How to Select the Right Roots Blower for Your Application: A Step by Step Specification Guide

Why Getting the Specification Right Matters

An undersized roots blower will not achieve the required flow at the operating pressure, leading to process underperformance. An oversized blower wastes energy, increases capital cost, and in pneumatic conveying applications, can drive conveying velocity above the optimum range, causing product degradation and pipeline wear. The roots blower is also a constant volume machine, meaning its flow rate changes with speed but is largely independent of pressure within the rated range. This characteristic makes it important to specify correctly because there is less self correcting behaviour compared to a centrifugal machine.

Step 1: Define the Required Process Parameters

The first step is to collect the process data that drives the specification. These parameters must be measured or calculated at the actual process conditions, not estimated from handbook values:

• **Required flow rate** in normal cubic metres per hour at inlet conditions, or alternatively in actual cubic metres per hour at the blower inlet temperature and pressure
• **Required differential pressure** in bar or kilopascals, which is the pressure difference between the blower discharge and its inlet. For pressure systems this is the system resistance plus the vessel or pipeline back pressure. For vacuum applications it is the difference between atmospheric pressure and the required vacuum level
• **Inlet air temperature** in degrees Celsius at the blower inlet flange. Blower rated capacity is typically stated at 20 degrees Celsius, 1.013 bar absolute. Higher inlet temperatures require a derating to actual flow
• **Inlet air pressure** at the blower inlet, which is atmospheric pressure minus any inlet filter pressure drop or minus vacuum in a vacuum system
• **Gas composition** if the blower is handling a gas other than air. For biogas, nitrogen, or CO2 duty, the molecular weight affects the power requirement
• **Ambient temperature** and **altitude above sea level**, both of which affect the density of cooling air for air cooled machines and the motor cooling

Step 2: Convert to Standard Conditions for Blower Selection

Blower manufacturers publish performance curves in terms of inlet volume flow at standard conditions (typically 20 degrees Celsius and 1.013 bar absolute) against differential pressure. To select a blower correctly, the required flow must be expressed in the same standard conditions.

The conversion from actual to standard conditions uses the ideal gas relationship:

This standard flow value is then used to select from the Acme roots blower performance charts.

Step 3: Select the Blower Frame Size and Speed

Acme Air Equipments publishes performance data for each roots blower model showing the delivered flow in normal cubic metres per hour as a function of operating speed in RPM, with separate curves for each differential pressure. The selection procedure is:

• Enter the performance chart at the required differential pressure on the X axis
• Move vertically to the flow curve at the required standard volume flow on the Y axis
• The intersection of these two values identifies the operating point
• Read off the required blower speed in RPM from the speed curve passing through that operating point
• If the required speed is within the permissible range for the frame size, the selection is confirmed
• If the required speed is outside the range, select the next larger frame size and repeat

For the Acme standard roots blower range, permissible operating speeds range from 300 RPM to 3,600 RPM depending on the model. The maximum speed is limited by tip velocity and bearing speed ratings. The minimum speed is limited by the need to maintain adequate sealing between the lobe tips and the casing.

Step 4: Verify Power Requirement and Motor Selection

The shaft power consumed by the roots blower is the sum of the theoretical compression power and the mechanical losses from timing gear friction, bearing friction, and seal friction. The theoretical power for an isothermal compression process is:

Theoretical power (kW) = inlet volume flow (m3/s) multiplied by inlet absolute pressure (kPa) multiplied by natural log of (discharge absolute pressure divided by inlet absolute pressure)

Roots blowers are not isentropic or isothermal compressors; they are positive displacement machines with internal leakage. The actual shaft power is therefore higher than the theoretical isothermal power by an efficiency factor. Typical overall efficiencies for roots blowers range from 75 to 90 percent depending on the pressure ratio and machine size.

The selected motor must be rated for the maximum shaft power that will be demanded under all operating conditions, with a service factor of 1.15 to 1.25 applied. The motor frame, enclosure rating (IP55 minimum for industrial applications), efficiency class (IE3), and power supply voltage must also be specified at this stage.

Step 5: Check Noise and Temperature Limits

For installations near occupied areas or in noise sensitive environments, the calculated sound pressure level of the selected blower at the installation point must be checked against the site noise limits. Acme’s roots blowers are supplied with inlet silencers and discharge silencers as standard. The sound reduction provided by these silencers is stated in the product data sheet for each model.

If additional noise reduction is required, acoustic enclosures or inline discharge silencers of greater attenuation can be specified. For particularly noise sensitive locations, a variable speed drive running the blower at reduced speed reduces noise by approximately 3 dB for each 10 percent reduction in speed.

Discharge air temperature should also be verified. The discharge temperature of a roots blower depends on the differential pressure and can be estimated as:

Discharge temperature (degrees C) = Inlet temperature + (differential pressure in bar multiplied by approximately 50 degrees C per bar for a twin lobe design, or 45 degrees C per bar for a tri lobe design)

If the discharge temperature will exceed the temperature rating of the downstream system components or the conveyed material, an intercooler or aftercooler should be included in the specification.

Step 6: Specify Accessories and Ancillaries

A complete roots blower installation requires several ancillary components that must be specified alongside the machine:

• **Inlet air filter**: rated for the particle size of the environment, with a differential pressure indicator and a pressure drop specification that triggers maintenance before flow is significantly affected
• **Inlet silencer**: matched to the blower model and required sound attenuation. Acme supplies inlet silencers as standard equipment
• **Discharge silencer**: typically higher attenuation than the inlet silencer since discharge pulsation pressure is higher
• **Flexible connections**: at both inlet and discharge to isolate piping vibration from the blower and to accommodate thermal expansion
• **Safety relief valve**: set at 110 to 115 percent of the maximum system operating pressure to protect the blower and piping from overpressure
• **Pressure and temperature instrumentation**: discharge pressure gauge and temperature indicator at minimum, with sensors for remote monitoring where required
• **Non return valve**: on the discharge connection to prevent backflow through the blower when the machine is stopped

Frequently Asked Questions