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 4 of an eight-part article.
Attendees 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.
Capacity calculation for cooling and heating coils
When evaluating the impact of altitude on HVAC systems, especially cooling and heating coils, it’s crucial to understand how the capacity changes. Capacity can be determined using the following equations:
Cooling Capacity:
Capacity = 𝑚˙ × Δℎ
Where 𝑚˙ is the mass flow rate, and Δℎ is the enthalpy difference.
Heating Capacity:
Capacity = 𝑚˙ × 𝐶𝑝 × Δ 𝑇
Where 𝑚˙ is the mass flow rate, 𝐶𝑝 is the specific heat capacity, and Δ𝑇 is the temperature difference.
At higher altitudes, the mass flow rate decreases due to lower air density. Consequently, the capacity of cooling and heating coils also decreases. The reduction in capacity is proportional to the reduction in mass flow rate, but the actual reduction is slightly less than the density ratio due to changes in outlet conditions, such as temperature.
Example calculations and analysis
Cooling Coil:
Density ratio: The density at Johannesburg is 0.98 kg/m³ compared to 1.2 kg/m³ at sea level. This results in an 18% reduction in density.
Estimated capacity reduction: based on the density ratio, we would initially expect a similar 18% reduction in capacity. However, due to the impact on outlet temperatures, the actual reduction is less severe.
For example, consider a cooling coil with:
- Original outlet temperature: 9.25°C at sea level
- New outlet temperature: 8.84°C at high altitude
- Reduction in capacity: 15.3% at altitude, which is slightly less than the 18% density reduction
Heating coil:
- Density ratio: Similar density change applies
- Estimated capacity reduction: The reduction observed was approximately 14%, which is again less than the 18% due to changes in outlet temperatures.
- For the heating coil:
- Original outlet temperature: 32°C
- New outlet temperature: 42.7°C
- Reduction in capacity: About 14%, reflecting the same trend of being less severe than the density reduction
