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 1 of an eight-part article.
Greg Grobbelaar. Johannesburg Centre chairman, 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.
In HVAC design, most equipment is developed with the assumption of sea-level conditions, where atmospheric pressure is about 101.3 kPa. However, Johannesburg, situated approximately 1 700m above sea level, presents a unique challenge due to its significantly lower atmospheric pressure and air density compared to coastal regions.
The pressure exerted by the atmosphere decreases with altitude. At sea level, the pressure is approximately 101.3 kPa, but at higher altitudes such as Johannesburg, it drops to around 82.5 kPa. This decrease in pressure is accompanied by a corresponding reduction in air density. Sea level air density is about 1.2 kg/m³, while at Johannesburg’s altitude, it is roughly 0.98 kg/m³.
For practical calculations, a simplified rule of thumb can be applied where air density is approximated as 1.0 kg/m³ at high altitudes and 1.2 kg/m³ at sea level. While temperature also affects air density, this discussion focuses on the altitude-induced changes.
Impact on HVAC systems
- Cooling capacity: The lower air density at higher altitudes means there are fewer air molecules available to absorb and transfer heat. This reduction in heat transfer efficiency affects the cooling capacity of HVAC systems. For instance, a cooling coil designed for sea-level conditions might underperform in high-altitude locations because the reduced density of the air decreases the coil’s effectiveness in removing heat from the air.
- Efficiency and performance: As altitude increases, the cooling efficiency of HVAC systems can diminish. This is primarily due to the fact that cooling systems must work harder to achieve the same level of cooling output. Consequently, HVAC systems in high-altitude areas might require adjustments or even modifications to maintain optimal performance.
- Design considerations: When designing HVAC systems for high-altitude locations, engineers must account for these differences. This includes recalibrating equipment to accommodate lower air density and potentially increasing the size or capacity of cooling components to achieve desired performance levels.
