When the air exchange process is accomplished with air moving fans, it is called mechanical ventilation. When done by utilizing buoyancy and wind forces it is referred to as natural ventilation.

This paper will discuss the principles of mechanical ventilation and the application of these principles to the design of systems for pig facilities in Minnesota. Readers may want to refer to a Midwest Plan Service publication entitled Mechanical Ventilating Systems for Livestock Housing (MWPS-32) which is available from the University of Minnesota.

Insulation Requirements

Figure 1 Insulation prevents condensation on inside surface

Figure 2 Insulation R-values of 13 inch walls, 25" in the ceiling and 6" around the foundation

To successfully ventilate a pig housing facility, one needs a well insulated building shell. The major function of insulation in a mechanically ventilated barn is to prevent condensation on the building's inside surfaces. This is accomplished, as shown in Figure 1, by providing a sufficient amount of insulation to maintain surface temperatures above the dew point temperature. 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 (Figure 2). Higher "R" values are sometimes used in facilities in cold climates like the northern U.S. and Canada. However, a small percentage (20%) of the total heat is lost through the walls and ceiling in most pig facilities. The majority of total heat is lost by the cold weather air-exchange that is necessary to control moisture and maintain acceptable air quality. The insulated building shell also reduces solar heat gain in the summer, especially insulation that is located on the underside of the roof or in a flat ceiling with an attic space above. Since the primary function of insulation in pig facilities is to prevent condensation, excessively large insulation values (R values greater then 25 in the walls and 40 in the ceiling) have limited benefit.

Of almost greater importance than the amount of insulation is the type of insulation used and how it is installed in a pig housing facility. To protect the insulation from the moisture produced in a pig barn, some type of vapor retarder (formerly called vapor barrier) is necessary. 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 through the insulation and condensing inside the wall. Polyethylene film or sheets should always be used even if existing insulation has attached vapor retarders, i.e. aluminum foil backing on fiberglass blankets. The large moisture loads in pig facilities can cause significant moisture problems with even very small breaks or 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 pig housing facilities, but is necessary to safeguard the insulation. Crushed rock around the perimeter of a building to prevent rodents from burrowing under walls or maintaining a bait and trap system throughout the farm to hold down rodent populations, are highly recommended to prevent insulation deterioration in walls and ceilings.

Another important building location to insulate is the foundation of a pig barn. By providing perimeter insulation, as shown in Figure 3, floors along outside walls will be much more comfortable for pigs during cold weather since surface temperatures will be warmer. Perimeter insulation also eliminates condensation and frost in these areas. Rigid board insulation is recommended with an R value between 6 and 8, extending 2 or 3 feet below ground level.

Stud wall to concrete slab floor Post wall to concrete slab floor
Figure 3. Foundation or perimeter insulation for both stud wall and post construction.

Mechanical Ventilation

Figure 4 Types of mechanical ventilation systems

The air pressure difference, created by fans, between the inside of the barn and the outside is the reason why air is exchanged in a mechanically ventilated pig facility. The most common system is shown graphically in Figure 4A where the exhaust fan(s) create a slight negative pressure or vacuum in the barn which causes air to enter the barn through designed inlets. Positive pressure systems do the opposite (Figure 4B). A fan blowing air into a barn create a positive pressure and air escapes through designed outlets. This system is fairly uncommon, since it often causes deterioration of building materials from moisture moving through cracks in the building shell. A third type, which has gained popularity recently, is a neutral pressure system as seen in Figure 4C. Push-pull systems and most commercial heat exchangers operate under neutral pressure, at least at the continuous or cold weather ventilation rates. A neutral pressure system has both an exhaust fan and an inlet fan, which create a zero or approximate neutral pressure difference between the inside and outside. Such a system typically becomes a negative pressure system when other larger exhaust fans operate during warmer weather. Most of the remaining discussion in this paper will describe and outline the negative pressure system because of its common use and consistency from minimum to maximum air exchange, although the neutral pressure and positive pressure will be referred to when appropriate.

