Plate Freezer Products

Plate freezers achieve optimal performance when processing products with specific geometric and physical characteristics. Product selection directly influences heat transfer efficiency, freezing time, equipment utilization, and final product quality.

Product Suitability Criteria

Geometric Requirements

Plate freezing operates most efficiently with products meeting specific dimensional criteria:

Optimal Product Characteristics:

• Flat parallel surfaces for maximum plate contact
• Uniform thickness throughout product cross-section
• Rectangular or square footprint
• Minimal surface irregularities
• Consistent product dimensions within batch

Contact Surface Quality: Surface contact area directly affects heat transfer coefficient. Products with rough or irregular surfaces reduce effective contact from 90-95% to 60-70%, increasing freezing time by 30-50%.

Dimensional Constraints

Thickness limits result from:

• Extended freezing time beyond economic viability (>4 hours)
• Increased core temperature hold time
• Excessive pressure requirements for contact
• Plate spacing limitations in standard equipment

Fish Products

Fish Blocks

Fish blocks represent the primary application for horizontal plate freezers in the seafood industry.

Groundfish Blocks:

• Minced cod, haddock, pollock
• Block dimensions: 406 x 203 x 64 mm (16 x 8 x 2.5 in)
• Weight: 7.3 kg (16 lb) standard
• Freezing time: 90-120 minutes at -40°C plate temperature
• Heat transfer coefficient: 200-250 W/(m²·K)

Block Formation Process:

1. Filleting and deboning
2. Mechanical mincing or grinding
3. Form filling in cardboard cartons
4. Plate loading with carton
5. Freezing under pressure (0.5-1.0 bar)

Heat Transfer Calculation:

Freezing time approximation using Plank’s equation modified for plate contact:

$$t_f = \frac{\rho L}{T_p - T_f} \left[\frac{P a}{h} + \frac{R a^2}{k}\right]$$

Where:

• t_f = freezing time (s)
• ρ = product density (kg/m³)
• L = latent heat of fusion (kJ/kg)
• T_p = plate temperature (°C)
• T_f = final product temperature (°C)
• a = thickness (m)
• h = surface heat transfer coefficient (W/(m²·K))
• k = thermal conductivity (W/(m·K))
• P = 1/2 (for infinite slab)
• R = 1/8 (for infinite slab)

Typical Fish Block Values:

• ρ = 1040 kg/m³
• L = 280 kJ/kg (includes sensible and latent heat)
• k_frozen = 1.8 W/(m·K)
• k_unfrozen = 0.5 W/(m·K)
• Initial temperature = 4°C
• Moisture content = 75-80%

Fish Fillets

Product Specifications:

Fillet Preparation:

• Individual quick frozen (IQF) or block frozen
• Skin-on or skinless affects contact
• Trimmed to uniform thickness
• Glazing after freezing recommended

Quality Factors:

Fillet quality in plate freezing depends on:

1. Pressure Control: 0.3-0.8 bar contact pressure

   • Insufficient pressure: poor contact, slow freezing
   • Excessive pressure: product damage, moisture loss

2. Freezing Rate: Target 5-25 mm/hour through -1 to -5°C zone

   • Faster rates: smaller ice crystals, better texture
   • Slower rates: drip loss on thawing increases

3. Temperature Uniformity: ±2°C across plate surface

   • Non-uniform temperature creates quality variation
   • Affects yield and appearance

Breaded Fish Products

Formed breaded fish sticks and portions:

• Pre-cooked or raw
• Uniform rectangular shape
• Thickness: 10-15 mm
• Breading provides flat surface
• Freezing time: 15-30 minutes
• Ideal for horizontal plate freezers

Meat Products

Formed Meat Patties

Hamburger Patties:

Thermal Properties of Ground Beef:

Fat content significantly affects freezing characteristics:

Higher fat content requires longer freezing time due to:

• Lower thermal conductivity
• Lower latent heat per unit mass
• Depressed freezing point

Sausage Products:

• Breakfast sausage patties: 50-75 mm diameter, 8-12 mm thick
• Formed links in rectangular trays
• Pre-cooked products freeze faster
• Interleaving paper prevents sticking
• Freezing time: 15-25 minutes

Portion-Cut Steaks

Specifications:

• Thickness: 20-35 mm maximum for plate freezing
• Individual vacuum packaging required
• Flat surface essential for contact
• Boneless cuts only
• Freezing time: 60-90 minutes
• Applications: portion steaks, sandwich steaks

Processed Meat Products

Sliced Luncheon Meats:

• Stacked in rectangular blocks
• Interleaving film between slices
• Block height: 40-80 mm
• Excellent plate contact
• Freezing time: 30-50 minutes
• Used for food service distribution

Vegetable Products

Block Frozen Vegetables

Block freezing in plate freezers serves as intermediate storage before retail packaging.

