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热点推荐：Air pressure and wind
Air Pressure and Wind
Air Pressure
We know that standard atmospheric pressure is 14.7
pounds per square inch.& We also know that air pressure decreases
as we rise in the atmosphere.
1013.25 mb = 101.325 kPa = 29.92 inches Hg = 14.7
pounds per in2 = 760 mm of Hg = 34 feet of water
Air pressure can simply be measured with a barometer
by measuring how the level of a liquid changes due to different weather
conditions.&
In order that we don't have columns of liquid many feet tall, it is
to use a column of mercury, a dense liquid.
The aneroid barometer measures air pressure
without the use of liquid by using a partially evacuated chamber.&
This bellows-like chamber responds to air pressure so it can be used to
measure atmospheric pressure.
Air pressure records:
1084 mb in Siberia (1968)
870 mb in a Pacific Typhoon
An Ideal Gas behaves in such a way that the
relationship between pressure (P), temperature (T), and volume (V) are
well characterized.& The equation that relates the three
variables, the Ideal Gas Law, is
PV = nRT with n being the number of moles of gas, and R being a
constant.& If we keep the mass of the gas constant, then we can
simplify the equation to (PV)/T = constant.& That means that:
For a constant P, T increases, V increases.
For a constant V, T increases, P increases.
For a constant T, P increases, V decreases.
Since air is a gas, it responds to changes in
temperature, elevation, and latitude (owing to a non-spherical Earth).
Air pressure decreases naturally as we rise in the
atmosphere, or up a mountain, we must make correction to the air
pressure owing to elevation
above sea level.& These corrections are easily made by adding the
air pressure that would be exerted by the air column at that
elevation.& For example, in the figure below, at sea level no
correction is needed.& At 1000 m elevation, a correction of 99 mb
is required so that the adjusted sea level pressure of station B is
1014 mb.& For an elevation of 1800 m, a correction of 180 mb is
needed.& So, for a temperature of 20&C, the elevation
correction is approximately equal to 0.1 x elevation.& Note, this
correction is good only for approximately 20&C.& See appendix
D for additional temperatures.
Air pressure corrections owing to elevation, using a
temperature
of approximately 20&C.
Figure 6.6 in The Atmosphere, 8th edition, Lutgens and Tarbuck,
edition, 2001.
An aside:& flying in commercial airliners.
Usually when you fly on a commercial airline, the pilot
on the loudspeaker and announces thank you for flying their airline,
estimated time of arrival (ETA) and the height you'll be flying, e.g.,
Well, they are not exactly telling you the truth.& Since pressure
from place to place, owing to weather systems, temperature, and
elevation,
airliners will fly at a constant air pressure rather than constant
altitude.&
So, for example, if the pilot sets the airline to fly at 265 mb, that
be approximately 10 km (32,800'), but the actual elevation above sea
is variable.
Wind results from a horizontal difference in air
pressure and since the sun heats different parts of the Earth
differently, causing pressure differences, the Sun is the driving force
for most winds.
The wind is a result of forces acting on the
atmosphere:
Pressure Gradient Force (PGF) - causes
horizontal pressure differences and winds
Gravity (G) - causes vertical pressure
differences
Coriolis Force (Co) - causes all moving
objects, such
as air, to diverge, or veer, to the right in the Northern
Hemisphere and to the left in the Southern Hemisphere.
Friction (Fr) - very little effect on air high
the atmosphere, but more important closer to the ground.
Centrifugal Force (Ce) - objects in motion tend
travel in straight lines, unless acted upon by an outside force.
The Net force = PGF + G + Co + Fr + Ce
Pressure Gradient Force, PGF
The Pressure Gradient Force (PGF) is the direct result
different air pressures.& As we have done for temperature by
drawing isothermal maps, we can do for pressure and draw isobaric
maps.& Lines on these maps connect points of equal pressure.
Pressure Gradient Force (PGF) resulting in winds generated between
differences.& Solid lines are isobars - lines of constant
Figure 6.9 in The Atmosphere, 8th edition, Lutgens and Tarbuck,
edition, 2001.
The magnitude of the pressure difference and the
distance between the two points in question will essentially determine
the velocity of the PGF wind.& That is, if the stations are far
apart and the pressure difference is great, then the winds will be less
than if the stations were close together and the pressure difference
where the same.& On the figure
above, figure 6.9 from our book, you can see that the winds are
away from the high pressure region, "H," and towards the low pressure
"L."& Note, however, that the direction of the winds are not
from high to low pressure.
Gravity, G
The vertical pressure gradient is much larger than the
horizontal
pressure gradient (~100 x), yet winds don't blow straight up.&
Gravity acts to stop, or slow, the vertical flow of air, so vertical
are much less than horizontal winds.& Most vertical winds are on
order of 1 mph, however some downdrafts and updrafts can be up to 60
Coriolis Force, Co
Since the Earth rotates, objects that are above the
Earth apparently move or are deflected if they are already moving,
owing to it's rotation.& This apparent motion is caused by the
Coriolis Force, Co.& In the Northern Hemisphere objects will be
deflected to their right, while in the Southern Hemisphere objects will
be deflected to their left.& The magnitude of the deflection is
also a function of distance from the equator
and velocity.& So, the farther from the equator the object is, the
the deflection, and the faster an object is moving, the greater the
deflection.&
These "objects" can be anything from airplanes, to birds, to missiles,
parcels of air.
Coriolis Force (Co), results in objects being deflected owing to
rotation of the Earth beneath them.
Figure 6.11 in The Atmosphere, 8th edition, Lutgens and
Tarbuck, 8th
edition, 2001.
The effect of the Coriolis Force (for various latitudes).
Figure 6.12 in The Atmosphere, 8th edition, Lutgens and
Tarbuck, 8th
edition, 2001.
