October 12, 2007

Producer Question

Situation: I have a 27-foot diameter bin 18 feet to the eave. It has a full mesh floor but only a 5 HP aeration fan. I will need to dry corn in this bin this year.

Question 1. Should I buy a 10 HP fan?

Question 2. Would there be any advantage if I connected a second 5 HP fan to the first one instead of buying a 10 HP fan? (Authors note: This is known as putting two fans in series.)

Question 3. Would it pay to install a second transition duct and put a second 5 HP fan on the opposite side of the bin instead of buying a 10 HP fan? (Authors note: This is known as putting two fans in parallel.)

Response

All fan models have their own unique operating characteristics but we can make some comparisons using a 5 hp axial flow fan and a 10 hp axial flow fan from a popular manufacturer. I will use the FANS program from the University of Minnesota to make the comparisons.

Answer to Question 1. If the bin is filled with corn to a depth of 17.5 feet, one 5 HP fan could be expected to produce about 0.96 cubic feet per minute (cfm) per bushel. In Nebraska, we recommend 1 cfm/bushel as a minimum for natural air drying corn with a starting moisture content of 17% or less. The fan you have now produces close to the minimum airflow at full depth. Do not start with moisture contents over 17% if you are filling the bin to the eave when using this fan and consider putting less than the full depth of grain in the bin (more on this later).

If we look at replacing the 5 HP fan model with a 10 HP fan, how much difference will it make? According to the FANS program, you could expect to push around 1.13 cfm/bushel through 17.5 feet of corn in this bin. The extra 0.17 cfm/bu airflow brings you up over the minimum airflow and will result in a somewhat shorter drying time as compared to the 5 HP fan.

The difference in airflow is surprisingly small when comparing a 5 HP fan and a 10 HP fan. Part of the difference is the 5 HP fan model chosen for this comparison is actually using 5.57 HP and the 10 HP fan model is only using 8.25 HP under these conditions, so the actual HP requirement of the larger fan is not twice that of the smaller fan. The other factor is more static pressure is required to push greater rates of airflow through the grain. The 5 HP fan must produce 2.76 inches of static pressure to push 0.96 cfm/bu while the larger fan must produce 3.49 inches of static pressure to push 1.13 cfm/bu.

Answer to Question 2. Connecting two 5 HP fans in series will result in about the same airflow characteristics as one 10 HP fan. Using the FANS program, I found two of the 5 HP fans in series had an output of 1.16 cfm/bu at 3.62 inches static pressure whereas one 10 HP fan produced 1.13 cfm/bu at 3.49 inches static pressure. The two smaller fans in series are using 8.8 HP while the 10 HP fan is using 8.25 HP.

This could be a cost effective option to increase airflow, especially compared to buying a 10 HP fan or adding a second transition duct.

Answer to Question 3

Adding a second transition duct and installing a second 5 HP fan would increase airflow slightly more than any of the other alternatives. According to FANS, two 5 HP fans in parallel would produce 1.22 cfm at 3.89 inches static pressure while using 9.55 HP. I question whether the advantage in airflow would be great enough to get payback on the cost of installing a second transition duct and the extra wiring as compared to a series arrangement.

A Fourth Option

There is an additional option in this case. If the producer were to fill the bin to a depth of 15.5 feet, it could achieve the same airflow per bushel (1.13 cfm/bu) from a single 5 HP fan that it would if he changed to a 10 HP fan and filled the bin to the full 17.5 feet depth. The time to dry the grain would be the same in either scenario because the airflow (cfm/bushel) is the same in either scenario. The difference is, at a full 17.5 feet depth, he would dry 8016 bushels and at 15.5 feet depth he would dry 7100 bushels — an 11% difference in bushels dried per batch. The cost of electricity for fan operation would be 34% less, however, and he would have no additional equipment costs. Depending on how tight the on-farm drying situation is, it seems to me this is the most cost effective alternative.

A final comment. You need at least a square foot of exhaust opening per 1000 cfm of airflow to avoid restrictions that can add to the static pressure the fan must overcome. Hooded air vents designed for a 5 HP fan may not be adequate if you change to a bigger fan or multiple fans. You may need to open the center hatch and/or access hatch to properly vent the air passing through the grain.

Tom Dorn
Extension Educator, Lancaster County