Spinach Blocks:

Blanching Impact:

Blanching affects freezing performance:

• Enzyme inactivation preserves quality
• Cell structure disruption increases thermal conductivity
• Moisture content changes: 85-92%
• Air removal improves plate contact

Other Block Vegetables:

Void Space Considerations:

Particulate vegetables contain interstitial air:

• Reduces effective thermal conductivity
• Increases freezing time by 20-40%
• Compression during freezing minimizes voids
• Pressure: 0.5-1.0 bar typical

Vegetable Purees

Suitable Products:

• Squash puree
• Sweet potato puree
• Pumpkin filling
• Tomato paste blocks

Advantages:

• Excellent plate contact (no voids)
• Uniform heat transfer
• Faster freezing than particulate vegetables
• Freezing time: 60-90 minutes for 50 mm thickness

Product Thickness Limits

Maximum Economical Thickness

Thickness limitation driven by freezing time economics:

Freezing Time vs. Thickness:

For a fish block at -40°C plate temperature:

Note: Freezing time increases approximately with square of thickness (Plank’s equation), while throughput remains relatively constant until excessive thickness reduces productivity.

Practical Thickness Ranges:

Minimum Thickness

Extremely thin products also present challenges:

• Difficult to achieve uniform pressure
• Product damage from excessive pressure
• Poor economics (low weight per plate area)
• Minimum practical thickness: 6-8 mm

Package Requirements

Carton Specifications

Material Selection:

Carton Thermal Resistance:

Typical carton adds thermal resistance:

$$R_{carton} = \frac{t_{carton}}{k_{carton}}$$

Where:

• t_carton = 1.5-2.5 mm thickness
• k_carton = 0.05-0.08 W/(m·K)
• R_carton = 0.019-0.050 m²·K/W

This resistance increases freezing time by 10-25% depending on product thickness.

Structural Requirements:

• Wet strength sufficient for handling
• Dimensional stability under pressure
• Flat surfaces for plate contact
• Corner reinforcement for stacking
• Moisture barrier for product protection

Vacuum Packaging

Film Properties:

Advantages for Plate Freezing:

• Eliminates air gaps between product and package
• Improves plate contact
• Reduces freezer burn
• Extends shelf life
• Minimal thermal resistance (film thickness 0.05-0.15 mm)

Vacuum Level:

• 98-99.5% vacuum for irregular products
• Ensures conformance to plate surface
• Critical for products with surface irregularities

Interleaving Materials

For stacked products (patties, fillets):

Material Options:

Function:

• Prevents product adhesion
• Allows individual piece removal
• Must not interfere with heat transfer
• Adds negligible thermal resistance

Freezing Time by Product Type

Calculation Methodology

Modified Plank’s Equation:

$$t_f = \frac{\Delta H_e}{T_m - T_c} \left[\frac{P a}{h_1 + h_2} + \frac{R a^2}{k_f}\right]$$

Where:

• ΔH_e = effective volumetric enthalpy change (kJ/m³)
• T_m = mean freezing temperature (°C)
• T_c = coolant/plate temperature (°C)
• h_1, h_2 = surface heat transfer coefficients (W/(m²·K))
• k_f = thermal conductivity of frozen product (W/(m·K))

Enthalpy Change:

$$\Delta H_e = \rho \left[c_1(T_i - T_m) + L + c_2(T_m - T_f)\right]$$

Where:

• c_1 = specific heat above freezing (kJ/(kg·K))
• c_2 = specific heat below freezing (kJ/(kg·K))
• T_i = initial temperature (°C)
• T_f = final center temperature (°C)
• L = latent heat of fusion (kJ/kg)

Comparative Freezing Times

Standard Conditions: Plate temperature -35°C, initial product temperature 4°C, final temperature -18°C

