Piping System
The compressed air distribution system connects the compressor room to points of use. Inadequate design causes pressure drops, condensation, and energy losses.
Network Configurations
Loop System
┌────────────────────────────────────────┐
│ │
│ ○────○────○────○────○────○────○ │
│ │ │ │
└────┴────────────┬───────────────┴─────┘
│
Compressor
Room
| Advantage | Description |
|---|---|
| Bidirectional flow | Air arrives via two paths |
| Lower pressure drop | Flow splits, lower velocity |
| Redundancy | If one section blocks, alternative exists |
| Uniform pressure | Balance at all points |
Application: Large plants, distributed demand.
Branch System
Compressor
Room
│
┌──────────┼──────────┐
│ │ │
○ ○ ○
│ │ │
┌──┴──┐ ┌──┴──┐ ┌──┴──┐
○ ○ ○ ○ ○ ○
| Advantage | Description |
|---|---|
| Lower initial cost | Less total piping |
| Simple to install | Direct routes |
| Easy to expand | Add branches |
| Disadvantage | Description |
|---|---|
| Higher pressure drop | Unidirectional flow |
| No redundancy | Failure affects entire branch |
| Variable pressure | Distant points have less pressure |
Application: Small plants, concentrated demand.
Combined System
┌────────────────────────────────────────┐
│ │
│ ○────○────○────○────○────○────○ │ ← Main loop
│ │ │ │ │ │
└────┴─────────┼──────────────┴────┴─────┘
│
┌──────┼──────┐
○ ○ ○ ← Secondary branches
Combines main loop with branches to specific areas. Optimal for medium and large plants.
Pipe Sizing
Calculation Factors
| Parameter | Unit | Description |
|---|---|---|
| Flow (Q) | CFM, m³/min | Maximum system demand |
| Pressure (P) | PSI, bar | Operating pressure |
| Length (L) | ft, m | Total distance including fittings |
| Allowable drop (ΔP) | PSI, bar | Maximum 3% of pressure |
| Velocity (V) | ft/s, m/s | Maximum 20 ft/s (6 m/s) in mains |
Recommended Velocity
| Line Type | Maximum Velocity |
|---|---|
| Main line | 20 ft/s (6 m/s) |
| Distribution line | 30 ft/s (9 m/s) |
| Service line | 50 ft/s (15 m/s) |
Velocities above recommendations cause:
- Higher pressure drop
- Excessive noise
- Erosion at elbows and fittings
- Condensate entrainment
Quick Sizing Table
Based on: 100 PSI, 3% drop, 100 ft length
| Flow (CFM) | Minimum Diameter |
|---|---|
| 10 | 1/2" |
| 25 | 3/4" |
| 50 | 1" |
| 100 | 1-1/4" |
| 200 | 1-1/2" |
| 400 | 2" |
| 750 | 2-1/2" |
| 1,000 | 3" |
| 2,000 | 4" |
| 4,000 | 6" |
For longer lengths: Increase one size for each doubling of length.
Equivalent Length of Fittings
Fittings add flow resistance. Expressed as equivalent length of straight pipe:
| Fitting | Equivalent Length (in diameters) |
|---|---|
| 90° standard elbow | 30 D |
| 90° long radius elbow | 20 D |
| 45° elbow | 16 D |
| Tee (straight flow) | 20 D |
| Tee (90° flow) | 60 D |
| Ball valve (open) | 3 D |
| Gate valve (open) | 8 D |
| Globe valve (open) | 340 D |
| Swing check valve | 100 D |
| Reducer | 10 D |
| Expander | 25 D |
Example: A 90° elbow in 2" pipe equals 30 × 2" = 60" = 5 ft of straight pipe.
Piping Materials
Material Comparison
| Material | Max Pressure | Max Temp | Corrosion Resistance | Relative Cost |
|---|---|---|---|---|
| Black steel | 300+ PSI | 400°F | Low | $ |
| Galvanized steel | 300+ PSI | 400°F | Medium | $$ |
| Copper type L | 250 PSI | 400°F | High | $$$ |
| Aluminum | 200 PSI | 150°F | High | $$$$ |
| Stainless steel | 300+ PSI | 400°F | Very high | $$$$$ |
| PVC/CPVC | See note | See note | High | $ |
PVC and CPVC are NOT approved for compressed air per OSHA. They can fail catastrophically due to:
- Cyclic pressure fatigue
- UV degradation
- Impact
- Oil contact
Only use plastic systems specifically designed and certified for compressed air.
Black Steel (Schedule 40)
| Advantage | Disadvantage |
|---|---|
| Economical | Internal corrosion |
| High pressure | Scale contaminates air |
| Easy to source | Requires exterior paint |
| Weldable | Heavy |
Use: Main lines, limited budget.
Extruded Aluminum
| Advantage | Disadvantage |
|---|---|
| No corrosion | Lower max pressure |
| Lightweight | High initial cost |
| Quick connections | Requires special tools |
| Smooth interior | Limited to 150°F |
| Modular |
Use: New installations, plants requiring frequent modifications.
Copper Type L
| Advantage | Disadvantage |
|---|---|
| No corrosion | High cost |
| Decades of proven use | Requires brazing |
| Smooth interior | Connections take time |
Use: Permanent installations, instrument air.
Installation
Slope
All horizontal lines must slope toward drain points:
Compressor Drain
○━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━◇
↘ 1-2% slope ↘
| Slope | Application |
|---|---|
| 1% (1/8" per foot) | Minimum acceptable |
| 2% (1/4" per foot) | Recommended |
Drain Points
Install at:
- Low points in the network
- Before regulators
- Before critical equipment
- End of dead-end lines
- Every 100-150 ft in main lines
Main line
━━━━━━━━━━━━━━━━━━━━━━━
│
│ Drop leg
│ (pocket)
▼
┌───┐
│ D │ ← Automatic drain
└───┘
Air Takeoffs
Connection to equipment should be from the top of the pipe:
CORRECT: INCORRECT:
○ ← Takeoff ━━━━━━━━━
━━━━━┿━━━━━ │
│ ○ ← Collects
↓ condensate
Equipment and dirt
Supports
| Diameter | Maximum Support Spacing |
|---|---|
| 1/2" - 1" | 6-8 ft (2-2.5 m) |
| 1-1/4" - 2" | 8-10 ft (2.5-3 m) |
| 2-1/2" - 4" | 10-12 ft (3-3.5 m) |
| 6" and larger | 12-15 ft (3.5-4.5 m) |
Acceptable Pressure Drop
Pressure Drop Budget Distribution
| Component | Maximum Drop |
|---|---|
| Aftercooler | 3 PSI (0.2 bar) |
| Dryer | 3-5 PSI (0.2-0.35 bar) |
| Filters | 3-5 PSI (0.2-0.35 bar) |
| Main piping | 1-2 PSI (0.07-0.14 bar) |
| Distribution piping | 1-2 PSI (0.07-0.14 bar) |
| Point of use FRL | 5-10 PSI (0.35-0.7 bar) |
| Total system | 15-25 PSI (1-1.7 bar) |
Cost of Pressure Drop
For every 2 PSI of additional system drop, approximately 1% more compressor energy is required.
| Extra Drop | Energy Increase | Annual Cost* |
|---|---|---|
| 5 PSI | 2.5% | $625 |
| 10 PSI | 5% | $1,250 |
| 15 PSI | 7.5% | $1,875 |
| 20 PSI | 10% | $2,500 |
*Based on 100 HP compressor, $0.10/kWh, 8,000 h/year