How do chillers, cooling towers and fan coils work together?

Last Updated on May 6, 2022 by Shawn Lee

The main system components of central air conditioning cooling equipment are:

Chiller Air Handling Unit (AHU) Cooling tower water pump chillers are usually located in the basement or roof, depending on the type of chillers used. Roof chillers are usually “air-cooled”, while underground chillers are usually “water-cooled”, but they all have the same function, that is, to generate cold water for air conditioning by removing excess heat from buildings. The only difference is how the chillers dissipate excess heat.

An air-cooled chiller will use a fan to blow cold air through the condenser to remove heat from the system. This type does not use a cooling tower. You can click here to learn about the system. In the rest of this paper, we will focus on water-cooled chillers and cooling towers.

A water-cooled chiller has two large tanks, one called an evaporator and the other called condenser.

Chilled water: The evaporator of the refrigerator is the place where “chilled water” is produced. The “chilled water” leaves the evaporator at a temperature of about 6 C (42.8 F) and is pushed around the building by the chilled water pump. Chilled water flows to each floor along with the height of the building through pipes called “risers”. These pipes are called risers regardless of whether the water flows upward or downward.


The cooling water is branched from the riser into a pipe with a smaller diameter, which leads to the fan coil unit (FCU) and the air handling unit to provide air conditioning. AHU and FCU are basically boxes with fans inside, which suck air from buildings and push it through heating or cooling coils to change the temperature of the air, and then push the air back into the buildings. The chilled water enters the AHU/FCU and passes through the cooling coils (a series of thin tubes), where it will absorb the heat of the blown air. The chilled water is heated, and the air blowing through it is cooled. Now, when the cooling water leaves the cooling coils, the temperature will rise to 12 C (53.6 F). Then, the warm-cold water returns to the evaporator through the backwater riser. Once entering the evaporator, the refrigerant will absorb the excess heat and transfer it to the condenser. Then, the chilled water will keep cool again, ready to circulate around the building and collect more excess heat. Note: Coldwater is called “cold water” whether it is cold or hot.

Condenser water: The condenser of water chillers is where excess heat is collected and then sent to the cooling tower. The refrigerant passes between the evaporator and condenser to remove all unnecessary heat. Another water cycle called “condenser water” passes through the cycle between the condenser and cooling tower. The refrigerant collects heat from the “cold water” circuit in the evaporator and transfers it to the “condenser water” circuit in the condenser.

The condenser water enters the condenser at a temperature of about 27 C (80.6 F) and will pass through and collect heat on the way. Upon leaving the condenser, the temperature will be about 32 C (89.6 F (Fahrenheit)). Condenser water and refrigerant never mix, they are always separated by the tube wall, and heat is only transferred through the tube wall. Once the condenser water passes through the condenser and absorbs the excess heat, it will flow directly to the cooling tower to dissipate the heat and return to the cooler to prepare for collecting more heat.

Horizontal concealed ultra-thin fan coil unit
Horizontal concealed ultra-thin fan coil unit

Cooling tower: The cooling tower is usually located on the roof, which is the final destination of excess heat in the building. The cooling tower contains a large fan, which blows air through the equipment. The condenser water is pumped to the cooling tower and then sprayed into the air stream. Cool ambient air will enter the condenser water spray and come into direct contact with it (in the open cooling tower), which will transfer the heat of the condenser water to the air, and then blow the air into the atmosphere. Then, the water in the condenser will be collected and then returned to the chiller # heat exchanger to clean the condenser, ready to collect more heat.


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