Free area varies widely by size, options (special shapes and screens), and louver model. Each of these has an impact on the free area and should be considered when selecting louvers for your application.
Additional information on the impact of free area on louver selection, please visit the linked page how louvers work.
Size plays the most significant role in free area variation. Most people think of percentage free area as a constant, regardless of size. This is far from true, however. For example, the frame of the louver will take up more of the free area (as a percentage) as the louver size reduces (see Figure 1.1 below).
Even with a thin frame style (3/4" on all Architectural Louvers models) the percentage free area is significantly reduced on smaller size louvers. Keep in mind that we still need to subtract the blade obstruction from these numbers.
The shape of the louver will have an effect on the free area percentage. A slope or curvature eliminates some functional portion of each blade at the connection to frames, thereby reducing the free area. We have selected a comparison between a rectangular and triangular louver to demonstrate this reduction.
Because the blade is sloped from front to rear AND it is being mitered to fit flush to an adjoining frame, some of the free area is eliminated (the rear edge of each blade is shorter than the front edge). The angle slope of the blade and the pitch of the triangle will determine the free area reduction. As the slope of the blade increases (45 degree blade slope vs. 35 degree blade slope) or the pitch of the triangle decreases (3/12 pitch vs. 8/12 pitch), the "DEAD" portion of the blade increases, reducing the overall free area of the louver.
Many assumptions are made regarding rear screens on louvers. A very limited amount of industry testing has been done and the data is only relevant for new, clean screens. Dirty screens will eventually reduce the free area significantly.
In general, the free area of a louver is unaffected by a clean screen. AMCA (Air Movement and Control Association) tested and published results for the effect of screens on louver airflow and water penetration in their January 1994 Techspecs document. These tests demonstrated a slightly positive or negative effect on airflow through the louver (depending on the type of screen). Pressure loss increased 11% to 17%. Water performance improved 12% to 33%. But screens under airflow will eventually get embedded with dirt and grime, so most of the industry data is not relevant to real world application.
For small mesh screens (under 1/2" mesh) used at air intake locations, the buildup of dirt and grime can reduce the free area 10% to 90%. If left without maintenance, the screens will eventually block most of the airflow. If access to the screens is limited or maintenance is questionable, large mesh screen should be used.
Some good practices in screen selection:
Every louver model has a different blade and blade spacing, so the free area will vary based on the characteristics inherent in that design. While free area is not the only consideration in selecting which louver to use, it can have a dramatic impact on performance.
In general, shallow slope blades allow for higher free areas based on simple trigonometry. To prohibit sight through between blades, the shallow slope will also require narrow blade spacing (more blades for the same height). This increases the cost of the louver.
Most commercial louvers have free areas between 30% and 60%. There are many residential style louvers that have free areas below 30%. Architectural Louvers makes many styles of louvers. The free areas are listed below:
Performance Table is based on a test size of 48" wide x 48" high for comparison purposes:
|Product Model||Free Area||First Point of Water Penetration (free area velocity)||Overall Performance (C.F.M.)||Pressure Loss at this velocity (inches water gauge)|
|E6JN||69.1%||915 fpm||9014 cfm||0.12|
|E4DP||59.3%||930 fpm||8826 cfm||0.12|
|E4JP||58.4%||960 fpm||8976 cfm||0.13|
|E6DP||57.7%||1046 fpm||9655 cfm||0.13|
|E6JP||57.3%||1123 fpm||10298 cfm||0.18|
|E4DS||56.0%||930 fpm||8333 cfm||0.13|
|E4WS||56.0%||346 fpm||3100 cfm||0.02|
|E6JF||54.4%||1020 fpm||8884 cfm||0.18|
|E2WV||53.8%||889 fpm||7645 cfm||0.24|
|E6WH||51.4%||>1250 fpm||10275 cfm||0.21|
|E6WF||51.1%||1081 fpm||8832 cfm||0.14|
|E4WH||50.6%||>1250 fpm||10113 cfm||0.25|
|E4JS||50.4%||888 fpm||7157 cfm||0.15|
|E2DS||49.4%||889 fpm||7032 cfm||0.12|
|E2JS||48.7%||725 fpm||5648 cfm||0.08|