Air Compressor Station Energy Efficiency Retrofit

120 air-jet looms in the workshop, each using 0.7 m³/min, working pressure 4.5 bar, simultaneous use at 90%, total demand 75.6 m³/min. Three shifts, 7,500 hours per year.

Original station had 3 units of 55 kW fixed-speed screw compressors, 10 m³/min each, 7.5 bar, with 3 refrigerated dryers. Produced 7.5 bar air, then pressure regulators at the workshop dropped it to 4.5 bar.

Post-retrofit daily consumption 2,350 kWh. Pre-retrofit 3,100 kWh. Monthly bill from about $4,550 down to about $3,450, at $0.10/kWh. Annual savings around $13,200.

New setup: 2 units of 90 kW low-pressure VFD screw compressors, 22 m³/min each at 4.5 bar, plus 1 unit of 55 kW low-pressure fixed-speed as backup, 15 m³/min. Dryers swapped to 2 low-pressure units. Pressure regulators gone. Direct 4.5 bar supply.

Installed power went from 165 kW to 235 kW. Output actually went down. Two VFD units together, 44 m³/min.

Before, all 3 machines ran full tilt, loading above 95%, never unloading. Paper capacity 30 m³/min, demand 75.6 m³/min, looks like a huge gap. High-pressure air expands after pressure reduction, barely enough. Barely enough means no slack. One machine down and production takes a hit.

Equipment $120,000, old equipment trade-in $17,000, installation $11,000, net investment $115,000. Payback 4.3 years.

Power went up, electricity went down. The specific power numbers look confusing. Pre-retrofit: 165 divided by 75, roughly 2.2. Post-retrofit: 180 divided by 75, roughly 2.4. Just looking at that, the retrofit seems pointless.

Got a backup now. Primary machines need service or break down, production doesn't have to stop and wait.

Monthly average from 93,000 kWh down to 70,500 kWh. Screw compressors last 10 to 15 years. After payback, $13,200 a year is pure savings.

Compressor station electricity is 15 to 25 percent of textile workshop costs. Electricity prices go up, this gets more and more visible.

VFD machines soft-start, less current surge, less motor wear, last a few extra years. Partial load, speed is lower, noise is lower. Network pressure fluctuation down, VFD responds fast. Pressure regulators gone, one less failure point, one less maintenance item.

Looms need 4.5 bar, make 4.5 bar. Output follows the load. This approach works across air-jet loom workshop compressor stations. Lots of old stations run the high-pressure production, pressure-reduction supply setup. Payback 4.3 years. Similar stations can use this as a reference.

Low-pressure dedicated machines have rotor profiles and compression ratios designed for low pressure. Regular machines running low pressure, noticeably worse efficiency.

Equipment running full load long-term wears faster, more breakdowns, higher maintenance. The original 3 machines were exactly in that state. Any little demand swing and the network pressure wobbled.

Net investment $115,000, annual savings $13,200, 4.3-year payback. Every year after is profit. Textile industry has plenty of these stations. Similar setup, similar problems.

Back to the specific power thing. Pre-retrofit 2.2, post-retrofit 2.4. Surface level, efficiency looks worse. That number is installed power divided by output, doesn't account for pressure reduction losses or how the machine runs under fluctuating loads. Fixed-speed at low load, specific power looks terrible, because power doesn't drop but output does, number shoots up. VFD at low load, power and output drop together, specific power stays relatively flat.

Another thing easy to miss. High-pressure air expands after pressure reduction. So 30 m³/min at 7.5 bar converts to more than 30 at 4.5 bar. Conversion factor is roughly 7.5 divided by 4.5 times a correction coefficient, varies a bit with temperature and humidity. Original setup met the 75.6 demand because of this expansion. Cost was 21% of compression energy wasted.

Dryers also changed. Low-pressure refrigerated dryers need capacity matched to the compressors. The original 7.5 bar dryers handling 4.5 bar air volume didn't match right, dew point control would have issues. This cost was lumped into the $11,000 installation budget.

Monthly savings of about $1,100 is from actual measurement, not calculation. Ran one month pre and one month post, compared meter readings. $0.10/kWh is the local industrial rate, peak and off-peak averaged.

Textile workshop demand is relatively stable. Looms running, consumption is consistent. Swings mainly from shift changes and servicing. This load profile, VFD savings aren't as dramatic as in applications with wild swings. Electricity savings mainly come from killing that 21% pressure reduction loss. VFD contributes roughly 5 to 8 percent, depends on how different day and night demand is.

Backup machine, 55 kW fixed-speed, normally stays off. Steps in when a primary needs service. Can also run alongside the VFDs during peak demand for extra capacity. Fixed-speed not VFD because the backup runs so few hours, VFD pricing advantage doesn't show up.

Stable network pressure, hard to put a dollar figure on. Used to swing between 4.3 and 4.7 bar. After retrofit, basically 4.5 plus or minus 0.1. Air-jet looms are touchy about pressure. Too low, weft insertion force not enough. Too high, wasted air. Stable pressure helps fabric quality. Can't turn that into dollars.

Nobody measured how many decibels the noise dropped. Noticeably quieter in the compressor room though. VFD at low speed, clearly quieter than fixed-speed at full load. For the people on duty in there, pretty tangible.

Screw compressor lifespan mainly depends on the airend, those screw rotors. VFD airends wear less than fixed-speed, because a lot of the time they're not at full speed. Fixed-speed startup has a big current spike, hurts motor windings and bearings. VFD soft-start, no such problem. Lifespan-wise, VFD lasts a few extra years. Wasn't factored into the payback either.

Air-jet loom workshop stations basically all share these traits: pressure not high, mostly 4 to 5 bar; flow is big, each loom 0.6 to 0.8 m³/min; load relatively steady. Old station thinking was buy standard 7.5 bar screw machines, plenty of headroom, just reduce it. Cheap electricity days, no big deal. Prices go up, that 21% really sticks out.

Low-pressure dedicated VFD machines, more models these past few years, prices came down too. Retrofit economics better than before. Major brands all have 4 to 5 bar low-pressure lines. Plenty of options. Technical difficulty low, mainly equipment swap and piping adjustment. No construction needed. Short downtime.