Running Like the Wind
By Shawn Conley
Choosing the right fans for your building
By Shawn Conley
There are many input costs that must be considered in the management of a poultry building, and one of the biggest factors affecting input cost is fan configuration. The obvious cost lies with the equipment itself; however, the electricity used to run the fans will, over time, cost much more than the initial setup cost because bird performance is directly influenced by air quality, temperature and wind speed. Whether you have an existing building, or are considering building something new, it’s very important to get the fan selection right and configure them in the barn for ideal performance. Spending an extra couple of thousand dollars up front can make you tens of thousands or hundreds of dollars down the road.
There are two parts to the fan sizing equation: minimum ventilation fans, and cooling fans (which could be for cross or tunnel ventilation). This is really where electricity cost is going to factor into the decision. There are some general rules of thumb for establishing the numbers and sizes of fans that should be used in different configurations. Our minimum ventilation fans should handle about one CFM (cubic feet per minute) per square foot of floor space, and will be run by time, not variable speed, for two reasons. Variable speed fans are extremely inefficient, and variable speed almost always results in air dropping from inlets directly to the floor. Once we have the correct minimum ventilation fans, the remaining fans should all be 48” or larger to maximize efficiency and minimize the number of openings in the wall that would allow air leaks. We will need to be in the vicinity of about 10 CFM per square foot for cross ventilation, and the numbers for tunnel will be based on the desired air speed, which should range anywhere from 500 – 800 feet per minute depending on bird density and farm location. This would mean as much as 12-14 CFM per square foot.
The larger the fan, the more efficient it is. CFM / Watt is the best measure of a fan’s usage of energy. A 12” fan can be as inefficient as 6 CFM / Watt, a 24” is around 13, 36” around 16-17 CFM / Watt, and fans over 48” can range from 20 all the way up to 27 CFM / Watt. So, what does this translate to in electricity cost? Assuming a cost of $0.20 / Kw hr, and maintaining a constant 100,000 CFM of exhaust for 24 hours straight, it would cost nearly $60 to run 12” fans, and as little as $15 to run fans larger than 48”. Running 24 to 36” fans can range from $30-35. The clear conclusion we can draw here is that it is significantly less expensive to run larger fans, 75 per cent or more in some cases. If we could get away with running only 48” plus, that would be great, but in most buildings (with the exception of large turkey finishers) we need some smaller fans.
Minimum Ventilation Fans
To accomplish the goal of 1 CFM per square foot, we need to know that a 12” fan only produces about 1250 CFM at 0.10”wc, 18” is about 3500, 24” is about 6500, and a 36” is close to 10,000 CFM. These days, almost all fans are sold with cones on the outside, so about 10 per cent more CFM can be attained, although I tend to dismiss this number because fans are rarely perfectly clean and maintained. Larger fans can be from 20,000 – 25,000 CFM at 0.10”wc. It is extremely important to look at the ratings provided by manufacturers and prorate them to actual static pressures in a barn. Many manufacturers will provide fan capacities at 0.05, when actual numbers are 0.08 – 0.12 or more. This can translate to a 15 per cent or greater reduction in capacity that needs to be calculated when deciding how many fans to install.
The take home message on minimum fans is that we need 1 CFM / square foot, and that we want to utilize the biggest fans possible while allowing flexibility to run as little as 50 CFM per 1,000 birds if heaters are exhausted outside, and 75 if they exhaust indoors. We don’t want to run a timer fan less than 20 seconds out of five minutes. For example, for easy calculation, if we take a 50 x 500’ barn (25,000 square feet), and place 25,000 birds in the front half of the barn, keeping in mind that air exchange and some heat is needed in the non brood area, we will need about 25,000 CFM for total minimum ventilation, and as little as 1,875 CFM the first day. The best configuration here is four 24” fans, totalling 26,000 CFM in capacity, but capable of running a pair of fans at 43 seconds out of five minutes. A second, but less symmetrical configuration would be three 36” fans at 30,000 CFM total capacity, and running a single fan at 56 seconds out of five minutes. This is a little less desirable for even distribution of air, but it does maximize efficiency, saving about $6.40 per day when starting birds.
Larger Exhaust Fans
Our larger fans are, in a way, more simple. We want to use the largest fans we can fit in the wall, and make sure we reach our target for air movement. The most common sizes being used in new construction are now 54” fans that will move about 25,000 CFM of air. Using our previous example, in a cross ventilated barn, a 10 CFM per square foot target would require 250,000 CFM in total. Ten fans evenly spaced throughout the sidewalls, five per side, would adequately exchange the air. In a tunnel configuration with a target air speed of 600 feet per minute, we need to calculate the cross sectional area of the end wall and multiply by the air speed to get the CFM requirement. Assuming a 10’ average ceiling height, 50’ x 10’ x 600’ / min = 300,000 CFM, so we need 12 of our 54” fans at the exhaust end to pull the air. There is an option in this case to go to a 50,000 CFM, 72” exhaust fan, requiring only 6 large fans, although it may be beneficial to substitute two 54” fans for one 72” fan to more smoothly transition to the remaining five 72” fans from the four 24” minimum fans. It is not ideal for fan life to run the 72” fans for the transitional ventilation, which should run up to about 3 – 5 CFM per square foot though the sidewall vents.
Take Home Message
Configuring fans is one of the most effective ways to increase profits by reducing long-term input costs. Spending a few extra dollars per month on financing to get the most efficient fans pays off many times over when considering running cost vs. initial setup cost. Take the time to get it right, and invest in the future profitability of your business.