Temperature correction of pneumatic conveying airflow: a defensible assessment must connect process data, material behavior and equipment boundaries. Temperature changes the volume and density of conveying air. A roots blower selected from a reference airflow must therefore be checked against the actual inlet temperature, while pipeline and receiver calculations need the local gas temperature at their own positions.
This page addresses temperature correction of airflow. Machine thermal limits are covered in Roots Blower Pressure Ratio and Discharge Temperature in Pneumatic Conveying, and product-contact air quality is covered in Managing Roots Blower Discharge-Air Quality for Pneumatic Conveying.
Use Absolute Temperature
At approximately the same absolute pressure and for the same dry-air mass flow, volume changes in proportion to absolute temperature:
Temperatures must be expressed in kelvin. A comparison based on Celsius values gives an incorrect result because the Celsius zero is not absolute zero.
When both pressure and temperature change, the combined gas relationship described in Volumetric Airflow Reference Conditions in Pneumatic Conveying should be used. The correction must always state which location and reference condition it represents.
Blower Inlet Temperature Changes Displacement Needs
A roots blower displaces actual inlet volume. Warm inlet air is less dense, so a given air mass requires a larger inlet volume. Cold air is denser and places different demands on mass flow, motor cooling and downstream process temperature.
The blower enquiry should include minimum, normal and maximum inlet temperatures rather than one annual average. Intake location matters: air drawn from a hot equipment room may be substantially warmer than outdoor design data.
An inlet filter adds restriction, and a heavily loaded filter can reduce inlet absolute pressure. Temperature and pressure should therefore be evaluated together when converting process air demand to blower inlet flow.
Local Pipeline Temperature Affects Velocity
Discharge air leaving a roots blower is warmer than inlet air when the machine operates against pressure. The gas then exchanges heat with supply pipework, the conveyed product and the environment. Temperature at the pipeline inlet may differ from temperature at the receiver.
Because local gas volume affects local velocity, a critical section should use a temperature estimate appropriate to that section. Assuming one temperature throughout a long indoor-outdoor route can distort both minimum-velocity and maximum-velocity checks.
Aftercoolers can reduce pipeline temperature but add pressure loss and may create condensate. Their clean and fouled resistance, outlet-temperature range and drainage must be included in the duty.
Product and Filter Limits
Warm air may soften polymer particles, dry a moist product, change coating behavior or alter flowability. Cooling humid air can wet hygroscopic powder. The allowable range must come from the product owner and representative tests where the consequence is significant.
Receiver filters, flexible hose, gaskets and seals also have temperature limits. The filter supplier should receive the maximum actual receiver-air temperature, including abnormal but credible operating conditions.
Air temperature alone does not determine final product temperature. Solids rate, particle size, contact time, pipe heat loss and moisture phase change all contribute. A simple specific-heat calculation should not be presented as a guaranteed conveying result.
Commissioning and Seasonal Verification
Instrumentation should distinguish blower inlet, blower discharge, conveying-line inlet and receiver temperature. Trending these values with pressure, speed and airflow can identify ventilation changes, filter restriction, cooler fouling or an operating point outside the expected range.
Seasonal checks are valuable where ambient temperature varies widely. A setting commissioned in winter may deliver a different air mass and pipeline velocity in summer even when the displayed blower speed is unchanged.
Glossary
Absolute temperature: Thermodynamic temperature measured from absolute zero, normally expressed in kelvin.
Cold governing case: Lowest-temperature operating condition that controls a flow, power or process check.
Hot governing case: Highest-temperature operating condition that controls a flow, displacement or thermal check.
Local gas temperature: Actual conveying-gas temperature at a specified point in the system.
Seasonal correction: Adjustment of airflow or operating limits for expected ambient-temperature variation.
Temperature correction: Conversion of a gas volume or related duty between stated thermal conditions.