Pressure Drop in Pneumatic Conveying

What Is Pressure Drop?

When gas flows through a pipeline, it encounters resistance from the pipe wall, bends, fittings and connected equipment. When solid particles are transported with the gas, additional pressure requirements arise from particle acceleration, gas-solid interactions and the way the material moves through the pipeline.

Pressure drop generally refers to the pressure difference between two defined locations. For an entire conveying system, the required pressure difference must account for the relevant pipeline sections and equipment, using a consistent pressure reference and operating condition.

In a real pneumatic conveying installation, the required pressure is not determined by pipeline length alone. Air flow, pipe geometry, material loading, elevation, conveying regime and equipment losses can all influence the result.

Why Pressure Drop Matters

The pressure requirement affects the selection and operating conditions of the blower, compressor or vacuum source. An estimate that is too low can lead to unsuitable equipment selection, while an overly conservative estimate may result in unnecessary equipment cost or energy use.

Main Sources of Pressure Loss

The overall pressure requirement may include contributions from several parts of the system. These contributions should be identified according to the actual conveying arrangement.

• Gas friction along straight pipeline sections
• Pressure losses through bends, fittings and changes in direction
• Pressure requirements associated with vertical conveying
• Energy required to accelerate solids into the conveying stream
• Additional resistance associated with transporting the solids
• Losses through feeders, separators, filters and other equipment
• Pressure changes associated with gas density and operating conditions

These contributions do not always behave as independent, constant values. In gas-solid conveying, particle behavior, gas velocity and solids loading can interact, so the chosen calculation method must suit the material and operating regime.

Effect of Pipeline Length

Pipeline length is an important factor because longer conveying routes generally create more frictional resistance and expose the gas-solid mixture to a greater length of pipeline.

However, pressure requirement should not be estimated by multiplying length by a universal pressure-loss value. The relationship depends on the gas flow, pipe diameter, material feed rate, conveying regime and the distribution of straight sections, bends and elevation changes.

When comparing alternative layouts, engineers should evaluate the complete route and identify whether changes in length also change the number of bends, the vertical lift or the operating conditions.

Effect of Pipe Diameter

Pipe diameter affects gas velocity, pipeline resistance and the behavior of the conveyed material. For a given actual volumetric gas flow rate, the average gas velocity is related to the internal cross-sectional area of the pipe.

Where v is average gas velocity in m/s, Q is actual volumetric gas flow in m³/s, A is internal pipe area in m², and D is internal pipe diameter in metres. The flow rate and pipe area must use compatible units.

For the same actual gas flow, a smaller internal diameter produces a higher average gas velocity. This can increase gas friction losses and may change the solids-conveying behavior. A larger diameter reduces average gas velocity at the same flow rate, but it does not automatically produce a better conveying system: solids transport, capacity, installation cost and operating conditions must also be checked.

Pipe diameter should therefore be selected as part of the overall conveying design, not from pressure drop alone. See How to Select Pipe Diameter for Pneumatic Conveying for more detailed guidance.

Effect of Air Velocity

Gas velocity influences both the pressure requirement and the ability of the system to transport the material. The appropriate operating range depends on the material, particle-size distribution, solids loading, pipeline geometry and conveying regime.

If the velocity is too low for the application, particles may settle, accumulate or form deposits. If the velocity is unnecessarily high, gas friction losses, energy use, particle degradation or pipeline wear may increase.

There is no single velocity limit that can be applied reliably to every material and conveying arrangement. Dilute-phase and dense-phase systems may operate under different gas-solid flow conditions. For a comparison of these approaches, see Dense Phase vs Dilute Phase Pneumatic Conveying .

Effect of Bends and Vertical Elevation

Bends change the direction of the gas-solid mixture and introduce additional resistance. Their effect depends on bend geometry, radius, orientation, gas velocity and the behavior of the transported solids. Simply counting bends does not fully describe their pressure-loss contribution.

Vertical conveying also requires consideration of the energy needed to lift the solids. The pressure requirement associated with elevation depends on material throughput, the conveying arrangement and the gas-solid flow conditions.

For preliminary evaluation, record the horizontal route, vertical lift, bend locations and other significant pipeline features. A detailed estimate should use a calculation method appropriate to the actual layout.

Material Loading and Bulk Solids Properties

Gas-solid pressure loss depends on the properties and quantity of material being conveyed. Two powders with similar bulk densities can behave differently because their particle sizes, shapes, moisture levels, cohesion and flow characteristics differ.

