4.1. The driving forces of natural ventilation
Three forces can move the air inside buildings:
The first two forces are explained in the following sections. Natural forces drive natural ventilation, while mechanical fans drive mechanical ventilation. Mechanical force can be combined with natural forces in a hybrid, or mixed-mode, ventilation system.
4.1.1. Wind pressure
When wind strikes a building, it induces a positive pressure on the windward face and negative pressure on the leeward face. This drives the air to flow through windward openings into the building to the low-pressure openings at the leeward face (see ). It is possible to estimate the wind pressures for simple buildings. Wind flows around buildings are complex and the subject of a number of textbooks, for example Aynsley, Melbourne & Vickery (1977) and Liu (1991).
Wind-induced flow directions in a building.
For single-sided ventilation with the rooms otherwise hermetically sealed, there is no contribution from mean wind pressures, only from the fluctuating components (see ). Etheridge & Sandberg (1996) covered the topic of unsteady pressures in some detail. This is a common design; however, over time, there becomes significant leakage around doors and other room penetrations. It must be remembered that just because a window is open, sufficient air changes per hour (ACH) may not necessarily be achieved.
Fluctuating components contributing to single-sided airflow.
The wind pressure generated on a building surface is expressed as the pressure difference between the total pressure on the point and the atmospheric static pressure. Wind pressure data can usually be obtained in wind tunnels by using scale models of buildings. If the shape of the building, its surrounding condition and wind direction are the same, the wind pressure is proportional to the square of outdoor wind speed. Thus, the wind pressure is usually standardized by being divided by the dynamic pressure of the outdoor wind speed. The standardized wind pressure is called the wind pressure coefficient and symbolized as Cp. The outdoor wind speed is usually measured at the height of the eave of the building in the wind tunnel:
where:
Cp = wind pressure coefficient (–)
PT = total pressure (Pa)
PAS = atmospheric static pressure at the building height (Pa)
ρ = density of air (kg/m3)
VH = wind velocity at a remote site from surrounding influences at the building height (m/s).
4.1.2. Stack (or buoyancy) pressure
Stack (or buoyancy) pressure is generated from the air temperature or humidity difference (sometimes defined as density difference) between indoor and outdoor air. This difference generates an imbalance in the pressure gradients of the interior and exterior air columns, causing a vertical pressure difference.
When the room air is warmer than the outside air, the room air is less dense and rises. Air enters the building through lower openings and escapes from upper openings.
The flow direction reverses, to a lesser degree, when the room air is colder than the outside air; the room air is denser than the outside air. Air enters the building through the upper openings and escapes through the lower openings.
Stack (or buoyancy) driven flows in a building are driven by indoor and outdoor temperatures. The ventilation rate through a stack is a function of the pressure differential between the two openings of that stack.
Pressure differential can be calculated as follows:
where:
Ps = stack (or buoyancy) pressure (Pa)
ρo = density of outdoor air (kg/m3)
ρi density of indoor air (kg/m3)
g = gravity acceleration (9.8 m/s2)
H = height between two openings (m)
Ti = indoor air temperature (°K)
To = outdoor air temperature (°K)
4.2. Ventilation flow rate
As a rule of thumb, wind-driven natural ventilation rate through a room with two opposite openings (e.g. a window and a door) can be calculated as follows:
Ventilation rate (l/s) = 0.65 × wind speed (m/s) × smallest opening area (m2) × 1000 l/m3
provides estimates of the ACH and ventilation rate due to wind alone, at a wind speed of 1 m/s, assuming a ward of size 7 m (length) × 6 m (width) × 3 m (height), with a window of 1.5 × 2 m2 and a door of 1 m2 × 2 m2 (smallest opening).
Estimated air changes per hour and ventilation rate for a 7 m × 6 m × 3 m ward.
The wind speed refers to the value at the building height at a site sufficiently away from the building without any obstructions (e.g. at an airport).
For stack (or buoyancy) natural ventilation, the ACH can be calculated as:
Advanced design tools for both analysis and opening sizing are also available (CIBSE, 2005).
4.3. Summary
Before designing a purely natural ventilation system, designers need to understand the main driving forces of natural ventilation — wind pressure and stack (or buoyancy) pressure. These forces control how air moves within and through a building, and they can be combined, as needed, to design an optimal natural ventilation system.