By the way, the Coriolis Force has nothing
whatsoever to
do with water the direction that water drains down sinks and toilets.
Friction, F
Friction is most important near the ground and less
important higher in the atmosphere.& If we consider winds aloft,
an important wind
is the geostrophic wind.& The geostrophic wind is a wind
parallels the isobars.& At first this may seems incorrect, but
think about it for a moment.& If the PGF forces winds from high to
pressure and the Co deflects the winds, there may come a time when the
are deflected 90& from their initial direction, directly toward the
pressure system.& If the PGF exactly balances the Co, the the
geostrophic
winds will flow parallel to the isobars.
The formation of geostrophic winds by a careful balance between the Cf
PGF on winds aloft.
Figure 6.13 in The Atmosphere, 8th edition, Lutgens and
Tarbuck, 8th
edition, 2001.
Winds near the surface are influenced by the
ground.& This influence is in the form of friction.& Friction
acts to retard the
motion of the wind -- it is always in the direction opposite the wind
velocity.&
Friction acts to oppose the flow of the air.& The air will slow
reducing the Coriolis force.& This results in an imbalance of
The atmosphere adjusts, to regain a balance, by turning the wind toward
pressure.& A new balance is achieved when the sum of the Friction
Coriolis forces balance the horizontal pressure gradient force.
But, the air must go somewhere!
Winds are directed towards low pressure, which
results in:
Directional convergence
Lifting of air
"Bad" Weather
Winds are directed away from high pressure, which
results in:
Directional divergence
Sinking of air
"Good" Weather
The effect of friction on winds in low pressure (cyclonic flow) and
pressure (anticyclonic flow).
Figure 6.15A and 6.15B in The Atmosphere, 8th edition, Lutgens
Tarbuck, 8th edition, 2001.
At low elevations, friction will slow the air, and
hence the Co will be less effective in its deflection of the wind.
The effect of friction on winds at high versus low elevations.
Figure 6.16 in The Atmosphere, 8th edition, Lutgens and
Tarbuck, 8th
edition, 2001.
Centrifugal Force, Ce
Newton's First Law of Motion: Objects at rest will
remain at rest and objects in motion will remain in motion, at the same
direction, unless acted upon by an outside force.& Therefore,
even though they may be acted on by gravity, the Coriolis Force, and
pressure gradient force will tend to move in straight paths.& This
best illustrated by swinging an object on a string and then letting the
loose.& The object will travel straight, tangent to the circle it
once following, and will no longer follow a curved path.
Wind Measurement
How do we measure the wind speed?& With
anemometers.
Pitot tube - used on aircraft and in wind
Drag cylinder or sphere - not used too often
Heat dissipation - reliable, fast, rugged
Speed of sound - expensive, reliable, fast
Cups and Propellers - most widely used, easy to
reliable except in low wind conditions, problems with freezing rain.
Wind direction
Wind vanes - the "rooster on the roof" is one
they always point into the wind.
Aerovanes - combination wind vane and propeller
anemometer
Wind socks - used at airports
So, a NW wind indicates a wind coming out of the
northwest - that usually means cooler, drier weather, as opposed to a
SE wind (warmer, more humid air).
Puzzling Questions
Why does it take longer to fly from New York to
Angeles than it takes to fly from Los Angeles to New York?
Why do storms (low pressure systems) usually
from west to east?
Why are the 500 mb winds very different from
at the surface?
Why are the upper level winds much faster than
at the surface?
The answer to all of these questions questions is a
very specific wind called the Jet Stream.
Recall the horizontal temperature effects on the
pressure and the balance of forces at each level:
What have we found?
A horizontal temperature difference causes a
horizontal
pressure difference aloft.
The isobars tilt, being higher in the warm air.
Because the tilt increases with height, the
horizontal
PGF increases with height.
The geostrophic winds increase with height.
The result is a thermal wind that is created by
a change in the geostrophic wind with height that is caused by a
horizontal temperature variation.
The thermal wind is a difference of winds at
two different heights.
The thermal wind is parallel to the isotherms.
Cold air is to the left of the thermal wind.
The strength of the thermal wind depends on the
vertical wind shear.
The vertical wind shear depends on the
horizontal
temperature gradient.
What does this tell us about the real winds?
The winds blow from the west aloft.
Faster air trip from L.A. to New York than
The winds aloft can change direction if the
horizontal
temperature gradient changes direction.
The winds aloft are strongest near the largest
horizontal temperature gradient.
The strongest band of winds aloft is called the
The stronger the horizontal temperature
gradient, the
stronger the thermal wind.
The stronger the vertical wind shear
Winds increase more rapidly with height
Stronger winds Aloft
Jet Stream
A region of increased wind speeds.
Typically found above the largest horizontal
temperature gradient.
Stronger in the winter when the temperature
are the largest.
As you go higher in the atmosphere, above the jet
stream, the horizontal temperature gradient reverses.& So the jet
stream weakens.& There is a maximum to the westerly winds with
height. This is the jet stream.
We'll talk more about the jet stream when we discuss
global circulation in more detail.Natural-air corn drying in the upper Midwest : Harvest : Corn : University of Minnesota Extension
University of Minnesota Extensionwww.extension.umn.edu612-624-1222
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& & Natural-air corn drying in the upper Midwest
Natural-air corn drying in the upper Midwest
William Wilcke and R. Vance Morey
On this page
This publication is for corn producers, educators, consultants, and equipment dealers who are interested in natural-air corn drying in Minnesota and neighboring states. Answers to some common questions provide a description of natural-air drying, its advantages and disadvantages compared to other types of drying, equipment requirements, management recommendations, and expected energy use.
Does natural-air corn drying really work in northern states?
