Dehydration Processes

Dehydration processes remove moisture from vegetables to extend shelf life, reduce weight, and preserve nutritional content. HVAC systems for dehydration facilities must provide precise temperature and humidity control, handle high moisture loads, recover waste heat, and maintain appropriate conditions in packaging and storage areas.

Dehydration System Overview

Vegetable dehydration facilities require specialized HVAC systems that integrate with drying equipment to control air temperature, humidity, and flow rates while maximizing energy efficiency.

System Functions

Primary Requirements:

• Supply heated drying air at controlled temperature and humidity
• Remove moisture-laden exhaust air from drying chambers
• Recover heat from exhaust air streams
• Maintain appropriate conditions in preparation and packaging areas
• Control odors and particulate emissions
• Provide makeup air for combustion equipment

Operating Parameters:

• Drying air temperatures: 50°C to 70°C (122°F to 158°F) for most vegetables
• Relative humidity: 10% to 30% in drying chambers
• Air velocity: 1 to 3 m/s (200 to 600 fpm) over product
• Moisture removal rates: 50 to 200 kg/hour per dryer
• Exhaust air moisture content: 60% to 80% RH at dryer outlet

Drying Air Temperature Control

Precise temperature control prevents product degradation, ensures consistent moisture removal, and optimizes energy consumption.

Heating Systems

Direct-Fired Burners:

• Natural gas or propane combustion
• Efficiency: 90% to 95%
• Temperature control: ±2°C (±4°F)
• Products of combustion mixed with drying air
• Requires high-quality fuel for food safety
• NOx emissions below 30 ppm required

Indirect Steam Heat Exchangers:

• Finned tube coils in air stream
• Steam pressure: 100 to 150 psig
• Temperature control: ±1°C (±2°F)
• No combustion products in drying air
• Higher capital cost than direct-fired
• Requires steam generation infrastructure

Electric Resistance Heaters:

• Used for small-scale operations
• Precise temperature control: ±0.5°C (±1°F)
• Operating cost 3 to 4 times higher than gas
• No combustion products or emissions
• Modular design for staged capacity

Temperature Control Strategies

Multi-Stage Drying:

• Initial stage: 70°C to 75°C for rapid moisture removal
• Intermediate stage: 60°C to 65°C for continued drying
• Final stage: 50°C to 55°C to prevent case hardening
• Stage-specific temperature control within ±2°C

Product-Specific Requirements:

Humidity Management in Dryers

Controlling humidity in drying chambers optimizes moisture removal rates and product quality while preventing condensation in exhaust systems.

Humidity Control Methods

Fresh Air Introduction:

• Outdoor air at ambient conditions
• Heating reduces relative humidity
• Example: 20°C, 60% RH heated to 60°C yields 8% RH
• Maximum moisture carrying capacity increases with temperature
• Psychrometric calculations determine required air volume

Exhaust Air Recirculation:

• Partial recirculation reduces heating load
• Typical recirculation: 20% to 40% of total air volume
• Mixed air humidity must not exceed 30% RH
• Reduces fresh air heating requirement by 15% to 25%
• Requires careful control to prevent over-humidification

Desiccant Dehumidification:

• Used when outdoor air is too humid
• Reduces inlet air to 5% to 10% RH
• Regeneration temperature: 120°C to 150°C
• Energy penalty: 15% to 20% increase
• Enables consistent operation in humid climates

Moisture Load Calculations

Evaporation Rate Determination:

Moisture removal rate (kg/hour) = Product feed rate × (Initial moisture - Final moisture)

Example calculation:

• Feed rate: 1000 kg/hour fresh vegetables
• Initial moisture: 85% wet basis
• Final moisture: 5% wet basis
• Moisture removal: 1000 × (0.85 - 0.05) = 800 kg/hour water

Required Drying Air Volume:

Air volume (m³/hour) = Moisture removal rate / (Humidity ratio change × Air density)

For 800 kg/hour moisture removal with 15 g/kg humidity ratio change:

• Required air: 800 / (0.015 × 1.2) = 44,444 m³/hour (26,100 cfm)

Exhaust Air Handling

Exhaust air from dryers contains high moisture levels, odors, and particulate matter requiring proper handling before atmospheric discharge.

