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Technical
• Compressor power : The power draw of the compressor will also be affected . At higher altitudes , the system must work harder to achieve the same cooling effect , leading to changes in power consumption .
For a dummy HVAC system designed for specific conditions : Sea level design :
• Cooling capacity : 7.16 kW
• Compressor power : 1.97 kW
• Outdoor design temperature : 35 ° C
• Indoor design temperature : 27 ° C
When moving this system to a higher altitude :
• Cooling capacity : Expect a decrease due to reduced coil capacity and overall system performance .
• Compressor power : Likely to increase as the compressor must work harder to maintain cooling performance in lower air density .
CASE STUDY RESULTS 1 . 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 for the coil 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 .
2 . 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 , in this example 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 . Typically , it is expected that the power draw will increase slightly .
Image supplied by SAIRAC Image supplied by SAIRAC
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RACA Journal I January 2025
3 . 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 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
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