Calculating cooling range for cooling tower design modification

[edwardmp2009]

At our company, we currently have a 3-cell counter current flow induced draught cooling tower used for cooling service water for vacuum pumps. Literature states that vacuum pumps operate at maximum capacity when service water temperature is 15 degrees Celsius and below. The design data available for the service cooling tower is shown below:

Number of cells = 3
Capacity = 87 cubic meters / h of water in each cell (total = 261 cubic meters / h)
Type = induced draught cooling tower
Dimensions of each cell:Length = 1 m; width = 1 m; height = 1.5 m
Current cooling water temperature in summer = 35 degrees Celsius
Cooling tower return water temperature = 47 degrees Celsius
Maximum ambient air temperature = 42 degrees Celsius
Inlet air wet bulb temperature = 24.5 degrees Celsius

From the above data, it can be seen that the temperature approach is 10.5 degrees Celsius and the cooling range is 12 degrees Celsius.

The following air data is not available at this stage:
1. air flow rate
2. dry bulb temperature of outlet air
3. wet bulb temperature of outlet air

We wish to modify the cooling tower by adding a 4th cell of similar dimensions. How can we calculate the cooling range or temperature approach for the new design, that is the cooling water temperature under the new design modification?

 

[edwardmp2009]

I guess, the only way out would be to make intelligent estimates of the air flow rate, dry bulb temperature and wet bulb temperature of outlet air. Assuming that the outlet air has 100 % humidity, then wet bulb temperature = dry bulb temperature

 

[CH5OH]

if cooling capacity was adequate, the lowest obtainable water temperature would be the wet bulb temperature of the air.
the cooling capacity you are short off:
heat water 35deg (kJ/kg)- heat water 24.5deg
find the delta heat in the cooling water from inlet and outlet temp (kJ/kg) (mollier water,steam table)
next, the heat load from the flow (kg/s) x kJ/kg = kj/s
next find the cooling capacity of 1 cell (heat load/3)
the new temperature outlet cooler:
heat you're short off (kJ/kg) - cooling capacity 1 cell (kJ/s) / flow (kg/s)
from table:
temperature cooler out= temperature @ (heat water 24.5deg (kj/kg) + above (kJ/kg))
temperature cooler in= temperature @ (heat water @ temp out + heat load)

 

[KiwiMace]

If you add another identical cell, your approach temp will not change. Approach is a function of the design of the tower. You can somewhat vary the flow of a tower to obtain a lower supply temp, but only within the operating minimum flow limits of the hot water distribution.

Based on what you have written above, i suggest you might ask your vendor to at least verify the distribution will work with 3/4 of the basis flow and if not reduceor change the nozzles so that it is. They will be able to give you new performance figures including supply temp. This is difficult to estimate by yourself because of the fill characteristics unknown.


As mentioned above, the inlet air wetbulb is your minimum theoretical water temp, but the design is asymptotic at this number so practically unattainable. You might get 15 on non design days though.

 

[edwardmp2009]

Thanks for your responses and compliments of the season.
CH5OH - May you elaborate a little bit more on the last 2 statements about temp cooler out and temp cooler in. Also note that Perry's Chemical Engineers' handbook quotes the minimum design approach temperature as 2.8 deg C which means practically, the lowest achievable cooling water temp is (24.5 + 2.8)= 27.3 deg C.
KiwiMace - i strongly believe that adding an identical 4th cell will reduce approach temp because of:
1. Increase in cooling surface area by 33.33 %
2. Increase in G/L ratio by 33.33 % due to reduction of water load per cell.
I'm still to contact the vendor and seek more clarification on your point of view.

 

[sshep]

Hey Edward,

The GPSA manual has a nomograph which is used to calculate an existing performance factor, and then assess changes based on heat load, circulation rate, air rate, L/G ratio, wetbulb temp change, etc. Since it isn't clear if you will change the circulation rate proportionally or spread the existing circulation over 4 cells, etc, you should work through your specific case.

I have tried to make an attachment just to show you what it will look like, but cannot see at this time if it has attached properly. GPSA gives instructions for how to make different assessments.

bnest wishes,
sshep

 

[sshep]

Hey Edward,

Something isn't working with my computer to open files from engineeering.com but I hope it worked.

Since the question related to the range and approach, I just want to add that I thik the range is basically a function only of the duty being removed from the process and the CW circulation rate since duty=circulation rate*range. I think that range has essentually nothing do with the tower geometry itself.

It is the approach which is the unknown in your case. When you find the approach, the supply and return temperatures will also be easily calculated.

best wishes,
sshep