Natural Air Grain Drying

October 2008

Acknowledgments

Personnel of other government agencies and universities, farmers, contractors, commercial firms and many others contribute information for use in Saskatchewan Agriculture publications. Brevity does not permit credit to each source. We thank all contributors and reciprocate by offering this publication for public benefit. In this publication, imperial units are used as the primary units of measurement. The SI (metric) units in brackets are not necessarily exact conversions, but are equivalent practical conversions and are values that would be used if the SI system was used alone.

ISBN 0-88656-521-9

Introduction

To complete harvest as quickly as possible, with the lowest possible crop loss and with minimal cash costs, many farmers have, or are considering, natural air drying.

Natural air drying differs from hot air drying in many ways, yet it may accomplish the same end result if the user understands the process and its limitations.

The main advantages of natural air drying are flexibility and low operating costs. The disadvantages include reliance on ambient air conditions, extended operating time and associated grain handling requirements.

Natural air drying also differs from aeration. In many cases the two terms are used inter-changeably however aeration is the movement of a small quantity of air, approximately 1/10 cfm/bu. (1.3L/s·m³), through stored grain. The main purpose of aeration is to equalize the grain and ambient air temperatures. Aeration will, within limits, maintain, grain condition. Aeration is not drying, although some drying may occur under certain conditions.

When considering natural air drying or aeration, it is also necessary to contact Saskatchewan Power to ensure that adequate electrical power is available at the site.

Principle of Operation

Natural air drying works on the principle that when the moisture content (m.c.) of the grain is greater than the equilibrium moisture content (Fig. 1), moisture will move from the grain into the air until equilibrium is reached. The lower the relative humidity (R.H.) of the air, the lower the equilibrium moisture content. Equilibrium moisture content is the condition when there is no movement or transfer of moisture between the grain and air. Figure 1 gives the approximate equilibrium moisture contents for cereal grains and for oilseeds.

Figure 2 also illustrates that the combination of high grain moisture content and high grain temperature drastically reduces the allowable storage time and the allowable time for drying. Under these conditions, it is critical that the natural air drying system have the capability to complete the drying process before the grain begins to spoil.

Below is the allowable storage time for grains without forced air movement through the grain mass. Airflow through the grain, as with aeration or natural air grain drying, will increase the allowable storage time.

 

SAFE STORAGE TIME CEREAL GRAINS
Grain Temp. ° C	Grain Moisture Content
	14%	15%	16%	17%	18%	19%	20%	21%	22%	23%	24%	25%
< -5	SAFE 80 - >240 days
5					80-120	40-60	40-60	40-60	40-60	20-30	20-30	10-15
10				80-120	40-60	40-60	40-60	20-30	20-30	10-15	10-15	10-15
15			80-120	40-60	40-60	20-30	20-30	20-30	10-15	10-15	5-8	5-8
20		80-120	40-60	40-60	20-30	10-15	10-15	10-15	5-8	5-8	3-5	3-5
25	80-120	40-60	20-30	20-30	10-15	5-8	5-8	3-5	3-5	3-5	3-5	3-5
30	40-60	20-30	10-15	10-15	5-8	3-5	3-5	3-5	3-5	3-5	3-5	3-5
NOT SAFE

Estimated maximum duration in weeks of storage of field pea with respect to germination.

Storage Temperature	% Moisture Content
	11%	12%	13%	14%	16%	18%	21%	24%	29%
25oC	40	30	20	13	7	4	2	1
20oC	80	55	38	26	15	8	4	2	1
15oC	160	110	70	45	26	15	7	3	1
10oC	350	230	150	95	55	30	16	6	1
5oC	700	480	300	200	120	60	30	12	3

Source:  Alberta Agriculture and Rural Development

Average moisture content throughout the bind does not determine the storage ability of the grain.  Spoilage may occur in isolated locations in the bind where moisture is high.

Careful drying of pulse crops is necessary.  Drying too rapidly can damage the grain.  Drying too slowly can create conditions favourable for mould growth.

If the grain is to be used for seed purposes, and if moisture content is to be reduced by five per cent or more, then drying should be done in two stages.  Drying air temperature should also be lower for grain to be used for seed.  An aeration bin should be used alongside a heated air dryer for pulse crops.  The product is dried to within two per cent of the desired final moisture content and then transferred with caution, while hot, to the aeration bin.  It is allowed to sit in the aeration bin for at least six hours.  It is then cooled to outdoor temperature.  During this time, approximately two per cent moisture will be removed.  An airflow of 0.5 to one cubic foot per minute (CFM) per bushel should be used to cool the grain.  This system allows the grain to temper and then cool slowly to produce less stress on individual seeds and less cracking.

