Chapter 7 - Natural Ventilation Systems
Chapter 7 - Natural Ventilation Systems
Natural ventilation, as the name implies, is a system using natural forces to supply a building with fresh air. Air exchange is accomplished through designed inlets and outlets in a building. It is important to recognize that naturally and mechanically ventilated buildings operate under different principles. Mechanically ventilated buildings use fans to exchange air, which can be controlled to provide the desired air exchange rate. Thermal buoyancy and wind are both dependent on uncontrollable weather. This makes natural ventilation control different.
Natural ventilation is an attractive management technique because fan and fan maintenance expenses are eliminated. The roof design, the design of major openings for ventilation, building orientation, the occupants, and finally the outside water are all factors influencing the results of the ventilation process. Openings located along the sidewalls are termed ‘sidewall openings’, and the opening at the roof peak, or ridge, is called the ‘ridge opening.’ Variation exists in the shape of the interior roof itself due to slope and style and these in turn can influence the ‘chimney’ effect that develops in the building. The occupants (poultry in this case) also influence the performance of naturally ventilated buildings. Bird age and population density affect the response of a building to ventilation changes as well as fresh air distribution in the building. The simultaneous effects of all these components determine the success of naturally ventilated buildings.
Proper ventilation of naturally ventilated buildings requires that the sidewall and ridge openings work in harmony to deliver and distribute the required fresh air to the building. Both high and low openings are needed in naturally ventilated buildings. At least two openings on opposite sides or ends of the buildings are needed for air distribution within the structure and to avoid short-circuiting of airflow. Providing proper distribution of fresh air within the building is frequently neglected, and this can cause many problems. For example, if the building is managed in such a way that all the fresh air passes through the sidewall opening and is ‘short-circuited’ to the ridge opening, then the fresh air provides very little benefit to the birds. Another common mistake is to attempt to ventilate with only one opening, as through one sidewall curtain, which does little to promote internal air distribution. It is essential that naturally ventilated buildings adhere to sound fresh air distribution principles.
Comparing naturally to mechanically ventilated buildings:
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Advantages |
Disadvantages |
Several key advantages and disadvantages of naturally ventilated buildings compared to mechanically ventilated buildings are listed below. The initial high cost of fans along with the elimination of fan operating expenses makes natural ventilation an attractive option.
It is more difficult to consistently achieve the desired house environment with natural ventilation. Rapid changes in wind speed, wind direction, and outside temperature require that sidewall and ridge openings be constantly changed to ensure adequate fresh air exchange rates and proper fresh air distribution within the building. If an inlet controller cannot properly adjust openings in response to weather changes, then extreme fluctuations of inside temperature, humidity or ammonia level will occur. An intelligent inlet controller responds effectively to weather influences and can drastically reduce this disadvantage usually associated with naturally ventilated buildings.
Orientation and building ‘footprint’ dimensions have to be carefully planned for naturally ventilated buildings. It is extremely important to orient the building so that it is exposed to prevailing winds during the hottest part of the year. If a naturally ventilated building is improperly oriented relative to warm-weather winds, the building will be under ventilated, resulting in inside temperature and humidity levels outside the bird comfort zone.
Naturally ventilated buildings require more land space, which includes surrounding free space, in order to take advantage of warm weather winds. The building itself may be no larger than a mechanically ventilated house, but a requirement for unobstructed airflow near and around the building rules out close siting to other structures. Obstructions such as building, trees, and other large, wind-deflecting obstacles affect wind patterns and reduce wind energy available for ventilating a building. Precautions must be taken during planning and construction to adhere to both orientation and physical spacing guidelines.
Wind induced and buoyancy induced pressure forces
Naturally ventilated buildings deliver fresh air to the birds through two basic forces: wind induced and buoyancy induced pressure forces.
As illustrated in Figure 7.13, wind induced ventilation is the processes by which wind, acting on a building, will pressurize openings relative to the building’s inside and thereby induce fresh airflow through the building. Buoyancy-induced ventilation is oftentimes referred to as the ‘chimney-effect’ or ‘thermal buoyancy.’