Yes! Natural-air corn drying has been used successfully for many years by researchers at a number of agricultural experiment stations and by thousands of corn producers. The process works best under cool (40 to 60 degrees F), dry (55 to 75% relative humidity) conditions. Since average fall temperature and humidity are in these ranges in the Upper Midwest, natural-air drying usually works quite well.
How does it work?
Figure 1. Natural-air drying bin equipped with grain spreader, exhaust vents, fan, and full-perforated floor.
Natural-air drying, also called ambient-air drying, near-ambient drying, unheated-air drying, or just air drying, is an in-storage drying method that uses unheated, outdoor air to dry corn to a safe storage moisture (13 to 15%). Instead of using heat energy from fossil fuels to remove moisture, natural-air drying uses electricity to operate fans, with energy for removing moisture coming primarily from the drying potential of outdoor air. Natural-air drying of shelled corn is similar in principle to the drying that takes place in cribs of ear corn, except that, because the airflow resistance for shelled corn is greater than for ear corn, fans rather than wind pressure move air through the bin.
Natural-air drying is basically a race between drying progress and growth of the fungi (commonly called molds) that cause grain spoilage. The bin is usually filled in a few days and the fan is started as soon as bin filling begins. Drying takes place in a one- to two-foot thick drying zone (also called a drying front) that moves slowly up through the bin (Figure 1). Grain below the zone is generally dry enough to be safe from spoilage, while grain above the zone remains at its initial moisture until the zone passes. (Note that positive pressure, or upward airflow, is recommended for natural-air drying so that wet grain is at the top of the bin. There it is easier to watch for signs of mold and to move moldy corn out of the bin if necessary.)
Drying time depends on initial grain moisture, airflow per bushel provided by the fan, and weather. If corn moistures and airflows recommended in Table 1 are used, drying usually takes four to eight weeks depending on the weather. In cool, damp falls, the drying zone doesn't reach the top of the bin before winter and drying is completed in spring.
Table 1. Airflow and moisture recommendations for natural-air corn drying in the Upper Midwest.
Airflow (cfm/bu)*
Initial corn moisture (% wet basis)
Northern Minnesota, North Dakota, South Dakota
Southern Minnesota, Iowa, Wisconsin
Values are for bins filled rapidly (in a day or two) in mid-October. Use of these airflows should result in drying without spoilage, or need to move corn to prevent spoilage, at least 90% of the time.
*cfm/bu = cubic feet of air per minute per bushel of grain
Does grain ever spoil before it dries?
There is some risk of grain spoilage, but if airflow is matched to grain moisture and bins are monitored closely, spoilage can be avoided.
Table 2 shows the allowable storage time for shelled corn. This is the approximate amount of time that corn can be held at different temperatures and moisture contents before there is enough mold damage to cause price discounts or possible animal feeding problems. At temperatures higher than 60 degrees F, corn dries fast, but mold grows faster & especially if corn moisture is higher than 22%. Mold growth is very slow at temperatures lower than 40 degrees F, but drying is also slow. Thus, the best temperatures for natural-air drying are about 40 to 60 degrees F. Drying is slow and expensive when fall weather is colder than normal, but the greatest risk of spoilage comes during unusually warm falls.
Table 2. Allowable storage time (days) for shelled corn. This is the approximate number of days corn can be held before there is enough mold growth to cause price discounts or feeding problems.
Corn temperature(degrees F)
Moisture content (% wet basis)
The key to success is to provide enough airflow to move the drying zone all the way through the bin before any spoilage occurs. Because wet grain spoils faster, it is important to use more airflow per bushel for wetter corn.
Regardless of airflow, check the condition of grain at the top of the bin every few days during drying. If heating, musty or sour odors, or moldy kernels are detected, move some or all of the wet grain out of the bin and feed, sell, or dry it in another dryer.
What equipment is necessary for natural-air drying?
A fan, a bin, and an air distribution system are required, and exhaust vents and a grain spreader are desirable. Sometimes grain stirrers, heaters, or both, are added, but they aren't normally necessary for successful drying.
What size fan is necessary?
Figure 2. Probability of success, or percentage of years that corn is expected to dry without spoilage for different harvest moistures and airflows. (Mid-October harvest near St. Paul, MN, 16 ft corn depth.)
Figure 3. Average energy use per bushel for natural-air corn drying with different harvest moistures and different airflows. (Mid-October harvest near St. Paul, MN, 16 ft corn depth.)
Required fan size depends on corn moisture, corn depth in the bin, and the desired probability of success. Probability of success is the percentage of drying seasons, or number of years out of 100, that the drying zone is expected to move all the way through the bin before spoilage occurs, or before you need to move corn out of the bin to prevent spoilage.
One way to increase probability of success for any corn moisture is to use a higher airflow per bushel (Figure 2). Greater airflow means faster drying and less time for spoilage to occur. Disproportionately larger fans, which draw more electrical power, are necessary to deliver higher airflow, and energy use per bushel increases (Figure 3). Use Figures 2 and 3 to compare expected probability of success and energy consumption for different airflows and grain moistures. Although weather data for St. Paul, MN were used to prepare Figures 2 and 3, the results apply to much of the Upper Midwest.
Information like that in Figure 2 was used to develop Table 1. If moisture and airflow recommendations in Table 1 are followed, grain should dry without spoilage at least 90% of the time. This means that moving grain to prevent spoilage shouldn't be necessary more than about 10 years out of 100.
To select a fan, use the desired airflow per bushel and the normal drying depth to determine the expected static pressure (Table 3). Next, multiply airflow per bushel by the number of bushels in the bin to get total airflow (cfm). Finally, use fan manufacturers' catalogs to select a fan that will provide the desired airflow (cfm) at the expected static pressure (inches of water, or in. water).