Caution - High Temperature Drying of Pulses

Pulses need special attention when they are dried in high temperature grain dryers.  The most important aspect is extreme susceptibility of these grains to mechanical and thermal stresses.  Mechanical stresses are those that happen during movement of grain in and out of the dryer and when the grain is recirculated in the dryer.  The grain is extremely susceptible to breakage when it is hot.

The exterior part of pulse seed is covered with a waxy material and is impervious to moisture.  The pathway for moisture flow into and out of the seed is through the point of the seed connection to the pod.  Because of the limited pathway for moisture, the seed undergoes internal pressures during high temperature drying and may crack.  Therefore the recommendation is not to dry the seed by more than four to five points in one pass.  This means that if moisture is to be dropped by 10 points, the drying should take place in two to three passes.  The recommendation is also that the seed left to be tempered for about eight hours between each pass.  For larger seeds such as chickpeas and beans, this resting time should be increased to 24 hours.

Grain moisture content limits the maximum drying temperature.  More caution is needed at higher moisture content.  The maximum drying temperature is usually less than 45^oC.  If the grain moisture content is more than 24 per cent the maximum temperature should be reduced by 5^o to 7^oC.

Operation

When using natural air drying as an integral component of an overall harvesting system, harvest should begin when the grain reaches 18 to 20 per cent m.c. Thus, maximum advantage may be taken of the low relative humidity which normally occurs in late August and early September. Natural air drying should not be considered as an emergency drying system.

The grain placed in the drying bin should be relatively clean. Fines, chaff, small or broken seeds and weed seeds increase the grain's resistance to airflow. Fines also tend to accumulate in the center of the bin and may spoil due to lack of conditioning.

The use of a grain spreading device in large diameter bins is recommended. These devices will help in distributing the grain, including fines, evenly in the bin. The grain surface should be kept level, or if possible, with a slight dish in the center. The above conditions will result in more uniform conditioning due to an even airflow through the entire grain mass. If the grain is clean, a spreading device may not be necessary provided the grain is manually leveled.

The fan should be started when the perforated flooring or tubes are completely covered with grain. Grain should be checked frequently to determine the location and the rate of movement of the drying front.

Checking should continue after the fan has been stopped and when the grain has been transferred to another bin. The grain may still need to be aerated just as if it had been harvested dry. During winter, check the top center one to three feet (300 to 900 mm) of grain for moisture build up or for an increase in temperature. Air forced through from the bottom, picks up moisture and carries it out through openings in the top of the bin (Fig. 3). Grain above the drying front remains at or near its initial moisture content. Grain within the drying front is between the initial moisture content and equilibrium moisture content. Grain below the drying front has a moisture content at or near the equilibrium moisture content.

The key to natural air drying is to move the drying front through the grain as quickly as possible. Actual times may vary from one to six weeks.

Management of Natural Air Grain Drying Systems

The flexibility of natural air grain drying systems will allow the operator to manage the system in several ways. Improper management usually results in lost income, not lost grain due to spoilage. The term "dry" defines the maximum moisture content at which the Canadian Wheat Board buys grain without penalty and is the moisture content at which grain can be stored for long periods of time.

The "dry" moisture content for various grains are:

wheat	14.5%
barley	14.0% - malt
	14.8% - feed
oats	14.0%
rye	14.0%
triticale	14.0%
buckwheat	9.5%
canaryseed	12.0%
corn	15.5%
sunflower	9.5%
flax	10.0%
canola	10.0%
mustard	9.5%
beans	18.0%
peas	16.0%
lentils	14.0%
faba beans	16.0%
soybeans	14.0%
safflower	9.5%

The fan(s) should be started when the perforated flooring or tubes are completely covered with grain. Be sure enough grain is on the flooring to hold it in place when the fan is turned on. The fan should then be operated continuously until one of the following situations occur:

a. Drying Front is Completely Through the Grain Mass

In this case, the fan has been operated continuously until the drying front has been forced through the grain mass. The grain at the bottom will dry to equilibrium grain moisture content, which may be in the 10 to 11 per cent range before the top reaches 14.5 per cent. Some rewetting of the bottom layer may occur during evenings or wet rainy spells, but a moisture content variation, similar to that shown in Figure 4.

