Discharge coefficients, free, effective and equivalent areas

https://www.gov.uk/government/publications/classvent-and-classcool-school-ventilation-design-tool

[edit] Introduction

The terms free area, effective area, equivalent area and discharge coefficient are similar, and can at times be poorly, incorrectly or mistakenly calculated. This has the potential to become increasingly significant as the impacts of climate change and the need for adaptation increase.

The Building Regulations Part O comes into force from June the 15th 2022, and refers to all three terms, requiring their consideration in meting the requirements of the approved document Part O.

[edit] Free area

Free area (Afree) is the geometric open area of a ventilation opening. It is quite simply the height and width of the opening multiplied to give the open area. W x H = A or Afree. 'This area assumes a clear sharp-edged orifice that would have a coefficient of discharge (Cd) of 0.62'

[edit] Effective area

Effective area (Aeff) is the area through which air flows after the resistance of airflow has been taken into account. The effective area is the free area described above multiplied by the discharge coefficient (Cd). Afree x Cd = Aeff. The discharge coefficient includes the angle of the opening as one of its factors.

[edit] Equivalent area

Equivalent area (Aeq) is a measure of the aerodynamic performance of an opening. It is the area of a sharp-edged circular orifice through which air would pass at the same volume flow rate, under an identical applied pressure difference, as through the opening under consideration. This is the effective area divided by the orifice discharge coefficient (Cd0) which assumes a clear sharp-edged orifice would have a coefficient of discharge (Cd) of 0.62. or Aeff / Cd0 = Aeq)

[edit] Discharge coefficient

In practice, a constant value of discharge coefficient is often used for simplification, this is basically a ratio of the actual discharge to the ideal discharge. However, a constant value of discharge coefficient can lead to deceptive airflow estimations in different window designs.

[edit] Orifice discharge coefficient

The orifice discharge coefficient (Cd0) is somewhat more accurate and gives a figure for a specific window considering also the slope or angle of plane and other adjustments.

[edit] Recommended tool

The buildings regulations recommend using a tool to assist in assessing these figures when it comes to the simplified method of Part O. The tool is referred to as the 'discharge coefficient calculator ' and was written in 2018 as part of the guidance on school design that formed BB101.

The simple tool can be downloaded here: https://www.gov.uk/government/publications/classvent-and-classcool-school-ventilation-design-tool

[edit] Related articles on Designing Buildings

• Air change rates.
• Approved Document O.
• Approved Document F.
• Background ventilator.
• Cross ventilation.
• Displacement ventilation.
• Equivalent area.
• Natural ventilation.
• Stack effect.
• Thermal comfort.
• Ventilation.
• Whole building ventilation.