Dilution Ventilation for Solvent Vapor Control

Dilution Ventilation Fundamentals

Dilution ventilation provides the primary safety mechanism in printing facilities by maintaining solvent vapor concentrations below 25% of the Lower Explosive Limit (LEL) throughout occupied spaces. Unlike local exhaust ventilation which captures emissions at source, dilution ventilation uses large volumes of fresh air to reduce vapor concentrations to safe levels through mixing and displacement.

The fundamental principle derives from steady-state mass balance: solvent vapor generation rate equals removal rate through ventilation. This creates the basis for all dilution ventilation calculations in printing operations handling toluene, MEK (methyl ethyl ketone), acetone, ethyl acetate, isopropyl alcohol, and other volatile organic compounds.

OSHA 29 CFR 1910.106 establishes mandatory requirements for ventilation where Class I flammable liquids are handled, while ACGIH Industrial Ventilation Manual provides engineering guidance on airflow calculations and safety factors. NFPA 86 specifically addresses ovens and dryers processing flammable materials.

Mass Balance Equation Development

Basic Dilution Equation

The steady-state mass balance for a well-mixed space yields:

$$\dot{m}{generation} = \dot{m}{removal}$$

Expanding both sides:

$$G = Q \times \rho_{air} \times C_{target}$$

Where:

• $G$ = Solvent evaporation rate (lb/min)
• $Q$ = Ventilation airflow (ft³/min)
• $\rho_{air}$ = Air density (lb/ft³), typically 0.075 lb/ft³ at standard conditions
• $C_{target}$ = Target concentration (lb solvent/lb air)

Conversion to Volumetric Concentration

Using the ideal gas law at standard conditions (530°R, 1 atm), convert mass concentration to volumetric parts per million:

$$PV = nRT$$

For one mole of ideal gas at 70°F (530°R):

$$V = \frac{nRT}{P} = \frac{1 \times 1545 \times 530}{14.7 \times 144} = 387 \text{ ft}^3$$

This yields the universal constant 387 ft³ per lb-mole at standard conditions.

Converting the mass balance equation to volumetric concentration:

$$Q = \frac{G \times 387 \times T}{MW \times C_{ppm} \times 10^{-6}}$$

Where:

• $T$ = Absolute temperature (°R)
• $MW$ = Molecular weight of solvent (g/mol)
• $C_{ppm}$ = Target concentration (parts per million by volume)

Simplified Design Equation

For room temperature applications (530°R), the equation simplifies:

$$Q_{dilution} = \frac{403 \times G}{C_{ppm}}$$

Where:

• $Q_{dilution}$ = Required dilution airflow (cfm)
• $G$ = Solvent evaporation rate (lb/min)
• $C_{ppm}$ = Target concentration (ppm)

Factor 403 = 387 × 530 / (MW × specific gravity conversion factors).

LEL-Based Design Criteria

Lower Explosive Limit Physics

Flammable vapors ignite only when vapor-to-air ratio falls within the flammable range between LEL and UEL (Upper Explosive Limit). Below LEL, insufficient fuel exists for combustion propagation. Above UEL, insufficient oxygen exists for combustion.

Common printing solvents LEL data:

Critical observation: Lower LEL values (heptane at 1.05%, toluene at 1.2%) require greater dilution ventilation for equivalent safety margin.

Safety Factor Selection

Industry standards mandate maintaining concentrations well below LEL to provide safety margin for:

Equipment limitations:

• LEL sensor accuracy: ±10-15% of reading
• Response time lag: 10-30 seconds from concentration change to alarm
• Calibration drift between maintenance cycles

Operational factors:

• Non-uniform concentration distribution (dead zones, stratification)
• Transient emission spikes during startup, cleaning, spills
• Simultaneous failure scenarios (ventilation reduction + emission increase)

OSHA and NFPA requirements:

OSHA 29 CFR 1910.106(e)(2)(iii) requires ventilation maintaining vapor concentration below 25% of LEL in areas handling Class I flammable liquids.

NFPA 86 Section 5.7.2 requires continuous monitoring with automatic shutdown at 25% LEL for ovens and dryers processing flammable materials.

Conservative design criteria:

$$C_{target} = \frac{LEL}{SF}$$

Where $SF$ = Safety factor:

• $SF = 4$ (25% LEL): Minimum for OSHA/NFPA compliance
• $SF = 10$ (10% LEL): Recommended for higher hazard operations
• $SF = 20$ (5% LEL): Used in critical areas with ignition sources

Design Concentration Calculation

For toluene with LEL = 1.2% = 12,000 ppm, applying 25% LEL criterion:

$$C_{target} = \frac{12,000}{4} = 3,000 \text{ ppm}$$

Most facilities design for additional conservatism:

$$C_{design} = \frac{12,000}{8} = 1,500 \text{ ppm}$$

This provides 12.5% LEL operating point, allowing concentration to double during upset conditions while remaining below 25% LEL regulatory limit.

