Read the axes first
The horizontal axis may show delivery or percentage load, not necessarily speed. The vertical axis may show input power, specific power or efficiency. Lower specific power is better; higher efficiency is better. Graphs at different pressures or scales cannot be directly compared visually.
Hours spent at each point are decisive
For each operating period, determine the required delivery, corresponding input power and duration. Energy is the sum of input power multiplied by time. Include operation below the minimum control range, shutdown and unloaded running. Annual specific energy consumption is then total energy divided by total volume, not the arithmetic mean of the values on the graph.
What a catalogue graph does not tell you
The graph alone does not describe transient behaviour, filter condition or coordination with other machines. Continuous operation at the upper permissible limit is not in itself evidence of harmful operation if the manufacturer allows it. Equally, without data it is not valid to claim that peak efficiency always occurs between 40 and 80% speed.
What to check in practice
- Request delivery–input power points at the same operating pressure.
- Measure the proportion of operating modes across shifts, breaks and the season.
- Check behaviour below the lower limit and the available reserve.
Example scenario
A purely illustrative comparison at the same pressure: machine A draws 20 kW at 3 m³/min, while machine B draws 22 kW. At 6 m³/min, they draw 40 and 38 kW respectively. Over 3,000 hours at the first point and 1,000 hours at the second, A consumes 100,000 kWh and B 104,000 kWh. The advantage at maximum output was therefore not decisive on its own.
| Delivery | Input power A | Input power B | Annual operating time |
|---|---|---|---|
| 3 m³/min | 20 kW | 22 kW | 3 000 h |
| 6 m³/min | 40 kW | 38 kW | 1 000 h |
What to watch out for
- Applying one machine's optimum to all brands and pressures.
- Reading delivery percentages as input power percentages.
- Comparing one machine's maximum output with a different flow on the other.
Why is there no single general efficiency graph?
Without a documented model, pressure and test conditions, the graph would appear more precise than it actually is. The basic principle is explained under variable-speed control, but an investment comparison must use specific performance data.