EMC Upgrade Halves Specific Power at a Seawater Desalination Plant

The client operates a large seawater desalination plant in Tianjin. Its compressed air supply has two very different duties: instrument air at about 0.5 MPa for valves and controls, and low-pressure air at about 0.1 MPa for membrane backwash and blow-off. Demand ranges from roughly 10 to 90 m³/min, and the station runs without interruption for more than 8,000 hours a year.

The station relied on six ageing 90 kW screw compressors and was facing four problems:

• falling efficiency and high electricity costs;
• expensive maintenance;
• overheating in summer;
• instrument-air dew points that did not meet specification.

Because one group of machines served both pressure levels, the low-pressure duty was being supplied at instrument-air pressure, wasting energy.

The station was rebuilt under an energy management contract (EMC). The plant made no capital investment and shares the verified savings, and ownership of the equipment transfers to the plant when the contract ends.

• Separate pressure systems. Instrument air and backwash air were split into independent systems, each run at its own pressure.
• Instrument air: two SIND DUALLUBE-VPM series two-stage, permanent-magnet variable-speed screw compressors, 16.6 m³/min each, with a SIND ChillDry CD refrigerated dryer to bring the dew point into specification.
• Backwash and blow-off air: three SIND PureFlow series oil-free screw compressors, 28.5 m³/min each. Oil-free air prevents contamination of the filtered water and protects reverse-osmosis membrane performance.
• Heat-tolerant design: the whole system is rated for 52 °C ambient, which removes the summer overheating problem.
• Full-process monitoring of air volume, power, pressure and dew point, so savings can be verified.

Initial monitoring data show savings above expectations:

• Specific power reduced from 0.1717 to 0.08623 kWh/m³, an initial energy saving of 49.78%.
• About CN¥1,000,000 (≈US$140,800) saved in electricity costs per year.
• More than CN¥100,000 (≈US$14,100) saved in maintenance costs per year under the EMC service scope.
• Instrument-air dew point within specification and no summer overheating.

Separating the two pressure levels and matching each to the right compressor technology halved the station’s specific power. (USD figures at CN¥7.1/US$.)