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0807USAtunnell1
Thermal image of curtain-ventilated house with outside temperature in the mid 90s.
on June 24, 2009

Reducing electricity usage in tunnel houses

There are practical things growers can do to minimize their electric bills while keeping their flocks comfortable in hot weather.

Tunnel ventilation has proven to be one of the most effective methods of keeping birds cool during hot weather. With air exchange rates of less than sixty seconds, air speeds of 500 feet per minute or greater and evaporative cooling pads capable of decreasing the incoming air temperature 10 F to 20 F, tunnel ventilation can produce effective temperatures in the seventies when outside temperatures are well into the nineties. Whereas 20 years ago significant bird losses were not uncommon during a heat wave, today, barring some type of mechanical failure, tunnel ventilation has made heat stress related losses a fairly rare occurrence.

The tremendous cooling capacity of a tunnel ventilated house does come at a cost. A tunnel ventilated house requires more fan power than is typically used in curtain-ventilated houses. Though this tends to result in higher operating costs during hot weather, there are ways of keeping electricity usage to a minimum without sacrificing bird cooling. Minimizing power usage in a tunnel-ventilated house during hot weather basically comes down to three things: proper ventilation equipment selection, house and equipment maintenance, and proper ventilation system and bird management.

Tunnel fan selection

Probably the most effective way to keep summertime electricity bills to a minimum is to install fans that are very energy efficient. It is important to keep in mind that the most energy efficient fan is not necessarily the fan that uses the least amount of power, but rather the fan that moves the most amount of air with the least amount of power.

Selecting a fan based on fan motor size won't necessarily result in reduced electricity usage. How much power a fan motor uses depends on how much work it is asked to do. If you have a 48 inch fan with a 1 horsepower motor and replace it with a 1.5 horsepower motor, the fan's power usage will be essentially the same as it was with the smaller motor, provided the same motor pulley is used, because the amount of work required by the motor to spin the fan blades has not changed. But, increase the size of the motor pulley, which increases fan speed and work, then the power required will increase substantially whether the fan has a 1 or 1.5 horsepower motor. Power usage is not determined by motor size but by the amount of work required of the motor.

Just because a fan uses more power doesn't mean that it is less energy efficient. If two fans move the same amount of air and one uses 20 percent more power than the other it is obviously less energy efficient. But, if a fan uses 20 percent more power and moves 40 percent more air than another fan it is actually more energy efficient. This is why the amount of air that a fan can move with each watt of power, cubic feet per minute per watt of power (cfm/watt), should be used when comparing fans for energy efficiency. The higher the cfm/watt rating of a fan the less it will cost to operate. A fan's energy efficiency rating can only be obtained from independent fan test data such as that provided by The University of Illinois BESS Laboratory (www.bess.uiuc.edu).

Another common misconception is that if a fan moves more air, then you will need fewer of them, and therefore will have lower operating costs. This way of thinking has led to significantly increased operating costs on a large number of broiler farms. For instance, the standard version of a very popular 52 inch galvanized fan with a butterfly shutter moves 24,500 cfm at 0.10 inches static pressure and has an energy efficiency rating of 20.8 cfm/watt. The high capacity version of the same fan model has a slightly different blade angle which increases the air moving capacity of the fan to 27,300 cfm at 0.10 inches static pressure (a 10 percent increase). Though fewer fans would be required to tunnel ventilate a house, the downside is that the energy efficiency rating of the high capacity version of the fan drops to 18.0 cfm/watt.

In order to keep energy costs to a minimum during hot weather growers should only purchase fans that have an energy efficiency rating of at least a 20 cfm/watt or better at 0.10 inches of static pressure. A tunnel fan can easily use more than $8,000 in electricity over its lifetime. Spending a hundred dollars on a more energy efficient fan that could reduce electricity usage by 20 percent or more is a very good investment.

Tunnel fan operation

Just because a house may have ten tunnel fans doesn't mean you have to use all of them during hot weather. Keep in mind that for the most part all the tunnel fans are only required during the last two to three weeks of the grow-out when birds are at their greatest size. Younger birds that are producing less heat, are poorly feathered, and more spread out in the house require significantly less fan capacity to keep them comfortable during even the hottest of weather. Though using all the tunnel fans in a house on younger birds may not be harmful during hot weather, it will lead to significantly higher electricity usage.

As a general rule producers should increase the number of tunnel fans used as bird age increases. For instance, many producers have found that by only using a maximum of 40 to 50 percent of their tunnel fan capacity (200-400 feet per minute) during the first third of a flock can keep their birds comfortable. During the middle third of the flock this often needs to increase to approximately 75 percent (400-500 feet per minute). During the last third of the flock all the tunnel fans are typically required to keep the birds comfortable during hot weather (500 feet per minute plus).

Tunnel fan and pad maintenance

Dirty fan shutters and screens can reduce the air moving capacity of exhaust fans from between 10 to 30 percent. Since each fan moves 10 to 30 percent less air, 10 to 30 percent more fans may have to be used during hot weather to provide the necessary level of air exchange and velocity to keep birds comfortable.

Evaporative cooling pads are essentially a wet air filter that the tunnel fans pull air through to decrease incoming air temperature. If the pads become clogged with dust, feathers, algae, and/or minerals it makes it harder for the fans to pull fresh air into the house not only reducing the air moving capacity of the fans but increasing the power usage as well. The end result is that a producer may be using ten fans, using the power of ten, to move the air that eight fans could have done had the pads been kept clean.

Bird distribution

One of the keys to keeping birds cool during hot weather is to keep them spread out as evenly as possible throughout the house. Space between the birds reduces the air speed required to remove heat from the birds. When 20 to 30 percent more birds end up on the tunnel inlet end of a house because migration fences were not properly installed, more fans will have to be used to keep these crowded birds comfortable. Keeping birds evenly distributed in the house saves electricity and improves bird performance.

House tightness

To maximize bird cooling and keep electricity usage to a minimum it is very important that a house is tight so all the air brought into the house by the tunnel fans enters through the house's evaporative cooling pads. The looser a house, the greater the temperature difference between the two ends. This is because as the air travels from the pad end to fan end of the house it will heat up from the birds and from hot air entering through cracks/holes in the side walls and ceiling. The increased heat will cause the air at the tunnel fan end of the house to become significantly warmer causing additional tunnel fans to operate. A loose house will result in a higher electric bill and can reduce bird performance as well.

Totally enclosed houses

Though the more important benefit of totally enclosing a house is the reduced fuel usage during cold weather, there are significant summertime benefits, one of which is reduced electricity usage. The side wall curtains on a tunnel-ventilated house are a significant source of heat gain and hot air leakage during hot weather. When a house is totally enclosed these sources of heat are eliminated resulting in less temperature rise between the pad and fan end of the house. A smooth side wall, often made by totally enclosing a house, will increase air velocities near the side wall 20 percent or more resulting in significantly increased cooling for birds near the side wall.

Tunnel ventilation has proven to be an effective method of keeping birds cool during hot weather. If you are considering building a new house, retrofitting an older house to tunnel ventilation or simply adding or replacing tunnel fans, make sure you consider purchasing only fans that have an energy efficiency rating of at least 20 cfm/watt at 0.10 inch static pressure or better. If you have a tunnel house make sure that you keep your fans and pads properly maintained, your house tight, your birds spread evenly throughout the house and set up a program to limit the number of tunnel fans that can operate with younger birds. Following these steps will not only keep electricity usage to a minimum during the hot summer months but could easily improve bird performance as well.

0807USAtunnel2
Thermal image of a tunnel-ventilated house on the same farm taken at the same time as the image of the curtain-ventilated house.
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