27 replies to this topic

[PingPong]

What air flow does Hysys calculate?

 

Air flow required depends on number of theoretical stages in the column. With 3 stages less air is required than with 2 stages.

 

Obviously the air leaving the column at the top can not be warmer than the incoming water, which is at 45 ^oC.

Let's assume that a realistic air outlet temperature is 40 ^oC, then the maximum possible water content of that air can be calculated using a steam balance.

You know the water content of the air entering the column.

You can also calculate the duty of the cooling tower from the cooling water flow rate and the inlet and outlet temperatures.

Using the heat of vaporization of water you can estimate how much water has to vaporize.

Then finally you can calculate how much air is required to carry that amount of water vapor out of the column.

[mastergii]

 

 

Air flow required depends on number of theoretical stages in the column. With 3 stages less air is required than with 2 stages.

 

 

What other effects is important to think about when deciding how many stages to use?

[PingPong]

This cooling tower does not exist yet, so anything you do is just a guess. We do not know what the cooling tower vendor will choose with respect to the air outlet temperature. My 40 ^oC (5 ^oC lower than incoming water) was just a guess.

Number of theoretical stages in your simulation is just a means to achive an objective: to cool the water down to 30 ^oC (or whatever).

 

 

The cooling tower vendor will probably use another, more complicated, calculation method to achieve this objective, taking also into account correlations to calcultate required tower height, taking into account a U-value for heat transfer and an A based on the type of column fill used. He may come up with a different air flow than you, and achive a different number of theoretical stages (which he will not report to you) but that does not really matter.

 

What you do now is no more than making a simplified model, a black box. Suppose you find out that using 3 stages and a certain amount of air results in cooling water of 30 ^oC and exit air of 40 ^oC, then you can keep both of those constant and see what the impact will be if you change wet bulb, or water inlet temp, or air inlet temp, or water flow.

 

If you would ask the vendor the impact of the same operating changes, he will come up with results that will not be much different from yours, even though his calculation method is different. That is simply because both your method and his method both give exactly the same result at the design point.