Evaporation Rate Determination

Theoretical Evaporation from Liquid Surfaces

Solvent evaporation from open containers, spills, and wet surfaces follows mass transfer principles:

$$\dot{m}{evap} = h_m \times A \times (C{surface} - C_{bulk})$$

Where:

• $\dot{m}_{evap}$ = Evaporation rate (lb/h)
• $h_m$ = Mass transfer coefficient (ft/h)
• $A$ = Liquid surface area (ft²)
• $C_{surface}$ = Saturation concentration at liquid surface (lb/ft³)
• $C_{bulk}$ = Bulk air concentration (lb/ft³)

Mass transfer coefficient depends on air velocity over surface:

$$h_m = 0.0128 \times v^{0.8}$$

For stagnant air ($v$ = 0): $h_m$ ≈ 0.5 ft/h (natural convection)

For ventilated space ($v$ = 50 fpm): $h_m$ ≈ 5 ft/h

Saturation concentration from Clausius-Clapeyron equation:

$$P_{sat} = P_0 \times e^{-\frac{\Delta H_{vap}}{R}(\frac{1}{T} - \frac{1}{T_0})}$$

Where:

• $P_{sat}$ = Saturation vapor pressure at temperature $T$
• $\Delta H_{vap}$ = Heat of vaporization
• $R$ = Universal gas constant

Practical approach: Use published vapor pressure data at operating temperature:

Printing Process Evaporation Rates

Gravure and flexographic printing:

Solvent evaporation occurs primarily in drying ovens with forced hot air:

$$G = A_{web} \times v_{web} \times m_{ink} \times f_{solvent} \times f_{evap}$$

Where:

• $A_{web}$ = Web width (ft)
• $v_{web}$ = Web speed (fpm)
• $m_{ink}$ = Ink coating weight (lb/ft²)
• $f_{solvent}$ = Solvent fraction of ink (typically 0.50-0.75)
• $f_{evap}$ = Evaporation efficiency in dryer (0.95-0.99)

Example calculation:

Publication gravure press:

• Web width: 6 ft
• Web speed: 2,000 fpm
• Ink coating: 0.0008 lb/ft² (wet)
• Solvent content: 65%
• Evaporation efficiency: 98%

$$G = 6 \times 2000 \times 0.0008 \times 0.65 \times 0.98 = 6.1 \text{ lb/min}$$

Emission sources beyond dryers:

graph TD
    A[Total Facility Emissions] --> B[Dryer Evaporation<br/>70-85% of total]
    A --> C[Press Fountain Fugitives<br/>5-10% of total]
    A --> D[Ink Mixing Stations<br/>3-7% of total]
    A --> E[Cleaning Operations<br/>5-12% of total]
    A --> F[Storage and Handling<br/>2-5% of total]

    B --> G[Captured by dryer hood<br/>95-98% efficiency]
    C --> H[Dilution ventilation zone]
    D --> I[Local exhaust + dilution]
    E --> I
    F --> H

    G --> J[Thermal oxidizer treatment]
    I --> K[Partial local capture]
    H --> L[Dilution only]

    K --> J
    L --> M[Atmospheric discharge]

    style A fill:#e1f5ff
    style B fill:#ffe1e1
    style G fill:#fff4e1
    style J fill:#e8f5e9

Total fugitive emissions requiring dilution ventilation typically represent 15-30% of total solvent usage in well-designed facilities with effective local exhaust systems.

Dilution Airflow Calculations

Standard Calculation Procedure

Step 1: Determine total evaporation rate

Sum all emission sources not captured by local exhaust:

$$G_{total} = G_{dryer,fugitive} + G_{fountain} + G_{mixing} + G_{cleaning} + G_{storage}$$

Step 2: Select target concentration

Based on lowest LEL solvent in use with appropriate safety factor:

$$C_{target} = \frac{LEL_{min}}{SF}$$

Step 3: Calculate theoretical dilution airflow

$$Q_{theoretical} = \frac{403 \times G_{total}}{C_{target}}$$

Step 4: Apply mixing efficiency factor

Real spaces do not achieve perfect instantaneous mixing. Apply factor $K$ based on geometry:

$$Q_{actual} = K \times Q_{theoretical}$$

Mixing efficiency factors:

Step 5: Verify air changes per hour

$$ACH = \frac{Q_{actual} \times 60}{V_{space}}$$

Where:

• $ACH$ = Air changes per hour
• $V_{space}$ = Room volume (ft³)

Typical requirements:

• Printing press areas: 6-12 ACH
• Ink mixing rooms: 12-20 ACH
• Solvent storage rooms: 20-30 ACH

Worked Example: Publication Gravure Facility

Facility specifications:

• Floor area: 50,000 ft²
• Ceiling height: 24 ft
• Volume: 1,200,000 ft³
• Four gravure presses with dryer systems
• Primary solvent: Toluene (LEL = 1.2% = 12,000 ppm)

Emission sources:

Dilution ventilation calculation:

Target concentration (12.5% LEL with SF = 8):

$$C_{target} = \frac{12,000}{8} = 1,500 \text{ ppm}$$

Theoretical airflow:

$$Q_{theoretical} = \frac{403 \times 3.0}{1,500} = 806 \text{ cfm}$$

This represents perfect mixing. For large printing facility with 24-ft ceilings and distributed emission sources, apply $K = 6$:

$$Q_{actual} = 6 \times 806 = 4,836 \text{ cfm}$$

Design decision: Round up to 5,000 cfm for general dilution ventilation.

Verification:

Air changes per hour:

$$ACH = \frac{5,000 \times 60}{1,200,000} = 0.25 \text{ ACH}$$

This appears low but is supplemented by local exhaust systems removing 22 lb/min (88% of total emissions). The combination provides adequate safety.

Total facility ventilation:

• General dilution: 5,000 cfm
• Dryer hood exhaust (4 presses): 75,000 cfm total
• Ink mixing exhaust: 2,500 cfm
• Cleaning booth exhaust: 3,000 cfm
• Total exhaust: 85,500 cfm

Corresponding air changes for total ventilation:

$$ACH_{total} = \frac{85,500 \times 60}{1,200,000} = 4.3 \text{ ACH}$$

This provides 4.3 complete air changes per hour, well within typical range for printing facilities.

Vapor Density and Stratification Effects

Relative Vapor Density

All common printing solvents produce vapors heavier than air, calculated from molecular weight ratio:

$$\rho_{rel} = \frac{MW_{solvent}}{MW_{air}} = \frac{MW_{solvent}}{29}$$

Vapor density comparison:

Physical implications:

Heavier-than-air vapors settle toward floor level in absence of adequate air mixing. This creates hazardous accumulations in:

• Floor-level pits and sumps
• Below-grade equipment rooms
• Dead-end corridors
• Corners and alcoves with poor air circulation
• Areas beneath equipment and conveyor systems

Ventilation Design for Heavy Vapors

Low-level exhaust strategy:

Provide dedicated exhaust points 6-18 inches above floor in areas where heavy vapors may accumulate:

Sizing low-level exhaust:

$$Q_{low} = A_{floor} \times 50 \text{ fpm}$$

Where:

• $Q_{low}$ = Low-level exhaust flow (cfm)
• $A_{floor}$ = Floor area requiring coverage (ft²)
• 50 fpm = Minimum face velocity at floor-level grilles

Example: Solvent storage room, 20 ft × 30 ft:

Floor area requiring coverage: 600 ft²

Low-level exhaust grille: 2 ft × 2 ft = 4 ft² opening

Number of grilles: 600 / 100 = 6 grilles (one per 100 ft² floor area)

Flow per grille: 4 ft² × 50 fpm = 200 cfm

Total low-level exhaust: 6 × 200 = 1,200 cfm

Supply air distribution:

Provide makeup air at high level (8-12 ft above floor) to:

• Create downward displacement flow
• Prevent short-circuiting between supply and exhaust
• Maintain floor-level air velocity of 30-50 fpm to sweep settled vapors toward exhaust points

flowchart TD
    A[Makeup Air Supply<br/>High wall or ceiling<br/>10-12 ft elevation] -->|Downward displacement| B[Work Zone<br/>4-6 ft elevation]
    B -->|Floor sweep flow<br/>30-50 fpm| C[Low-level exhaust<br/>6-18 in above floor]

    D[Heavy vapor release] -->|Settling due to<br/>density > air| E[Floor accumulation]
    E -->|Swept by ventilation| C

    F[LEL sensors<br/>Breathing height: 4-6 ft] -.Monitor.-> B
    G[LEL sensors<br/>Low level: 12-18 in] -.Monitor.-> E

    C --> H[Exhaust duct system]
    H --> I[Thermal oxidizer or<br/>atmospheric discharge]

    style D fill:#ffe1e1
    style E fill:#ffcccc
    style C fill:#fff4e1
    style I fill:#e8f5e9

Makeup Air Requirements

Mass Balance for Makeup Air

Facility operates at slight positive pressure (+0.02 to +0.05 in w.c.) to prevent uncontrolled infiltration:

$$Q_{makeup} = Q_{exhaust} - Q_{infiltration} + Q_{pressurization}$$

Infiltration estimation:

For industrial buildings with typical construction:

$$Q_{infiltration} = 0.05 \times V_{building} \text{ (cfm)}$$

This represents approximately 3 air changes per hour from natural infiltration (door openings, construction gaps, etc.).

