We live in a world today with scenery dominated by complex machinery and intriguing equipment, most of which, the general public has little to no knowledge of in terms of what they are or how they work. More importantly, people don’t know why they are so important to the quality of life we are afforded today.
One of these fascinating pieces of equipment is a cooling tower. Cooling towers are used for, well… cooling. They are massive heat exchangers used to manage and dispose of waste heat from other industrial or manufacturing processes. This allows for the equipment, or process that produces the excess heat, to be used more frequently and for longer periods of time. Aside from the obvious cooling benefit of preventing an explosion or fire while you run your plant’s machinery, cooling towers help to cut long term depreciation costs for the manufacturers by extending the life of their equipment.
So, what exactly is a cooling tower? In searching for a cooling tower definition, one will find many resources that define it loosely as a unique type of heat exchanger in which air and water are brought into contact with each other. Ultimately, cooling towers are used to cool and treat the water stream coming in from industrial equipment or processes.
As it circulates hot water coming in from machinery or condensers, cooling towers disseminate the water over a large “fill” that enables rapid evaporation to take place releasing the heat back into the atmosphere. There are a few cooling tower processes and working principles involved to make this happen (including what a “fill” is), but we’ll touch on those a little later.
The main process of a cooling tower involves the hot water from a condenser or other industrial process being pumped directly into the cooling tower through affixed piping. The water is then dispersed evenly across the fill to allow for evaporation. Evaporative cooling is the main working principle involved in industrial cooling towers and is the reason they are so effective. A major contributor to how effective evaporative cooling is, relates to the fill the Cooling Tower contains or what the fill is comprised of. I know you are already thinking, what possible type of filler these massive towers could contain, but this fill is a little different than what you may think.
Fill, or fill media, refers to the collection of slats/boards/PVC inside the cooling tower that are designed to evenly distribute the water as quickly and widely as possible. To accomplish this, when the hot water is first pumped into the cooling tower, there are nozzles inside designed to spray the hot water over the fill media evenly. By spreading out the water’s surface area like this, the air now has the maximum air to water contact opportunity, which in turn causes the smaller volumes of water that are spread out over the fill to evaporate much faster.
At the other end, after spreading the water surface area as wide as possible, cooling towers use motor-driven electric fans to constantly drive air flow through the tower itself. As the water gets exposed to the air, evaporation begins to take place and the heat is released into the flow of air created by the electric fan. Finally, the heat is pumped out of the top of the tower, almost like an exhaust system, while collecting the now cooled water in a collection basin at the bottom of the tower. The cool water will now be treated and re-routed back to cool the machine it started with.
As the system water evaporates, where only pure water is lost, all of the elements in the system water are left behind. This dictates a need for makeup water, which is just a water source designed to refill the system water as needed, to help retain the volume of system water that is needed for operation. This makeup water will also contain additional elements to add into the system water when refilled. As the makeup water begins to replenish the system water, the elements contained in the system water begin to rise and accumulate. To reduce this concentration of elements, cooling towers have a bleed off valve that will open periodically based on the volume of system water. When the bleed off opens, it will drain a portion of the system water out as wastewater discharge, which will typically contain a high concentration of elements. This highly concentrated wastewater leaves the basin of the cooling tower via the bleed off valve, at which point more make up water is added into the remaining volume of system water. This slowly dilutes the concentration of elements in the system water due to it having less elements than the wastewater.
This dilution process is extremely important as poor water treatment practices can lead to a hazardous work environment or large-scale environmental damage. Often water treatment chemicals (such as chromium) are added to the system water to protect equipment and piping from corrosion, but more importantly to control bacteria and algae growth within the tower. One of the biggest reasons for this treatment is to prevent diseases.
To start, let’s assume we operate an industrial power plant and are desperately in need of a cooling solution for a new and very large reactor that will be installed this summer. Once any business determines they have a need for cooling, or a cooling tower specifically, they will typically call an HVAC or cooling tower Manufacturer’s Representative to determine what design, size, type, and quantity will be needed. These professionals are the intelligence behind your mission of a fully functional power plant with all the bells and whistles. Not only do they get you the lowest cost and ensure on-time delivery, but they are the ones who communicate directly with the mechanical engineers to make sure the cooling tower you get is the one you need.
The most common type of cooling tower is the crossflow cooling tower. These systems have water flowing vertically through the fill media crossing paths with air flow coming in horizontally. The inbound hot water is spread out using a distribution basin located at the top of the tower, using gravity only. These basins are industry standard and found on most if not all crossflow units in use today. Crossflow cooling towers are usually selected based on their efficiency and serviceability however, one of the downfalls to this design is that it has a bigger footprint. Footprint refers to the size of the equipment in relevance to the property size of the business, as crossflow designs are short and take up a lot of real estate.
In Counterflow cooling towers, unlike the crossflow, the air flows vertically through the fill to begin the cooling process. Unfortunately, this design prohibits the use of a gravity only method such as with crossflow towers. This is due to the change in airflow, as when the air flows vertically it is against the inbound flow of the water, preventing the water from evenly spreading and disseminating through the fill. Instead, they have pressurized nozzles at the top of the tower to spray the hot water onto the fill, ensuring that water is evenly distributed, and the fill is fully wetted.
The next two up for comparison on our list are forced draft cooling towers and induced draft cooling towers which focus more on the air flow creation process. As I’m sure you may have guessed, one of these uses a forced airflow derived by using blowers at the bottom of the air inlet louver, and the other tries to take advantage of the airs natural flow. Forced draft would be the prior type and seems to function normally aside from the obvious increase in maintenance from the addition of the blowers at the bottom of the tower. However, with induced draft, fans mounted on top of the tower are used to manipulate the airflow coming into the tower by pulling it up through the tower and releasing out of the top.
Last but certainly not least, these massive pieces of equipment are not the easiest of things to transport, and when FedEx isn’t an option, where do you turn? As it turns out, there are two ways to have a cooling tower installed and you generally choose when you are quoted on the product. Factory Assembled Product means just what it sounds like, assembled at the factory to the maximum level the tower’s design will allow for, and shipped in either one or a few shipments to the site for final assembly. Whereas Field Erected Product refers to the towers that too large to ship fully or mostly assembled. Field Erected Towers are shipped out more dispersed, parts come in marked for inspection and assembly in many different smaller shipments. Most manufacturers provide Labor and Supervision for assembly to ensure the equipment will function properly and has all necessary parts accounted for.
Industrial Cooling Towers are by far some of the most unique and effective pieces of equipment I have had the pleasure of learning about. So, I can tell you with certainty… next time I’m driving down I-95 next to these massive towers attached to plants and factories, I will be wondering if they are crossflow or counterflow, and forced draft or induction.