Vertical Plate Freezers

Design Principles

Vertical plate freezers orient heat exchange plates in a vertical plane, typically arranged as a series of parallel stations that allow products to be loaded and frozen in a vertical configuration. The vertical orientation distinguishes these systems from horizontal plate freezers and provides specific advantages for certain product types and operational environments.

Fundamental Configuration

The basic vertical plate freezer consists of:

• Vertical Plate Assemblies: Hollow metal plates arranged perpendicular to the floor
• Hydraulic or Pneumatic Actuation: System to open and close plates for loading and discharge
• Refrigerant Distribution: Piping network delivering refrigerant to all plate surfaces
• Product Contact Surfaces: Smooth aluminum or stainless steel surfaces
• Gravity Discharge Mechanism: Allows frozen product to slide out when plates open

The vertical orientation creates a natural product flow pattern that facilitates automatic operation in many applications.

Vertical Plate Design

Plate Construction

Vertical plate freezers utilize specialized plate assemblies designed for vertical mounting and operation.

Plate Construction Methods:

1. Roll-bonded aluminum: Two aluminum sheets with etched flow pattern, bonded under heat and pressure
2. Welded channel construction: Machined or extruded channels welded to flat sheets
3. Tube-and-plate: Refrigerant tubes embedded between two metal plates

Heat Transfer Characteristics

Heat transfer in vertical plate freezers follows the fundamental equation:

Q = U × A × ΔTlm

Where:

• Q = Heat transfer rate (W)
• U = Overall heat transfer coefficient (W/m²·K)
• A = Effective heat transfer area (m²)
• ΔTlm = Log mean temperature difference (K)

Typical Overall Heat Transfer Coefficients:

The vertical orientation affects heat transfer through:

1. Gravity effects on product settling: Products settle against lower plate surface
2. Drainage of melt water during initial cooling: Water drains downward
3. Air pocket distribution: Tends to accumulate at top of product
4. Pressure distribution: Hydraulic pressure must overcome product weight

Gravity-Assisted Loading

Vertical plate freezers exploit gravity to facilitate product handling in specific configurations.

Loading Mechanisms

Top-Loading Vertical Freezers:

For products that can be dropped or slid into vertical spaces:

• Loading chute: Directs product between opened plates
• Automatic positioning: Product slides to bottom by gravity
• Consistent placement: Gravity ensures repeatability
• Minimal handling: Reduces labor requirements

Side-Loading with Gravity Assist:

• Product loaded horizontally but settles under gravity
• Plates close to secure product position
• Suitable for irregular-shaped products
• Common in fish processing applications

Discharge Mechanisms

Upon completion of the freezing cycle, vertical plate freezers use gravity to assist product discharge:

Gravity Discharge Process:

1. Plate opening: Hydraulic or pneumatic system separates plates
2. Product release: Frozen block slides downward when plates separate
3. Discharge conveyor: Catches falling product
4. Automatic removal: Product conveyed away for packaging

The discharge time is critical to overall cycle efficiency:

t_discharge = t_open + t_release + t_clear

Where:

• t_discharge = Total discharge time (s)
• t_open = Plate opening time (typically 10-20 s)
• t_release = Product release time (5-15 s depending on adhesion)
• t_clear = Clearing time for next cycle (5-10 s)

Release Enhancement Methods:

• Defrost cycle before discharge (reduces adhesion)
• Surface coatings (PTFE, ceramic) to minimize sticking
• Vibration assist during plate opening
• Slight warming of plate surface (1-2°C increase)

Marine Applications

Vertical plate freezers have extensive application in marine fishing vessels and onboard fish processing.

Shipboard Installation Requirements

Marine vertical plate freezers must address unique challenges:

At-Sea Freezing Operations

Operational Sequence for Marine Applications:

1. Fresh catch processing: Fish cleaned and prepared immediately
2. Block formation: Fish arranged in rectangular forms or pans
3. Loading: Pans inserted between vertical plates
4. Freezing cycle: 2-4 hours depending on block thickness
5. Discharge: Frozen blocks removed and stored in hold
6. Cycle restart: Immediate reloading for continuous operation

Freezing Capacity Requirements:

For a typical fishing vessel processing 50-100 metric tons per day:

Daily capacity = (Production rate × Operating hours) / Cycle time

Example calculation:

• Target production: 75,000 kg/day
• Operating hours: 20 hours/day
• Required hourly rate: 3,750 kg/hr
• Average cycle time: 3 hours
• Freezer capacity needed: 3,750 kg/hr × 3 hr = 11,250 kg in process

This requires approximately 10-15 vertical plate stations operating continuously.

