The Balancing Act
Total Systems Approach to Air Exchange
Air movement, whether statically or mechanically induced can be a double-edged sword if components are not sourced in a complimentary, calculated design. Pain points exist when too much emphasis is focused on exhaust rather than intake resulting in a vacuum like space drawing external moisture and contaminants into an otherwise perfectly constructed building. The Bureau of Labor Statistics reports that over 15 million people were employed in the manufacturing sector in 2016 with most people working in ventilated, but unconditioned low-rise metal buildings throughout the United States. Ventilated buildings are designed to exchange contaminated air from the inside with fresh air from the outside in an effort to promote health and safety for the occupants. The quantified combination of exhaust fan and intake louver technologies is the most feasible and effective way to control and accomplish air exchange.
Public Service Announcement
Decades ago, manufacturing plants relied primarily on open windows and doors to naturally ventilate work spaces. Weather, location and opening sizes led to inconsistent performance that plagued air quality. Initiatives that promote worker safety led to the development of ventilation equipment that is applied to large spaces to provide a controllable environment yielding more consistent production. The size, number of occupants and use of a building determines the amount of make-up air from the outside required to properly ventilate a space. Generally speaking, most manufacturing spaces require a minimum of 4 air changes per hour (ACH) but that number can be as high as 50 depending on the type of materials and their production processes. Governance was organized by Air Movement and Controls Association International (AMCA) to establish performance and test data on behalf of consumer welfare for the component technologies that facilitate ventilation.
Fair and Balanced
Fans that exhaust unwanted air work in conjunction with louvers that intake air to accomplish circulation. These common components bear AMCA ratings from reputable manufacturers and are typically applied to building systems in sizes proportionate to the rate of ventilation for the space. The volume of air moved by a fan assembly is measured in Cubic Feet per Minute (CFM) to establish performance. Determining the number of CFMs required to accomplish air exchange for sizing fan equipment can be calculated by dividing the total cubic area of a building by the number of air changes per hour. The most critical determination applies to the quantity and size of louvers necessary to balance the air withdrawn from a space. Louvers consist of slats mounted at regular intervals that allow the transfer of air across a wall boundary and have performance ratings based on water penetration speed and air flow effectiveness through the louver obstruction. Feet per Minute (FPM) is the threshold metric for water infiltration through the louver boundary based on the air intake velocity. The number of louvers required to fairly offset the exhausted air is calculated by dividing the total building CFM into the louver FPM Rating and subsequently dividing the resulting square feet of free area by the Efficiency Rating percentage of the louver. This resulting number yields the total square feet of louvers specific to the FPM and efficiency ratings required to offset the air transferred from the space via the exhaust fans.
The balance of airflow in and out of a space is critical to avoid deterioration of the space and occupant health. When make-up air equipment is undersized in a sealed building, a vacuum like environment is created which transports nearby particles heavier than air such as undesirable moisture and contaminants into the space. Water will take the path of least resistance compromising the integrity of unrelated building components not designed for negative pressure such as skylights, insulation, windows, doors and building structural elements. Increased wear on the exhaust fan motors and bearings has been documented as the result of differential pressure that seemingly chokes the equipment. Poor air quality results from the influx of moisture which is known to produce fungi such as Stachybotrys Chartarum commonly referred to as Black Mold that severely damages the respiratory system of exposed occupants. These are just a few examples of consequences resulting from shortcuts in ventilation design.
As soon as the building envelope is sealed, negative pressure will reveal itself. Intake must be greater than or equal to exhaust to avoid a vacuum condition within a space that results in water infiltration. Exhaust fans and louvers when sized appropriately, working in conjunction with one another are the epitome for quality, feasible ventilation in large spaces such as manufacturing environments. Shield your building by finding the system equilibrium--put in what you take out.
Written by: Clifton Reasor and featured in Metal Construction News' Know Your Products section of the August 2017 Issue