As illustrated in Figure 7.14, the basic principle is that hot air rises. Openings positioned low and high in a structure are particularly important to this process. Housed poultry release a great deal of heat resulting in increased temperatures around the bird. Warmed air will rise, and with properly designed ridge openings, this rising air will escape from the building. As heated air escapes, fresh outside air will replace it through sidewall openings.
In cold weather conditions, when a naturally ventilated building’s openings are nearly closed, buoyancy-induced ventilation is often the primary mechanism of air exchange. With any wind acting on the structure, wind forces will quickly override buoyancy effects. In warm weather conditions with more sidewall and ridge area open on the building, wind-induced ventilation becomes the primary mechanism for ventilation. In regions where extreme heat and cold exist throughout the year, both wind and buoyancy-induced ventilation operate to deliver required fresh air to the birds.
Wind acting on an opening differences in pressure across the opening, which forces air to flow through it. The buoyancy effect is dependent on a temperature difference between warm inside and cooler outside ambient conditions. The height difference between inlet and outlet also contributes to airflow.
The relative importance of wind acting directly on an opening may be either an advantage or a disadvantage, depending on the time of year. During warm weather conditions wind becomes an advantage and techniques are used to take full advantage of the prevailing wind direction. During cold and mild weather conditions, relatively high wind speed can function as a serious disadvantage. During cold weather periods, the goal is to ventilate the building at minimum rates for controlling moisture and noxious gas levels. If sidewall openings are left too far open, do not seal tightly, or are torn, infiltration from wind can be substantial, resulting in uncontrolled drafty conditions and, potentially, excessive use of supplemental heat.
Wind also affects the rate of air flow through the ridge opening. Wind blowing over the ridge opening reduces pressure at the opening. This suction force draws air out of the ridge vent.
Ventilation requirements
The ventilation requirements for naturally ventilated buildings are expressed in terms of the fresh air exchange rate. The fresh air exchange rate defines the number of times the volume of air in the building is replaced. If we concern ourselves with the volume (sidewall height x building width x building length), then the ventilation system must replace this inside air with fresh air over a specified period of time.
The maximum ventilation rate requires a compromise in design. As outside temperature increases, required fresh air exchange rates for controlling temperature increases rapidly. In fact, without evaporative cooling, the inside temperature will always be higher than the outside temperature, a concept that is often misunderstood.
Naturally ventilated building design
During warm and hot weather periods, naturally ventilated buildings rely almost completely on wind to generate the required fresh air movement through the building. Building orientation is best determined using local wind patterns. To take advantage of warm weather winds, the building ridge axis should be perpendicular to the prevailing warm weather winds. In lieu of localized wind patterns, ‘wind roses’ can be used to position naturally ventilated buildings so as to take advantage of warm weather winds. Wind roses are the summaries of wind patterns and wind speeds for various weather stations across the U.S. Since winds generally shift between seasons of the year, it is important that patterns for summer winds be selected. The percent time of calm days is a very important parameter in relation to naturally ventilated buildings. Significant periods of calm days combined with warm temperatures result in inadequate fresh air entering the building and an unacceptable increase of inside temperature.
Sidewall and ridge openings
Any single opening in a naturally ventilated building, acting alone, will behave as both an inlet and an exhaust. If an opening should behave as an inlet, adequate openings must be provided to behave as exhaust points, and vice versa. The naturally ventilated building thus ‘breathes’ to maintain pressure forces at levels consistent with exterior pressure acting on the building.
A properly designed naturally ventilated building provides opening sizes that maximize airflow through the building during periods of extreme heat. To accommodate periods of cold weather, opening sizes need to be adjusted quickly and appropriately to control the quantity of outside air entering the building.
