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What is a Cooling Tower in HVAC and How Does It Work?

Posted on Friday Jun 19, 2020 at 08:15AM

This Cooling Tower blog is the third in a series of blogs that will explore how Veris Industries products can be used to monitor and regulate heating, ventilation, and air conditioning (HVAC) applications.

In a commercial chiller, water is used to extract heat from the refrigerant on the condenser side of the chiller before the refrigerant is recirculated to cool water on the air handler side of the chiller. If the heat extracted from the refrigerant cannot be used in other building processes, it remains in the water. This heated water is pumped to a cooling tower.

What is a Cooling Tower?

A cooling tower is a heat rejection device that extracts waste heat to the atmosphere by cooling a stream of hot water in the tower. Cooling towers remove heat through evaporation from water to lower the water’s temperature.

Because the cooling tower utilizes an evaporative process, it must be placed outside of the building. Cooling towers can be found on the roofs of large buildings or on a cement pad or risers.

How Do Cooling Towers Work

The type of heat rejection that cooling towers use is termed "evaporative" because it allows a small portion of the water being cooled to evaporate into a moving air stream. This heat removal then provides significant cooling to the rest of the water stream. The heat that is transferred from the water stream to the air stream raises the air's temperature and its relative humidity to 100%, and this air is discharged to the atmosphere. The remaining cooled water is pumped back to the condenser side of the chiller. Additional water is added to the cooled water stream to compensate for volume losses due to evaporation.

Cooling Tower Diagram

Types of Cooling Towers

Cooling towers are defined by various factors. There are two types of cooling towers: Natural Draft Cooling Towers and Mechanical Draft Cooling Towers. Cooling towers can further be defined by how the air and water interact with each other. These two types are counter flow and crossflow.

Natural Draft Cooling Towers

What is a Natural Draft Cooling Tower?

A Natural Draft Cooling Tower does not use fans or mechanics to create air flow. They use convective flow to provide air circulation, causing hot air to rise within the tower. This can be called the stack effect. Hot water from the system is cooled via direct contact with fresh air.

How does a Natural Draft Cooling Tower Work?

The differences in density between the entering air and internal air within the tower causes air movement. The warm air (more dense) will naturally rise through the tower while cool air (less dense) from the outside will fall. This creates a constant cycle of airflow.

Natural Draft Cooling Tower Applications

Natural Draft Cooling Towers are used for applications that there is a high demand for constant cooling over many years. Examples of these types of applications are: thermal power plants, power plants new load centers, oil refineries, petrochemical plants, natural gas plants

Natural Draft Advantages and Disadvantages

Advantages

Disadvantages

Cost Saving: low power costs and maintenance costs are not high as they have less moving parts
Power Saving: does not require cooling fans which saves on power consumption
Efficient components
Easy to operate and maintain: less wear and tear and less moving parts when maintenance does need to occur
No noise
Low system losses: they lose les than 1% of total water flow
No recirculation of air due to high stack that prevents the warm air from re-entering the tower
Smoke released at the top is Environmentally friendly
Large cooling capacity and can handle a large amount of water flow

High initial cost due to being expensive to construct.
Require a large area to build due to how tall and large they are
Difficult to get planning permission due to the structing impacting the aesthetics of the the area
Not as reliable due to performance relying on wind, velocity, and direction that can affect airflow and heat transfer
Susceptible to freezing problems in cold climates due to ice forming on the fill material and cold water basin

Mechanical Draft Cooling Towers

What is a Mechanical Draft Cooling Tower and how does a Mechanical Draft Cooling Tower work?

Unlike a Natural Draft Cooling tower, Mechanical Draft Cooling towers use a single fan or multiple fans to create air flow through the tower. Typical fans that are used are propeller fans and centrifugal fans. There are two types of Mechanical Draft Cooling towers: induced draft cooling towers and forced draft cooling towers.

Mechanical Draft Cooling Tower Applications

They are commonly used in petroleum, chemical metallurgical and other industries.

Mechanical Draft Advantages and Disadvantages

Advantages

Disadvantages

More reliable and stable than natural draft towers due to the air flow not being affected by external factors due to being manipulated according to the cooling load required
Compact structure makes it less of an initial investment
Light Weight
Convenient Installation
Higher cooling efficiency
Higher Water Spraying Density
Force of the induction means the air is moving at a high velocity when it exits the tower. That high velocity sends the air far enough away to prevent unwanted recirculation.

Higher operating costs due to more power consumption

Types of Mechanical Draft cooling towers

Induced Draft Cooling Tower

The fan is installed after the heat exchanger (located at the top) and draws air from up through the tower against a strong downward flow of water. Coldest water is at the bottom and comes in contact with dry air while the warmest water is at the top of the tower coming in contact with the moist air. This causes a quick and efficient heat transfer. These are widely used in industrial plants that need stable performance. Some advantages for the induced draft cooling tower: Low absorbed capacity, requires little space for installation. Disadvantages are that it is more noisy than a forced draft cooling tower.

Forced Draft Cooling Tower:

The fans are located before the heat exchanger in the air intake at the sides or the base of the tower. Air is pushed directly into the tower taking air in at a high velocity but discharging it at a lower velocity. Forced Draft Cooling Towers are best used for Indoor applications where high static pressure is a concern. Advantages are: handles high pressure well, Easy to maintain, low noise, low absorbed capacity. Some disadvantages are: more susceptible to undesirable air recirculation due to lower velocity output, costly design due to requiring more power to operate, more susceptible to freezing.

What is a cross flow cooling tower?

With a cross Flow cooling tower air flows horizontally across the downward flow of water. They use splash fills to allow incominig air to flow horizontally while gravity sends the hot water down from the distribution basins from the top of the tower. This eliminates the need for a pressurized spray system.

Advantages:

Easy to maintain: distribution system can be sectionalized and serviced separately, access to nozzles available anytime
Cost effective: uses smaller pumps due to use of gravity, no need for pressurized water system
More variable water flow due to spray being non pressurized

Disadvantages:

More prone to freezing
More inefficient
More likely to become clogged with dirt or debris
Distribution basin prone to biological fouling

What is a counterflow cooling tower?

The air and water enter from opposite ends of the tower, air moves upward while water moves downward. These towers have a pressurized spray distribution system and it is not possible to use gravity flow basins.

Advantages:

Greater efficiency and saves energy due to being more compact size
Smaller area than Cross flow
More resistance to freezing due to spray distribution
Heat transfer more efficient due to spray distribution improving water droplet size

Disadvantages:

Higher energy and operational costs due to large pumps required
Less variable water flow
More noisy

How Cooling Tower Systems Work with Veris Products

Product Family	Why it is used on a Cooling Tower
Flow Meters	Monitors flow of the supply and return heating loop. The water utility will charge the end user based on the amount of water supplied. The end user is also charged for the amount of water dispelled into the sewer system. In some climates, evaporation can be as much as 50% = 60%. Utilizing a flow meter on both the supply and the sewer drain will save the end user money by capturing the actual amount of water sent down the drain. The difference between the amount of water supplied and the amount of water sent down the drain is the volume of water lost due to evaporation. With this data from the supply and the drain flow meters in hand, the end user will receive a credit for the evaporation losses.
Temperature	Immersion temp in the cooling tower is a control input for fat rotation. The cooling tower fan removes a large amount of the heat to prepare the water for conditioning (via coolant) in the chiller.
Current Sensors	Monitors run status of pumps and fans to ensure that they are working when needed. Proof of functionality. No need to manually check each fan or pump. (If current is flowing, the fan or pump is working.)