For safe storage many food and feed crops require drying. Drying floors and racks use
solar energy but losses can occur when it rains. Losses also occur during long periods of
damp weather. Conventional grain bin dryers are heated with high cost fuel plus the deep
layer of grain calls for powerful fans. My low cost inflated plastic solar heated dryer
design not only overcomes these problems but also can be used to store crops. In the oft
season it can be used as a greenhouse.
Materials required; 1. Clear plastic film designed for greenhouses, 2. A squirrel cage fan
also called a blower. 3. Power for the fan, an electric or diesel motor. 4. Netting to
support the crop being dried. 5. Support for the netting.
To make a grain dryer from a 40 feet by 100 feet sheet of 6 mill greenhouse plastic I sew
the edges to a 30 foot by 90 foot sheet of netting that will allow air to freely pass but
will not allow the grain to fall through. Starting at the center of each side attach the
plastic to the netting to form an envelope like structure. I have used an electric sack
sewer for this job but hand sewing is an alternative. Where the plastic is larger than the
netting it is necessary to pucker or pleat the plastic over the last 7 feet from the
corners of the netting. Next place the envelope netting side down on used shipping pallets
or bamboo poles layered at right angles to form a floor that air can pass readily to the
edges. Reinforce three or four areas with gummed tape in the middle of the plastic large
enough to accept the discharge from an inclined grain auger. Cut a hole for the auger
discharge and attach the plastic to the elevator so the grain can fall through the roof
onto the netting. If an auger is not available a funnel can be used to accept grain
directly from the discharge of a combine. Patch the holes before the fan is turned on. In
hand harvested crops a door can be used to carry the grain into the dryer.
Reinforce a corner with gum tape and cut a hole to match the discharge from a squirrel
cage fan. A door can be installed in another corner. I have used zippers salvaged from
sleeping bags as doors. A wooden door can also be constructed. After the house is inflated
it is easy to determine the shape of a door frame that will fit in a corner. Use a band
saw or hand saw to cut this shape in a thin sheet of plywood. Then cut a door in the
plywood. Use thin wooden nailing strips to attach the plastic to the door frame avoiding
stress points in the plastic.
If four feet of grain is placed on the netting then 10,800 cubic feet of grain will be on
the netting and 2,000 additional cubic feet will slope down onto the plastic around the
edges. Rough rice has a density of 36 pounds per cubic foot so this dryer will hold about
230 tons of rice.
The cost of 6 mill plastic for this dryer is $275 US and is designed to last 36 months in
the sun. Netting cost ranges from 10% to 20~/o of the plastic depending on it's life. Used
squirrel cage fans from old air conditioners are often available. Therefore plastic and
netting for a dryer could cost less than $2.00 per ton capacity and could dry and store
three to ten crops.
To support the roof of a plastic dryer or greenhouse requires between 50% and 75% of on
inch of water pressure. This same pressure will move one cubic foot of air through 4 to 5
feet of grain each minute. The type of grain and the trash mixed with the grain will
change the air flow somewhat, but four feet deep grain is a good place to start. If the
grain is too thin the air will move out through the grain too fast and the roof will be
too soft and may be stressed in the wind. Black plastic can be placed on top of the grain
to force all the air to pass through uncovered areas, causing the uncovered areas to dry
faster and at the same time increase the air pressure in the house.
I think this dryer would make a fine insect free fish or fruit dryer but I have not tried
it.
The same design, without a floor makes a fine frameless air supported greenhouse.
Just attach the edges to the ground and make sure you pucker the corners so the roof
can form a parabola.
For more information contact:
Robert J. Buker, Ph.D.
4124 East State Road 225
West Lafayette, Indiana 47906
INFLATED PLASTIC STRUCTURES FOR SOLAR DRYING OR GRAIN
For safe storage many food and feed crops require drying. Drying floors and racks use
solar energy but losses can occur when it rains. Losses also occur during long periods of
damp weather. Conventional grain bin dryers are heated with high cost fuel plus the deep
layer of grain calls for powerful fans. My low cost inflated plastic solar heated dryer
design not only overcomes these problems but also can be used to store crops. In the off
season it can be used as a greenhouse.
