To make sure an aerospace subcontractor never stops for lack of air, the safest solution is to split the demand across two screw compressors that take turns, each able to cover essential production on its own. A shop using 2.5 m³/min can run on two S 150 units at 2.2 m³/min each: in normal operation they share the load, and if one is being serviced, the other keeps the essentials running. Traceable records of servicing and air quality complete the set-up for customer audits.
Why is continuity of air critical at a subcontractor?
At an aerospace subcontractor, compressed air feeds machines that stop without it: machining centres, sheet metal presses, valves on surface treatment lines, spray booths, assembly tooling. Losing air causes three problems:
- Late delivery, in a supply chain where lead times are tight.
- Work in progress, which can become non-conforming: a part left too long in a tank, a paint job interrupted, a machining operation stopped mid-cycle.
- A file to justify to the customer: cause of the stoppage, parts affected, actions taken.
Compressed air is therefore one of the production resources to keep under control, just like the machines.
How do you size two compressors for redundancy?
Start from the total demand, then identify what absolutely must keep running.
- Total demand, measured or calculated: take a machining and surface treatment subcontractor with an average consumption of 1,650 l/min.
- 25% safety margin: 1,650 × 1.25 = 2,063 l/min.
- 20% expansion margin: 2,063 × 1.2 = 2,475 l/min, or about 2.5 m³/min.
- Essential share: the machining centres and the treatment line, without the blow guns or the spray booth, account for around 1,300 l/min, so with the same margins 1,300 × 1.25 × 1.2 = 1,950 l/min.
Two S 150 units at 2.2 m³/min each cover the 2.5 m³/min together, and either one alone covers the essential 1.95 m³/min. Another option: an S 220 PM at 3.6 m³/min for production, with an S 110 at 1.7 m³/min for top-up and partial backup. The S series is variable speed: the lead compressor follows demand and the second one starts when demand exceeds it.
| Solution | In normal operation | If one compressor stops |
|---|---|---|
| A single compressor of 3.6 m³/min | everything covered | total shutdown |
| Two compressors of 2.2 m³/min | 4.4 m³/min available | 2.2 m³/min: the essentials keep running |
| 3.6 m³/min + 1.7 m³/min | 5.3 m³/min available | 1.7 or 3.6 m³/min depending on which machine stops |
Our article on the backup compressor and redundancy explains how to sort the air users.
The compressor is not the only weak point
Doubling up the compressor is pointless if all the air passes through a single dryer or a single receiver. Identify the single points in the chain:
- The dryer: a failed dryer lets water through. Provide a bypass with filtration, or one dryer per compressor. The 4-in-1 compressors in the K series each have their own built-in dryer.
- The filters: fit them in parallel on critical stations so you can change an element without shutting off the air.
- The network: a ring main with isolating valves lets you isolate one section without stopping the rest. See a leak-free compressed air network.
- The power supply: two compressors on the same feeder trip together.
How do you organise servicing without stopping production?
Screw compressors are serviced at 500 hours, then every 1,500 hours. With two machines, you service one while the other produces. On three shifts, 1,500 hours come round in less than three months: plan these interventions like those on the machine tools and keep a service kit in stock. The screw compressor maintenance guide details each operation.
For an unexpected breakdown or a peak in workload, compressor hire of screw compressors from 5.5 to 132 kW keeps you going while the repair is done. Prepare a tapping point on the network in advance, with a valve and a coupling, so a hire machine can be connected in a few minutes.
What does an auditor check on compressed air?
During a customer audit or a quality audit, compressed air is often examined where it touches the product: painting, surface preparation, blowing off parts, surface treatment. The auditor looks for evidence that quality is controlled over time:
- The required air class to ISO 8573-1, station by station, and the treatment installed to achieve it. See our guide to ISO 8573-1 air quality.
- The maintenance log for compressors, dryers and filters, with the dates and part numbers of replaced parts.
- Dew point records and, where relevant, quality measurements at the point of use.
- The procedure to follow in the event of a stoppage or drift: who is informed, which parts are quarantined.
Our technicians carry out a compressed air audit of your installation: actual flow, leaks, pressure, air quality. It is a sound basis for building that file, and for checking that your two compressors are sized on real consumption rather than an estimate. The leak cost calculator shows what leaks represent.
The traps of redundancy
- The backup that will not start. A machine that never runs is found to be faulty on the day you need it. Swap the lead machine every week.
- Two compressors fighting each other. Badly set, they start and stop one after the other. Stagger the set pressures.
- A backup that is too small. Check that it really covers the essentials using the calculation above, not just "part of it".
For every use of air in your trade, see our page on the compressor for aerospace. Send us your consumption and your specification with a quote request, or start with the configurator.
For your trade: Compressor for aerospace, aircraft maintenance and subcontracting.