Cooling Tower Effectiveness

[grant464]

Hi guys,

I currently have process water @ 500gpm that is cooled via a cooling tower from about 110 down to 90 F. Our process requirements have changed, and we now need the process water to be at a lower temperature, approx. cooled from 100 down to 75-80 F.

My concern is whether a cooling tower is capable of reaching the 75-80 degree temperature year round. The plant is located in Toronto, Ontario, Canada, so the wetbulb temperature can climb into the 60's for 2-3 months of the year. Also, the wetbulb can hit into the 70's during the late afternoon hours in July-August.

My question:
Will adding additional cooling tower capacity meet my requirements, or does the cooling tower's performance severly degrade when the wetbulb temp is so close to my design cooling temp?

Maybe for this design temperature, it would be better to purchase a chiller to couple with the cooling tower?

Some general discussion on the topic would be great, I'm not looking for an easy answer.

Thanks,

Grant

 

[danberry]

Your heat load is apparently the same but since you are closer to ambiant WB, more capacity is needed. A typical cooling tower approach is around 15F so your outlet could be at 75F (2-3 month) to 85F on those hot days.

I suspect that with a bit of over-design you may get a 10F approach and 80F outlet. Talk to your cooling tower vendor.

 

[ChrisConley]

I'm assuming the same flowrate? If you can get away with a reduced flowrate you'll improve the ability of the cooling tower to achieve your approach.

How critical is the 75F number? A small, side stream air-cooled chiller chiller might be a solution for high wetbulb days.

 

[grant464]

Yes, the same flowrate is required.

When you say small side stream chiller, what is defined as small? Considering my flowrate, my initial impression is that a massive chiller would be needed. (The cooling tower is 500 tons.)

What about installing a water-cooled chiller inconjuntion with the existing cooling tower. When it is cool enough outside, the chiller could be bypassed?

- Grant

 

[ChrisConley]

I was thinking that a 500 ton W/C chiler would be expensive.

Essentially you're be looking for a chiller that could drop your outlet temp from 85F to 80F on a design day (Depending on the performance of you cooling tower).

5F drop on 500gpm is ~110 tons of cooling. With a side-stream 110 ton air-cooled chiller you would only turn it on when outdoor wetbulb was high and your cooling tower couldn't make up. The sidestream would chill say 180 gpm ~15F to 70F, when mixed with 320 gpm at 85F mixes to 500 gpm at 80F. *

The high discharge temp off the air-cooled chiller would help it's efficiency as well.

A W/C is probably an easier solution (fewer controls), but the sidestream would be much cheaper $$.

[small]*All calculations and numbers are 'back of a napkin' and should be verified.[/small]

 

[grant464]

I appreciate the help, I will investigate both to see what makes most sense.

- Grant

 

[davefitz]

Yu would probably need to have the tower reviewed by a CT vendor to confirmthe following but:

a) for the same thermal load ( ie GPM * ( Ti-To) ), the CT outlet temp will be above the current wet bulb temp by the same amount. If you plot current To-Twb and find it is a nearly constant number .AND. greater than 5 F, then there is potential to upgrade the CT to get beter performance.

b) Many large towers were built to allow retrofit with either a more efficient fill or able to add about 20% more fill. This conversion would require modifying or replacing the fan blades and reducing the margin on the motors. The cost and power consumption of a CT retrofit is likely far less than adding a chiller.

c) As I recall, CTI will not permit a tower vendor to guarantee performance with less than a 5F pinch between Twb and To; if the current pinch is 15 or 20F, there may be more cability inthe tower with the upgrade or additon of fill.

d) clean out the fill once ina whiel with a dose of hydrogen peroxide, if permited by env permit.

 

[imok2]

Why not look at installing a parallel 120 ton cooling tower
with VSD fan and isolation valves to come on line if wet bulb gets too high for main tower.

 

[grant464]

imok2, what I'm concerned about with adding a second cooling tower is whether it will be capable of meeting the load when the wetbulb rises in the summer, when the approach temperature becomes very small.

I think that a far larger second cooling tower would be needed.(far larger than 120 tons, is my speculation).

- Grant

 

[KiwiMace]

This is about a 520RT cooling load, and the approach looks pretty good for the majority of the time.

If 80degF is OK, and the wetbulb goes up to 70degF, you shouldn't have a problem. Most standard cooling tower selections I have seen only start to get nervous with a 6degF approach.

Note that cooling tower size/cost is v sensitive to the approach (design wetbulb temp - desired water outlet temp) and a higher approach in the design will net a cheaper smaller tower. Your latitude is good for cooling towers!

 

[imok2]

If you size the cooling tower at a 76 wet bulb you should be OK

 

[quark]

Go through this excellent and extensive thermal design manual of Cooling Towers with example spreadsheets.

You have all the derivations for calculations there and if I remember well, the spreadsheets were prepared by J-Walk. Run the parameters and check for yourself.

I was searching for this manual ever since the hanafos website was pulled down and luckily I came across it yesterday.

PS: After accessing the website, click on Principle of Cooling menu at left bottom and once you go to that page, take your scroll to the right and click on Professional Engineering Publication

 

[grant464]

Thanks again for all the help guys,

- Grant