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Improving Compressor Efficiency: Is a Rerate the Right Solution?

Improving Compressor Efficiency: Is a Rerate the Right Solution?

Procuring a new centrifugal compressor package can be expensive and time intensive. Most organizations require minimum bid procurement, many channels for budget approval, and permits for a new compressor. 

However, an alternative exists to help customers increase their plant air capacity without purchasing a completely new compressor: rerating. 

What is a Rerate?

When a compressor is rerated, changes are made to an existing compressor package to: 

  1. Increase design flow and capacity. 
  2. Increase design discharge pressure. 
  3. Increase compressor efficiency. 

Choosing a rerate can have significant advantages over purchasing a new air compressor, but you must determine which solution is best for your specific needs: a new air compressor or rerate. 

Centrifugal compressors are designed for specified design conditions using a combination of usually 2-4 stages, utilizing impellers, diffusers, and inlet casings (volutes). While centrifugal compressors can operate within a certain range, their capabilities are limited by the conditions they were designed for when the compressor was originally manufactured. 

Rerating offers the ability to increase performance by as much as 30-50% by changing some of the internal components of the compressor but without replacing the entire skid assembly. 

If a unit is older, hot day temperatures may have increased over time, resulting in reduced performance, and a higher chance of the compressor surging. Rerating to newer atmospheric conditions can get the flow back to its original level and possibly increase efficiency utilizing newer aero hardware technology.

Pros of a Rerate

Things to Consider

Minor adjustments can make a significant impact on discharge pressure. 

In addition to airend components, items such as inlet and unloading valves, unloading silencers, or drivers may also need to be upgraded to accommodate the rerate.

Lead-time is much shorter compared to new equipment.

Accessory changes may require some modifications to the customer site or utilities and may incur more downtime for the unit.

Lower upgrade costs in comparison to an entire new unit.

Is the airend already at the top end of the frame’s flow capabilities?

If the entire airend is replaced, original airends can be kept as emergency spares. 

 

Purchase can be categorized as maintenance rather than capital budget in some cases. 

 

 

Case Study #1: Major Automotive Manufacturer 

With constant vehicle models added to their production line over the years, keeping up with air demand was a challenge for a major automotive company located in the Southern U.S. Purchasing an additional air compressor was under consideration, but after reaching out, the option of a rerate to achieve their air needs was proposed. 

The airends on their existing air compressors provided 4,000 CFM each. By rerating these airends, their maximum capacity could be increased to 5,600 CFM, each which would offer the air supply that their plant needed for their increased production.  

The original design of the two compressors was carefully reviewed, and a full proposal was offered to show the increased inlet flow rate and capacity increase. The proposal outlined that the airends would be identical in design to the original air compressors and maintenance parts used and would be free of any silicone. Full specifications and the proposed performance curve were included with the proposal.

Rerate Proposal Summary

Original Spec

Proposed Rerate

Inlet Flow Rate 

4000 CFM/
airend  

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 located in the Northeastern U.S. was concerned about the performance of their 1970s centrifugal compressor as well as their plant’s energy efficiency. They were seeking a solution where they could reduce energy but maintain the original performance needed for production.

With the advancement in parts and aero technology, a rerate offered the same level of performance as their aging existing air compressors but utilized 18.1 less horsepower. Specifically, the horsepower went from 680HP to 661.9HP with the rerate but maintained the same inlet capacity of 3,050 ICFM. 

To put this in perspective, if this machine runs 24 hours a day/7 days a week for the entire year, this unit will use about 122,700 less kWh per year. Based on the average US energy rate of 12.84 cents per kWh, this results in savings of approximately $15,700 a year.

This customer not only was able to get the full capacity of their aging compressor back but also able to decrease the energy costs required to maintain that airflow.

For more information on airend rerates, and how these options might assist you in maintaining flawless airend performance while reducing energy consumption, contact your authorized FS-Elliott Channel Partner or Representative today!

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