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End User Focus
Figure 3A. Variation of electric power requirement for the fan-motor system under different ambient temperatures.
Figure 3B. Month wise variations in ambient temperatures.
Evaluation:
• Airflow requirement: Required airflow and static pressure are obtained from different ambient temperatures from detailed simulation of the ACHE’ s design using actual design software. Typical results are shown in Figure 4.
• Power requirement: For fans with fixed blade angle air flow, rotational velocity of the fans is not changed to control the air flow, but the air flow is throttled to reduce it. This is the most energy-inefficient operation of the fan motor system and practically results in no savings.
For auto-variable fans, the blade angle at clevis is changed to match the required airflow and static pressure for different ambient conditions, keeping the original speed of the fan same. Performance equation of motor fans have been used to calculate the power requirement by an autovariable fan operating under different ambient conditions. The rpm of a fan operating with VSD is changed to match the required airflow and static pressure with variable ambient temperature, keeping the original blade angle at clevis of the fan the same. Reducing velocity using a variable frequency drive does not result in a change in efficiency since the velocity and the rotational speed are reduced by the same proportions. The pitch, therefore, remains constant.
Required airflow and static pressure under different ambient temperatures.
Saving in power due to change in rpm is estimated using the characteristic and performance equations of Moore Fan.
Energy savings potential: Refineries can reduce fan speed by 20-30 % using VSDs, leading to 25-35 % energy reduction.
• Example: Speed control vs on / off cycling; Demand = 50 % Power2 =( 1 / 2) Power1
Motor Configuration:
• Typically, 50 % of fan motors are manual on / off and 50 % are variable speed.
• If needed, 100 % of motors can be variable speed for enhanced efficiency.
Conclusion A rated capacity saving of at least 20 % could be realized on account of seasonal temperature variation and duty variation owing to different processed crude. In case of units not operating normally at a rated capacity throughout the year, power saving will still work out to be higher. The payback period for other refineries( capacity and location) may vary depending on site-specific conditions such as seasonal temperature variations and the varying types of crude being handled. In addition to energy saving as a result of reductions in speed, tremendous reduction in the ACHE’ s noise level is also of great advantage.
42 Heat Exchanger World October 2025 www. heat-exchanger-world. com