Understanding Geometric Free Area vs. Aerodynamic Free Area in Smoke Vents

Here’s a quick guide to the essential terms:

• Geometric Area: The actual physical opening in the roof or wall, measured perpendicular to the airflow, without considering any installed ventilation products.
• Coefficient of Discharge (Cv): A measure of a smoke vent’s efficiency, determined through rigorous testing.
• Aerodynamic Free Area: The effective area for smoke extraction, calculated by multiplying the Geometric Area by the Coefficient of Discharge.
• Free Area: The unobstructed area of the vent, often referenced in building regulations like Approved Document B.

But which of these matters most for your project? And how do they impact building safety and compliance? Let’s dive into the details and clear the smoke on this critical aspect of building design.

When specifying smoke ventilation systems, the distinction between geometric area and aerodynamic area is paramount. This knowledge ensures regulatory compliance and guarantees effective performance in the event of a fire, potentially saving lives. Let’s explore these concepts in depth to help you make informed decisions for your building projects.

Official Guidelines on Free Area of Smoke Ventilators

According to the Approved Document B (fire safety) volume 2: Buildings other than dwellings (2019 edition incorporating 2020 and 2022 amendments), specifically on page 171, the free area of a smoke ventilator should be measured by either of the following methods:

1. The declared aerodynamic free area in accordance with BS EN 12101-2.
2. The total unobstructed cross-sectional area (geometric free area), measured in the plane where the area is at a minimum and at right angles to the direction of air flow.

Diagram D7: Free area of smoke ventilators (Source: Approved Document B)

What is Geometric Area?

The geometric area, also known as Geometric Free Area (Gfa), refers to the physical size of the opening provided by a smoke vent when it is fully open. It is calculated by multiplying the width and the length of the opening (e.g., in square meters). This measurement does not account for any obstructions or the efficiency of airflow through the vent.

For example, a smoke vent with an opening size of 1.26m x 1.26m has a geometric area of:

Geometric Area = Width x Length = 1.26m x 1.26m = 1.5876 m²

What is Aerodynamic Area?

The aerodynamic area, also known as Aerodynamic Free Area (Afa), represents the effective area through which smoke can be extracted. It is typically larger than the geometric area and is determined under dynamic test conditions as specified in BS EN 12101-2. The aerodynamic area is calculated by multiplying the geometric area by a coefficient factor, which accounts for the vent’s design, opening angle, and the presence of any wind deflectors or obstructions.

Aerodynamic Area = Geometric Area x Coefficient Factor

The coefficient factor is determined through rigorous testing and typically ranges between 0 and 1, representing the efficiency of the vent in allowing air to pass through. A higher coefficient means better airflow efficiency.

Application in Building Design and Regulations

The use of geometric area or aerodynamic area in building design depends on the specific approach and regulations being followed:

• Geometric Area (Gfa): This is typically used in buildings designed according to prescriptive guidance, such as Approved Document B in the UK. In most standard cases, geometric area is the referenced measurement for smoke ventilation requirements.
• Aerodynamic Area (Afa): This is usually employed in fire-engineered buildings, where consultants design the structure with specific fire suppression systems and fire safety systems. When a smoke vent has been specified by a fire engineer, it will usually be given as an Aerodynamic Free Area (Afa).

It’s important to note that when smoke ventilation is calculated as Aerodynamic Free Area (Afa), the corresponding Geometric Free Area (Gfa) will be smaller. This is because the Afa takes into account the vent’s efficiency in smoke extraction, which is typically higher than its simple geometric opening.

Why is the Difference Important?

Understanding the distinction between geometric and aerodynamic areas is crucial for several reasons:

1. Compliance: Building regulations may specify requirements in terms of either geometric or aerodynamic area, depending on the design approach and local codes.
2. Performance: Aerodynamic area provides a more accurate representation of a vent’s smoke extraction capability, which is critical in fire safety design.
3. Specification: When selecting smoke vents, it’s essential to know whether the required area is geometric or aerodynamic to ensure the correct product is chosen.

Simply meeting the geometric area requirement may not suffice if the building design requires a specific aerodynamic area for effective smoke extraction. Therefore, specifying a vent that meets or exceeds the required area (whether geometric or aerodynamic) is essential for compliance and safety.

SRHP/AOV 140° and SRHE-AOV Smoke Vent Specifications

Our smoke vents are designed to provide both smoke ventilation and roof access. Below are the key specifications relevant to understanding their geometric and aerodynamic areas:

Note: The SRHP-AOV can be manufactured up to a maximum size of 1550 x 1050 mm. Sizes marked with * are only available in the SRHE-AOV (Bespoke Smoke Vent) model. Aerodynamic Area values are based on official test results with wind deflectors installed, as per FIRES-AT-001-14-AUNE test report.

For specific details on larger sizes or custom requirements, please contact our sales team. We can assist you in selecting the most appropriate model and size for your specific project needs.

Key Features of SRHP/AOV 140° Smoke Vent

• Thermal Performance: 50mm (2″) fully insulated kerb and lid, corner welded and reinforced, providing a U-value of 0.43 W/m²K with polyisocyanurate insulation.
• Upstand: 300mm (12″) in height with external curb liner, thermal break, and insulation options.
• Operation: 24V DC/4A Actuator tested for 10,000 openings and 1,000 comfort ventilation cycles according to EN12101-2.
• Wind Deflectors: Aluminum wind deflectors included to enhance aerodynamic performance.
• Finish: Mill finish aluminum with powder coating available as an optional extra.
• Hinge: Heavy-duty continuous hinge for reliable operation.
• Seal: Double unbroken bubble & blade seal to lid for improved insulation and weather resistance.

Selecting the Right Size for Your Project

When specifying smoke vents for your project, consider the following:

1. Building Regulations: Ensure compliance with local building codes and regulations regarding minimum free area requirements, whether geometric or aerodynamic.
2. Project-Specific Needs: Consider factors such as the building’s size, layout, and expected occupancy when determining the required ventilation capacity.
3. Installation Location: The SRHP/AOV 140° can be installed on roofs with pitches up to 15°, offering flexibility in placement.
4. Dual Functionality: This smoke vent also provides roof access, which may be beneficial for maintenance or other purposes.

Compliance and Certification

The SRHP/AOV 140° smoke vent is tested and certified according to the EN12101-2 standard, ensuring reliability and performance in smoke ventilation applications. This certification involves rigorous testing of various performance characteristics, including:

• Aerodynamic free area
• Reliability (opening cycles)
• Performance under low ambient temperatures
• Wind load resistance
• Snow load capacity
• Resistance to heat

By choosing a certified product like the SRHP/AOV 140°, you can be confident in its ability to perform as required in emergency situations.

Summary

Understanding the difference between geometric and aerodynamic areas is essential when specifying smoke vents. The SRHP/AOV 140° smoke vent offers a range of sizes to meet various project requirements, combining effective smoke ventilation with thermal performance and durability. By carefully considering your project’s needs, local regulations, and the specific free area requirements (geometric or aerodynamic), you can select the appropriate size and configuration to ensure both compliance and optimal safety for building occupants.

For more information or assistance with specifications, please contact our sales team:

• Email: [email protected]
• Phone: +44 (0) 1922 711185