On a recent plant visit our engineers followed a rumbling noise from a backroom to discover a large air compressor running flat out. The 300 horsepower (HP) system was pushing 100psi air to a set of production lines located at the other end of the facility. Serious industrial production stuff.
To the layman the slight hissing sound heard outside the room jumped out as an energy efficiency opportunity. Repairing the leak would save wasted air pressure and translate into energy dollar savings.
But there was a bigger missed opportunity:
Only one of the company’s six production lines was running.
In manufacturing compressed air often powers critical production processes. Pressurized air can actuate valves, move product down conveyors, pick-up product cases, clean off surfaces or power pneumatic tools. Both for volume and redundancy, multiple compressors are often staggered around a large plant, connected to a piping header that distributes the air, with ceiling “drops” located wherever air is needed.
But just as HVAC contractors oversize home air conditioning systems for the hottest day, most manufacturers take the same approach with compressed air, modeling designs for the plant running at full capacity, their ultimate production objective. Sizing is based on every line running at once, with production output at the highest volume. The oversized systems often run all day long, all year round.
So what does it cost?
At full load the simple calculation is the product of total Brake Horsepower (BHP is slightly higher than nameplate HP) times .746 (the conversion factor for HP to kilowatts) times the plant’s annual operating hours times the cost per kWh divided by the motor’s efficiency rating. At $0.10 kWh the 300HP system our engineers uncovered consumed $217k per year in electricity.
Thinking about efficiency, on first pass the variables don’t seem easy to manipulate. BHP is the cumulative sum of the compressors’ horsepower. The plant’s operating hours are what they are, as is their cost per kWh and the motor’s efficiency rating.
Beyond repairing leaks, where’s the opportunity?
Our team focuses on redefining “full capacity.”
At full production compressed air will likely be maxed out, but at most other times the system’s output can be scaled back. Stunningly, $187k of the $217k in electricity costs come from overproduction of air and generating waste heat.
Reducing total HP running at any moment has a 1:1 impact on the numerator in the cost equation. Sequencing controls can bring online only the optimal number of compressors based on varying production levels while shutting down pipe runs to idled production equipment.
Running individual compressors at reduced speeds can also have a big savings impact. Variable speed drives (VFDs) enable constant torque piston and screw compressors to be slowed down, ramping speed back up when air pressure demand increases.
And waste heat can be reclaimed from the motor, producing zero cost therms for drying out the moisture buildup typically occurring in compressed air systems.
The simple paybacks for these types of upgrades are fast – almost always under three years, and usually closer to one.
But before making any suggestions we always start by installing metering.
No eager, sustainability-minded engineer can have a credible conversation with a plant manager without actual usage data. Production correlated flow and power metering for all critical processes allows us to have a fact-based discussion. Only then can we consider a redesign, resizing or upgrade that reduces energy consumed withoutimpacting production.
In the end we’re aiming to convert “full capacity” to “on-demand,” with a system that intelligently produces air only where and when necessary. And apart from a little less hissing sound, the principal difference should only appear in next month’s electricity bill.
