Roots Blower Compression Ratio
Roots Blower Compression Ratio
Roots blower compression ratio is the ratio of discharge pressure to inlet pressure – a critical parameter that determines discharge temperature, efficiency, and operating limits. Unlike screw compressors, roots blowers have no internal compression – the compression ratio is determined by system resistance, not by rotor geometry. Higher compression ratios mean higher discharge temperatures and lower efficiency.
Based on field data, compression ratio is the single most important factor in discharge temperature. At 8 psig, pressure ratio is 1.54 – discharge temperature 185–200°F. At 15 psig, pressure ratio is 2.02 – discharge temperature 210–240°F. At 20 psig, pressure ratio is 2.36 – discharge temperature 250–280°F.
This guide covers compression ratio calculation, effect on performance, temperature rise, and operating limits.
Table of Contents
What Is Roots Blower Compression Ratio?
How Compression Ratio is Calculated
Compression Ratio and Temperature
Compression Ratio and Efficiency
Operating Limits
Compression Ratio vs Pressure
Altitude Effect
Selection Guide
Frequently Asked Questions
Final Thoughts
What Is Roots Blower Compression Ratio?
Roots blower compression ratio is the ratio of absolute discharge pressure to absolute inlet pressure. It is a dimensionless number that indicates how much the pressure is increased across the blower.
Compression ratio formula:
Compression Ratio = Pdischarge (absolute) / Pinlet (absolute)
Example:
Inlet: 14.7 psia (sea level)
Discharge: 8 psig = 22.7 psia
Compression Ratio = 22.7 / 14.7 = 1.54
Key points:
Roots blowers have no internal compression
Compression ratio is created by system resistance
Higher compression ratio = higher discharge temperature
Higher compression ratio = lower efficiency
Based on field data, typical compression ratios for roots blowers are 1.2–2.0. Above 2.0, efficiency drops significantly and temperature rises rapidly.
How Compression Ratio is Calculated
Absolute pressure:
Inlet absolute = atmospheric pressure (14.7 psia at sea level)
Discharge absolute = gauge pressure + atmospheric pressure
Formula:
R = (P2 + Patm) / Patm
Where:
R = compression ratio
P2 = discharge pressure (psig)
Patm = atmospheric pressure (psia)
Examples:
| Discharge Pressure (psig) | Discharge Absolute (psia) | Compression Ratio |
|---|---|---|
| 3 | 17.7 | 1.20 |
| 5 | 19.7 | 1.34 |
| 8 | 22.7 | 1.54 |
| 10 | 24.7 | 1.68 |
| 12 | 26.7 | 1.82 |
| 15 | 29.7 | 2.02 |
| 20 | 34.7 | 2.36 |
At altitude:
At 5,000 ft, atmospheric pressure = 12.2 psia
8 psig = 20.2 psia
Compression Ratio = 20.2 / 12.2 = 1.66
Higher ratio than sea level for same gauge pressure
Compression Ratio and Temperature
Discharge temperature formula:
Tdischarge = Tinlet × R^((γ-1)/γ) + ΔTmechanical
Where:
Tdischarge = absolute discharge temperature (°R)
Tinlet = absolute inlet temperature (°R)
R = compression ratio
γ = specific heat ratio (1.4 for air)
ΔTmechanical = mechanical heating (30–50°F)
Theoretical temperature rise:
| Compression Ratio | Theoretical Temp Rise (°F) | Actual Typical (°F) |
|---|---|---|
| 1.20 | 27 | 50–60 |
| 1.34 | 48 | 75–90 |
| 1.54 | 73 | 105–120 |
| 1.68 | 90 | 125–145 |
| 1.82 | 107 | 145–170 |
| 2.02 | 132 | 175–210 |
| 2.36 | 158 | 240–270 |
Key insight:
Temperature rise increases with compression ratio
At 8 psig (R=1.54): 185–200°F
At 15 psig (R=2.02): 210–240°F
At 20 psig (R=2.36): 250–280°F
Temperature limits:
Below 220°F: normal operation
220–250°F: monitor closely
Above 250°F: oil degradation
Above 275°F: risk of rotor contact
Compression Ratio and Efficiency
How compression ratio affects efficiency:
| Compression Ratio | Efficiency (3-lobe) |
|---|---|
| 1.20 | 72–77% |
| 1.34 | 72–78% |
| 1.54 | 72–78% |
| 1.68 | 70–76% |
| 1.82 | 68–74% |
| 2.02 | 65–72% |
| 2.36 | 60–68% |
Why efficiency drops:
Higher compression ratio = more slipback
More slipback = more leakage
More leakage = lower volumetric efficiency
Lower volumetric efficiency = lower overall efficiency
Best efficiency range:
Compression ratio 1.3–1.7 (5–10 psig)
Lowest slipback
Moderate temperature
Peak efficiency
Efficiency comparison:
| Pressure | Compression Ratio | Efficiency |
|---|---|---|
| 5 psig | 1.34 | 72–77% |
| 8 psig | 1.54 | 72–78% |
| 10 psig | 1.68 | 70–76% |
| 12 psig | 1.82 | 68–74% |
| 15 psig | 2.02 | 65–72% |
Operating Limits
Compression ratio limits:
| Blower Type | Max Compression Ratio | Max Pressure |
|---|---|---|
| Standard | 2.0 | 15 psig |
| High pressure | 2.5 | 20–25 psig |
| Intermittent | 2.7 | 25 psig |
What limits compression ratio:
