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Technical
THE IMPACT OF ALTITUDE ON HVAC COOLING PERFORMANCE
Compiled by Eamonn Ryan based on a SAIRAC Johannesburg Centre Tech Talk by Jannie Potgieter
Altitude significantly affects the cooling performance of HVAC systems .
© RACA Journal
The presentation by Jannie Potgieter , a consulting engineer at
Thermologika with advanced degrees in engineering , addressed how altitude influences cooling performance , particularly in highaltitude 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 / m ³, while at Johannesburg ' s altitude , it is roughly 0.98 kg / m ³.
For practical calculations , a simplified rule of thumb can be applied where air density is approximated as 1.0 kg / m ³ at high altitudes and 1.2 kg / m ³ 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 .
Attendees at one of the in person SAIRAC Johannesburg Centre Tech Talks last year .
• 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 highaltitude 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 .
INTRODUCTION TO FAN SYSTEMS In HVAC systems , the fan is the primary component directly affected by changes in air density due to altitude variations . Understanding how altitude influences fan performance requires a review of fundamental fan laws and their application in different conditions .
FAN TERMINOLOGY AND LAWS For clarity , several abbreviations are used throughout the discussion :
• V˙ \ dot { V } V˙ : Volume flow
• m˙ \ dot { m } m˙ : Mass flow
• ΔP \ Delta PΔP : Static pressure difference
• PPP : Power
• ppp : Capacity
It is important to note that unless otherwise specified , the temperature is assumed to be constant .
Among HVAC components , the fan is most directly influenced by changes in air density . The effects on other components , such as coils , are secondary and largely depend on how changes in fan performance affect overall airflow .
To understand the impact of altitude on fan performance , it ' s essential to revisit the fundamentals . The fan laws describe how fan performance parameters — such as airflow , static pressure , and power consumption — vary with changes in system conditions .
KEY FAN LAWS : Volume flow : Volume flow ( V˙ \ dot { V } V˙ ) remains constant regardless of changes in air density . This property is a fundamental characteristic of fans , which are designed as constant volume flow devices . Static pressure : The static pressure ( ΔP \ Delta PΔP ) that a fan can achieve is impacted by changes in air density . The static pressure is influenced by the density difference , and as altitude increases and
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RACA Journal I January 2025 www . refrigerationandaircon . co . za