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Boosting Compressor Efficiency: Unveiling the Ideal Solution - Rerate or Replace?

Boosting Compressor Efficiency: Unveiling the Ideal Solution - Rerate or Replace?

Rerating offers an effective and cost-efficient solution to enhance plant air capacity, bypassing the need for purchasing a new centrifugal compressor package. This approach eliminates the complexities associated with minimum bid procurement, budget approval, and permits for acquiring a new compressor. Organizations can increase capacity and efficiency by rerating existing equipment while saving time and money. 


What is a Compressor Rerate? 


A compressor rerate refers to the process of modifying an existing compressor package to accomplish the following: 


  • Increase design flow and capacity. 
  • Increase design discharge pressure. 
  • Improve compressor efficiency. 


When deciding between a rerate or purchasing a new air compressor, it is crucial to consider the specific requirements of your situation. While centrifugal compressors are engineered with 2-4 stages, impellers, diffusers, and inlet casings to meet particular conditions, their performance is confined to their original design limitations. Rerating presents a compelling advantage as it allows for performance enhancements of up to 30-50% by modifying internal components without replacing the entire skid assembly.  


When an older unit is experiencing decreased performance due to rising temperatures over time, rerating can restore the flow to its original level and potentially enhance efficiency by leveraging advancements in aero hardware technology to accommodate current atmospheric conditions. 


Benefits of Rerating: 


  • Even minor adjustments can significantly impact discharge pressure. Alongside airend components, upgrades to inlet and unloading valves, unloading silencers, or drivers may be necessary to accommodate the rerate effectively. 
  • Rerating offers a much shorter lead time compared to acquiring new equipment. However, changes to accessories may require modifications to the customer site or utilities, which could result in additional downtime for the unit. 
  • Rerating is more cost-effective than purchasing an entirely new unit, especially if the airend already operates near the upper limits of the frame's flow capabilities. 
  • By replacing the entire airend, the original can be retained as an emergency spare. 
  • In certain cases, the rerate can be categorized as a maintenance expense rather than a capital budget expenditure.


Case Study #1: Major Automotive Manufacturer 

A major automotive company in the Southern U.S. faced the challenge of meeting the increased air demand by adding new vehicle models to its production line. While considering the purchase of an additional air compressor, they were presented with the option of rerating to fulfill their air needs. 

The existing air compressors' airends provided 4,000 CFM each. Through rerating, the maximum capacity of each airend could be increased to 5,600 CFM, effectively meeting the plant's increased production air supply requirements. 

A thorough review of the compressors' original design was conducted, and a comprehensive proposal was offered, demonstrating the increased inlet flow rate and capacity. The proposal outlined that the rerated airends would have the same design as the original air compressors and would utilize maintenance parts free of silicone. The proposal included full specifications and the proposed performance curve. 

Rerate Proposal Summary

Original Spec

Proposed Rerate

Inlet Flow Rate 

4000 CFM/

5600 CFM/airend 

Discharge Pressure

150 PSIG

150 PSIG

Airend Power Requirements Per Airend

1015 HP

1207 HP

Currently Installed Driver Nameplate Horsepower  

1250 HP

No change required. 

Capacity Increase (Per Airend)


1600 CFM/40%

Total Capacity Increase


8000 CFM/40%


Case Study #2: Food & Beverage Manufacturer 


A large food manufacturer in the Northeastern U.S. had concerns about the performance of their aging centrifugal compressor from the 1970s and the overall energy efficiency of their plant. They were searching for a solution that would allow them to reduce energy consumption while maintaining the necessary performance for production. 


With advancements in parts and aero technology, a rerate presented an opportunity to achieve the same level of performance as their existing air compressors while reducing horsepower usage. Specifically, the horsepower requirement decreased from 680 HP to 661.9 HP with the rerate while maintaining the same inlet capacity of 3,050 ICFM. 

To put this into perspective, if the compressor operates 24/7 throughout the year, this rerated unit would consume approximately 122,700 kWh less per year. Based on the average U.S. energy rate of 12.84 cents per kWh, this results in annual savings of roughly $15,700. 

This customer regained the full capacity of their aging compressor and achieved significant energy cost reduction while maintaining the required airflow for their operations. 

In conclusion, when it comes to improving compressor efficiency, a rerate can be a viable solution to increase capacity, discharge pressure, and overall efficiency without needing a new compressor.  

Organizations can substantially improve performance by modifying the existing compressor package while minimizing costs and downtime. However, it is essential to carefully assess each case's specific needs and conditions to determine whether a rerate or a new compressor is the right solution. Consultation with industry experts and authorized representatives can provide valuable insights and guidance in making this decision. 

For more information on airend rerates and how these options can help you maintain optimal airend performance while reducing energy consumption, we encourage you to contact your authorized FS-Elliott Channel Partner or Representative today

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