www . refrigerationandaircon . co . za RACA Journal I January 2025 41
Technical 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
Gregory Grobbelaar . Previous Johannesburg Centre chairman .
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 ( m ³/ 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 m ³/ 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 .
© RACA Journal
• Mass flow : Reduces from 2.33 kg / s to a lower value because the density is lower at altitude .
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 : Density ratio : The density at Johannesburg is 0.98 kg / m ³ compared to 1.2 kg / m ³ 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 .
www . refrigerationandaircon . co . za RACA Journal I January 2025 41