In general, the ridge opening needs to accommodate a low wind condition during periods where mild weather ventilation rates (about 15 air exchanges per hour). During warm and hot weather conditions, the usefulness of the ridge opening for ventilation is very limited because the buoyancy effect is low due to similar indoor and outdoor temperatures. Some have argued that the ridge opening is unnecessary for this reason. In cold weather, however, the ridge opening is often the primary mechanism for moving fresh air through the building. The ridge opening in cold weather can be thought of as the exhaust fan for moving air with inlets occurring along the sidewalls. Even during warm weather when little wind is present, the ridge performs an important function in allowing state warm air to rise up out of the building. The ridge also provides a high opening on which the wind can act. Because the wind blows faster higher off the ground, there is a benefit from enhanced wind suction ventilation at the ridge.
In contrast to sidewall openings, the ridge opening size is usually not adjustable. Generally, it is not necessary to control the ridge opening. Although the ridge opening usually remains open year-round without adjustment, some control is possible with internal panels or a pipe that partially covers the ridge opening. The ridge opening should never be completely sealed, and any ridge opening baffle should allow air exchange, even when the baffle is in its closed position.
The above discussion of the sizing of ridge opening assumed that the ridge opening was a completely clear, unobstructed opening. Often, however, barriers designed to prevent snow and rain from entering the building are designed into ridge vents. Such barriers also have the unwanted effect of reducing the effective opening size or airflow path. The influence of barriers on airflow capacity can be substantial. A ¾ inch screen or bird netting will not obstruct airflow through a ridge opening. However, substantial reductions in airflow result from diversions placed as a component of the ridge. The common practice of placing a ridge-cap, for example, has been shown to significantly reduce airflow capacity; often by 50%. Ridge caps must provide enough space under the cap to allow free air movement. A better approach than ridge caps, one that minimizes airflow obstruction, is to place a trough under the ridge opening. Slope the trough at ¼ inch per foot and provide a drainage discharge. When placing the trough, minimize any obstruction effects by making sure that the total opening between the interior roof line and the trough edge is at least twice the full ridge opening.
An undersized ridge opening can be improved by the use of upstandswhich accelerate wind speed at the ridge allowing for higher suction forces at the opening. For example, a 6-inch wide ridge opening with a 6-inch vertical upstand will have quadrupled the airflow capacity of a simple ridge opening of that size. Therefore, an undersized ridge opening can be improved by incorporating upstands. Some ridge vent baffles, used to control ridge opening size, also open to the outside in a way that provides a ‘built-in’ upstand; an added benefit rarely recognized.
Curtains
Sidewall opening size is commonly adjusted using moveable curtains. Cables and drop cords, along with an automated winch assembly, can provide the needed adjustment of curtain openings. Sidewall curtains open from the top down, so that with the smallest opening, cold, fresh air enters at about an 8-foot height. Suppliers often hem the material with a pocket at the top sized to accommodate a standard 1¼ inch outer diameter curtain rod or a 2¼ inch outer diameter rod. A bottom hem can be provided at an additional cost. Good pocket construction includes double stitching of a folded under hem to prevent fraying and tearing.
Flexible curtains come in a variety of types and insulating values. Ultraviolet (UV) light resistance of the material is essential for poultry house curtains as they will be exposed to sunlight. Thicker materials or higher thread counts are typically stronger and usually cost more. Often multiple layers of materials are laminated together into a single curtain for enhanced wind and water resistance. Single- and multiple- layer curtains woven from polyester or polyethylene are common. Vinyl curtains will often crack in cold conditions. Some fiberfill curtains are available, but these can be difficult to fold during periods where maximum sidewall opening is needed. Clear, white, blue, and black-out curtain colors provide a range of levels of light transmission. Some curtains have a reflective covering.
