Web Press HVAC Systems

Web offset printing facilities present unique HVAC challenges due to high-speed printing operations, substantial heat-set dryer loads, solvent vapor emissions, and stringent environmental control requirements. Proper system design requires careful integration of process exhaust, makeup air delivery, and environmental control while maintaining code compliance and operational efficiency.

Heat-Set Dryer Loads

Heat-set web presses utilize gas-fired or electric dryers operating at 350-450°F to cure inks containing petroleum-based solvents. These dryers represent the dominant thermal load in the facility.

Dryer Heat Load Components:

The dryer exhaust system must handle 8,000-15,000 CFM per press unit at temperatures of 275-350°F. This exhaust contains 25-40% LEL (Lower Explosive Limit) solvent vapors requiring proper dilution and control.

Solvent Vapor Control Systems

Web offset inks contain volatile organic compounds that vaporize during the drying process, creating both safety and environmental concerns.

Exhaust System Design:

1. Primary dryer exhaust - Captures high-concentration vapors directly from dryer chamber at 10,000-12,000 CFM per press
2. Secondary cooling exhaust - Removes residual vapors from cooling rollers at 3,000-5,000 CFM per press
3. General dilution ventilation - Maintains pressroom concentrations below 25% LEL at 0.5-1.0 CFM per square foot

The combined exhaust typically operates at negative pressure (-0.10 to -0.25 inches w.c. relative to atmosphere) to prevent vapor migration to adjacent spaces.

NFPA Code Compliance:

Per NFPA 86 (Ovens and Furnaces), heat-set dryers require:

• Minimum 8,000 CFM exhaust per dryer section regardless of size
• Interlocked burner shutdown on exhaust fan failure
• Continuous vapor concentration monitoring with alarm at 50% LEL
• Purge cycle providing 4 complete air changes before burner ignition
• Explosion relief venting sized per NFPA 68

NFPA 91 (Exhaust Systems for Air Conveying of Vapors, Gases, Mists, and Particulates) governs ductwork design, requiring Class I or II materials with explosion relief panels at 20-foot intervals maximum.

Vapor Abatement Technology

Environmental regulations typically require destruction or recovery of solvent vapors rather than direct atmospheric discharge.

Thermal Oxidizer Systems:

Catalytic or regenerative thermal oxidizers achieve 95-99% destruction efficiency:

• Operating temperature: 1,400-1,600°F (thermal) or 600-800°F (catalytic)
• Residence time: 0.5-0.75 seconds minimum
• Fuel consumption: 2-8 MMBtu/hr depending on VOC concentration
• Heat recovery: 85-95% through regenerative beds or heat exchangers

The oxidizer requires auxiliary natural gas firing when solvent concentration drops below auto-thermal operating point (typically 8-12% LEL).

Condensation Recovery:

High-concentration streams (>40% LEL) may justify solvent recovery via refrigerated condensation:

• Cooling to 35-45°F condenses 70-85% of solvents
• Recovered solvent returns to ink manufacturer
• Reduced oxidizer fuel consumption
• Capital cost recovered in 3-5 years at high production rates

Pressroom Environmental Control

The printing process requires tight environmental control to maintain registration accuracy and prevent web breaks.

Design Conditions:

The space experiences significant sensible heat gain from dryers (150,000-300,000 Btu/hr radiant), press drives (50-100 HP total), and lighting (1.0-1.5 W/ft²).

Air Distribution Strategy:

Low-velocity displacement ventilation works effectively:

• Supply air at 65-68°F from floor-level diffusers
• Return air extraction at ceiling level captures heat stratification
• Perforated duct distribution along press line maintains uniform conditions
• Dedicated outdoor air system provides 0.15-0.20 CFM/ft² for pressurization

The pressroom operates at +0.05 to +0.10 inches w.c. positive pressure relative to warehouse areas to prevent dust infiltration into the printing environment.

Makeup Air Integration

The substantial process exhaust (15,000-20,000 CFM per press) requires equivalent makeup air introduction without disrupting pressroom conditions.

Makeup Air System Design:

Direct-fired makeup air units provide economical heating:

• 80-85% thermal efficiency
• Temperature control ±3°F via modulating burner
• Evaporative or mechanical cooling for summer operation
• HEPA filtration (85-95% efficiency) to remove outdoor particulates

The makeup air delivery must account for dryer exhaust temperature. Energy recovery is generally impractical due to contaminated exhaust stream, though air-to-air heat exchangers may recover 40-60% of sensible heat if justified economically.

Capacity Calculation:

Total makeup air = Dryer exhaust + Cooling exhaust + Building infiltration - Thermal expansion factor

For a typical 6-unit press facility:

• Dryer exhaust: 60,000 CFM
• Cooling exhaust: 18,000 CFM
• Infiltration: 8,000 CFM
• Thermal expansion credit: -12,000 CFM (due to temperature rise)
• Required makeup air: 74,000 CFM

Fire Protection Considerations

Beyond NFPA 86 requirements, additional fire protection measures include:

• Automatic CO₂ or dry chemical suppression in dryer chambers
• Fusible link dampers in exhaust ducts (165°F activation)
• Emergency shutdown systems accessible from multiple locations
• Segregated electrical classification (Class I, Division 2) within 5 feet of dryer openings
• Sprinkler protection throughout press area with early suppression fast response (ESFR) heads

Proper HVAC design for web press facilities requires integration of high-volume exhaust systems, vapor abatement technology, precision environmental control, and comprehensive code compliance. The engineering approach must balance process requirements, energy efficiency, and regulatory obligations while maintaining safe and productive operating conditions.

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