1. Temperature.
Higher ratio = higher temperature
Above 250°F: oil degrades
Above 275°F: rotor contact risk
2. Slipback.
Higher ratio = more slipback
Reduced flow
Lower efficiency
3. Bearing load.
Higher pressure = higher bearing load
Reduced bearing life
4. Motor power.
Power = flow × pressure
Higher pressure = more power
Compression ratio vs operating limits:
| Compression Ratio | Pressure (psig) | Temperature | Recommended |
|---|---|---|---|
| 1.3–1.7 | 5–10 | <220°F | Continuous |
| 1.7–2.0 | 10–15 | 220–250°F | Monitor |
| 2.0–2.3 | 15–20 | 250–280°F | Water cooling |
| >2.3 | >20 | >280°F | Not recommended |
Compression Ratio vs Pressure
Understanding gauge vs absolute:
| Gauge Pressure (psig) | Absolute Pressure (psia) | Compression Ratio |
|---|---|---|
| 5 | 19.7 | 1.34 |
| 8 | 22.7 | 1.54 |
| 10 | 24.7 | 1.68 |
| 12 | 26.7 | 1.82 |
| 15 | 29.7 | 2.02 |
At altitude:
At 5,000 ft (12.2 psia):
| Gauge Pressure (psig) | Absolute Pressure (psia) | Compression Ratio |
|---|---|---|
| 5 | 17.2 | 1.41 |
| 8 | 20.2 | 1.66 |
| 10 | 22.2 | 1.82 |
| 12 | 24.2 | 1.98 |
| 15 | 27.2 | 2.23 |
Key insight:
Same gauge pressure = higher compression ratio at altitude
Higher compression ratio = higher temperature
Derate blower at altitude
Altitude Effect
Atmospheric pressure at altitude:
| Elevation (ft) | Atmospheric Pressure (psia) | Correction Factor |
|---|---|---|
| 0 | 14.70 | 1.00 |
| 1,000 | 14.17 | 1.04 |
| 2,000 | 13.66 | 1.08 |
| 3,000 | 13.17 | 1.12 |
| 4,000 | 12.69 | 1.16 |
| 5,000 | 12.23 | 1.20 |
Altitude effect on compression ratio:
Lower atmospheric pressure = higher compression ratio
Higher compression ratio = higher discharge temperature
Derate blower at altitude
Altitude derating:
5,000 ft: compression ratio 8% higher
10,000 ft: compression ratio 18% higher
Reduce pressure or add cooling
Selection Guide
Step 1 – Determine compression ratio.
Calculate based on required pressure and site altitude.
Step 2 – Check temperature.
Calculate discharge temperature based on compression ratio. Ensure below 220°F for continuous operation.
Step 3 – Verify efficiency.
Check efficiency at compression ratio. If efficiency too low, consider alternative technology.
Step 4 – Consider altitude.
Correct compression ratio for altitude. Higher altitude = higher ratio = higher temperature.
Step 5 – Specify upgrades.
If compression ratio >1.7: consider C4 bearings, stainless rotors, water cooling.
