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End User Focus required. Therefore, auto-variable fans are preferred over manually adjustable fans. These fans automatically adjust pitch in response to a preset operating variable( mainly, product outlet temperature). The most frequent complaints about variable pitch involve its reliability. Many instances of the fan blade getting stuck due to various reasons have been reported. Furthermore, with variable pitch fans, the motors run with the same speed but at lower load, adversely affecting its efficiency.
• By changing speed of the fan: Changing the fan’ s rotational speed also changes the fan’ s characteristics and reduces airflow with substantial power savings and reduction in noise level. Variable voltage DC motors, using transformers and rectifiers may be used for controlling the fan’ s rotational speed. However, such DC motors require frequent maintenance, and their part load efficiency is very poor. By changing the number of poles of the fan’ s induction motor rotational speed, operating with pole changing motors may be controlled with only a few discrete values and may cause system control problems. Therefore, full potential of power saving is not realized in actual practice by the above measures.
Inherent characteristics of fan-motor system The energy-saving potential of different air flow controls mechanisms, discussed above, are limited due to their inherent characteristics, briefly discussed below.
• Fan efficiency: Efficiency for a fan of particular design is a function of the fan’ s geometry( such as diameter, hub diameter and tip seal) and airflow( i. e., pitch). For auto-variable fans, the blade angle at clevis is changed to adjust the pitch. At or near full load, a fan’ s efficiency is changed very little. Reducing the velocity by reducing the blade angle results in a change in efficiency because velocity is reduced while the RPM remains constant.
• Motor efficiency: Three-phase induction motors are used mainly to drive the fans. As the air flow requirement decreases due to reduction in ambient temperature, the motor operates at part-load conditions. Efficiency and powerfactor of an induction change with the loading of an induction motor. Until 50 % loading, variation in motor efficiency and power factor is negligible. However, the efficiency and the power factors are significantly lower than their full load values for induction motors operating with 40 % of the design load or less. This results in lower efficiency for fixed blade and autovariable fan-motor systems. Typical electrical motor efficiency under different load condition is shown in Figure 2. It is evident from the above that the total efficiency of the motor and fan system decreases significantly at lower than design load.
Figure 2. Variation of electric motor efficiency under different loading for constant speed-motors.
VSDs as energy-saving devices VSDs provide the best way to control air flow, in terms of energy efficiency and operational flexibility. Variable frequency drive technology has been underutilized and its potential not yet realized to convert power in refineries. Optimum system performance can be achieved if the fan, the VSD-motor unit, and the accessories are selected with adequate precaution. Use of VSD allows power savings on account of the following:
• Change in process duty due to change in throughput and / or type of crude being processed
• Seasonal and daily variations in ambient temperature
The power saving potential is fully exploited by maintaining motor efficiency at reduced load. Most of the saving will occur when the variation in type of crude and seasonal temperature variation are accounted for. As mentioned above, the ACHEs are typically designed for maximum ambient temperature encountered in the geographical area. Even in the middle of the summer, ambient temperature varies significantly between day and night. Hence, power saving is possible daily. The typical power requirement for fixed blade fans, auto-variable pitch fans, and fans with VSD at different ambient temperatures is shown in Figure 3A.
Case study An air temperature of 39 ° C is considered where ACHEs are followed by water coolers. When the ambient temperatures rise to 45 ° C, the ACHEs will underperform. But downstream water cooler is designed to take care of additional duty under such an eventuality. Approach: Based on the site average climatological data presented in Figures 3A and 3B, the whole year is divided into four temperature zones by combining those months where the maximum and minimum temperatures are similar. In addition to seasonal temperature changes, daily temperature variation also takes place and is accounted for. www. heat-exchanger-world. com Heat Exchanger World October 2025
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