Supplementary information to the Harvest Maxi-Stirrer instruction booklet.
With the introduction of the Harvest Maxi-stirrer to bulk storage systems drying techniques have dramatically changed with the ability to thoroughly mix the grain in
store it is now possible to achieve faster drying without the risk of capping the grain with excessive moisture not clearing the crop previously safe drying in bulk storage was achieved by ventilating with air of controlled humidity which was not over dry so preventing the air from becoming saturated and not clearing the top of the crop and depositing more moisture within the store the ability to keep the air moving through the full depth of the grain has led to faster drying and the ability to use heated air as opposed to humidity control this has led to the option of two different methods of drying with the choice of humidity or temperature control.
Humidity Drying:
The continual movement of grain increases the ventilation rate the previous figure of 100cfm of air per tonne no longer applies and many new stores are fanned at only 50cfm per tonne although this is adequate in most situations in extreme circumstances the ventilation rate may have to increased when excessive amounts of moisture need to be expelled from the crop. If it is found that moisture is building up in the store a higher rate needs to be employed to expel moisture already removed which is still sitting within the stack so the air flow needs to be increased by ventilating a smaller area.
These circumstances should only occur when excessively high moisture content is encountered. Although the control of humidity is not as critical when stirrers are employed it is not economical to set too low humidity level as this will not lead to any faster drying and only consume more gas even with the stirrers humidity drying cannot exceed ½% moisture removal per 24hrs.so settings should only be made to remove up to 2% moisture content then reset for the next reduction using an excessively low r/h setting will only raise the air temperature slightly not enough to give any faster drying as you would get with temperature control.
If ½% per 24hrs is not achieved check that :-
You have at least 50cfm per tonne ventilation rate eg: fan output 30000cfm @ 50cfm per tonne will dry a 600tonne batch, calculate store capacity and open air doors relevant to that amount (do not ventilate or stir down the angle of repose as excessive air will be lost through lower depth)
Check accuracy of humidity sensor to ensure that a lower enough setting is achieved. Also compensate if ambient temperature is below 15%c (reduce rh setting at least 1% for every 1degree C below 15degree C ambient temperature). If excessive moisture is found on the surface of the crop this is due to inadequate ventilation within the building allowing moisture to form due to condensation above the crop.
Temperature Drying:
With the ability to keep the crop turning over it is possible to dry with heated air to give a higher rate of drying than with humidity control.
Air Temperature:
It has been found that a grain temperature of 30degrees C will enable the crop to give up its moisture at a higher rate. The temperature lift will depend on the temperature of the in coming grain in good harvesting conditions grain can come off the field at 25degrees C+ at other times the temperature lift will have to be greater in general if it is only required to remove approx.
2% it is probably more economical to use humidity drying as it can take at least 24hrs to get grain temperature up to 30degrees C using probably as much gas as would be burnt during humidity drying.
Stirrers should not be run continuously during the initial heating period as increased ventilation will only prolong the process once the grain has reached 30degrees the stirrer can be ran but still paying attention to the grain temperature if this starts to fall then either the air temperature must be increased or the stirrer ran intermittently to maintain 30degrees C.
An air temperature of least 50degrees C is required, exposure to this heat without stirring will not cause any problems, depending on store and grain conditions an air temperature in excess of 50degrees may be required to give the right grain temperature.
Air Flow:
50cfm per tonne is the maximum in most cases considerably lower air flows are sufficient although difficult to measure in most cases if an air flow can be discerned it is usually sufficient (Cassela air flow meter just registering air flow disc just off bottom stop). The larger tonnage that can be ventilated the lower the drying cost.
The lower the air flow the slower the air speed through the crop consequently the heated air has longer contact with the crop removing more moisture from the crop.
Maintaining Crop Temperature:
This can be done by:
Adjusting air flow (ventilating more crop)
Increasing temperature setting.
Running stirrer intermittingly.
