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Farm Ventilated Bin Drying Plant

Sufficient information was gained at Askham Bryan during the grain bin ventilation experiments of 1946 to enable an installation to be designed to cover the requirements of a farm growing about 150 acres of grain; but before a plant of this kind could be recommended for general use it was desirable to erect a full sized installation in order to study the economic side and the technique of drying grain in bins on a farm, For various reasons it was impracticable to build a large storage plant at the Institute itself; so to make further progress it seemed best to work at this stage with a farmer who wished to use the ventilated bin system, who had sufficient confidence in to invest the necessary capital, and who was willing to help in costing and to allow experimental work to be done. The plant to be described was, in fact, built by a farmer who co-operated in this way, and undertook a great deal of detailed design work.

A combined drying and storage plant of this kind has several advantages. It provides the necessary storage space for the grain growth on the farm, as well as a means of drying it, for a cost very much lower than that of a conventional drier and storage of equivalent capacity. The hot air temperatures used are so low that there is no risk of damaging the grain by excessive heat and, because the drying process is spread over a period which may extend if necessary to ten days, it is far easier to obtain an even and precise moisture content in the finished product than with a high temperature drier. The system enables the degree of drying to be adjusted by a simple method of varying the relative humidity of the ventilating air, and owing to the slow movement of air through the grain the relative humidity of air leaving the grain surface at the top of the bin may be used as a means of measuring the moisture content of the grain at this point, and of determining the time when ventilation can be safely discontinued. Furthermore at any time during storage a check on the condition of the whole bulk of stored grain can easily be made by ventilating each bin for a short period.

Against the above advantages must be set the following limitations. When full bins are dried with an air velocity of 10 feet per minute and the air is conditioned to a relative humidity of about 60%, the average drying rate for the whole bulls of grain is approximately ½% per 24 hours' ventilation. This means that the system cannot be used where much grain has to be harvested at 21%m.c. or over, as mould will develop on grain stored at high humidity for too long a period, A small batch of wet grain, however, can be dried out quickly enough if placed in an empty bin; but this involves extra handling.

Main Features of the Design

It was decided that the storage space should be divided into 6 units, each of 25 tons capacity-filled by bucket elevator and belt conveyor and emptied by a pneumatic elevator-and as ventilation would normally end before the bins needed emptying, it was decided that one electric motor could be used for driving either the ventilating fan or alternatively the fan of the emptying conveyor.

Previous experimental work has shown that such a plant should have the following features in its design:-

1. The bins should have flat floors.

2. Grain should be stored to a height not exceeding 10 ft, above floor level.

3. The fan should be capable of ventilating up to two thirds of the stored grain at any one time, at a velocity of 10 ft. per minute, the grain being pre-cleaned to facilitate the passage of air through it.

Construction

A ventilated bin drying plant for grain embodying the main features was erected on a farm during 1947. The main structure was built entirely by the labour on the farm during slack periods with the help of a bricklayer and his labourer, as this appeared to offer the cheapest method of construction. The plant which is shown in figure 1 (Side view of silos) was made up of six circular bins, 11 ft. high and 13 ft. In diameter, and had a total capacity of about 150 tons of wheat. The bins were fitted with a porous floor made of foam slag blocks, and the air v/as led to them through 2 ft. square section concrete ducts. The air was supplied by a medium speed reverse curve fan driven by a 10 h.p. electric motor and heated by 21 KW electric heater placed in the main duct. During 1947 all grain entering and leaving the bins was conveyed pneumatically by air from a second fan driven by the same motor; although a system of mechanical conveyors had been designed for filling the bins but could not be completed in time. The plant itself was built at a total cost of £699-13-9 and the whole installation including the Dutch barn covering it cost £1009-13 - 9.