System Components

When designing a mechanical ventilation system, there are three major components to conside: FANS, OPENINGS and CONTROLS. Fans and openings control the amount of air exchange in a mechanical ventilation system. The openings also determine the air distribution or mixing in a mechanically ventilated pig housing unit. Controls are needed to adjust ventilating rates (fan controls) and air velocities through openings, as weather conditions, age and number of pigs change. These three components will be discussed individually.

Fans

Fans are used in mechanical ventilating systems to exchange the desired amount of air in a pig housing unit. In a negative pressure system, fans are installed to exhaust stale or used air from the building and bring fresh, clean air into the barn. It is very important to use only fans that have been rated. The ratings of the exhaust fans are given in cubic feet of air per minute (cfm), or in SI units - cubic meters per second, at specific static pressure levels. Fan ratings are given in table form, similar to Table 1. Look for certification by an organization like the Air Movement and Control Association (AMCA) when purchasing fans. With rated fans there is some assurance that the numbers given in the table are valid. The cfm ratings of an individual fan depends upon the horsepower and rpm of the motor, shape of the blades, and design of the shroud around the blades. It is very difficult to accurately determine a fan's cfm capacity when it is already in place in an existing facility. Therefore, it is very important that fans that are used in pig ventilation systems be rated so that the air exchange or ventilation rate is known.

Air delivery in cubic feet per minute (cfm) 
at indicated static pressure
-----------------------------------------------------------------
 (Free Air)
Diameter RPM HP 0" 1/10" 1/8" 1/4" 3/8" 1/2"
-----------------------------------------------------------------
 8" 1650	 1/50 400 316 289 --- --- ---
 10" 1550	 1/50 594 457 413 --- --- ---
 12" 1550	 1/30 730 --- --- --- --- ---
 12" 1600	 1/12 1188 1073 1035 827 --- ---
 16" 1140	 1/12 1675 1440 1374 --- --- ---
 16" 1725	 1/3 2534 2392 2353 2142 1890 1635
 18" 1140	 1/6 2686 2460 2395 --- --- ---
 18" 1725	 5/8 4065 3920 3880 3682 3445 3195
 21" 1140	 1/4 3812 3599 3540 --- --- ---
 21" 1725	 3/4 4914 4770 4740 4510 4320 3920
 24" 855	 1/3 4691 4310 4180 --- --- ---
 24" 1140	 7/8 6254 5990 5920 5470 4810 4220
-----------------------------------------------------------------

As seen in Table 1, the maximum cfm delivery of any fan is at the zero static pressure or free air level. As static pressure increases (similar to having a wind blow against the fan), a fan's cfm or air delivery capacity decreases. This is seen as one moves to the right in the table. For mechanically ventilated pig barns, it is suggested that the cfm value be chosen at either 1/10" or 1/8" static pressure for design purposes when selecting fans. Most mechanical ventilating systems, including negative, positive, and neutral pressure systems operate at static pressures slightly below these levels. However, when fans are selected at static pressures slightly above operating conditions, a small safety factor is provided, to make sure that sufficient air exchange is provided in the building. Other criteria, such as electrical energy usage by the fan, can also be used to select fans. Typically input energy usage rates are given in values of cfm per watt. Electrical energy efficiency is most critical in larger capacity fans that operate primarily during warm weather. Energy efficiency criteria for the continuous or cold weather operating fan or fans is less important because they move a small percentage of the total air flow in the ventilation system. If a multi-speed fan is selected for a mechanical ventilation system, one should make sure that the ratings are at the revolutions per minute (rpm) range that the fan will be used. Typically, cfm values are greatly reduced when the fans rpm's are low, as shown in Table 2. For this reason it is recommended that single speed fans be used, especially for the cold weather or continuous rate, rather than multi- speed fans because of their ability to "buck the wind" better and provide a more consistent air exchange.

