Planning a Compressed Air System – Air Compressor Manufacturers

Planning a compressed air system is calculating a ten-year bill.

Many people planning compressed air systems focus all attention on equipment purchasing. This machine costs how much, that machine costs how much, comparing quotes back and forth. This thinking is wrong from the start.

Compressed air system cost structure is different from most industrial equipment. Equipment purchasing only accounts for two to three tenths of total ownership cost. Remaining seven tenths or more is electricity.

Buy a compressor, run it ten years, money paid to the power company is several times the cost of buying the machine.

Specific power, this parameter, many people don't pay much attention. Figure a difference of a few tenths doesn't matter. Do the math and you'll know if it matters: specific power difference of 0.5 kW/(m³/min), for a 350 CFM machine, means 5 more kWh every hour. Run 8,000 hours a year, that's 40,000 kWh a year. Industrial electricity at seven cents, $2,800 more per year. Ten years is $28,000. Saved $5,000 at purchase buying a machine with higher specific power, ten years you're out twenty-plus thousand.

Many people don't do this math, or do it and don't take it seriously. Equipment purchasing goes through capital expenditure, electricity goes through operating cost. Two pots of money come from different pockets. Decision makers might not even be the same people. Purchasing department completed their task. Electricity cost is production department's problem. By the time production department notices electricity cost is sky high, equipment's already been bought.

So planning phase must get this math done clearly, and make sure decision makers see this math.

Industrial compressor system — Electricity is the big cost—typically 70% or more of total ownership

These years VFD compressors are hot. Manufacturers advertise 30%, 40% energy savings, sounds tempting. Many people hear energy savings and go VFD, regardless if their conditions are suitable.

VFD machines save electricity by adjusting speed based on air demand. High demand, high speed. Low demand, low speed. Avoids fixed-speed machine's load/unload cycling. Fixed-speed machine unloading doesn't produce air, but motor still spinning, wasting electricity. VFD machines save this waste.

Problem is, VFD itself has losses from the inverter. Full load operation, efficiency actually slightly lower than fixed-speed. Stable load conditions, fixed-speed running full load, efficiency highest. High load fluctuation conditions, VFD advantage shows.

What counts as high load fluctuation? Morning shift air demand twice the night shift, that's high fluctuation. Morning full load afternoon only half, that's high fluctuation. Air demand throughout the day changes less than 20%, that's stable. Going VFD won't save much money.

How to know if your plant has high load fluctuation? Measure. Clamp a current recorder on existing compressor power wires. Collect one week or even one month of data. Current curve shape tells everything: curve flat like a straight line, fixed-speed is enough. Curve jumping up and down, VFD can save money.

New projects without historical data can look at similar processes in same industry for reference. Really can't find a reference, rent equipment first and run a few months to get baseline. Better than guessing. Select equipment wrong, ten years of electricity bills paying for that mistake.

Multi-unit systems have one configuration worth considering: one VFD plus several fixed-speed. VFD specifically handles load fluctuation, speed follows air demand. Fixed-speed either runs full load or stops, always working at optimal efficiency point. System pressure stabilized by VFD, fixed-speed doesn't participate in pressure regulation. This configuration brings out advantages of both machine types.

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Multiple compressors must have sequencing control. Can't emphasize this enough.

Compressor control system — Sequencing control coordinates multiple machines for optimal efficiency

System without sequencing control, what's that like? Each machine has its own pressure switch. Pressure below lower limit, load. Above upper limit, unload. Three machines each minding their own business. System pressure dips slightly, all three load together. Pressure spikes slightly, all three unload together. Pressure curve swinging wildly. Load/unload count hits hundreds per day. High electricity, short equipment life, pressure still unstable.

Installing a sequencing controller costs how much? Few thousand dollars. Annual electricity savings? Maybe several times that amount. Several machines coordinating together. How many should run, run how many. Each one's loading level, orderly. System pressure stable, load/unload count greatly reduced.

Some plants to save that few thousand dollars on controller, annual extra electricity cost is far more than that amount.

Air-using equipment in the plant won't all have same pressure requirement. Most equipment 85 psi is enough. A few need 115 psi or even 145 psi. Easiest approach is configure to highest requirement, whole plant at 145 psi. Cost of this decision: all equipment, all the time, using pressure 60 psi higher than actually needed. Compressor to produce this extra 60 psi, uses nearly 30% more electricity. A few high pressure pieces of equipment, making whole plant pay for them.

Correct approach is configure main system at 85 psi. Those few high pressure equipment, install boosters in front of them. Boosters raise 85 psi to 145 psi, solve problem locally. Booster equipment investment plus operating power consumption, way less than raising whole system pressure.

Pipe pressure drop must be factored in too. Compressor station outlet 100 psi, pipe runs long, many elbows, pipe diameter too small, by the time it gets to workshop end might only be 80 psi. Planning phase estimate this loss, don't wait until system is built to find end pressure isn't enough.

Industrial air system — Equipment configuration depends on load curve shape

Air demand calculation is starting point for all calculations. Adding up nameplate consumption then multiplying by simultaneous use factor is standard practice. Factor usually 0.6 to 0.8. Measured load curve more reliable, especially for retrofit projects. Historical data right there, waste not to use it.

Reserve margin depends on company's own growth plans. No standard percentage. Aggressive expansion, leave more. Steady progress, leave less. Leave too much, equipment runs inefficiently long-term. Leave too little, production expansion gets bottlenecked.

Post-treatment equipment, enough is enough. Paint spray sensitive to oil. Electronics manufacturing sensitive to particles. Ordinary pneumatic tools not that picky. Distinguish applications. What should be high, high. What can be low, low. Don't configure whole plant to highest standard "for insurance." Desiccant dryer regeneration eats 10% to 15% of compressed air. Configure too much and that's waste.

Machine room ventilation must be good. Compressors generate lots of heat. Inlet should avoid pollution sources. Leave space for future equipment additions. Pipe network run loop if possible, end pressure more even, also has redundancy. Do these by standard practice, won't have big problems.

Equipment configuration, big and small mixed or uniform spec, depends on load curve shape. Base load proportion high, configure more big machines. Fluctuation severe, increase small machine proportion. Exactly how to configure has no formula. Must look at measured data.