Large centralized AC systems require heat rejection to the outside of the building. This heat rejection is obtained via the use of a cooling tower.
A cooling tower is a device that rejects heat to the atmosphere by cooling a working fluid (typically water) to a lower temperature. In a chiller plant, this water is part of the condenser water loop, and is responsible for removing heat from the chiller, and rejecting it to the outside atmosphere.
Cooling towers are usually found on the top of large buildings that have central AC systems. If a building has a water-cooled chiller, then there is a good chance that it has a cooling tower outside. Cooling towers can also be found on the ground behind buildings if there is enough space.
How does a Cooling Tower work?
Cooling towers work by using air to cool a working fluid (water). There are primarily three different methods in which a cooling tower can remove heat from water:
1. Direct Cooling – Open Loop
This is done by bringing the water into direct contact with air. As the cool, dry air comes into contact with the warm water, some of the water evaporates, producing an evaporative cooling effect. When some of the water evaporates, it removes a large amount of heat with it. Because the water comes into contact with the outside atmosphere, this is known as an open-loop system.
2. Indirect Cooling – Closed Loop
This is similar to the direct cooling open-loop system, however, the water that is being cooled never comes into direct contact with the surrounding air. Instead, this water flows through a coil in a closed-loop system. To remove heat, a separate, open-loop system flows over the coil to remove heat from the water in the closed-loop.
3. Direct Cooling – Closed Loop
This is similar to the indirect cooling closed-loop system, in that the water that is being cooled flows through a heat exchange coil in a closed-loop system. To remove heat, air flows over this coil and takes heat away from the water inside. This is the only method that does not use evaporative cooling to remove heat.
Classification of Cooling Towers
Now that we have discussed the different heat exchange methods of cooling towers, let’s go over the specifics of how cooling towers remove heat from water.
Classification by Airflow Generation Method
All three methods of heat removal in cooling towers require airflow to remove heat from the working fluid (water). However, there are a few different methods that are used to achieve the airflow movement that is required for heat removal.
This is the simplest form of airflow generation. There are no fans or anything of that sort in a natural draft cooling tower. Natural draft is achieved by utilizing the lower density of warm, moist air. This warm, moist air rises naturally upward and out of the cooling tower, inducing an upward current of air through the cooling tower.
Mechanical draft cooling towers utilize fans to move air through the cooling tower. There are two types of mechanical draft:
- Forced Draft. A forced draft cooling tower has a fan at the cooling tower’s air intake. The fan blows air into the cooling tower.
Pros: Able to work with high static pressures, so can be installed in tight spaces where airflow may be restricted.
Cons: Airflow exiting the cooling tower is low velocity, so it can be prone to recirculating back into the tower, reducing the tower’s efficiency. Also, the fan can be damaged if the atmospheric air is cold enough, since the fan is on the inlet of the cooling tower.
- Induced Draft. An induced draft cooling tower has a fan at the cooling tower’s air discharge. The fan sucks air into the cooling tower.
Pros: Airflow exiting the cooling tower is high velocity, so there is less chance of recirculating back into the tower.
Cons: Needs to be installed in a large open space. Can sometimes be very noisy.
Classification by Direction of Air-to-Water Flow
We can also classify cooling towers based on the direction that the atmospheric air interacts with the fluid in the cooling tower.
Crossflow Cooling Tower
In crossflow cooling towers, the water flows downward vertically through the cooling tower, while the air flows horizontally through the fill in the cooling tower. This means that the atmospheric air interacts with the water flow in the cooling tower in a perpendicular direction.
Counterflow Cooling Tower
In counterflow cooling towers, the water flows downward vertically through the cooling tower, while the air flows upward vertically through the fill in the cooling tower. This means that the atmospheric air interacts with the water flow in the cooling tower in the opposite direction.
Cooling Tower Components
Compared to other equipment in HVAC, cooling towers are relatively simple machines that are easy to understand. This is because there are a minimal amount of moving parts in a cooling tower – usually the fan is the only moving part!
Cooling Tower Fan
Mechanical draft cooling towers have fans. The purpose of the cooling tower fan is to induce a draft through the cooling tower to aid in heat transfer. In forced draft cooling towers, the fan can be either axial or centrifugal. In induced draft cooling towers, the fan is always axial. Most cooling towers are induced draft with an axial fan.
Heat Exchanging Coil
Closed loop cooling towers have a coil that the working fluid (water) flows through. The purpose of the coil is to cool the working fluid that flows through the inside of it. A coil consists of a pipe with thin fins around it to absorb heat efficiently. In a closed loop system with indirect cooling, a separate open loop system flows over the coil to cool the working fluid inside of the coil.
In an open loop cooling tower, fill is the primary surface which facilitates the transfer of heat from water to air. Fill works by dispersing the water so there as much surface area as possible to aid in heat transfer. Fill is usually made of some type of plastic such as PVC or polypropylene. There are two types of fills:
- Splash Fill
Splash fill works by disintegrating the water into smaller and smaller drops. More drops = more surface area = more heat transfer.
- Film Fill
Film fill works by grooming the water to flow down through the cooling tower in a thin, wide film. Thinner film = more surface area = more heat transfer.
Drift eliminators are usually installed on the perimeter of the cooling tower, around the fill. The purpose of the drift eliminator is to reduce the loss of already-cooled working fluid to hot air in the atmosphere.
Water Treatment System
The purpose of the water treatment system is to remove minerals from the water to prevent scaling and fouling on the cooling tower. Additionally, water treatment systems usually have filtration to remove particulates, and chemicals to prevent growth of biological organisms such as algae.
Cooling towers are an integral part of a central AC system. Even though they are fairly simple machines, there is a lot of science behind the implementation of cooling towers. There are many different types of cooling towers depending on the application and the needs of the system that they are a part of.