Roots blower selection for soil vapor extraction requires more than summing the expected airflow from extraction wells. The blower must provide the required gas volume at actual inlet vacuum while overcoming losses in the wellfield, manifold, separator, filters and vapor-treatment system.
Roots blower selection for soil vapor extraction (SVE) must account for changes in the wellfield and treatment train during remediation. Soil moisture, groundwater elevation, well configuration, and equipment resistance can change the required duty. The selection should therefore define the normal operating point and the full range of credible operating conditions.
Define the SVE Duty
A blower enquiry should specify required inlet airflow, inlet vacuum, discharge pressure, inlet-gas temperature, site altitude, and gas composition. It should also state expected operating hours and the minimum and maximum operating cases that the system must handle.
Airflow and vacuum must be evaluated together. A blower that provides the required flow at low vacuum may deliver less flow at a deeper vacuum as internal leakage increases. The duty point should be stated at the blower inlet, not only at the extraction wells.
Wellfield airflow depends on how many wells operate at the same time and how much gas each contributes. Define the total number of wells, the maximum and minimum numbers expected to operate simultaneously, individual well airflow and vacuum, and the zones with different permeability. Include planned well-cycling and future expansion. Summing every well’s maximum measured flow can oversize the blower if those wells will not operate together. Using only current average flow can undersize it if later operating stages require more capacity.
Pilot testing provides site-specific data for preliminary selection. Record airflow at several applied vacuum levels, wellhead and observation-point vacuum, contaminant concentration, moisture carryover, groundwater level, soil temperature, stabilization time, and response after shutdown. Testing at multiple points shows whether additional vacuum produces useful airflow or mainly increases system resistance.
Wellhead vacuum is the pressure needed at an extraction branch to produce the intended subsurface airflow. It depends on soil permeability and moisture, extraction depth, screen design, well spacing, surface sealing, groundwater conditions, and interaction between wells. Suction-side pressure losses mean the blower must operate at a greater vacuum than the wellhead reading indicates. Calculate those losses at the required airflow.
Pressure losses occur through branches, balancing valves, flexible connectors, headers, fittings, separators, filters, and silencers. Calculate each loss on the side where the component is installed: inlet-side losses increase required blower vacuum, while discharge-side losses increase required discharge pressure. Check both maximum simultaneous flow and credible partial-flow cases. With a fixed-speed positive-displacement blower, closing several branches can increase vacuum if the blower continues to run at the same speed.
Use clean, normal, and maximum acceptable filter and separator pressure losses in the operating analysis. Do not size the blower for a completely blocked filter; set differential-pressure monitoring and maintenance limits to prevent operation in that condition. Include vapor-treatment resistance on the correct side of the blower. Carbon-bed resistance can rise as the bed collects moisture or dust, so the maximum serviceable pressure loss belongs in the duty range.
Inlet Conditions and Flow Reference
A roots blower displaces a nominal volume of gas per rotor revolution. Actual inlet flow also depends on speed, internal leakage, differential pressure, and inlet conditions. Gas mass flow depends on inlet density, which varies with absolute pressure, temperature, and composition.
State the flow measurement location and reference conditions. Actual volume at the blower inlet, wellfield flow, standard flow, normal flow, and free-air delivery are not interchangeable without conversion. Specify the reference pressure and temperature for standardized values; do not assume that “standard” and “normal” use the same conditions. The supplier and remediation designer should use the same basis when comparing performance.
SVE vacuum is often shown as negative gauge pressure, while pressure ratios and gas-density calculations use absolute pressure. Provide site atmospheric pressure, blower inlet absolute pressure, discharge absolute pressure, and differential pressure across the blower. At high altitude, lower atmospheric pressure changes inlet density and can change the pressure ratio for the same gauge-vacuum reading.
Soil-gas temperature may differ from outdoor ambient temperature and can change as gas passes through exposed piping. Define the expected normal and maximum credible inlet temperatures, including recirculated heat or inlet conditioning if used. Higher inlet temperature reduces gas density and can raise discharge temperature. Check the blower at the highest credible inlet temperature and the site altitude; sea-level data should not be applied automatically.
The extracted gas may contain air, water vapor, and contaminants. Composition affects gas properties, flammability, toxicity, corrosion, and seal compatibility. Provide expected normal and maximum contaminant concentrations. A clean-air performance curve should not be accepted for a chemically aggressive vapor stream without confirming compatibility and performance for the specified gas.
SVE gas is often humid and may approach saturation. Water can condense when the gas cools below its dew point in exposed piping or a cool separator; changes in gas conditions can also affect condensation. Estimate the expected condensate load and ensure the blower receives gas rather than liquid droplets or slugs. Separator and drainage details are addressed in the package design.
Blower, Motor, and Capacity Control
Blower speed is a primary means of adjusting airflow, but the machine must remain within its approved speed range. Higher speed can raise flow and absorbed power; the resulting noise, vibration, and discharge temperature depend on the model and operating point. Some models have minimum-speed limits for cooling, lubrication, or mechanical reasons, so verify the permissible range for the selected unit.
Check motor power at credible combinations of airflow, inlet vacuum, discharge resistance, speed, inlet-gas temperature, and active wells. Do not combine independent maximum values unless they can occur at the same time. Confirm that the motor can handle the highest credible absorbed power and that overload settings protect the motor without being used to justify operation beyond blower limits. A larger motor does not increase blower capacity unless speed or operating conditions change.
