Isentropic Efficiency: What It Means to YOU

About 75% of a compressor’s total lifetime cost is energy. That's the reason that compressor providers constantly work to improve compressor designs and controls to improve efficiency. It’s also why power providers offer financial incentives for users to upgrade to more efficient technology, and why compressor sales professionals educate customers on improving ROI by investing in greater efficiency.

CAGI, the unified voice of the compressor industry and supported by all the major compressor manufacturers, developed model-specific Compressor Data Sheets that help us compare key aspects of performance for different compressors, including energy use. Now CAGI data sheets include another helpful way to compare compressors: Isentropic Efficiency.

What’s Isentropic Efficiency?

Stated simply, Isentropic Efficiency indicates how efficiently a compressor converts electrical energy into compressed air potential energy. The higher the percentage, the better the compressor is at converting electrical energy into compressed air. It’s a simple ratio:

                                                       Energy (kW) for the Ideal Compression Process

Isentropic Efficiency (%) =    --------------------------------------------------------------

                                                                       Actual Energy (kW) Consumed

Isentropic Efficiency is stated as a percent of an ideal compressor. Of course an ideal compressor doesn’t exist, because real world machinery encounters friction losses, electrical losses, inertial losses, pressure drop, mass-flow losses and more. CAGI created the Isentropic Efficiency calculation to provide a simple way to directly compare the efficiency of different machines. (CAGI produced a 9-minute video that describes the concept in detail - watch it here).

What About Specific Power?

For years, compressor manufacturers and users relied on the measure of Specific Power (line 12 on a CAGI Compressor Data Sheet). This is the ratio of input electrical energy in kW required to deliver an output flow at 100 cfm at a given discharge pressure:

                                       kW

Specific Power =  -------- x 100 (at a given discharge pressure)

                                       CFM

Importantly, Isentropic Efficiency and Specific Power may point to different compressors as being more energy efficient. That’s because Specific Power calculations use full load operating pressures based on kW/100 CFM, and those full load pressure values may not be the same for different compressors being compared. Isentropic Efficiency normalizes the comparison to simplify the evaluation of efficiency in like sized compressors.

Comparing Convertibles to SUVs

Consider an analogy. Say you’re out comparing new cars and focusing on those with a certain level of efficiency: 30 miles per gallon. The window stickers required in new cars show you which ones meet that standard, and those that do include a 2-seat convertible, a subcompact SUV, a compact sedan, a midsize hybrid. While all provide transportation and the same fuel efficiency, their feature sets and advantages vary significantly. In much the same way, every 200-hp air compressor with the same rating in a given category won’t necessarily provide the same features and advantages.

Energy efficiency continues to be a key factor in evaluating compressors, and while a difference in Isentropic Efficiency may be small, it does establish a certain benchmark. From there, you can  evaluate other key factors, such as compression technology, connectivity and monitoring, sound levels, footprint, system integration, maintenance requirements, access to service, acquisition cost, financing options, utility incentives, manufacturer reputation, and more. You also can incorporate the insights and data gathered from any air system audits that are conducted (more details on audits here). 

Get all the details about CAGI and isentropic efficiency here.