Plate Freezers

Plate freezers utilize contact freezing principles to achieve rapid freezing of packaged foods through direct conduction heat transfer. Refrigerant-cooled metal plates press against product surfaces, creating thermal contact that eliminates air gaps and maximizes heat removal rates. This method produces freezing rates 5-10 times faster than air blast freezing for suitable products.

Contact Freezing Principles

Contact freezing achieves superior heat transfer coefficients by eliminating convective resistance between the cooling medium and product surface.

Heat Transfer Mechanism:

The overall heat transfer coefficient in contact freezing combines:

• Conduction through plate material: 40-60 W/(m²·K)
• Contact resistance at plate-product interface: 200-500 W/(m²·K) with pressure
• Conduction through product: variable, 1-2 W/(m·K) for frozen food
• Conduction through packaging: 10-50 W/(m²·K) depending on material

Applied pressure between plates and product reduces contact resistance by:

• Eliminating air gaps at the interface
• Deforming product slightly to increase contact area
• Compressing packaging material against product surface
• Maintaining consistent contact during freezing volume changes

Total heat transfer coefficients in plate freezers reach 150-300 W/(m²·K), compared to 15-25 W/(m²·K) in air blast systems. This 10-fold improvement reduces freezing time proportionally, with a 4-inch thick product freezing in 2-4 hours versus 12-24 hours in air blast.

Freezing Rate Calculation:

Plank’s equation estimates freezing time for slab-shaped products:

t_f = (ρ·L_f / (T_i - T_p)) · (P·a/h + R·a²/k_f)

Where:

• t_f = freezing time (s)
• ρ = product density (kg/m³)
• L_f = latent heat of fusion (334 kJ/kg for water)
• T_i = initial freezing point (°C)
• T_p = plate temperature (°C)
• P, R = shape factors (0.5, 0.125 for infinite slab)
• a = product thickness (m)
• h = surface heat transfer coefficient (W/(m²·K))
• k_f = thermal conductivity of frozen product (W/(m·K))

For contact freezing, the high h value minimizes the first term, making conduction through the product (second term) the dominant resistance.

Horizontal Plate Freezers

Horizontal configurations feature vertically stacked refrigerated plates with product loaded between adjacent plates from the front face.

Design Configuration:

• Plate spacing: 50-150 mm adjustable via hydraulic rams
• Typical dimensions: 1.5-2.5 m wide × 3-6 m long per plate
• Station count: 12-40 plates in commercial units
• Hydraulic pressure: 2-10 bar applied to compress plates against product
• Refrigerant passages: internal channels 10-15 mm diameter, 50-75 mm spacing

Loading Sequence:

1. Plates open hydraulically to maximum spacing
2. Product cartons manually loaded or conveyor-fed onto each plate level
3. Plates close to contact product with controlled pressure
4. Refrigerant flow initiated through plate circuits
5. After freezing cycle completion, plates open for product removal

Heat Transfer Design:

Horizontal plate freezers optimize heat transfer through:

• Bilateral heat removal from both top and bottom product surfaces
• Aluminum plates with high thermal conductivity (200-250 W/(m·K))
• Internal refrigerant evaporation at -40 to -45°C
• Refrigerant channels designed for uniform temperature distribution
• Plate surface flatness maintained within ±1 mm across plate area

Product Suitability:

Horizontal plate freezers excel for:

• Fish fillets and blocks
• Meat patties and formed products
• Prepared meals in rectangular packaging
• Vegetable blocks
• Juice concentrates

Product requirements include:

• Rectangular or flat geometry to maximize contact area
• Packaging that withstands compression (2-10 bar pressure)
• Thickness typically 25-100 mm for optimal freezing time
• Consistent thickness across product for uniform freezing

Performance Characteristics:

Vertical Plate Freezers

Vertical configurations position plates horizontally with product loaded between vertically separated plates, allowing continuous or semi-continuous operation.

Design Configuration:

Vertical plate freezers employ:

• Horizontally oriented plates mounted on movable carriages
• Plate spacing: 20-150 mm hydraulically adjustable
• Loading from top or sides depending on design
• Automatic indexing systems for continuous throughput
• Single-sided or double-sided refrigeration depending on product

Operational Modes:

Batch Mode:

1. Plates open to receive product
2. Product loaded manually or automatically
3. Plates close with controlled pressure
4. Fixed freezing cycle (1-4 hours)
5. Plates open and product discharged
6. Cycle repeats

Continuous Mode:

1. Product enters loading zone
2. Conveyor indexes product through freezing zone
3. Progressive freezing as product advances
4. Discharge at exit with fully frozen product
5. Throughput: 500-5000 kg/hr depending on capacity