Table 3. Static pressure (inches of water) for airflow through shelled corn.*
Corn depth (ft)
Airflow (cfm/bu)
*Airflow resistance values have been multiplied by 1.5 to give table values. This accounts for fines and packing in the bin. If corn is stirred, airflow resistance is reduced, so divide table values by 1.5.
There are three basic types of grain drying fans & axial-flow, centrifugal, and in-line centrifugal. Any of the three types can be used, but axial-flow fans are most common for natural-air corn dryers because they are the least expensive and the most efficient type at the low static pressures encountered in corn drying. Also, heat given off by the motor is captured by the drying air.
Table 4 provides a rough estimate of fan power requirements (horsepower, or hp) for different airflows and corn depths. Power requirements increase drastically as depth and airflow per bushel increase. High-power fans are expensive to install and operate. Thus, natural-air drying is most economical if corn depth is less than 18 ft and corn moisture is less than 23% (full bin airflow is less than 1.25 cfm/bu). Short, large-diameter bins are more expensive to build than tall, slender ones, but the energy savings for short drying bins makes the extra initial investment worthwhile.
Table 4. Approximate fan power requirements (hp per 1000 bu) for natural-air corn drying.
Corn depth (ft)
Airflow (cfm/bu)
hp per 1000 bu
For more information on fan selection, get a copy of the bulletin Selecting Fans and Determining Airflow for Crop Drying, Cooling, and Storage, FO-5716, or the FANS computer program from the Minnesota Extension.
Should full perforated floors be installed in natural-air drying bins?
Yes. Drying is much more uniform when air is distributed through a full perforated floor that is set at least a foot above the concrete pad. It might be possible to dry relatively low moisture corn using a duct system to distribute air, but airflow and drying in the grain above the ducts are not uniform & especially at the higher airflows necessary for higher moisture corn.
Are roof vents necessary?
An exhaust area of about one square foot (sq ft) per 1000 cfm of airflow is needed above the grain to prevent excessive condensation under the bin roof and to prevent back pressure that would reduce airflow delivery by the fan. Calculate the total area that is likely to be open when the fan is operating, and if it is less than 1 sq ft/1000 cfm, install extra exhaust vents. When calculating exhaust area, include openings such as the gap at the eave, which in some cases can provide sizable exhaust area.
Area of eave gap (sq ft) = [gap width (in.) / 12 in./ft] x 3.14 x bin diameter (ft)
Should corn be cleaned before it goes into the bins?
Fines (small pieces of broken grain, dirt, chaff, and weed seeds) cause problems in drying and storage bins because they restrict airflow, and they are more susceptible to spoilage than whole kernels. Worse yet, fines tend to concentrate in areas directly under the spout used to fill the bin. Running corn through a cleaner to remove fines during bin filling is one of several options for managing fines.
Figure 4. Periodically withdrawing grain during bin-filling to reduce fines from bin center.
First try to minimize production of fines. Set combines for minimum damage and maximum cleaning. Handle grain gently by using grain conveyors that are easy on grain (bucket elevators, drag conveyors) and by reducing drop heights and number of times grain is handled. Augers and pneumatic conveyors have the potential to cause a lot of grain damage. Reduce damage potential by operating augers slowly and full, and by using gentle curves in tubing and the proper air/grain ratio in pneumatic conveyors.
Cleaning grain to remove fines before natural-air drying is the best management choice. Fines removal reduces spoilage risk, and it reduces drying cost by allowing the fan to deliver greater airflow. An Iowa State University study indicated that it takes more than three times as much fan power to move the airflow needed for natural-air corn drying through non-cleaned vs. cleaned corn.
If a cleaner isn't available, or if cleaning creates too great a bottleneck in grain handling, periodically remove some grain from the bin center during bin filling to remove fines (Figure 4) . If you can't feed or sell fines, or choose not to remove them, then at least use a grain spreader to distribute them more uniformly throughout the bin.
Should the grain surface in natural-air drying bins be leveled?
Yes. The grain surface should always be leveled after grain is put into or removed from a bin in which natural-air drying is in progress. If grain depth over the drying floor is not uniform, airflow will be greater and drying will be faster in areas that have shallower grain depths. But the fan must be kept running until the drying front moves through areas with greater grain depth, which increases drying cost per bushel for the whole bin.
The situation is even worse if grain is peaked under the spoutline. Drying is very slow in grain peaks due to the low airflow caused by the greater grain depth and greater concentration of fines.
Would adding heat reduce drying time?
Adding heat speeds drying slightly, and it slightly increases the chance of completing drying in fall. The main effects of adding heat, however, are to increase drying cost and to overdry corn at the bottom of the bin.
Heaters are not generally needed on natural-air corn dryers in the Upper Midwest. Most years, corn dries to a safe storage moisture in fall or the following spring, without the use of supplemental heat. Table 5 gives corn equilibrium moisture values for typical drying season air conditions. Values in the table are moisture contents that corn approaches when exposed to the different combinations of temperature and humidity.
Table 5. Equilibrium moisture content (% wet basis) of shelled corn. These are the moisture contents that corn would reach if exposed to the listed combinations of temperature and humidity for very long periods of time.
Temperature (degrees F)
Relative humidity (%)
203040506070
14.813.913.112.511.911.4
16.115.214.513.813.312.7
17.616.716.015.414.814.3
19.418.617.917.316.816.3
22.221.120.520.219.719.3
Because heat increases temperature and decreases relative humidity of drying air, the effect on final corn moisture is surprisingly large (Table 6). Even small amounts of heat will almost always dry corn to well under 15% moisture. Drying corn to less than 15% is expensive, not only because of the extra energy required to get it that dry, but also because the extra water removal results in less weight of corn available for sale.
Table 6. Example of the effect of supplemental heat on air relative humidity and final corn moisture.
