Process gas conveying and boosting in chemical plants depends on gas composition, required flow, pressure rise and destination conditions. These gases may be inert, combustible, corrosive, toxic, condensable or contaminated with fine particles and liquid droplets. Their properties can also change as process temperature, pressure and composition vary.
A purpose-designed roots process gas blower can provide reliable gas conveying, circulation and pressure boosting where the required pressure ratio remains within the practical range of positive-displacement rotary-lobe technology. Its nearly constant volumetric displacement makes it particularly useful for applications requiring stable gas flow despite moderate changes in downstream resistance.
This service must be distinguished from conventional air-blowing. A standard roots air blower is not automatically suitable for hydrogen, hydrocarbons, hydrogen sulphide, solvent vapours or other hazardous process gases. Chemical service requires a blower specifically engineered for the gas composition, operating conditions and permissible leakage.
What Is Process Gas Boosting?
Process gas boosting means increasing the pressure of a gas stream sufficiently to overcome the resistance of downstream equipment and piping. The blower does not necessarily compress the gas to a high final pressure. Its purpose may be to provide only the differential pressure required to maintain flow through the process.
A process gas roots blower may be used to overcome pressure losses through:
- Reactors
- Absorption columns
- Gas scrubbers
- Heat exchangers
- Condensers
- Gas filters
- Adsorption systems
- Catalytic treatment units
- Separation equipment
- Control valves
- Long pipelines
- Recovery headers
- Vapour-treatment systems
- Fuel-gas distribution lines
The discharge pressure is created by system resistance. A roots blower transfers gas volume, while the connected process determines the operating pressure.
Typical Chemical Process Gases
Depending on its construction and material compatibility, a specialized roots blower may handle gases or gas mixtures such as:
- Nitrogen
- Carbon dioxide
- Argon
- Hydrogen-containing mixtures
- Synthesis gas
- Methane and hydrocarbon mixtures
- Ethylene-containing gases
- Butadiene-containing streams
- Styrene off-gas
- Hydrogen sulphide-containing gas
- Solvent vapours
- Reactor off-gas
- Vent gas
- Flare-recovery gas
- Recycled process gas
Inclusion in this list does not mean that one blower design is suitable for every gas. Hydrogen requires different leakage considerations from carbon dioxide, while wet hydrogen sulphide presents substantially different corrosion risks from dry nitrogen.
The complete gas composition, including trace contaminants, must be defined before equipment selection.
Operating Principle of a Roots Process Gas Blower
A roots blower contains two synchronized lobed rotors rotating in opposite directions. Gas enters the inlet, becomes trapped between the rotors and casing, and is transported toward the discharge.
The rotors do not normally contact each other or the casing. Timing gears maintain the necessary rotor relationship, while bearings support the shafts outside the gas chamber.
A roots blower does not perform substantial internal compression in the same way as a screw compressor. When a trapped volume reaches the discharge port, higher-pressure gas from the discharge side flows back into the rotor chamber. The resulting pressure equalization is sometimes described as external or isochoric compression.
This operating principle produces:
- Approximately constant displacement per revolution
- Stable volumetric flow
- Rapid response to speed changes
- Pressure pulsations
- Increasing internal leakage as differential pressure rises
- Increasing discharge temperature with higher pressure ratio
- A defined thermal and mechanical operating limit
The machine should therefore be selected from both required flow and maximum differential pressure.
Flow Rate at Actual Inlet Conditions
Blower displacement must be evaluated at the actual inlet pressure and temperature. A flow value stated in normal or standard cubic metres per hour cannot be used directly as the blower inlet volume.
For a real gas:
Gas Flow Conversion to Actual Conditions (Standard)
- Qactual = volumetric flow at actual inlet conditions
- Qstandard = flow at the stated standard reference conditions
- Tactual = actual absolute inlet temperature
- Tstandard = standard absolute temperature
- Pactual = actual absolute inlet pressure
- Pstandard = standard absolute pressure
- Zactual = compressibility factor at inlet conditions
- Zstandard = compressibility factor at standard conditions
Temperatures must be expressed in kelvin, and absolute pressure must be used.
The reference conditions associated with Nm³/h or Sm³/h should always be stated. Different standards may use different reference temperatures and humidity definitions.
Gas Density
Gas density affects mass flow, power consumption and system pressure loss. It can be estimated from:
Gas Density
- ρgas = gas density
- Pabs = absolute gas pressure
- M = molar mass of the gas or gas mixture
- Z = compressibility factor
- Ru = universal gas constant
- T = absolute gas temperature
The gas mass flow is then:
Gas Mass Flow Rate
- ṁgas = gas mass flow rate
- Qactual = actual volumetric flow
The molar mass of a mixture can be estimated from its mole fractions:
Molar Mass of Gas Mixture
- Mmix = average molar mass of the mixture
- yi = mole fraction of component i
- Mi = molar mass of component i
A changing gas composition can therefore change density even when the measured volumetric flow remains constant.
Required Differential Pressure
The blower differential pressure must be calculated from the complete process path:
Total Differential Pressure
- ΔPsuction = inlet-system pressure loss
- ΔPpiping = pipeline and fitting losses
- ΔPequipment = resistance of process equipment
- ΔPcontrol = pressure loss across control devices
- ΔPdischarge = pressure required at the destination
The calculated value should represent the most demanding credible continuous operating condition. Filter fouling, variable liquid levels, process contamination and control-valve position may increase resistance above the clean-system value.
A reasonable engineering allowance may be included, but excessive pressure margin can result in an unnecessarily large motor and poor energy performance.
Absolute Pressure Ratio
Compression ratio must always be calculated using absolute pressure:
Pressure Ratio
- P1,abs = absolute blower inlet pressure
- P2,abs = absolute blower discharge pressure
Gauge pressure must not be used in this equation.
For example, a blower taking gas from a process below atmospheric pressure may experience a meaningful pressure ratio even when the discharge gauge pressure appears low. Evaluating only gauge differential pressure can underestimate thermal loading.
Volumetric Capacity and Speed
The theoretical displacement of a roots blower is related to its displacement per revolution and rotational speed:
Theoretical Volumetric Flow
- Qtheoretical = theoretical volumetric flow
- Vrev = displaced volume per revolution
- N = rotational speed
The actual delivered flow is lower because of internal leakage:
Actual Volumetric Flow
- ηv = volumetric efficiency
Volumetric efficiency depends on rotor clearances, gas properties, speed, differential pressure and operating temperature. Low-molecular-weight gases can produce greater internal leakage than air, reducing delivered capacity.
Final flow should therefore be obtained from performance calculations for the actual gas rather than by applying an air performance curve without correction.
Glossary
Absolute suction pressure: Inlet pressure measured relative to a perfect vacuum.
Actual inlet flow: Volume flow stated at the machine’s real inlet pressure and temperature.
Compressibility factor: Ratio used to relate a real gas to ideal-gas behavior at stated conditions.
Differential pressure: Discharge pressure minus inlet pressure across the machine.
Gas density: Mass per unit volume of a gas at stated composition and conditions.
Gas throughput: Pressure multiplied by pumping speed at a stated vacuum condition.
Reference state: Specified pressure and temperature used to report a gas volume or flow rate.
Shaft power: Mechanical power delivered to the machine shaft.