Printing Plant HVAC Systems

Printing Plant HVAC Systems

Printing operations require precise environmental control to maintain paper dimensional stability, optimize ink transfer and drying, eliminate static electricity interference, and manage solvent vapor concentrations. Paper dimensional changes of 0.5-2.0% occur across the 30-70% RH range, causing registration errors in multi-color printing where tolerances demand alignment within 0.010 in (0.25 mm). Ink viscosity, tack, and drying rates vary significantly with temperature (±5°F impacts transfer characteristics), while static charges exceeding 3-5 kV disrupt web handling and sheet feeding. High-speed lithographic and gravure presses generate substantial solvent vapor loads requiring 100% outdoor air systems with thermal oxidizers, contrasting with water-based flexographic operations using recirculation with dedicated exhaust.

Paper Conditioning Requirements

Dimensional Stability Fundamentals

Paper is hygroscopic, absorbing or releasing moisture to achieve equilibrium with ambient conditions. Dimensional change follows:

$$\Delta L = L_0 \times \alpha_{RH} \times \Delta RH$$

Where:

• $\Delta L$ = Length change (in)
• $L_0$ = Original dimension (in)
• $\alpha_{RH}$ = Hygroscopic expansion coefficient (% per %RH)
• $\Delta RH$ = Relative humidity change (%RH)

Typical expansion coefficients:

Critical observation: Cross-grain expansion is 3-4× greater than with-grain, necessitating grain direction consideration in press layout and sheet cutting.

Registration tolerance analysis:

Four-color process printing requires each color plate to align within 0.010 in (0.25 mm). For a 40-in wide sheet:

Allowable dimensional change: $$\Delta L_{max} = \frac{0.010 \text{ in}}{40 \text{ in}} = 0.025% = 250 \text{ ppm}$$

Corresponding RH tolerance (coated stock, α = 0.015% per %RH): $$\Delta RH_{max} = \frac{0.025%}{0.015%} = 1.7% RH$$

Conclusion: Maintain press area RH within ±2%RH for acceptable registration in high-quality work. Tighter control (±1%RH) required for premium commercial and packaging printing.

Conditioning Zone Specifications

Paper storage acclimation:

Paper arrives at moisture content corresponding to manufacturing conditions (typically 45-55% RH). Requires acclimation period before printing:

Acclimation time: $$t_{acclim} = \frac{t^2 \times \rho \times c_p}{D \times h_{mass}}$$

Where typical values yield 24-72 hours for full equilibration through a ream stack.

Storage room conditions:

Press room conditions:

Seasonal adjustment: Some facilities maintain 50% RH year-round; others adjust to 45% RH (winter) and 55% RH (summer) to reduce humidification/dehumidification loads while staying within tolerance.

Temperature Control for Ink Performance

Ink Rheology Temperature Dependence

Ink viscosity, tack, and flow characteristics are highly temperature-sensitive:

Viscosity-temperature relationship (Andrade equation):

$$\mu(T) = \mu_0 \times e^{B/T}$$

Where:

• $\mu$ = Dynamic viscosity (Pa·s)
• $T$ = Absolute temperature (K)
• $B$ = Material constant (1000-2000 K for printing inks)

Practical impact:

10°F (5.6°C) temperature change causes 15-25% viscosity change in offset inks, affecting:

• Ink transfer uniformity
• Dot gain (halftone reproduction)
• Color density consistency
• Drying rate

Ink fountain temperature control:

High-volume presses use chilled water circulation (60-70°F) through ink fountain jackets to:

• Remove frictional heat from roller train
• Maintain consistent ink temperature (±2°F)
• Stabilize color density throughout run

Press room thermal loads:

Design approach: High internal sensible loads (dryers, motors) typically require year-round cooling. Economizer operation provides “free cooling” during moderate outdoor temperatures.

