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Energy optimization with VSD in air coolers
Variable Speed Drives( VSDs) are emerging as one of the most effective solutions for optimizing energy use in refinery air coolers. By precisely matching fan speed to process and environmental conditions, VSDs minimize unnecessary power consumption, improve efficiency, and reduce noise. This article examines their advantages over traditional control methods and highlights the potential savings achievable through their application in ACHE systems.
Figure 1. Energy usage profile in CDU / VDU.
By Uma Sankar Khan, Technical Professional Leader, KBR
Refineries are designed to handle a wide range of crudes to have greater operational flexibility. These crudes usually differ widely in their characteristic, resulting in quite different yield patterns. As final product destination conditions are broadly outlined, equipment is specified and designed catering to various operating conditions. The Air-Cooled Heat Exchangers( ACHE) are no exceptions. ACHEs are typically designed and optimized to cater to a variety of operating conditions usually under various stringent scenarios( i. e., maximum process duty while encountering maximum ambient temperature) because of the seasonal variations in the ambient temperature during the year and the change in type of crude processed. ACHEs without proper control of airflow will supply too much cooling air most times of the year, leading to higher power consumption. ACHEs are extensively employed in refineries to reduce the temperature of process streams to about 60-65 0C, which is then followed by a water cooler to cool the process streams to their respective desired temperature. ACHEs typically account for about 10-15 % of the total power required in primary distillation units, which translates to approximately 2-3 % of the total energy required in these units.
As air coolers are predominantly operated at lower heat duty and ambient temperature than their design conditions, there exists extensive potential for energy saving. In this paper, energy conservation opportunities in refinery ACHEs are addressed using variable speed drives / variable frequency drives( VSD).
Regulation of air flow A fan without a flow controller supplies too much cooling air most of the year and thus often leads to unwanted sub-cooling of the products. Furthermore, the power consumption of the fan supplying the full air volume is unnecessarily high. Reducing airflow as conditions permit can result in substantial power saving and reduced noise level, since former goes with the third power of the airflow requirement. To match the system requirements with the fan output, one of two basic approaches may be adopted. One approach is to change the system curve and the other is to change the fan performance curve. Some of the most commonly employed approaches are described below along with their advantages and disadvantages.
• By placing damper: For flow throttling, a damper is placed upstream or downstream of the fan, and the system curve is modulated to give the desired airflow rate. This mode of controlling fan capacity is energy inefficient, noisy, and may also lead to flow instabilities.
• By shut down of individual fan: This is achieved by shutting off one of the two fans in each bay. On-off fan control is simple, but it can cause problems in ACHEs such as water hammering, tube-to-header leakage, nozzleflange leakage and so forth. Power saving depends solely on operators’ judgement, and greater operator interventions are also required in this case. Moreover, shutting off a fan causes step change leading to instabilities in the process. Due to its very nature, it does not permit savings of full potential.
• By controlling pitch of the fan: For energyefficient control of fan capacity, it is always preferred to change the system curves. The fan’ s blade angle or rotational speed may be changed to adjust the fan’ s performance curve. To change the blade angle, thereby controlling the fan capacity, either manually adjustable or auto-variable fans are adopted. However, energy conservation for manually adjustable fans depends solely on the efficiency of the operator; for manual adjustment of the pitch, a temporary shutdown of the fan may be
40 Heat Exchanger World October 2025 www. heat-exchanger-world. com