The Vacuum That Ran Out of Air: Ejector Consumption vs Plant Capacity
A vacuum gripper with four ejectors. The plant compressor could keep up with one ejector, but four ejectors running simultaneously drained the air system. The pressure dropped. Other machines slowed. The ejectors were consuming 80 Nl/min each. Four of them: 320 Nl/min. The plant’s spare capacity was 200 Nl/min. The vacuum system was overloading the compressor.
How much air does an ejector use
A Venturi ejector consumes compressed air to produce vacuum. The air consumption is rated at the supply pressure. A typical small ejector (10 Nl/min vacuum flow) consumes about 15-20 Nl/min of compressed air. A larger ejector (30 Nl/min vacuum) consumes 40-60 Nl/min.
For four ejectors at 20 Nl/min each: 80 Nl/min total. But they don’t all run continuously. They run during the pick (0.5 seconds), then shut off. The duty cycle is about 20%. Average consumption: 80 × 0.2 = 16 Nl/min. That’s small. But during the pick, the peak is 80 Nl/min. If the plant can’t supply 80 Nl/min instantly, the pressure drops.
The options
| Option | Air consumption | Cost |
|---|---|---|
| Standard Venturi ejector (4x) | 80 Nl/min peak | Low |
| Energy-saving ejector (with shutoff) | 20 Nl/min average | Medium |
| Central vacuum pump (oil-sealed) | 0 (electric, not compressed air) | High |
| Vacuum reservoir + timed ejector | 10 Nl/min average | Low |
What we did
1. Energy-saving ejectors. We replaced the standard ejectors with energy-saving ones. These have a vacuum switch that shuts off the air once the vacuum reaches -60 kPa. The ejector runs only until the vacuum is built, then stops. The reservoir holds the vacuum through the move. Average consumption dropped from 80 Nl/min to 15 Nl/min.
2. Vacuum reservoir. We added a 1 L reservoir between the ejectors and the cups. The reservoir pre-charges the vacuum. The ejector runs for 0.2 seconds to fill the reservoir, then shuts off. The cups draw from the reservoir during the pick. The air consumption is minimal.
3. Dedicated air line. We ran a dedicated 10 mm line from the compressor to the vacuum gripper. The gripper doesn’t share air with the rest of the plant. The pressure stays stable during the pick.
The calculation
For a vacuum system, the air consumption is:
Q_air = Q_ejector × duty_cycle
Where Q_ejector is the ejector’s rated consumption (Nl/min) and duty_cycle is the fraction of time the ejector is running. For a standard ejector running 100% of the time: Q = 20 Nl/min. For an energy-saving ejector running 20% of the time: Q = 4 Nl/min. The energy-saving ejector uses 5x less air.
The vacuum air rule: Venturi ejectors consume compressed air. Use energy-saving ejectors with vacuum shutoff. Add a reservoir. Run a dedicated air line. If the vacuum system is draining the plant, switch to energy-saving ejectors or a central vacuum pump. The ejector that ran continuously was wasting air.