Advantages Over Horizontal Configuration:

• Floor space efficiency with vertical orientation
• Easier automation integration
• Simpler product loading/unloading mechanisms
• Better drainage of condensate
• Easier access for cleaning and maintenance

Design Variations:

Double-Contact Vertical Freezers:

• Product frozen between two plates (bilateral freezing)
• Maximum heat transfer rate
• Suitable for packaged products 20-100 mm thick

Single-Contact Vertical Freezers:

• One refrigerated plate, one insulated restraint plate
• Lower energy consumption
• Suitable for products with one flat surface

Plate Freezer Refrigeration Systems

Plate freezers operate as direct expansion systems with refrigerant evaporating within the plates.

Refrigerant Selection:

Refrigerant Distribution:

Plate circuits require careful design:

• Individual expansion valves per plate for temperature control
• Superheat maintained at 5-10 K at evaporator outlet
• Refrigerant velocity: 3-8 m/s in channels for good heat transfer
• Oil return considerations in low-temperature applications
• Hot gas defrost capability integrated into circuit design

Defrost Systems:

Frost accumulation on plates requires periodic removal:

• Hot gas defrost: Reverse refrigerant flow at +20 to +40°C for 10-20 minutes
• Electric defrost: Resistance heaters at 500-1500 W/m² plate area
• Water defrost: Applicable only in higher temperature applications
• Defrost frequency: Every 6-24 hours depending on ambient conditions
• Defrost efficiency improves with automatic termination based on plate temperature

Heat Transfer Optimization

Maximizing heat transfer in plate freezers requires attention to multiple factors.

Contact Pressure Optimization:

Applied pressure must balance competing requirements:

• Higher pressure reduces contact resistance
• Excessive pressure damages product or packaging
• Pressure uniformity across plate area critical
• Typical optimization: 3-5 bar for most applications

Plate Surface Design:

Surface characteristics affecting heat transfer:

• Surface roughness: Ra = 1.6-3.2 μm optimal
• Surface flatness: ±1 mm maximum deviation
• Material: Aluminum alloy 5000-series for corrosion resistance
• Coating: Anodized or special coatings for cleanability
• Thickness: 8-15 mm for mechanical strength and thermal performance

Refrigerant-Side Enhancement:

Internal plate design maximizes refrigerant-side heat transfer:

• Channel geometry: circular or rectangular passages
• Channel diameter: 10-15 mm for good distribution
• Channel spacing: 50-75 mm for temperature uniformity
• Refrigerant distribution headers ensure equal flow to all channels
• Vapor quality management prevents liquid carryover

Thermal Performance Monitoring:

Critical measurements for performance verification:

• Plate surface temperature: infrared or embedded sensors
• Refrigerant saturation temperature at inlet and outlet
• Product core temperature during freezing
• Freezing time for standard test products
• Energy consumption per kg product frozen

Product Quality Considerations

Contact freezing affects product quality through freezing rate and mechanical pressure.

Crystal Size Control:

Rapid freezing in plate systems produces:

• Ice crystal size: 30-100 μm typical
• Minimal cell damage compared to slow freezing
• Better texture retention upon thawing
• Reduced drip loss: 1-3% versus 5-10% for slow freezing

Physical Product Effects:

Plate pressure impacts:

• Product density increase: 2-5% typical compression
• Package deformation: designed into packaging specifications
• Surface marking: minimized by smooth plate surfaces
• Thickness uniformity: pressure equalizes variations

Packaging Requirements:

Suitable packaging must provide:

• Mechanical strength to resist 2-10 bar compression
• Thermal conductivity for heat transfer
• Moisture barrier properties
• Flat surfaces for good plate contact
• Thickness typically 0.1-0.5 mm for polymer films

Performance Parameters

Comprehensive performance metrics for plate freezer specification:

System Selection Criteria

Plate freezer selection depends on multiple product and operational factors:

Product Characteristics:

• Flat or block-shaped geometry required
• Consistent thickness across product
• Packaging able to withstand compression
• Product thickness 20-150 mm range

Production Requirements:

• Batch versus continuous processing preference
• Required throughput capacity
• Available floor space constraints
• Labor availability for loading/unloading

Economic Considerations:

• Capital cost: $150,000-$800,000 depending on capacity
• Energy efficiency advantages over air blast
• Maintenance requirements and costs
• Product quality improvements justify premium cost

Plate freezers represent the most efficient freezing method for suitable products, offering rapid freezing rates, excellent product quality, compact footprint, and favorable energy consumption compared to alternative freezing technologies.

Sections