| name | air-preparation |
| description | Compressed air preparation — filtration/regulation/lubrication (FRL), drying (refrigerant/desiccant), air quality ISO 8573-1, dew point, pressure drop, compressor selection, ISO 4414. |
| metadata | {"priority":7,"promptSignals":{"phrases":["air preparation","FRL unit","compressed air quality","air dryer","ISO 8573","desiccant dryer","refrigerant dryer"],"minScore":3}} |
Compressed Air Preparation — Complete Skill
ISO 8573-1 Air Quality Classification
Three quality parameters:
| Class | Particles (max) | Water (pressure dew point) | Oil (total) |
|---|
| 1 | 0.1 μm; 0.1 mg/m³ | -70°C | 0.01 mg/m³ |
| 2 | 1 μm; 1 mg/m³ | -40°C | 0.1 mg/m³ |
| 3 | 5 μm; 5 mg/m³ | -20°C | 1 mg/m³ |
| 4 | 15 μm; 8 mg/m³ | +3°C | 5 mg/m³ |
| 5 | 40 μm; 10 mg/m³ | +7°C | 25 mg/m³ |
Notation: ISO 8573-1:2010 [solid:water:oil] e.g., Class 1.2.1
Application requirements:
| Application | Typical class |
|---|
| Pneumatic cylinders (no lube) | 3.4.3 |
| Spray painting | 1.2.1 |
| Food contact | 1.2.1 |
| Air bearing | 1.1.1 |
| Instrument air | 2.2.2 |
| Breathable air | 1.2.1 + CO check |
Compressor Selection
Reciprocating (piston):
Flow: 0.5–30 m³/min; Pressure: up to 40 bar; Pulsating flow → needs receiver tank
Oil-lubricated or oil-free (for clean air applications)
Rotary screw:
Flow: 0.5–500 m³/min; Pressure: 6–13 bar (single stage); most common industrial
Oil-flooded (oil separator required downstream) or oil-free (no separator needed)
Centrifugal (turbo):
Flow: > 100 m³/min; Pressure: 6–10 bar; oil-free; best for large industrial plants
Power estimation:
P_comp = (m_dot × c_p × T_in / η_iso) × [(P_out/P_in)^((γ-1)/γ) - 1] [kW; c_p = 1.005 kJ/(kg·K); γ = 1.4 for air]
η_iso = 0.65–0.80 (isentropic efficiency)
Receiver tank volume:
V_tank = (Q_comp - Q_demand) × Δt / ΔP [m³; Q in m³/s at working pressure; ΔP = pressure band [Pa]]
Minimum: V_tank ≥ 5–10 × Q_comp [L; Q in L/min] for pressure stabilization
Filtration
Coalescing filter:
Removes liquid water and oil aerosols; 0.01–5 μm particle removal
Efficiency: 99.999% at rated flow; replace element every 4000–8000 hr
Pressure drop: 0.1–0.3 bar new; replace at > 0.5 bar
Particulate filter:
Removes solid particles; 5–40 μm rating typical
Pre-filter before coalescing; protects expensive downstream elements
Adsorption filter (activated carbon):
Removes oil vapors; reduces oil to < 0.003 mg/m³
Place downstream of coalescing filter; limited capacity → replace regularly
Filter sizing:
Use manufacturer flow curves; verify ΔP < 0.2 bar at max flow
Size for 110–120% of nominal flow to extend element life
Drying
Refrigerant Dryer (most common)
Operation: cool air below dew point → condensate out; reheat to reduce downstream humidity
Pressure dew point (PDP) achievable: +3°C to +10°C (ISO Class 4 or 5 moisture)
Power: 0.5–5 kW (depends on flow)
Limitations: cannot achieve PDP < +3°C; not suitable for outdoor or below-freezing conditions
PDP at system pressure:
PDP_at_P = PDP_at_1bar + ΔT_correction [ΔT = 15°C per bar increase approximately]
At 7 bar: if dryer gives +3°C PDP, actual PDP at line ≈ +3°C (dryer already accounts for pressure)
Desiccant Dryer (heatless or heated)
Heatless (PSA): two towers with activated alumina or molecular sieves; one drying, one regenerating
Purge air: 15–20% of inlet flow used to regenerate (net output = 80–85% of inlet)
PDP achievable: -20°C to -70°C (ISO Class 2 or 1)
Cycle time: typically 5–10 min per tower
Heated desiccant (HOC — heat of compression): uses compressor heat for regeneration; zero purge air loss; energy efficient
Desiccant life: 2–5 years (activated alumina); replace when PDP rises 5°C above spec
PDP calculation (desiccant):
PDP_required = lowest ambient operating T - 10°C safety margin [e.g., outdoor at -20°C → PDP = -30°C → Class 2]
Pressure Regulation
Pressure regulator:
Set at 10% below system minimum required pressure (to account for line pressure drop)
Select Cv for ΔP at maximum flow: Q = Cv × √(ΔP/SG) [Q in US gal/min; SG = 1 for air]
System pressure drop budget:
Filter: 0.1–0.3 bar; dryer: 0.1–0.2 bar; regulator: 0.2–0.5 bar; piping + fittings: 0.2–0.5 bar
Total: allow 1.0–1.5 bar pressure drop from compressor to point of use
Example:
System requires 6 bar at tools; losses 1.2 bar → set compressor to 7.5–8 bar
Lubrication (FRL — Filter/Regulator/Lubricator)
For lubricated pneumatic tools only:
Oil mist injected into air stream; flow-dependent injection
Setting: 1 drop/min per 10 L/min flow (typical starting point)
When NOT to lubricate:
Servo valves; air bearings; food/pharma; any instrument that will be damaged by oil
Modern pneumatic cylinders (ISO 15552) often rated for unlubricated service
Piping and Distribution
Velocity limits:
Mains: 6–9 m/s; branches: 3–6 m/s; drops: 1–3 m/s
Pipe diameter (simplified):
D = √(Q_max × L / (490 × ΔP_allowed)) [mm; Q in m³/hr; L in m; ΔP in bar]
Use ring main (loop) topology to reduce pressure drop and allow bidirectional flow
Slope: pipe pitched 1:200 toward drain points (condensate removal)
Isolation valves: every 30–50 m for maintenance; quick-connects at point of use
Standards
| Standard | Scope |
|---|
| ISO 8573-1 | Air quality classes |
| ISO 8573-2 | Particle measurement |
| ISO 4414 | Pneumatic fluid power safety |
| CAGI/Compressed Air Challenge | Best practices for energy efficiency |
Output
Provide: ISO 8573-1 air quality class required [solid.water.oil], pressure dew point [°C], compressor type and delivery pressure [bar], flow rate [m³/min], receiver tank volume [L], filtration stages (particulate → coalescing → activated carbon), dryer type (refrigerant/desiccant) and achievable PDP [°C], regulator set pressure [bar], system pressure drop budget [bar], pipe sizing [mm], and applicable standard (ISO 8573-1, ISO 4414).
