One of the first, crucial steps in designing an aeration system is sizing the fans. Fans that are misapplied or sized incorrectly can lead to failure of the entire system.
The first step in fan selection is deciding how much airflow is required (see table below). Typically, this is 1/10 cfm per bushel for dry grain in storage. To obtain the total airflow, calculate the total volume of grain in the storage structure in bushels, then multiply by the cfm per bushel rate. This will determine the total cfm required for the system.
Next, decide how many fans are required. It is important to know the design of the air distribution system, if one exists. In a new system, it is imperative to know how many air inlets are in the air distribution system.
Now determine the static pressure for the desired airflow rate. Static pressure is the pressure, expressed in inches of water column, created when air is forced through the grain mass. Different grains have different resistance to airflow, so it is critical to know what type of grains will be stored in the silo. Grain depth also is crucial when determining static pressure.
There is a common misconception that grain depth and type are the only factors needed to determine static pressure. The airflow rate also makes a big difference. For example, 1/10 cfm per bushel on wheat for a 60-foot grain depth would be 9.42 inches of water column.
Finally, determine the airflow rate required for each fan. Take the total airflow and divide by the quantity of fans in the system. For example, 10,000 cfm divided by four fans would equal 2,500 cfm per fan.
DIFFERENT FAN TYPES. Deciding what fan type to use is one of the most important decisions when designing an aeration system. Often, the wrong fan is chosen because of a misunderstanding of how each fan type performs. This may result in problems as bad as total system failure or, more likely, inefficiencies.
Tube axial or vane axial fans used in the grain conditioning industry have a propeller-type blade. Most axial fans used for aeration are direct drive and use a 3,450 rpm motor.
Tube axial fans are normally smaller, typically 3 hp or smaller. Vane axial fans normally range from 3 to 15 hp and include air straightening vanes in the housing to reduce the turbulence created by the propeller. Tube axial fans do not have these vanes because of housing size restrictions.
Axial fans are very loud compared to other fan types. This can be an important consideration. Local zoning codes should always be consulted.
Axial fans are considered high performance, low static pressure, and are normally used on aeration systems with low grain depths or on deeper grain depths with grains having low resistance to airflow, such as corn or soybeans.
Inline centrifugal fans are similar in construction to axial fans. The major difference is that the inline fan uses a centrifugal impeller. Most manufacturers use a backward inclined airfoil design for the impeller. This type of fan typically ranges from 2 to 15 hp and has a direct drive with a 3,450 rpm motor.
Inline fans are considered medium performance, medium static pressure fans and are most often used on aeration systems with higher grain depths or on lower grain depths with grains having high resistance to airflow, such as wheat or sorghum. These fans are often misapplied and sold as a "quiet axial" when often the axial fan will outperform it.
Centrifugal fans with a 1,750 rpm motor are designed so air enters the fan from the side and turns 90° to the fan discharge. Most manufacturers use a backward inclined airfoil design for the impeller.
These fans typically have an electric direct motor drive with a horsepower of 3 to 100. The 1,750 rpm centrifugal fans are considered high performance, medium static pressure fans, and are normally used on aeration systems with higher grain depths or on lower grain depths with grains having high resistance to airflow (wheat, sorghum). These fans can often be used in place of a 3,450 rpm axial fans because their performance characteristics are similar at low static pressures.
Centrifugal fans with a 3,450 rpm motor are similar to the 1,750 rpm type, with an electric direct motor drive, but the housing and blade are much smaller for the same horsepower motor, typically from 3 to 60 hp.
The 3,450 rpm centrifugal fan is considered medium performance, high static pressure fans. They are typically used on aeration systems with very high grain depths and grains having high resistance to airflow (wheat, sorghum). Because these fans are less expensive than the 1,750 rpm type, they are often misapplied, and do not perform nearly as well at low static pressures for the same horsepower.
SELECTING THE RIGHT FAN. When selecting a fan for your aeration system, first check the performance tables supplied by the manufacturer.
To select a fan, take the static pressure and the airflow per fan and find the fan most closely matching the requirements. Several different fans may match your requirement, so pick the one with the lowest horsepower.
For best performance, pick a fan that performs in the middle of its static pressure range. Fans operating at the upper end of their static pressure range have a possibility of stalling (ceasing to move air). For example, the design calls for a fan producing 4,000 cfm at 10 inches of water column. A 10 hp inline fan or a 10 hp 3,450-rpm centrifugal fan will work (see table on next page), but the centrifugal fan is more in the middle range of its static pressure capabilities.
Sometimes, the system design may be unreasonable for available fans, and it will be impossible to find a fan to fit your application. If this happens, fans can possibly be reconfigured to fit the application.
Fans can be run in series (piggy-backed) to increase the static pressure capabilities. Axial fans are fairly easy to install in series as the housings can be bolted together, back to back. It is more difficult to run centrifugal fans in series. A transition must be built to direct the discharge of one fan into the inlet of the other.
When running fans in series, the airflow does not change much at low static pressures. It is also critical to provide adequate roof exhaust when designing an aeration system. Improperly designed exhaust systems can cause supply fans to not provide as much air as intended because of static pressure losses. The excess static pressure can also put undo stresses on the roof.
A properly designed exhaust system should allow the air to escape at fairly low velocities so there are not excessive losses. A common misconception is that the air coming out of the exhaust should "blow your hat off" if the system is working properly. In reality, if this is the case, additional exhaust should be investigated.
Although exhaust area should be designed into the system to provide one square foot of opening for every 1,000 cfm of supply air. Roof hatches should probably not be considered as vent area unless you are sure they will always be open while conditioning grain. Some openings at the eave can be counted on but steps should be taken to make sure it is unobstructed. For example, 25,000 cfm provided by supply fans divided by 1,000 would require 25 square feet of opening.
Roof exhausters are used throughout the industry to provide ventilation as well as exhaust in large commercial tanks. Power exhausters are normally low static pressure, high air volume axial fan designs.
There are different theories on how to size power exhausters. Most companies provide the exhausters primarily for ventilation of the large open area under the roof. The formula used by most companies is: (Design cfm of supply fans x 1.5) divided by cfm rating of exhauster equals the quantity of exhausters. For example, (30,000 (cfm of supply fan cfm) x 1.5) divided by 10,000 (cfm of exhauster) equals 4.5 exhausters. (Round down to 4.)
Normally, no static roof exhaust area is eliminated when adding power roof exhausters.
Randy Sheley is technical sales manager for GSI International, Assumption, Illinois, U.S. This article was presented at the Grain Elevator and Processing Society's 1999 conference.