Open Vs Closed Circuit Cooling Towers

[EngrDesign1856]

I am new to the industry, so I have very limited knowledge of the subject. I have been recently asked to have a have an old BAC cooling tower replaced with little to no down time to the factory. After doing a little research I came across various closed circuit cooling towers, and I was wondering if anyone has had experience switching from an older open circuit system to a closed circuit system, and what problems should I expect. The current operating conditions are at a max of 800 GPM, from 120ºF to 80ºF with (worst case) wet bulb at 75ºF. Also, if anyone might have relative operating costs, that would be helpful also.

 

[timbones]

I guess the first question is, why would you want to put in a closed circuit cooling tower?

In my experience, the open circuit system is by far the more common way to go. I can't imagine going to a closed system unless I had some compelling reason such as keeping the cooling fluid seperate from air or fouling was a concern. Neither of these are likely an issue in your case as you are thinking about converting an existing open circuit.

As for operations, you will still need cooling water circulation pumps, but now you will need another set of pumps to circulate the cooling medium as well. So your plant is more complex and your pumping horsepower is probably higher. Your fans will probably need to be higher power because you have to suck or blow air past both the tower fill and the closed loop exchangers. You may have to contend with plugged spray nozzles and external scale on the tube bundles. Efficiency will probably be lower (due to the extra delta T between the coil and the outside of the tubes) and the tower probably bigger. The whole set-up sounds more expensive.

Where is the upside in this application? I am no expert on these things, so what have I missed?

Tim

 

[EngrDesign1856]

Timbones, thanks for your reply. The only reason why I was contemplating the closed loop system was because I was told that a closed system can be placed on the ground next to a building. Do you or anyone else know if this is true, or do you still have the same air recirculation problems associated with the open towers? The other thought was that if it could be installed on the ground, then your pumps wouldn’t have to be as large. I understand that I will need additional pumps for cooling water, but I thought power wise, I would still be ahead. If I am still overlooking something, please explain.

Is it possible to place an open or closed system on or near the ground if it is away from all other buildings, if so then how far away does it have to be?

The other reason for all of this is that the owners are remodeling the plant, and do not like the huge monstrosity of a system that we currently have now. Any thoughts on appearances?

Thanks again!!

 

[timbones]

You can put either system on the ground. In fact in my lines of work, which is oil, gas and power generation, the open loop cooling towers are almost always on the ground.

Aesthetically, it is better to have them on the roof of a building because (out of site)=(out of mind). Cooling tower plumes are not the greatest thing to look at. The advantage of the closed system in this regard is you can put a dry air cooled exchanger at the top of the tower (above the wet section) to cut down the size of the plume.

As for distance away from the building, I'm not really sure. Probably depends a lot on the tower design. I know BAC has a number of designs that can be put almost right up against the side of the building. Just make sure you know the prevailing wind direction and choose the side of the building accordingly!

 

[Deltathx]

BAC typically manufactured (realtively speaking) smaller cooling towers which would lend themselves to be roof mounted. Cooled loop cooling towers can be smaller and be self contained with an integral basin but as timbones suggested there would usually be a driver to use a closed loop cooling tower (particularly a smaller unit).

There are a few common drivers;
Energy savings can be achieved if you are replacing a cooloing tower heat exchanger combination with a closed loop system. Pumping the cooled being cooler to a closed loop cooler might require a little more energy but a fraction of the water is typically required and usually less air so your auxiary losses are much less than a cooling tower / heat exchanger system.

Less water means less water treatment and because the water is not flowing inside a heat exchanger, water quality is not as important. Often spent water or water of poor quality can be used.

Closer approch temperatures can be achieved by making direct use of the evaporative cooling.

A closed loop system does not suit every application but it sound as though you should at least consider one for your application. I suggest you look at

http://www.niagarablower.com

for a good explantion of the how closed loop evaporative coolers work and the benefits.

Tom
Delta T Heat Exchangers

http://www.deltathx.com