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Grilles (Diffusers, Terminals)

Grilles are key components in the HVAC systems that manage airflow into or out of a space. 

Their results include Flow Rate, Pressure, and Pressure Drop, which can be reviewed and edited in the properties of each Grille.

Flow Rate

  • Represents the airflow passing through the Grille, either into or out of a space.

  • The Flow Rate is directly specified in the properties of the Grille.

    • Note, the Flow Rate on the system may be higher than what is in the Diffusers if Increase Flow Rate Based on the Heat Load ? is selected as Yes in the Heating tab of the Air Handling Unit properties. This increases the Flow Rate if more flow is required to heat the room.

  • Navigate to the properties of the Grille to verify and adjust the flow rate based on system design or room requirements.

Pressure

  • This value reflects the cumulative pressure drop along the ductwork, Fittings, Valves, Dampers, and other components, leading to or from the AHU.

To troubleshoot or adjust the Pressure: refer to related sections of the knowledge base for Ducts, Fittings & components affecting system pressure.

Pressure Drop

  • This represents the resistance to airflow through the Grille.

  • The Pressure Drop value is specified in the properties of the Grille and is determined by the zeta Coefficient or a fixed pressure drop you have inputted.

  • Check the Properties of the Grille to ensure the pressure drop value aligns with the design and system requirements. Adjust the zeta value or fixed Pressure Drop as needed.

Pressure Drop Formula (zeta Coefficient):

Ξ”P = ΞΆ Γ— (ρ Γ— vΒ²) / 2

Where:

Ξ”P: Pressure drop (Pa or in Hβ‚‚O)

ΞΆ: Zeta coefficient (dimensionless, input in the properties; higher ΞΆ values result in higher pressure drops, lower ΞΆ values result in lower pressure drops)

ρ: Air density (kg/m³ or lb/ft³)

v: Velocity of air (m/s or ft/s)

A higher zeta (ΞΆ) value increases the pressure drop across a component. This is because the zeta coefficient represents the resistance to airflow, so a higher value indicates greater resistance, resulting in more energy loss and a larger pressure drop. Conversely, a lower zeta value indicates less resistance and a smaller pressure drop.