How to size roots blower
How to Size Roots Blower
Knowing how to size roots blower correctly is the difference between reliable operation and chronic problems. An undersized blower cannot deliver required flow – processes fail. An oversized blower wastes energy, short-cycles, and costs thousands annually in unnecessary electricity.
Based on sizing experience across hundreds of aeration, conveying, and vacuum applications, I have seen every mistake: using SCFM instead of ACFM, forgetting altitude correction, ignoring diffuser fouling margin, undersizing motor safety factor. Each mistake costs money.
This guide provides a step-by-step methodology for sizing roots blowers. It covers flow calculation, pressure determination, motor selection, and common pitfalls. Use it to size blowers correctly the first time.
Table of Contents
What Does It Mean to Size a Roots Blower?
Step 1: Determine Required Flow
Step 2: Correct Flow for Altitude and Temperature
Step 3: Determine Required Pressure
Step 4: Select Motor Power
Step 5: Consider Application-Specific Factors
Step 6: Select Blower Type and Configuration
Common Sizing Mistakes
Performance and Engineering Calculations
Sizing Examples by Application
Sizing Checklist
Frequently Asked Questions
Final Thoughts
What Does It Mean to Size a Roots Blower?
Sizing a roots blower means selecting the correct combination of flow capacity (ACFM), pressure rating (psig or inches Hg), and motor power (HP) for a specific application. The goal is to match blower output to system demand with appropriate margin – not too little, not too much.
Based on field data, correctly sized blowers operate at 70–90% of rated capacity. They run efficiently, maintain stable pressure/flow, and last 10+ years. Incorrectly sized blowers operate below 50% capacity (wasting energy) or above 100% capacity (overloading motors, overheating).
The sizing process follows a logical sequence: define system flow requirement, correct for operating conditions, determine system pressure, calculate required power, and select the blower. Each step requires engineering judgment – not just plugging numbers into formulas.
Step 1: Determine Required Flow
Define system flow requirement. This is the volume of air the blower must deliver at operating conditions. Flow depends on the application:
Wastewater aeration: Calculate from oxygen demand. Typical: 0.5–1.5 SCFM per 1,000 cubic feet of basin volume.
Pneumatic conveying: Calculate from material flow rate and solids loading ratio.
Vacuum system: Calculate from system air removal requirement.
Industrial ventilation: Calculate from hood capture velocity and duct area.
Important: Flow must be expressed as ACFM (actual cubic feet per minute) at operating conditions, not SCFM (standard cubic feet per minute). SCFM is a reference condition – it does not reflect actual volume at your site.
Example – wastewater aeration:
500,000 gallon basin (66,800 cu ft). Desired airflow: 1.0 SCFM per 1,000 cu ft. Required SCFM = 66.8 × 1.0 = 66.8 SCFM.
Step 2: Correct Flow for Altitude and Temperature
SCFM to ACFM correction formula:
ACFM = SCFM × (14.7 / local atmospheric pressure in psia) × (local absolute temperature in °R / 520°R)
Local atmospheric pressure at altitude:
Sea level: 14.7 psia
1,000 ft: 14.2 psia
2,000 ft: 13.7 psia
3,000 ft: 13.2 psia
4,000 ft: 12.7 psia
5,000 ft: 12.2 psia
Absolute temperature:
°R = °F + 460
Example continuation:
Plant at 3,000 ft elevation (13.2 psia), 90°F ambient (550°R).
ACFM = 66.8 × (14.7/13.2) × (550/520) = 66.8 × 1.114 × 1.058 = 78.8 ACFM.
The blower must deliver 78.8 ACFM at site conditions – 18% more than SCFM.
Why this matters: Sizing based on SCFM without correction undersizes the blower. At 5,000 ft, the correction is 20% – a significant error.
Step 3: Determine Required Pressure
Calculate system backpressure. The blower must overcome all system pressure losses:
Pressure components:
Static head (if discharging into liquid): depth (ft) × 0.433 psig/ft
Piping losses: depends on pipe size, length, fittings
Filter/diffuser losses: manufacturer data or estimate
Silencer pressure drop: typically 0.5–1.0 psig per silencer
Fouling margin: 1–2 psig for aeration, conveying filters
Safety margin: 15–20% of total
Example – wastewater aeration:
Static head: 15 ft water = 6.5 psig
Piping losses: 0.5 psig
Diffuser clean loss: 0.5 psig
Silencer loss: 0.5 psig
Fouling margin: 2.0 psig
Subtotal: 10.0 psig
Safety margin (15%): 1.5 psig
Total design pressure: 11.5 psig (specify 12 psig)
Vacuum pressure conversion:
Vacuum in inches Hg: 1 inch Hg = 0.491 psia
Required vacuum = 10 inches Hg = 4.91 psia absolute = 9.79 psig below atmospheric.
