The intensification and mechanisation of arable farming in Britain over the years since the Second World War has seen crop yields soar. Even in recent years, grain harvesting at some of the more efficient arable operations in this country has rocketed from around 15 tons per hour off the field to something approaching 100 tons per hour.
The majority of this production is stored on site prior to being shipped for milling and processing, and this intermediate storage may have a duration of anything from a few weeks to six months or more. Storing grain over this length of time requires very careful management if it is to remain in the peak condition required by downstream processors.
The key to successful grain storage is to maintain its moisture content and temperature levels within certain limits. The optimum is 14% moisture and an ambient storage temperature of below 15ºC, at which levels grain can be adequately conserved for months at a time, without risk of insect infestation, mould growth or other forms of deterioration.
But harvest conditions vary greatly and these will have a big impact on the moisture content of grain coming off the field. In many instances, moisture levels must be reduced and this is achieved by passing the grain through a purpose-built dryer as soon as it has been harvested.
For some, more modest operations, it is perfectly adequate for the dryer to be monitored by a member of the farm staff and all interventions carried out manually. But when grain production reaches the levels mentioned above, a more sophisticated approach is needed, and highly productive units often demand continuous materials handling processes that will run unmanned, if necessary.
A leading designer and manufacturer of continuous flow grain dryers is York based Kentra Grain Systems, and one of its recent projects has been to supply a fully automated, 20 tons per hour dryer for a new grain store at the Creslow, Buckinghamshire farm of H B & LJ Lear. The main contractor and designer of the installation was Tripp Batt of Bury St Edmunds, Suffolk, and the electrical power installation (which included a 170kVA feed from a nearby former MoD listening station, now owned by Mr Lear) was done by D P Electrical of Granborough, Buckinghamshire.
The drying process
Grain is deposited at a reception pit from where it is transferred via two elevator/conveyors to an initial separation process. Here, unwanted seeds and other debris are sieved from the grain, the finer dust and particulates being captured via a cyclone filter before the clean grain is transferred to the top of the drying chambers. The heat source is provided by two diesel-fuelled burners, and heated air is drawn into the drying column inlet air ducts by forced ventilation. Air exhaust ducts alternate with the inlet ducts in successive layers within the column and are configured in such a way that each inlet duct is surrounded by four outlet ducts.
Grain falls under gravity between these triangular-shaped ducts and the latters’ special configuration allows the drying air to be dispersed both with, and also against the grain flow direction (a drying technique commonly known as ‘Mixed Flow’). This arrangement ensures that the grain is in contact with the drying air for only a very short period, allowing much higher temperatures to be employed – up to 125ºC for feed cereals – without risk of damage to the grain.
Higher operational temperatures mean higher drying efficiencies. Moreover, the furnace design offers a very low resistance to air flow, enabling the use of lower power extraction fans. The grain is cooled before it exits the dryer to ensure that it is at optimum temperature for storage. Ventilation fans in the bulk storage area help to maintain this temperature.
The inherently efficient drying action of Kentra’s innovative Mixed Flow process requires a relatively sophisticated control system – not just to ensure the safe and accurate operation of the dryer, but also to fulfil the farmer’s often complex grain drying schedule, involving a mix of manned and unmanned operation with remote monitoring and event messaging. However, any complexity within the control system is transparent to the user, as Kentra has managed to design a straightforward and simple-to-use operator interface that virtually any member of the farm staff can use with confidence.
The control system
Attaining optimum moisture level within the grain depends upon the measurement and control of several process parameters: the rate at which grain is fed into the drying column and three temperatures - the process drying air temperature, the grain temperature within the drying column itself and a secondary grain temperature taken at the hottest point in the process (which is used as a safety cut-out in the event that it exceeds a preset maximum). No moisture measurements are taken other than an occasional manual instrument check at the dryer outlet; flow rate and temperature are sufficient for the control system model to determine the final moisture content of the grain.
The main control elements of the system are two Parker SSD 3kW ac drives that control the speed of the grain intake conveyors; a Parker SSD dc drive to control grain discharge from the dryer, and two IDEC Electronics PLCs – an FC4A-D20RKI ‘MicroSmart’ unit that controls the grain input rate and an FC4A-C24R2 unit to oversee the dryer. The latter is the master control unit, with the elevator control PLC acting as the slave. Linking the PLCs gives the drying function hierarchical control over specific elements within the system, including various motorised valves and interlocks. IDEC digital/analogue modules are also used to provide the necessary I/O points for the PLCs. The main operator interface is provided by an IDEC HG2F touch screen HMI, as Kentra’s managing director, Barry Higginbottom explains.
“One of the most useful features of this HMI is its ability to hold a whole season’s worth of drying history within its memory – no other memory devices are required. It was also easy to customise the HMI for this application as programming is straightforward; the menu tree, including the grain flow route, is accessed in a logical sequence and we were able to plot the all-critical temperature/time log in a graphical format. When the plot levels off, this is a signal to the operator that the drying process has stabilised and he or she is able to switch from the initial manual start-up sequence to fully automatic discharge control.”
The HG2F also enables the initial entry of grain type, process drying temperature selection, as well as providing real time indication of grain level, operating speed, temperature readings and alarm events. A second HMI (an IDEC HG1F unit) is installed to interface with the elevator control PLC.
Other elements within the control system include all necessary relays and motor contactors and the three Pt100 resistance thermometers. Interestingly, only two channels are used for these devices to save I/O costs. The process drying air thermometer has a dedicated channel, but the remaining two share a single channel via a multiplexer. A PID loop in the master PLC is tuned to control the drying rate, based on the grain feed rate and process temperature.
There are a number of safety features built into the system to protect both dryer and grain. These include excessive drying air temperature, excessive grain temperature, loss of grain feed, grain outlet blockage, electric motor overload, and burner flame failure. All such events are logged for quality control purposes, as well as to provide a useful tool for service and maintenance. Kentra has also included a GSM module to provided SMS alarm and event messaging to mobile ‘phones during periods of unmanned operation.
The main control console, which houses all control elements, including the relays, contactors, drives, PLCs I/O modules and HMIs, also features an analogue mimic panel. Mr Higginbottom explained that, while this time-honoured form of machine monitoring is not strictly required because of the use of modern HMIs, many users are happy to incur the additional expense because the mimic provides a complete overview of the machine status at-a-glance.
The green field project at H B & L J Lear’s farm was unusual in that it presented a blank canvas for Tripp Batt’s design team. “This is most unusual, these days,” says Mr Higginbottom. “Normally, we would be appointed to retrofit machines or shoe horn them into existing facilities. This new 20 ton per hour grain dryer and storage facility was designed from the foundation up and gave Tripp Batt a prime opportunity to ensure that it met all of Mr Lear’s requirements.”