How to Increase Roots Blower Pressure

Increasing roots blower pressure is possible within limits – but there are significant engineering and safety considerations. A roots blower delivers constant volume; pressure is created by system resistance. To increase pressure, you must increase system resistance, increase speed, or upgrade the blower. But each method has limits: temperature rise, motor overload, and component stress.

Based on field data, increasing pressure from 8 psig to 10 psig increases power by 25% and discharge temperature by 20–30°F. Exceeding the blower's design pressure causes bearing failure, rotor contact, and motor overload. Understanding the limits is essential for safe operation.

This guide covers methods to increase pressure, system modifications, component upgrades, and safety considerations.

Table of Contents

• Can You Increase Roots Blower Pressure?

• How Pressure Is Created

• Method 1: Increase System Resistance

• Method 2: Increase Speed (RPM)

• Method 3: Upgrade Components

• Method 4: Stage Blowers in Series

• Limits of Pressure Increase

• Safety Considerations

• When to Upgrade to a Larger Blower

• Frequently Asked Questions

• Final Thoughts

Can You Increase Roots Blower Pressure?

Yes – but within limits. A roots blower delivers constant volume. Pressure is determined by downstream system resistance. To increase pressure, you must increase the resistance the blower pushes against – or increase speed to deliver more volume against the same resistance.

Methods to increase pressure:

1. Increase system resistance (restrict flow)

2. Increase speed (RPM)

3. Upgrade components (higher pressure design)

4. Stage blowers in series

Based on field data, pressure can be increased by 2–3 psig by increasing speed or adding restriction. Beyond that, component upgrades or staging are required. Exceeding the blower's design pressure causes:

• Higher discharge temperature (oil degradation)

• Higher motor current (overload)

• Increased bearing load (shorter life)

• Thermal expansion (rotor contact)

How Pressure Is Created

Roots blower pressure generation:

• The blower delivers constant volume (ACFM) at a given speed

• The downstream system (pipes, valves, diffusers, filters) creates resistance

• Pressure = resistance × flow

• The motor draws power proportional to pressure × flow

Key relationship:

• Pressure is determined by the system, not the blower

• Increasing system resistance increases pressure

• Increasing speed increases flow, which increases pressure (if system resistance is fixed)

The limitation:

• The blower is rated for maximum pressure

• Exceeding rating causes damage

• Higher pressure = higher temperature = higher power

Method 1: Increase System Resistance

How it works:
Restrict the flow downstream of the blower. The blower pushes against the restriction, creating higher pressure.

Methods to increase resistance:

1. Partially close a discharge valve

2. Add a flow restriction (orifice plate)

3. Increase filter/diffuser resistance

4. Smaller diameter piping

Effect:

• Pressure increases

• Flow decreases (slightly – slipback increases)

• Power increases (pressure × flow)

• Temperature increases

Example:

• Blower delivering 500 ACFM at 8 psig

• Close discharge valve 20%

• Pressure rises to 10 psig

• Flow drops to 480 ACFM (slipback)

• Power increases 25%

• Temperature rises 20–30°F

Limitations:

• Cannot exceed blower's maximum pressure rating

• Motor will overload if pressure too high

• Temperature rise may damage oil and bearings

Warning: Do not close the discharge valve completely. This causes overpressure, motor overload, and potential blower damage.

Method 2: Increase Speed (RPM)

How it works:
Increase blower speed – more volume per minute. Higher flow against the same system resistance creates higher pressure.