Static Control Systems

Electrostatic Charge Generation

Paper moving over rollers and guides generates static charge through triboelectric effect:

$$V_{static} = \frac{Q}{\varepsilon_0 \varepsilon_r A}$$

Where:

• $V_{static}$ = Surface potential (V)
• $Q$ = Charge accumulated (C)
• $\varepsilon_0$ = Permittivity of free space (8.85×10⁻¹² F/m)
• $\varepsilon_r$ = Relative permittivity of paper (2-4)
• $A$ = Surface area (m²)

Charge accumulation increases with:

• Web speed (more friction events per unit time)
• Low relative humidity (reduced surface conductivity)
• Synthetic materials (higher resistivity than paper)
• Dry conditions (less charge dissipation)

Static-related problems:

Humidity as Static Control

Surface resistivity vs. relative humidity:

Paper surface resistivity decreases exponentially with increasing RH:

$$\rho_{surface}(RH) = \rho_0 \times e^{-k \times RH}$$

Typical values:

Static control strategy:

Maintain RH > 45% for passive charge dissipation through moisture film on paper surface. Below 40% RH, static problems escalate rapidly regardless of active ionization systems.

Active Static Elimination

Ionizing bars:

Corona discharge ionizers produce balanced positive and negative ions that neutralize surface charges:

Placement:

• Before sheet feeders (prevent feeding errors)
• After dryers (charge buildup during heating)
• At web unwind/rewind (high-speed charge generation)
• Pre-sheeter on web presses (static cutting issues)

Performance criteria:

• Discharge time: < 3 seconds to neutral
• Balance: ±50V residual voltage
• Coverage: 1 ionizing bar per 36-48 in width

Maintenance: Weekly cleaning of emitter points (dust accumulation degrades performance). Monthly high-voltage supply verification.

Solvent Vapor Management

Vapor Generation Rates

Solvent-based inks (gravure, flexo, screen) and coatings release VOCs during drying:

Solvent evaporation rate:

$$\dot{m}{solvent} = A{web} \times v_{web} \times C_{solvent} \times f_{evap}$$

Where:

• $\dot{m}_{solvent}$ = Evaporation rate (lb/h)
• $A_{web}$ = Web area printed per unit length (ft²/ft)
• $v_{web}$ = Web speed (fpm)
• $C_{solvent}$ = Solvent coverage (lb/ft²)
• $f_{evap}$ = Fraction evaporated in dryer (0.95-0.99)

Example calculation:

Publication gravure press:

• Web width: 72 in = 6 ft
• Web speed: 2000 fpm
• Ink coverage: 0.001 lb/ft² (wet film)
• Solvent content: 60% by weight
• Evaporation fraction: 0.98

$$\dot{m}_{solvent} = 6 \text{ ft} \times 2000 \text{ fpm} \times 0.001 \text{ lb/ft}^2 \times 0.60 \times 0.98 = 7.1 \text{ lb/min} = 426 \text{ lb/h}$$

Ventilation requirement:

NFPA 86 and local codes typically require maintaining solvent concentration below 25% of Lower Explosive Limit (LEL).

For toluene (common gravure solvent):

• LEL = 1.2% by volume = 12,000 ppm
• Maximum allowable: 3,000 ppm (25% LEL)
• Safety factor: Design for 1,500 ppm maximum

Required airflow:

$$Q_{exhaust} = \frac{\dot{m}{solvent} \times 387 \times T}{MW \times C{max}}$$

Where:

• $Q_{exhaust}$ = Exhaust rate (CFM)
• $\dot{m}_{solvent}$ = 426 lb/h = 7.1 lb/min
• $MW$ = Molecular weight of solvent (92 for toluene)
• $T$ = Temperature (°R)
• $C_{max}$ = Maximum concentration (1,500 ppm)

$$Q_{exhaust} = \frac{7.1 \times 387 \times 530}{92 \times 1500} = 10,570 \text{ CFM}$$

Actual design: Use 15,000-20,000 CFM to provide safety margin and account for non-uniform mixing.