Air Ducts

Work began with the construction of the ventilating ducts. Those were pre-cast in wooden shuttering, the concrete used consisting of three parts of gravel, two parts of sand and one of rapid hardening cement. There were five types of duct. each being 2 ft. square on the inside, 3 ft. 6 in. long with two-inch walls. They were arranged as shown in figure 2 (Foundations and floor arrangements) and are detailed in figures 3A (duct type A -8off, 3B (duct type B -4off) , 3C (duct type C -6off) , 3D (duct type D -1off) & 3E (duct type E -1off .

Type A was a straight pipe with male and female bevelled joints.

Type B has openings in two sides to form the branch joints to each bin. It was considered that it would weaken the pipe too much to leave a 2 ft. Square hole in opposite sides of any one pipe, and a hole 1 ft. by 2 ft, was left at one end of each of two pipes to form the outlet branch joints. Thus two of those pipes provide openings for a pair of duct branches. Care was needed to ensure that these holes were left at the female end of one pipe and the male end of the other to give the required air seal.

Type C was used as the end of each branch and was cast with a blind end and with a 2 ft. square hole as the outlet to the bin.

Type D gave the branch joint at the end of the main duct and was cast with one blank end and a 2 ft. square hole on each side.

Type E has a 2 ft. square hole in the centre of the top of the pipe to take the heating unit. An angle frame was set round the hole to form an airtight seating for the unit and at one end was a flat steel face to form a flush fitting for the outlet of the fan.

Shuttering

Each of these duct sections were cast in the same shuttering (see figure 4 (duct casting box ) which was modified as required to make the different duct sections. It consisted of a flat baseboard and two slots of shuttering. The bottom of the inner set was located by pins driven into the baseboard and was kept in shape by removable struts. The inner shuttering was in four sections and hinged at the corners to make it easy to remove. The four outer sections wore located by battens nailed to the baseboard, and wore bound with wire to keep them in position. Where a blind end v/as necessary this was formed by pouring concrete on to the baseboard with the outer shuttering in position. The inner shuttering was then inserted, and any hole required was formed by nailing 2 in, strips in the appropriate positions to prevent concrete from flowing where it was not needed. Two separate bases were used to enable duct units to be made at the rate of one every three days. The moulds were filled and left for three days before the shuttering was removed, and the duct sections were not moved from the base until after the sixth day. Covering slabs for the duct chambers were also cast in concrete and were reinforced because being under the false floor of the bins they had to support the weight of the grain, they were 4 in. thick, 3 ft. 4 in. long and 2ft. wide.

Valves

Special sections of ducting 4½ in. long were cast round the valves for shutting

4. Duct-work should be of large cross sectional area to avoid pressure losses, and arranged so that any one bin or combination of bins could be ventilated as required; the necessary valves should be airtight, accessible and cheap.

5. Air heaters should be capable of heating the maximum volume of air to be used by 10°F above atmospheric temperature, their output being under control to suit variations in the volume of air required and in atmospheric condition off the air from the bins, and are shown in figure 9A (air valves frame). Each valve was made up of an angle iron frame, a slide, a sealing wedge, and a cover for use when the slide was removed. Each frame was welded up in a jig to make sure that it was of the same size and would, therefore, take any of the slides or sealing wedges. The inside of the frame was 25½ in. wide at the top tapering to 23 in. wide at the bottom and made as shown in figure 9A. The slides were made from ⅛ in. thick mild steel plate and were 26½ in. wide at the tops, 24 in. wide at the bottom and were 30 in. long. A handle was riveted to the top as shown in figure 9B (air valve slide) , Steel plate was used because it was considered likely that wood would shrink or warp when exposed to the hot air. Further, it was found that when the slides were cut out with an oxyacetylene cutter they buckled slightly so that it was necessary to cut them out with a shear or saw. For sealing, a strip of rubber insertion 2 in. wide, and ⅛ in. thick was fitted round the bottom and sides of each slide so that it projected ½ in. beyond the edge. It was held in place by a mild steel strip 1¼ in. wide and ⅛ in. thick, secured by aluminium rivets at 3 in. intervals, (aluminium rivets were used because they were less likely to cause distortion of the rubber by excessive pressure), A cross section of the edge of the slide, and a section of the frame with the slide in place, showing the way in which the air pressure helps the sealing are also given in figure 9B.

