Open Vs Closed Circuit Cooling Towers

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Open Vs Closed Circuit Cooling Towers

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Open Vs Closed Circuit Cooling Towers

EngrDesign1856

(Mechanical)

(OP)

I am new to the industry, so I have very limited knowledge of the subject. I have been recently asked to have an old BAC cooling tower replaced with little to no downtime 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 120°F to 80°F with (worst case) wet bulb at 75°F. Also, if anyone might have relative operating costs, that would be helpful also.

RE: Open Vs Closed Circuit Cooling Towers

timbones

(Mechanical)

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 separate 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

RE: Open Vs Closed Circuit Cooling Towers

EngrDesign1856

(Mechanical)

(OP)

Timbones, thanks for your reply. The only reason why I was contemplating the closed loop system was that I heard it could 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? Another 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, how far away does it have to be? Another reason for 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!!

RE: Open Vs Closed Circuit Cooling Towers

timbones

(Mechanical)

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 sight) = (out of mind). Cooling tower plumes are not the greatest thing to look at. The advantage of the closed system in this regard is that 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. It 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!

RE: Open Vs Closed Circuit Cooling Towers

Deltathx

(Mechanical)

There are a few common drivers; Energy savings can be achieved if you are replacing a cooling tower heat exchanger combination with a closed loop system. Pumping the coolant 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 auxiliary 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 approach temperatures can be achieved by making direct use of the evaporative cooling. A closed loop system does not suit every application, but it sounds as though you should at least consider one for your application. I suggest you look at Tom, Delta T Heat Exchangers (www.deltathx.com). BAC typically manufactured relatively smaller cooling towers that would lend themselves to be roof-mounted. Cooled loop cooling towers can be smaller and 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).

Energy savings can be achieved if you are replacing a cooling tower/heat exchanger combination with a closed loop system. Pumping 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 auxiliary losses are much less than a cooling tower/heat exchanger system. Less water means less treatment, and because the water is not flowing inside a heat exchanger, water quality is not as important. Often spent or poor-quality water can be used. Closer approach temperatures can be achieved by making direct use of evaporative cooling. A closed loop system does not suit every application, but it sounds as though you should at least consider one for your application. I suggest you look at www.niagarablower.com for a good explanation of how closed loop evaporative coolers work and their benefits. Tom, Delta T Heat Exchangers.

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The Difference Between Closed and Open Circuit Cooling Towers

Which Cooling Tower System Is Best for You?

Cooling towers are used to cool and circulate water in settings such as oil refineries, petrochemical, and other chemical plants, thermal power stations, and HVAC systems for cooling buildings. A cooling tower extracts waste heat into the atmosphere through the evaporative cooling of a water stream to a lower temperature.

There are two different types of industrial or HVAC cooling tower designs: Closed Circuit Cooling Tower systems and Open Circuit Cooling Tower systems. Both types of towers serve the same purpose, but they go about cooling the water in different ways.

How Does an Open-Circuit Cooling Tower Work?

An open-circuit cooling tower is an enclosed structure that distributes warm water over a labyrinth-like packing, or fill, which provides an expanded air-water interface for heating of the air and evaporation to take place. The water is cooled as it falls through the fill and is then collected in a cold water basin below. The heated moisture-laden air leaving the fill is discharged into the atmosphere.

More about Closed-Circuit Cooling Towers

A closed-circuit cooling tower, or dry cooling tower, involves no contact between the air and the fluid being cooled. This tower has two separate fluid circuits, one in which the fluid is recirculated on the outside of the second circuit, which is a bundle of tubes through which the hot water is flowing. The air drawn through this cascading water provides evaporative cooling similar to an open cooling tower, except that the cooled water never makes direct contact with the air. Closed-circuit cooling towers require much more energy to achieve the same cooling as an open circuit cooling tower.

Different Types of Closed-Circuit Cooling Tower Systems

Another closed-circuit cooling tower system involves a separate heat exchanger package along with an open cooling tower. The fluid on the hot side of the heat exchanger also never makes direct contact with the air. This allows the heat exchanger to be put inside, protected from the outdoor weather elements along with other system advantages. This closed system will last much longer than the outside coil type of closed-circuit cooling tower.

Are Both Cooling Towers Effective?

There are so many factors and details to consider. You may be wondering: “Are both systems equally effective?” The answer to that question is “yes!” Both open and closed-circuit cooling towers are effective for cooling water, but they each work in different ways and consume different amounts of energy.

Still Have Questions?

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