Supplemental heat (degrees F)
Drying air temperature (degrees F)
Drying air relative humidity (%)
Corn moisture content (%)
15.714.913.712.0
*Axial-flow drying fans heat the air 2 to 3 degrees F.
Another reason that supplemental heat is not usually needed is that heat from the motor and impeller of axial-flow fans increases drying air temperature at least 2 to 3 degrees F, which is enough to reduce corn moisture about 0.8 percentage points more than would be expected based on outdoor air conditions.
People often ask about the practicality of using solar collectors to heat air for grain drying. Research in the late 1970s and early 1980s showed that properly sized solar collectors had about the same effect as electric or gas heaters. That is, adding solar collectors increased the drying rate slightly, but they also increased total drying cost and the amount of overdrying compared with natural-air drying.
If reducing drying time is the objective, a larger fan is usually a better investment than a heater. But if for some reason it is necessary to add supplemental heat, size the heater to increase the air temperature no more than about 5 degrees F, and use a humidistat to turn the heater off during dry weather. It can be difficult to find gas heaters in this size range. Portable gas space heaters might be one option. Avoid kerosene or fuel-oil heaters because they could leave an oily or smoky odor on the grain. Electric heaters are more common, but they are expensive to install and operate. Consult your local power supplier before installing an electric heater.
Calculate heater size as follows:
Gas heater (Btu/hr) = 1.1 x (temperature rise, degrees F) x (airflow, cfm)
Electric heater (kW) = 1.1 x (temperature rise, degrees F) x (airflow, cfm) / 3412
Are there advantages to stirring corn when drying without heat?
Grain stirrers are an essential component in some types of heated-air dryers, but the situation is different for natural-air drying and drying where the air is heated less than 10 degrees F. Stirring grain provides advantages for natural-air drying although the value is probably not enough to justify the cost of buying new stirring equipment. If you have a natural-air dryer that already contains stirring equipment, don't operate the stirrers continuously. Stirring too much will sift fines to the floor where they could restrict airflow. Also, stirring too frequently will reduce drying efficiency. Natural-air drying is most efficient when there is a layer of wet grain at the top of the bin and drying air is nearly saturated when it leaves the bin. The drier the top layer is, the less saturated the air and the less efficient the drying process.
If grain stirrers are available, operate them as follows:
During bin filling, stir to loosen grain and boost airflow provided by the fan. Stop stirring within 24 to 48 hours after the bin is full.
If harvest moisture was greater than 20%, stir again when the average moisture in the bin is 18 to 20%. Stirring blends overdry corn from the bottom of the bin with wet corn at risk of spoilage from the top. This reduces overdrying and spoilage risk, but still leaves wet enough corn on top to maintain reasonable drying efficiency.
Finally, stir again when average moisture in the bin reaches the desired value (usually 14 or 15%). Toward the end of drying, the top layer is still wetter than the average bin moisture and the bottom is usually drier than average. Stirring allows you to turn off the fan sooner because it blends corn uniformly, top to bottom, to attain the recommended average moisture.
Should the fan be stopped at night or during humid weather?
Leave the fan running if the bin contains corn wetter than about 16% and the temperature is warmer than about 40 degrees F. If the corn is warm and wet and the fan is off for very long, mold growth might cause the corn to heat. Also, some operation during humid weather is needed to rewet corn at the bottom of the bin that overdried during dry weather. If the fan only operates during the driest weather, corn will be badly overdried. Remember, fan heat reduces air relative humidity and allows corn drying even under fairly humid conditions.
If corn is nearly dry and the temperature is low, there is little risk of corn spoilage and it is safe to stop the fan during humid weather. Stopping the fan will save some energy. Regardless of corn moisture, it is usually best to stop the fan during heavy snowfall to avoid plugging holes in the perforated drying floor.
How can I tell when drying is finished?
Grain at the top of the bin remains near its initial moisture content until the drying zone moves all the way up through the bin (Figure 1). The top grain will finally start to dry when the drying zone reaches the grain surface. Continue drying until the top grain reaches the desired final moisture content (usually 14 to 15%). Check the moisture at several locations on the surface because movement of the drying front might not be completely uniform due to irregularities in airflow. By the time the drying zone has reached the grain surface, corn below the surface has normally dried to a safe storage moisture.
Locate the drying front and check drying progress every week or so. This can be done by using a sampling probe to pull grain samples from various depths and by measuring the moisture content of the samples. It is also possible to locate the drying front by pushing a small diameter rod down into the grain. The rod will push hard through the wet grain above the drying zone, but suddenly push more easily when it reaches dry grain in the drying zone. Watch out for overhead electrical powerlines when handling long metal rods at the top of grain bins.
When the condition of the air entering a natural-air drying bin is constant for long periods of time, it might be possible to use temperature measurements to locate the drying zone. Grain cools as it gives up moisture, so a transition from warm to cool grain can indicate the location of the drying zone. Because outdoor air temperature changes frequently, however, drying bins can have warming and cooling zones moving through them in addition to the drying zone. Thus, in practice, it is difficult to use grain temperature to track drying fronts.
What if drying isn't completed before winter?
Just turn off the fan, restart it as needed during winter to keep grain cooled to about 30 degrees F, and then resume drying in spring. It is not economical to continue natural-air drying in winter because drying is very slow and the equilibrium moisture is such that corn wouldn't dry any further than about 17% moisture anyway. It is safe to turn the fan off during winter, because mold growth is very slow at temperatures less than 30 degrees F. Grain temperature can be checked by using permanently mounted temperature cables that hang from the bin roof, by using thermometers or temperature sensors mounted on the ends of probes, by quickly measuring the temperature of grain samples pulled to the surface of bins, or by measuring the temperature of exhaust air. Measure exhaust temperature by placing a thermometer about a foot below the grain surface while the fan is running.
