Roots blower pressure ratio: reliable decisions depend on stated operating conditions, material evidence and equipment limits. Differential pressure is a central roots blower selection value, but it does not describe the complete thermal duty. The blower also responds to inlet absolute pressure and temperature. Together, inlet and discharge absolute pressure define the pressure ratio that influences internal backflow, motor power and discharge temperature.
Gauge Pressure Is Not the Whole Picture
A line operating at the same gauge pressure can impose different pressure ratios at sea level and at high altitude. Inlet-filter restriction also lowers the pressure entering the blower. The machine then works across a larger ratio even if the discharge gauge appears unchanged.
The enquiry should therefore provide site atmospheric pressure or altitude, inlet restriction, inlet temperature and required discharge pressure. These conditions are needed to select the model and motor correctly.
Why Discharge Temperature Rises
As each trapped gas volume opens to the higher-pressure discharge region, gas flows backward briefly to equalize pressure. This external compression process adds energy and temperature. Higher differential pressure, hotter inlet air and greater pressure ratio generally increase discharge temperature.
Actual temperature also depends on model, speed, internal clearances, gas properties and heat dissipation. A simple theoretical estimate is useful for screening, but manufacturer performance data and permissible limits control final selection.
Effects on the Conveying Process
High gas temperature can affect:
- Heat-sensitive powders, pellets or additives.
- Flexible hose, gaskets and filter media.
- Lubricant condition in separated bearing and gear compartments.
- Thermal expansion and rotor clearances.
- Gas density and downstream conveying velocity.
- Moisture evaporation, condensation after cooling and product quality.
The maximum acceptable temperature may therefore be set by the product or downstream equipment before the blower reaches its own limit.
Operating Cases That Raise Pressure
Normal pressure can increase because of a dirty receiver filter, higher solids rate, a more restrictive route, feeder problems or progressive pipe deposits. Closed branch valves and pipeline blockage are abnormal cases that require protection.
The motor and blower should be checked at the highest credible operating pressure, while a relief valve and shutdown prevent exposure beyond permitted limits. Continuous operation on the relief valve is not acceptable capacity control.
Inlet Conditions and Filtration
An undersized or contaminated inlet filter reduces inlet absolute pressure and can raise pressure ratio. Hot intake locations further increase discharge temperature and reduce inlet-air density. The intake should be clean, cool and sized for the required flow with a maintainable filter.
Filter differential pressure is therefore both a maintenance value and a blower-duty input. Replacing a filter with a finer element without checking area and loss can change the operating point.
Cooling and Process Limits
An aftercooler can reduce gas temperature entering the conveying line, but it adds pressure loss and can form condensate. It does not authorize the blower to operate above its pressure, speed, power or internal temperature limits.
Where cooling is required, the project must define allowable product temperature, coolant conditions, separation of condensate and the cooler’s clean and fouled resistance. The blower is selected upstream of that added loss.
Pasifik Blower Selection Boundary
Pasifik Blower publishes a portfolio range of approximately 30–9,360 m³/h and up to 1,000 mbar across its two-lobe and three-lobe positive-pressure models. The 1,000 mbar figure is a portfolio ceiling, not a rating for every model, speed, inlet condition or gas.
Final selection requires the chosen model’s flow-pressure and power diagrams and verification of discharge temperature at the actual site. If the conveying process requires a pressure beyond the verified Pasifik envelope, a different air-source category should be selected instead of staging or overspeeding standard blowers without an approved design.
Glossary
Aftercooler: Heat exchanger reducing gas temperature after compression.
Discharge temperature: Gas temperature measured at or near the blower outlet under defined conditions.
Fouled resistance: Pressure loss of equipment after deposits or contamination have accumulated.