Optimising the performance of vertical aeration systems

SUMMARY

INTRODUCTION

The basis of good storage practice is adequate cooling and drying and HGCA have funded research to develop IPM systems for feed grain (Armitage et al., 1992 ) and malting barley (Armitage and Woods, 1997), based on cooling dry grain to as low a temperature as practical and top-dressing with a pesticide to deal with upward-moving insects and surface mite infestations caused by absorption of moisture by the grain surface layers. These strategies have been based on studies of horizontal aeration.

In order to conform to ACCS standards, there has been a recent upsurge in installation of aeration systems. However, rather then the conventional, systems, blowing air upwards from under-floor or under-grain horizontal ducts, many farmers have preferred to use vertical duct or 'pedestal' systems where air is drawn down through the grain and exhausted from the duct. These are manufactured by several firms, Lishmann, Brice-Baker and Polypipe Civils being the market leaders. The capital costs of vertical ventilation systems, based on current rule-of-thumb, are lower than conventional horizontal duct systems. Vertical systems are less prone to damage during loading out so they can be expected to have a longer service life. Vertical systems do not require holes to be cut into grain support walls and can be used effectively in grain bulks that are not level-loaded.

These vertical aeration systems raise a number of new problems. The air flow, working pressure and the region of effective cooling are difficult to predict. The novel duct system and direction of airflow are likely to affect moisture translocation and insect and mite distributions. Maximum economic benefit from these systems depends on selecting the largest practical spacing for the units and managing the ventilation to ensure effective cooling of the whole grain bulk by sharing fans between a number of duct units. Current practice is to base unit spacings on 'rule-of-thumb' and experience. Often suction is recommended for these systems on the basis of lower resistance and increased airflow, although in reality the resistance to suction or blowing is likely to be the same. In some cases there may be advantages to be gained from blowing.

This study sought to gather the technical data that is required to develop user guidelines for the available systems and to validate and quantify their ability to cool and dry grain. It would identify any advantages that attach to sucking and blowing.

The aims of this study were:-

• To establish some principles of vertical airflow based on computerised fluid dynamics modelling.
• To measure airflow rates, air distribution patterns and cooling speed profiles in several commercial vertical duct installations.
• To make a systematic study of the effect of perforation size, outlet area and duct diameter on airflow and pressure drop in grain ventilation ducts.
• To use the measured data to develop and validate a computer model of vertical ventilation systems and to use this model to develop practical user guidelines on unit spacing.

The development of systematic guidelines for the spacing of vertical ventilation systems can be expected to ensure effective cooling in all parts of the bulk, minimise capital investment by recommending the widest practical spacing and minimise fan power requirements.

Abstract

Cooling by aeration is the basis of safe storage but, rather then the conventional systems, blowing air upward from horizontal ducts, many farmers have preferred to use vertical duct systems where air is drawn down through the grain and exhausted from the duct. These vertical aeration systems raise a number of new problems.

Aims- The aims of this study were 1) to establish some principles of vertical airflow based on computerised fluid dynamics modelling, 2) to measure airflow rates, air distribution patterns and cooling speed profiles in several commercial vertical duct installations, 3) to make a systemic study of the effect of perforation size, outlet area and duct diameter on airflow and pressure drop in grain ventilation ducts and 4) to use the measured data to develop and validate a computer model of vertical ventilation systems and to use this to develop practical user guidelines on unit spacing.

Conclusions- Computational modelling showed that if the aerator fan is set to blow rather than suck then the volume of grain cooled is up to 20% greater and better cooling is obtained near the floor. The optimum spacing of vertical aerators was calculated for different grain bed depths. Measurements made in six grain stores with vertical ventilation systems showed that the volume of air moved by the fan was up to 20% more when it was blowing than when it was sucking. Diurnal temperature fluctuations were used to estimate the speed of the cooling zone which was linked to the pressure gradient and this relationship used to estimate cooling times . Using a purpose built test rig, it was shown that horizontal pressure drop was about 50% of that for vertical flow. Ducts with >30% outlet area offered little resistance to flow at working flow rates. Ducts with <10% outlet area restricted the airflow. Duct wall resistance was 16% lower when blowing wheat than when sucking. Seed size affected the duct outlet pressure drop in ducts with round outlets but had little influence on the pressure drop with slot outlets. A simple model, based on cooling front velocities and the pressure and flow characteristics obtained from the farm tests and trials with ducts was developed to enable routine design of new systems in a short time and a user guide has been drafted.

Implications- The development of systematic guidelines for the spacing of vertical ventilation systems will ensure effective cooling in all parts of the bulk and minimise capital investment by recommending the widest practical spacing and minimise fan power requirements.