From classical loops and swirls to modern geometric patterns, KEES decorative grilles add a much-appreciated level of style to any installation. But beyond matching a designer or architects’ creative vision, decorative grilles serve the functional purpose of letting air flow in or out with minimal noise disturbance. How do you balance the aesthetics and performance of a decorative grille?
How Do Decorative Grille Patterns Affect Performance?
The pattern of a decorative grille affects performance through its free area or open area percentage, the percentage of the pattern open to airflow versus obstructed by grille material. A pattern with a high free area allows air to pass through easily and quietly; a pattern with low free area forces air through fewer channels, making the process noisier.
The noise coming from HVAC equipment is quantified by Noise Criterion (NC) levels. NC is a relative value measuring the sound pressure levels of a space. Places with low recommended NC levels, like libraries, hotel rooms, and houses of worship, expect air moving in and out of grilles to be quiet.
In an ideal world, architects or interior designers planning a space with a low NC level would choose a pattern with high free area to maximize performance. In reality, their choice is influenced solely by the aesthetics of a pattern. Once that pattern is baked into the plans for a building, engineers and contractors can’t necessarily replace it with a different looking pattern that may perform better.
What engineers and contractors can do is adjust the size of the grille and the opening it covers. A larger size allows more air through more easily, offsetting inefficiencies in the pattern’s design.
What Goes Into the Performance of a Decorative Grille?
Three factors go into determining the performance of a decorative grille:
Volume flowrate of air: The amount of air, in cubic feet per minute (CFM), that moves through the grille.
Face velocity at pattern area: The speed of the air as it passes through the grille pattern, in feet per minute (FPM). To calculate face velocity, divide the volume flowrate of air by the pattern area dimensions from the KEES product submittal.
Free area or open area percentage: The amount of a decorative grille pattern that is open to airflow versus obstructed by grille material. This percentage is found on the KEES product submittal.
Free area and face velocity are inversely related. The less free area the pattern has, the higher the face velocity and the higher the NC level. By increasing the free area via making the grille and the duct opening it covers larger, the face velocity is decreased, improving performance.
It is important to know that while the size of the grille and wall opening can be customized, the pattern area is set by given increments based on the pattern itself. The number of vertical and horizontal patterns must be expressed in whole numbers.The KEES sizing chart found in decorative grille product submittals (see Fig 3 below) gives exact dimensions depending on the number of patterns expressed.
Why Size Matters: A Tale of Two Installations
Imagine the same decorative grille pattern is being installed in two locations: an open office plan and a church sanctuary. Considering the appropriate NC level for each setting, what would be the decorative grille and wall opening size?
The pattern chosen is KEES Square Link (SQ125), with a free area of 40%. Additionally, both installations will be moving 500 CFM.
The Office: 12” x 12”
For an open office plan, where workers collaborate in a shared space and total silence is not expected, a reasonable NC level is around 40. What would be an acceptable size for the office’s decorative grilles?
Looking at the chart above from the KEES decorative grille performance data manual, we see that a wall/ceiling opening size of 12”x12” at a face velocity between 500 FPM and 600 FPM would put the noise level between 37 and 43, meeting the needs of the installation.
500 CFM ÷ 500 FPM = 1 sq ft per 144 sq inch, 12” x 12”
500 CFM ÷ (144 sq inch per sq ft ÷ 12” x 12”) = 500 FPM
The Church: 16.25” x 16.25”
In contrast to a busy office, silence is golden in a church sanctuary. A NC level of 25 or less would give churchgoers a quiet place to worship.
Looking at Fig. 2, if we want our grille to meet 25 NC, we cannot exceed 300 FPM. We can use the following equation to determine a baseline for the grille and wall opening size:
500 CFM ÷ 300 FPM = 1.67 sq ft/240 sq in, approximately 15.49” x 15.49”
By converting these dimensions back into square feet and dividing it by the CFM, we can obtain the approximate face velocity to measure against our target NC level:
500 CFM ÷ (144 ÷ 15.49” x 15.49”) = 331 FPM
331 FPM is higher than the target face velocity of 300, thus the grille and wall opening needs to be larger than 15.49” x 15.49”. To determine how much larger, let’s review the pattern area sizing chart from the SQ125 product submittal:
According to this chart, our initial dimension value is between 14.75” (pattern area 10) and 16.25” (pattern area 11). As we are looking to make the grille larger, let’s put 16.25” x 16.25” into the equation and measure the resulting face velocity:
500 CFM ÷ (144 ÷ 16.25” x 16.25”) = 273 FPM
The acceptable size for church sanctuary grilles and openings would be 16.25” x 16.25”, just under the standard size of 18” x 18”. That's 74% larger than the optimal size for the office. When it comes to decorative grilles, size really matters.
Work with KEES to Maximize Decorative Grille Performance
KEES understands that aesthetics and performance go hand-in-hand. That’s why we work with designers and architects to produce decorative grilles that maximize aesthetics and performance. Any of our 120 decorative grille patterns can be sized to fit an installation’s needs. We also offer custom patterns to bring unique visions to life, helping clients add a personal touch to their projects. Let us do the heavy lifting when it comes to determining the appropriate size for your decorative grille—so you can get right to installation.