Methods to increase speed:

1. Increase motor speed (if using VFD)

2. Change motor (higher RPM)

3. Change pulleys (belt drive)

Effect:

• Flow increases (flow ∝ RPM)

• Pressure increases (system resistance × flow)

• Power increases (power ∝ RPM³ at constant pressure)

• Temperature increases

Example:

• Blower at 1,800 RPM delivering 500 ACFM at 8 psig

• Increase to 2,000 RPM (11% speed increase)

• Flow increases to 555 ACFM (11%)

• Pressure rises to 8.9 psig (system resistance)

• Power increases 37% (RPM³)

• Temperature rises 15–20°F

Limitations:

• Maximum speed limited by bearings and rotor stress

• Higher speed = higher wear

• Motor may need upgrade

• VFD required for speed control

VFD speed increase:

• VFD can increase speed up to motor maximum

• Typically 10–20% speed increase possible

• Check motor and blower maximum speed ratings

Method 3: Upgrade Components

How it works:
Upgrade blower components to handle higher pressure. This allows the blower to operate at higher pressure without damage.

Component upgrades:

Effect:

• Blower can operate at higher pressure safely

• Higher pressure capability

• Extended component life

Limitations:

• Cost: 30–50% of new blower cost

• Labor: significant

• May be more cost-effective to buy a new blower

Method 4: Stage Blowers in Series

How it works:
Two or more blowers connected in series. Each blower adds pressure. First stage compresses to intermediate pressure, second stage to final pressure.

Series arrangement:

• Blower 1: inlet to 5 psig

• Blower 2: 5 psig to 10 psig

• Total pressure: 10 psig

Effect:

• Higher total pressure

• Each blower operates within its rating

• Intercooling between stages possible (reduces temperature)

Limitations:

• Higher cost (two blowers)

• More complex system

• Requires intercooling for high pressure

• More maintenance

When to use staging:

• Pressure above 15 psig

• Single blower cannot achieve required pressure

• Need to stay within blower ratings

• Long-term continuous operation

Limits of Pressure Increase

Maximum pressure limits:

What limits pressure:

1. Temperature.

• Higher pressure = higher discharge temperature

• At 15 psig: 210–240°F

• At 20 psig: 250–280°F

• At 250°F: oil degrades rapidly

• At 275°F: shutdown recommended

2. Power.

• Higher pressure = higher power

• Power ∝ pressure

• Motor may overload

3. Bearing life.

• Higher pressure = higher bearing load

• Bearing life halves for every 25°F above 200°F

4. Thermal expansion.

• Higher temperature = more rotor expansion

• Tip clearance decreases

• Risk of rotor contact

5. Relief valve.

• Relief valve set at maximum pressure

• If pressure exceeds setting, valve opens

• Cannot exceed relief valve setting

Safety Considerations

Before increasing pressure:

1. Check motor capacity.

• Motor amps will increase

• Verify motor has capacity

• Check overload protection setting

2. Check discharge temperature.

• Monitor temperature during pressure increase

• Stay below 220°F for continuous operation

• Above 250°F: shut down

3. Check relief valve setting.

• Set at maximum operating pressure + 2 psig

• Ensure relief valve is working

4. Check system components.

• Piping rated for higher pressure

• Valves rated for higher pressure

• Silencers rated for higher pressure

5. Monitor vibration.

• Higher pressure = higher vibration

• Check vibration levels

Pressure increase checklist:

• Motor capacity verified

• Temperature monitored

• Relief valve set correctly

• System components rated for higher pressure

• Vibration monitored

When to Upgrade to a Larger Blower

Signs you need a larger blower:

• Pressure increase exceeds 2–3 psig

• Motor is running at 95%+ of nameplate amps

• Discharge temperature exceeds 220°F

• Vibration has increased significantly

• Need pressure above 15 psig continuous

Larger blower benefits:

• Designed for higher pressure

• Proper motor sizing

• Proper cooling

• No risk of damage

Cost comparison:

• Modifying existing blower: 30–50% of new cost

• New larger blower: 100% of new cost

• But new blower provides proper design and longer life

Frequently Asked Questions

1. Can I increase roots blower pressure?
Yes – within limits. Increase system resistance (restrict flow) or increase speed (RPM). But exceeding design pressure causes higher temperature, motor overload, and component damage. Maximum pressure is 15 psig standard, 25 psig with upgrades.

