ......or ....heavier ....more saturated ................ the better...............