The sealing wedge is shown in figure 9C (wedge for valves guides) and was made from hard wood with a strip of ⅛ in. rubber insertion in the centre, When a slide was in place the wedge was pushed hard into the frame in order to prevent escape of air from the top, figure 9C, The cover strip, figure 9D (cover for valves slide) , was also made from hard wood backed with a length of 1¾ in by ¼ in. angle iron and faced on the bottom with rubber insertion padded with resilient material nailed in place. It was held down by wing nuts on the two studs welded to the frame as shown in figure 9A,

Foundations

Because the water table was near to the surface on the site chosen, it was not possible to excavate deep enough to take the whole of the channels for the ducting and its insulation. Shallow trenches only deep enough to obtain a firm level foundation, therefore, were excavated, and the soil removed was placed in the spaces between them to form the core of a platform to carry the silos. On the floor of the trenches a layer of concrete 4 in. thick was laid.

The sides of the duct channels were built from 4½ in. brickwork 2 ft. 10 in, high to give finished channels 3 ft. wide. The layout of the channels is given in the plan in figure 2, and an early stage of their construction is shown in figure 5 (photo base construction) . It will be seen that, in order to keep the ducting as short as possible the channels did not run to the centre of the two end silos. The two outside walls of the silo platform were also of 4½ in, brickwork with 9 in. piers, laid on a 6 in. concrete foundation, and were built up so that they wore level with the walls of the duct channels. At the ends of the plant, the walls were curved to follow the line of the silos; but at the sides they were straight and set back 15 in. from the outer edges of the silos, as shown in figure 2. The duct sections were laid on single bricks, the top joints being sealed from the outside and the remainder from inside the pipes. The duct-work was completed by packing

down the sides and on top of the air duct pipes. The space under the ducts was left clear in case any moisture should collect there and give trouble by destroying the insulating properties of the sawdust. The whole system of duct work was finally covered with the reinforced slabs; these came half-way over each side wall, leaving a ledge to support the concrete covering the silo platform. The slabs which spanned the duct junctions were placed parallel to the main air duct and were supported by 2½ in. angle set in the walls.

Before concrete for the silo platform was laid, the space between the walls forming the sides of the duct channels and those at the outsides was filled with well rammed soil until the surface was about 2 in. below the tops of the walls. This area was then concreted over 6 in. thick. The overhanging portions of the outsides were made by setting up suitable shuttering at these points, and were reinforced by substantial bars running well back under the silos at right angles to the line of the walls. A drain pipe with an outside diameter of 7 in. Wrapped in paper was set in each one, and withdrawn when the concrete had set leaving holes into which the grain outlet pipes were afterwards cemented. As the concrete covering of the silo platform was being put on, the outline of the silos was determined and strong wire loops were bedded into the concrete all round it. These were about 1½ to 2 in. high, and formed the anchorage for reinforcement in the silos themselves. The edges of the holes whore the ducts came through the concrete surface of the silo platform were chamfered off to give a bevelled outlet 3 ft. square at floor level and four channels were formed in the concrete 6 in. wide and 6 in. deep, radiating out from the corners of each of the duct outlets to within 2 ft. of the silo wall. Those can be seen in figure 8 (bin details) , and helped to distribute the air evenly over the whole of the silo floor.

Silos

For the silos themselves, a cylinder of reinforcement was erected and plastered first with concrete and then with 2 layers of cement mortar; the reinforcement was surplus air-strip tracking 10 ft. 7 in. wide. It consisted of 3in. Mesh galvanised wire netting reinforced with ⅜ in, diameter mild steel rods placed vertically at 8 in. intervals. Two cylinders of this reinforcement wired together were used for each silo and were set one inside the other so that the ⅜ in. diameter rods did not coincide with each other. The reinforcement was anchored to the silo platform by means of the loops inserted for the purpose when the concrete surface was applied. The complete cylinder of reinforcement was then wound with ¼ in. diameter steel balloon cable to give added strength: the bands cable were 4 in. apart at the bottom, the interval increasing gradually to 12in, at the top. Figure 6 (photo bin reinforcing) shows the plant with the silo platform completed and the silo reinforcement in course of erection.

