Chapter 7 - Combined Mechanical and Natural Ventilation Systems

Most curtain-sided poultry houses employ both mechanical and natural ventilation systems. These are entirely separate systems that operate independently at different times in the grow-out cycle. The system uses depends on the priorities for the season and situation. Generally, mechanical ventilation is used for cold weather when young birds are involved. Natural ventilation is used for mild and warm weather. Tunnel ventilation is used for hot weather with evaporative cooling being added for extreme hot weather.

The decisions will depend on the temperature extremes in the area and the size of broiler grown. Although they are used mostly for broiler production and the grow-out phase of turkey production, combined or hybrid systems may also be used for breeding flocks. At certain times, fine control of the minimum ventilation rate is needed, and it is best achieved using mechanical ventilation. At other times, temperature control is not as critical, and ventilation can be provided more cost-efficiently by using natural ventilation.

The two ventilation systems, mechanical and natural, differ mainly in the level of control they give a grower over house temperature, air distribution, and air quality. Natural ventilation offers a relatively low level of control over air exchange and distribution and is therefore best suited to times when outside conditions are close to the conditions desired inside. During cold weather, when there is a need to carefully control air exchange rate to maximize fuel use efficiency, proper air distribution, and warmth at bird level, then mechanical ventilation is desirable.

The advantages of the combined system are a combination of the strengths of the individual systems. They are:

• When using supplemental heating, the ventilation exchange rate is controlled using mechanical ventilation, which conserves heating fuel.
• Natural ventilation can be used during warm weather and during periods when birds are mature, thereby saving electrical energy for fans while still providing a high level of air quality.
• Tunnel ventilation and/or evaporative cooling can be used during hot weather to enhance cooling.
• One facility is flexible enough to handle birds for the entire grow-out period, no matter what season of the year, and to meet their changing needs.
• During power outages, curtain drops may be used to prevent excessive mortality during warm or hot weather.

The disadvantages of the combined systems are:

• More equipment may be required for each building because it must operate as both a mechanical and a natural ventilation system. Therefore, some specialized equipment may not be fully utilized.
• Some cold weather heating inefficiencies are built into the system due to the use of sidewall curtains, which allow infiltration and have a lower R-value than an insulated wall.
• A sophisticated control system is needed to effectively make transitions between mechanical and natural systems.
• Poor air quality and potential heat stress problems may occur during transition periods when fans are not running and sidewall curtains are closing or opening

In combined ventilation systems, two sets of sidewall inlets are usually needed: adjustable baffle inlets for mechanical ventilation and sidewall curtains for natural ventilation. A third inlet configuration will be needed if an evaporative cooling or tunnel ventilationfunction is included in a hybrid system.

The three options for mechanical ventilation inlets for cold weather use are adjustable baffle inlets, a curtain crack, and a fixed board crack. These inlets are similar in that they bring air uniformly into the house. They differ considerably in their ability to control the direction of airflow and the mixing of cold entering air with warmer inside air.

The most desirable air distribution is provided by the adjustable baffle inlet, which provides more uniform air mixing throughout the house by directing air along the ceiling. This mixes the cold incoming air with the warm air that has stratified near the ceiling, and the mixed air is then circulated back down toward the bird for improved fresh air distribution and efficient heat use.

In contrast with the air distribution provided by the adjustable baffle inlet, a curtain cracked open less than an inch will direct air down the floor, chilling the birds, and triggering furnaces or brooders to turn on. The result is uneven air distribution, wasted fuel, and poor air distribution. For these reasons, curtain crack inlets are not recommended.

Fixed board openings are a better option than a curtain cracked open. Fixed board opening design provides intermittent inlet holes along the top overlap board of the curtain. These openings direct air straight into the house while providing more uniform air distribution throughout the long length of curtain. Notched boards can be positioned to allow exposure of increasing inlet opening as the curtain drops or intermittently leaving a board off can provide the desired configuration of openings.

As the curtain drops, these spaces will act as inlets while the rest of the curtain remains closed. Although fixed board openings are a simple alternative to the mechanical complexity of an adjustable baffle inlet, they do not function quite as well. Although they will not provide the air distribution and direction of adjustable baffle inlets, fixed board openings are sometimes used effectively in mild climates and are preferred over curtain cracks.

Hot weather combined systems incorporate cooling strategies into the ventilation system in order to reduce heat stress effects on broilers. Such systems are natural ventilation during cold and mild weather until the temperature reaches a setting at which the curtains are closed and mechanical ventilation takes over for enhanced cooling. The most common types of hot weather combined system use tunnel ventilation to supply airflow for effective evaporative cooling.

Tunnel ventilation is designed to provide high-velocity air to cool broilers (i.e., the windchill effect). To accomplish this, the natural ventilation sidewall curtains must be closed while the inlet for the tunnel ventilation system is opened at one end of the house. Fans on the opposite end provide the air exchange needed to create the tunnel airflow. The difficult portion of the transformation is the coordination of the closing of sidewall curtains, the opening of tunnel inlet curtains, and the switching on of fans. If the fans are running before the sidewall curtain is fully up, the static pressure created will suck the curtains into the sidewall structure resulting in slow or halted curtain movement during the transformation. A poorly coordinate transition can thus result in torn curtains or broken winching equipment, as well as incomplete transformation to tunnel ventilation system. Evaporative cooling systems are often used in conjunction with tunnel ventilation system.

Generally it is best to mechanical ventilate when it is 15°F cooler outside than the desired indoor temperature. With smaller birds, which have less sensible heat loss and are more susceptible to drafts, a 10°F differential is a better criterion.

One disadvantage often encountered in naturally ventilated buildings is the low level of insulating value provided by the sidewall curtains, in addition to the infiltration leaks commonly caused by careless curtain installation and maintenance. Mechanical ventilation recommendations are based on maintenance of certain static pressure differences for proper function. House tightness affects the ability of the ventilation system fans to create the desired static pressure difference. Efficient mechanic ventilation requires a tight house.

A simple way to determine the level of house tightness requires a manometer or static pressure gauge. This should already be part of the mechanical ventilation controls. If the house lacks a static pressure gauge, use a manometer. Close up the house entirely by closing all sidewall curtains and mechanical ventilation inlets (in addition to all doors and windows, of course). Turn on two 36-inch exhaust fans or one 48-inch fan. These are the timer fans used for coldest weather ventilation with nominal capacity of 20,000 CFM. A house with no leakage should have a static pressure difference of about 0.2 inches. A static pressure difference between 0.09 - 0.10 inches is considered excellent; 0.06 - 0.08 inches is good; and 0.01 – 0.02 inches is poor.

To improve house tightness, small openings in the structure need to be sealed. Smoke bombs can help visually identify where air is entering the house during the tightness test. Most cracks can be eliminated at minimal cost. Inspect the following:

• Curtains
  • Tighten straps to hold curtain tight against the side of the house.
  • Provide a minimum of 6-inch curtain overlap with sides and top.
  • Keep bedding out of the floor area below the curtain so it closes completely.
  • Repair holes with curtain tape.
• Fans
  • Remove dirt so that shutters close completely.
  • Insulate or plastic wrap the inside of any fans not used during winter.
  • Caulk gaps around fan housing.
• House
  • Caulk and fix leaky side and end wall doors.
  • Cover holes in side and end walls.
  • Fix holes in ceiling vapor barrier and/or insulation.