Compressed-Air Baseline and Sequencing at a Metal Fabricator
A metal fabricator runs three compressed-air compressors: two 150 hp fixed-speed load/unload units and one 100 hp variable-speed unit. The system pressure is 110 psig, and the plant operates 8,000 hours per year, five days a week. Average air demand is about 1,100 cfm, but no flow meter is installed. The compressors draw roughly 250 kW in total. The compressed-air dryers are refrigerated, and the dew point is 40°F. Production reports random pressure drops at the end of a 500 ft header, so the maintenance team raised the pressure setpoint from 100 to 110 psig. A leak survey two years ago tagged 40 leaks, but only half were repaired. The compressor room is hot, there is no receiver storage beyond a 400-gallon tank, and the VSD unit often runs at 100% while one fixed-speed unit unloads. Electricity costs $0.09/kWh, and the demand charge is $12/kW.
1. Design a measurement plan to separate leak and artificial demand from production demand and establish a baseline.
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Install a thermal-mass flow meter and a power logger on the compressor bus for at least two weeks. Log pressure at the header and at the far end, compressor amps, load/unload status, VSD speed, and dryer dew point. Run an off-shift test: with production valves closed, measure the flow needed to hold pressure; that is the leak plus artificial demand. Repeat at 100 and 90 psig to see how leakage changes with pressure. Calculate specific power in kW per 100 cfm for each compressor and for the system.
Map high-pressure uses and identify any need for a booster. Use run-hour and amp data to reconcile the 250 kW total. Without this baseline, a VSD or storage proposal is only a guess, and leak savings cannot be separated from production changes.
2. Recommend system improvements and a control sequence. Explain the priority and why the existing VSD is not the main answer.
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Priority: repair leaks, then reduce pressure, then improve storage and sequencing, then optimize the VSD. Leaks are cheap and reduce demand at all hours. Lower pressure reduces unregulated leak flow and compressor power, but confirm end-use minimum pressure and add regulators or a booster for high-pressure tools. Add receiver storage near the peak load and set a tight pressure band so the VSD can trim and fixed-speed units do not run unloaded. Sequence the VSD as the trim machine, not as a base-load machine fighting a fixed-speed unit. Fix the hot compressor room ventilation and check dryer dew point; do not over-dry.
The VSD already exists, so the first gain is controls and storage, not another drive. The current symptom of one unit unloading while the VSD runs at 100% shows a sequencing problem.
3. Estimate the savings and payback for leak repair, pressure reduction, and storage/sequencing. What would you verify after implementation?
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Use the measured baseline. If total demand is 1,100 cfm and 250 kW, average specific power is about 0.227 kW/cfm. Reducing leakage from 25% to 10% cuts 165 cfm, or about 37 kW. At 8,000 hours and $0.09/kWh, savings are about 300,000 kWh and $27,000 per year; a $15,000 leak program pays back in under a year. A 10 psi pressure reduction at about 1% per 2 psi saves roughly 5% of compressor power, about 12.5 kW, or $9,000 per year at low cost. Storage and sequencing may save another 5-10%, about 12-25 kW, or $9,000-$18,000 per year.
Check demand charges and interaction with the existing VSD. Present these as screened estimates, then verify with a post-repair off-shift test. The risk is that production adds a new end use and masks the savings.