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 3 of an eight-part article.
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: sea level vs high altitude
Let’s consider a fan selected at sea level with the following performance parameters:
- Operating Point: Point 4
- Flow Rate: 1.94 cubic meters per second (m³/s)
- Pressure Difference: 180 Pascal (Pa)
- Power Consumption: 1.05 kilowatts (kW)
- Mass Flow: 2.33 kilograms per second (kg/s)
These values are based on a density of 1.2 kg/m³ (typical for sea level). To assess the impact of moving this fan to a higher altitude, such as Johannesburg, where the density is approximately 0.98 kg/m³, we need to estimate how each parameter will change.
- The fan is a constant volume flow device, meaning its volume flow rate remains unchanged regardless of altitude. Therefore, the flow rate of 1.94 m³/s will stay the same even at higher altitudes.
- The pressure difference the fan can achieve decreases with reduced air density. The lower density results in a reduced static pressure capability of the fan.
- Power consumption decreases because the fan moves less mass at higher altitudes. With lower air density, the mass flow rate decreases, leading to lower power requirements.
- Mass flow decreases due to the lower air density. At sea level, the mass flow rate was 2.33 kg/s, but this will reduce at higher altitudes.
When analysing fan performance, it’s essential to consider both the fan curve and the system curve. The system curve represents the resistance against airflow, which depends on the pressure drop across the system components. The pressure drop can be expressed as the product of a resistance coefficient, air density, and velocity squared. This means that as air density decreases, the pressure drop across the system also reduces.
The intersection of the fan curve with the system curve determines the operating point. At sea level, the operating point is where the blue fan curve intersects the system curve. At high altitude, the operating point shifts to where the orange fan curve intersects the new system curve.
Impact analysis:
- Volume flow: Remains constant at 1.94 m³/s, despite changes in altitude.
- Pressure drop: Reduced at high altitude due to lower air density. This reduces the fan’s ability to maintain the same pressure difference.
- Power consumption: Decreases because the fan is moving less mass. The reduction in mass flow due to lower density means the fan requires less power.
- Mass flow: Reduces from 2.33 kg/s to a lower value because the density is lower at altitude.