How does the air flow rate controlled by the orifice differential pressure affect the make up rate of water?

cooling towerHow does the air flow rate controlled by the orifice differential pressure affect the make up rate of water?
What orifice are you referring to?





The make up rate of a cooling tower is dependent on the heat load on the tower. Fans pull ambient air through the tower, contacting the water droplets of the tower spray from the distribution nozzles. The cooling does not necessarily occur by just lowering the temp of the water. Most of the cooling occurs by the process of evaporation of the circulating water. Water is not blown out the top of the tower. Water vapor is exiting the top of the tower. For every pound of water evaporated, you remove about 1000 BTU of heat from the cooling water system.





Suppose that a steam plant is operating a steam turbine generator, with a steam flow of 300,000 lbm/hr. The exhaust of the steam turbine is dumped to a condenser to be condensed back into water to be reused by thte boiler. To condense the steam, you must remove 1000 btu (this is called the latent heat of vaporization) from each pound of steam. This heat is transferred to the cooling water flowing through the condenser. Its flow rate (in pounds/hr) is much higher than that of the steam. Now, the heat must be removed from the cooling water in the tower. In fact we must remove the heat equal to the latent heat that was present in the steam before it was condensed. That means that we must evaporate 300,000 pound/hr of our cooling water, which means we are losing about 600 gallons of water per minute. That's a lot of water to be made up!How does the air flow rate controlled by the orifice differential pressure affect the make up rate of water?
The air flow rate is controlled by the orifice. Change in the orifice causes change in the differential pressure, which is how it controls the flow of air. More differential pressure, less air, less differential pressure, more air.


In a cooling tower, the make up water is the water used to replace the water lost in the cooling process.


So there are two mechanism by which the air flow rate could alter the rate of water make up.


The first is that the more air there is, the more water is caught up in the air flow, and leaves the tower.


The second is that the more air there is, the more cooling is done by air, and not the water, so the demand for water decreases.


It depends on the design of the tower whether the first or second mechanism has the greater effect.


If the tower is relatively large and the air velocity small, this ensures that very little water escapes, the second mechanism is dominant, and as air increases, water usage decreases.


If it is small, and the air velocity high, more water will be drawn out of the tower if more air is used, so the consumption goes up and not down.


Simple