When managing natural air grain drying system in this manner, a substantial loss in saleable product weight may result. For example, about 4200 lb. (1.9 t) of saleable water is removed from a 19 ft. (5.9 m) diameter by 14 ft. (4.3 m) high bin containing 3280 bu. (89.4 t) of wheat, by drying the complete grain mass to an equilibrium moisture content of 12.5 per cent. At $8.50/bu. ($312.30/t) this amounts to $595 of lost revenue plus the cost of operating the fan longer.

b. Drying Front is Partially Through the Grain Mass

This case is similar to "a." except the fan is stopped before the drying front is pushed completely through the grain mass (Fig. 5). The bottom layers are below dry, but the upper layers are still above dry. At this stage, the grain is mixed. Moving the grain from one bin to another storage bin may mix the below-dry and the above-dry grain. After a period of time, the moisture contents equalize, hopefully at "dry". Experience will be required to determine when the fan should be stopped and the grain moved. The grain should be probed to determine if aeration is required.

In both cases, a. and b. the operator must know the grain condition and what effect the flow of ambient air will have on the grain. This information will allow the operator to decide if the fan should be operating.

For example, barley at 19 per cent m.c. was harvested in late August when ambient conditions are 30ºC and 40 per cent relative humidity (R.H.) The fan was started as soon as the flooring was covered. The drying front has moved part way through the grain mass.

After several days of continuous fan operation, the grain condition is similar to Figure 5. The weather now turns to 20ºC and 90 per cent R.H. The grain mass will be cooled to 20ºC and the lower section (nine per cent grain) will begin to take on some moisture both changes are desirable. The upper grain (19 per cent m.c.) will not change as the equilibrium moisture content at 90 per cent R.H. is approximately 20 per cent. It has been shown that grain re-wets at a much slower rate than it dries.

If the high humidity conditions continue for more than two or three days, it would be advisable to turn the fan off as little is gained by allowing it to operate longer. Barley will safely store for about 20 days at 20ºC and 19 per cent m.c. (Fig. 2). The grain should be probed and checked for trouble spots.

 

c. Drying Front is Partially Through the Grain Mass and the Entire Grain Mass Temperature is -5ºC.

This situation occurs when weather conditions are unfavorable and there is insufficient time to move the drying front through the grain is described in b. The fan must be allowed to operate to drop the grain temperature to below freezing. This prevents the grain from heating and/or spoiling during winter months. The entire bin may be left frozen until spring or dried in a hot air dryer.

Drying With Supplemental Heat

The drying rate of a natural air grain drying system can be slowed or completely stopped by adverse weather conditions. Cool air can only hold a small amount of moisture and moisture movement from grain to air is very slow at temperatures less than 10ºC. Heating the inlet air reduces the air relative humidity and increases the temperature of the grain. This increases the drying rate.

The rule of thumb relating temperature increase to relative humidity decrease is: A temperature increase of 10ºC reduces the relative humidity by one-half. For example, air at 0ºC and 70 per cent relative humidity heated to 10ºC results in a relative humidity of 35 per cent; heated an additional 10º to 20ºC will reduce the R.H. to 17 per cent.

Using supplemental heat results in the continuation of drying but the disadvantages of using supplemental heaters are the increased capital and operating costs and possible over drying costs.

The following recommendations, the result of research conducted in Saskatchewan, will minimize drying costs when incorporating supplemental heat:

• use heaters if drying is not complete and the night temperatures drop into the 0º to 5ºC range or if the relative humidity remains above 70 per cent for extended periods - airflow rates - minimum of one cfm/bu (13 L/s³)
• limit air temperature increase to 10ºC
• minimum air temperature (after heater) 5ºC
• maximum air temperature (after heater) 24ºC
• "average dry" bin (management item b.), mix grain then cool
• natural gas should only be used if installation cost is low [estimated saving over propane of 4:1]

The following formula can be used to size a supplemental heater or to determine the approximate temperature rise that can be expected from a particular heater.

Heater output = change in temperature x airflow x constant
watts = temp. change (º) x L/s x 1.2
Btu/hr = temp. change (º) x cfm x 1.1

Costs Resulting From Selling Grain Other Than "Dry"

There is a cost for selling grain at a moisture content above "dry".  Grain terminals that have driers buy grain only on a dry basis.  In July 2008 the average drying costs at four grain elevators at 16 per cent moisture content was 18.5 cents/bu. ($6.80/t) and at 18 per cent moisture content was 34.8 cents/bu. ($9.47/t).

