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A Pacific Northwest Extension Publication Basic Ventilation Considerations for Livestock or Poultry HousingPNW 307 Do you want to print a copy? We recommend downloading the print version (352 K). First download the free Adobe Reader, if you don't already have it. Contents Heat balance Most producers would agree that all buildings that house livestock and poultry need ventilation. The real discussion begins when you try to determine how much ventilation you must provide. Any discussion of how much ventilation is needed must start by asking, "Why ventilate?" The answer varies with season, type of building and floor, number and age of livestock, and your waste-handling system. We ventilate to remove moisture, gases like ammonia and hydrogen sulfide, disease organisms, and heat. Ventilation air also replenishes oxygen consumed by gas brooders used to heat some poultry buildings. Under most conditions, we ventilate in the summer to remove heat and in the winter to remove moisture. When the ventilation rates are adequate to remove heat or moisture, the organisms and gases are usually well diluted and present no problem. But a lot depends on your type of waste-handling system and on how frequently you clean the building and empty your manure storage units. While ventilation is important at all times, perhaps winter ventilation is the most critical. We humans can measure heat (temperature) quite well when we determine summer ventilation needs, but we have some problem sensing moisture levels (humidity) when we try to regulate winter flow rates. Let's use a winter example to demonstrate how the heat balance determines the required ventilation rate and possible needs in the building for supplemental heat. The heat coming into the building takes two forms (excluding mechanical heat from lights, motors, etc.). The primary source, in most buildings, is generated by the animals. The second is supplemental heat from furnaces, brooders, or lamps. This quantity of heat must satisfy three heat losses to keep the room/ building at some selected temperature:
The heat added to the system tends to raise the air temperature. The heat lost to each of these three sources tends to cool the air. If the building temperature is to remain constant (for this example, 60OF) the heat produced or generated in the building must heat the incoming air to 60OF and provide enough other heat to equal these three losses. A further examination of these relationships reveals that we can easily calculate the heat lost from the building when we know the room size, type of construction, level of insulation, and the inside and outside temperatures. With the building size, we know the animal capacity, and we can look up both the heat and moisture generated by the housed animals. We can determine the heat lost in the ventilation air if we know inside and outside air temperatures and the recommended ventilation rate for the number of animals we house. Table 1 provides these values. So, for any given set of conditions--outside and inside temperature, building conditions, animal numbers--you can calculate whether you'll need supplemental heat to maintain the indoor temperature you select. If you do need supplemental heat--but choose not to provide it--the inside temperature will fall. This cooler air will reduce the amount of water carried from the building in the ventilation air, and humid conditions will prevail (see figure 1). Other solutions would be to increase the heat input (provide more animals) or reduce the losses (add insulation to the building). Unfortunately, too many operators try to save heat by reducing the ventilation rate--which usually causes odor and moisture problems. While you can't accomplish these calculations quickly, the concept is rather simple. You can calculate a heat balance for your set of conditions to answer questions about your ventilation system. This design logic works for evaluating the ventilation and heating needs of any livestock or poultry building. Researchers have measured the heat and moisture produced by the different kind, sizes, and species of livestock and poultry. Other researchers have studied the characteristics of air and have determined its moisture and heat-carrying capacity at different temperatures. The higher the air temperature, the greater is its ability to carry moisture in the form of water vapor. Figure 2 shows a simple relationship between temperature and water-holding ability. Complete properties of air are shown on a psychrometric chart. It's a good idea to have a chart of your own; you'll find that it can help you better understand the relationships between air and moisture. (I'll be glad to send you a chart.) You use the moisture-holding capacity of air when you calculate the quantity of air necessary for winter ventilation to carry water vapor (from breathing, urine, and spilled water) from the building.
A second important characteristic that you use in ventilation systems is that cool air sinks and warm air rises. We use this phenomenon to locate and size air inlets and outlets in natural ventilation systems. You can calculate an adequate air flow rate by using the number of animals in the building and outside air conditions. Researchers have done this for general conditions, and table 1 shows their recommended ventilation rates in cubic feet per minute (cfm) per head or per 1,000 birds. These rates are designed to remove adequate moisture in the winter and heat in the summer. They're calculated to remove the unwanted moisture (urine, spilled water, etc.) from a building with a solid floor. A different floor type, such as a slatted floor, removes some of the moisture and allows a reduction in winter ventilation rates. Natural and mechanical systems There are two possible systems that you can select to provide the required ventilation air-natural and mechanical. The natural system uses no fans; it relies only on the wind and animal heat thermal buoyancy (the fact that warm air rises) to move air. The advantages of this system are its low initial cost (no fans to buy) and its ability to move air without requiring any electricity (no operating cost). The disadvantage is that natural systems require some management to set the inlet openings that control the range of ventilation rates. The actual ventilation rate and building temperature cycle through the day. This range is influenced by air temperature, wind, insulation level, and animal numbers and their activity level in the building.