In ‘insulating curtains,’ multiple layers of fabric are sewn together in order to trap a small insulating air space between each layer. Such insulating curtains do not provide significant insulating value. For example, a single-layer curtain has an R-value of roughly R-1, and multilayer curtains provide very little additional insulating value at R-3 or R-4. One insulated curtain with seven layers of material has an R-3 rating, which can be compressed down to about 1/10th inch. Multilayer curtains can be beneficial though, especially in regions where cold weather infiltration is a problem. Multilayer curtains tend to reduce excessive cold air infiltration is a problem. Multiplayer curtains tend to reduce excessive cold air infiltration and ultimately uncontrolled, drafty exchange of cold outside air that necessitates supplemental heating. Insulating curtains also provide some protection from the surface condensation that is likely to occur on uninsulated curtains. The problems created by dripping water, which may pool and freeze near the bottom of the uninsulated curtain, must be compared to the advantages of more daylight transmission provided by uninsulated curtains. Air leakage can be a significant problem during cold weather when ice forms along the bottom of the curtain, pushing the bottom of the curtain away from the side of the house. For curtains which are permanently anchored at the bottom (not ‘double-hung’), install a wooden strip to prevent this air leakage and pooling of condensation. The strip should extend an inch or so above the bottom of the sidewall curtain opening to eliminate any pocket where shavings and water can accumulate.
One type of curtain material popular is poultry houses because of its low cost is a 4½ ounce clear or blue polyethylene fabric. This is among the lightest weight fabrics. Heavier 6 - 7½ ounce fabrics have an expected life of five to eight years. The lighter weight fabric is less strong, but with good care it should also last this long.
Curtains are susceptible to several types of damage and need to be replaced periodically. Their useful life is dependent on such factors as regional storm frequency or high wind exposure, fabric durability, and rodent populations surrounding the building. Some curtain material may rot and mildew from prolonged high humidity. Moisture from rain and damp barn air can corrode light metal or rot unprotected wooden components. Poultry houses often contain elevated ammonia levels, yet plated hardware is often sufficiently durable. Stainless steel hardware is used in highly corrosive environments, such as swine buildings. Replacement is usually needed, not because the curtain degrades, but because it has been torn by strong winds or heavy storms. Unsupported fabric will flap and tear sooner and unrepaired tears will make replacement necessary sooner. Tears can easily be repaired with tape chosen to match the fabric type and applied to both sides of the tear.
Components may be added to the curtain system to prolong curtain life. Double-hung curtains, those with rods at top and bottom, can be stored at the top of the sidewall opening space during prolonged hot weather. Such curtains can be fully lowered and disengaged from bottom rod fasteners. Top and bottom rods can then be bundled together with curtain material and the whole assembly raised and secured to the top of the opening for storage. Storing curtains at the top of the wall protects them from poultry, rodents, machinery, litter, weeds, water, and dirt. An additional protective measure is to unfurl curtains periodically during summer storage months to discharge water and dirt and to dislodge any nesting rodents and birds. Curbs and bump rails should be provided to keep tractors and equipment away from the curtain walls. Bottom brackets will hold the folded-down curtain and provide a rope attachment point to prevent billows. Curtain clips secure the drop cords to the top curtain rod without piercing the fabric.
Because winds blowing against curtain exert a lot of force, curtains that are allowed to flap around in the breeze can quickly be torn. To prevent such damage, curtains need to be supported and securely anchored both on the inside and outside. On the building interior, curtains can be given extra support form the wind effects by closely-spaced (4-foot) building studs, open mesh screening (bird netting, for example), or support ropes or straps. In poultry housing, bird netting is most often employed. Materials that are not UV-resistant will soon deteriorate.
Exterior curtain support prevents billowing. Curtain straps or support ropes are the most common choice in poultry housing. Anti-billow rope is laced through fasteners spaced every two feet, alternating form top to bottom of the curtain to form a continuous V-shaped support. Polypropylene rope is inexpensive and has UV inhibitors, which make it well-suited to this function. Curtain strapping is often polypropylene around 2-3 inches wide and is UV resistant. Strapping should be spaced no more than 5 feet apart down the length of the house, vertically over the curtain installation.