The first layer of plastering was made up of 3 parts of ¾ in. aggregate, 1 part of sand, and 1 part of cement. It was mixed to a fairly stiff consistency and a rapid hardening cement was used in order to make the work of plastering easier.

The first layer was applied from the outside of the silo, while a board about 3 ft. long and 1 ft. 6 in. wide shaped to the curve of the silo was held on the inside to prevent the concrete from falling away. The surface was left quite rough both inside and out in order to provide a key for the subsequent layers of plaster. Figure 7 (photo part finished bin) shows this first stage of plastering on one silo. The other layers of plaster were made from throe parts of sand, to one part of rapid hardening cement and applied with a plasterer's trowel in the usual way, the silos being wetted thoroughly beforehand. A single layer was put on each side to give a finished wall thickness of about 2½in.

Silo Floors

The porous silo floors were made from foam slag blocks 18 in. long by 9 in. wide and 2 in. thick. They were set on honey-comb brickwork giving a space between the underside of the floor and the silo platform 4½ in. deep. The duct outlets were bridged by reinforced concrete members 4½ in. by 3 in., 4 ft. long. A diagram of a section of the floor and its supporting brickwork is given in figure 8. The supporting bricks were set in cement to prevent them from rocking, and great care was taken to ensure that the upper surfaces of all the bricks were at the same level. Bricks and half-bricks wore alternated under the comers of the foam slag slabs, the joins of the latter were also staggered to prevent any four comers meeting at the same spot (the corners of these slabs were easily rubbed off during transport or subsequent handling). The slabs were not bound to the bricks in any way as the bricks were bedded-in level and binding the slabs was unnecessary.

The joints between them wore pointed where necessary with a special mortar made up of 5 parts of crushed foam slag block to 1 part of cement. The aim was not to point all the joints completely, but only to fill up any places where the edges of the blocks had been damaged during transport and there were gaps large enough to allow grain to fall through.

Before the silo floors wore completed outlet pipes 5 in. square in section and 20 in. long were made from 16 gauge mild steel plate and bedded with concrete into the holes left in the overhanging parts of the silo platform and, when in place, did not rely on the porous floor for support but were hold entirely by the platform.

The foam slag blocks were cut to be a rough fit round these outlet pipes and the spaces between them and the edge of the outlets were filled with the special mortar used for pointing the joints between the slabs. The method of bedding the pipes, and the construction of the slides are shown in figure 10 (duct for grain outlet) .

* Such silos are not waterproof, and if the walls are to be exposed to the weather, they must be treated with some form of waterproofing compound.

Machinery and Switch Gear

The motor, fans and electrical equipment were installed in a small building erected at the end of the pant and can be seen on the left of the photograph in figure 1. The ventilating fan was bedded down with the centre of the outlet in line with the centre of the main duct, and a sheet metal adaptor made to connect the fan outlet, which was 17 in, by 22 in., to the end of the concrete ducting, which was 2ft. square. The adaptor was bolted to section E of the ducting, figure 2 with a cork gasket to prevent loss of air. The fan was joined to the adaptor with a flexible coupling fabricated from sheet rubber insertion ⅛in. thick, the aim being to isolate the rigid concrete duct from any vibration of the fan. A section of the complete joint between fan and ducting is shown in figure 11 (trunking for duct connection) . The ventilating fan was driven by two 11/32 in. V belts from a 10 hp, 3 phase motor set on slide rails as shown in figure 2. The fan for the pneumatic conveying was set on a pedestal 2 ft. 3½ in. high so that the ducting could run above the concrete ventilating duct, and was so that belts of equal length could be used for driving each fan in order to avoid having to keep spare belts for both drives. The tension of the ventilating fan belts was adjusted by moving the motor on its slide rails, and that of the conveying fan bolts by inserting shims under the fan itself.

