Compressed Air System Audit

ISO 11011:2013 "Compressed Air Energy Efficiency Assessment" provides an international framework for system audits. But there's considerable distance between the standard text and actual practice.

Audit

System Energy Assessment

Equipment list looks simple, doing it is a hassle.

Machines sitting in the compressor room, model, power, output capacity, all on the nameplate. Problem is nameplate data and actual performance often don't match. A machine rated 350 CFM that's been running twelve years, actual output might only be 280 CFM. Record purchase year, because equipment from around 2012 has noticeable efficiency gap, related to technical iteration in screw rotor machining precision and motor efficiency classes at that time.

Collect 12 months of electricity bills. Bills from finance department usually only have total amount, need to get meter reading records from electrician or request usage details from power company. Peak-valley price difference in some regions approaches 3x, this info is needed later for economic analysis.

Maintenance record quality varies wildly. Well-managed factories have complete equipment ledgers, every oil and filter change has signature records. Most factories' maintenance records are just a few handwritten papers, scribbled writing, fuzzy dates, "changed filters" three words covering everything. Run into the latter situation, can only ask maintenance workers on site, their verbal descriptions are often way more detailed than written records.

Production shifts and air-using equipment list go together. Three-shift factories have relatively flat load curves. Single-shift factories have obvious start-stop fluctuations at 8am and 6pm. Equipment list takes the most time. Big factory with hundreds of use points, spray guns, cylinders, pneumatic valves scattered in every corner, surveying once might take two or three days.

Supply-side measurement focus is figuring out what each machine is doing.

Clamp-on ammeter reads current, compare to nameplate current, roughly know load rate. 100 hp motor full load current around 135A, measures 90A, means this machine isn't eating full. Just looking at current isn't enough, gotta squat next to the machine and time it.

Load, unload, load, unload. Stopwatch records duration of each state. This work is boring. Thirty minutes minimum, sometimes gotta squat over an hour to see the pattern. Some machines load four minutes unload two minutes, some load one minute unload seven or eight minutes. Latter has big problems. Fixed speed machine in unloaded state is idling, electricity still burning, no air produced, energy wasted right there.

Discharge pressure and temperature are routine items. Pressure gauge needs to be calibrated, factory's original gauge drifting a few psi is normal. Temperature, shoot the outlet with IR gun. Air-cooled machine normal discharge temp is 20 to 25°F above ambient. Over 35°F above, time to check the cooler.

Aftertreatment equipment pressure drop easily gets overlooked. Refrigerated dryer, filters, oil-water separators, gas passes through one checkpoint and loses a bit of pressure. Looking at each device separately 3 psi, 2 psi doesn't seem like much, strung together might exceed 9 psi. This loss ultimately needs to be compensated by raising compressor discharge pressure, and each 15 psi pressure increase means about 7% more energy consumption.

Demand-side measurement needs field work.

Main header end pressure is first data point. From compressor station outlet, follow the pipe all the way to the farthest shop, install a pressure gauge there or use portable instrument to measure. Compressor station outlet 100 psi, shop end 85 psi, where did the 15 psi in between go? Pipe too small, too many elbows, valve half-open, certain section of pipe inner wall scaled, cause might be one of these, might be all of them.

Branch line pressures need to be marked on a diagram. Shop A end 90 psi, Shop B 80 psi, Shop C 72 psi. Shop C operators might have been complaining about not enough air for a while, equipment running slow, but this info may not have reached management.

Measurement

Branch Line Pressure

Flow

Ultrasonic Meter

Leak testing is the part of the whole audit that produces most immediately visible results.

Find a production downtime window. Weekend, lunch break, or holiday works. Shut off all air-using equipment, leave only the pipe network pressurized. Start compressor, observe load-unload cycle. Under zero load condition, air the machine produces during loading is all going to replenish leaks. Loading 3 minutes unloading 7 minutes, leak rate 30%. This number means nearly one-third of compressed air is escaping through various gaps in the pipe network.

Detection

Ultrasonic Survey

Leak Points

Tagged for Repair

Is 30% leak rate high? Compressed Air Challenge gives North American factory average around 25%. Domestic factories are generally higher. Under 10% counts as well-managed. Over 20% needs serious attention. Over 30% is must-act-immediately level.

Locating leak points uses ultrasonic leak detector. SDT, CTRL, Fluke all have related products. Point instrument at fittings, valves, hoses, hear hissing sound and that's a leak point. A 90,000 square foot shop's first comprehensive survey, finding two or three hundred leak points is common. Tag and number them, record locations, grade by leak volume. Fix big leaks first. A 1/8" hole at 100 psi pressure leaks 53 CFM.

Energy analysis strings together the data collected earlier.

Core metric is specific power: how many kWh consumed to produce one cubic meter of air. Calculation is simple, total compressor station electricity consumption divided by effective air supply. Effective air supply has to subtract leak losses, so leak testing comes first.

What specific power is good? Modern VFD screw machines under rated conditions can do 0.10-0.11 kWh/m³. Old machines plus system losses, 0.15-0.17 kWh/m³ is also common. Compressed air system efficiency guides give tiered reference values, first tier efficiency, second tier, third tier each has corresponding ranges.

Identifying improvement opportunities doesn't need any fancy methodology. Just go through checklist item by item.

Leak rate exceeding standard is most universal problem, almost every factory has it. Pressure set too high ranks second. Many factories set pressure at 110 psi or even 115 psi, but 95% of pneumatic equipment works fine at 90 psi. Equipment loading rate too low ranks third. This problem is especially prominent in factories with big capacity fluctuations. Peak season machines at full load, off-season big machines running idle.

Heat recovery has been mentioned more in recent years. Compression heat accounts for about 90% of input energy. Air-cooled machine hot air, water-cooled machine hot water can both be recovered. Winter shop heating, process preheating, domestic hot water supply are all uses. Actual adoption isn't great. Main obstacle is heat recovery investment needs to match heat demand. Many factories have excess heat in summer with nowhere to use it, only need it in winter but that's short duration, economics don't work out.

Improvement plans need to do the math.

Leak repair has smallest investment and fastest return. Material cost few hundred dollars, labor by hours. $3,000-5,000 can complete one comprehensive repair, payback usually under six months. No dispute on this type of project, should start immediately.

Pressure optimization costs almost nothing, just adjusting setpoints. But execution resistance is often bigger than expected. Maintenance staff worry equipment will have problems if pressure drops, prefer to set it conservatively high. Individual stations really do need high pressure, correct approach is local boosting not raising everything, this requires installing boosters, some investment.

VFD retrofit is bigger investment. One 100 hp VFD screw machine landed price around $50,000. Energy saving effect depends on how inefficient original system was. If replacing severely oversized fixed speed machine, two to three years can recover. If original system configuration was already reasonable, payback might stretch to over five years.

Verify after completing changes.

Same testing methods done again: leak rate dropped from 28% to how much, specific power dropped from 0.14 to how much, end pressure stable at how many psi. Data comparison goes in the report. This is both closing out this round of audit and leaving baseline for next round.

Continuous tracking is more valuable than one-time audit. Leaks will bounce back. Fitting fixed today might start leaking again in three months. Establish monthly inspection routine. Do focused area recheck every quarter. Do comprehensive leak detection annually. These actions become routine, then compressed air system efficiency level can be stably maintained.