Fast Dry – An Alternative to Conventional Drying

by Dave Wall

FARMERS KNOW THAT grain dries quickly in the field on a hot or windy fall day. So why not duplicate the process in a grain bin? I call this process Fast Dry.

Over the years, aeration systems have helped get crops off in good condition. In the early 1980s, farmers installed 12-inch and 18-inch aeration tubes in bins to handle grain at 18 percent moisture. It worked for the conditions at that time.

But today, farms, combines and bins are much bigger. This raises more questions for producers. “Why doesn’t my oId natural air system work fast enough? Why does it work one year but not the next? How can I speed up the process and design my present bins and future bins to reduce drying costs? If I design it right, can I save the cost of a dryer that I may not even need?”

Four steps to duplicate an ideal summer/fall drying day…..

Step one

Let’s go over some Fast Dry technical data to understand what happens. Look at the amount of moisture removed by various temperatures and humidity and by airflow. Cubic feet per minute (cfm) simplifies later calculations.

Table 1. Drying temperatures Air temperature

Air temperature Air Humidity Pounds of water removed per 1,000 cfm
 (%) (per hour) (per day)
 17°C (60°F)  65.0  7.25  173.8
 22°C (70°F)  45.0  12.44  298.5
 27°C (80°F)  32.0  18.33  440.0
 38°C (100°F)  18.0 32.20  59.10
 60°C (140°F)  5.8  59.10  1418.0

The ideal hot August day is 25 to 40 C. We want as much airflow as possible to remove as much moisture as possible. Fan airflow will vary because of static pressure, which is measured in kilopascals or inches of water.

People in the fan business can calculate the static pressure on any given bin size, grain depth, type of grain and the most critical factor in a Fast Dry system, the amount and kind of perforations in the bin.

The more square feet of perforation, the bigger the drying front created in the bin. The more air exposed to the mass of grain, the faster a drying front is sent through the bin.

For example, with a duct system in a hopper bin, I may have only 40 to 50 square feet of drying area. What happens in this design is called channeling. Air takes one path and grain is over-dried in that one area. That’s a small drying front compared to 500 square feet of perforation found on a 24- foot or 27-foot flat-bottom bin.

Under excellent conditions, I can only move about 40 cubic feet of air through one square foot of perforated metal. I use 30 cfm for calculations because small seeds, especially in a canola floor, may block a small percentage of holes.

With a seven-hp fan, we need a floor at least 200 square feet. That figure comes from dividing the required 6,000 cfm airflow through the bin by 30 cfm for each square I foot of perforated metal.

The fans are all centrifugal. Whether they’re in-line or straight centrifugal doesn’t matter as much as getting the air to the best moisture carrying capacity and having a matched package to go with it.

Step two

Let’s look at how much air most fans move in cfm. Then we can find out how many pounds of water can be removed per day.

Table 2. Increased airflow increases drying

Fan Size Airflow (1,000 cfm) Pounds of water removed per day
 27°C  38°C
 3hp 3,500rpm  2-4  880-1,760  1,500-3,000
 7hp 3,500rpm  4-6  1,760-2,640  3,000-4,500
 10hp 3,500rpm  5-7  2,200-3,080  3,750-5,250
 10hp 1,750rpm  7-12  3,080-5,280  5,250-9,000

Source: Dave Wall

Step three

A bushel of 18 percent wheat has three pounds of excess water. 20 percent wheat has four pounds of

water. Barley and oats are lessbecause of lighter bushel weights.

With a 7,OOO-bushel bin of 18 percent wheat, there are 21,000 pounds of water to remove. A seven-hp fan at 27 C and 32 percent humidity would take more than eight days to remove three points of moisture. At night and during many days, we don’t get that low humidity, so in reality this could mean 20 or more days.

If the humidity is controlled and the fans operate at 38 C and 18 percent humidity, we can dry the 7,OOO-bushel bin in less than fivedays. 7,000 bushels times three pounds of excess water equals 21,000 pounds. Then divide 21,000 pounds by 4,500 pounds per day to get 4.66 days.

Step four

This involves taking charge of controlling the humidity or moisture carrying capacity of air. Here is another formula and chart to show how to lower humidity and determine proper burner size. For every 2.2 degrees C (4°F) increase in temperature, I reduce humidity by 10 percent.

Table 3

Increasing air temperature lowers humidity
Starting at: 5.6 ( (42 F) and 90% humidity

Temperature Humidity Temperature Humidity
7.8°C (46°F) 81% 25.4°C (78°F) 36%
10.0°C (50°F) 73% 27.6°C (82°F) 32%
12.2°C (54°F) 66% 29.8°C (86°F) 29%
14.4°C (58°F) 60% 32.0°C (90°F) 26%
16.6°C (62°F) 54% 34.2°C (94°F) 23%
18.8°C (66°F) 49% 36.4°C (98°F) 21%
21.0°C (70°F) 44% 38.6°C (102°F) 19%
23.2°C (74°F) 40%

Source: Dave Wall

You can see by this chart that there isn’t any point in adding heat once you reduce the humidity to 20 percent. In most high humidity conditions, I need a temperature rise of 25 to 33 degrees C or 40 to 50 degrees F to dry to optimum conditions.