Exhaust System Design

Ductwork Sizing:

• Velocity: 15 to 20 m/s (3000 to 4000 fpm)
• Material: Stainless steel 304 or 316
• Insulation: 50 mm minimum to prevent condensation
• Slope: 1% toward drain points
• Access doors every 6 meters for cleaning

Condensate Management:

• Drip legs at low points in ductwork
• Automatic drain traps with sight glasses
• Condensate collection capacity: 10% of moisture removal rate
• Routing to wastewater treatment system

Fan Selection:

• Type: Centrifugal backward-curved for efficiency
• Material: Stainless steel construction
• Motor: TEFC, 1.15 service factor
• Controls: VFD for flow modulation
• Static pressure: 500 to 1000 Pa (2 to 4 in. wg)

Emission Control

Particulate Removal:

• Cyclone separators for particles >10 microns
• Bag filters for particles 1 to 10 microns
• HEPA filters for particles <1 micron (specialty products)
• Pressure drop: 250 to 500 Pa across filters
• Regular cleaning prevents buildup

Odor Control:

• Thermal oxidizers for high-odor products (onions, garlic)
• Operating temperature: 650°C to 750°C
• Residence time: 0.5 to 1.0 seconds
• Destruction efficiency: >99%
• Biofilters for lower odor loads

Heat Recovery Opportunities

Heat recovery from exhaust air significantly reduces operating costs and improves energy efficiency in dehydration facilities.

Heat Recovery Systems

Air-to-Air Heat Exchangers:

• Plate heat exchangers between exhaust and fresh air
• Effectiveness: 60% to 70%
• Energy savings: 30% to 40% of heating load
• Payback period: 2 to 4 years
• Requires periodic cleaning due to particulate buildup

Waste Heat Boilers:

• Generate low-pressure steam from exhaust air
• Steam pressure: 15 to 50 psig
• Used for process heating or space heating
• Applicable when exhaust temperature >150°C
• Economically viable for large operations (>5,000 kg/hour)

Heat Pump Integration:

• Extract heat from exhaust air at 50°C to 70°C
• Raise temperature to 80°C to 100°C for process use
• COP: 3 to 4 in this application
• Reduces primary heating by 50% to 60%
• Higher capital cost, 4 to 6 year payback

Heat Recovery Performance

Packaging Room Conditioning

Post-drying packaging areas require controlled temperature and humidity to prevent moisture reabsorption and maintain product quality.

Environmental Requirements

Temperature Control:

• Setpoint: 18°C to 22°C (64°F to 72°F)
• Tolerance: ±2°C (±4°F)
• Cooling load: 150 to 250 W/m² floor area
• Heat sources: equipment, lighting, personnel, product

Humidity Control:

• Setpoint: 35% to 45% RH
• Tolerance: ±5% RH
• Dehumidification capacity: 5 to 10 kg/hour per 100 m²
• Prevents moisture pickup by hygroscopic products

Air Quality:

• Filtration: MERV 13 minimum
• Air changes: 15 to 20 per hour
• Pressurization: +10 to +15 Pa relative to adjacent areas
• Dust concentration: <0.5 mg/m³

HVAC System Design

Dedicated Outdoor Air System (DOAS):

• Handles latent load from outdoor air
• Dehumidification to 40% RH or lower
• Heating/cooling for temperature control
• Energy recovery on exhaust air (60% effectiveness)

Recirculation Units:

• Handle sensible cooling load
• MERV 13 filtration
• Variable air volume for zone control
• Fan energy: 0.4 to 0.6 W/cfm

Dehumidification Strategy:

• Overcool and reheat method for precise control
• Cooling coil: 7°C to 10°C (45°F to 50°F)
• Reheat: Hot water or electric resistance
• Dew point control: 8°C to 12°C (46°F to 54°F)

Dryer Type Specifications

Different dryer configurations require specific HVAC considerations for optimal performance.