Steel cable is most often used to move curtains in sections that may be 100 – 200 feet long. Hand winches or automatic curtain machines are used to move the cable, which runs the entire length of the curtain section. Drop cords attached to the cable operate through a fixed pullet for raising and lowering the curtain. Pulleys are positioned every 4-6 feet along the curtain with drop cords of polyester rope. Polyester rope has a lower elongation factor than other common ropes for less stretch.
Winches, cables, and pulleys used to move the drop cords must be rugged. Consider how often you will be adjusting curtains when choosing operating methods and hardware. In the process of changing the direction of the cable, the pulley is also bending that cable. A curtain may be adjusted hundreds of times a day. On a five-minute timer, curtains or inlets will be opened 288 times per day and closed another 288 times per day. Sections of cable are being bent thousands of times per month. How much the pulley affects cable life is largely dependent on the pulley diameter. Larger pulleys bend the cable less; doubling the pulley diameter can increase cable life up to thirteen times. A second benefit of larger-diameter pulleys is a decreased likelihood that the cable will slide over the pulley surface, which leads to excessive wear and eventual breakage. It is important, however, that pulleys and cables be properly matched, since the type and size of the cable determine pulley diameter to minimize wear. Smaller-diameter cables require smaller-diameter pulleys. Whether the cable slides over the surface or turns the pulley is a function of surface area against which the cable acts on the pulley. Most steel cable breaks are due, not to insufficient cable strength, but to improper matching of cables topulleys. Manufacturers have found that many cable breaks are caused by insufficient cable-to-pulley surface contact.
Flexibility is an important determinant of how long a cable will last. The more flexible the cable, the less likely it will break. Steel strand cables common in curtain applications are 7 x 7 or 7 x 19. A 7 x 7 cable has seven bundles with seven wires per bundle. Seven x 19 cables are preferred; since they are usually more flexible and so do not need as large a pulley as 7 x 7 cables.
To further reduce cable wear, the pulley should have a smooth interior surface as well as bearings to minimize cable slippage over the pulley. Wear against the side of the pulley will dramatically reduce pulley life. Cables must also be properly aligned with the pulley groove, or they will not last long despite other precautions.
Controlling naturally ventilated buildings
Automatic controls are needed to maintain the indoor temperature and provide air exchange as weather changes hourly and seasonally. Natural ventilation system controllers are available to regulate air exchange, by adjusting inlet and outlet opening sizes. Controllersalso regulate the supplemental heating rate. Sold state controllersand computer systems capable of controlling the inlet and outlet opening and supplementalheaters are available. They can use both time and temperature to provide the desired ventilation strategy. Thermostatic control is typically used to turn on and off supplemental heaters as needed. Automatic curtain controllers are preferred for controlling the inlet openings in naturally ventilated buildings because they typically assure adequate air exchange though circulation fans. Manual control is discouraged to avoid rapid drops or rises in interior temperature and moisture content.
Automatic curtain controllers are set with thermostats to control inside temperature by adjusting the sidewall openings. When temperature falls outside the thermostat set point range, the curtain machine will start and the curtain will open or close a set distance, 3 inches for example, then stop and wait, for perhaps four minutes, before any further adjustments to curtain position are made. This four-minute cycle is necessary to allow the building environment and thermostat to react to any temperature to react to any temperature change resulting from the new curtain position.
The inlet and outlet openings are adjusted to control the air exchange rate. The inlets need to provide the minimum air exchange necessary for moisture control during cold weather when supplemental heat is needed. In mild and warm weather the inlets and outlets need to provide sufficient air exchange to maintain the desired inside temperature. Various devices can be used to adjust the opening size; pneumatic systems’ either manual or motorized cable and winch systems; and motorized mechanical arms. Typically the opening control units make small adjustments, either increasing or decreasing the sidewall and ridge opening size, on a frequent (every 10 minutes) basis to either increase or decrease the air exchange.