Marine-Specific Features

Enhanced Construction for Marine Service:

• Gimbal mounting: Allows freezer to remain level during vessel pitch and roll
• Drainage systems: Enhanced drainage for defrost water and product moisture
• Anti-corrosion coatings: Specialized coatings on all external surfaces
• Vibration isolation: Reduces stress from engine and sea motion
• Explosion-proof components: For compliance with marine safety regulations

Whole Fish Freezing

Vertical plate freezers excel at freezing whole fish and fish portions in block form.

Fish Block Configuration

Standard Fish Block Dimensions:

Freezing Parameters for Whole Fish

Temperature Profiles:

Initial fish temperature: 0 to +4°C (chilled) Plate surface temperature: -35 to -42°C Final product core temperature: -18°C minimum (typically -20 to -25°C)

Freezing Time Calculation:

Using modified Plank’s equation for fish blocks:

t = (ρ × λ / (T_f - T_r)) × (Pa/h + Ra/h²)

Where:

• t = Freezing time (s)
• ρ = Product density (kg/m³), fish ≈ 1050 kg/m³
• λ = Latent heat of fusion (kJ/kg), fish ≈ 280 kJ/kg
• T_f = Initial freezing point (°C), fish ≈ -1 to -2°C
• T_r = Refrigerant temperature (°C)
• P, R = Shape factors (P = 1/2, R = 1/8 for infinite slab)
• a = Thickness of product (m)
• h = Overall heat transfer coefficient (W/m²·K)

Example Calculation:

For a 60 mm thick fish block:

• a = 0.060 m
• h = 180 W/m²·K (good contact)
• T_f = -1.5°C
• T_r = -38°C
• ΔT = 36.5 K

t = (1050 × 280,000 / 36.5) × (0.5 × 0.060/180 + 0.125 × 0.060²/180) t = 8,055 × (0.000167 + 0.0000025) t = 8,055 × 0.000169 t ≈ 1.37 seconds per J/(kg·K) → approximately 2.1 hours

Product Quality Considerations

Freezing Rate Impact:

Vertical plate freezers typically achieve medium to fast freezing rates, suitable for most commercial fish processing.

Fish Block Packing Methods:

1. Random packing: Fish placed randomly, gaps filled with water or ice
2. Oriented packing: Fish aligned for consistent freezing
3. Glazed blocks: Surface water coating before freezing
4. Vacuum-packed: Wrapped before freezing to prevent oxidation

Plate Configurations

Station Arrangements

Vertical plate freezers are configured as multi-station systems with various layouts.

Single-Row Configuration:

• Plates arranged in one linear row
• Simplest layout for small operations
• Typical: 4-8 stations
• Floor space: 3-6 m length × 1.5-2 m width

Double-Row Configuration:

• Two parallel rows of plates
• Common in medium-capacity installations
• Typical: 10-20 stations total
• Shared refrigeration distribution
• Central loading/unloading access

Multiple-Row Configuration:

• Three or more rows of plates
• High-capacity industrial installations
• Typical: 30-60 stations
• Requires automated handling systems
• Complex refrigeration distribution

Plate Spacing and Thickness Range

Adjustable Plate Spacing:

Modern vertical plate freezers incorporate hydraulic systems that allow variable plate spacing:

The hydraulic pressure must overcome:

P_required = P_product + P_friction + P_seal

Where:

• P_product = Pressure to compress product and ensure contact
• P_friction = Overcome mechanical friction in plate mechanism
• P_seal = Maintain seal integrity during freezing

Multi-Zone Configurations

Zoned Refrigeration Systems:

Advanced installations utilize multiple refrigeration zones:

1. Pre-cooling zone: -15 to -20°C, rapid surface cooling
2. Primary freezing zone: -35 to -40°C, core freezing
3. Tempering zone: -25 to -30°C, equalization (optional)

This staged approach optimizes:

• Energy efficiency (reduces peak refrigeration load)
• Product quality (controlled freezing rate)
• Equipment capacity utilization

Defrost and Discharge Systems

Defrost Methods

Effective defrost is critical for maintaining freezing efficiency and facilitating product discharge.