Important: Always add fouling margin. Systems clog over time. Sizing exactly at clean conditions means blower will overload when filters/diffusers foul.
Step 4: Select Motor Power
Calculate required brake horsepower (BHP):
BHP = (ACFM × psig) / (229 × ηmechanical × ηmotor)
Where:
ACFM = actual flow at operating conditions
psig = discharge pressure (gauge)
229 = constant (includes conversion factors)
ηmechanical = mechanical efficiency (0.85–0.90 for roots blowers)
ηmotor = motor efficiency (0.91–0.95 for IE3/IE4)
Example:
ACFM = 78.8, psig = 12, ηmechanical = 0.88, ηmotor = 0.94
BHP = (78.8 × 12) / (229 × 0.88 × 0.94) = 945.6 / (229 × 0.827) = 945.6 / 189.4 = 5.0 HP
Add safety factor: Multiply BHP by 1.15–1.20 for motor sizing.
Required motor HP = 5.0 × 1.15 = 5.75 HP → select 7.5 HP motor (next standard size).
Motor efficiency impact:
IE2 (standard): 91% efficiency
IE3 (premium): 94% efficiency
IE4 (super premium): 96% efficiency
Rule of thumb for field estimation:
At 8 psig, three-lobe blower requires approximately 18–20 HP per 100 ACFM.
Example quick estimate:
100 ACFM at 8 psig → 18–20 HP
50 ACFM at 8 psig → 9–10 HP
Step 5: Consider Application-Specific Factors
Aeration:
Add diffuser fouling margin: 1–2 psig
Consider VFD for variable organic loading
Plan for multiple blowers (redundancy)
Pneumatic conveying:
Add margin for line plugging pressure spikes: 2–3 psig
Specify hard-chrome rotors for abrasive materials
Add 20% motor safety factor
Vacuum:
Tighter tip clearance required (0.05–0.10 mm)
Vacuum-rated seals (labyrinth preferred)
Vacuum-rated inlet filter
Biogas:
Stainless steel rotors (316L)
Explosion-proof motor
Temperature monitoring (shutdown at 275°F)
High temperature:
Derate motor for altitude (1% per 1,000 ft above 3,300 ft)
Consider C4 bearings for high temperature
Water cooling above 12 psig continuous
Step 6: Select Blower Type and Configuration
Lobe count:
Twin lobe: lower cost, lower efficiency (65–72%)
Three lobe: industry standard (72–78%)
Helical: lower pulsation, quieter (73–79%)
Drive type:
Direct coupled: most efficient, lower maintenance
Belt driven: variable speed without VFD, 3–5% efficiency loss
Pressure rating:
Standard: 2–15 psig
High pressure: 10–20 psig (thicker casing, C4 bearings)
Accessories:
Inlet silencer: required for noise reduction
Discharge silencer: required for pulsation damping
VFD: for variable flow applications
Inlet filter: 10-micron minimum, 2-micron for dusty
Common Sizing Mistakes
1. Using SCFM instead of ACFM
Most common mistake. At 5,000 ft elevation, SCFM undersizes blower by 20%. Always correct for altitude and temperature.
2. No altitude correction
Many plants at elevation. Atmospheric pressure at 5,000 ft is 12.2 psia vs 14.7 at sea level. This is a 17% difference.
3. No fouling margin
Systems clog. Sizing exactly at clean conditions guarantees overload. Add 15–20% margin.
4. Forgetting silencer pressure drop
Each silencer adds 0.5–1.0 psig. Inlet silencer is on suction – adds to vacuum load or reduces inlet pressure.
5. Undersizing motor safety factor
Use 15–20% safety factor. Conveying lines plug. Filters foul. Motors overload.
6. Oversizing blower
Oversized blower wastes energy and short-cycles. Operate at 70–90% of rated capacity for best efficiency.
7. Ignoring VFD
Fixed-speed blowers waste energy in variable flow applications. VFD saves 20–30%.
8. Not considering multiple blowers
Single blower has no redundancy. Multiple smaller blowers provide turndown and backup.