Use the following criteria to decide when to stop drying in fall:
Turn the fan off when moisture of corn at the top of the bin is less than 15.5%. Restart the fan to cool the corn to about 30 degrees F as soon as the weather gets cold enough.
After November 1, turn off the fan when corn moisture at the top of the bin is less than 17% and corn temperature is less than 30 degrees F. If the weather forecast calls for a period of warm weather, resume drying until average temperatures drop below 30 degrees F again.
After November 15, turn the fan off when corn moisture at the top of the bin is less than 18% and corn temperature is less than 30 degrees F.
After December 1, turn the fan off when corn moisture at the top of the bin is less than 19% and corn temperature is less than 25 degrees F.
After December 15, turn the fan off when corn temperature at the top of the bin drops below 25 degrees F, regardless of corn moisture.
People often refer to operating the fan at air temperatures lower than 32 degrees F as "freezing" corn and ask if that practice causes any problems. Although free water freezes at 32 degrees F, corn does not. Drying is quite slow at temperatures lower than 32 degrees F, but as long as there is no condensed water in the bin, running the fan at these temperatures does no harm.
If drying isn't completed in fall, and the drying zone is at least halfway through the bin, and corn moisture at the top of the bin is less than about 23%, drying can usually be completed in spring. If there is too much wet corn going into spring, or if the corn is wetter than 23%, spoilage is likely during spring drying. In these cases, feed or sell at least part of the wet corn, or dry it in another type of dryer before spring.
When should I resume drying in the spring?
Use the following information to determine how to manage natural-air dryers in spring:
If corn at the top of the bin is wetter than 19%, run the fan continuously starting about March 15 until the corn is dry.
For 17 to 19% moisture corn, run the fan continuously starting about April 1 until the corn is dry.
For corn less than 17% moisture that is to be dried to 14% or less, run the fan continuously starting about April 15 until the corn is dry. If you are shooting for a final moisture of 15%, turn the fan off during the warmest, driest weather or the corn will get too dry.
Notice that the wetter the corn at the top of the bin is, the earlier you need to start drying. This is to make sure the drying front reaches the top of the bin before outdoor temperatures get high enough that the corn would mold. Also, corn at the bottom of the bin will be badly overdried (less than 13% moisture) if drying continues into late spring.
What if the corn is too wet at harvest to fill natural-air bins?
Some years, weather conditions are such that corn just doesn't dry in the field to safe levels for full-bin natural-air drying. Also, some producers have so many acres to harvest that they can't afford to wait until corn dries in the field to levels that are safe for natural-air drying. In both cases, layer filling or combination drying allows harvest at corn moistures greater than those recommended for full-bin drying.
Figure 5. Airflow produced by a typical 10-hp axial-flow fan on a 30-ft diameter bin for different depths of corn.
Layer filling is filling a natural-air bin slowly over a period of several weeks, instead of in a day or two. This can be done by filling on a regular schedule (for example, one quarter of the bin depth per week), or by putting in a few feet of grain at a time and waiting for the drying zone to reach the top layer before adding more grain. Regardless of filling schedule, make sure the top surface is level after each layer is added to the bin.
Layer filling works because airflow per bushel is much higher when a bin is only partly full. For one thing, the airflow provided by the fan serves fewer bushels when the bin isn't full. For another, fans, especially axial-flow fans, deliver much greater total airflow when grain depth is shallow. The fan in Figure 5 , for example, delivers about 1 cfm/bu when the bin contains 18 ft. of corn and almost 7 cfm/bu when the bin contains 4 ft. of corn. When airflow per bushel is high, drying is fast and reliable even for corn in the 24 to 26% moisture range.
Layer filling works best for producers who have several natural-air drying bins and can conveniently switch filling from one bin to another. It also works well for producers who are not in a big hurry to finish harvest.
Combination drying uses a gas-fired dryer to dry corn to 20 to 22% moisture before starting the natural-air drying process. Almost any kind of gas-fired dryer can be used for the first drying stage & including a bin-type dryer where the burner is simply switched off when corn moisture reaches 20 to 22%. The natural-air portion of combination drying is managed just like normal natural-air drying.
Advantages of combination drying include:
Producers can start harvesting corn at any moisture. This allows starting natural-air drying earlier in the season and makes natural-air drying possible in cool, wet falls.
Final grain quality is much better than that of grain dried completely in a gas-fired dryer.
Switching to combination drying greatly increases the capacity (bushels per hour) of the gas-fired dryer. If an existing gas-fired dryer can't keep up with the current harvest rate, switching to combination drying might eliminate this harvest bottleneck.
What about using grain preservatives or mold inhibitors in natural-air dryers?
Preservatives or mold inhibitors slow mold growth and allow more time for drying corn. Propionic acid is an example of a preservative that can be used on shelled corn. It is most commonly used to store wet corn temporarily for animal feeding in situations where drying is not feasible. It could also be used to reduce mold growth in natural-air dryers that have lower than recommended airflow. But use of propionic acid has some disadvantages. These include cost of application, corrosion of metal equipment, and the fact that treated corn can only be used for animal feed.
Anhydrous ammonia also inhibits mold growth on shelled corn. It can be used in a trickle ammonia process, where small amounts of ammonia are periodically injected into the drying air downstream from the fan (between the fan and the bin) on natural-air dryers. The ammonia slows mold growth and slightly increases the corn's protein content. Disadvantages of ammonia include corrosion of electrical components, handling of a potentially hazardous material, and treated corn that can only be fed to animals. Contact the Minnesota Extension for more information on the trickle ammonia process.
Other preservatives or mold inhibitors, either chemical or biological, might become available for use in natural-air corn dryers. Before using any of these products, though, make sure they have been approved for use on corn. Consider cost, safety, corrosion, and whether potential buyers will accept treated corn.
