Ohio State University Fact Sheet

Food, Agricultural and Biological Engineering

590 Woody Hayes Dr., Columbus, OH 43210

Manure Pit Ventilation Systems for Swine Operating Principles

AEX-150-98

Ventilation of shallow gutters and deep manure pits in confinement housing can increase animal performance and worker safety by exhausting pit gases from the building before they can enter the animal zone. This improves air quality in the animal area and results in a cleaner, healthier environment inside the building. Also, floors are warmed and dried by air that is pulled from the animal space into the manure pit. Several designs can be used to incorporate pit ventilation in livestock buildings. This fact sheet describes these designs and should help livestock producers understand how the designs can be utilized.

Pit ventilation is usually accomplished through one of three basic designs: a plenum; a ventilation duct; or a pit annex. The plenum and ventilation duct designs are most commonly used in farrowing and nursery buildings with shallow manure gutters or hairpin gutters. These designs normally provide the winter air exchange needs of the building to control odor and maintain proper relative humidity. In recent years, ventilation ducts have gained popularity in both mechanically and naturally ventilated swine finishing and gestation buildings as a means of providing minimum winter ventilation rates and exhausting pit gases year-round.

A pit annex, or dog-house design, is normally used only with deep manure pits. This design removes air directly from the pit, without the use of a ventilation duct. Annexes are most often used in breeding, gestating or finishing facilities where the winter ventilation rates can be several thousand cubic feet per minute.

Manure pit ventilation systems must be properly constructed and managed to operate correctly in a very corrosive environment. Ventilation system components will be exposed to moisture, ammonia, hydrogen sulfide, methane, and dust. Fans and other equipment used in these systems should be corrosion-resistant, preferably utilizing stainless steel, plastic, and polymers. The use of corrosion-resistant equipment will reduce maintenance and down-time of the ventilation system.

Air Plenums

An air plenum is usually utilized in farrowing and nursery buildings that have shallow or hairpin gutters. In this system, air is withdrawn from the head space of the gutters through holes in the upper portion of the gutter walls (Figure 1). A plenum is formed by enclosing the airspace along a gutter or between two gutters (space below a walkway) so that air can only enter this space through the holes in the gutter walls. The holes are constructed along the length of the plenum when the concrete gutter walls are being poured. The holes must be placed and sized to provide uniform air flow along the length of the gutters. An exhaust fan is located at the end of the plenum opposite the loading and unloading side of the building. Thus, incoming pigs are not exposed to contaminated air being exhausted from the pit.

Figure 1. Air plenum design under construction.

In modern swine buildings, air plenums are usually sized to provide the minimum winter ventilation rate. In some instances plenums can be designed to deliver the mild weather ventilation rate also.

It is essential the holes through which air is drawn into the plenum are properly sized and located. If the holes are too large, the side of the building close to the fan will be over-ventilated, while the air at the far end becomes stale. This same problem can result if the holes are evenly spaced along the length of the plenum. Proper distribution places few holes near the fan and increasingly more holes in the plenum walls as one moves away from the fan. Designs shall be obtained from reputable builders and should include specifications for construction of the plenum. It is a good practice to ask a builder to verify that their pit ventilation system has been designed by a qualified engineer and will be installed as designed. It is usually during construction of the air plenum that mistakes, which reduce the system's effectiveness, are made.

Air plenum holes become plugged with manure if the pit becomes too full. Livestock producers utilizing plenums or ventilation ducts should understand that some manure storage space is lost utilizing these ventilation systems. Add depth to the manure storage pit if more storage space is desired. Holes should be no smaller than 1.5 inches in diameter to reduce the chances of being clogged by manure.

Air plenums work well to remove pit gases from manure storage areas that are relatively narrow. When pulling air from a wide manure pit using an air plenum, it is important to realize that the entire pit area may not be fully ventilated. Areas of a pit more than five feet from the air plenum may not be well-ventilated and pit gases from these regions may enter the animal zone.

Ventilation Ducts or Tubes

Pit ventilation tubes (Figure 2) are commonly used in farrowing and nursery buildings because they provide more precise pit ventilation and work very well at the low winter ventilation rates needed within these buildings. As more swine producers seek improved environments in finishing buildings, pit ventilation tubes are becoming more common in these facilities also. Pit ventilation tubes are constructed using PVC pipe, usually six to 12 inches in diameter, which has holes drilled into it. Engineers usually size pit ventilation tubes to carry only minimum winter ventilation rates to eliminate the expense of large diameter PVC pipe that would be required at higher air flow rates. In addition, the larger the pit ventilation pipe, the greater the loss of storage capacity in the manure pit, which is a significant concern to most livestock producers.

Figure 2. Manure pit with ventilation tube in place.

Pit tubes are usually hung from the flooring directly above the manure pit using stainless steel straps or some other corrosion-resistant product. Ventilation tubes are popular with hairpin gutters as the duct can be located directly above the center wall and can provide reasonably effective ventilation of the entire pit space.

Most ventilation ducts extend from the manure pit through the building's foundation and have a single-speed fan that runs continuously. When two or more ventilation tubes are joined prior to exiting the pit, every effort should be made to use Y-type joints with angles of no greater that 45 degrees. There can be a significant loss of ventilation capacity if sharp angles are used.

As with air plenums, hole sizing and spacing is very important for this design to correctly operate. Holes should be 1.5 inches or greater in diameter to prevent clogging. Space and distribute holes properly (more hole area away from the fan) to provide for an even pull of air along the length of the tube.

Building Annex/Doghouse

Pit annexes are most commonly used with deep pits and are commonly found in finishing, breeding, and gestation buildings. They do not perform as precisely as ventilation ducts, but can handle much higher ventilation rates and are simpler and less expensive to build.

Figure 3. Exhaust view of an operational pit annex.

Special consideration must be given to the flooring system used with an annex. Flooring systems that have 50 to 60 percent open area, such as wire mesh or coated plastic, result in poor air distribution as too much air is pulled down through the floor close to the fan. Annexes should be used with flooring systems that have 15 to 20 percent open area, such as concrete slats. This allows the space below the flooring to be used as an air duct and offers improved air distribution. Also note that as the level of manure approaches the storage capacity of the pit, the effectiveness of the pit annex ventilation system is greatly reduced. As a general rule, keep at least one foot of separation between the manure level in the pit and the bottom of the slats.

Pit annexes are usually sized to provide minimum winter ventilation needs just as with the air plenum and duct systems. The fan in this design does not normally operate at as high a static pressure as the previous two designs because air is being drawn through a larger cross-sectional area. Thus, variable-speed fans are not uncommon in this design.

Some pit annex designs allow for the removal of the fan housing to access the manure pit for agitation or pumping. If this is done, replace the fan housing properly so the ventilation system is not short-circuited by air leaking around the fan.

Reviewed by:

Dale Ricker, State Swine Extension Associate

Terry Mescher, MNM Technician, Darke County SWCD