Because the wind is one of the driving forces, the building orientation influences the design of inlets, outlets, and controls. This orientation must reflect the needs of the building in a natural ventilation system. This type of system is most often used to ventilate housing for rabbits, mature chickens and turkeys, weaned lambs, ewes, calves, finishing hogs, and gestating sows-situations where building temperature variations and drafts cause few, if any, problems. Mechanical ventilation uses fans to provide the required air flow rate. These systems are common in poultry houses and in swine-farrowing and nurseries units. Their advantages include:
The major disadvantages are cost (initial purchase and operating expenses) and the serious problems you could have during hot weather if a power outage stops all fans in a building full of livestock or poultry. When fans pull air from the building and exhaust it outside, this is called negative pressure. In this design, the static air pressure inside the building is lower than outside (atmospheric) pressure, so it's negative. Negative pressure designs are commonly used in livestock housing. In a positive pressure design, fans blow outside air into the building, creating an increase in pressure inside. The major disadvantage is that warm, moist air is under pressure and moves into all the cracks in the building walls and ceiling (seeking openings to the lower pressure outside). In the winter, air cools as it moves toward the outside, and moisture condenses in the walls and attic. This makes the insulation wet, causing it to get heavy and compress, which in turn reduces the effectiveness of the insulation. An additional disadvantage is that this moisture will eventually cause structural problems. Unless you have a vapor barrier, the recommendation is that you do not use fiberglass, mineral wool, and cellulose insulation with positive pressure systems. The term static pressure means the difference between the air pressure inside and outside the building. It's easy to measure this pressure--and knowing it is necessary when you select a fan and adjust inlets. Figure 3 shows a simple manometer, a section of clear plastic tubing partially full of water bent into a "U" shape. You can place this U-tube inside or outside the building. Note, however, that you always expose one end of the tube to the outside air pressure and the other to the inside conditions. You can make a manometer with a piece of tubing (as in figure 3) or buy one from a fan supplier. In a negative pressure design, the air is moving from the outside into the building. In figure 3, the manometer is inside the building, and the outside air is pushing the water up the tube that's exposed to the pressure inside the building. This room has less pressure inside than outside, a negative pressure system.
Your system operates best when the manometer shows a reading between 1/16 and 3/16 inch of water. Pressures below 1/16 inch make it difficult to uniformly distribute the incoming air. Pressures above 3/16 inch (generally caused by inadequate inlet area) reduce the fan capacity, reduce efficiency, and increase operating costs. Ventilation inlets and outlets Inlets. In both the natural and mechanical systems, the inlet controls the direction, path, and distribution of fresh air into the room (see figure 4). Inlets are perhaps the most important part of your ventilation system. By opening or closing the inlet, you regulate the thickness of the air jet, the air velocity, and (therefore) the distance the air flows into the room. See figure 5. When you need more air because of seasonal changes, adjust the inlet to maintain the air flow path and velocity. Inlets can be self-adjustable (for example, by using weighted curtains), or you can adjust them manually as required by temperature changes through the season. In poultry buildings with large rooms, the inlets are designed with the building. In swine units, plan the locations of the inlets after you've selected the sleeping and dunging areas. You need the "best" air in the sleeping area; direct the poorest air into the dunging area. The best air will be the warmest in the winter and coolest in the summer. Feeders and waterers also influence inlet selections. Summer inlets should provide lots of turbulence and volume-their purpose is to cool the building and animals. Winter air flows are smaller, but proper control and distribution are quite important. The large winter temperature differences between inside and outside air can make drafts a big health problem. In most cases, you'll need a different inlet system for summer and winter. In some natural ventilation systems, however, you can adjust the same inlets to function properly in all seasons. Outlets in mechanical systems can usually be located on any wall. In swine and calf units, air should not have to move more than 40 feet to get to a fan. Poultry units use larger fans and can increase this distance to 50 feet. Locating the fan on a wall away from the wind or providing a fan shield will ensure uniform and consistent air flow rates. The outlet in natural systems should be at the ridge to ensure that warm, moist air is exhausted from the building. This takes advantage of the fact that warm air rises and usually carries additional moisture with it as it leaves the building. In single-sloped buildings, the outlet should be along the top of the tallest wall. Proper ventilation of livestock facilities can reduce disease and death loss and can improve your rate of gain and feed-conversion efficiency. Unfortunately, most livestock and poultry buildings need improved ventilation systems.
Prepared by James A. Moore is Extension agricultural engineer at Oregon State University. Reprinted June 1993. |
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Published and distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914, by the Oregon State University Extension Service, Washington State University Extension, University of Idaho Cooperative Extension System, and the U.S. Department of Agriculture cooperating. The three participating Extension Services offer educational programs, activities, and materials--without regard to race, color, religion, sex, sexual orientation, national origin, age, marital status, disability, and disabled veteran or Vietnam-era veteran status--as required by Title VI of the Civil Rights Act of 1964, Title IX of the Education Amendments of 1972, and Section 504 of the Rehabilitation Act of 1973. The Oregon State University Extension Service, Washington State University Extension, and University of Idaho Extension are Equal Opportunity Employers. Copyright © 1995- Oregon State University. Disclaimer. |