The Positives and Negatives of Negative and Positive Pressure Filtration

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Sam Hueser, Systems Design Engineer – Automated Production (AP)

Many producers are continuing to consider adding filtration to their sow farms in order to limit PRRS breaks, but there has been much debate over the best method for achieving it. Positive and negative pressure filtration systems each have unique challenges in terms of construction, management, maintenance, and control system complexity. This article describes common building designs for negative and positive pressure filtered barns, typical control schemes, how they react to room conditions and finally a few advantages that each system has over the other.

Negative pressure filtration

Negative pressure filtration is often accomplished by pulling fresh air through evaporative cooling systems and into the attic before it enters the room through filtered ceiling inlets. Filters are inserted in boxes installed above each inlet. The air then exits the room through actuated exhaust curtains and exhaust fans that are grouped together in banks. This is a “negative pressure” design because the air chambers, attics, animal spaces and fan banks are all under a slight vacuum relative to the outside.

In this design, the air chambers and attics are considered dirty because the air in those spaces has not yet been filtered. The fan banks are also considered dirty due to potential for back-drafting of air through leaky fan shutters. For this reason, it is advisable to have more than one fan running in each fan bank at all times so unfiltered air is still exhausted when one of the fans turns off. As a consequence of negative pressurization, clean spaces must be sealed air tight to prevent unfiltered air from leaking into them via cracks and gaps in the building structure and cladding. Maintenance staff must enter the attic and navigate a series of catwalks to reach each inlet in order to change filters but fan maintenance is completed at ground level. The trusses are often designed with a taller heel so inlets near the sidewalls can be accessed more easily.

Fans and inlets are staged the same as any other power ventilated room where fans are turned on as the temperature in the room rises and inlet opening is increased to match the fan’s airflow. The only difference is the exhaust curtains are staged to match the fan’s airflow as well. When the temperature in the room decreases, fans are turned off and the inlets and exhaust curtains are closed. This fan-inlet staging system is simple and relatively easy to troubleshoot.

Positive Pressure Filtration

Positive pressure filtration is achieved by pulling the air through an evaporative cooling system and a filter wall before it enters the attic where fans push it through ceiling inlets and into the room. The air then exits the room through actuated exhaust openings in the walls. This is a “positive pressure” design because the attic and animal spaces are being slightly pressurized relative to the outside. This pressurization prevents unfiltered air from entering the clean spaces.

In this design, the air chambers are considered dirty while the filter bank, attic and animal spaces are considered clean. The filter bank space must be sealed air tight because it is under vacuum but the attic and animal spaces do not need to be sealed as well because the air is being forced out of those spaces. As a result of the pressurization, special building materials are sometimes used to prevent moist air in the animal spaces from permeating the structural components and causing them to rot. Filters are changed at or near ground level in centralized locations but fan maintenance requires staff to enter the attic.

In a positive pressure system, the ceiling inlets operate according to the temperature in the room while the fans are staged according to the static pressure in the attic and the exhaust curtains operate according to the static pressure in the room. As the temperature rises, the inlet opening is increased which causes the attic static pressure to decrease because more air is forced out through the inlets. This means more fans need to be turned on in order to maintain the static pressure set-point of the attic. At the same time, the static pressure in the room increases meaning the exhaust curtains need to be opened further to maintain the static pressure set-point of the room. When the temperature in the room decreases the system operates in reverse. Inlets are closed which causes a simultaneous decrease in the room static pressure and an increase in the attic static pressure so exhaust curtains are closed and fans are turned off until the static pressure set-points are satisfied in both the attic and the room. This system is a constant balancing act of room temperature, and attic and room static pressure.

Advantages of negative pressure over positive pressure

System Simplicity: Negative pressure is an effective, relatively simple method for providing ventilation that has been used successfully for decades. Most farm staff will have experience with negative pressure systems and will therefore be more likely to notice when the system is not operating as intended and be able to fix issues as they come up. The fans are installed in banks at ground level so they are visible from the animal spaces meaning the staff can verify that they are operating correctly. Positive pressure systems are more complex, comparatively. Fans, inlets, and exhaust curtains all operate independently according to different environmental conditions so their effects on each other are often unapparent making positive pressure systems hard to troubleshoot.

Weather Effects: Managing minimum ventilation in a positive pressure system is a major challenge because the fans operate according to the attic static pressure so if inlets are not properly managed, the room will be over ventilated resulting in excessive heating. Decreasing the minimum ventilation rate in a negative pressure system is accomplished by simply lowering the speed of the minimum ventilation fans making negative pressure systems much easier to manage during cold periods. Positive pressure systems are also more susceptible to wind if not properly protected. Ventilation can become backed up when strong winds blow directly at exhaust openings, preventing warm air from escaping the room. This causes an increase in both room and attic static pressures resulting in fans turning off and exhaust curtains closing. The consequence is a higher room temperature and a significant reduction to the ventilation rate. Wind is less likely to cause backups in a negative pressure system because fans and inlets operate according to the room temperature and not static pressure. So if a strong wind were to stall some of the fans, more would turn on as the room temperature increased.

Advantages of positive pressure over negative pressure

Pressurized Clean Spaces: Keeping unfiltered, potentially pathogen ridden air away from the animals is the purpose of filtered farms. Pressurized spaces do not need to be sealed air tight and fans are not installed in fan banks which decreases upfront investment costs and maintenance costs as the site ages. In a negative pressure system, every gap and crack in the structure must be sealed and all spaces with doors to the outside must be pressurized to prevent unfiltered air from infiltrating animal spaces and potentially causing a break. Maintaining an airtight structure requires constant inspection and upkeep, the cost of which can be considerable over the life of a farm.

Filter Cost: Filters are a substantial part of the total cost of a filtered site. Minimizing the total number of filters that need to be maintained and replaced can save a significant amount of money in the long term. Positive pressure systems are typically designed to operate at higher static pressures than negative pressure systems meaning fewer filters are needed to achieve the same ventilation rate. The filters are also grouped together at or near ground level so they are more visible to farm staff and can be changed without entering the attic. Another benefit of filter walls is that fewer ceiling inlets are required because the maximum airflow through an inlet is limited by the inlet itself rather than the combination of the filter box and inlet like in a negative pressure system.

Industry Perspective

Chet Mogler is the Farm Manager of Pig Hill Farms, a 4,400 head negative pressure filtered sow farm in northwest Iowa: “Positive pressure filtering was still in its infancy when we began planning construction of the site in 2014. We were also very concerned about moist air from the pig spaces getting into the structure and shortening the building lifespan.” Chet went on to say, “The only plausible solution we could find was building the entire structure out of concrete which would have been too expensive.”

Dr. Brett Ramirez is an Assistant Professor in the Agricultural Engineering Department at Iowa State University: “Regardless of ventilation style, the total static pressure the fans must overcome is often very high due to evaporative cooling pads, filters, inlets, shutters, backdraft prevention, etc., and is different in each unique configuration. This increased static pressure must be accounted for when selecting fans. Designing for anything less than 0.15 in. W.C. is not going to be sufficient.”

Summary

Maintaining a healthy environment is of the utmost importance in sow farms. The intent of this article is to give an unbiased look at the benefits as well as challenges of both systems. There is no “best” system for everyone. Each producer must consider all the factors discussed along with many others unrelated to ventilation that were not mentioned, in order to determine which system is best for them.

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