Types and design of ventilation

Ventilation is the supply of fresh air to, and removal of stale air from, an enclosed space.

Types of ventilation

1. Natural ventilation: Uses wind pressure and stack (thermal) effect through openings.
   • Wind-driven (cross) ventilation: windows on windward and leeward sides.
   • Stack effect: warm air rises and leaves through high openings (ventilators, ridge, chimney), and cool air enters low.
2. Mechanical (forced) ventilation: Fans and ducts are used.
   • Extract (exhaust) system: fan removes air; used in kitchens, toilets.
   • Plenum / supply system: fan forces fresh air in.
   • Balanced system: both supply and exhaust fans.
3. Combined (mixed mode).

Mechanical ventilation is preferred where natural ventilation cannot give the needed rate: basements, large halls, interior rooms, kitchens, workshops.

Design methodology

1. Find the number of occupants and the use, then the required fresh-air rate (e.g. 25 to 30 m³/h per person) or the air change rate (ACH) from the code.
2. Required flow: $Q = \frac{V \times N}{3600}$ (m³/s), where $V$ is room volume and $N$ is ACH.
3. Natural: Wind flow $Q = E A V$ (m³/s), with $E$ effectiveness (0.5 to 0.6 for wind perpendicular to opening, 0.25 to 0.35 diagonal), $A$ open area, $V$ wind speed (m/s). Stack flow: $Q = C_d A \sqrt{2 g h \frac{T_i - T_o}{T_i}}$.
4. Fix opening areas (generally 15 to 20 % of floor area) and locate inlets low and outlets high, on opposite walls.
5. Mechanical: Choose fan capacity for $Q$, size ducts for 3 to 6 m/s, and select grilles.
6. Check the result against the Nepal National Building Code (NBC 206).