Air delivery in cfm at indicated static pressures
 RPM 0" 0.05" 0.10" 0.125" 0.15" 0.20" 0.25"
------------------------------------------------------
 1675 2172 2112 2028 1988 1932 1840 1480
 1355 1775 1622 1473 1385 1298 1005 585
 855 1110 816 273 --- --- --- ---
 586 482 83 --- --- --- --- ---
------------------------------------------------------

Ventilation Rates

The amount of air exchange required in a pig housing facility is given in Table 3, which is taken from MWPS-32, Mechanical Ventilating Systems for Livestock Housing. Table 3 lists three ventilation stages or rates, which vary depending upon pig size. The first stage is the cold weather or continuous rate which provides a base level of air exchange through the barn to maintain proper air quality and moisture control under winter conditions. The next ventilation stage is called the mild weather or fall/spring rate, which controls the barn's temperature during milder winter days and cool spring and fall days. This ventilation rate is generally regulated by a thermostat or temperature controller on fan or fans which operate intermittently during the year. Finally, the third ventilation stage is the hot weather rate which is designed to limit the temperature rise in the facility during the summer. Fans to provide this rate are also controlled with a thermostat or temperature controller to automatically activate them during hot conditions.

---------------------------------------------------------------
 Cold Mild Hot
 weather weather weather
Animal Weight rate(a) rate rate(b)
 lb Unit -----------cfm/unit-----------
---------------------------------------------------------------
Swine
 Sow and litter 400 hd 20 80 500
 Prenursery pig 12-30 hd 2 10 25
 Nursery pig 30-75 hd 3 15 35
 Growing pig 75-150 hd 7 24 75
 Finishing pig 150-220 hd 10 35 120
 Gestating sow 325 hd 12 40 150
 Boar/Breeding Sow 400 hd 14 50 300
---------------------------------------------------------------

Air Openings

The most overlooked component in any mechanical ventilation system are the air openings. Air openings need to be properly sized and adequately distributed. In a negative pressure ventilation system, the openings are the air inlets.

In order to size air inlets, you must assume a certain air velocity through the openings. Maximum design velocity through air inlet openings in a negative pressure system, may range from 1,000 to 600 feet per minute (fpm). For design purposes, inlet size can be determined by using an 800 fpm velocity. If this assumption is made, then the inlet area in square feet (sq ft) is equal to the maximum air exchange rate in cfm divided by 800 fpm. As an example, if the maximum air exchange rate is 4,000 cfm, then the inlet area is equal to 4,000 cfm / 800 fpm or 5 sq ft.

The inlet area needs to be well distributed throughout a mechanically ventilated pig barn to obtain good air mixing. Exhaust fan location has little to do with air distribution. Other than locating continuous running fans to exhaust from manure pit areas, fan placement is primarily determined by the need to avoid prevailing winds. A row of inlets, either a continuous slot (Figure 5) or a series of ceiling or box inlets (Figure 6), should be provided for every 20 feet of building width. This means that for the common 24 to 36 ft. wide barn there should be two rows of inlets while the newer 45 to 60 ft. wide barn should have three rows.

Figure 5. Slot inlet detail

Figure 6. Ceiling inlet detail

During cold weather, it is generally recommended that air be drawn from a plenum, such as an attic or a hallway, to prevent wind from having an adverse affect on the ventilation system. During the winter time, the static pressure level in a barn will be relatively low (between .02 and .05 inches of water gauge). If air is brought directly in from the outside, it can be subject to wind pressures that can adversely affect the mechanical ventilation system even with wind speeds of only 10 miles per hour. When this occurs, control of the ventilation system is lost, with either too much or not enough air exchange in the pig facility. During the summer time, it is advantageous to pull air directly in from the outside, thereby avoiding additional heat that may be collected from an attic. A seasonal compromise is to remove air from underneath the eaves directly through a slot inlet system during warm weather and from the attic in the winter time (see Figure 4).

Controls

A mechanical ventilation system needs controls, which regulate the air exchange rate during the day, as well as throughout the year. The most common type of temperature controller for a fan is a thermostat. There are 3 stages of ventilation recommended for pig facilities, controllers are only needed for fans which regulate the last 2 stages of ventilation. No thermostat controller is needed for the fan, which provides the winter rate, since it runs continously. There is, however, benefit from a special type of flow rate controller on this fan. It consists of a plywood, metal, or plastic duct built around the fan large enough to provide the winter rate, which draws air from near the floor.

