How a Chiller, Cooling Tower and AHU Work Together in a Chilled Water HVAC System

For chiller distributors or suppliers, understanding how a chiller, cooling tower, and air handling unit work together is essential for proper system design, troubleshooting, and energy optimization. This article breaks down the working relationship between these three critical HVAC components.

Key Components of a Central HVAC Cooling System

A centralized HVAC system is composed of a chiller, a cooling tower, an air handling unit (AHU), and water pumps. They always function as a linked machine to move heat. Here is a simple introduction to these core parts for you.

What is a Chiller?

A chiller is a specialized refrigeration system designed to remove heat from a liquid (usually water or a water/ glycol mixture) and provide cooling for large buildings or industrial processes. The liquid cooled by the chiller unit is then transported to the areas requiring cooling.

What is a Cooling Tower?

A cooling tower is an evaporative heat rejection device designed to reject heat from your building or industrial process into the atmosphere. It serves as the final stop for unwanted heat in your water-cooled HVAC system.

The cooling tower is necessary for a water-cooled chiller. It helps make your cooling more energy-efficient compared to using air-cooled fans alone.

What is an Air Handling Unit?

The Air Handling Unit (AHU) is the “lungs” of your building’s HVAC system. It is a centralized device, usually placed in a large metal box, which collects, processes, and recycles air through a series of pipeline networks for use throughout the facility.

How a Chiller, Cooling Tower and Air Handling Unit Work Together

Working Process Step-by-Step

The whole working process of a chiller, cooling tower, and AHU can be divided into 4 steps. In this section, we explain how chilled water system works for you step by step.

Step 1: The Chiller Produces Chilled Water

First, the chiller produces chiller water in the evaporator, which is the heat exchanger where the actual cooling of the water occurs.

• The warm water (typically around 12°C) from your building or devices returns to the evaporator. Here, the water passes through pipes containing low-temperature, low-pressure liquid refrigerant.
• Refrigerant is the medium used to “pick up” the heat from the water loop. Due to the extremely low boiling point of the refrigerant, it absorbs heat from the warm water, causing the refrigerant to boil and turn into a gas, while the water loses its heat.
• Then the cooled water (typically cooled to 6°C – 7°C) is discharged from the evaporator and pumped to your building’s air handling unit (AHU).

Step 2: The AHU Uses Chilled Water to Cool Air

Second, the cooled water produced by the chiller is introduced into the air handling unit (AHU) to cool your building’s occupants.

• The cooled water from the evaporator arrives at the AHU and enters a network of copper tubes known as the Cooling Coil.
• The AHU’s large fan draws in a mix of “Return Air” (warm air from inside the rooms) and “Fresh Air” (outdoor air for ventilation). This mixed air goes through a set of filters to remove dust and particles.
• The filtered warm air directly passes over the external cooling coil surface of the cooling coils, and the heat in the air is absorbed by the cooling water inside the coils. As a result, the air temperature drops significantly (typically to around 13 °C). Since the water inside the coils absorbs the heat from your building, its temperature rises (typically back up to around 12 °C).
• When warm air comes into contact with the cold cooling coil surface, the moisture in the air condenses into liquid water. These water droplets fall into a collection pan and are then discharged. This process reduces humidity and makes the indoor environment more comfortable.
• The cool and dry air is sent through the ventilation ducts of your building into offices or rooms. Meanwhile, the warmed cooling water is pumped back into the chiller to be cooled again.

Step 3: Heat Returns to the Chiller

Third, the HVAC cooling tower system cycle comes back to your machine room. The warmed water from AHU returns to the chiller to be processed.

• The water from AHU returns to the chiller’s evaporator, and it transfers its heat to the cold liquid refrigerant inside the evaporator tubes. So the water is cooled again and heads back to the AHU.
• The warmed refrigerant is sucked into the scroll compressor or screw compressor. The compressor squeezes the refrigerant gas, making it extremely hot and high-pressure, which is much hotter than the outside air.
• The superheated refrigerant gas enters the condenser. Here is a completely separate loop of water, which is called the Condenser Water Loop, that flows through this section. The heat transfers from the hot refrigerant gas into this new loop of condenser water.
• After losing the heat, the refrigerant turns back into a liquid to repeat its internal cycle, while the condenser water, which is loaded with all your building’s heat plus the heat generated by the compressor motor, is pumped out of the chiller and up toward the roof.

Step 4: The Cooling Tower Rejects Heat

In the final step, the condenser water, which is now carrying all the heat collected from your building, reaches the cooling tower to be cooled down by the outside air.