Selection example:
| Parameter | Value |
|---|---|
| Required pressure | 12 psig |
| Site altitude | 0 ft (14.7 psia) |
| Compression ratio | 1.82 |
| Discharge temperature | 210–230°F |
| Efficiency | 70–74% |
| Recommendation | Standard blower with monitoring |
High altitude example:
| Parameter | Value |
|---|---|
| Required pressure | 12 psig |
| Site altitude | 5,000 ft (12.2 psia) |
| Compression ratio | 1.98 |
| Discharge temperature | 230–260°F |
| Efficiency | 68–72% |
| Recommendation | High pressure design, water cooling |
Frequently Asked Questions
1. What is roots blower compression ratio?
Compression ratio is the ratio of discharge absolute pressure to inlet absolute pressure. It indicates how much pressure is increased across the blower. Roots blowers have no internal compression – the ratio is created by system resistance.
2. How is compression ratio calculated?
Compression Ratio = (discharge pressure + atmospheric pressure) / atmospheric pressure. Example: 8 psig at sea level = (8 + 14.7) / 14.7 = 1.54.
3. How does compression ratio affect temperature?
Higher compression ratio = higher discharge temperature. At 8 psig (R=1.54): 185–200°F. At 15 psig (R=2.02): 210–240°F. Temperature rise is approximately 20–30°F per 0.1 compression ratio increase.
4. What is the maximum compression ratio?
Standard blowers: 2.0 (15 psig). High pressure: 2.5 (20–25 psig). Above 2.0, efficiency drops and temperature rises rapidly. Above 2.5, screw compressors are more efficient.
5. How does compression ratio affect efficiency?
Efficiency drops at higher compression ratios. At R=1.54: 72–78%. At R=2.02: 65–72%. At R=2.36: 60–68%. Best efficiency at R=1.3–1.7.
6. What is the effect of altitude on compression ratio?
Altitude reduces atmospheric pressure – compression ratio increases for same gauge pressure. At 5,000 ft, 8 psig = R=1.66 vs 1.54 at sea level. Higher ratio = higher temperature. Derate blower at altitude.
7. How does compression ratio affect slipback?
Higher compression ratio = more slipback (leakage through tip clearance). More slipback = reduced volumetric efficiency. Tighter clearance reduces slipback.
8. What is the compression ratio at 10 psig?
At sea level: (10 + 14.7) / 14.7 = 1.68. At 5,000 ft: (10 + 12.2) / 12.2 = 1.82. Altitude increases compression ratio.
9. Why do roots blowers have no internal compression?
Roots blowers trap a fixed volume and move it – they do not reduce volume. Compression occurs only when air is discharged against system pressure. This is why compression ratio is determined by system resistance.
10. What is the relationship between compression ratio and pressure?
Compression ratio increases with pressure. For a given atmospheric pressure, higher gauge pressure = higher compression ratio. The relationship is linear but not proportional.
11. How does compression ratio affect motor power?
Power = flow × pressure / efficiency. Higher compression ratio = higher pressure = more power. Power increases linearly with pressure (for same flow).
12. What is the compression ratio for vacuum operation?
Vacuum compression ratio is less than 1.0 (inlet below atmospheric). Vacuum ratio = Pinlet / Patm. Example: 10 inches Hg vacuum = 9.79 psia / 14.7 = 0.67.
13. How do I reduce compression ratio?
Reduce discharge pressure. Increase inlet pressure (not possible). Change operating point. Use larger blower at lower pressure.
14. What is the effect of compression ratio on bearing life?
Higher compression ratio = higher pressure = higher bearing load. Bearing life decreases with pressure. At 15 psig, bearing life is 60% of normal. Use C4 bearings for high pressure.
15. When should I use a screw compressor instead of roots blower?
When compression ratio >2.0 (15 psig). Screw compressors have internal compression – more efficient at high compression ratios. At R=2.0+, screw efficiency is 5–10% higher.
Final Thoughts
After decades of roots blower compression ratio analysis, here is my practical advice:
Compression ratio drives temperature. Higher ratio = higher discharge temperature. At 8 psig (R=1.54): 185–200°F. At 15 psig (R=2.02): 210–240°F. At 20 psig (R=2.36): 250–280°F. Monitor temperature closely.
Efficiency drops at high compression ratio. At R=1.54: 72–78%. At R=2.02: 65–72%. Above R=2.0, efficiency penalty is significant. Consider screw compressors for high compression ratios.
Altitude increases compression ratio. At 5,000 ft, compression ratio is 8% higher for same gauge pressure. Higher ratio = higher temperature. Derate blowers at altitude. Zhanggu and other manufacturers provide altitude correction data.
The bottom line. Roots blower compression ratio is a critical performance parameter. Zhanggu and other manufacturers specify maximum compression ratios. Stay within limits. Monitor temperature. Add cooling for high ratios. The investment in proper selection pays back through reliable operation.