Performance and Engineering Calculations
Flow correction formula:
ACFM = SCFM × (14.7 / Patm) × (T / 520)
Where:
Patm = local atmospheric pressure (psia)
T = local absolute temperature (°R = °F + 460)
Pressure correction for altitude:
Patm = 14.7 × (1 – 0.000006875 × altitude)^5.256
Power formula:
BHP = (ACFM × psig) / (229 × ηmechanical × ηmotor)
Motor power:
Motor HP = BHP × safety factor (1.15–1.20)
Volumetric efficiency:
ηv = (actual flow) / (theoretical displacement) × 100%
Typical: 92–96% for new blowers
Discharge temperature:
Tdischarge = Tinlet × (Pdischarge/Pinlet)^0.286 + 30–50°F
Sizing Examples by Application
Example 1: Wastewater Aeration
Basin: 500,000 gallons (66,800 cu ft)
Required: 1.0 SCFM per 1,000 cu ft
SCFM = 66.8 SCFM
Site: 3,000 ft, 90°F
ACFM = 66.8 × 1.114 × 1.058 = 78.8 ACFM
Pressure: 15 ft depth = 6.5 psig + 2.0 psig losses + 1.5 psig margin = 10.0 psig
BHP = (78.8 × 10) / (229 × 0.88 × 0.94) = 4.2 HP
Motor = 4.2 × 1.15 = 4.8 HP → 5 HP
Select: 5 HP three-lobe direct-coupled blower with VFD
Example 2: Pneumatic Conveying
Material: Cement, 10 tons/hr
Solids loading ratio: 10
Air required: 10 tons/hr × 2,000 lb/ton / (10 × 60 × 0.08 lb/ACF) = 416 ACFM
Pressure: 12 psig + 2 psig margin = 14 psig
BHP = (416 × 14) / (229 × 0.86 × 0.94) = 31.5 HP
Motor = 31.5 × 1.20 = 37.8 HP → 40 HP
Select: 40 HP three-lobe high-pressure blower with hard-chrome rotors
Example 3: Vacuum System
Required: 200 ACFM at 10 inches Hg
Vacuum: 10 inches Hg × 0.491 = 4.91 psia
BHP = (200 × 10 × 0.491) / (229 × 0.85 × 0.94) = 5.4 HP
Motor = 5.4 × 1.15 = 6.2 HP → 7.5 HP
Select: 7.5 HP three-lobe vacuum blower with labyrinth seals
Sizing Checklist
Before you start:
Define application (aeration, conveying, vacuum, etc.)
Determine required SCFM or ACFM
Know site altitude and ambient temperature
Determine system pressure/vacuum requirements
Identify any special conditions (dust, corrosion, explosion)
Flow calculation:
Calculate required SCFM from process requirements
Correct SCFM to ACFM using altitude and temperature
Add flow margin (15–20%)
Pressure calculation:
Sum all system pressure components
Add fouling margin (1–2 psig)
Add silencer pressure drop
Add safety margin (15–20%)
Motor selection:
Calculate BHP using ACFM and psig
Select motor efficiency class (IE3 minimum)
Add safety factor (15–20%)
Round up to next standard motor size
Blower selection:
Select lobe count (three-lobe standard)
Select drive type (direct coupled standard)
Verify pressure rating meets requirements
Specify accessories (silencers, VFD, filters)
Verification:
Verify flow and pressure on blower curve
Check discharge temperature
Verify motor power matches curve
Review with supplier
Frequently Asked Questions
1. What is the most common sizing mistake?
Using SCFM instead of ACFM. SCFM is at standard conditions (14.7 psia, 60°F). At altitude or high temperature, ACFM is higher than SCFM. Sizing with SCFM undersizes the blower. Always correct to ACFM using local conditions. At 5,000 ft, the correction is 20% – a significant error.
2. How much pressure margin should I add?
Add 15–20% pressure margin to calculated system pressure. Systems clog over time – filters load, diffusers foul, conveying lines build deposits. Sizing exactly at clean conditions means the blower will overload when the system fouls. Add margin for reliability.
3. What motor efficiency class should I specify?
IE3 minimum for continuous duty. IE2 saves $2,000 upfront but loses $4,000+/year in energy. Payback for IE3 is 18–24 months. For 24/7 operation, IE3 is mandatory. IE4 for high energy cost or very long duty.
4. How much motor safety factor should I use?
Use 15–20% safety factor for motor sizing. Blowers see pressure spikes from line plugs, filter loading, and start-up conditions. Undersized motors trip on overload. Oversized motors cost more but prevent nuisance trips. In conveying and aeration, 20% is recommended.
5. Can I oversize a blower for future expansion?
Yes, but within reason. Oversizing by 20–30% is acceptable. Oversizing beyond 50% wastes energy and causes short-cycling. For future expansion, consider multiple blowers – add capacity without oversizing existing units. VFD helps manage oversized blowers.
