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Roof Exhausters Key Element In Tank Aeration

Exhausters prevent condensation build up on roof

Powered roof exhauster fans are a key element to a storage tank aeration system. Mounted on the top of the tank, these relatively low-horsepower fans pull air that has been pushed up through the grain mass by the big fans at the bottom out of the tank. In the process, they can help prevent the formation of condensation at the top of the tank that can lead to spoilage.

Grain Journal spoke recently with grain elevator operators about the importance of roof exhausters.

Dave McIntosh | Campus Manager | Heritage Cooperative | Marysville, OH

“I started in the industry in 2001 after working on farms. Whether it’s for commercial operations or smaller farm bins, I have always liked roof exhausters. I believe in moving air up through the grain. Roof exhausters also help with taking condensation off the inside of the roof.

“Keeping the fan screens clean is probably the biggest issue with roof exhausters. It’s not something that has to be done every year but definitely something that needs to be kept in mind over the long haul. Getting to the fans is probably the big issue, but the newer bins have a ring close enough that makes maintenance a bit easier. You are still required to use fall protection and having someone who is comfortable with heights out there with you.”

Derek Patterson | Manager of Grain Operations | CMI Terminal IV | Naicam, SK

“We have a number of 3,000-tonne (110,000-bushel) steel tanks with aeration, and these also have roof exhausters on them. We’re forcing all of this air up through the grain mass, and it has to get out somehow. I suppose without roof exhausters, the air would blow the roof off the place.

“Our roof exhausters are basically maintenance-free. Just keep them clear of obstruction. They’re 100 feet up in the air, so they would be hard to reach.

“The only problem we’ve ever had with our roof exhausters is that sometimes in the winter, they’ll ice up. Then the ice melts and can drop down into the tank. Usually, though, as soon as the sun comes out, it will melt the ice. We haven’t had any problems with them in the summer.”

Ron Groskreutz | Grain Originator | Heartland Co-op Grain | West Des Moines, IA

“The number of roof exhausters used is determined by the size of the grain bin. The majority of our grain bins are 78 or 90 feet in diameter and about 96 feet tall at the eave. They typically hold about 400,000 bushels each.

“While I am sure there is a formula that can be employed to determine the optimum number of roof fans, we don’t use one. Typically, we just use three 2-hp fans on the 78-foot bins and four 2-hp fans on the 90-foot bins. Each 78-foot bin is already equipped with 12 gravity vents, and there are 13 top vents on the 90-foot bins. The horsepower, speed, and brand of the fans affect the amount of air you are pushing into the bin from the bottom, as well as how much air you need to get rid of at the top.

“The fans pretty much are maintenance-free. They use sealed bearings and have a good history of running for years without failure.

“I absolutely recommend roof exhausters, because you need to get good air in your bin, and you need the roof exhausters to get some of that air out of your bin. You can’t install enough gravity vents to move the number of cubic feet of air you’re pushing in with your bottom fans – that’s why you need roof exhausters.

“Our fans supply 1/6 to 1/7 cfm per bushel, which is fairly high, so we’re probably going to think about adding more roof exhausters.

“Roof exhausters diminish the condensation, which can be a problem when you’re pushing all that air through the grain mass, and it condenses on the peak that drips onto the grain. So the more air you get out of the top, the less sweating there is inside.”

Dan McBride | Vice President-Operations South | Central Valley Ag | York, NE

“There are two main reasons for having roof exhausters on your bins. The first is to prevent condensation forming on top of the grain mass, which can cause spoilage. The second is when you’re using a high level of aeration, you need the exhausters to pull all of that air through the bin; you can’t handle that volume of air with vents alone.

“The number of exhausters you need depends on several factors. These include the depth of the grain, the diameter of the tank, and the amount of air you are trying to move. On a 105-foot-diameter steel tank, you c might use anywhere from four to eight 2-hp roof exhausters.

“There isn’t a lot of maintenance that needs to be done or that you can do at all on roof exhausters. The only problem that happens with them is when a motor burns out and needs to be replaced.

“On a steel tank, access to the roof exhauster can be a real problem. They typically are located on the bottom 20% of the roof, so to reach them, you need a lift and a bucket.

“It’s a lot easier on a concrete tank, since you have a flat roof to walk around on.”

Lance Falk | Location Operations Manager | The Mennel Milling Company | West Point, VA

“I recently switched companies, but with my previous employer, we had roof exhausters on every one of our silos from when we built them starting in 1996 all the way up to the present. On the average bin, about 500,000 bushels or smaller, we had five roof exhausters on them. Some larger bins, about 1.5 million bushels, had up to 14.

“For my new company, I’m installing roof exhausters. I went from concrete silos to metal silos, and I switched environments, too. I was in northern Indiana, and now I’m in Virginia right on the Pamunkey River, and the humidity is insane. The first thing I noticed was we didn’t have very many roof exhausters. Now, we have 10 roof exhausters on a 750,000-bushel bin that we’re hooking up right now, actually. c

“As far as determining the number of exhausters to install, I recommend talking with the supplier. Their engineers have the experience and knowledge to help make that decision.

“There is some maintenance that needs to be done, of course. You need to check the fans periodically. They have a tendency to build up dust over time, which can get them out of balance. They’ll shed it usually, but not when it’s particularly humid. When it’s humid, the condensation builds up, and that will cause dust to stick more. So we’ll have to go up and clean them every once in a while. This isn’t something that you’re going to do on a regular basis, though.

“Another thing that helps is if you have good louvers over your fans, top and bottom; that helps prevent buildup. Other than that, make sure that you grease the bearing on your motor once a year.

“Overall, I’m a firm believer in roof exhausters. They help you move air through the whole grain column, all the way up from the bottom to the top of the silo, in a short amount of time. They also help maintain the headspace without having to run your bottom fan and prevent getting that sweat sprout at the top.”

Mason Cady | Location Leader | Ag Partners | Albert City, IA

“When determining how many roof exhausters to use, it’s all relative to the size of the bin. And to be honest, everything that we have was designed by somebody else, so we haven’t actually added any of them to our bins ourselves. We have a number of them on our bins, but I’d have to look at the facility layout to even know for sure how many we have on each one. It’s all been determined by whoever built the bin – it’s been designed by somebody who knows exactly what they’re doing, rather than us, so we rely on their formula to get the exact number we need.

“As far as maintenance goes, we check them every year to make sure that they’re in working order. We check that the outlet is clean and doesn’t have a bunch of dust buildup. We’ll clean that out if need be. But outside of that, there is not much maintenance needed.

“Roof exhausters are critical, especially when you’re running the bottom aeration fans, because you’re pushing moisture up through the grain mass, and if you can’t get that and the heat out the top, you’re going to get condensation on the top of the grain. You’re going to spoil the entire top part of your bin. Roof exhausters prevent that condensation and spoilage.

“Overall, we’re definitely satisfied with our roof exhausters.”

Reprinted from Grain Journal July/August 2018 Issue