ISO 4174:1998

Overview

ISO 4174:1998 specifies a laboratory method to measure unit pressure loss (pressure drop per metre) for one‑dimensional air flow through bulk grain - including cereals, oilseeds and pulses. The standard defines the test apparatus, filling procedures, measurement parameters and calculation steps needed to derive a flow equation (based on the Kozeny‑Carman approach) that links pressure loss to air velocity and grain properties (porosity, specific area, density).

Key topics and requirements

• Scope: Measurement of pressure loss in the grain column so total ventilation pressure loss (duct + grain + entry) can be calculated; guidance on when duct/entry losses may be neglected (typical velocity limits: ~8–10 m/s main duct, 4–5 m/s secondary duct, ~0.25 m/s entering grain).
• Apparatus: Smooth‑walled cylindrical test cell with two pressure taps 500 mm apart, fine mesh support, fan/compressor, shock‑absorbing bottle, flow meter, manometer (U‑tube) and a micromanometer (accuracy ±0.1 Pa). Instruments for temperature, humidity and barometric pressure measurement; a pyknometer (ISO 3507) for particle density.
• Filling methods: Two standard fill procedures to simulate typical field conditions - loose fill (spout) and sprinkle fill (spreader) - to capture packing/settling effects.
• Measurements & parameters: Repeated tests with increasing and decreasing flows to allow for settling; record flow rate, gauge pressures, temperature, humidity, ambient/barometric pressure, bulk and particle density, and cell cross‑sectional area.
• Calculation & modelling: Derive unit pressure loss vs. inlet velocity curves, apply the Kozeny‑Carman regression to obtain characteristic coefficients (specific area S, Burke‑Plummer constant hb) and use a wall‑correction for true specific area (see clause 5.4). Constants and procedures for density and viscosity corrections are specified.
• Reporting: Full sample identification, method reference, whether corrected flow equation was used, results and any non‑standard conditions or incidents.

Practical applications and users

• Grain storage and silo designers for ventilation sizing and fan selection
• Post‑harvest engineers and aeration system designers to predict energy use and ensure uniform drying/aeration
• Grain technologists and research labs evaluating permeability, porosity and packing effects
• Quality assurance and test labs performing standardised measurements for regulatory or comparative studies

Practical outputs include pressure‑loss curves, fitted Kozeny‑Carman parameters, and predictive equations to estimate pressure loss for different apparent bulk densities and storage heights.

Related standards

• ISO 3507:1976 - Pyknometers (referenced normative standard for particle density measurements)
• Annex A of ISO 4174 (informative) - procedures for determining grain specific area and the Burke‑Plummer constant from grain geometry

Keywords: ISO 4174, unit pressure loss, bulk grain, airflow, cereals, oilseeds, pulses, Kozeny‑Carman, porosity, specific area, silo ventilation, pressure drop measurement.