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 6 of an eight-part article.
Question time 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.
Case study results
- Energy balance analysis: Upon relocating the HVAC system to Johannesburg, the cooling capacity decreased to 6.66 kW from the original 7.16 kW, while the compressor power draw increased to 2.06 kW from the original 1.97 kW. This change highlights a discrepancy between the expected and actual performance reduction. While the cooling coil capacity dropped significantly (approximately 15%), the overall system performance only decreased by 6.7%.
This discrepancy arises because the system components, including the compressor, must adjust to new operating conditions. In a high-altitude environment, the evaporator temperature and condensing temperature shift. For instance:
- Evaporator temperature: Reduced from 9.5°C to 8.4°C.
- Condensing temperature: Increased from 50.8°C to 52.5°C.
These temperature changes necessitate a new balance point for the compressor, which affects the overall system efficiency.
- Compressor and system performance: The compressor, specifically a scroll compressor in this case, maintains a fixed displacement and continues to pump a constant volume of refrigerant. However, the pressure ratio across the compressor increases due to the changes in evaporator and condensing temperatures. This results in a higher power draw to maintain refrigerant flow, despite the decreased cooling capacity.
Interestingly, while the compressor power draw increased, the overall system power consumption remained relatively stable. This balance was achieved because the increase in compressor power was offset by a reduction in fan power consumption.
- Energy efficiency ratio (EER): The system’s Energy Efficiency Ratio (EER) also declined. Initially, the EER was 2.82 at sea level, but it dropped to 2.63 at high altitude. This reduction in EER indicates that the system now provides less cooling capacity for the same amount of power consumed. The decrease in efficiency underscores the impact of altitude on overall system performance.
Design considerations and adjustments
To mitigate the effects of altitude on HVAC performance, several strategies can be employed:
- Compressor sizing: Consider using a larger compressor to compensate for reduced cooling capacity and maintain system performance. However, this approach may further increase condensing temperatures.
- Coil oversizing: Increase the size of cooling and heating coils to enhance their capacity and offset the loss due to altitude.
- Airflow adjustments: If feasible, increase the airflow through coils. This can help maintain capacity by ensuring that more air passes over the coils. Note that airflow increases may be limited by design constraints or existing system configurations.
- Software and manufacturer support: Use advanced HVAC design software to simulate system performance at different altitudes. Many manufacturers provide updated performance data or correction factors for altitude adjustments. Consulting these resources can help in accurately predicting and compensating for performance changes.
- Altitude correction factors: Manufacturers often provide correction factors for altitude in their equipment manuals. For instance, older manuals may list factors like a 3% reduction in capacity and 1.03 times increase in compressor power at 1 700m. These factors, however, are specific to particular units and designs and may not be universally applicable.
Conclusion
Understanding and addressing the impact of altitude on HVAC system performance is crucial for maintaining efficiency and effectiveness. By analysing system components and employing strategic adjustments, such as compressor sizing and coil oversizing, the performance impact can be managed. Utilising manufacturer data and design software further aids in achieving optimal system performance in high-altitude environments.
