EngiMetric Arena / Diagnose the Failure
Why is the compressor cycling every 25 seconds?
Competency Fault diagnosis primary Systems and balances supporting Quantitative reasoning supporting
What you should take away: Distinguish receiver storage from compressor capacity by comparing the available run time with the required run time.
The situation
What is happening
A compressed-air network draws 30 L/s of free air. The receiver is 500 L and cycles between 8 bar g and 6.5 bar g at 30 C. The compressor keeps up on average, but it loads and unloads about every 25 seconds, and the required receiver run time is 2 minutes. What is the plausible cause?
Given
| Quantity | Value | Unit | Note |
|---|---|---|---|
| Free-air demand | 30 | L/s | Network demand at standard conditions. |
| Receiver volume | 500 | L | Installed vessel. |
| Maximum pressure | 8 | bar g | Top of the pressure swing. |
| Minimum pressure | 6.5 | bar g | Bottom of the pressure swing. |
| Required run time | 2 | min | Network run time between compressor cycles. |
Assumptions this challenge makes
- The receiver discharges isothermally, which is the conventional sizing assumption.
- Atmospheric pressure is 1.013 bar absolute and the gas temperature is 30 C.
- The compressor capacity is adequate on average; the complaint is cycling, not low pressure.
Your call
Your answer
Hints
- Compute the free air the receiver releases over the 1.5 bar gauge swing. Remember to use absolute pressures.
- Divide the free-air volume by the 30 L/s demand to get the available run time.
- Compare the available run time with the 2 minute requirement. Does the compressor capacity enter that comparison?
Your answer Engine result
Where that comes from
Each limit, checked
Why
- The pressure swing is 1.5 bar. At 1.013 bar absolute reference, the receiver releases 500 x 1.5 / 1.013 = 740 L of free air.
- At 30 L/s, that lasts 740 / 30 = 24.7 s, not the required 120 s.
- The volume needed for 120 s is 30 x 120 / (1.5 / 1.013) = 2 431 L.
- The installed 500 L receiver is therefore about one-fifth of the required storage. The compressor can be adequately sized and still cycle rapidly if the receiver is too small.
- The calculation is isothermal and does not model control strategy, leaks or compressor unloading. Those are separate checks, but they do not change the storage shortfall.
What this challenge does not establish
- The receiver is modelled isothermally; real charge and discharge are closer to adiabatic.
- The compressor control strategy, unloading time and duty cycle are not modelled.
- The 30 L/s demand is an input; actual demand and leakage should be measured.
- Pressure-vessel code allowances, drains and pipe sizing are separate design checks.