Two loaded 37 kW screw compressors are not a small problem. In one field-audit example from the local Panrui energy-saving training file, a plant needed 9.9 m³/min at about 6.4 bar, and the two fixed-speed units consumed 75.1 kWh every hour. The existing machines had a combined specific power of 7.47 kW per m³/min. A screw air compressor replacement calculation with one right-sized 55 kW variable-speed drive cut the same load to 56.4 kWh/h, saving 18.7 kW in the first hour and 107,760 yuan over one 300-day year. That is the gap between “the compressors still run” and “the compressed-air system is under control.”

Why Two Small Screw Compressors Cost More Than One Right-Sized Unit

The hidden cost is not the second compressor. It is the way fixed-speed machines match a fluctuating air demand. When load drops, a fixed-speed unit keeps running unloaded or starts and stops, and each transition wastes motor and control energy. Two separate units also mean two air ends, two motors, two oil systems, and two sets of pressure settings to keep aligned.

In the example plant, the audit recorded a 37 kW machine delivering about 6.3 m³/min and a second 37 kW machine delivering about 6.0 m³/min. Together they supplied 9.9 m³/min, but the measured electrical load was 75.1 kW/h. Divide 74 kW of installed motor power by 9.9 m³/min and the result is 7.47 kW per m³/min—the number that determines the electricity bill, not the nameplate alone.

Before choosing a replacement, buyers should benchmark their current compressed-air cost. The full sizing and audit method is explained in the rotary screw air compressor guide.

The Replacement Math: Two 37 kW Units vs One 55 kW VSD

• Existing load: 9.9 m³/min total, supplied by two 37 kW compressors at about 6.4 bar and 6.1 bar.
• Existing power: 75.1 kWh/h combined, or 7.47 kW per m³/min.
• Replacement selection: a 55 kW permanent-magnet VSD machine rated at 12.8 m³/min and 7 bar.
• Replacement power: 5.7 kW per m³/min × 9.9 m³/min = 56.4 kWh/h.
• Hourly saving: 75.1 − 56.4 = 18.7 kW.
• Daily saving: 18.7 kW × 24 h = 449 kWh.
• Annual saving: 449 kWh × 300 days = 134,700 kWh.
• Annual cost saving: 134,700 kWh × 0.8 yuan/kWh = 107,760 yuan.

The 55 kW unit is not undersized. At 9.9 m³/min against a 12.8 m³/min rating, it runs at roughly 77% load with room for demand spikes. The key advantage is that a VSD can follow the real air load instead of forcing the motor to stay near full speed. More detail on how variable-speed control changes part-load efficiency is covered in the variable speed screw air compressor page.

What Buyers Get Wrong About Replacement Savings

A frequent objection is that the 55 kW machine will not handle the existing two-machine peak. The calculation above uses the measured load, not the nameplate. The plant was not using 74 kW of compressed air; it was using 9.9 m³/min. Once the real flow and pressure are recorded, the new unit can be selected against those numbers rather than against a salesman’s guess.

Another mistake is comparing purchase price before specific power. A cheaper fixed-speed replacement may have a lower invoice but the same 7.47 kW per m³/min penalty. The long-term cost is hidden in loaded hours, unloaded hours, and the actual kWh meter, not in the initial quotation.

Finally, do not trust a savings claim without a baseline. Record current flow, pressure, loaded hours, and kWh over at least one full production day. Then ask the supplier to rerun the calculation with the same input data. If the supplier cannot show the arithmetic, the “saving” is not verifiable.

Your Next Step: Turn the 75.1 kWh Baseline Into a Quote

Start with the loaded-hours and kWh data from your existing screw air compressor, then request a replacement calculation based on measured air demand. Ask for three numbers in writing: rated flow, specific power, and annual kWh at your load. Compare those figures against your current 7.47 kW per m³/min baseline before approving the purchase.

For a model recommendation and a field-audit calculation for your plant, contact the Panrui technical team. Bring one full day of compressor kWh and pressure data, and the team can calculate whether a single VSD replacement makes the same 18.7 kW per hour difference in your operation.