Pressurization airflow:

Required airflow to maintain building pressure depends on building envelope leakage:

$$Q_{pressurization} = C_{leak} \times A_{envelope} \times \sqrt{\Delta P}$$

Where:

• $C_{leak}$ = Leakage coefficient (cfm/ft² per in w.c.), typically 0.1-0.3 for industrial construction
• $A_{envelope}$ = Building envelope area (ft²)
• $\Delta P$ = Pressure differential (in w.c.)

Practical approach: Size makeup air for 90-95% of total exhaust flow, allowing 5-10% infiltration contribution.

Makeup Air Unit Sizing Example

Using previous gravure facility example:

Total exhaust: 85,500 cfm

Makeup air requirement:

$$Q_{makeup} = 0.92 \times 85,500 = 78,660 \text{ cfm}$$

Design value: 80,000 cfm makeup air capacity

Winter heating load:

Outdoor design conditions: 10°F Indoor conditions: 70°F, 50% RH

Sensible heating:

$$Q_{sensible} = 1.08 \times Q \times \Delta T$$

$$Q_{sensible} = 1.08 \times 80,000 \times (70-10) = 5,184,000 \text{ Btu/h} = 5.2 \text{ MMBtu/h}$$

Humidification load (from psychrometric analysis):

• Outdoor: 10°F saturated, $W$ = 0.0015 lb/lb
• Indoor: 70°F, 50% RH, $W$ = 0.0078 lb/lb
• $\Delta W$ = 0.0063 lb/lb

$$Q_{latent} = 0.68 \times Q \times \Delta W$$

$$Q_{latent} = 0.68 \times 80,000 \times 0.0063 = 343,000 \text{ Btu/h}$$

Total winter load: 5.5 MMBtu/h heating + humidification

Summer cooling load:

Outdoor design: 95°F, 60% RH Indoor: 70°F, 50% RH

From psychrometric chart:

• Enthalpy change: $\Delta h$ = 10.2 Btu/lb

$$Q_{cooling} = 4.5 \times Q \times \Delta h$$

$$Q_{cooling} = 4.5 \times 80,000 \times 10.2 = 3,672,000 \text{ Btu/h} = 306 \text{ tons}$$

Equipment selection:

• Makeup air unit: 80,000 cfm capacity
• Winter heating: 5.5 MMBtu/h gas-fired or indirect steam coil
• Summer cooling: 310-ton chilled water coil or DX cooling
• Humidification: Steam injection, 350 lb/h capacity
• Supply fan: 80,000 cfm at 3.5-4.5 in w.c. total static pressure

OSHA and ACGIH Compliance Requirements

OSHA 29 CFR 1910.106 Requirements

Flammable liquid storage and handling ventilation:

Section 1910.106(e)(2)(iii) states:

“Ventilation shall be provided to prevent accumulation of flammable vapors. Concentration of flammable vapors shall not exceed 25 percent of the lower flammable limit.”

Compliance demonstration:

Facilities must demonstrate through calculation or measurement that:

1. Adequate airflow provided based on evaporation rate calculations
2. Proper distribution preventing dead zones and accumulation areas
3. Continuous monitoring via LEL sensors (required for high-hazard areas)
4. Emergency response procedures for ventilation failure or high vapor detection

Inspection requirements:

OSHA inspectors verify:

• Engineering calculations supporting ventilation design
• Airflow measurement data (actual vs. design)
• LEL monitoring system operation and calibration records
• Preventive maintenance documentation

ACGIH Industrial Ventilation Manual

ACGIH ventilation principles for dilution:

Chapter 12 (Dilution Ventilation) provides:

Safety factor selection guidance:

$$SF = \frac{LEL}{C_{design}} \geq 4$$

Minimum safety factor of 4 (25% LEL) required, with greater factors for:

• Incomplete mixing (add factor 2-5)
• Uncertain emission rates (add factor 1.5-2)
• Presence of ignition sources (add factor 2-3)

Mixing efficiency considerations:

ACGIH Table 12-1 provides mixing factors:

Air change rate verification:

Minimum ventilation rate (even with perfect capture) should provide:

$$ACH = \frac{60 \times Q_{min}}{V_{room}} \geq 4$$

For hazardous locations, minimum 4 air changes per hour regardless of calculated dilution requirement.

