One of the key factors to consider when creating an industrial ventilation system is whether the facility needs to maintain a positive or negative pressure environment in order to ensure the safety of people, products, and processes. In this week’s blog post, we will explore the advantages of each option, the factors that should be taken into account, and the appropriate sizing of ventilation equipment to achieve positive or negative pressure in your facility.
Negative Pressure
To establish a negative pressure environment, fans are utilized to exhaust air, while supply air is regulated through louvers or doors. When the exhaust fans are activated, a suction effect is created in the structure, drawing air in via the supply air openings. The level of negative pressure within the building is influenced by the resistance of air flow caused by the supply openings. Excessive negative pressure can result in the sudden opening or closing of doors, posing a potential danger if fingers are caught in the door.
In cases where louvers are utilized in the supply air openings, it is crucial to ensure that the size of the open area in the louver is appropriate in order to maintain a safe air velocity at the louver face, thus preventing excessive negative pressure. Many manufacturers offer charts that display the pressure drop in relation to face velocity for their specific louvers.
If a man or truck door is used as the main source of air supply, it is necessary to have a policy in place that requires at least one door to be open while the exhaust fans are running. Failure to do so may result in a hazardous negative pressure level. To prevent this, it is advisable to have a backdraft damper installed in the area, which will release the pressure when all doors are closed.
There are two advantages of using a negative pressure ventilation system. The first benefit is the ability to regulate the placement of the exhaust air. This is particularly useful in situations where there are sources of heat, fumes, or dust within the building. By strategically placing the exhaust fans, the system can effectively remove the heat and contaminants away from individuals, goods, or operations. The second advantage is that a negative pressure system can prevent the infiltration of heat and contaminants into nearby locker rooms, break rooms, or offices.
One disadvantage of a negative pressure system is the inability to have complete control over the location of supply air. Although the desired route for supply air may be through louvers or doors, it is inevitable that air will also enter through other openings or gaps in the building. Due to the principle of least resistance, if there are other openings or gaps near the exhaust fan, the air will simply bypass the intended route and flow directly from the opening to the fan without any positive impact. Additionally, negative pressure systems have the drawback of introducing external dust and impurities into the building, as the system essentially draws air from the outside into the building.
Positive Pressure
To establish a positive pressure environment, fans are utilized to supply air while louvers or doors are used to exhaust air. This results in the building being pressurized, similar to blowing up a balloon. The same concerns exist for utilizing louvers and doors to exhaust air in a positive pressure environment as in a negative pressure environment when supplying air. If there is excessive resistance to the exhaust air flow, it can lead to a hazardous level of positive pressure, causing man doors to unexpectedly open or close. Therefore, the same approach should be followed for determining the appropriate size of louvers and keeping doors open in a positive pressure environment.
One of the main advantages of having a positive pressure setting is the ability to control the location of the supply air, thereby preventing the entry of air-borne contaminants. Instead of outside air seeping in through openings and crevices in the building, the positive pressure will expel air and safeguard people, products, and processes from external pollutants. For optimal results, it is crucial for the supply air fan to have a filter. The type and efficiency of the filters needed for the particular situation will depend on the smallest particle size that needs to be filtered out. Hence, the primary drawback of a positive pressure environment is the initial expense of acquiring filter housings and the recurring cost of replacing filters.
Conclusion
When deciding whether to use positive or negative pressure ventilation in an industrial setting, a useful guideline is to consider the location of potential heat or air borne pollutants that could impact people, products, or processes. If these sources are situated within the building, a negative pressure system is recommended to effectively remove the pollutants through exhaust ventilation. On the other hand, if the sources are outside the building, a positive pressure system can be used to prevent the infiltration of air borne pollutants, as long as proper filters are installed on the supply fans. For assistance in setting up a suitable pressure system for your facility, please reach out to a ventilation specialist at Eldridge.