Materials required: 1. Clear plastic film designed for greenhouses. 2. A squirrel cage fan
also called a blower. 3. Power for the fan, an electric or diesel motor. 4. Netting to
support the crop being dried. 5. Support for the netting.
To make a grain dryer from a 40 feet by 100 feet sheet of 6 mill greenhouse plastic I sew
the edges to a 30 foot by 90 foot sheet of netting that will allow air to freely pass but
will not allow the grain to fall through. Starting at the center of each side attach the
plastic to the netting to form an envelope like structure. I have used an electric sack
sewer for this job but hand sewing is an alternative. Where the plastic is larger than the
netting it is necessary to pucker or pleat the plastic over the last 7 feet from the
corners of the netting. Next place the envelope netting side down on used shipping pallets
or bamboo poles layered at right angles to form a floor that air can pass readily to the
edges. Reinforce three or four areas with gummed tape in the middle of the plastic large
enough to accept the discharge from an inclined grain auger. Cut a hole for the auger
discharge and attach the plastic to the elevator so the grain can fall through the roof
onto the netting. If an auger is not available a funnel can be used to accept grain
directly from the discharge of a combine. Patch the holes before the fan is turned on. In
hand harvested crops a door can be used to carry the grain into the dryer.
Reinforce a corner with gum tape and cut a hole to match the discharge from a squirrel
cage fan. A door can be installed in another corner. I have used zippers salvaged from
sleeping bags as doors. A wooden door can also be constructed. After the house is inflated
it is easy to determine the shape of a door frame that will fit in a corner. Use a band
saw or hand saw to cut this shape in a thin sheet of plywood. Then cut a door in the
plywood. Use thin wooden nailing strips to attach the plastic to the door frame avoiding
stress points in the plastic.
If four feet of grain is placed on the netting then 10,800 cubic feet of grain will be on
the netting and 2,000 additional cubic feet will slope down onto the plastic around the
edges. Rough rice has a density of 36 pounds per cubic foot so this dryer will hold about
230 tons of rice.
The cost of 6 mill plastic for this dryer is $275 US and is designed to last 36 months in
the sun. Netting cost ranges from 10% to 20% of the plastic depending on it's life. Used
squirrel cage fans from old air conditioners are often available. Therefore plastic and
netting for a dryer could cost less than $2.00 per ton capacity and could dry and store
three to ten crops.
To support the roof of a plastic dryer or greenhouse requires between 50% and 75% of on
inch of water pressure. This same pressure will move one cubic foot of air through 4 to 5
feet of grain each minute. The type of grain and the trash mixed with the grain will
change the air flow somewhat, but four feet deep grain is a good place to start. If the
grain is too thin the air will move out through the grain too fast and the roof will be
too soft and may be stressed in the wind. Black plastic can be placed on top of the grain
to force all the air to pass through uncovered areas, causing the uncovered areas to dry
faster and at the same time increase the air pressure in the house.
I think this dryer would make a fine insect free fish or fruit dryer but I have not tried
it.
The same design, without a floor makes a fine frameless air supported greenhouse.
Just attach the edges to the ground and make sure you pucker the corners so the roof
can form a parabola.
MY FIRST SOLAR DRYER BUILT IN 1977 WITH POWER POLES TO FORM A RIDGED
FRAME 25 FEET BY 94 FEET.
A shipping pallet covered with plastic netting. Wet corn is on the netting. This
netting can be used only one time but costs only $.0075 per square foot.
Adding the last corn to the dryer. Note the plywood extension on the auger to reach the
center of the dryer. In the foreground are three squirrel cage fans and the gasoline motor
to power them. The netting and plastic roof are both attached to the power poles, the end
of one pole can be seen at the right.
Air supported dryer resting on shipping pallets so that the moist air that has passed
down through the grain can escape. The wood frame plastic house covers the fans and engine
so that the engine heat is also used to dry grain.
I am standing on about three feet of grain inside the dryer.
Loading the last koad of grain into a wagon
For more information contact:
Robert J. Buker, Ph.D.
4124 East State Road 225
West Lafayette, Indiana 47906
INFLATED PLASTIC STRUCTURES FOR SOLAR DRYING OR GRAIN
Each Fall corn growers attempt to balance the ever increasing field loss against the high
cost of dry, early harvested corn. Cost and availability of grain transportation and
storage must enter this decision, as does fluctuating weather and grain price. As energy
and capital costs increase, we need to look at creative solutions to this problem.