2. What is the maximum pressure for a roots blower?
Standard three-lobe: 15 psig continuous. High-pressure design: 25 psig. Above 15 psig, efficiency drops and temperature rises. Above 25 psig, screw compressors are the better technology.

3. How does increasing speed affect pressure?
Higher speed = higher flow = higher pressure (against fixed system resistance). Flow ∝ RPM. Power ∝ RPM³. Speed increase of 10% increases power 33% – motor may overload.

4. How does increasing system resistance affect pressure?
Closing a valve or adding restriction increases pressure. The blower pushes against higher resistance. Pressure increases, flow drops slightly (slipback), power increases.

5. What is the temperature increase with higher pressure?
At 8 psig: 185–200°F. At 10 psig: 200–220°F. At 12 psig: 210–230°F. At 15 psig: 230–260°F. Temperature rise approximately 20–30°F per 2 psig increase.

6. What happens if I exceed the maximum pressure?
Discharge temperature rises (oil degradation), motor overloads, bearings fail from increased load, thermal expansion causes rotor contact. Exceeding design pressure causes progressive damage – not immediate failure, but reduced lifespan.

7. Can I use VFD to increase pressure?
VFD increases speed, which increases flow and pressure. But speed is limited by blower and motor ratings. VFD typically provides 10–20% speed increase – pressure increase of 2–3 psig.

8. How do I know if my motor can handle higher pressure?
Monitor motor amps. If amps exceed nameplate rating, motor will overload. Power ∝ pressure – 10% pressure increase = 10% power increase. Check motor service factor.

9. What upgrades are needed for higher pressure?
C4 bearings (for thermal expansion), stainless steel rotors (lower expansion), water cooling (temperature management), larger motor (power). Cost: 30–50% of new blower cost.

10. Is staging blowers in series an option?
Yes – for pressure above 15 psig. Two blowers in series – each adds pressure. Requires intercooling between stages. Higher cost but allows higher pressure within ratings.

11. What is the pressure limit for continuous operation?
15 psig standard, 20 psig with upgrades. Above 20 psig, screw compressors are more efficient. Continuous operation above 15 psig requires careful thermal management.

12. Why does temperature rise with pressure?
Higher pressure = higher pressure ratio. The air is compressed more during backflow. Tdischarge = Tinlet × (Pdischarge/Pinlet)^0.286 + ΔTmechanical.

13. Can I just close the discharge valve to increase pressure?
Yes – but do not close it completely. Partial closure increases pressure. Complete closure causes overpressure and motor overload. Use a valve with position indicator.

14. How does altitude affect pressure capability?
Altitude reduces atmospheric pressure – pressure ratio for same gauge pressure is higher. At 5,000 ft, 10 psig = pressure ratio 2.36 vs 1.68 at sea level. Higher ratio increases temperature – derate the blower.

15. When should I buy a larger blower instead of modifying?
When pressure increase exceeds 2–3 psig, when motor is near capacity, when temperature exceeds 220°F, or when pressure above 15 psig continuous. New blower provides proper design and longer life.

Final Thoughts

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

Pressure is created by system resistance. To increase pressure, increase system resistance (restrict flow) or increase speed. But each method has limits – temperature, power, and component stress.

Monitor temperature. Higher pressure = higher discharge temperature. Stay below 220°F for continuous operation. At 250°F, oil degrades. At 275°F, shutdown.

Check motor capacity. Power ∝ pressure – 10% pressure increase = 10% power increase. Motor may overload. Monitor amps during pressure increase.

Know the limits. Standard blowers: 15 psig. High-pressure designs: 25 psig. Exceeding limits causes damage. If you need higher pressure, consider staging or a new blower.

The bottom line. Increasing roots blower pressure is possible – but must be done carefully. Zhanggu and other manufacturers specify pressure ratings. Stay within ratings. Monitor temperature and power. When in doubt, consult the manufacturer. The cost of damage far exceeds the cost of proper engineering.