Compiled by Eamonn Ryan based on the SAIRAC Johannesburg Centre Tech Talk on September 19 by Michael Young, mechanical engineer
As we move forward in our discussion, let’s delve deeper into these factors and explore real-world examples of data centres that successfully integrated liquid cooling and those that thrived with traditional air cooling. This is Part 8 of a ten-part series.

Michael Young, mechanical engineer. © RACA Journal
Let’s dive into the concept of free cooling, particularly as it applies to chillers and their role in data centre efficiency.
What is free cooling?
Free cooling systems incorporate standard components such as compressors, expansion valves, evaporators, and condensers. The unique aspect of free cooling chillers is the addition of an extra row of coils on the condenser side. Depending on the operational load, either water or refrigerant is circulated through these coils to optimise cooling.
Then there are climate considerations. Using Johannesburg as a reference, we can examine the design based on ASHRAE standards. For Johannesburg, the ambient temperature is set at 34.3°C, according to the ASHRAE 50-year rating.
When we consider a W1 class cooling system, the supply water temperature on the facility side can be set at 17°C. This results in a 10°C temperature rise; the water exits the system at 27°C.
Heat exchange dynamics
According to thermodynamic principles, heat transfers from a hotter fluid to a colder one. In our example, if the incoming water is 27°C and the ambient air is 34.3°C, the heat exchange is not favorable. The air will actually heat the water rather than cool it. Therefore, the system redirects the flow of water entirely into the evaporator of the chiller.
In this scenario, the chiller relies on a secondary fluid—refrigerant— to facilitate heat transfer. As a result, the system recognises that trying to use the free cooling coils would be ineffective, leading to a decision to use only the evaporator.
As we shift to a W2 class cooling system with a supply water temperature of 27°C and a return temperature of 37°C, the dynamics change. Here, we can achieve some degree of heat transfer from the water to the ambient air. However, the system will likely require the compressor to operate to meet the full cooling demand, leading to what is known as hybrid mode.
Moving to a W3 class, we observe even better conditions with a supply temperature of 32°C and a return of 42°C. In this case, the system can effectively utilise the free cooling coils, maximising heat transfer without engaging the compressor.
Location and ambient temperature
The location of the data centre plays a critical role in the viability of free cooling systems. For instance, in hotter climates like Dubai, where ambient temperatures can exceed 45°C, the effectiveness of free cooling diminishes significantly. This highlights the importance of designing cooling systems based on local climate conditions, which ASHRAE guidelines help to define.
While free cooling offers significant advantages in terms of energy efficiency, its effectiveness is highly dependent on ambient temperatures and operational parameters. Understanding these variables allows for better design and implementation of cooling solutions tailored to specific environments.