In many cases it may be cheaper to have grain dried at an elevator if it can be sold, rather than dry it on the farm.

The penalty for selling grain below "dry" is not as well known and varies directly with moisture content.  Lowering the water content reduces the weight of the grain mass being sold.  Penalties can be minimized if the grain is blended to average "dry."

Table 1 illustrates the value of wheat at various moisture contents.

Table 1. Value of wheat at various moisture contents
Moisture Content %	Grain volume at 14.5% Bu. (T)	Price per Bu. (T)	Grain Return	Penalty Difference
10	950 (25.85)	$4.00 (317.84)	$8217.50	$ -432.500
12	972 (26.45)	4.00 (317.84)	8407.80	-242.20
14.5	1000 (27.22)	4.00 (317.84)	8650.00	-
16	1018 (27.71)	3.81 (306.08)	8476.94	-188.33
18	1043 (28.38)	3.61 (292.12)	8287.04	-362.96
Based on price of dry wheat at $8.65/bu. ($317.84/T) less shrinkage and average drying cost at four elevators:  16 per cent at 18.5 cents/bu. ($6.80/T) 18 per cent at 34.8 cents/bu. ($9.47/T)

Tools Required for Monitoring a Natural Air Grain Drying System

When setting up a natural air grain system, the following tools are essential:

Grain probe - used to obtain grain samples for determining moisture content. It is not easy to push a grain probe into 15 ft. (4.5 m) of tough grain. Probes vary is size and shape. Normally a long, thin tapered design is the easiest to operate for sampling. Probes with large flightings must be augured in and out. The capacity of the probe should be large enough that only one sample is required for the moisture meter.

Grain moisture meter - used to determine the grain moisture content which is necessary information for proper management of the system. Check Prairie Agricultural Machinery Institute (PAMI) reports regarding the performance of moisture meters.

Manometer - used to measure the static pressure developed by grain in a bin when forcing air through the grain. Manometers may be purchased or made. A manometer consists of a transparent tube, with a minimum inside diameter of 1/4 inch (six mm), bent into a "U" shape and a ruler; all mounted on a board (Fig. 6).

The tube is filled with water to the level shown in Figure 6. One end of the tube is left open and the other end is connected to the transition.

Factors to be Considered in System Design

When selecting the equipment for a natural air grain drying system, several points must be considered.

Location

The province is divided into three zones (Fig. 7) due to the various average fall weather conditions. Different airflow rates are required for a system in each zone because of the different weather conditions.

Table 2. Recommended airflow rates for various grain moisture contents and temperatures
% Moisture Content Above Dry	Grain Temp Low → High	Grain Temp Low → High
	cfm/bu	L/s. m3
→ 4	0.5 → 0.75	7 → 10
4 → 6	0.75 → 1.0	10 → 13
6 → 8	1.0 → 1.5	13 → 20

In zone one, the airflow rates given in Table 2 apply. In zone two, the rates will have to be increased by 1.25 times. The increased rates are required due to the higher relative humidity in zone two.

There are more risks involved in using natural air grain drying in zone three because the mean relative humidity for September is above 70 per cent. Drying is possible (Fig. 1), but the time required will be longer. If natural air drying is used in zone three, the grain should be monitored closely and the airflow rates will have to be increased by 1.5 times.

Grain Moisture Content and Temperature

The maximum expected moisture content of grain to be harvested will partially determine the required airflow rate. Grain with high moisture content will require a greater airflow rate than grain with a lower moisture content. The greater the airflow rate is necessary to increase the drying rate so that the grain is conditioned before it begins to spoil.

Zone 1 - Use airflow rates given in Table 2.
Zone 2 - Use 1.25 times the rates given in Table 2.
Zone 3 - Natural air grain drying is questionable. Use 1.5 times the rates given in Table 2.

Note:   60●(11.6)

• 60 = September mean relative humidity (%)
• ● = Location
• (11.6) = September mean temperature (^oC)

Grain temperature also influences the airflow rate. Grain put into the bin with a high temperature will require a higher airflow rate. This higher airflow rate is necessary to quickly reduce the grain temperature before the grain begins to spoil.

Figure 2 illustrates the safe storage times for grains at various moisture contents and temperatures. Table 2 provides the recommended airflow rates for various grain moisture contents and temperatures. The higher airflow rates should be used for grain above 22^oC.