Figure 7. Duct with sliding damper to throttle continuous fan

It includes an adjustable sliding damper which can be used to regulate the fan's air flow according to the size and number of pigs in the building (see Figure 7). The cross-sectional area of the duct should be determined by using the same 800 fpm air velocity criteria discussed earlier. Thus, if a 800 cfm fan is used, the duct should have 800 cfm/800 fpm or 1 sq. ft. of area (dimensions of 1 ft x 1 ft or 6" x 24"). It is suggested to not close the sliding damper more than half way so the fan is not restricted too much. This can be a very simple but useful "controller" for a pig nursery, whose minimum ventilation rate may vary significantly as the pigs grow. This offers an alternative to the use of variable speed fans, which have limited air exchange capacity at low rpms due to the fan characteristics discussed earlier.

Controllers for air inlets or openings are possible and can eliminate much of the manual adjustment necessary from season to season. The most common control design is a gravity baffle, which automatically opens when a static pressure occurs in the barn. Gravity baffle control produces a relatively constant static pressure level in the facility with a negative pressure ventilation system. The operating static pressure level can be adjusted by a movable sliding weight (Figure 8) on a rod or bar to counterbalance the pressure force to open the inlet baffle. As mentioned previously, manual control of inlets can work if properly managed. In many facilities in the upper midwest, a constant air inlet setting through the cold weather period (November through March) will provide an acceptable range of inlet velocities from the minimum condition to the mild weather or fall/spring rates. As the weather warms in the spring, a summer time opening must be provided and left through the summer until fall and cooler temperatures. Obvious difficulties do occur during the spring and fall when there are warm days and cold nights. Other more sophisticated inlet controls exist, which either sense static pressure or temperature and adjust openings with electrical damper motors. This approach is more expensive and does require more mechanical maintenance but can provide more precise control of air openings.

Figure 8. Counter weight gravity baffle inlets from Canada

Ventilation Design

To demonstrate the mechanical ventilation principles discussed in this paper, a design for a 200 pig nursery will be presnted. First of all, the ventilation rates for pigs this size are listed in Table 3. Assume that the 200 head nursery receives pigs weighing 12 lbs and will hold them until 60 lbs. Minimum ventilation rates vary from 2 to 3 cfm per animal, mild weather rates are +8 to +12 cfm per pig more for the fall/spring periods and vary from 10 to 15 cfm per pig, while the hot weather rates are +15 to +20 cfm per pig more for summer that vary from 25 to 35 cfm per pig. Since pigs are relatively large before they leave this facility, we will have to use the larger ventilation rates when sizing the ventilation fans for this facility. This is done by simply doing the multiplication, as shown below:

 Cold weather rate = 3 cfm per pig X 200 = 600 cfm fan

 Mild weather rate = +12 cfm per pig X 200 pigs = 2400 cfm fan

 Hot weather rate = +20 cfm per pig X 200 = 4000 cfm fan

Once these calculations are completed, the fans can be selected from a table similar to Table 1. As previously mentioned, fans should be selected at or slightly above the calculated cfm values at either the 1/10 or 1/8 inch static pressure level. For this example, let's recommend a fan rated at 600 cfm or slightly greater (up to 800 cfm) to control the continuous or cold weather rate. Let's also build a duct around this fan with a sliding damper, as shown in Figure 7, to throtte or regulate the air exchange rate of the fan. When the pigs are first moved into this unit, a 2 cfm per pig or even slightly lower ventilation rate is required because of the small average pig weight. One could initially throttle back this fan by adjusting the sliding damper approximately half way open, and then gradually increase the exchange rate of the fan by opening the damper as the pigs begin to grow. These adjustments might be made weekly or every 2 weeks. The two larger fans could have a single 3 stage controller with separate stages for each of the fans and another stage for a unit heater to maintain proper room temperature.

Cold Weather Inlets

Figure 9. Tempered hallway inlet method

The inlet area for this particular system could be done in two stages, first for the cold weather operation and then for the warm or summer ventilation rate. If one adds the continuous fan and the mild weather fan, you would obtain a total cfm rating of 3000 cfm (600 cfm + 2400 cfm). Dividing this number by 800 fpm, results in an inlet area of approximately 4 sq ft. Therefore, in this facility, 4 sq. ft. of inlet area should be provided during cold weather operation, or roughly from October through April. If this is a negative pressure ventilation system, then either ceiling inlets, or a slot inlet could be built to draw air from the attic space into the nursery room. Another, more common alternative for nursery facilities is to draw this air from a tempered hallway as shown in Figure 9. If a tempered hallway is used, a 4 sq. ft. opening is needed in the partition wall separating the room from the hallway. If the room is longer than 20 ft, a distribution duct should be connected to this opening to provide better air mixing in the individual room. Air must also be brought into the hallway either from the attic (as shown) or from the outside through a similar sized (4 sq. ft.) inlet. During cold weather hallway temperature should be no warmer then 50Á F otherwise excessive fuel usage will result when pigs in the nursery are near their maximum weight.

