# Examining Four Options for Adding a Bin Fan

### 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.