Keeping cool’s key to 26,000t grain store’s success

PX Farms’ gleaming new grain store faced a baptism of fire in harvest 2008, having to cope with drying as much as 95% of the incoming crop. It passed with flying colours, says James Peck of PX Farms.


“The combined capacity of the drier and wet bin means we can hold over 250t in the drying queue at any one time, or tip on the concrete pad and bucket the grain in when there’s space,” says Mr Peck. “It never caused any bottleneck in tipping or delay in harvesting – and hopefully we won’t get another year like that.”


Leased to merchant Wellgrain, the store contains not only the substantial output of the host farm’s crops but also that of 30 local growers. It has an intake rate of more than 150t/hr using on-floor tipping and a 120t/hr central overhead conveyor.


This has a “travellator” that moves forward in 1m steps to form a neat 60m-long pile, 14.5m deep at its centre and almost 7m at the edges. The store has clearly been built for capacity and efficiency.


But of equal importance in the specification is the monitoring and cooling systems designed to combat potential insect pest infestation.


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Gary Milner of Robydome checks the computerised control and monitoring set-up for the ventilation/cooling and drying systems.


With such a huge amount of grain in one heap – and the responsibility that comes with managing the output of the 30 local farms, let alone PX Farm’s own in-hand and contract farming operations – outstanding store management is imperative to maintain quality and operating throughput.


The process begins with accurate monitoring of incoming grain moisture, then drying as necessary to 15% moisture content. Each load is rigorously sampled and recorded on the intake office computer.


“While moisture content is important, we also recognise how crucial it is to get the grain cool enough to store in good condition and free from insect pests,” says Mr Peck. “That means getting the temperature down from a typical 32ºC at intake to the target store temperature of 6ºC as quickly as possible.”


The ventilation system is designed to cool the grain rapidly to less than 10C, at which point the insects will not breed; at single figures they will die.


As the 30m wide store is loaded, six rows of ventilation ducts are laid and ratchet-strapped to fixings on the floor. They start at 900mm diameter, then step down to 750mm, 600mm and finally 450mm to give the even air flow calculated by Silsoe-based independent grain storage engineer, David Bartlett.


Mr Peck emphasises the need to get the air flow right: “There is a temptation in most farm grain stores to over-engineer and over-specify,” he says. “But that risks over-drying the grain; when a 1% loss on a store this size equates to over £20,000, it pays to get the sums done and get the right air flow.”


The ventilation system operates in three zones, each with a pair of high-capacity, two-stage fans and ducting.


In the central zone, seven Robydome probes, fixed to the ducting tie-downs and incorporating three monitoring points each, check grain temperature throughout the depth of the pile and feed the results to a central computer.


Two zones down either side have 13 single-point probes each. These are suspended from the roof so they can be pushed into the grain as the store is filled but, more importantly, can be removed and safely coiled back up to the roof as the store is unloaded.


“We wanted to avoid mobile probes that involve walking across the grain or that could be left in the grain and possibly lost or damaged as the store is emptied,” explains Gary Milner of Robydome.


The large scale of the PX Farms store called for a custom design of monitoring probes but the principle can be scaled down to any on-farm store, from individual bins to small on-floor storage.


“The computerised controller for differential cooling is equally applicable to any storage situation, bringing about the same benefits in energy and automation efficiency and assuring the quality of the grain,” Mr Milner adds. “It can be incorporated into any existing system or new store build.”


Once grain starts pouring into the store, the cooling fans are run intensively to equalise the heap to a common temperature. Then a sophisticated differential cooling programme in the control system kicks in to bring the temperature down to the required levels.


The Robydome monitoring and control programme continuously tracks grain temperatures throughout the store and compares it with the outside air temperature and humidity. The fans are set to come on when ambient temperature is 2C below that of the stored grain and relative humidity is below 65; any rise above these parameters automatically switches the fans off.


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Having sufficient ventilation capacity to rapidly cool grain via floor ducting for effective quality and insect pest control was given a high priority in the store’s specification.


Automatic control ensures air carrying excess moisture is not pushed through the grain – otherwise it will be wetted rather than dried and a build-up of caked fusty grain alongside the ventilation ducts will interfere with air flow and cooling performance.


“The parameters are selected to give the most efficient possible cooling whilst ensuring the optimum humidity is maintained for good storage,” says Mr Peck. “The record of fans switching on and off throughout the day and night clearly shows it would be impossible to achieve this level of accuracy and efficiency with manual operation.”


Given the volume of air being pumped into the store, it is important to think about extraction capacity. After all, as Mr Peck points out, moist air condensing on a cool roof will drip moisture back onto the grain and create a crust of damp material highly conducive to insect activity.


“You need to take out an equivalent volume of air that’s gone through the grain as quickly as it’s blown into the pile,” he emphasises. “The eight large extractor fans in the roof automatically start up with the ventilation fans – but we can also switch them on manually to keep the working environment clean and virtually dust free when filling the store and out-loading lorries.”


With bespoke administration software recording grain movements and linked to Wellgrain’s own system, growers renting space on a stored volume basis know precisely how much grain they have to sell – and do not have the responsibility of managing stored crop at a busy time of year.


“From experience, most grain storage problems occur in October and November, once the grain is believed to be safely in the store but when most farmers are too busy with field work to regularly monitor and manage the store,” Mr Peck points out. “For growers using our facility through Wellgrain, all risks and concerns are removed; they can concentrate on the new crop in the ground.”

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