By Norm Grusnick, PEng
Commercial Sales Rep, PACAIRE HVAC Suppliers Ltd.
Grilles, registers, and diffusers (GRD) are designed to enable the efficient distribution of conditioned air in a given space. Each type of GRD product offers unique advantages and disadvantages, making them suitable for specific applications. When it comes to selecting GRD products, engineers carefully evaluate the requirements of the space to ensure optimal thermal comfort for the occupants. ASHRAE defines thermal comfort as “a condition of the mind which expresses satisfaction with the thermal environment.” The primary objective for engineers is to create an environment where most are comfortable. ASHRAE defines a thermally comfortable environment as below:
- perceived temperature: 23-25 degrees C
- relative humidity: 30-60%
- air velocity: Less than 50 feet per minute
GRD manufacturers publish performance data to provide engineers with the information they need to correctly select. The airflow quantity is measured in cubic feet per minute (CFM). Typically, the engineer would have a performance in mind, so they would use the performance tables to determine the proper size. For GRD products, three important metrics are throw distance, static pressure, and noise criteria.
Throw distance
First, the throw distance is an important measure of how far the air will travel. Engineers aim to design a space with the most effective air distribution while maximizing occupant comfort. The average person cannot feel any air movement with a velocity of less than 50 feet per minute (FPM), thus this is the threshold throw goal. The throw distance is influenced by a couple of important factors, including the initial velocity and the temperature difference between conditioned air and average air temperature. GRD manufacturers provide throw distance data to assist designers in understanding how far the air will travel before its velocity drops to the desired threshold. Many manufacturers list at 150,100, and 50 FPM velocities. By examining the throw distance data provided by manufacturers, engineers can determine the suitable placement and spacing of GRD products to obtain the desired air distribution pattern. It’s important to note that throw distance is not the only factor. Other variables are layout, shape and design of the diffuser, and specific requirements of the project.
Static pressure
The static pressure is a critical factor to consider when evaluating the performance of GRD products. It refers to the resistance that the airflow encounters as it passes through the grille, register, or diffuser. Pressure drop affects the overall system efficiency. The pressure drop is typically measured in inches of water gauge (in. wg). It is important to accurately estimate the total pressure drops in the system to size out the proper fan. Most GRD manufacturers publish pressure data for their products. This data is typically presented in the form of a table that shows the pressure drop across the product at various airflow rates. By selecting a GRD with the appropriate pressure drop, designers can ensure that the HVAC system operates effectively.
Noise criteria
Noise criteria, often abbreviated as NC, is a measure of the acoustic performance of GRD products. It quantifies the level of noise generated by the airflow passing through the grille, register, or diffuser. Noise level is an essential consideration, as excessive noise can significantly impact occupant comfort and productivity. Manufacturers conduct laboratory tests and measurements to determine the noise criteria for their products for a range of airflows. Noise criteria is typically expressed as a numerical value with lower values indicating quieter operation. Engineers need to consider the noise criteria when selecting GRD products to ensure that the resulting noise levels are within acceptable limits for the intended space.
Specifying GRD products
Now that we have discussed the metrics of throw distance, static pressure, and noise criteria it is crucial to understand the importance of considering these metrics in the selection of GRD products. By taking all three of these performance metrics into account, engineers can make informed decisions that ensure optimal performance, energy efficiency, and occupant comfort.■