Temperature Profile During Freezing

Characteristic Stages:

1. Cooling Phase: Product surface to freezing point

   • Duration: 5-10% of total freezing time
   • Rapid temperature drop
   • High heat flux

2. Phase Change: Latent heat removal

   • Duration: 70-80% of total freezing time
   • Temperature plateau at freezing point
   • Constant heat flux
   • Ice crystal formation

3. Tempering Phase: Frozen product to final temperature

   • Duration: 10-20% of total freezing time
   • Slower temperature drop
   • Decreasing heat flux

Time-Temperature Example (50 mm fish block):

Quality Considerations

Ice Crystal Formation

Crystal Size Impact:

Freezing rate directly affects ice crystal size:

Plate Freezer Performance:

Typical plate freezing achieves 2-6 cm/hr through critical zone (-1 to -5°C), producing good to excellent quality.

Critical Zone:

• Temperature range where maximum ice formation occurs
• Residence time in this zone determines crystal size
• Plate freezing minimizes this time through high heat transfer

Texture Preservation

Cell Structure Damage:

Ice crystal formation causes mechanical damage:

• Large crystals rupture cell walls
• Smaller crystals cause less damage
• Rapid freezing preserves texture
• Slow freezing creates mushy texture on thawing

Product-Specific Concerns:

Moisture Migration

Sublimation Prevention:

During frozen storage, moisture sublimes from product surface:

• Appears as freezer burn
• Reduces product weight
• Degrades appearance and texture
• Prevented by proper packaging

Plate Freezing Advantages:

• Rapid freezing minimizes surface exposure time
• Smooth frozen surface reduces sublimation sites
• Package contact during freezing creates barrier
• Lower surface temperature differential during storage

Pressure Effects

Contact Pressure Benefits:

Optimal pressure range: 0.3-1.0 bar depending on product

Benefits:

• Eliminates air gaps (reduces thermal resistance)
• Improves heat transfer coefficient from 50 W/(m²·K) to 200+ W/(m²·K)
• Maintains product shape
• Creates flat, uniform surface finish

Excessive Pressure Problems:

• Product deformation
• Moisture expression from product
• Equipment damage
• Uneven pressure distribution

Product-Specific Pressure:

Equipment Specifications

Plate Design Requirements

Surface Finish:

Smoother surfaces provide better contact, especially with irregular products.

Plate Spacing:

Adjustable spacing accommodates various product thicknesses:

• Minimum spacing: 15-20 mm
• Maximum spacing: 150-200 mm
• Adjustment increment: 5-10 mm
• Manual or hydraulic adjustment

Refrigeration System Requirements

Evaporator Temperature:

Plate temperature driven by product requirements:

Refrigeration Capacity:

Average heat load calculation:

$$Q = \frac{\dot{m} \cdot \Delta H}{t_{cycle}}$$

Where:

• Q = refrigeration capacity (kW)
• ṁ = product mass flow rate (kg/s)
• ΔH = enthalpy change (kJ/kg)
• t_cycle = cycle time including loading/unloading

Example Calculation:

Horizontal plate freezer specifications:

• Plate area: 20 m² total
• Product: fish blocks, 64 mm thick
• Loading: 12 kg/m²
• Freezing time: 105 minutes
• Loading/unloading: 15 minutes
• Cycle time: 120 minutes

Product throughput: (20 m² × 12 kg/m²) / (120 min / 60) = 120 kg/hr

Enthalpy change for fish: 280 kJ/kg

Refrigeration load: (120 kg/hr × 280 kJ/kg) / 3600 s/hr = 9.3 kW average

Peak load during initial cooling: 15-20 kW

Product Handling Systems

Automatic Loading:

For high-volume operations:

• Robotic cartridge loading systems
• Pneumatic plate opening/closing
• Automated product transport
• Reduces cycle time by 5-10 minutes
• Improves consistency

Cartridge Systems:

Pre-loaded cartridges with products:

• Slide into horizontal plate freezers
• Eliminates manual loading
• Reduces labor requirements
• Improves safety
• Standard cartridge: 400-600 mm wide

Quality Control Integration:

Inline systems monitor:

• Product weight (±1% accuracy)
• Dimensional compliance
• Temperature verification
• Reject non-conforming products before freezing