Guide to Chilled Water Systems and Improving Efficiency

What is a chilled water system?

A chilled water system is typically found in large to medium sized buildings, and consists of plant equipment that provides cooling for the building.

It is rare for residential buildings to have chilled water systems, but they are widely used in commercial and industrial buildings. Cooling is most commonly achieved by a chilled water system in large buildings like hotels, hospitals, airports, etc.

High-heat environments such as data centers or server farms are also cooled by chilled water systems, as are many industrial processes and machinery.

As opposed to air conditioning systems that rely on large volumes of refrigerant chemicals, a water cooled chiller system distributes cooling and returns heat from the building by using cooling water.

Chilled water systems can come in different sizes and complexity, scaling based on building size or application.

 

How do chilled water systems work in buildings?

Buildings, particularly medium to large ones, generate a lot of heat. Large numbers of people, computers, monitors and large glass windows allowing the sun to shine in are all factors that contribute to high building temperatures. In order to combat heat, buildings need to generate cooling.

Chilled water systems contain various types of plant equipment, such as industrial chillers, air handling units, distribution pumps and cooling towers.

The primary purpose of a chiller system is to transport heat from inside a building (or from process equipment) out of the building while cooling the internal temperature.

A refrigerant charge is used in industrial chillers to run a refrigeration cycle via the heating and cooling of compounds, turning them from gas to liquid and vice versa.

A chiller has two separate parts: the condenser and the evaporator.

The evaporator produces chilled water and is equipped with a compressor, which consumes a lot of energy. Cooled water is pulled out of the evaporator by a pump (another large energy user) and is distributed to air handling units throughout the building via a riser and pipework system.

An air handling unit in a water-cooled chiller system draws warm ambient air from the space using a fan. The chilled water from the evaporator is pumped through coils in the air handling units.

 

Cooling is achieved by passing the warm air pulled into the air handling unit over the coil, which then transfers the cooling to the air being blown out of it, providing cooling to the room or process. At the same time, the warm air passes through the coil and transfers heat to the water, which exits the air handling unit at a higher temperature.

Warm return water flows back into pipework and risers, which feed back to the evaporator of the chiller, to be chilled all over again.

The chiller produces heat as part of the refrigeration cycle that cools the evaporator water. The chiller absorbs heat into the condenser side, which results in warm water being pumped out.

By means of risers and pipework, the warm water is pumped to a cooling tower usually located on the roof of the building. A cooling tower collects the warm condenser water and sprays it inside the tower, allowing it to flow down and be collected at the bottom.

In the cooling tower, large fans are used to draw in ambient air from outside the building and introduce it to the warm condenser water to cool it. Warm air is generated from the condenser water that is then exhausted out of the cooling tower into the outside air.

Cooled water at the bottom of the cooling tower is collected and returned to the condenser where it will run over coils and again be used to transfer heat away from the chiller, continuing the process.

A chilled water system is composed of two closed loops of pipework; no water passes between the evaporator and condenser loops, just the refrigerant.

 

Chilled water system diagram

 

Are Chilled Water Systems Energy Efficient?

Water chilling systems are relatively energy efficient because the chiller, which consumes the most electricity, only switches on when the temperature in the reservoir reaches a certain level.

Nevertheless, the energy used for cooling is a sizeable part of the total energy consumption and utility costs of a building. As a result, any optimizations or efficiencies that can be implemented in order to deliver energy savings are very welcome, especially for high demand users.

A further concern related to the energy efficiency of water-cooled chiller systems are the emissions produced as a by-product of energy consumption. Climate change is a very real threat, and reducing energy use directly correlates with a reduction of greenhouse gasses, particularly carbon emissions (CO₂e).

 

How to improve performance?

There are several options available to make sure a chilled water system is as efficient as possible:

 

1. Continual maintenance and repair.

For larger or more demanding systems, routine checks and maintenance is needed to ensure optimum performance.

2. Cleaning and inspecting condenser coils is necessary.

The coils of the whole chiller system must be free to allow both air and water to pass freely, since heat transfer is the core element of the system. Efficiency of the system is directly related to the transfer occurring at the coils.

3. The refrigerant used in the chiller must also be charged at the correct levels for optimal system performance.

 

4. Free-flowing and contamination-free condenser water is essential.

If there is any build up or scale within the pipework, water flow will reduce, and efficiency will decrease.

5. Building Management Software

There are building management systems that offer software that monitors sensors in the system to ensure that it is functioning properly, and in some cases, even predicts problems before they occur. Unfortunately, this is a costly option.

 

Another way to reduce energy consumption and emissions

The transfer of heat and cooling that occurs within the coils is crucial to how well a chilled water system performs.

Further optimization can be achieved by improving the thermal contact between the chilled water and the pipework and coils. The thermal contact at these points allows for a more efficient transfer of cooling and absorption of building heat into the return water to the evaporator.

The surface tension of water (which appears like skin on the surface) can reduce the thermal contact made. By adding EndoCool to the system water in the chilled water loop (at a ratio of 1:100 litres), the surface tension of the water is reduced which allows better thermal contact while remaining thermally stable.

Additionally, the fluid within the closed loop of the evaporator in a chilled water system is typically dosed with glycols, inhibitors, or other anticorrosion and anti-freeze compounds. This can make the fluid more viscous, altering the efficiency of the distribution pumps.

EndoCool makes the fluid “wetter” and works in conjunction with inhibitors and anti-freeze chemistries to improve thermal contact and efficiency.