Grain Type and Depth

The type of grain to be dried will affect the static pressure developed and thereby the fan required. Grains with small seeds create a higher resistance to airflow and resulting higher static pressure.

The grain depth will also affect the static pressure, the greater the depth, the greater the static pressure. The following charts below give the expected static pressures for various depths and for different grains.

 

Example	Bin "A"	Bin "B"
Diameter	22 ft. (6.7 m)	18 ft. (5.5 m)
Grain depth	10 ft. (3.1 m)	15 ft. (4.5 m)
Grain volume	3000 bushels (110 m3)	3000 bushels (110 m3)
Expected static pressure	3.5 in. (875 Pa)	7.5 in. (1875 Pa)
@ 0.75 cfm/bu (10 L/s m3)	@ 2250 cfm (1100 L/s)	@ 2250 cfm (1100 L/s)
Typical fan required	5 hp. (3.73 kW) axial or 3 hp. (2.24 kW) centrifugal.	5 hp. (3.73 kW) centrifugal (beyond the range of an axial fan).

Natural Air Drying Equipment

Satisfactory performance of a natural air drying system depends upon the selection, installation and management of equipment. The following must be considered to ensure satisfactory performance.

Bin Selection

Figure 8 illustrates two bins which contain the same amount of grain. The difference in grain depths is due to the difference in bin diameters. Both bins will require the same airflow, but will require different fans to deliver this airflow. The deeper grain mass results in a higher static pressure and, consequently, a higher powered fan. In an attempt to keep the static pressure as low as possible, low, large diameter bins should be selected for natural air drying systems.

Fan Selection

The fan selected (Fig. 9) must be able to deliver the desired amount of air at the expected static pressure. Consult the manufacturer's specifications and independent fan test reports for fan performance.*

* - Alberta Farm Machinery Research Centre
- Prairie Agricultural Machinery Institute

Centrifugal type fans should be considered when static pressures exceed four in. (1000 Pa.)(Fig. 10 ) or if operating noise is a consideration.

In-line centrifugal fans have performance characteristics similar to conventional centrifugal fans. The in-line centrifugal fan's main advantages are lower cost and it can bolt directly to transitions made for axial fan.

Vane axial fans are more efficient than tube-axial fans. Externally they look identical; however, vane-axial fans have internal vanes which catch and direct the air as it leaves the fan blades (Fig. 12).

Figure 11. In-line centrifugal and centrifugal fans.

Figure 9.  Natural Air Drying Fans Figure 12.  Tube and vane axial fans

It is advisable to have a manometer mounted on each bin to measure the static pressure developed. By matching the fan output to the grain volume, the

desired airflow rate (cfm/bu. or L/s. m³) may be achieved. Difficulties have been encountered in accurately measuring the static pressure at the transition. To overcome the possibility of inaccurate readings, one of the following methods is recommended:

1. Place a length of tubing extending near the centre of the bin in or under the perforated material (Fig. 13). Connect one end to the manometer. If the transition is short and abrupt ("A" Fig. 14), add one inch (250 Pa.) to the manometer reading to give static pressure at the fan. If the transition is relatively long and tapered ("B" Fig. 14), add 1/2 inch (125 Pa) to the manometer reading.
2. Drill a series of holes in the transition as shown in Figure 14. Two holes at 90 degrees to each other is a minimum, four holes is preferred. Individual manometer readings should be averaged or the holes may be joined by using "T" and one reading taken. If possible, drill these holes large enough to install valve stems (without the valve) and keep capped when not in use.

Sizing of Transitions, Ducts, Vents and Perforated Floor Area

The transition from the fan assembly to the duct should be as smooth as possible. Abrupt changes in transition cross-section should be avoided. Transition "B", In Figure 14, is more desirable than Transition "A". Skilled sheet metal workers are capable of fabricating this type of equipment. The duct size required may be determined from the following, or from Table 3.

Cross-section area =	total air volume         *maximum duct air velocity
Cross-section area(ft.2) =	total air volume (cfm)               1500
Cross-section area(m2) =	total air volume (L/s)               7500
* maximum duct velocity = 1500 fpm (7.5 m/s)

The vent area in the bin roof should be approximately 1.5 times the duct cross-sectional area.

Selection of Perforated Material

The perforations in the ducts and/or flooring must be sized to prevent the seed from passing through and yet large enough to offer the least possible resistance to the airflow through the perforations.