Is natural-air drying cheaper than heated-air drying?
Energy cost is what usually comes to mind when drying cost is mentioned. But energy is only total cost includes labor, equipment (the dryer plus auxiliary holding bins and handling equipment), repairs, maintenance, taxes, and insurance.
Electrical energy use for natural-air drying depends on initial grain moisture, weather, airflow per bushel, and fan efficiency. For average weather conditions in the Upper Midwest, mid-October harvest, typical fan and motor efficiency, and about 16 ft of 20 to 22% moisture corn, average electrical energy use is about one kilowatt hour per bushel of corn dried (1 kWh/bu). Energy use is lower for earlier harvest, more efficient fans, shallower depths, or lower moisture. It is higher for later harvest, less efficient fans, deeper bins, or higher harvest moisture. Energy use can easily be 0.5 times the long-time average value in warm, dry years and 1.5 times the average value in cool, wet years. Electric heaters often draw as much power as the fan, so use of electric supplemental heat can easily double energy cost.
To calculate energy cost, multiply energy use (kWh/bu) times electricity cost per kilowatt-hour ($/kWh). To estimate how much it costs per day to operate a natural-air dryer, multiply the electrical demand of the fan in kilowatts (kW), times 24 hours per day, times the cost of electricity ($/kWh). As a rough approximation, fans draw about one kilowatt of electrical power per rated horsepower (1 kWh/hp).
$/day = fan hp x 1 kW/hp x 24 h/day x $/kWh
For comparison, gas-fired dryers use 0.015 to 0.025 gal of propane per bushel per percentage point of moisture removed. Gas-fired dryers also use some electrical energy per bushel, but this is small compared to the gas cost.
Gas cost for gas-fired dryer ($/bu) = points of moisture removed x gal/bu/point x gas cost ($/gal)
If grain is going to be stored on farm anyway, the only extra equipment costs for natural-air drying are a larger fan, drying floor, and perhaps a spreader and extra roof vents. Labor requirements during harvest are low, but the bins need to be checked every day or two while the fan is operating. Equipment for gas-fired drying includes the dryer and perhaps extra holding bins and handling equipment. Labor requirements can be high during drying, but once grain is dried, storage bins only need to be checked every week or two. So which drying method is cheaper? Because the answer depends on local gas and electricity costs, equipment costs, storage needs, and availability of farm labor, producers need to do cost calculations for their own situations.
Are there ways to reduce natural-air drying costs?
Here are some possible ways to reduce costs:
Plant earlier so that corn matures earlier in the fall when weather is warmer and drying is faster.
Plant corn varieties that dry faster in the field. Study variety trials for varieties that give good yields at low moisture.
Consider switching to shorter season varieties, but keep in mind that there might be a yield penalty. Agronomists say producers lose about 1 bu/acre for each relative maturity unit. For example, on the average, 105-day corn would yield about 5 bu/acre more than 100-day corn.
Remove fines. This reduces airflow resistance and provides more air and faster drying from a given fan.
Install short, large-diameter bins. It takes less fan power to deliver the same airflow per bushel through shallow grain depths (and thus less electrical energy for drying). This same principle also applies to layer filling. If a bin is filled slowly over a period of several weeks, the bottom layers dry faster and less energy is used.
Take advantage of electrical rates that reduce cost per kilowatt-hour for electricity. A few power suppliers offer special rates specifically for grain drying. Some offer off-peak rates for any electrical load. Producers must shut off the fan at times of peak electrical load to take advantage of these rates, but as long as the fan isn't off more than about two hours per day, there isn't a significant effect on the drying process.
Use lower-than-recommended airflow and accept greater risk of spoilage. Careful managers can successfully use lower airflow for natural-air drying (which reduces fan size and energy cost), but the penalty is that corn has to be moved more often to prevent spoilage.
Feed corn that doesn't dry in the fall during winter months.
If supplemental heat is being used, disconnect the heater, or at least install a humidistat that turns the heater off during dry weather.
Are there safety hazards involved in natural-air grain drying?
Yes. Dangers include falls while climbing bins, suffocation in grain, and breathing mold spores.
Management of natural-air dryers includes frequent (every day or two) climbs to the top of the bin to inspect grain. Install safety cages around ladders and guardrails around the opening into the bin. To make climbing safer and easier, consider installing stairs instead of ladders on bins.
Every year, a number of people die from suffocation in grain bins. This happens when they are pulled under flowing grain, when steep piles of moldy, caked grain collapse on them, or when they fall through bridges of moldy grain that sometimes remain at the top of partially emptied bins. To avoid these hazards, stay out of bins when grain unloading equipment is operating, and use long poles to knock down grain piles or bridges from a safe distance.
Mold spores can cause short- and long-term health problems. To work safely around moldy grain, wear a dust mask or respirator that is capable of filtering mold spores. Single-strap, disposable dust masks will no at a minimum, tight-fitting, two-strap masks are necessary to be safe.
Get a copy of Safe Storage and Handling of Grain, FO-0830, from the Minnesota Extension for more complete safety recommendations.
Compared with higher-temperature, gas-fired drying methods, natural-air drying:
Requires less equipment. All that's necessary is a bin with a full perforated floor, a properly sized fan, and a grain conveyor to fill the bin.
Requires less labor at harvest. During harvest, the only labor requirement is to fill the bin and turn on the fan. Most of the drying takes place after harvest.
Doesn't slow harvest. This is because drying takes place in storage. There is no need to wait for completion of drying before transferring grain to storage, as is the case for other drying methods. Once corn moisture is down to the recommended level, bins can be filled as fast as the corn is harvested.
