When it comes to big fans and their benefits for agricultural ventilation, the old logic that bigger is better has hit a snag. Researchers at BESS Lab operated by the University of Illinois at Urbana-Champaign have tested a number of 60-inch fans only to discover that, at a certain point, energy efficiency does not always continue to increase.
Morgan Hayes, Ph.D., P.E., formerly at the University of Illinois, and now at the University of Kentucky as a Livestock Systems Extension Specialist, believes the problem may lie in the dead air space of a fan’s standard design.
“A fan pushes most effectively at the end of the propeller blade,” she explained. “That’s where it can push the most air, on the outside of those blades. The companies that design fans constantly work on the propeller geometries and dimensions to get the most air through a fan. I think with these very large fans the center portion, the sort of dead area of that fan, grows as well and they haven’t successfully been able to move through that dead area to get the efficiency as high as they would like it to be.”
That’s not to say that fan companies won’t find a solution to the problem. They are working on it. But in the meantime, you might want to consider the alternatives.
“The bigger-is-better philosophy is somewhat true, but sometimes you’d be better to have a few more smaller fans depending on the set-up of your barn,” Hayes said.
Bottom line, the 48- and 54-inch size fans, prevalent for tunnel ventilation systems, is still optimum size for efficiency.
“The assumption was always that going from an 18 to a 24 inch, or a 36 to a 48 inch fan, you’re going to move more air and you’re going to do it more efficiently. The trick is, when you get to 60 inches, you can move more air but you can’t do it more efficiently,” Hayes explained.
The industry most affected by the 60-inch fan conundrum is the dairy industry. “The dairy industry has moved the most aggressively to the 60-inch fans, probably because they want air speed with big animals,” Hayes said. “They think: big animals, big fans. The goal is always to move more air through the same size opening by improving that design on the propellers, and to do it with less energy. Energy efficiency has become very important in the last couple years.”
Just remember that, as a general rule, bigger fans are more efficient than their smaller counterparts until you hit that 60-inch mark when efficiency flat-lines.
Not all fans that fit in a given opening perform equally. To determine the most efficient fans available, the BESS Lab is a good place to keep an eye on testing results. The lab was developed in the 1990s in response to the need for testing agricultural fans. Previously, testing was available at a facility that was geared primarily towards commercial and industrial fans. Fan companies that were focused on the ag industry felt a need for a more ag-focused option.
Hayes explained that the lab is “basically a large wind tunnel where we can precisely control air speeds and pressures to determine how well the fan performs. … It’s designed to give you a standardized way to evaluate your fans and to confirm the performance values the company is suggesting a fan can do.”
BESS Lab does not test every fan available, it relies on the fan companies to request their testing services, but because the testing is highly regarded in the field, there are a number of companies that seek the testing to help validate their product claims. Tests conform to official protocol set by the AMCA (Air Movement and Control Association International, Inc.)
Always reference performance data rather than assuming bigger is better. “The data of BESS Labs should give you a best-case scenario of how that fan will perform in the field,” Hayes said, noting that the results should be considered only a guide. “Realistically, your [fan] performance won’t be as good because dust and dirt will build up, but at least it will give you a baseline about how the fans can perform.”
Results are available online. Find them at: http://bess.illinois.edu. Other performance test results can be found there for circulating fans and poultry house light traps. RB