6. How does altitude affect blower sizing?
Altitude reduces air density. For the same mass flow, you need more volume flow. ACFM = SCFM × 14.7 / Patm. At 5,000 ft (12.2 psia), the correction is 1.20 – you need 20% more ACFM. Motor cooling also decreases at altitude – derate motor 1% per 1,000 ft above 3,300 ft.
7. How does temperature affect blower sizing?
Higher temperature increases air volume. ACFM = SCFM × (T/520). At 100°F (560°R), correction is 1.077 – 7.7% more volume. Higher temperature also increases discharge temperature (pressure ratio effect). For hot applications, specify larger blower or intercooling.
8. What is the rule of thumb for motor sizing?
At 8 psig, three-lobe blower requires approximately 18–20 HP per 100 ACFM. Example: 500 ACFM at 8 psig → 90–100 HP. Add 15–20% safety factor → 105–120 HP → select 125 HP motor. This rule works for quick estimates but always use detailed calculations for final sizing.
9. How do I size for VFD?
VFD reduces flow proportionally to speed (flow ∝ RPM). Power ∝ RPM³. At 80% flow, power is 51% of full. Size blower for maximum required flow. VFD provides turndown. Minimum speed for roots blower: 30% of rated (some designs). Below 30%, efficiency drops significantly.
10. How do I size multiple blowers?
Total capacity = sum of individual capacities. For redundancy, design for N+1 (e.g., three blowers where two are duty, one standby). For turndown, use multiple sizes – one large, one medium. Standard aeration design: three blowers (two duty, one standby) sized for 50% of peak each.
11. Should I size for clean or fouled conditions?
Size for fouled conditions. Diffusers foul, filters load, conveying lines accumulate deposits. Size pressure for 15–20% above clean conditions. This ensures blower continues delivering flow as system fouls. Sizing for clean conditions means overload within 6–12 months.
12. What is the effect of silencers on blower sizing?
Each silencer adds 0.5–1.0 psig pressure drop (discharge) or vacuum loss (inlet). Include silencer pressure drop in system pressure calculation. Inlet silencer on suction reduces inlet pressure – this increases the pressure ratio for the same discharge pressure.
13. How do I size a vacuum blower?
Vacuum sizing is similar to pressure sizing but using inches Hg instead of psig. Power formula: BHP = (ACFM × inches Hg × 0.491) / (229 × ηmechanical × ηmotor). Vacuum blowers require tighter tip clearance (0.05–0.10 mm) and vacuum-oriented seals. Add 20% margin for filter loading and system leakage.
14. How do I check if my blower is correctly sized?
On operating blower: measure discharge pressure and flow. If pressure is below design and flow is above design, blower is oversized (or system has changed). If pressure is at design and flow is below design, blower is undersized. Record baseline performance after commissioning – use for comparison.
15. What should I tell the supplier when getting quotes?
Provide: required flow (ACFM at operating conditions), system pressure (psig or inches Hg), site altitude and temperature, motor voltage and enclosure, special conditions (dust, corrosion, explosion), accessories (silencers, VFD, filters). The more information you provide, the better the sizing. Zhanggu and other manufacturers can size from complete specifications.
Final Thoughts
After decades of sizing roots blowers, here is my practical advice:
The process is methodical. How to size roots blower follows a logical sequence: flow → pressure → power → selection. Each step has critical details. Flow must be ACFM, not SCFM. Pressure must include fouling margin. Motor power must include safety factor. Skip any step and the sizing is wrong.
Add margin. The most common mistake is undersizing. Add 15–20% margin to flow, pressure, and motor power. Systems change – filters foul, diffusers clog, conveying lines plug. The blower sized for clean conditions will fail when the system fouls. Margin is not waste – it is reliability.
Consider the full lifecycle. A blower that costs $2,000 less but uses 5% more energy costs $4,000 more per year in electricity. Over 10 years, that's $40,000. Buy efficiency, not just first cost. IE3 motor, three-lobe design, VFD capability – these features pay back.
Get professional help. Sizing a blower for a critical application is not a guessing exercise. Provide complete specifications to the manufacturer. Zhanggu and other established manufacturers have sizing software and application engineers. Use their expertise. The cost of the blower is small compared to the cost of wrong sizing.
Verify after installation. After commissioning, record flow, pressure, and motor amps. Compare to the blower curve. If the blower is running at 100% capacity continuously, it may be undersized. If it's running at 50% capacity, it's oversized. Baseline data helps with future troubleshooting.
Sizing correctly the first time saves money and avoids headaches. Take the time to do it right.