Produces better quality grain. Test weight and germination are higher, and stress cracks and breakage susceptibility are lower than for corn that has been dried at higher temperatures. This does not mean that nutritional value is higher, but in some cases it can mean that test weight and BCFM (broken corn and foreign material) discounts can be avoided.
Uses fewer units of purchased energy per unit of water removed. Keep in mind, however, that the energy for natural-air dryers is electricity, which is more expensive per unit of energy than gas.
Natural-air drying has disadvantages that limit its usefulness for some producers.
Many years, drying is not complete before winter. This is not a problem in terms of spoilage because grain can be kept cold through winter and drying can be finished in spring, but it does limit fall and winter marketing opportunities.
In years with unusually warm weather, which leads to rapid mold growth, some grain from the top part of the bin might have to be moved to prevent spoilage.
It is possible to estimate average drying time and energy use, but actual values are highly weather-dependent and vary greatly from year to year. In dry falls, drying is fast a in cool, wet years, drying is slow and energy use is high.
Corn moisture should be less than about 23% for safe, full-bin drying. Layer drying and combination drying (described previously) can be used if harvest moisture is greater than 23%, but these might not be acceptable options for all producers.
Corn at the bottom of the bin overdries (to less than 15% moisture) in dry years. Because corn is usually sold on a 15% moisture basis, drying corn to less than 15% moisture results in weight and revenue loss.
Natural-air drying increases electrical demand, which could result in extra costs for increased electrical demand charges or for upgrading electrical service. Electrical demand for natural-air drying fans is roughly 1 kilowatt (kW) per 1000 bushels.
Airflow resistance: pressure required to for usually measured in inches of water.
Allowable storage time: amount of time that grain can be stored at a specified temperature and moisture content before there is enough mold growth to reduce the grain's value.
Axial-flow fan: a crop drying fan that has the motor and a multibladed impeller inside a barrel-shaped housing. Air flows over the motor and through the impeller in line with the motor shaft. These fans are very noisy, but they are usually the most efficient type for natural-air corn drying.
Btu: B a unit of energy.
Bu: bushel. Bushels can be calculated using volume or weight. Using the volume definition, shelled grains occupy 1.25 cubic feet. Using the weight definition, one bushel of shelled corn weighs 56 lb at 15.5% moisture (wet basis).
Centrifugal fan: a type of fan that has a wheel-type (sometimes called squirrel cage) impeller. Air enters the side of the wheel and then turns 90 degrees. The motor is usually outside the air stream. These fans are quiet and work well at high static pressures.
Cfm: cubic fe total amount of air flowing through a bin of grain.
Cfm/bu: cubic feet of air per minute per bushel calculated by dividing total airflow by bushels.
Combination drying: using a gas-fired dryer to quickly dry wet corn to a moisture content that is safe for natural-air drying.
Equilibrium moisture content: moisture content that grain reaches if exposed to air of constant temperature and relative humidity for a long period of time.
Exhaust vent: screened opening that allows air to exhaust from the top of grain bins. Enough vents are installed to provide a total of 1 sq ft exhaust area per 1000 cfm.
Fines: small pieces of broken grain, chaff, dirt, and weed seeds that are mixed with whole grain.
Fungi: scientific name for a group of chlorophyll-free plants that includes grain storage molds.
Grain spreader: a gravity- or motor-powered device mounted in grain bins just
purpose is to prevent formation of cone-shaped grain piles and the concentration of fines that normally develops in the center of cone-shaped piles.
Grain stirrers: devices that vertically mix grain in bins. They consist of one or more vertical, bare screws mounted on a horizontal arm that moves along a track attached just below the top of the bin wall. The rotating vertical screws pull grain up from the bin floor as they move around the bin.
Hp: used to indicate the output power of fan motors.
In-line centrifugal: a fan that uses a wheel-shaped (sometimes called squirrel cage) impeller inside a barrel- for a given horsepower, performance is usually between that of axial-flow and ordinary centrifugal fans.
In. water: unit used to measure pressure developed by fans, or the amount of pressure needed to force air through grain.
kW: unit rate of use of electrical energy.
kWh: kilowatt- unit of electrical energy.
kWh/bu: electrical energy used to dry a bushel of grain.
Layer filling: a natural-air drying method where the bin is filled slowly over a period of several weeks. This speeds drying and allows drying of wetter corn because airflow per bushel is much greater when the bin is only partly full.
Off-peak rates: reduced electrical rates offered in an attempt to shift electricity use from busy times of the day when power suppliers experience peak demand on their system to times when there is less demand on the system.
Pneumatic conveyors: systems that use air pr such systems usually include a high-pressure blower, air lock, cyclone separator, and a number of pipes or tubes.
Probability of success: This is the number of years out of 100 that you could expect successful drying (no spoilage or need to move grain to prevent spoilage) for a given grain moisture and airflow per bushel.
Static pressure: in grain systems, the pressure measured at the wall of a duct or plenum, or perpendicular to a moving stream of air. Static pressure indicates airflow resistance of grain.
Temperature cables: cables that hang from bin roof to floor and are used to measure tempera consist of steel cables that have electronic temperature sensors attached every few feet along their length and electronic read-out boxes that can be plugged into the cable to read temperature of each sensor.
Wet basis (moisture content): moisture content calculated by dividing weight of water in a sample by the total, or wet weight, of the sample. Grain moisture is usually expressed on a wet basis.
The authors, William F. Wilcke and R. Vance Morey , are associate professor and extension engineer, and professor and head of the University of Minnesota's Department of Biosystems and Agricultural Engineering, College of Agricultural, Food, and Environmental Sciences.
Thanks to Extension Plant Pathologist Richard Meronuck, Extension Engineers Fred Bergsrud and John Shutske, and Meeker County Extension Educator David Schwartz, for reviewing this publication, and Diedre Nagy, John Molstad, and Michael White of the Educational Development System for producing it.
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