Heat Exchanger Option

Another alternative method for providing ventilation to this nursery during the winter time uses an air-to-air heat exchanger (Figure 9). If a heat exchanger was used in this facility, the size of the heat exchanger would be determined by the size of the continuous ventilation rate (600 cfm for our example). The heat exchanger would then replace the continuous running fan. As in any nursery ventilation design, an additional exhaust fan is required for the mild weather rate. For our example this is a 2400 cfm fan which would provide air exchange if the barn became too warm. If this occurs, then inlets for that size fan (2400/800 or 3 sq. ft.) need to be provided in the nursery. These inlets will automatically open when the exhast fan is activated and a negative pressure is created. This is shown schematically in Figure 10 and is absolutely necessary in a barn that has a heat exchanger. You would also need the same total fan capacity as in a conventional ventilation design so a third fan providing the hot weather rate (4000 cfm) would be necessary.

Figure 10. Heat exchanger during cold and mild weather conditions

Supplemental Heat

Finally, the design of the cold weather ventilation system for the nursery unit will require supplemental heat. There is generally a need for two types of heat in small pig facilities. One is radiant heat to maintain warm surfaces or floor temperatures for young piglets. The other is space heat needed to maintain room temperature during cold outside temperatures (Table 4). Because young animals produce small amounts of heat, extra heat sources are needed during cold outside temperatures to maintain adequate inside conditions. The only way to maintain adequate room temperature is to have a supplemental heater. Unit heaters provide this additional heat to maintain air temperatures, and are typically hanging gas fired furnaces or electric convective heaters. Radiant heaters, on the other hand, provide heat for animal comfort and are used to heat surfaces. Typical radiant heaters are heat lamps, infrared and catalytic heaters or fiberglass heat mats. These units only heat surfaces and not the air directly. It is not recommended to try and heat the air with radiant heaters. In a small animal housing unit like a pig nursery, it is necessary to have both types of heat, especially during the first week or two after weaning.

Animal unit Supplemental Heat per Animal Unit
---------------------------------------------------------------------
 Inside Slotted Bedded/
 Temp, F Floor Scraped Floor
 -------Btu/hr/unit-------
---------------------------------------------------------------------
 Swine
 Sow and litter 80 4,000 ---
 70 3,000 ---
 60 --- 3,500
 Prenursery pig (12-30 lb) 85 350 ---
 Nursery pig (30-75 lb) 75 350 ---
 65 --- 450
 Growing-finish pig (75-220 lb) 60 600 ---
 	
 Gestating sow/boar 60 1,000 ---
---------------------------------------------------------------------

Warm Weather Inlets

Completing our design example during warm weather, the additional 4000 cfm fan is activated by the thermostat control. Additional inlet area is needed for this ventilation stage of 4000 cfm / 800 fpm = 5 sq. ft. This inlet area could draw air directly from the outside in 4 or 5 different wall inlets. If the tempered hallway system is used, additional inlets in the partition separating the room from the hallway would be required. Control on wall inlets could be done manually or with a gravity baffle or a static pressure sensing unit to open and close when the large fan is activated.

Summary

To successfully mechanically ventilate a pig housing facility, one needs to understand ventilation principles for purposes of design and management. Components of the ventilation system need to be chosen wisely, based on known capacities and using proper design criteria. The fans and heaters openings, and controls need to function as a system to avoid components such as heaters, from running when larger exhaust fans operate. Finally, with a well designed and manageable mechanical ventilation system, a good indoor environment can be maintained not only for the animals housed but also for people who work in these facilities.

Source: University of Minnesota, U. S. Department of Agriculture, and Minnesota Counties Cooperating - January 2004