Plastic film has been used to protect harvested grain from rain and snow. It has also been
used as a solar collector to heat air to dry grain in conventional bins. This paper
reports an on-farm attempt to combine these uses into an inflated solar grain dryer and
storage structure.
Starting on September 24, 1977 I constructed a low-cost solar grain dryer. First, a layer
of 6 mil black plastic was placed on the ground at the edge of my corn field. The ground
was sloping to provide water drainage. Recycled shipping pallets, some in poor condition,
were placed on the plastic about 4" apart. Next, light-weight plastic netting
resembling one-quarter inch hardware cloth was placed over the pallets. Used power poles
were placed on the edge of the pallets to form a rectangle 25' x 94'. At one end the power
pole was supported by posts 2' high and a plywood wall was constructed under it. A 2' x 2'
access door and the discharge ducts from the squirrel cage fans were installed in this
wall.
The grain was dumped directly from the combine to form a free standing stack. The stack
was almost 5' deep at the center and filled an area 25' x 84' and contained about 3,400
bushels.
To form an air control valve a 6' wide sheet of black plastic was centered over the pole
which laid on the pallets. By sealing the outside edge of this valve to the ground air
escape could be selectively stopped. This plastic valve extended under the grain about 2'
around the edge- where the grain was not deep.
Reinforced clear plastic was then attached to the power poles using 2" x 2"
nailing strips. The plastic was puckered at the corners to avoid stress points. When the
fans, which were powered by a gasoline motor, were started, the house was inflated to form
a parabola 10' high in the center.
The fans produced about one-fourth inch of water pressure which held the plastic roof up
and at the same time forced air through the grain. The sun's rays passed through the
plastic and warmed the air as it was forced down through the grain, into the duct formed
by the pallets, and was discharged to the outside The motor and fans were sheltered by
clear plastic supported by a simple frame. This shelter caused the intake air to be drawn
over the gasoline engine thereby collecting the waste heat of the engine.
Ten feet of the house which was not filled with grain added to the solar collection area.
The total solar collection area was 25' x 94' or 2,350 square feet. One researcher in
Kansas calculated that a square foot could be equal to 186 kilowatts. At 2.6 cents per
kilowatt, this amounts to $11.12 per day.
The fan was on for 32 days between October 10 and November 19, 1977. Based on the fans
ratings I estimated 4,000 cu.ft. per minute of air passed through the grain. The moisture
content of the grain was 22.5% when the fans were started and dropped to 17.5% by November
19, 1977. The grain dried, in the shallow areas first. Therefore, these areas were covered
with black plastic to prevent air from moving through these dry areas.
On October 1 the first load of corn dumped into the dryer was covered with plastic without
air for ten days. Condensation of the underside of the plastic caused the kernels on the
surface to sprout and mold to form. Four bushels of moldy and sprouted corn were removed
and used as cattle feed. The rest of the molded corn was distributed across the surface of
the stack where it quickly dried without further deterioration.
The gasoline engines used have proved unreliable and the air supported structure has
deflated over 20 times. Wind pulled the plastic free from the power poles and ripped it on
two occasions. Repairs were made. Wind never damaged the structure while the fans were
operating. Electric power would decrease engine failure which was the most serious
prob-lem encountered.
The costs of this dryer are as follows. The plastic roof, made of a reinforced clear
sheet, 32' x 100', cost $170.95. The plastic one-fourth inch hardware cloth cost just over
one cent per square foot or $36.64. Other plastic came to $59.30. Nails, tape, rat bait
and starter fluid added $15.73. Gas and oil for the engine powering the fans cost $117.73.
These expendable and short-lived items total $400.35 or 12.4 cents per bushel. The power
poles cost 30 cents per foot or $72.80. The 9 hp engine was $202.49. The three fans, one
new and two used, cost $52.00. Pulleys, belts and hose clamps and lumber added $28.58. The
total for capital items was $355.87 or 10.5 cents per bushel. Total cost for drying and
storage was 22.9 cents per bushel. Gas and oil contributed less than 3 cents per bushel,
indicating an energy efficient system.
