Compressed air has been an essential utility right back to the industrial revolution. It still provides a safe and easily transported power source for a multitude of operations, from underground mining to semiconductor manufacturing.
It is often referred to as the “fourth utility”, I like to think of it more as the “forgotten utility”. The generation and treatment packages usually site in a compressor room tucked away in the corner of a plant and left to “do its thing” in the background. Surprising when you consider it is eight times more expensive than electricity as a utility!
What is not clearly understood by most owners operating a compressed air system is the true cost of ownership. There are four cost areas to owning and operating a compressed air system. All of which need to be considered together, to deliver an efficient and reliable compressor air supply around a plant. When you consider each in turn, the “elephant in the compressor room” is clearly the energy consumption, as this 10 year cost analysis demonstrates.
Looking at them individually. Most owners focus on keeping the capital costs of the equipment as low as possible, believing this will give the fastest return on investment. Ironically, it often has the opposite effect. Purchasing a low cost, energy hungry compressor can dramatically increase the amount of energy a compressed air system consumes and the plants overhead costs. SCR Comp are bucking this trend with their EPM range of compressors which are extremely energy efficient and cost effective, reducing capital costs and improving Return On Investment.
The next area of focus is usually the maintenance costs. Whilst maintenance is important for reliability and efficiency, they have little impact on the generation efficiency of an air compressor. Whilst it is important maintenance is adequately cover, the cost is easily fixed with extended warranty and service packages available from most equipment manufacturer.
Installation costs are a small portion of the total costs, but a critical one. This is the stage where you can make your nice new energy efficient generation set inefficient by simply laying it out incorrectly. One aspect that often gets overlooked is intake air temperatures. Think back to physics at school, with that balloon that expands and contracts with temperature changes. The same is true of intake air to compressors, lower intake air temperatures reduce the energy a compressor uses to get up to the required pressure. Amongst other things, this can be as simply as ducting cool intake air directly to a compressor, to avoid sucking its own waste heat back in (remember the balloon scenario?).
In reality, the energy costs of a compressor should be the first priority when designing a system. Especially with the dramatic increases in energy cost of recent years. We measure energy efficiency of a compressor as Specific Power Consumption in kW/m3/min, i.e. how much energy it takes to generate a fixed volume of compressed air at a stated pressure. When designing an energy efficient system this figure should be between 5 and 6 kW/m3/min for a typical 7Barg general purpose compressed air system.
The “baby elephant in the compressor room” is the amount of waste heat an air compressor generates. This is as high as 80% of your input energy, despite having bought an “energy efficient” compressor with a low Specific Power Consumption. Waste heat is a massive opportunity for energy recovery. A typical 22kW compressor will exhaust the equivalent of seventeen 1kW residential space heaters all running together. There are numerous ways to recover heat from a compressor, which is a lengthy discussion for another time.
Once your system is generating efficiently, one further aspect of compressed air ownership that needs considering is the efficient use of the expensive compressed air. There are numerous ways to prevent the wastage compressed air once it has been generated, like leakage reduction, pressure drop optimisation venturis etc.
Finally, it is also worth noting these principles can also be applied to gas compressors for things like nitrogen and carbon dioxide, not just compressed air.
Mike Knowles began his career in the 1990’s as a qualified chemist and he became interested in environmental monitoring. This led to optimising the energy efficiency of compressed air systems.
Now, 30 years on he still enjoys saving thousands of tons of CO2 and energy for companies like Muller, JLR and Kelloggs.
Contact Mike: [email protected]
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