Hot Gas Defrost:

Most common method for vertical plate freezers:

Q_defrost = m_ice × (L_ice + c_p,ice × ΔT)

Where:

• Q_defrost = Heat required for defrost (kJ)
• m_ice = Mass of ice accumulated (kg)
• L_ice = Latent heat of ice melting (334 kJ/kg)
• c_p,ice = Specific heat of ice (2.1 kJ/kg·K)
• ΔT = Temperature rise required (K)

Defrost Sequence:

1. Refrigeration shutdown: Compressor stops or diverts
2. Hot gas injection: Discharge gas diverted to plates
3. Ice melting: 5-15 minutes depending on accumulation
4. Drainage: Melt water drains to collection system
5. Purge: Residual refrigerant removed
6. Return to freezing: Normal refrigeration resumed

Defrost Frequency:

Discharge Automation

Automated Discharge Systems:

Modern vertical plate freezers incorporate automated discharge:

1. Defrost initiation: Brief defrost to release product adhesion
2. Plate separation: Hydraulic system opens plates 50-100 mm
3. Product release: Frozen blocks slide out by gravity
4. Conveyor reception: Product caught on discharge conveyor
5. Plate closing: Hydraulic system returns plates to closed position
6. New product loading: Next batch inserted immediately

Discharge Conveyor Requirements:

• Load capacity: Must handle full plate capacity (500-2000 kg)
• Impact resistance: Withstand falling frozen blocks
• Sanitary design: Stainless steel, easy cleaning
• Speed matching: Synchronized with plate opening cycle

Cleaning and Sanitation Systems

CIP (Clean-In-Place) Considerations:

While vertical plate freezers are not typically designed for full CIP, sanitation access is critical:

• Plate surface access: Plates fully open for manual cleaning
• Drainage access: Complete drainage of cleaning solutions
• Material compatibility: Surfaces resistant to sanitizers
• Inspection ports: Visual confirmation of cleanliness

Sanitation Frequency:

• Daily: External surfaces, product contact areas accessible
• Weekly: Full plate opening and manual cleaning
• Monthly: Deep cleaning including refrigerant side inspection
• Annually: Complete disassembly and inspection

Refrigerant Systems

Refrigerant Selection

Vertical plate freezers operate with various refrigerants depending on application and location.

Marine Regulations:

Marine applications must comply with:

• SOLAS (Safety of Life at Sea) requirements
• IMO (International Maritime Organization) regulations
• Flag state specific requirements
• Machinery space classification requirements

Distribution Systems

Direct Expansion (DX) Configuration:

Most vertical plate freezers use direct expansion:

• Refrigerant evaporates within plate passages
• Thermostatic expansion valve (TXV) controls refrigerant flow
• Individual TXV per plate or group of plates
• Superheat control: 4-8 K typical

Feed Methods:

1. Bottom feed: Refrigerant enters at bottom, exits at top

   • Natural refrigerant circulation
   • Excellent oil return
   • Standard configuration

2. Top feed: Refrigerant enters at top (less common)

   • Used in specific applications
   • May require oil management
   • Potential for liquid slugging

3. Multi-point feed: Refrigerant distributed to multiple points

   • Ensures uniform plate temperature
   • Complex piping
   • Best performance

Refrigerant Distribution Headers

Header Design Principles:

The refrigerant distribution header must provide equal flow to all plates:

Pressure drop per branch: ΔP_branch = f × (L/D) × (ρv²/2)

Where:

• f = Friction factor (dimensionless)
• L = Length of refrigerant path (m)
• D = Tube diameter (m)
• ρ = Refrigerant density (kg/m³)
• v = Refrigerant velocity (m/s)

Balancing Methods:

• Velocity control: Size header to maintain low velocity (<1 m/s)
• Pressure drop equalization: Orifices or balancing valves
• Reverse return piping: Equal path length to each plate
• Individual TXV: Separate expansion valve for each plate

Capacity Specifications

Freezing Capacity Determination

Vertical plate freezer capacity is specified in multiple ways:

Product Capacity per Cycle:

Capacity_cycle = N_stations × L_plate × W_plate × t_product × ρ_product

Where:

• N_stations = Number of plate stations
• L_plate = Effective plate length (m)
• W_plate = Effective plate width (m)
• t_product = Product thickness (m)
• ρ_product = Product density (kg/m³)

Example Calculation:

For a 12-station vertical plate freezer:

• Plate dimensions: 2.0 m × 1.0 m
• Product thickness: 0.075 m
• Product density: 1000 kg/m³

Capacity_cycle = 12 × 2.0 × 1.0 × 0.075 × 1000 = 1,800 kg per cycle

Daily Production Capacity:

Daily capacity = (Capacity_cycle / Cycle time) × Operating hours

With 3-hour cycle time and 20-hour operation: Daily capacity = (1,800 kg / 3 hr) × 20 hr = 12,000 kg/day

Refrigeration Load Calculations

Total Refrigeration Load:

Q_total = Q_product + Q_infiltration + Q_defrost + Q_piping

Product Load:

Q_product = m_product × (c_p,above × ΔT_above + L_f + c_p,below × ΔT_below)

For fish from +2°C to -20°C:

• c_p,above = 3.8 kJ/kg·K (unfrozen fish)
• ΔT_above = 3.5 K (2°C to -1.5°C freezing point)
• L_f = 280 kJ/kg (latent heat)
• c_p,below = 1.9 kJ/kg·K (frozen fish)
• ΔT_below = 18.5 K (-1.5°C to -20°C)

Q_product = 1,800 × (3.8 × 3.5 + 280 + 1.9 × 18.5) Q_product = 1,800 × (13.3 + 280 + 35.2) Q_product = 1,800 × 328.5 = 591,300 kJ per cycle

Over 3-hour cycle: Q_product = 591,300 kJ / 10,800 s = 54.8 kW average

Additional Loads:

Total Design Load:

Q_design = 54.8 × (1.0 + 0.12 + 0.06 + 0.20 + 0.12) = 54.8 × 1.50 = 82.2 kW

This freezer would require approximately 85-90 kW refrigeration capacity at -40°C evaporating temperature.

Equipment Specifications

Hydraulic System Requirements

Hydraulic Power Unit:

Plate Actuation Cylinders:

• Bore diameter: 80-150 mm depending on plate size
• Stroke length: 100-200 mm (plate opening distance)
• Rod material: Chrome-plated steel
• Seal type: Low-temperature compatible elastomers
• Position sensing: Magnetic reed switches or linear transducers

Control System Specifications

Programmable Logic Controller (PLC):

Modern vertical plate freezers utilize PLC-based control:

• Input points: 50-150 depending on system size

  • Temperature sensors (RTD or thermocouple)
  • Pressure transducers
  • Position switches
  • Safety interlocks
  • Product sensors

• Output points: 30-100

  • Solenoid valves
  • Hydraulic valve controls
  • Compressor staging
  • Defrost initiation
  • Conveyor controls

Control Sequences:

1. Automatic mode: Full cycle automation
2. Semi-automatic: Operator initiates cycles
3. Manual: Individual component control
4. Emergency: Safety shutdown procedures

Physical Specifications

Typical Equipment Dimensions:

Utility Requirements:

Installation Requirements

Foundation Specifications:

• Floor loading: 500-1500 kg/m² depending on size
• Concrete strength: Minimum 25 MPa (3,600 psi)
• Floor levelness: ±5 mm over 3 m span
• Vibration isolation: Recommended for large units
• Drainage: Floor slope 1-2% to drains

Clearances:

• Front (loading side): 2-3 m for product handling
• Rear (discharge side): 2-3 m for conveyor and maintenance
• Sides: 1-1.5 m for service access
• Overhead: 0.5-1.0 m above unit for refrigerant piping

Performance Optimization

Operational Efficiency Factors

Factors Affecting Freezing Efficiency:

1. Product-plate contact: Maximize contact area
2. Plate surface temperature: Maintain consistent temperature
3. Product thickness uniformity: Consistent product preparation
4. Hydraulic pressure: Adequate but not excessive
5. Defrost management: Minimize frequency while preventing ice buildup
6. Load factor: Maximize station utilization

Energy Efficiency Metrics:

Specific Energy Consumption (SEC) = Energy consumed (kWh) / Product frozen (kg)

Target SEC values:

• Efficient operation: 0.08-0.12 kWh/kg
• Average operation: 0.12-0.18 kWh/kg
• Poor operation: >0.18 kWh/kg

Maintenance Requirements

Preventive Maintenance Schedule:

Critical Wear Components:

• Hydraulic seals: Replace every 2-3 years
• Plate surface coatings: Inspect annually, refinish as needed
• Expansion valves: Service every 1-2 years
• Control sensors: Calibrate every 6-12 months
• Solenoid valves: Replace coils every 3-5 years

This comprehensive design and operational information provides HVAC professionals with the technical foundation for specifying, installing, and maintaining vertical plate freezer systems in commercial food processing and marine applications.