Important properties to investigate include:

• Particle size and particle-size distribution
• Particle density and bulk density
• Moisture content and tendency to agglomerate
• Particle shape, surface characteristics and abrasiveness
• Flowability, cohesion and permeability
• Material feed rate and solids-to-gas loading

Increasing solids throughput at an unchanged gas flow can alter conveying behavior and increase the pressure requirement. The relationship is not universal and should not be represented by an unverified material-independent coefficient.

The ConveyCalc Bulk Solids Materials Database can be used as a starting point for material research. Reference information should be checked against the actual material and project data before it is used for engineering decisions.

Gas-Only Pressure Loss vs Gas-Solid Conveying

A gas-only pipeline calculation estimates pressure loss caused by gas flow through a pipe and its fittings under specified conditions. It can be useful for understanding the air-side contribution, but it does not by itself calculate the complete pressure requirement of a pneumatic conveying system.

When solids are present, the model may also need to account for particle acceleration, particle-wall interactions, gas-solid momentum exchange, vertical conveying and the selected conveying regime. The importance of these effects varies with the material and system.

Consequently, adding a generic percentage to a gas-only pressure loss does not guarantee a valid gas-solid estimate. The method and any empirical coefficients should have a traceable technical basis and a defined range of applicability.

Preliminary Pressure Drop Estimation

During early-stage engineering, a preliminary pressure assessment can help compare possible pipeline layouts and identify missing information. Before calculating, define the system boundary and the operating conditions to which the pressure values refer.

Typical information required for an initial assessment includes:

• Required material conveying capacity
• Material properties and particle-size information
• Gas flow rate and its reference conditions
• Pipe internal diameter and pipeline length
• Vertical elevation and bend configuration
• Conveying regime and operating pressure
• Feeder, filter, separator and other equipment losses
• The calculation method and its validated range

If critical information is unavailable, the estimate should identify the assumptions and limitations instead of presenting a single result as a verified design value. The next engineering step may involve material testing, vendor data, reference cases or a more detailed model.

Pressure Drop and Blower Power

Pressure requirement and gas flow are important inputs when estimating the power needed by a blower or compressor. For an incompressible-flow approximation, the power transferred to the fluid can be expressed as:

Where Q is the volumetric flow rate in m³/s at the relevant operating condition and Δp is the pressure rise in pascals (Pa). The result is in watts (W).

A simplified estimate of input power may divide fluid power by an appropriate overall efficiency. However, gas compressibility, inlet conditions, pressure ratio, blower type and the manufacturer's performance data can materially affect actual power demand. This simplified expression must not be treated as a final blower shaft-power or motor-sizing calculation.

For related background, see Estimating Blower Power for Pneumatic Conveying .

Common Pressure-Drop Estimation Mistakes

• Estimating pressure loss from pipeline length alone without considering the full system configuration.
• Using nominal pipe diameter instead of the actual internal diameter in velocity calculations.
• Mixing standard volumetric flow with actual in-pipe volumetric flow without converting between the corresponding gas conditions.
• Treating a gas-only straight-pipe calculation as the complete gas-solid conveying pressure requirement.
• Applying one empirical coefficient to different materials and conveying regimes without validation.
• Selecting a blower based only on a preliminary pressure estimate without checking its operating point and performance data.

Preliminary Design Checklist

Before using a pressure-drop estimate to compare conveying arrangements, review the following items:

• Confirm the required material conveying capacity.
• Define the conveying route and system boundaries.
• Record pipe internal diameter, length and bend geometry.
• Identify the vertical lift and conveying direction.
• Define gas flow rate and the associated reference conditions.
• Collect relevant material properties and feed-rate data.
• Identify the conveying regime and operating conditions.
• Account for significant feeding and separation equipment losses.
• Document the calculation method, assumptions and uncertainties.
• Verify the air-source operating point before equipment selection.

These checks help distinguish a useful preliminary estimate from a calculation that is ready to support detailed engineering. Each important result should be traceable to its input data, method and stated limitations.

Summary

Pressure drop in pneumatic conveying is influenced by gas flow, pipeline geometry, vertical elevation, solids loading, material properties and equipment resistance. Gas-only calculations can provide useful information about part of the system, but they do not automatically represent the complete pressure requirement when solids are conveyed.

A reliable preliminary assessment begins with clear operating conditions, representative material data and a defined system layout. The calculation method should match the intended application, and any assumptions or unverified parameters should remain explicit until they can be checked.