How Cooling Towers Work

Table of Contents

How Cooling Towers Work - Cooling towers use evaporation cooling to cool condenser water from a chiller. Water is pumped to the top of the cooling tower, where the water flows down some flow plates to the cooling tower basin. As the water is flowing down the flow plates air is pulled through the plates and over the water where it releases heat through evaporation.

Large Cooling Towers Serving a Data Center and Commercial Building

After the water has released the heat, it is then pumped back through piping to a chiller barrel, where it absorbs more heat and then returns to the cooling tower. Because of evaporation, the cooling tower system is equipped with a float that controls the level of water in the basin to prevent the water from evaporating away and giving the system a shot of freshwater called make-up water.

When the float drops below a predetermined level, a valve is opened, introducing make-up water into the cooling tower system. Some cooling towers utilize a solid-state type of water leveling system, and these water leveling devices need to be calibrated to maintain precise levels in the tower. The make-up water is generally supplied from the city water supply or even a well.

VFD’s or Variable Frequency Drives

High-efficiency cooling towers have variable frequency drives that can be controlled by a reset schedule based on outside ambient air temperature. This feature saves energy by allowing the fan to run at lower speeds based on lower temperatures outside. What type of fan for the system and the complexity of the fan type will depend on the type of cooling tower and its intended design?

HVAC Cooling towers also require water treatment chemicals and regular maintenance for cleaning, belt, and motor maintenance. Some cooling towers use filters to filter unwanted particles from the water. Most cooling towers require some water treatment. Whether it is glycol for operation in colder temperatures or water quality, it is necessary to treat algae or other biological matter that can grow in water.

How Cooling Towers Work - Applications

Cooling Towers are used in many commercial and industrial applications to cool condenser water from a chiller where the chiller provides cold water for a chilled water system. The chiller, using the process of refrigeration, absorbs heat from the chilled water side of the loop and transfers the heat to the condenser water. Then it is delivered to the cooling tower to remove the heat from the condenser water. Therefore, using the process of evaporation as described above.

Free Cooling Application

Cooling towers can also be used in the wintertime to provide free cooling to a chilled water system. It does this by cooling the water down to a set point using the cooling tower and delivering the cool water to a heat exchanger. The condenser water that was cooled in the cooling tower absorbs heat from the chilled water loop and returns to the cooling tower to release the heat into the atmosphere. That requires a specific control system and sensors. The control system locks out the chiller. Therefore, the customer saves on energy costs by getting chilled water without mechanical cooling.

How Cooling Towers Work | Technical Terms, Classification, and Factors

Approach - the difference between the entering air wet bulb temperature and the leaving water temperature.

Bleed - Poor quality water usually drained to increase the quality of water in the system.

Drift - Moist air or water droplets discharged from the cooling tower. It is a necessary but undesired effect, and cooling tower design tries to minimize this effect.

Plume - the hot moist air that forms a dense fog when discharged by the cooling tower under certain conditions.

The Process of Evaporation - it is how cooling towers work as described above.

Range - The difference between the leaving water temperature and entering water temperature or the Δ T.

How Cooling Towers Work - Types of Cooling Towers

There are different types of cooling towers available in the commercial market, and the type used will depend on the design engineer who designs the system. Cooling Towers can be broken down into different classifications depending on your description of cooling towers. There are natural draft cooling towers that do not use fans, and there are mechanical draft cooling towers that require fans for the heat exchange process.

Mechanical draft cooling towers are more precise for capacity control. Additionally, they can be modulated based on demand. Therefore, on a hot day, when the heat loads are higher, the system (pumps and fans) can run at 100%, but on cooler days when the heat load is not so high, the system can be modulated to a lower running capacity based on the lower demand. That all depends on the controls of the cooling and how it is set up. Furthermore, the classification of cooling towers are:

Induced Draft - Cross Flow or Counter Flow
Forced Draft - Counter Flow
Ejector Parallel Flow

There are pros and cons for the various types of cooling towers used based on the type of use. However, the design engineer who designed the system should have chosen the best type of cooling tower system for the intended use. All considerations and factors should have been taken into account when the system was designed. However, we all know that sometimes things are missed in the initial design, construction, and commissioning process when things are built.

Factors and Considerations

Here are some factors and considerations for cooling towers from lessons learned:

Mechanical cooling towers are often much smaller than natural draft cooling towers.
Since natural draft cooling towers require outside air they need to be outside. Mechanical draft cooling towers can be located inside buildings.
For all cooling towers located outside considerations need to be taken into account for colder weather. Steps should be taken to prevent freezing especially in the Northern climate regions (in the Northern hemisphere). Vice versa for the Southern hemisphere.
Fans that are in the exit air stream need to be weatherized and protected. Protected from corrosion and other harmful effects of moist air.
Maintenance factors of various cooling towers differ a lot.
Noise transmission of the cooling towers needs to be taken into account.
Plume discharges from the cooling tower can cause serious problems. This is true if located too close to inlet air ducts for building make-up air.
Parking lots in the path of the discharge plume need to be taken into account. The discharge can cause liability issues with vehicles and paint jobs on those vehicles.
Drift rates can be excessive even with drift eliminators. Some municipalities will credit for high water usage for cooling towers. Therefore, check with the local government for any benefits.