Exhaust Systems Design and Implementation

Exhaust systems remove contaminated air, heat, moisture, and odors from building spaces, maintaining acceptable indoor air quality and occupant comfort. Proper design requires understanding airflow physics, contaminant characteristics, and system performance parameters.

Types of Exhaust Systems

General Exhaust Ventilation

General exhaust systems dilute and remove airborne contaminants uniformly distributed throughout a space. These systems rely on room air mixing to transport contaminants to exhaust points.

Applications:

Office buildings and commercial spaces
Restrooms and locker rooms
Storage areas and mechanical rooms
Light manufacturing facilities

General exhaust airflow requirements follow ASHRAE 62.1 minimum ventilation rates based on occupancy and floor area:

$$V_{\text{exhaust}} = R_p \times P + R_a \times A$$

Where:

$V_{\text{exhaust}}$ = required exhaust airflow (cfm)
$R_p$ = people outdoor air rate (cfm/person)
$P$ = design occupant density (people)
$R_a$ = area outdoor air rate (cfm/ft²)
$A$ = floor area (ft²)

Spot Exhaust Systems

Spot exhaust captures contaminants at or near their source before dispersal into the general space. This approach provides higher efficiency than general exhaust for localized sources.

Common Applications:

Kitchen range hoods
Laboratory fume hoods
Welding stations
Printing operations
Soldering benches

Capture velocity determines the required exhaust flow rate based on hood configuration and contaminant characteristics:

$$Q = V_c \times A_{\text{face}}$$

Where:

$Q$ = exhaust airflow (cfm)
$V_c$ = capture velocity (fpm)
$A_{\text{face}}$ = hood face area (ft²)

Typical capture velocities range from 50-200 fpm for canopy hoods to 100-500 fpm for lateral exhaust hoods, depending on contaminant thermal characteristics and generation rate.

Process Exhaust Systems

Process exhaust systems handle industrial contaminants, hazardous materials, or high-temperature waste streams. These systems require specialized design to manage corrosive gases, explosive dusts, or toxic substances.

Design Considerations:

Material compatibility with contaminants
Explosion-proof construction when required
Minimum transport velocities for particulates
Spark-resistant construction for combustible dusts
Corrosion-resistant materials for chemical processes

Exhaust System Configuration

graph TB
    subgraph "General Exhaust System"
    A[Room Space] --> B[Exhaust Grilles]
    B --> C[Duct Trunk]
    C --> D[Exhaust Fan]
    D --> E[Discharge Stack]
    end

    subgraph "Spot Exhaust System"
    F[Contaminant Source] --> G[Capture Hood]
    G --> H[Duct Branch]
    H --> I[Main Duct]
    I --> J[Fan]
    J --> K[Stack/Filter]
    end

    subgraph "Multiple Source System"
    L[Source 1] --> M[Hood 1]
    N[Source 2] --> O[Hood 2]
    P[Source 3] --> Q[Hood 3]
    M --> R[Branch 1]
    O --> R
    Q --> R
    R --> S[Main Duct]
    S --> T[Exhaust Fan]
    T --> U[Discharge]
    end

Ductwork Sizing and Pressure Loss

Exhaust ductwork sizing balances initial cost against operating efficiency. Undersized ducts increase pressure loss and energy consumption; oversized ducts waste material and space.

Velocity Selection

Recommended duct velocities depend on system type and transported materials:

Application	Velocity Range (fpm)
General exhaust	1,200 - 1,800
Kitchen grease-laden air	1,500 - 2,000
Light dust	2,500 - 3,000
Heavy dust	3,500 - 4,500
Abrasive materials	4,500 - 5,500

Pressure Loss Calculation

Total system pressure loss includes friction losses in straight duct, fitting losses, and exit losses:

$$\Delta P_{\text{total}} = \Delta P_{\text{friction}} + \Delta P_{\text{fittings}} + \Delta P_{\text{exit}}$$

Friction loss in straight duct:

$$\Delta P_{\text{friction}} = f \times \frac{L}{D} \times \frac{\rho V^2}{2}$$

Where:

$f$ = friction factor (dimensionless)
$L$ = duct length (ft)
$D$ = duct diameter (ft)
$\rho$ = air density (lbm/ft³)
$V$ = air velocity (ft/s)

Fitting losses use loss coefficients:

$$\Delta P_{\text{fitting}} = C \times \frac{\rho V^2}{2}$$

Where $C$ is the fitting loss coefficient from ASHRAE Fundamentals.

Fan Selection

graph LR
    A[Calculate System Airflow] --> B[Determine Total Pressure Loss]
    B --> C[Select Fan Type]
    C --> D[Centrifugal Fan]
    C --> E[Axial Fan]
    D --> F[Backward Inclined]
    D --> G[Forward Curved]
    D --> H[Radial Blade]
    E --> I[Vane Axial]
    E --> J[Tube Axial]
    F --> K[Review Fan Curve]
    G --> K
    H --> K
    I --> K
    J --> K
    K --> L[Verify Operating Point]
    L --> M[Check Motor HP]

Fan Static Pressure

The fan must overcome total system pressure at design flow:

$$SP_{\text{fan}} = \Delta P_{\text{total}} + VP_{\text{discharge}}$$

Where:

$SP_{\text{fan}}$ = fan static pressure (in. w.g.)
$VP_{\text{discharge}}$ = velocity pressure at discharge (in. w.g.)

Fan Power Requirement

Brake horsepower determines motor sizing:

$$\text{BHP} = \frac{Q \times SP_{\text{fan}}}{6356 \times \eta_{\text{fan}}}$$

Where:

$\text{BHP}$ = brake horsepower
$Q$ = airflow (cfm)
$SP_{\text{fan}}$ = fan static pressure (in. w.g.)
$\eta_{\text{fan}}$ = fan total efficiency (decimal)

Select motor horsepower 10-15% above calculated BHP to account for system variations and belt drive losses.

System Balancing

Exhaust systems require balancing to achieve design airflows at each exhaust point:

Measure actual flows at each terminal using flow hood or pitot traverse
Calculate adjustment based on flow-pressure relationship: $Q_2/Q_1 = \sqrt{P_2/P_1}$
Adjust dampers at branches to redistribute airflow
Verify fan performance against manufacturer curves
Document final settings and flows for maintenance reference

Design Standards and Guidelines

ASHRAE 62.1: Ventilation for Acceptable Indoor Air Quality
ACGIH Industrial Ventilation Manual: Hood design and capture velocities
NFPA 91: Exhaust Systems for Air Conveying of Vapors, Gases, Mists, and Particulate Solids
IMC Chapter 5: Exhaust Systems

Critical Design Parameters

Material Selection:

Galvanized steel for general exhaust
Stainless steel for corrosive environments
PVC or FRP for chemical exhaust
Aluminum for abrasive materials (non-sparking)

Discharge Location:

Minimum 10 ft above roof level
10 ft from air intakes
Consider prevailing winds and building wake effects
Rain caps for outdoor terminations

Controls:

Variable speed drives for energy optimization
Interlocked with supply air systems
Pressure monitoring for system verification
Alarm for inadequate exhaust flow

Proper exhaust system design integrates airflow calculations, material selection, fan performance, and control strategies to maintain safe, comfortable indoor environments while minimizing energy consumption.