Chapter 9 - Fogging and Misting Systems

There are a few things to keep in mind when designing a fogging system for a poultry house to ensure maximum evaporation of water. These include droplet size, nozzle placement, and fogging system flexibility.

It is important that the water droplets produced by the fogging system be kept aloft as long as possible to ensure maximum air temperature reduction in a poultry house. The longer a droplet floats around a house the greater the amount of water that evaporates off the droplet. If the droplet stays suspended long enough, it will totally evaporate before coming in contact with the floor. Once a droplet hits the floor, very little additional reduction in house temperature will occur.

One of the keys in keeping droplets aloft is to make them as small as possible. The smaller the water droplet, the more it is suspended by air movement. Generally speaking, at a constant pressure, the lower the nozzle flow rate, the finer the mist produced. The magnitude of the differences varies with manufacturer. The 2 gal/hr nozzles of some manufacturers put out droplets nearly 30% larger than their 1 gal/hr nozzles.

Another factor that affects droplet size is water pressure. With most nozzles the greater the water pressure the finer the mist produced. At 40 psi the typical misting nozzle producers a 72 micron droplet. To give you an idea of how small a droplet we’re talking about, the period at the end of this sentence is 500 microns in diameter. At 200 psi the droplet size is decreased to 32 microns. At either pressure, we’re talking about a very small droplet; however, there is a substantial difference between the two. The smaller droplet stays aloft much longer than the larger droplet and completely evaporates more than twice as fast as the larger droplet. Theoretically there are advantages to increasing water pressure above 200 psi, but the cost of specialized fittings, pipe, and pumps usually limits most fogging systems to 200 psi and below.

The other factor that determines how long a droplet will stay aloft is air movement. Without air movement a droplet emitted from a nozzle will stay suspended only a few seconds before hitting the ground. This suspension time can be increased dramatically by air currents by circulating fans. In addition, the circulation fans help mix the droplets with all the air in the house, not just the air in the immediate vicinity of the misting nozzles. It is important to note that the larger the water droplet the more crucial it is to have good air movement to keep the droplets suspended.

To get maximum suspension time it is best to deposit the droplets into the top of an air stream. This would mean that the droplets would have to pass through the air stream before they could reach the floor. For instance, let’s say a naturally-ventilated house had a row of 36” circulation fans 50 feet apart blowing down the center line of the house; the fans would produce a significant amount of air movement within seven feet of either side of the fans. To get maximum suspension time, nozzles should be installed near the ceiling no further than seven feet from the center line of the house. Nozzles placed outside this area would be more likely to cause floor wetting. If two rows of paired fans were used, the nozzles could be placed in a wider area.

Care must be taken not to place nozzles directly in front of or behind a circulation fan. Nozzles should be placed at least 15 feet away from any circulation fan because the air velocity leaving a fan can disturb the cone of water droplets emitted from a nozzle, leading to the formation of large water drops that quickly fall to the floor. Nozzles placed too close to the intake side of the fan can also lead to the wetting of the fan, increasing the collection of dust and feathers on screens and fan blades and increasing the chance of an electrical short.

In mechanically-ventilated houses using exhaust fans and eave air inlets, fogging nozzles should be placed near the wall inlets so that the air is cooled as soon as it enters the building. Furthermore, placement of the nozzles near the sidewall will ensure that the fog is introduced into an area of high air movement, aiding evaporation and minimizing house wetting.

In order to maximize cooling and minimizing floor wetting, the amount of moisture added to the air should be matched with the relative humidity in the house. The lower the relative humidity, the greater the amount of water that can be added and the more cooling that can be produced. On very humid days, only a limited amount of water can be added. Ideally, a system would monitor house temperature as well as relative humidity and then turn on enough fogging nozzles to cool the air, but not so many as to wet the house. Though this is not practical in many instances, it is possible to design a system that has some flexibility.

A two- or three-staging fogging system can offer a wide degree of flexibility at minimal cost. For instance, two lines of 1 gal/hr nozzles on 20’ centers could produce the initial cooling at 83°F. On humid days, the grower could have the flexibility to turn on just this set of nozzles to avoid floor wetting. On warmer days, above 85°F, if the humidity allows for more cooling, two additional lines of 1 gal/hr nozzles, situated near the other two lines, could also be turned on. It is even possible to employ a third system to be used during the hottest days of a summertime grow-out.

Though fogging lines can run lengthwise in a naturally ventilated house, they are easier to manage if they are installed in multiple lines running from sidewall to sidewall. For instance, if the circulation fans are located 40 feet apart down the center line of a house, a row of fogging nozzles could be placed 15 feet downwind of each fan. The first row of nozzles would be viewed as stage one. A second row of nozzles would be placed 10 feet from the first and used as stage two.

Fogging pads

Fogging-pad cooling systems have become popular for tunnel-ventilated broiler houses, since they keep the house interior drier than traditional fogging systems; are easy to manage; and are relative relatively inexpensive. Fogging nozzles are used instead of water recirculating system to wet the pads. Some disadvantages of fogging-pad systems are that fogging is subject to wind, which can reduce the wetting effect, and that water dripping out of the pads is not re-used. Advantages include their lower cost and cooling effects of up to 10-13°F. A gutter should convey water dripping out of the pads away from the building wall and causing structural damage. Fogging pad system development is continuing. System manufacturers and dealers will have the latest information on pad specifications, nozzle types, and placement, etc.

One successful method of installing fogging pad systems is to place the pad 8-10 inches away from the sidewall. The sidewall curtain is positioned between the pad and sidewall. Advantages of installing fogging pads 8-10 inches off the sidewall include the following:

• Pad life is increased since the curtain is not dragged across the pad surface as it is opened or closed.
• Rodents are discouraged from nesting in the pads in winter since the pads are outside and cold.
• The gap between the pads and sidewall reduces the amount of water entering the house.
• The sidewall curtain can protect the pad from dust and mechanical damage from debris originating from inside the house.
• The gap enables placement of a 5-foot pad on a 4-foot curtain opening or 6-foot pad on a 5-foot curtain opening. This increases the air speed through the smaller curtain opening, which reduces dead-air space at the inlet end of the house.
• When the pad and curtain openings are the same size, the gap allows the curtain opening to be closed 8-12 inches, which increases air speed and reduces dead-air zones.