Problems that can be encountered when temperature drying:
When drying crops with moisture content in excess of 19-20% particularly in adverse ambient conditions if it is found that moisture is being removed from the crop but not expelled from the stack it may be necessary to increase the ventilation rate when a dramatic drop in moisture content will be observed this will be water that has been removed from inside the kernel but has remained on the surface of the grains because air flow is saturated and unable to carry anymore moisture away. If this situation continues it may be necessary to reduce the drying rate until upper levels of moisture have been removed.
Condensation:
Condensation within the store can be a problem when high moisture contents are being removed upper levels of moisture are easier to remove as water is present nearer the surface of the kernel and can be evaporated easily this can lead to saturation of the air flow and if there is inadequate extraction from the building condensation will take place. If ambient temperatures are low so the building structure is cool water will be deposited back on the crop which will have to be dried again.
Extreme care must be taken while temperature drying under these circumstances at may be even necessary to revert to slower humidity drying to allow the air flow to carry the moisture away until the grain is below 20%.
Ventilation and Extraction Fans:
Extraction fans can reduce condensation problems but with the increased drying depth of floor stores (4.2m) and eaves height of around 9m roof extraction fans are becoming further away from the surface of the crop and it may not be possible to keep all the moisture in suspension while reaching roof level. Side extraction fans fitted in gable ends or in the side cladding of the building can certainly be beneficial or a mixture of both types can be utilised. Free flow ventilation with louvers has not proved satisfactory in many cases.
When ambient conditions ( high humidity low temperature) are causing an increase in condensation problems the drying process can be halted over night. Turning off fans, heaters and stirrers it has been found that very little temperature is lost over night as grain is a good insulator and the heat within the stack will allow the moisture to continue traveling towards the surface of the kernel to be easily removed when ventilation and stirring recommence in the morning using this regime will not result in drastically slowing the drying process but will give a reduction in energy consumption that can reduce the overall drying cost if maximum speed is not essential.
Frequently Asked Questions:
Q. How long does it take to reach a grain temperature of 30degrees C?
A. Depending on grain temperature leaving the field it can take 24 hours to achieve, do not stir during this initial period and make sure that the air temperature is at least 50degrees C.
Q. We cannot get grain temperature of 30degrees C irrespective of how long we heat?
A. Air flow is too high giving excessive air speed so not transferring heat to the crop.
Reduce fan output or increase the area being ventilated, airflows of 20-30cfm per tonne are enough when temperature drying. Make sure that heaters are capable of holding an air-tunnel temperature of at least 50degrees C even during the night when ambient temperatures drop and the temperature lift has to be far greater. Most problems encountered with temperature drying are due to not holding 30degrees C grain temperature or incomplete moisture removal from the building leading to rewetting of the crop.
Q. Should I stir continuously during temperature drying?
A. Monitor grain temperature when first starting to stir if temperature drops only run intermittently to maintain temperature of 30degrees C.
Q. What is the difference in cost between humidity and temperature drying?
A. If both systems are used at maximum efficiency there should be virtually no difference in cost per tonne. Humidity method will consume less heating energy but take considerably longer so gas prices will be low but fan running hours much longer. Temperature drying will consume more gas but with a drastic reduction in fan running costs, if multiple or two stage fans are used it is often possible to switch units off to give lower air flows.
Usually the largest consideration with temperature drying is speed enabling batches to be dried and removed from store making way for further incoming crop.
Q. When humidity drying should I run stirrers continuously?
A. In most cases yes stirred grain enhances air flow and gives even moisture content through out the stack. If difficulty is encountered when trying to attain the final moisture content and accuracy sensors has been checked it can be helpful to only stir for short periods of 1-2hours then run continuously for at least 24hours when final moisture content is achieved.
Q. When using humidity control should the same settings be employed as with a static store?
A. It is not so important to take moisture levels down only 2-3% at a time but a setting to give 5-6% removal will not be energy efficient so if high moisture contents are encountered it still pays to reset the humidity controller once or twice during the drying regime. Check the accuracy of the humidity sensor this is the cause of most problems when unable to finish drying.
Always remember to compensate humidity setting during periods of low ambient temperature (below 15degrees C.).
