Fundamentals of Ventilation

1. What is Ventilation?

Ventilation is the process of supplying fresh outdoor air into an indoor space and removing stale indoor air, in order to maintain:

• Indoor Air Quality (IAQ)
• Thermal comfort
• Health & safety of occupants

In HVAC design, ventilation is provided using systems such as FAHUs, AHUs, exhaust fans, and natural ventilation methods.

2. Purposes of Ventilation

• Dilution of contaminants: Removes CO₂, VOCs, odors, and smoke.
• Oxygen replenishment: Maintains breathable air quality.
• Moisture control: Helps prevent mold growth and condensation.
• Temperature and humidity regulation: Works with HVAC systems to condition air.
• Pressurization: Maintains positive/negative pressure in sensitive areas (hospitals, labs).

3. Types of Ventilation

🔹 a. Natural Ventilation

• Achieved by openings such as windows, vents, or louvers.
• Driven by wind pressure and thermal buoyancy.
• Limitations: uncontrollable, climate-dependent, poor in high-rise or sealed buildings.

🔹 b. Mechanical Ventilation

• Driven by fans and duct systems.
• Types:
  • Exhaust ventilation → removes stale air (kitchens, toilets).
  • Supply ventilation → provides filtered, conditioned outdoor air (FAHU).
  • Balanced ventilation → supply + exhaust with heat recovery (ERV/HRV).

4. Ventilation Air Components in HVAC

• Fresh Air (Outdoor Air): Clean air drawn from outside.
• Return Air: Air taken back from occupied spaces.
• Exhaust Air: Contaminated air discharged outdoors.
• Relief Air: Excess air vented to balance indoor pressure.

5. Ventilation Requirements & Standards

Ventilation rates depend on occupancy type, density, and building codes.

• ASHRAE 62.1 – Ventilation for Acceptable IAQ: Defines minimum outdoor air per person and per area.
• WHO & Local Codes: Often stricter for healthcare and industrial spaces.

🔹 Rule of Thumb:

• Offices: 8–10 L/s per person (≈15–20 CFM/person)
• Hospitals: 12–15 ACH (Air Changes per Hour) for patient rooms
• Labs: 6–12 ACH depending on use

6. Psychrometric Process of Ventilation

When outdoor air enters a building:

1. It is filtered to remove dust and contaminants.
2. It passes through cooling/heating coils to adjust temperature.
3. It may undergo dehumidification or humidification.
4. It is distributed via ductwork into occupied spaces.

The psychrometric chart helps visualize air properties (dry-bulb temperature, wet-bulb temperature, humidity ratio, enthalpy) during these processes.

7. Pressurization through Ventilation

• Positive Pressure: Supplying more air than exhausted → prevents infiltration of hot/humid/polluted air (offices, malls, clean zones).
• Negative Pressure: Exhausting more air than supplied → isolates contaminants (toilets, isolation rooms, labs).

8. Energy & Ventilation

• Ventilation consumes significant energy because outdoor air must be fully conditioned.
• Solutions to improve efficiency:
  • Energy Recovery Ventilators (ERV/HRV)
  • Demand-Controlled Ventilation (DCV) using CO₂ sensors
  • Variable Air Volume (VAV) systems

9. Practical Example

• Office Building: FAHU supplies 20% outdoor air → mixed with 80% return air in AHU → delivered to occupied zones.
• Hospital Operating Room: FAHU provides 100% outdoor air → passes through HEPA filters → maintains positive pressure relative to surrounding corridors.