Cabinet Tray Dryers

Batch Operation:

• Capacity: 50 to 500 kg per batch
• Air flow: 1 to 2 m/s (200 to 400 fpm) through trays
• Heating: Direct gas or electric
• Temperature control: Manual or automated staging
• Drying time: 6 to 12 hours per batch

HVAC Requirements:

• Exhaust flow: 2000 to 5000 m³/hour per unit
• Makeup air heating: 50 to 150 kW per unit
• Intermittent operation profile
• Simple on/off control acceptable

Tunnel Dryers

Continuous Operation:

• Capacity: 200 to 2000 kg/hour
• Multiple zones with temperature staging
• Counter-flow or parallel-flow air movement
• Product transport: Belt or truck system
• Length: 10 to 40 meters

HVAC Design:

• Zone-specific temperature control
• Fresh air injection at product exit end
• Exhaust extraction at product entry end
• Recirculation loops within zones
• Heat recovery between zones

Belt Dryers

High-Capacity Continuous:

• Capacity: 500 to 5000 kg/hour
• Multi-pass belt configuration (3 to 8 passes)
• Air flow perpendicular to belt travel
• Residence time: 2 to 4 hours
• Compact footprint

HVAC Specifications:

• Air volume: 40,000 to 200,000 m³/hour
• Heating capacity: 500 to 2500 kW
• Multiple exhaust points for moisture removal
• Sophisticated controls for uniform drying
• VFD fan control for energy optimization

Fluidized Bed Dryers

Particulate Product:

• Suitable for diced vegetables (3 to 10 mm)
• Residence time: 20 to 60 minutes
• High air velocity: 2 to 5 m/s (400 to 1000 fpm)
• Excellent heat transfer efficiency
• Compact design

HVAC Considerations:

• High-pressure air supply (1000 to 2000 Pa)
• Cyclone separator on exhaust required
• Fine particulate control essential
• Air filtration on supply prevents contamination
• Noise attenuation on fan discharge

Energy Efficiency Optimization

Implementing energy-saving strategies reduces operating costs while maintaining product quality.

Efficiency Measures

Variable Speed Drive Application:

• Supply and exhaust fans
• Energy savings: 30% to 50% at part load
• Maintains proper air balance
• Soft start reduces mechanical stress
• Payback: 1 to 2 years

Waste Heat Recovery:

• Heat exchangers on all major exhaust streams
• Preheat combustion air with exhaust
• Generate hot water for facility use
• Combined efficiency improvement: 25% to 35%

Control Optimization:

• Real-time monitoring of product moisture
• Automatic adjustment of temperature and air flow
• Minimize over-drying that wastes energy
• Data logging for continuous improvement
• Reduce drying time by 10% to 15%

Insulation Enhancement:

• Dryer chamber insulation: 100 to 150 mm
• Ductwork insulation: 50 to 75 mm
• Removable insulation on serviceable components
• Reduces heat loss by 15% to 20%
• Payback: 2 to 3 years

Performance Monitoring

Safety and Code Compliance

Dehydration facilities must comply with fire, electrical, and food safety codes while protecting personnel.

Fire Protection

Dust Explosion Prevention:

• Ductwork grounding and bonding
• Explosion vents on dryers and cyclones
• Spark detection and suppression systems
• Regular cleaning of dust accumulation
• Ignition source elimination

Fire Suppression:

• Automatic sprinklers in packaging areas
• Dry chemical systems in dryers
• CO₂ systems for electrical rooms
• Fire dampers in ductwork penetrations
• Emergency shutdown systems

Personnel Safety

Temperature Protection:

• Lockout/tagout procedures for hot equipment
• High-temperature alarms and interlocks
• Thermal insulation on accessible surfaces
• Personal protective equipment requirements
• Burn hazard signage

Air Quality:

• Continuous monitoring of combustion gases
• CO detectors in enclosed spaces
• Adequate ventilation for personnel areas
• Emergency exhaust systems
• Respiratory protection program for cleaning operations

Proper HVAC system design, installation, and operation ensure efficient vegetable dehydration while maintaining product quality, worker safety, and regulatory compliance throughout the facility.