Compiled by Eamonn Ryan based on a SAIRAC Johannesburg Centre Tech Talk by Jannie Potgieter
Altitude significantly affects the cooling performance of HVAC systems.
Attendees at one of the in person SAIRAC Johannesburg Centre Tech Talks last year. © 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 high-altitude 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/m³, while at Johannesburg’s altitude, it is roughly 0.98 kg/m³.
For practical calculations, a simplified rule of thumb can be applied where air density is approximated as 1.0 kg/m³ at high altitudes and 1.2 kg/m³ 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.
- 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 high-altitude 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 density decreases, the fan’s ability to generate static pressure is reduced.
Power: The power (PPP) required by the fan also changes with air density. A reduction in air density requires the fan to work less to maintain the same volume flow, reducing its power consumption.
In high-altitude locations like Johannesburg, where air density is lower, the fan laws help estimate the performance of fans under these conditions. The fan laws allow engineers to:
- Estimate new fan curves: By applying the fan laws, it is possible to generate new fan curves that reflect performance at different densities.
- Determine operational speeds: Engineers can calculate the new operational speed of the fan needed to achieve desired performance under varying altitude conditions.
- Calculate power requirements: The fan power can be recalculated to account for the decreased workload due to lower air density.
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)
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
- Pressure drop: Reduced at high altitude due to lower air density. The fan curve and system curve was reduced by the same ratio.
- Volume flow: Remains constant at 1.94 m³/s, despite changes in altitude. The new operational point is at a lower static pressure but at the same flow rate.
- 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.
Gregory Grobbelaar. Previous Johannesburg Centre chairman. © RACA Journal
Capacity calculation for cooling and heating coils
When evaluating the impact of altitude on HVAC systems, especially cooling and heating coils, it’s crucial to understand how the capacity changes. Capacity can be determined using the following equations:
Cooling Capacity:
Capacity = 𝑚˙ × Δℎ
Where 𝑚˙ is the mass flow rate, and Δℎ is the enthalpy difference.
Heating Capacity:
Capacity = 𝑚˙ × 𝐶𝑝 × Δ 𝑇
Where 𝑚˙ is the mass flow rate, 𝐶𝑝 is the specific heat capacity, and Δ𝑇 is the temperature difference.
At higher altitudes, the mass flow rate decreases due to lower air density. Consequently, the capacity of cooling and heating coils also decreases. The reduction in capacity is proportional to the reduction in mass flow rate, but the actual reduction is slightly less than the density ratio due to changes in outlet conditions, such as temperature.
Example calculations and analysis
Cooling Coil:
1.2 Density ratio: The density at Johannesburg is 0.98 kg/m³ compared to 1.2 kg/m³ at sea level. This results in an 18% reduction in density.
Estimated capacity reduction: Based on the density ratio, we would initially expect a similar 18% reduction in capacity. However, due to the impact on outlet temperatures, the actual reduction is less severe.
For example, consider a sensible cooling coil with:
- Original outlet temperature: 9.25°C at sea level
- New outlet temperature: 8.84°C at high altitude
- Reduction in capacity: 15.3% at altitude, which is slightly less than the 18% density reduction
Heating coil:
- Density ratio: Similar density change applies
- Estimated capacity reduction: The reduction observed was approximately 14%, which is again less than the 18% due to changes in outlet temperatures.
- For the heating coil:
- Original outlet temperature: 32°C
- New outlet temperature: 42.7°C
- Reduction in capacity: About 14%, reflecting the same trend of being less severe than the density reduction
Considering the entire HVAC system, including evaporator, condenser, and compressor, the overall performance is influenced by the combined effects of each component:
- Cooling capacity: The cooling capacity of a complete HVAC system will reduce as the altitude increases due to reduction in mass flow and subsequent decreased capacity of the coils.
- 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
- 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.
- 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.
- 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 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.
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.
-
- Image supplied by SAIRAC
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 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.
Conclusion and Q&A session
To wrap up the discussion, the presentation explored how altitude impacts HVAC systems, focusing on coil capacity and compressor power adjustments. By utilising equations and correction factors, we’ve been able to estimate changes in system performance under different conditions.
- Practical considerations and real-world variations: While theoretical calculations and correction factors provide a useful starting point, real-world performance may vary. Differences between theoretical predictions and actual results can occur due to factors such as:
- System specifics: Unique system configurations, including coil geometry and design, affect performance.
- Operational conditions: Variations in actual operating conditions compared to standard test conditions can influence outcomes.
Manufacturer data: Correction factors and performance estimates are often based on manufacturer-specific data and may not apply universally.
During the Q&A, the question about the difference between theoretical and practical correction factors highlighted the need to account for real-world testing and adjustments. It’s crucial to communicate any discrepancies between theoretical capacity and actual performance to customers, ensuring they understand the practical implications of operating at different altitudes.
