Ventilation System Design
Learning Objectives
- Calculate ventilation requirements using ASHRAE 62.1 methodology
- Design demand-controlled ventilation systems using CO2 sensors
- Analyze energy recovery ventilation (ERV/HRV) systems
- Apply computational fluid dynamics concepts to air distribution
- Balance ventilation, filtration, and energy in system design
Ventilation Fundamentals
ASHRAE 62.1 Ventilation Rate Procedure
Vbz = Rp x Pz + Ra x Az
- Vbz: Breathing zone outdoor airflow (CFM)
- Rp: Outdoor air rate per person (CFM/person)
- Pz: Zone population
- Ra: Outdoor air rate per unit area (CFM/sq ft)
- Az: Zone floor area (sq ft)
For classrooms: Rp = 10 CFM/person, Ra = 0.12 CFM/sq ft
Example: Classroom Ventilation Calculation
Given: Classroom with 30 students, 1 teacher, 900 sq ft
Calculate:
Vbz = (10 CFM/person x 31 people) + (0.12 CFM/sq ft x 900 sq ft)
Vbz = 310 + 108 = 418 CFM outdoor air required
Convert to ACH:
Room volume = 900 x 10 ft ceiling = 9000 cu ft
ACH = (418 x 60) / 9000 = 2.8 outdoor air changes per hour
Note: This is minimum outdoor air. Total supply air is typically 4-6x higher to maintain temperature and air distribution.
Demand-Controlled Ventilation (DCV)
DCV adjusts outdoor air based on actual occupancy using CO2 as a proxy:
CO2-Based Control Logic
Required outdoor air per person = 15 CFM / ((CO2indoor - CO2outdoor) / 700)
- Setpoint: Typically 800-1000 ppm CO2
- Control: Modulate outdoor air damper or fan speed
- Benefits: 20-40% energy savings vs. fixed ventilation
- Limitation: CO2 indicates occupancy, not all pollutants
Design consideration: Ensure minimum ventilation even when CO2 is low to address non-occupant-related pollutants.
Energy Recovery Ventilation
Heat Recovery Ventilator (HRV)
- Transfers sensible heat (temperature)
- 60-80% heat recovery efficiency
- Best for heating-dominated climates
- No moisture transfer
Energy Recovery Ventilator (ERV)
- Transfers heat and moisture (enthalpy)
- 50-70% total energy recovery
- Better for humid climates
- Reduces latent cooling load
Energy Savings Calculation
Qsaved = V x rho x cp x eta x delta_T
Where eta = recovery efficiency, delta_T = temperature difference between indoor and outdoor
Air Distribution Effectiveness
How air is distributed affects both comfort and contaminant removal:
| Distribution Type | Ez | Description |
|---|---|---|
| Ceiling supply, ceiling return | 1.0 | Mixed air, average effectiveness |
| Ceiling supply, floor return | 1.0 | Downward displacement |
| Floor supply, ceiling return | 1.2 | Displacement ventilation - better |
| Personal ventilation | 1.5-2.0 | Clean air direct to breathing zone |
Displacement ventilation: Cool air supplied at floor rises as it warms, carrying contaminants up and out. More effective for the same airflow rate.
Activity: Ventilation System Design
Design challenge: Design a ventilation system for a 1200 sq ft science classroom with:
- Maximum occupancy: 32 students + 1 teacher
- Variable occupancy (0-100%)
- Internal heat gains from equipment
- Climate: heating-dominated (5000 HDD)
- Calculate minimum outdoor air per ASHRAE 62.1
- Design a CO2-based DCV system with appropriate setpoints
- Select HRV or ERV and justify choice
- Estimate annual energy savings from energy recovery
- Specify supply diffuser locations for good air distribution
- Include in-duct filtration (specify MERV rating)
Deliverable: System schematic with component specifications and calculations.
Key Takeaway
Ventilation system design balances air quality, comfort, and energy. ASHRAE 62.1 provides the framework for determining outdoor air requirements. Demand-controlled ventilation optimizes energy use while maintaining air quality. Energy recovery ventilation captures waste heat and reduces operating costs. Good air distribution ensures clean air reaches the breathing zone where it matters most.