Q. What depth can rape be dried at?
A. The most common problem with slow drying of rape is insufficient depth of crop, having had to limit depth in static stores the same does not apply to stirring.
Shallow depths of rape will give very high air speed through the crop and unless exceptionally high moisture content is encountered rape can usually be put in store at maximum storage depth preferably no less than 8 foot (2.4metres).
Q. Will I rewet the crop when cooling after temperature drying for long term storage?
A. Early in the season the best cooling conditions occur at night which is also a time of high humidity. If the crop temperature is higher than the ambient air temperature air introduced into this area of higher temperature will have the moisture removed so no increase in grain moisture content will occur, but once the crop temperature reaches equilibrium with the outside air humidity controllers should be used set to the final moisture content.
Q. When cooling will I reduce the moisture content of the crop?
A. This depends on grain temperature and time elapsed before cooling. If cooling is started immediately after drying has finished very little further reduction in moisture occurs.
If the grain is left heated for 48 hours or more a further reduction in moisture content will occur when cooling, as remaining moisture in the kernel will migrate to the surface of the grain becoming easier to remove. Amount removed will depend on final grain temperature and time left before cooling.
Q. How many passes are required by the stirrer to move total area of crop?
A. Providing the unit travels up and down the building moving across its previous path and not returning in the same track and area 18m long by 10m wide will approx. 6 passes to move the total area. The timers in the control panel can be used to adjust the pattern of travel.
Any other problems or observations from users would be appreciated to help us obtain more knowledge mainly on temperature drying techniques. As you will appreciate this system is a relatively new development in bulk stores and we are still very interested in your comments. Please use our link on the web site to contact us or fax 01795 538524.
Harvest Maxi-Stirrer© Systems.
H.G.C.A suggests growers consider the use of stirrers
The home grown cereals authority publication “on-floor drying to minimise grain spoilage” which sets out guidelines for drying and reducing grain spoilage in bulk stores recommends considering installing grain stirring units to speed the drying of the wettest grain within the store.
In “static” stores it also suggest reducing the crop depth by almost 1m when moisture contents are in access of 20%, but with the current input rates and the need to maximize storage space this could be a problem.
Harvest Installations have found that stores fitted with the “Maxi-Stirrer” can be loaded to full depth even with moisture contents in access of 20% and the action of the stirrer will enable adequate air flow to continue drying, also rape and other crops giving a high resistance to air flow can be filled to much greater depths than with a static store so more making more efficient use of storage area.
Other information obtained from modelling projects show that grain up to 20% can be dried at lower cost with the use of stirring systems with crop storage to a depth of 4.8m.
Grain in access of 20% shows a slight increase in drying costs when using stirrers, but in a static store the bed depth has to be reduced by half to be able to dry, this means only half the capacity in store and when the crop is dry it will have to be reloaded to maximum storage depth obtaining full capacity but with increased handling charges.
With the increased use of temperature drying as opposed to humidity control in bulk stores there has been some concern as to the safe maximum air temperature, over the last few years many customers with stirrer units have used air temperature off the fan of 40-45c which gives a grain temperature of around 30c well below the maltsters and millers requirements that grain should not exceed 50c.
Stirrer units also give very quick and even cooling of the crop once dried so reducing the risk of infestation.
Floor storage trends
A survey of bulk grain stores that Harvest Installations have been involved in over the last three seasons has shown that the average capacity of new stores is now in excess of 2000 tonnes.
Storage depth is invariably 12’ (3.6m) or 14’ (4.2m) with the use of stirring systems there is no longer any problem with storage depths over 10’(3m) but consideration must be given to the provision of an adequate “pusher” to be able to achieve these greater depths when filling to maximum capacity.
Although store capacity has been increasing over the years fan size has not increased proportionately it appears that a fan capacity to dry approximately 500tonnes per batch fulfils most growers requirements this keeps energy requirements both electrical and gas at reasonable level for either humidity of temperature drying.