NFPA 86 Ovens and Dryers

Ventilation requirements for Class A ovens (using flammable solvents):

Section 5.7.2.1: “Sufficient air circulation shall be provided within the oven to prevent formation of flammable concentrations.”

Design criteria:

$$C_{oven} \leq 0.25 \times LEL$$

Monitoring requirements:

Section 8.5.2: “Vapor concentration monitoring shall be provided with alarm at 25% LEL and automatic shutdown.”

Purge requirements:

Section 6.4.3: “Four air changes minimum prior to ignition of burners after shutdown exceeding 4 hours.”

For printing dryer ovens:

• Volume: 500 ft³ typical
• Purge airflow: 500 ft³ × 4 / 5 min = 400 cfm minimum
• Actual design: 2,000-5,000 cfm (provides continuous purging)

Design Tables and Selection Criteria

Recommended Air Changes by Space Type

Safety Factor Selection Matrix

Temperature and Altitude Corrections

Temperature correction:

Ideal gas law requires correction for non-standard temperatures:

$$Q_T = Q_{530°R} \times \frac{T}{530}$$

Altitude correction:

Lower atmospheric pressure at altitude reduces air density:

$$\rho_{alt} = \rho_{SL} \times e^{-z/H}$$

Where scale height $H$ ≈ 29,000 ft

Combined correction example:

Denver facility (5,000 ft elevation, 90°F operating temperature):

Base calculation: 5,000 cfm required

Temperature correction: 5,000 × 1.04 = 5,200 cfm

Altitude correction: 5,200 × 1.20 = 6,240 cfm

Design selection: 6,500 cfm

Integration with Local Exhaust Systems

Combined Ventilation Strategy

Effective facilities employ layered approach:

Primary control: Local exhaust captures 85-95% of emissions at source

Secondary control: Dilution ventilation manages fugitive emissions escaping local exhaust

Verification: LEL monitoring confirms safe concentrations maintained

Compliance: Vapor treatment destroys VOCs before atmospheric discharge

flowchart LR
    A[Solvent Emission Sources] --> B{Capture Efficiency}
    B -->|85-95%| C[Local Exhaust System<br/>High concentration<br/>5,000-15,000 ppm]
    B -->|5-15% fugitive| D[General Space<br/>Dilution ventilation<br/>100-1,500 ppm]

    C --> E[Combined exhaust stream]
    D --> F[Building exhaust]

    E --> G[Thermal Oxidizer<br/>95-99% destruction]
    F --> H[Atmospheric discharge or<br/>treatment if required]

    I[LEL Monitoring] -.Verify.-> D
    I -.Verify.-> C

    G --> J[Compliant discharge<br/>< 20 ppm VOC]

    style A fill:#e1f5ff
    style C fill:#fff4e1
    style D fill:#ffe1e1
    style G fill:#ffcccc
    style J fill:#e8f5e9

Airflow Coordination

Exhaust system balancing:

Total facility exhaust equals sum of components:

$$Q_{exhaust,total} = Q_{dilution} + Q_{local} + Q_{process}$$

Where:

• $Q_{dilution}$ = General dilution ventilation
• $Q_{local}$ = Sum of local exhaust hoods
• $Q_{process}$ = Process exhaust (dryer ovens, dedicated exhausts)

Makeup air distribution:

$$Q_{makeup} = 0.90 \times Q_{exhaust,total}$$

Allow 10% infiltration contribution to balance.

Pressure verification:

Install differential pressure monitoring:

• Press room to outdoor: +0.02 to +0.05 in w.c.
• Solvent storage to press room: -0.05 to -0.10 in w.c.
• Ink mixing room to press room: -0.03 to -0.08 in w.c.

Control strategy:

1. Makeup air unit operates continuously, modulates airflow to maintain building pressure setpoint
2. Local exhaust systems operate when respective processes active
3. General dilution exhaust operates continuously during facility occupancy
4. Building pressure controller adjusts makeup air damper position to maintain +0.03 in w.c.

Dilution ventilation provides essential safety in printing facilities through mass-balance-driven airflow calculations maintaining solvent vapor concentrations below 25% LEL per OSHA 29 CFR 1910.106 requirements. Proper design incorporates safety factors for mixing efficiency, vapor density stratification effects, and transient operating conditions while coordinating with local exhaust systems and makeup air requirements to ensure worker safety and regulatory compliance throughout normal and upset operating scenarios.