Stack Effect in Building Ventilation

Stack Effect

• The stack effect is a natural process where warmer air rises and cooler air descends, creating air movement in enclosed spaces.
• Stack effect can be used for ventilation, particularly in buildings with chimneys, courtyards, and wind towers.
• Stack pressure (Ps) can be calculated using the formula: Ps = 0.042 × h × ΔT where:
  • Ps is the stack pressure in N/m2
  • h is the height of the stack in meters
  • ΔT is the temperature difference in degrees Celsius

Applications of Stack Effect

• Chimneys: Heated air rises through the chimney, creating negative pressure inside the building, drawing in cooler air.
• Courtyards: During the day, solar radiation heats the courtyard air, which rises, drawing cooler outdoor air into the spaces.
• Wind Towers: In windy conditions, cool night air is drawn into the tower and then forced downwards into living spaces.
• Wind Tower Operation:
  • Day: Hot ambient air cools when it contacts the cool upper part of the tower, becomes denser, sinks, and replaces hot air in the living spaces.
  • Night: The tower walls absorb heat during the day and release it at night, warming the cool night air.
• Wind Tower Considerations:
  • Wind towers may need to be shut off when cooling is not required.
  • Wind towers are typically used in hot and dry climates for cooling purposes.

Wind Shadow and Wind Towers

• Buildings can create wind shadows that affect ventilation.
• The wind shadow created by a building can be reduced by using a gap between the building face and the canopy.
• This gap creates a downward pressure, directing airflow into the living zone.
• Wind towers are commonly used to improve ventilation in low-rise, high-density settlements where wind access is limited.
• Wind towers capture wind above the building and draw it downwards, delivering fresh air.
• The orientation and position of louvers can significantly affect the indoor airflow and the wind shadow.

Cross-Ventilation

• Cross-ventilation optimizes airflow by placing openings strategically to guide air through a building.
• Optimal cross-ventilation is achieved with openings placed on opposite sides of a building.
• To ensure effective air movement, avoid placing openings directly opposite each other.
• Larger openings on the leeward faces of the building compared to the windward faces enhance cross-ventilation.
• Placement of inlets at high-pressure zones and outlet at low-pressure zones improves airflow.
• Obstacles like internal walls and furniture can reduce the speed of airflow through cross-ventilation.
• Partition screens can be used to maintain some airflow even when internal partitions are necessary.

Orientation and Wind Direction

• Buildings oriented at right angles to the wind direction experience the highest pressure and airflow.
• A 45-degree wind incidence reduces pressure by 50% but increases the average indoor air velocity.
• It is important to consider the wind direction and optimize the building's orientation for both wind and solar gains.
• In equatorial regions, a building's north-south orientation is generally preferable for solar exclusion, but often the wind is predominantly easterly.

Explore the stack effect and its applications in building ventilation. Understand how this natural process allows for effective air movement in spaces through chimneys, courtyards, and wind towers. Learn about calculating stack pressure and the impact of temperature differences on air circulation.