My plans called for the grain to be sold around January 1, but snow drifts limited access
to the dryer which was one-fourth mile from the road. As mentioned earlier, when the fan
failed during a storm two rips developed in the plastic cover. These were patched, but
snow later blew in these holes. After the fans were turned off and the plastic weighted
down with old tires, snow may have also blown under the dryer, wetting the stored corn
along the edge of the dryer. The snow was so deep that a calf walked across the dryer
(without causing damage). However, wild rabbits tunneled into the snow drifts and cut the
plastic. A few mice were noted, but rats were not a problem, suggesting the rat bait was
effective.
When the dryer was unloaded on April 1, 1978, small amounts of moldy grain were discovered
in the two places where the plastic had been ripped by high winds. Mold was also found
where wild rabbits had cut through the plastic cover, allowing some moisture to enter.
However, by far the largest amount of spoilage and high moisture grain was found on top of
the plastic sheet used as an air control valve that extended about 2' over the plastic
netting all the way around the edge of the structure.
The one area where mold was never found was at the center under the deepest part of the
stack, just over the netting where moist air exited. Therefore, most grain damage occurred
near the edges where the grain dried fastest, dropping to 14% soon after the fans were
started. Therefore, this damage was from rewetting by snow drifting under the dryer, or
possibly by condensation collecting on this plastic. The plastic netting functioned well
in that it supported the grain and did not trap melting snow or condensation.
The dryer was unloaded with a light weight auger. The auger was set into the grain where
it worked itself down to the pallets. The elevator was moved frequently and grain was
shoveled to it by hand. Three men loaded all the grain in less than 10 hours. Grain that
could not easily be shoveled off the pallets was left to feed my cattle. The plastic
sheeting under the pallets enabled me to retrieve every last grain by lifting one edge of
the plastic to roll the grain into piles.
It cost 23 cents per bushel to construct and operate this dryer which reduced the moisture
in the corn from 22.5% to 17.5% by mid November. On April 1, 1978, when it was sold, it
tested 18.3% moisture, indicating some rewetting had occurred.
Was the dryer an economic success? The local grain price when the dryer was filled was
$1.48 per bushel. The same elevator was offering $1.88 per bushel if delivery was delayed
until after January 1 or 40 cents per bushel for storage alone. Using the elevator's
standard shrinkage value of 1.3% for each 1% of moisture, I had 3,231 bushels of grain at
22.5% moisture at harvest and sold 3,131 bushels of 18.3% moisture on April 1. The price
on April 1 ranged from $2.31 to $2.33 per bushel.
At harvest, my 3,397 bushels of corn at 22.5% moisture, with a base price of $1.48 per
bushel would have netted $1.20 per bushel or $4,077. On April 19 I sold 3,231 bushels of
corn at 18.3% moisture containing 12.19% damage on a $2.31 to $2.33 per bushel market for
$6,832, or $2.11 net per bushel. My gross improved by $2,755 or 85 cents per bushel sold.
Had I sold at harvest for delivery after January 1 and delivered corn at 17.5% moisture
with no damage, I would have received 60 cents per bushel more than I would have obtained
for my corn at harvest. This value of 60 cents per bushel is probably a better value to
set on the gross earning of the structure.
The labor required to build, operate and inspect this structure has not been recorded for
much of it has been a design and sampling. Keeping the gasoline engine operating required
substantial labor also.
This dryer aided harvest as no labor was used to transport grain at harvest time. I own a
corn combine, but find grain trucks hard to rent at harvest.
SUMMARY AND CONCLUSIONS
I designed a solar grain dryer that required $756.22 to build and operate. The grain dried
in it suffered some damage, but sold for $2,755.00 more than it would have at harvest for
a return to design, labor and management of almost $2,000.00. Had it been sold in January
before the damage occurred, it would have sold for about $800.00 less. The labor required
to load the grain out of the dryer is partly offset by not having to rehandle grain from
the combine at harvest time when labor and trucks are in short supply.
RECOMMENDATIONS
1. The design should be modified to provide for vertical side walls 3' to 6' high and the
grain should be piled to a nearly uniform depth to promote uniform drying.
2. Plastic should never be placed directly under the grain.
3. Use electrically powered fans, as small gasoline engines are unreliable.
For more information contact:
Robert J. Buker, Ph.D.
4124 East State Road 225
West Lafayette, Indiana 47906