Compressed-air capacity versus pressure-drop in an electronics plant
An electronics assembly plant has three oil-injected screw compressors: two 132 kW fixed-speed and one 132 kW variable-speed, all rated at 7.5 bar(g). Average site demand is 21 m³/min. Logs show the VSD unit at 40% and one fixed-speed unit running unloaded for 35% of the time. Pressure is 7.2 bar(g) at the compressor discharge but 5.4 bar(g) at the furthest SMT line. A 0.6 bar drop occurs across the refrigerated dryer and filters. A leak survey estimates leakage at 22% of demand. The production manager wants a fourth 132 kW compressor and a larger receiver, quoting a supplier who says the system is 'undersized'. You are asked to review the proposal against the measured data. The plant operates 8,000 hours per year and pays MYR 0.45 per kWh.
1. Estimate installed and actual capacity, and determine whether the low line pressure is a capacity problem or a pressure-drop/control problem. Show your assumptions.
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Capacity check For a typical 132 kW oil-injected screw compressor at 7.5 bar(g), delivered capacity is roughly 23–25 m³/min. Take 23 m³/min each: installed capacity is about 3 × 23 = 69 m³/min, and connected demand is 21 m³/min, or about 30% of installed capacity. Even allowing for one unit as standby, two units provide about 46 m³/min, more than twice average demand.
Pressure-drop check Discharge pressure is 7.2 bar(g), line pressure 5.4 bar(g), so total drop is 1.8 bar. Of this, 0.6 bar is across the dryer and filters, leaving about 1.2 bar in the distribution network. That is excessive for a well-designed system and explains why the far SMT line is starved even though compressor capacity is available.
Control check One fixed-speed unit running unloaded 35% of the time, with the VSD at only 40%, shows poor sequencing and storage control. Unloaded running consumes power without useful air. The evidence therefore points to pressure drop, leakage, and control inefficiency, not insufficient compressor capacity. A fourth compressor would add capital and likely worsen part-load control.
2. Quantify the likely savings from leak reduction and pressure management, and rank the measures.
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Leak reduction Leakage is 22% of 21 m³/min = 4.6 m³/min. Using a specific power of about 6.5 kW per m³/min at this pressure, the leak load is roughly 4.6 × 6.5 ≈ 30 kW. At 8,000 h/year this is about 240,000 kWh/year, or MYR 108,000/year at MYR 0.45/kWh. A leak-tagging and repair programme can typically remove 50–80% of this over time, so the first-year saving may be partial but the annualised potential is substantial.
Pressure management Lowering compressor discharge pressure by 0.5 bar can reduce specific power by roughly 3% per bar, so about 1.5% of compressor power. For 132 kW that is roughly 2 kW per running unit, plus reduced artificial demand from leaks. The larger benefit is eliminating the 1.8 bar drop: replacing clogged filters and resizing the dryer/filter train and distribution mains restores line pressure without raising compressor pressure.
Ranking
- Leak repair and leak tagging - lowest cost, largest avoidable waste.
- Dryer/filter and distribution pressure-drop correction - restores production pressure and reduces artificial demand.
- Master controller and storage - stops unloaded running and stabilises VSD trim.
- Fourth compressor - poor economic choice until 1–3 are complete. Measure post-implementation flow and power to verify savings.
3. Specify the pressure/quality and controls changes you would require, including how to verify savings.
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Pressure and quality
- Set compressor discharge at the lowest pressure that satisfies the critical SMT nozzle pressure, not at the current 7.2 bar(g).
- Replace or clean the dryer and filters when differential pressure exceeds the manufacturer’s limit; select a low-pressure-drop dryer and filter train.
- Verify air quality against the plant’s ISO 8573-1 class for particulates, water, and oil; do not sacrifice dew point for pressure drop.
- Check receiver sizing and location; a receiver near the high-demand SMT area can buffer pressure fluctuations.
Controls
- Install a master controller with the VSD as trim and fixed-speed units as base load; set dead bands to prevent hunting.
- Lock out unnecessary unloaded running and sequence the standby unit automatically.
- Add flow, pressure, and power metering at the compressor room and at the SMT line.
Verification Establish a baseline of kW per m³/min and line pressure before changes. After each measure, re-log for at least two weeks at similar production. Verify leak reduction by repeat ultrasonic survey, pressure drop by differential gauges, and energy savings by metered kWh. Use a measurement-and-verification protocol such as IPMVP Option B (retrofit isolation) so savings are defensible.