Exhaust System Design

100% outdoor air systems:

Solvent-laden air cannot be recirculated. Requires:

• Makeup air units (100% OA, heating/cooling to setpoint)
• Exhaust fans (dryer hoods, direct capture at press)
• Energy recovery (when economically justified)

Dryer hood capture:

Enclose dryer ovens with hoods capturing heated, solvent-laden air:

• Capture velocity: 100-200 fpm at hood face
• Temperature: 200-400°F (dryer exhaust)
• Solvent concentration: 2,000-5,000 ppm typical

Thermal oxidizers:

Destroy VOCs before atmospheric discharge:

Regenerative thermal oxidizer (RTO):

• Combustion temperature: 1400-1600°F
• Destruction efficiency: > 98%
• Heat recovery: 90-95%
• Fuel consumption: Minimal (self-sustaining above ~10% LEL)

Catalytic oxidizer:

• Combustion temperature: 600-800°F
• Destruction efficiency: 95-98%
• Heat recovery: 70-80%
• Catalyst replacement: 3-5 years

Operating cost comparison:

High solvent load (> 25% LEL): RTO self-sustaining, minimal fuel Low solvent load (< 10% LEL): Catalytic more economical (lower temperature)

Drying System Integration

Heatset Web Offset

Drying process:

Heated air (400-500°F) impinges on freshly printed web, evaporating petroleum-based solvents (15-25% of ink weight):

Energy requirement:

$$Q_{dryer} = \dot{m}{web} \times c_p \times \Delta T{web} + \dot{m}{solvent} \times h{fg}$$

Where:

• $\dot{m}_{web}$ = Web mass flow rate (lb/h)
• $\Delta T_{web}$ = Web temperature rise (°F)
• $\dot{m}_{solvent}$ = Solvent evaporation rate (lb/h)
• $h_{fg}$ = Latent heat of vaporization (500-1000 Btu/lb)

Dryer exhaust treatment:

• Volume: 20,000-100,000 CFM (depends on press size)
• Temperature: 300-400°F (after web cooling)
• Solvent concentration: 1,000-3,000 ppm
• Treatment: Thermal oxidizer (typically RTO)
• Heat recovery: Preheat makeup air or generate hot water

Chill stand requirements:

Web exits dryer at 250-350°F, must cool to < 100°F before folder:

• Chilled water system: 45-55°F supply
• Cooling load: 100-500 tons (depends on press speed, web width)
• Chill rolls: Chrome-plated, internal water circulation

UV Curing Systems

Ultraviolet ink drying:

UV inks contain no solvents; cure instantly via photopolymerization when exposed to UV radiation (200-400 nm wavelength).

UV lamp systems:

Medium-pressure mercury lamps:

• Power density: 200-400 W/in of web width
• Peak emission: 365 nm (UV-A)
• Cooling: Air or water (lamps generate substantial heat)
• Life: 1,000-2,000 hours

LED-UV systems:

• Power density: 5-20 W/cm²
• Wavelength: 365, 385, 395 nm (specific to ink formulation)
• Cooling: Air-cooled (lower heat generation)
• Life: 20,000-40,000 hours
• Energy savings: 50-70% vs. mercury lamps

HVAC integration:

UV systems generate heat but no VOCs:

• No exhaust treatment required
• Sensible cooling load: 50-80% of UV lamp power
• Lamp cooling air: Typically exhausted (contains ozone from UV generation)
• Press area cooling: Conventional systems with recirculation

Ozone management:

UV radiation (< 240 nm) generates ozone (O₃) from atmospheric oxygen:

• Ozone concentration: 0.02-0.10 ppm typical near UV lamps
• OSHA PEL: 0.10 ppm (8-hour TWA)
• Mitigation: Exhaust lamp cooling air directly outdoors

Air Distribution Design

Pressurization Strategy

Press room pressurization:

Maintain slight positive pressure (+0.02 to +0.05 in w.c.) relative to adjacent spaces:

• Prevents dust infiltration from warehouse, shipping
• Reduces uncontrolled outdoor air leakage
• Maintains stable humidity control

Zoning approach:

1. Paper storage: 70°F, 50% RH, neutral pressure
2. Press room: 72°F, 50% RH, positive pressure
3. Finishing area: 70-75°F, 45-55% RH, neutral/negative
4. Dryer hoods: Negative pressure (exhaust capture)

Distribution Methods

Low-velocity displacement:

Supply conditioned air at low velocity (50-150 fpm) near floor level:

• Minimizes disruption to paper handling
• Reduces static generation from high air velocities
• Provides excellent temperature stratification

Typical layout:

• Supply: Floor-mounted diffusers or low sidewall grilles
• Return: High sidewall or ceiling (capture heat from dryers, lights)
• Design: 0.3-0.5 CFM/ft² press area

Overhead high-velocity:

For facilities with overhead crane access or limited floor space:

• Supply: Ceiling-mounted diffusers, 300-800 fpm discharge
• Neck down to 50-75 fpm in occupied zone
• Return: Low sidewall or floor trenches

Critical consideration: Avoid high-velocity discharge directly at press (> 100 fpm causes web flutter, feeding problems, static issues).

Humidity Control Systems

Humidification

Steam grid systems:

Distributed low-pressure steam injection maintains uniform humidity:

Steam requirement:

$$\dot{m}{steam} = \frac{Q{air} \times \rho \times (W_2 - W_1)}{60}$$

Where:

• $\dot{m}_{steam}$ = Steam flow rate (lb/h)
• $Q_{air}$ = Airflow rate (CFM)
• $\rho$ = Air density (0.075 lb/ft³ standard)
• $W_2 - W_1$ = Humidity ratio increase (lb water/lb dry air)

Example:

100,000 CFM makeup air, 20°F outdoor, 70°F indoor, 50% RH target:

From psychrometric chart:

• $W_1$ (20°F, saturated) = 0.0012 lb/lb
• $W_2$ (70°F, 50% RH) = 0.0078 lb/lb
• $\Delta W$ = 0.0066 lb/lb

$$\dot{m}_{steam} = \frac{100,000 \times 0.075 \times 0.0066}{60} = 0.825 \text{ lb/min} = 49.5 \text{ lb/h}$$

At 1000 Btu/lb latent heat: 49,500 Btu/h = 41.3 MBH humidification load.

Distribution:

• Duct-mounted steam grids (multiple injection points)
• Absorption distance: 10-15 ft minimum before discharge
• Control: Modulating valve, dewpoint or RH sensor

Dehumidification

Cooling-based dehumidification:

Cool air below dewpoint to condense moisture:

Summer design example:

90°F, 60% RH outdoor → 70°F, 50% RH indoor:

• Cooling coil: Chilled water 42-45°F supply
• Leaving air: 52-55°F (dewpoint control)
• Reheat: 52°F → 70°F supply (sensible heat)

Energy penalty: Simultaneous cooling (remove moisture) and heating (restore temperature to setpoint) reduces efficiency.

Desiccant dehumidification:

For applications requiring 35-40% RH or precise control:

• Rotary desiccant wheel (silica gel, molecular sieve)
• Regeneration: 200-300°F heated air
• Process air: Ambient → wheel → cooler → space
• Energy: Regeneration heat (typically natural gas)

Selection criteria:

• Cooling-based: Cost-effective for 40-60% RH range
• Desiccant: Required for < 40% RH or when cooling-based cannot achieve target

Printing plant HVAC systems provide precise temperature and humidity control essential for paper dimensional stability, ink performance optimization, and static elimination, while managing substantial solvent vapor loads and dryer heat rejection through specialized exhaust treatment and 100% outdoor air ventilation strategies that distinguish these facilities from conventional industrial applications.

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