An air-only system curve shows the pressure required to move gas through a pneumatic conveying network before material is introduced. It combines pipe friction and losses through bends, valves, supply components, receiver equipment and filters across a range of airflow.
The curve is valuable because a roots blower does not independently set discharge pressure. The system resistance creates the pressure at each blower speed and flow condition.
What Belongs in the Curve
The boundary should be stated clearly. A complete positive-pressure air-only curve may include:
- Blower inlet-filter restriction.
- Inlet and discharge silencers.
- Supply pipe and check valve.
- Empty conveying pipeline.
- Bends, diverters and transitions.
- Cyclone or receiver entry.
- Receiver filter and clean-air vent.
If a component is excluded, its loss must be added elsewhere in the duty. Mixing a line-only curve with a complete-package pressure requirement creates false margin.
Flow and Pressure Trend
For a fixed turbulent-flow geometry over a limited range, air-only pressure loss often follows an approximate square relationship:
The relationship is not exact for a long compressible-gas line. Density, absolute pressure, temperature, friction factor and local volume change along the route. It is still useful for recognizing why a modest airflow increase can consume a much larger share of available pressure.
Article 096 provides the component-level calculation method. The system curve assembles those component results at several flow points.
Use Compatible Flow Conditions
Supplier data for a valve, filter or cyclone may be stated at an actual inlet volume, standard volume or a test-gas condition. Every point must be converted to a consistent mass-flow or reference basis before losses are added.
The curve should also identify the assumed gas temperature and local atmospheric pressure. A curve developed for cool sea-level air may not represent a hot high-altitude installation.
Create More Than One Curve
One clean-condition curve is rarely enough. Useful cases include:
- Clean receiver filter.
- Maximum normal filter differential pressure.
- Alternative conveying routes.
- Purge configuration.
- Minimum and maximum process temperature.
- Defined wear or fouling allowance.
A named fouled curve is more transparent than adding an unexplained percentage to blower pressure. It can also support alarm settings and maintenance decisions.
Compare the Curve with Blower Data
A positive-displacement roots blower provides a flow characteristic at each speed. Where that characteristic and the system resistance correspond, the package reaches an operating point. Internal leakage causes effective flow to change with pressure, so final analysis must use the selected model’s performance data rather than assuming perfectly constant volume.
The air-only operating point is only the baseline. Adding solids creates particle acceleration, support, friction, elevation and gas-solid interaction losses. The model must retain pressure capacity for the loaded duty without exceeding motor or temperature limits.
Validate During Commissioning
An empty-line test at several approved speeds can be compared with the calculated curve. Pressure should be measured at meaningful locations so a discrepancy can be assigned to intake, supply pipe, conveying line or receiver filter.
If measured resistance is high, possible causes include a wrong bore, closed valve, kinked hose, construction debris, wet filter or inaccurate flow reporting. Increasing blower speed before identifying the restriction can raise pressure rapidly.
The accepted curve becomes a maintenance baseline. Future pressure at a known flow can be compared with the clean condition to detect deposits or component deterioration.
Glossary
Air-only system curve: Relationship between gas flow and total clean-gas resistance for a defined system boundary.
Clean-condition curve: System curve based on clean pipework and normal clean-component resistance.
Fouled-condition curve: System curve incorporating a declared maximum normal loading or deposit condition.
Operating point: Flow and pressure condition at which air-mover behavior corresponds to system resistance.
System resistance: Total pressure loss imposed by all components inside the defined boundary at a stated flow.