• The hot water (typically around 35°C now) is pumped to the top of the tower and sprayed through nozzles over the “Fill” (the internal honeycomb-like layers). This breaks the water into thousands of tiny droplets, maximizing the surface area exposed to the air.
• As the water droplets fall through the fill, a large fan at the top of the tower pulls outside air across them. A tiny fraction of that water (about 1%) evaporates into the air.
• The evaporation requires a massive amount of energy, so it absorbs the heat from the remaining water.
• The heat is now in the form of water vapor and is blown out the top of the tower into the atmosphere. The remaining water (around 29°C now, depending on design wet bulb temperature) collects in the basin at the bottom of the tower and is pumped back down to your chiller’s condenser to pick up another load of heat, starting the entire cycle over.

Further Learning the Two Water Loops in the System

From the basic cooling process above, you can discover that there are two separate water circuits involved in the system: the chilled water loop and the condenser water loop. Each loop handles a different part of the heat transfer process. Understanding these two loops helps you better assess and optimize cooling systems more effectively.

Chilled Water Loop

The chilled water loop is responsible for delivering cooling capacity to the AHU. It is a cooling loop on your building side. It transports cooling capacity and absorbs heat from indoor air.

• System path of the water: Chiller (6–7°C) → Pump → AHU → Return (12–13°C) → Chiller

Unlike traditional AC systems, a chilled water loop avoids running large amounts of high-pressure refrigerant through occupied spaces. Your single central chiller plant can easily serve multiple zones or even entire campuses by simply extending the piping.

Condenser Water Loop

The condenser water loop is responsible for transferring heat to your cooling tower. It is a heat rejection loop.

• System path of the water: Chiller → Pump → Cooling Tower (5.5°C) → Return → Chiller

In most large systems, the condenser water loop is an “open” loop. This means the water actually comes into direct contact with the outside air at your cooling tower. Because of this, it requires strict chemical water treatment to prevent algae growth, scaling, and corrosion.

System Comparison: Chilled Water Loop vs Condenser Water Loop

FAQ

Can an HVAC System Run Without a Cooling Tower?

Yes. Water-cooled chillers absolutely require a cooling tower to function, but an HVAC system can run without one only if it is air-cooled (uses powerful fans to reject heat directly into the ambient air) or geothermal (uses the earth as a heat sink). These methods can eliminate the water consumption, but the energy efficiency is also lowered.

How Does a Cooling Tower Work?

In the HVAC system, a cooling tower is used to remove waste heat from your building’s condenser water loop. The warm water is dispersed into small water droplets on the top of the tower to accelerate evaporation, thus absorbing a large amount of heat from the remaining water. The cooled water collects at the bottom of the tower and is recycled back to the cooling unit.

How Many AHUs Can One Chiller Support?

One chiller can support one to dozens of Air Handling Units. This is specifically decided by the total cooling capacity. Your chiller can distribute that water to as many AHUs as its total capacity and pump head allow, provided the combined heat load of those units does not exceed the chiller’s output.

AHU vs FCU: What is the Difference?

The functions of AHU and FCU are similar, but there are lots of differences in scale and scope.

AHU always serves multiple rooms or entire floors. It handles large volumes of air (typically 2,000 to 100,000 CFM) and manages fresh outdoor air intake. FCU always serves a single room or small area, which primarily recirculates indoor air and is much smaller and simpler in design.

What are the Common Applications of a Central HVAC Cooling System?

The central HVAC cooling system is widely used in various places that require large-scale, high-efficiency, and precise environmental control. Its most common applications include large commercial buildings such as hospitals, hotels, airports, and shopping centers.

It is also the core infrastructure of data centers, precision laboratories, and pharmaceutical factories.

In the industrial field, such as in the manufacturing of electronic semiconductors and large chemical plants, the central cooling system is the key to maintaining the normal operation of your production lines.

Why Proper System Integration Matters

The effective operation of the central air conditioning system relies on the close coordination among the chiller units, cooling towers, and air handling units. It ensures that each component achieves optimal energy efficiency under different loads through automated control strategies and prevents energy waste or premature equipment wear due to improper operation of a single component.

If the integration is inadequate, your system may encounter problems such as cold and hot counterbalancing, excessive pump power consumption, or uncontrolled indoor temperature and humidity.

How to Choose the Right Chiller for Your HVAC System?

When choosing a chiller, you need to consider cooling capacity, energy efficiency targets, chiller refrigerant types, and physical site constraints to ensure long-term operational success. The type of chiller (water-cooled or air-cooled chiller) is decided by your requirements.

Additionally, you must evaluate the specific compressor type, such as centrifugal for massive loads or screw/scroll for smaller applications.

Final Thoughts

A well-designed HVAC system depends on the seamless coordination between the chiller, cooling tower, and air handling unit, and you can find that among these components, the chiller plays a vital role. Selecting a high-performance and appropriately sized chiller can significantly enhance the system’s stability, energy efficiency, and long-term operating costs. Chillerone has over 20 years of experience in manufacturing chillers. If you are looking for high-quality chillers, just contact us.