Discharge temperature depends on differential pressure, pressure ratio, speed, inlet temperature, internal leakage, gas properties, and cooling. Compare predicted temperatures with the selected blower’s permitted limits at each planned duty point. Temperature monitoring and shutdown criteria should follow the equipment limits and the project’s protection strategy.
Excessive inlet vacuum can result from closed or isolated branches, wet or low-permeability soil, a restricted filter or separator, condensate in the manifold, incorrect valve settings, excessive speed, or operation with fewer wells than planned. A vacuum-relief valve can admit controlled air to protect the blower from excessive vacuum. Size and set it for the equipment limit; do not use it as the normal method of airflow control.
A variable-frequency drive can adjust output as the active wellfield and seasonal demand change. It may reduce unnecessary airflow or vacuum when the duty is lower, provided the blower remains within its permitted speed range. Coordinate the control strategy with wellfield balancing and vapor-treatment capacity. Main-header pressure can be a control signal, but one header value does not confirm uniform treatment at every well.
A single blower may suit a compact wellfield with a narrow operating range. Multiple blowers may be considered when the site has separate treatment zones, a broad demand range, high availability requirements, or planned expansion. Possible arrangements include duty/standby, staged operation, lead-lag control, or separate blowers for distinct zones. Staging can improve capacity matching, but energy performance should be compared at the actual duty points rather than assumed.
Some rotary-lobe packages are configured for both pressure and vacuum service, but their permissible conditions and performance are not necessarily identical. For SVE, verify the specific model’s vacuum performance, inlet and discharge pressures, differential-pressure limit, cooling, seals, and absorbed power. A positive-pressure curve alone is not sufficient for vacuum selection.
Select the normal operating point within the approved performance range, with enough capacity for credible seasonal changes, filter loading, and well combinations. Excess spare capacity can create its own control problems if the blower must run continuously at minimum speed, rely on heavy branch throttling, or discharge surplus flow through a relief device. Design margin should be justified by the site’s actual operating range.
Energy use should be assessed across the expected duty profile. Avoiding unnecessary vacuum and airflow, repairing leaks, maintaining filters, and using headers sized for the required flow can reduce avoidable system losses. Moving clean leakage air consumes blower and treatment capacity without improving contaminant removal.
Package Protection and Site Requirements
Roots blowers generate pulsating flow and mechanical noise. Depending on the installation, the package may require inlet or discharge silencers, an acoustic enclosure, flexible connectors, and vibration isolation. Silencers add pressure loss and must be included in the system calculation. Their materials and internal elements must tolerate moisture and the extracted contaminants.
Before specifying equipment for potentially flammable vapor, the project team should define the applicable hazardous-area classification and gas hazards under the site’s governing requirements. Provide the relevant gas or vapor group, temperature class, maximum concentration, and normal and abnormal release conditions to the package supplier. The complete package—including motor, instruments, and controls—must suit the classified area; a suitable motor alone does not establish package suitability. Grounding, ventilation, and instrument requirements should be resolved in the project design.
Check the compatibility of the blower casing, rotors, seals, flexible connectors, piping, and lubricants with the specified contaminants, temperature, concentration, and exposure duration. Potential failure modes include elastomer swelling, corrosion, lubricant degradation, polymer softening, deposits, and seal leakage. Mixtures of contaminants may require a different compatibility review than a single pure chemical.
A standard roots blower should not transport groundwater or liquid contaminant slugs. Inlet protection may include a suitably sized knockout vessel or separator, a demister, high-liquid-level alarm, automatic drain, and high-high-level blower shutdown. Provide secondary containment and inspection access where the site design requires them. If liquid cannot be reliably separated upstream, another vacuum technology may be needed.
Select inlet filtration for maximum actual inlet airflow, particle size, moisture, chemical compatibility, initial pressure loss, and the maximum permitted pressure loss. Differential-pressure monitoring allows service to be based on filter condition rather than a calendar interval alone.
Specify instrumentation from the control and protection strategy. Typical points include blower inlet vacuum, discharge pressure, inlet and discharge temperature, motor current and speed, filter differential pressure, separator level, and well-level airflow and vacuum. Critical alarms and trips should operate automatically where required by the risk assessment, not depend solely on operator observation.
Commissioning and Operating Verification
Commissioning should verify the blower and the complete SVE system at the normal operating point and credible partial-operation conditions. Record inlet airflow and vacuum, discharge pressure, inlet and discharge temperature, motor current and speed, separator level, filter differential pressure, individual well airflow and vacuum, and vapor-treatment pressure loss. Test protective devices before unattended operation. These readings establish the baseline for later troubleshooting and maintenance.
Glossary
Absolute Pressure: Pressure measured relative to a perfect vacuum.
Actual Inlet Airflow: The volumetric flow physically entering equipment at the real inlet pressure, temperature and gas composition.
Duty Point: The required combination of flow, pressure or vacuum and operating conditions used for equipment selection.
Gas Composition: The identity and concentration of gases, vapors, moisture and contaminants in the handled stream.
Gauge Pressure: Pressure measured relative to local atmospheric pressure.
Hazardous Location: An area classified because flammable gas, vapor or combustible dust may be present in an ignitable concentration.
Standby Capacity: Installed reserve capacity intended to maintain an approved duty after loss or maintenance of the operating unit.
Variable-Frequency Drive: An electronic drive that controls AC motor speed by varying supply frequency and voltage.