Ventilation Systems: Design and Performance

Fundamental Principles

Ventilation systems provide outdoor air to occupied spaces to dilute and remove contaminants while maintaining acceptable indoor air quality. The primary functions include:

• Contaminant Dilution: Reducing concentrations of occupant-generated pollutants (CO₂, bioeffluents, VOCs)
• Moisture Control: Managing latent loads from occupants and processes
• Odor Management: Maintaining acceptable sensory conditions
• Pressurization Control: Preventing infiltration and managing airflow patterns between zones

The dilution principle follows mass balance:

$$C = \frac{G}{Q} + C_o \left(1 - e^{-Qt/V}\right)$$

Where:

• $C$ = indoor contaminant concentration (ppm or mg/m³)
• $G$ = contaminant generation rate (mass/time)
• $Q$ = outdoor air flow rate (volume/time)
• $C_o$ = outdoor contaminant concentration
• $V$ = space volume
• $t$ = time

At steady-state equilibrium:

$$C_{ss} = \frac{G}{Q} + C_o$$

This relationship demonstrates that doubling outdoor airflow reduces the steady-state concentration increment by half.

Ventilation Rate Determination

ASHRAE Standard 62.1 establishes minimum ventilation rates using the Ventilation Rate Procedure (VRP):

$$V_{oz} = R_p \times P_z + R_a \times A_z$$

Where:

• $V_{oz}$ = outdoor air requirement for zone (cfm)
• $R_p$ = people outdoor air rate (cfm/person)
• $P_z$ = zone population
• $R_a$ = area outdoor air rate (cfm/ft²)
• $A_z$ = zone floor area (ft²)

For multiple-zone systems, the system outdoor air intake must account for the fraction of outdoor air in the supply:

$$V_{ot} = \frac{\sum_{all\ zones} V_{oz}}{E_v}$$

Where $E_v$ is the system ventilation efficiency, ranging from 0.6 to 1.0 depending on system configuration.

Ventilation System Categories

Mechanical Ventilation Systems

Constant Volume Systems Supply fixed airflow rates regardless of load variations. Outdoor air is typically controlled by dampers maintaining minimum position or CO₂ concentration.

Variable Air Volume (VAV) Systems Modulate supply airflow based on thermal load. Outdoor air control must compensate for varying supply rates:

$$%OA_{min} = \frac{V_{ot}}{V_{supply}}$$

As supply airflow decreases during part-load, the outdoor air damper opens to maintain required ventilation rate.

Dedicated Outdoor Air Systems (DOAS) Decouple ventilation from thermal conditioning. The DOAS handles 100% outdoor air, pre-conditioning it before delivery. Parallel terminal units manage sensible loads independently.

Natural Ventilation

Utilizes driving forces from wind and temperature differences (stack effect). Effective opening area required:

$$A_{eff} = \frac{Q}{\sqrt{2 \times \frac{\Delta P}{\rho}}}$$

Where $\Delta P$ represents wind or buoyancy-driven pressure difference.

Ventilation System Comparison

Air Change Rate Method

An alternative approach expresses ventilation as air changes per hour (ACH):

$$ACH = \frac{Q \times 60}{V}$$

Converting to outdoor air rate:

$$Q = \frac{ACH \times V}{60}$$

Typical requirements:

• Residential: 0.35 ACH minimum (ASHRAE 62.2)
• Offices: 4-6 ACH total, 15-20% outdoor air
• Laboratories: 6-12 ACH minimum, often 100% outdoor air
• Healthcare isolation rooms: 12 ACH with negative pressure

Ventilation Effectiveness

Not all ventilation air equally reaches the breathing zone. Ventilation effectiveness ($E_v$) accounts for short-circuiting and stratification:

$$E_v = \frac{C_e - C_s}{C_r - C_s}$$

Where:

• $C_e$ = exhaust contaminant concentration
• $C_r$ = breathing zone concentration
• $C_s$ = supply air concentration

Perfect mixing yields $E_v$ = 1.0. Displacement ventilation can achieve $E_v$ > 1.0, while poor distribution results in $E_v$ < 1.0.

Energy Considerations

Ventilation represents a significant energy load:

$$Q_{sensible} = 1.08 \times Q \times (T_o - T_s)$$

$$Q_{latent} = 4840 \times Q \times (W_o - W_s)$$

Where $Q$ is in cfm, temperatures in °F, and humidity ratios in lb_w/lb_da.

Annual ventilation energy in climates with heating and cooling seasons often exceeds 30% of total HVAC energy consumption. Energy recovery systems can reduce this by 50-80% depending on climate.

Pressurization Strategy

Building pressure relationships control contaminant migration:

• Positive Pressure: Supply > exhaust, prevents infiltration
• Negative Pressure: Exhaust > supply, contains contaminants
• Neutral Pressure: Balanced flows, minimal driving force

Pressure differential typically ranges from 0.02 to 0.05 in. w.c. between zones requiring separation.

System Integration

Modern ventilation systems integrate with:

• Building automation: Scheduling, reset strategies, demand control
• Energy management: Economizer cycles, heat recovery optimization
• Indoor air quality monitoring: CO₂, VOC, particulate sensors
• Life safety: Smoke control, emergency ventilation modes

flowchart TD
    A[Outdoor Air Intake] --> B[Filtration]
    B --> C{Energy Recovery?}
    C -->|Yes| D[ERV/HRV]
    C -->|No| E[Mixing Plenum]
    D --> E
    E --> F[Heating/Cooling Coils]
    F --> G[Supply Fan]
    G --> H[Distribution Ductwork]
    H --> I[Terminal Devices]
    I --> J[Occupied Zones]
    J --> K[Return Air Path]
    K --> L{Economizer Mode?}
    L -->|Yes| E
    L -->|No| M[Exhaust/Relief]

    style A fill:#e1f5ff
    style J fill:#fff4e1
    style D fill:#e8f5e9

Design Best Practices

1. Size outdoor air intakes for peak ventilation requirements with adequate free area (face velocity < 500 fpm)
2. Locate intakes minimum 25 ft from exhaust outlets, loading docks, and other contamination sources
3. Provide adequate mixing between outdoor and return air (minimum 5 ft mixing section)
4. Install airflow measurement at outdoor air intakes for commissioning and verification
5. Design for turndown in VAV systems maintaining minimum ventilation at all supply rates
6. Consider bypass for energy recovery during mild conditions when outdoor air is suitable for free cooling
7. Implement demand control only where occupancy varies significantly and unpredictably

These principles establish the foundation for effective ventilation system design that balances indoor air quality requirements with energy efficiency across all operating conditions.

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