Insulation
A well-insulated building shell is needed to successfully naturally ventilate a poultry house. Insulation helps prevent condensation on the inside surfaces, reduce heat loss in cold weather, and reduce solar heat gain in warm weather. Thermal buoyancy is enhanced by reducing building heat loss through the building shell.
Condensation occurs when the building’s inside surface temperature dips below the indoor air’s dew-point temperature. Condensation is prevented by providing sufficient insulation to maintain inside surface temperatures above the dew-point temperature. Insulation also reduces building heat loss; however, only 20% of the total heat (i.e., building and air exchange) is lost through the walls, ceiling, and perimeter in most poultry facilities in cold weather. The majority of total heat is lost through the cold weather air-exchange needed to control moisture and maintain acceptable air quality. The insulated building shell also reduces solar heat gain in the summer; especially insulation located on the underside of the roof.
The amount of insulation, as measured by the resistance or ‘R’ value, should be in the mid-teens for walls and the mid-twenties for ceilings and roofs. Higher ‘R’ values are sometimes used in facilities in cold climates like the northern U.S. and Canada. Since the primary function of insulation in poultry facilities is to prevent condensation, excessively large insulation values (R values greater than 25 in the walls and 40 in the ceiling) have limited benefit
Proper installation is critical for achieving a uniformly and completely insulated building. It is critical to protect insulation from the moisture produced in a poultry house with some type of vapor retarder(formerly called vapor barrier). Generally, the vapor retarder is a 4 or 6 mil thick polyethylene film that is placed on the warm side of the insulation. This prevents water vapor inside the barn from moving into the insulation and condensing in the insulation inside the cold wall. Polyethylene film or sheets should always be used even if the insulation has an attached vapor retarder (i.e., aluminum foil backing on fiberglass blankets). The large moisture load in a poultry facility can cause significant moisture problems with even very small breaks in cracks in the vapor retarder along studs, ceiling joists, and electrical outlets.
Protecting the insulation from rodents (mice and rats) is also very important. Rodent control is difficult in poultry housing facilities (see Chapter 13 for more information on pest management), but is necessary to safeguard the insulation. Crushed rock around the perimeter of a building to prevent rodents from burrowing under the walls and maintaining bait and trap systems throughout the farm to hold down rodent populations, are highly recommended for preventing insulation deterioration in walls and ceilings.
Building foundations are another important place to insulate. Perimeter insulation will keep concrete floors and inside wall surface temperatures warmer, making it more comfortable for animals during cold weather. Perimeter insulation also eliminates condensation and frost in these areas. Rigid board insulation is recommended with an R value betwee 6 and 8m extending 2 or 3 feet below ground level.
A common practice in cold climates is to completely close the opening on the sidewall that comes in contact with prevailing winter winds (usually the northern wall). This is not recommended since it will result in poor air quality in the windward side of the building. A better strategy would be to have a small opening on the windward side that is protected from direct wind exposure by a windshield.
Heaters
Supplemental heat is usually needed in naturally ventilated grower houses to maintain desired indoor temperatures during cold weather. Different types of heaters are used for supplemental heating in poultry houses including radiant, space, and make-up air heaters. Radiant heaters work well for improving bird comfort but do not heat the room air directly. Radiant heaters warm surfaces, which give up heat to warm the room air. Unit space heaters heat room air directly. Make-up heats heat incoming ventilation air.
Unvented heaters add both heat and the products of combustion into the building. The products of combustion include gases that can create health and safety problems within the building if gas concentrations accumulate. For this reason unvented heaters are often not recommended. Proper maintenance and burner adjustment is critical for effective heater operation and minimizing the amount of undesirable combustion products like carbon monoxide from being produced. If unvented heaters are used, increase the cold weather ventilating rate by 2.5 CFM/1,000 BTU/hr of heater capacity because of the moisture and products of combustion added to the building. For a 100,000 BTU/hr (typical) heater this would mean an increase of 250 CFM of airflow by the continuous ventilating fan.
