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Cooling Tower Thermal Selection With Large Approach


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#1 rajeev205

rajeev205

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Posted 04 July 2008 - 01:51 AM

I need an expert opinion on following issue related to cooling tower thermal selection:

I have selected a cooling tower for cooling 900 m3/h/cell of flow of water from 60 deg c to 45 deg c at design wet bulb temperature of 27.8 deg c. Tower selected is counterflow induced draft type and it is using optigrid type fill. Tower plan area (inside) is 6 m x 6 m. I have kept fill depth as 3 m with vertical spacing between fill layer as 200 mm. Air inlet height of cooling tower is 1.829 m. It is all around the tower which is having 2 cells. Total operating load is 1800 m3/h. Air inlet is all around cooling tower.
Fan dia is 3.657 m having hub of dia 0.9144 m. Fan is running at 315 RPM. Motor rated capacity is 11 kW and of 1500 RPM.

I am getting theoritically selection with L/G of 3.999 and KaV/L 0.744.

Water loading on fill is 25.48 m3/h/m2.

I need expert opinion on L/G of 3.999 which seems to be too high for splash cooling arrangement, Please let me know is it a correct selection?

Water loading also seems to be too high on per square m are of opti grid fill. Will it not affect the cooling?

Please help me in solving above problem.

#2 Jiten_process

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Posted 05 July 2008 - 01:38 AM

QUOTE (rajeev205 @ Jul 4 2008, 01:51 AM) <{POST_SNAPBACK}>
I am getting theoritically selection with L/G of 3.999 and KaV/L 0.744.

Water loading on fill is 25.48 m3/h/m2.

I need expert opinion on L/G of 3.999 which seems to be too high for splash cooling arrangement, Please let me know is it a correct selection?

Water loading also seems to be too high on per square m are of opti grid fill. Will it not affect the cooling?

Please help me in solving above problem.


hi rajeev

I am not an expert, however i have just done good exercise on thermal design of cooling tower.

neways, your approach is high, but i dont think it should have any prob. provided the design is robust.

Have u worked out height of cooling tower? what is the M.T. coefficient (Ka) value of the fill which you have selected (generally it depedns on type of fill and is vendor specific which they do not disclose)?

Can you give some data; dry bulb temp. of air, the outlet air temp. which you have calculated (as per my caln it is 53°C), height above MSL, Relative humidity? how did u select loading factor?
i just wanted to recalculate your values.

Loading factor too, seems too high but you got to cross check it with vendor, they are the right persons to ask for.

As per my caln your L/G should be 3.5 and KaV/L value is around 0.54. (assuming 70mtr above MSL).

reply....

#3 rajeev205

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Posted 05 July 2008 - 03:40 AM

QUOTE (Jiten_process @ Jul 5 2008, 12:08 PM) <{POST_SNAPBACK}>
QUOTE (rajeev205 @ Jul 4 2008, 01:51 AM) <{POST_SNAPBACK}>
I am getting theoritically selection with L/G of 3.999 and KaV/L 0.744.

Water loading on fill is 25.48 m3/h/m2.

I need expert opinion on L/G of 3.999 which seems to be too high for splash cooling arrangement, Please let me know is it a correct selection?

Water loading also seems to be too high on per square m are of opti grid fill. Will it not affect the cooling?

Please help me in solving above problem.


hi rajeev

I am not an expert, however i have just done good exercise on thermal design of cooling tower.

neways, your approach is high, but i dont think it should have any prob. provided the design is robust.

Have u worked out height of cooling tower? what is the M.T. coefficient (Ka) value of the fill which you have selected (generally it depedns on type of fill and is vendor specific which they do not disclose)?

Can you give some data; dry bulb temp. of air, the outlet air temp. which you have calculated (as per my caln it is 53°C), height above MSL, Relative humidity? how did u select loading factor?
i just wanted to recalculate your values.

Loading factor too, seems too high but you got to cross check it with vendor, they are the right persons to ask for.

As per my caln your L/G should be 3.5 and KaV/L value is around 0.54. (assuming 70mtr above MSL).

reply....

Dear Jiten,

Thanks for your reply.

KaV/L of fill is 0.676

I have designed the tower considering inlet air RH as 55% wbt 27.8 deg C. Hope dry bulb is worked out as 34.64 deg C. Exit air temperature is 54.64 deg C. Altitude consider is 0 m. I have space restriction that is why I have opted for 6m x 6 m space. Hence loading is coming around 25.48 m3/h/m2.

Regards

Rajeev

#4 Jiten_process

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Posted 05 July 2008 - 04:45 AM

QUOTE (rajeev205 @ Jul 4 2008, 01:51 AM) <{POST_SNAPBACK}>
Have u worked out height of cooling tower? what is the M.T. coefficient (Ka) value of the fill which you have selected (generally it depedns on type of fill and is vendor specific which they do not disclose)?

Can you give some data; dry bulb temp. of air, the outlet air temp. which you have calculated (as per my caln it is 53°C), height above MSL, Relative humidity? how did u select loading factor?
i just wanted to recalculate your values.


reply....


rajeev,

u still havent told me all what i have asked above. neways, for your case as per my understanding you should not worry with high L/G ratio. Second, try to carry out firm calculation of KaV/L, I assume that you must have used Simson's rule to solve the integration equation for tower characterstics, m i right?

once, you get final confirmed value of KaV/L, thenafter it becomes more vendor subject, because there are two unknowns in above value (i.e. Ka and L-loading factor). There are some well established charts available for calculating loading factor w.r.t. inlet-outlet water temp and w.b. temp. So far i have come across loading factor max. 8m3/hr/m2. However it is more vendor dependent since it depends on type, shape, arrgmnt of fill. Further for m.t. coeff. (ka) again it is purely vendor dependent value which i feel you cant find out. However you can have past project data for same vendor and fill, then you can back calculate it.

I dont know how did you take loading factor and Ka, but then only you would arrive at tower height. But theoratically it is clear, if you increase the loading factor your fill area will get down but your tower height(i.e. fill height) will increase which again lead to high fan motor rating.

One thing is more; for high L/G, loading and tower height be clear that if your site operating conditions varies slightly then effect will be considerable on performance. So design the tower and evaluate the performance for anticipated variation in input data to make the design robust. which you are suppose to do being a process engineer.

So, if you are working with some EPC company then try to consult some vendor.

I hope this makes some sense....
to be continued...




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