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 7 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.
Correction factors and practical considerations
When adapting HVAC systems for high-altitude conditions, applying correction factors for capacity and power is crucial but requires careful consideration. Correction factors, such as those provided in various industry resources, can give a rough estimate of performance changes, but they must be used cautiously as they are not universally applicable to all systems.
Correction factors, like those found in sources such as the Bartok PDF, include:
- Total capacity correction factor: This factor adjusts the total cooling or heating capacity based on altitude. For instance, at 1 800m, you might see a 5% reduction in total capacity.
- Sensible capacity correction factor: This factor adjusts the sensible (or temperature-related) capacity of the system. At the same altitude, a typical correction might show around a 13% reduction.
- Condenser intake temperature correction factor: This factor adjusts the condenser’s intake temperature to prevent overheating and ensure proper operation at high altitudes. It accounts for the reduced efficiency of the condenser in higher ambient temperatures.
These factors provide a starting point but are specific to certain systems and conditions. They are not a one-size-fits-all solution and can vary depending on the particular design and operational parameters of the HVAC system.
From the case study data, the following observations were made:
- Density correction factor: The density at high altitudes reduces, impacting the coil capacity. For example, at 1 700m, a 15% reduction in coil capacity was observed, while the density change was around 18%. The overall system capacity reduction was only 7%, indicating that while individual coil performance suffers, the system’s total capacity reduction is less severe due to compensatory adjustments.
- Compressor power correction factor: At high altitudes, the compressor power typically increases by about 5% to maintain performance, due to changes in condensing and evaporating temperatures.
- Recalculate system performance: Whenever possible, work with suppliers or use advanced software tools to recalculate system performance at high altitudes. This ensures more accurate adjustments and optimises system design for new conditions.
These results highlight that while individual components may show significant performance reductions, the overall system may not experience as drastic a decline. This is because of the interactions between different system components, including compensatory adjustments in temperature and pressure.
Adjust system components:
- Increase compressor size: This helps compensate for reduced cooling capacity but may increase condensing temperatures.
- Oversize coils: This can help maintain capacity despite reduced efficiency.
- Modify airflow: Increasing airflow through the coils can help maintain capacity, though this may be limited by existing design constraints.
- Use correction factors as guidelines: Apply correction factors to estimate performance changes but be aware that these are not precise and may not account for all system variables.
