Roots Blower Inlet Pressure

Roots blower inlet pressure is the absolute pressure at the blower inlet – typically atmospheric pressure at the installation site. Inlet pressure affects flow capacity, pressure ratio, and discharge temperature. Lower inlet pressure (high altitude) reduces mass flow and increases discharge temperature for the same gauge pressure.

Based on field data, inlet pressure is often overlooked in blower sizing. At 5,000 ft elevation, inlet pressure is 12.2 psia vs 14.7 psia at sea level – a 17% reduction. This affects flow correction, pressure ratio, and motor sizing. This guide covers inlet pressure effects, altitude correction, and practical applications.

Table of Contents

• What Is Roots Blower Inlet Pressure?

• Inlet Pressure and Flow

• Inlet Pressure and Pressure Ratio

• Inlet Pressure and Temperature

• Altitude Effect

• Inlet Filter Effect

• Inlet Piping Effect

• Selection Guide

• Frequently Asked Questions

• Final Thoughts

What Is Roots Blower Inlet Pressure?

Roots blower inlet pressure is the absolute pressure at the blower inlet port. For most applications, inlet pressure is atmospheric pressure at the installation site – 14.7 psia at sea level, lower at altitude. Inlet pressure affects density, mass flow, and pressure ratio.

Key concepts:

• Inlet pressure = absolute pressure at blower inlet

• Standard: 14.7 psia (sea level)

• Lower at altitude

• Affects flow, temperature, and power

Based on field data, inlet pressure is a critical factor in blower performance. A 10% drop in inlet pressure reduces mass flow by 10% – and increases discharge temperature by 5–10°F.

Inlet Pressure and Flow

Flow relationship:

• Volume flow (ACFM) is independent of inlet pressure (positive displacement)

• Mass flow is proportional to inlet pressure

Mass flow:
Mass flow = Volume flow × Density
Density ∝ Inlet pressure

Effect of lower inlet pressure:

• Same volume flow = less mass flow

• ACFM unchanged, but mass flow reduced

• Process performance may be affected

Example:

• Sea level: 500 ACFM, density 0.075 lb/ft³, mass flow = 37.5 lb/min

• 5,000 ft: 500 ACFM, density 0.062 lb/ft³, mass flow = 31.0 lb/min

• Mass flow reduction: 17%

Correction:
To maintain mass flow, volume flow must increase.
ACFM required = SCFM × (14.7 / Pinlet)

Inlet Pressure and Pressure Ratio

Pressure ratio formula:
R = Pdischarge (absolute) / Pinlet (absolute)

Effect of lower inlet pressure:

• Same gauge pressure = higher pressure ratio

• Higher pressure ratio = higher discharge temperature

Example – 8 psig discharge:

Effect on temperature:

• Higher pressure ratio = higher discharge temperature

• At 5,000 ft, discharge temperature 15–20°F higher than sea level

Inlet Pressure and Temperature

Discharge temperature formula:
Tdischarge = Tinlet × R^0.286 + ΔTmechanical

Effect of lower inlet pressure:

• Higher pressure ratio = higher discharge temperature

• Higher discharge temperature = oil degradation

Example – 8 psig, 80°F inlet:

Altitude effect:

• 3,000 ft: +5–10°F

• 5,000 ft: +10–15°F

• 10,000 ft: +20–30°F

Altitude Effect

Atmospheric pressure at altitude:

Altitude effects on blower:

Altitude correction:

• ACFM = SCFM × (14.7 / Patm)

• Pressure ratio = (Pdischarge + Patm) / Patm

• Motor derating: 1% per 1,000 ft above 3,300 ft

Inlet Filter Effect

Inlet filter pressure drop:

• Clean filter: 0.5–1.0 inches WC

• Loaded filter: 4–8 inches WC

• 1 inch WC = 0.036 psig

Effect on inlet pressure:

• Filter drop reduces inlet pressure

• Lower inlet pressure = higher pressure ratio

• Higher pressure ratio = higher discharge temperature

Example:

• Sea level: 14.7 psia

• Filter drop: 8 inches WC = 0.29 psig

• Effective inlet pressure: 14.41 psia

• Pressure ratio increase: 0.5–1%

Recommendation:

• Change filters at 6–8 inches WC

• Monitor filter delta-P

• Clean filters maintain inlet pressure

Inlet Piping Effect

Inlet piping losses:

• Friction losses reduce inlet pressure

• Losses increase with flow and pipe length

Design recommendations:

• Inlet velocity: <3,000 ft/min

• Short, straight piping

• No sharp bends

• Larger diameter reduces losses

Effect on performance:

• 1 psig inlet loss = 7% pressure ratio increase

• Higher pressure ratio = higher temperature

• Monitor inlet pressure

Inlet piping checklist:

• Velocity <3,000 ft/min

• Minimal bends

• Short as possible

• No restrictions

Selection Guide

Step 1 – Determine site altitude.
Atmospheric pressure from altitude table.

