Compiled by Eamonn Ryan based on a SAIRAC Johannesburg Centre Tech Talk by Jannie Potgieter.
Altitude significantly affects the cooling performance of HVAC systems. This is Part 2 of an eight-part article.
Robert Fox, SAIRAC National President, at an earlier in-person tech talk. ©RACA Journal
The presentation by Jannie Potgieter, a consulting engineer at Thermologica with advanced degrees in engineering, addressed how altitude influences cooling performance, particularly in high-altitude locations like Johannesburg.
Introduction to fan systems
In HVAC systems, the fan is the primary component directly affected by changes in air density due to altitude variations. Understanding how altitude influences fan performance requires a review of fundamental fan laws and their application in different conditions.
Fan terminology and laws
For clarity, several abbreviations are used throughout the discussion:
- V˙: Volume flow
- m˙: Mass flow
- ΔP: Static pressure difference
- P: Power
- p: Capacity
It is important to note that unless otherwise specified, the temperature is assumed to be constant.
Among HVAC components, the fan is most directly influenced by changes in air density. The effects on other components, such as coils, are secondary and largely depend on how changes in fan performance affect overall airflow.
To understand the impact of altitude on fan performance, it’s essential to revisit the fundamentals of fan laws. The fan laws describe how fan performance parameters—such as airflow, static pressure, and power consumption—vary with changes in system conditions.
Key fan laws:
Volume flow: Volume flow (V˙) remains constant regardless of changes in air density. This property is a fundamental characteristic of fans, which are designed as constant volume flow devices.
Static pressure: The static pressure (ΔP) that a fan can achieve is impacted by changes in air density. The static pressure is influenced by the density difference, and as altitude increases and density decreases, the fan’s ability to generate static pressure is reduced.
Power: The power (P) required by the fan also changes with air density. A reduction in air density requires the fan to work harder to maintain the same volume flow, impacting its power consumption.
In high-altitude locations like Johannesburg, where air density is lower, the fan laws help estimate the performance of fans under these conditions. The fan laws allow engineers to:
- Estimate new fan curves: By applying the fan laws, it is possible to generate new fan curves that reflect performance at different densities.
- Determine operational speeds: Engineers can calculate the new operational speed of the fan needed to achieve desired performance under varying altitude conditions.
- Calculate power requirements: The fan power can be recalculated to account for the increased workload due to lower air density.
