You replace an old compressor when it can no longer keep up, when breakdowns keep coming back, or when it uses a lot of electricity for the air it produces. The new one is not chosen by copying the old one's power: it is sized on the workshop's actual demand, which the old machine's meters let you estimate.
Signs that it is time
- Breakdowns keep coming back and parts are getting hard to find.
- Pressure drops at peak times: the compressor no longer keeps up, or the network loses too much air.
- A major service is coming up: compare its cost and what it brings with replacing the machine.
- A fixed speed compressor spends a lot of time running unloaded, or a piston compressor runs almost non-stop when it is made for intermittent use.
- The workshop has changed: new tools, more people, a new paint station.
One sign alone is not always enough. Two or three together justify running the numbers.
Step 1, read the old machine's meters
Most screw compressors show two meters: total hours and loaded hours, when the machine is actually producing air. Note them down, together with the flow and pressure on the rating plate.
The load factor is the ratio between the two: loaded hours / total hours. Multiplied by the old machine's flow, it gives an estimate of the workshop's average demand.
Example with an old 11 kW fixed speed screw compressor, rated 1,600 l/min at 8 bar:
- Meters: 24,000 total hours, of which 14,400 loaded.
- Load factor: 14,400 / 24,000 = 60%.
- Average demand: 1,600 × 0.60 = 960 l/min.
The average is not enough: the machine must also handle peaks. If the old one held pressure all day, the peak stays below its 1,600 l/min. If pressure dropped at busy times, the peak was higher, and it must be worked out again from the tools.
For a piston compressor, which often has only one meter, start from the list of tools, as in our guide on what size compressor you need. The configurator does this calculation and adds a 25% margin.
Step 2, fix leaks before sizing
A leaking network makes the compressor run for nothing, night included. If you size the new machine without fixing the leaks, you are buying flow to feed them. Walk the network when production is stopped, following our article on how to detect and repair compressed air leaks, then take the readings again if the repairs are significant.
Step 3, choose flow and pressure
Back to the example: 960 l/min on average, peaks below 1,600 l/min, at 8 bar. The S 110 delivers 1,700 l/min at 8 bar: it covers the peak. With variable speed, it follows the 960 l/min average by slowing down, instead of switching between loaded and unloaded running.
Do not copy the old machine's pressure without checking it either. Many compressors are set higher than needed "to be on the safe side". A common rule of thumb is about 7% more energy for each extra bar. Our article on calculating compressor pressure helps you set the right value. Watch the flow at high pressure too: the S 110 delivers 1.4 m³/min at 10 bar and 1.1 m³/min at 13 bar.
Finally, avoid oversizing just to be safe. A machine that is far too big, even with variable speed, spends time at minimum speed or stopped.
Step 4, what to keep and what to change
- The receiver: check its condition, its maximum pressure against the new setting, and its inspections, described in our guide on air receiver regulations. Recalculate its volume with the air receiver size calculator.
- The dryer and filters: they must be sized for the new compressor's flow, not the old one's.
- The type of machine: on a network that already has a receiver and dryer in good condition, a standalone S series compressor is enough. If everything needs redoing, a K series 4-in-1 combines compressor, receiver, dryer and filtration.
- Power supply and room: our screw compressors run on 380 V three-phase. Check the room ventilation too, especially if the new machine is more powerful.
What you gain, a worked example
Back to the old 11 kW fixed speed compressor. It runs 4,000 hours a year, 60% of them loaded, so 1,600 hours unloaded. Unloaded, a fixed speed screw compressor still uses about a quarter of its power:
- 11 kW × 0.25 = 2.75 kW used when unloaded.
- 2.75 kW × 1,600 h = 4,400 kWh a year used without producing air.
That is the share variable speed targets: the motor follows demand instead of running unloaded. Add the effect of a better pressure setting, about 7% less per bar. To put a price on those kWh with your electricity tariff, use the electricity cost calculator.
The other gains show less on the bill, but they count: maintenance that follows the hour meter, with a first service at 500 hours then every 1,500 hours, a 2-year warranty and spare parts available for 15 years.
Planning the replacement
- If you are unsure about sizing, our compressed air audit measures your demand over a week of production, weekend included, and simulates the options with your own figures.
- To keep production running, we offer screw compressor rental from 5.5 to 132 kW during the changeover.
- For the quote, send us a photo of the old machine's rating plate, its hour meters, the set pressure and the list of your equipment in a quote request: we size the replacement with you.