Step 2 – Correct flow for altitude.
ACFM = SCFM × (14.7 / Patm)

Step 3 – Calculate pressure ratio.
R = (Pdischarge + Patm) / Patm

Step 4 – Check discharge temperature.
Tdischarge = Tinlet × R^0.286 + ΔTmechanical

Step 5 – Derate motor if needed.
Motor capacity decreases at altitude.

Altitude selection example:

Frequently Asked Questions

1. What is roots blower inlet pressure?
Inlet pressure is the absolute pressure at the blower inlet. For most applications, it is atmospheric pressure at the installation site – 14.7 psia at sea level, lower at altitude. Inlet pressure affects flow, temperature, and performance.

2. How does inlet pressure affect flow?
Volume flow (ACFM) is independent of inlet pressure (positive displacement). Mass flow is proportional to inlet pressure – lower inlet pressure = less mass flow. At 5,000 ft, mass flow is 17% less than sea level.

3. How does inlet pressure affect pressure ratio?
Lower inlet pressure = higher pressure ratio (for same gauge pressure). At 5,000 ft, 8 psig = R=1.66 vs 1.54 at sea level. Higher pressure ratio = higher discharge temperature.

4. How does altitude affect blower performance?
Altitude reduces inlet pressure. Mass flow decreases, pressure ratio increases, discharge temperature increases. Motor cooling decreases. Correct flow and motor sizing for altitude.

5. What is the correction for altitude?
ACFM = SCFM × (14.7 / Patm). At 5,000 ft (12.2 psia), correction = 1.20 – need 20% more volume flow for same mass flow.

6. How does inlet filter affect inlet pressure?
Dirty filters cause pressure drop – reducing inlet pressure. 8 inches WC drop = 0.29 psig reduction. Lower inlet pressure = higher pressure ratio = higher discharge temperature. Change filters at 6–8 inches WC.

7. How does inlet piping affect inlet pressure?
Piping losses reduce inlet pressure. Design for velocity <3,000 ft/min. Short, straight piping minimizes losses. 1 psig loss = 7% pressure ratio increase.

8. How does inlet pressure affect motor power?
Lower inlet pressure = lower density = less mass flow = less power. Power decreases with inlet pressure. But motor cooling also decreases – derate motor at altitude.

9. What is the effect of inlet pressure on discharge temperature?
Lower inlet pressure = higher pressure ratio = higher discharge temperature. At 5,000 ft, discharge temperature is 10–15°F higher than sea level for same gauge pressure.

10. How do I size a blower for high altitude?
Correct flow: ACFM = SCFM × (14.7 / Patm). Calculate pressure ratio with local atmospheric pressure. Check discharge temperature. Derate motor 1% per 1,000 ft above 3,300 ft.

11. What is the effect of inlet pressure on volumetric efficiency?
Lower inlet pressure = lower density = more slipback = lower volumetric efficiency. Effect is small (1–2%) but noticeable at high altitude.

12. How do I measure inlet pressure?
Install pressure gauge or transducer at blower inlet. Measure absolute pressure. Compare to atmospheric pressure – difference indicates inlet losses.

13. What is the maximum inlet pressure?
Standard blowers are designed for atmospheric inlet. Vacuum blowers handle lower inlet pressure. High pressure inlet (boosted) requires special design – consult manufacturer.

14. How does inlet temperature affect inlet pressure?
Temperature affects density but not pressure. Higher temperature = lower density = lower mass flow (same volume). Correct for temperature separately.

15. When should I consider inlet pressure in blower selection?
Always – but especially at altitude (>3,000 ft), with long inlet piping, or with dirty filters. Correct flow and pressure ratio for inlet conditions. Zhanggu and other manufacturers provide altitude correction data.

Final Thoughts

After decades of roots blower inlet pressure analysis, here is my practical advice:

Inlet pressure matters. Lower inlet pressure (altitude, filters, piping) reduces mass flow, increases pressure ratio, and raises discharge temperature. 5,000 ft = 17% mass flow reduction. Correct sizing for site conditions.

Altitude correction is essential. ACFM = SCFM × (14.7 / Patm). At 5,000 ft, 20% more volume flow needed for same mass flow. Zhanggu and other manufacturers provide altitude correction data.

Monitor inlet pressure. Inlet filter pressure drop reduces inlet pressure. Change filters at 6–8 inches WC. Long inlet piping adds losses. Design for minimum losses.

The bottom line. Roots blower inlet pressure is a critical performance parameter. Zhanggu and other manufacturers provide data for altitude and inlet conditions. Correct flow for inlet pressure. Monitor filter pressure drop. The investment in correct sizing pays back through reliable operation.