The material should be at least 10 per cent perforated. Consult the various suppliers and manufacturers of perforated material to determine the one(s) most suitable. The perforated floor area should be sized such that the air velocity through this total area is 30 ft./min. (0.15 m/s) or less. Actual velocity of air through a perforation will be 300 ft./min. (1.5 m/s) with 10 per cent perforated material.

Layout of Perforated Ducts and Flooring

The layout of perforated material should provide as uniform as possible air distribution to the grain mass. Uniform air distribution is necessary when drying high moisture grain, grains at high temperatures, or when using high airflow rates.

Flush Floor Systems

The fully perforated floor is the most desirable layout for a natural air drying system, but is the most costly and, in some cases, not necessary. Fully perforated floors may be installed in existing bins. Fully perforated floors usually consist of perforated planking on manufactured supports, perforated sheet or roll material on either manufactured supports or user supplied supports. The distributor or supplier of the perforated material should be consulted regarding the support required.

Partially perforated floors, as illustrated in Figure 15, are being successfully used. The recommended maximum moisture content of the grain being dried with this system is six per cent above dry. Partially perforated floors are normally installed in new bins, however retrofitting an existing bin is possible.

Figure 16. Partially perforated floor.

Figure 16B.  Tube or half-round system.

 

Table 3. Sizing of Transitions, ducts, perforated floor area and vents
Air Flow		Minimum Cross-Sectional Duct Area		Perforated Floor Area		Minimum Roof Vent Area
cfm	L/s	ft2	m2	ft2	m3	ft2	m3
500	250	0.33	0.03	17.0	1.7	0.5	0.05
1,000	500	0.67	0.07	33.0	3.3	1.0	0.10
1,500	750	1.00	0.10	50.0	5.0	1.5	0.15
2,000	1,000	1.33	0.13	67.0	6.7	2.0	0.20
2,500	1,250	1.67	0.17	83.0	8.3	2.5	0.25
3,000	1,500	2.00	0.20	100.0	10.0	3.0	0.30
3,500	1,750	2.33	0.23	117.0	11.7	3.5	0.35
4,000	2,000	2.67	0.27	133.0	13.3	4.0	0.4

Above Floor Systems

The tube or half-round perforated systems (Fig. 16) are commonly used in existing bins. The maximum recommended grain moisture content of the grain being dried with this system is four per cent above dry. This moisture content may be increased by using a "Y" or "T" system of ducts to obtain a more uniform air.

The inverted "V" system (Fig. 17) is another above floor system, but does not use any perforated material. The result is reduced surface areas for air to penetrate the grain mass. The maximum moisture content of the grain being dried will depend on the duct length and layout.

Figure 18 illustrates a design suitable for a 12 x 14 x 8 ft. (3.6 x 5.2 x 2.4 m) wood grain bin at an airflow rate of one cfm/bu. (13 L/s³). All of the above floor systems must be moved or removed when the bin is being emptied or when a sweep auger is used.

Air Distribution Systems for Hopper Bottom Bins 

There are a variety of air distribution systems available for natural air drying in hopper bottom bins (Fig. 19). The majority of these units do not meet the guidelines set out in Table 3. However, test and field experience indicates these units have reasonably good air distribution patterns; those with ducts located in the upper part of the hopper rely on air leakage around the outlet port to allow air down through the grain in the hopper. The following points should be considered when selecting an air distribution system:

• largest perforated (or outlet) area (as close as possible to requirements provided in Table 3)
• ease of installation
• ease of operation
• adequate support for suspended components

Natural Air Drying Management Considerations

Small seeded grains such as canola and flax are harder to blow air through and as a result would require a larger fan than indicated in this study.  Wheat that is higher in moisture than 18 per cent would require a higher airflow than indicated in this study.  Higher airflow could be obtained by using a larger fan than the one indicated here or filling the bin only partly full of grain.  Another alternative to a larger fan is to run the fan for a longer period of time.  The time of year and air temperature have a significant influence on the time required to dry grain with natural air.  Grain dried in August with higher air temperatures and low humidity will dry faster than grain dried in October.  Grain that will not dry in the fall can be frozen in cold weather and dried in the spring.

Above floor screens have a lower investment and drying cost per bushel but do not provide the uniform airflow across a higher percentage of the bin floor as provided by a flush floor screen.  Natural air drying equipment installed in large diameter bins provides the lowest per bushel drying costs.  Purchasing a grain moisture meter and grain sample probe will assist you in managing your natural air drying system.  Temperature sensing cable systems are also available to assist in managing stored grain.