With this information I can determine the proper burner size. By using the formula of airflow in cfm times the temperature rise in of times 1.05 we determine the required BTUs. A seven-hp fan moves 5,000 cfm times 50F times 1.05 equals a 262,500 BTU burner.

Fast Dry 20,000 bushels

Now let’s design an ideal Fast Dry system. We’ll go with a 33-foot diameter 20,OOO-bushel bin with a full floor and U-trough system for fast unloads. This gives 854 square feet of drying area. Instead of the traditional in-line centrifugal fan, we use a 10-hp, l,750-rpm fan that moves more than 10,000 cfm.

Now we need a burner that produces at least 500,000 BTU. We arrive at that figure by taking 10,000 cfm times the temperature rise of 50 F. From the charts, I know this setup will take out more than 7,500 pounds of water per day.

If I only put in 15,000 bushels at 18 percent, which is 45,000 pounds of water, it takes six days or less to get the job done.

The simplicity of these formulas makes some happy customers. The cost of a complete package with fans and burners is $1.50 to $2 per bushel. This includes all construction, concrete and rebar, proper fan, burner and unload system.

The next farming generation

With two or three bins designed this way, the next generation on the farm will still be using them in 40 years. By placing them in a straight row, grain can be moved quickly with a few 10-inch or 13- inch augers. The 6,000 to 10,000 bushel hopper bins are convenient, but don’t allow proper drying and a person can’t use the 10-inch or 13-inch swing augers to unload them. So for less cost per bushel you can dry, aerate, cool and handle grain more efficiently with a flat-bottom bin.

What are the design criteria to make this work? The wider the bin or the more square feet of floor, the more grain is exposed to dry air. Bins that are not as high at the eave work better. The floor should be 100 percent perforated.

Last year I experimented with a solid flashing only six inches wide against the bin wall, preventing air from passing through that six-inch outer layer of grain. Some farmers complained that this outer grain actually started to sprout because of the heat, moisture and time in the bin. Now I recommend staying away from partial floors.

Manitoba Agriculture’s full-floor and partial-floor comparisons suggest that full bin floors reduce static pressure and give better results, especially when drying grain.

Bins must be designed with an open eave so in late fall, when moisture-saturated air hits a frozen roof, condensation runs out of the bin and not down the inside. This is a bin with no uprights or stiffeners, or has the uprights on the outside.

The unload mechanism is also important. The new Springland V-Trough that unloads up to 8,000 bushels per hour was covered in the May 1999 FARMING magazine.

A new bin can also be designed to add a stir system later by installing a track at the eave. If three-phase power is available, a stir system can reduce the drying time by half. By taking the air temperature up to 60 C (140 F) you double the moisture carrying capacity, but at that point the grain needs to be stirred to avoid over-drying.

Fast Dry is efficient because the air is generally blown through more than 10 feet of grain. This gives the air time to get saturated with moisture. The heat can be propane, natural gas or any other source.

Farmers love the simplicity and low cost of Fast Dry. For as little as $1,700, a farmer can buy a 40,000 to 500,000 BTU burner that converts one bin to Fast Dry, provided the bin was designed properly.

Even though propane and natural gas create water that is added to the air flow, the air still has more than enough moisture holding capacity to dry the grain. It’s easy to adjust the temperature higher to burn off the excess water. If natural gas is available, it’s cost effective to run lines to each bin and have a burner at each bin.

The open flame is not a concern because the flame, together with all the safety switches, is outside the bin. Even sunflower growers have used Fast Dry without fires because they designed their system properly.

The use of water radiators with a remote furnace that burns wood, coal or natural gas is not as efficient because there isn’t enough temperature rise to burn off the high humidity. As well, energy is lost with the transfer from open flame to liquid heat, then converting liquid heat to airflow.

Greg Hudye from Norquay SK has used Fast Dry since 1996 as reported in the October 1998 FARMING magazine. In 1999, he put grain in at 21 percent. His system takes out about one point of moisture per day. He ran his burner for eight days, then left the fan on to cool the grain and take out a few more points.

The driest grain in his Fast Dry system in 1999 was 12.7 percent. In his first year with the system, some grain went down to eight percent. Using 13-inch augers, he takes grain from some bins and transfers it to others for more even moisture samples.

His complete setup cost less than $2 per bushel to build, which gives him storage, drying and a simple grain handling system. Hudye says the system has more than paid for itself and fuel costs are pennies per bushel. He says the beauty of the system is that he can fill the bins and walk away without worrying.

This article was originally published in the Volume 3, Number 7, August 2000 issue of FARMING – a supplement to The Western Producer