The Compressor That Ran Hot
We sized an air compressor for a machine (article 135). The average demand was 500 NL/min. The compressor was rated for 500 NL/min (100% duty). On the floor, the compressor ran 100% (no rest). It overheated (the thermal protection tripped). The problem: the duty cycle. A compressor can’t run 100% (it needs to rest, to cool). The duty cycle (the % of time it runs) must be under 70–80%. We upsized the compressor (750 NL/min). It ran 66% of the time (500/750), resting 34%. It stayed cool. The mistake was sizing the compressor for the average demand (100% duty) without the duty cycle margin.
Air compressor duty cycle is the % of time the compressor runs. This article covers the sizing.
The Duty Cycle
A compressor runs (to fill the tank) and rests (when the tank is full). The duty cycle is the % of time it runs. For a small shop compressor, the duty cycle is 50–60% (it runs half the time). For a continuous-duty compressor (a rotary screw), it’s up to 100% (but still, margin helps). The average demand (article 135) divided by the compressor’s rating is the duty cycle.
Duty cycle = demand / compressor rating
For demand = 500 NL/min, compressor = 500 NL/min: duty = 100% (no rest, overheats). For compressor = 750 NL/min: duty = 66% (rests 34%). Stay under 70–80% (for a reciprocating compressor). For a rotary screw (continuous), 100% is OK (but margin for future).
The duty cycle rule: Size the compressor so the duty cycle is under 70%. The compressor that overheated was rated for the average demand (100% duty). Upsize (750 NL/min for a 500 demand). The duty cycle = demand/rating. Keep it under 70% (margin).
Step 1: The Receiver Tank (Article 135)
The receiver tank (article 135) cushions the compressor. The compressor fills the tank (runs), then rests (the tank supplies the demand). The tank allows the compressor to run in bursts (not continuously). For a high-demand, intermittent use (a cylinder that cycles every 10 s), the tank provides the air (the compressor refills between cycles). Size the tank (article 135).
Step 2: Reciprocating vs Rotary Screw
A reciprocating (piston) compressor has a duty cycle limit (50–70%). It overheats if it runs 100%. A rotary screw compressor runs continuously (100% duty). For a continuous demand (a 24/7 machine), use a rotary screw. For intermittent use (a small shop), a reciprocating is fine (cheaper).
Step 3: Future Demand
Leave margin (future machines, article 135). A compressor that’s at 70% duty today can’t handle a new machine (it goes to 90%). Size for the future (1.5× the current demand).
| Demand | Compressor Rating | Duty Cycle |
|---|---|---|
| 500 NL/min | 500 NL/min | 100% (overheats) |
| 500 NL/min | 750 NL/min | 66% (good) |
| 500 NL/min | 1000 NL/min | 50% (lots of margin) |
A Compressor Checklist
- What is the average demand? (Article 135?)
- What is the compressor rating? (NL/min?)
- Duty cycle = demand/rating? (Under 70%?)
- Is there a receiver tank? (Article 135?)
- Is it reciprocating? (Duty limit 70%?)
- Is it rotary screw? (Continuous?)
- Is there margin for future? (1.5×?)
- Does the compressor overheat? (Test?)
- Is the pressure holding? (Article 149?)
- Is the maintenance scheduled? (Oil? Filters?)
The Bottom Line
Air compressor duty cycle keeps the duty under 70%. The compressor that overheated was rated for the average demand (100%). Upsize (1.5× the demand). Use a receiver tank (article 135). The compressor that stayed cool wasn’t the smallest one — it had duty cycle margin.