Increasing number of stores are being fitted with a second stirrer unit enabling two batches to be treated at the same time this means conditioning, cooling or drying can take place at the same time when batches need to be moved on quickly. Although some stores are initially supplied with two stirrers most second units are fitted retrospectively.
Without exception ventilation and extraction fan units are fitted as standard to avoid excessive condensation problems that can occur with the enhanced rate of drying now available. Fans can either be roof mounted, positioned at the eaves or gable end these units also greatly improve the working environment within the store, current installation techniques avoid any ingress of rain making these fans completely waterproof. All extraction and air inlet areas can be fitted with acoustic cowls which have been used in noise sensitive areas.
Building structures now have further increased eaves height to allow for greater storage depths and to give clearance for ever increasing trailer size. Concrete retaining walling has now become almost universally used giving a durable and easy cleaning structure.
There is also an increasing trend to design buildings with side entry doors this give the advantage of segregating the storage area into four units if required.
Although various patterns have evolved over the last few years with the incorporation of stirrers into floor stores and drying units it is still imperative that each customers individual requirements are discussed in depth before a final specification is decided on.
Non standard temperature & humidity controllers
The manufacture of gas fired humidity and temperature controllers has been a substantial part of the business of HARVEST INSTALLATIONS over the years.
Recently however the need to supply burners built to give sufficient heat for ever larger grain and vegetable stores has meant that many controllers have to be designed and assembled which have outputs different to the stock range of units.
As grain and onion stores increase in capacity air requirements per tonne are calculated along with the BTU’s per hour required to give the sufficient temperature rise or humidity control. Single units of up to 4 million btu/hr have been supplied and multiple units complete with purpose built stand working from a single sensor supplying heat to multiple fan units have become normal practice within larger onion stores.
The required output for each specific controller is obtained by calculating the necessary gas jet sizes and not by changing the gas pressure, this maintains the optimum gas to air mix which ensures a clean efficient burn enabling the burners to accurately hold the required temperature or humidity.
As the standard stock range of units are built through out the year and held ready for delivery the special units can be built to order normally within the required delivery period.
For control of fan units before or after using heat HARVEST INSTALLATIONS have designed and market the ‘Advance Controller’ which uses the same humidity or temperature sensors as supplied with the burner units. This controller can be programmed to switch fans on or off between preset upper and lower limits of temperature or humidity, the unit is extremely simple to set up and has a clear digital display giving the sensor readings.
The most popular uses for this equipment are:- Reducing grain temperature when the ambient air is suitable. Use of ambient air without the application of heat when grain is in store at a high moisture content. Ventilation and conditioning of crops with suitable ambient air and the monitoring of crop temperature within the store.
Case Study - 2006 Installation
One of the largest stores completed this year holds approx: 4000 tonnes with Maxi-stirrers both sides of the Harvest steel main air tunnel and storage at 14 foot (4.2m). Due to the length of the building it was decided to have fan houses at both ends, an internal fan room at the entrance end was used to keep the doors and concrete apron clear of any obstruction. The store has drive over wooden flooring and concrete grain retaining wall.
Equipment for a complete new floor store in New Zealand is currently being manufactured ready for shipment. Maxi-stirrer, main air tunnel and burner units are going to join other CHC units which have been shipped during the last 12 months. We are fortunate in having a representative who had previous experience of selling and installing our equipment in the UK and is now using his expertise to introduce farmers over there to the benefits of using Harvest equipment.
Burner assembly has also started to fulfill existing orders and build up our usual stock for next season.
Burner service forms are being sent out in the NewYear for existing customers to book a fixed price service as we have done in the past years. Maxi-stirrer forms should reach owners during this November. With the increasing number of both CHC's and Maxi-stirrers in use an early start to servicing is important to enable all units to be inspected before next harvest. Any customers not receiving forms and wishing to book a service please contact the office.
News - new extraction fan model
Now available for installation our new extraction fan Model GEVE 900 for gable- end installation includes mounting frame and weather hood combines efficient extraction capabilities and low noise levels whilst running.
Call us for more information on the latest in the ever increasing Harvest product lines!