The electrical circuit used is given in figure 12 (21KW air heater) . It will be seen that the circuit is so arranged that the heater cannot be switched on unless the motor is running. This was an added precaution against the heater being switched on or left on when no air was passing through to cool it. The heater was wired in three banks of 7 KW, each being controlled by a separate switch so that the degree of heat used could be varied in three steps. A safety thermostat was built into this member and cut off the current if the temperature rose above a predetermined figure; in practice it was set to trip at 120 degrees F. The heater unit was 23½ in. square and 21 in. long, and built on to a plate which supported it in the duct E, figure 3E. The joint round the plate was sealed with a rubber insertion gasket ⅛in. thick and the heater was held in place by wing nuts. Two handles were fitted so that it could be removed and replaced easily. The cables leading to the heater were long enough to allow removal and are carried in a length of flexible conduit.

Conveying System

The pneumatic conveying system was designed primarily for emptying the bins and for turning over the contents from one bin to another during the drying process. During 1947, however, it was used also for filling the bins because the mechanical system could not be installed in time. From the conveying fan a 9 in. diameter duct could be taken out through either of the side walls of the fan house, as shown in figure 2, and run beneath the overhanging parts of the silo platform. A valve of the type shown in N.I.A.E. Drawing No. 1096 was fitted into the ducting under the outlet of the silo to be emptied or turned, and by the use of suitable bends and lengths of straight ducting the grain could be discharged wherever it was required. For filling, the ducting was simply re-arranged so that the valve was clear of the silos, a small hopper was set up over it, and the ducting was assembled to deliver into the correct silo. To empty a silo it was necessary to move nearly half of the contents by shovelling because the outlet was close to the wall and the floor was level. The work was not very difficult as most of the shovelling was on the level or down-hill. Self-emptying silos were not used because they would have been much higher and more expensive, and because it was practically impossible to ventilate them so that the airflow was the same in all parts.

The mechanical convoying system designed for filling the plant was to draw grain from a receiving pit placed between the last two silos as indicated in the plan in figure 2. From here the grain was to be elevated by a standard bucket elevator to a pre-cleaner for removing straws, green stuff, small seeds and dust. From the cleaner the grain was to be taken by a central mechanical conveyor above and between the two rows of silos and discharged into the centre of any one of them. It was planned to drive both of the conveyors and the pre-cleaner by a single electric motor and provision for this has boon made in the electrical system.

Buildings

The plant was covered, with a standard Dutch barn with a span of 30 ft. Three bays each 15 ft. wide were required to cover the plant itself, but a fourth bay was added to provide cover for discharging the grain trailers. The height of the barn from ground level to eaves was 15 ft. and from the silo platform to the centre of the roof 18 ft. 4 in. It should be noted that, as this was the widest standard Dutch barn that could be got at the time, the plant could not be any wider than 26 ft. 6 in. from one outer foundation wall to the other or there would not have been room for the 9 in. ducting of the pneumatic convoying system to pass under the overhanging part of the silo platform, between the outer foundations walls and the stanchions supporting the barn.

Conclusions

Although it was not possible to Test the plant on account of the dry season, the readings for air flow and power consumption taken in conjunction with the results of the experiments on bin ventilation already carried out at the Institute, show that the plant should deal efficiently with at least 150 tons of damp grain in a normal season. The cost of drying facilities and storage accommodation (including the necessary building) provided by this method of drying is very much less than that of a conventional drier and storage of equivalent capacity. The plant must be tested in a less favourable season before accurate figures for performance can be obtained and a precise assessment made of its practical possibilities.

Note:- This is an interim account of the ventilated bins at Pitstone Green Farm; it is intended that this and details of further work with this plant shall be included in a more comprehensive publication on bin ventilation to be produced at a later date.

Cost Analysis for Labour and Materials

Materials

SUMMARY OF COSTS.

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