In-depth analysis of sizing requirements for one of the toughest use-cases for compressed air; harvest blowdown.
With insurance companies cracking down, and more farmers seeking more efficient options, the use of compressed air for blowing down headers has increased dramatically over the last decade. Brüder Australia is launching a short series of articles addressing common misconceptions and to assist farmers with finding the right compressor for their needs…
How many cfm do I need? Common question, some say 90, others say 400. The answer to this seems to depend on who you ask. From our experience, the balance of performance versus cost effectiveness and convenience is around the 140cfm mark.
The reason is not so much to do with whether it will blow down your header properly, but more the mechanical side. A portable diesel compressor requires a certain pressure level to be maintained in the air/oil separator to prevent the oil from entering the air lines.
This is not such an issue with diesel compressors in the civil market where they are often used with a flow restricting tool (e.g. jack hammer) which provides back pressure. This is however, an issue in the agriculture market where the machines are being used at 'free-flow' and wide open throttle for a large percentage of their working life.
While this isn't going to kill your compressor on the first day, it does affect the performance and, along with the inconvenience of oil in your air lines, can create more significant longer term issues. This sweet spot of back pressure isn't an exact science, it varies from machine to machine and can even be affected by what oils are used and the ambient temperatures the compressor is being used in. However, as a rule of thumb, a 140cfm+ machine works well when used with 3/4″ hose.
Ideally we are looking for a pressure of above 4.0 bar when the valves are fully open. If your compressor cannot achieve this, a suggestion is to install a high flow regulator, or slightly close a ball valve until the pressure is maintained at this minimum level. This will provide the needed pressure for the separator to work.
The Brüder AG176 is our most popular machinery blowdown compressor which has a free air delivery of 176cfm, at which the pressure is normally maintained above 4 – 4.5 bar.
