Belt Freezers for IQF

Belt Freezer Overview

Belt freezers represent the most versatile IQF technology for freezing individual food pieces in a continuous process. These systems use mesh belt conveyors to transport product through a refrigerated airstream, achieving rapid freezing while maintaining piece separation. Belt freezers excel at processing small to medium-sized products where high air velocities and uniform exposure to cold air are critical for quality.

The fundamental principle involves spreading product in a monolayer on a moving mesh belt while directing high-velocity cold air perpendicular to the belt surface. This configuration maximizes heat transfer coefficient and minimizes freezing time while preventing product agglomeration.

System Architecture

Single-Pass Belt Freezers

Single-pass configurations utilize a straight horizontal belt running through an insulated tunnel. Product enters at ambient or refrigerated temperature and exits fully frozen.

Configuration characteristics:

• Belt length: 10-40 m typical
• Freezing tunnel width: 0.6-2.4 m
• Product residence time: 3-15 minutes
• Air velocity through product: 3-6 m/s
• Evaporator temperature: -35°C to -40°C

Single-pass systems offer several advantages:

• Simple product flow path reduces jamming risk
• Easy access for cleaning and maintenance
• Lower initial capital cost
• Minimal vertical space requirement
• Straightforward integration into processing lines

Limitations include larger floor space requirements and lower capacity per unit footprint compared to multi-tier systems.

Multi-Pass Belt Freezers

Multi-pass systems stack multiple belt tiers vertically within a single insulated enclosure. Product transfers between tiers via cascading chutes or transfer conveyors, multiplying effective belt length while minimizing floor space.

Typical configurations:

Multi-pass advantages:

• Compact footprint for high-capacity applications
• Reduced building envelope requirements
• Lower refrigeration load per unit capacity
• Enhanced energy efficiency through air recirculation

Disadvantages include increased mechanical complexity, higher maintenance requirements, and potential for product damage during tier transfers.

Air Flow Patterns and Distribution

Perpendicular Air Flow Configuration

Belt freezers employ perpendicular air flow where refrigerated air passes vertically through the mesh belt and product layer. This configuration maximizes the heat transfer coefficient by ensuring direct contact between cold air and all product surfaces.

Air flow parameters:

• Approach velocity (above belt): 4-7 m/s
• Velocity through product layer: 2.5-5 m/s
• Pressure drop across belt: 50-150 Pa
• Air temperature differential: 8-12°C (entering to leaving)

The velocity through the product layer depends on:

• Belt open area (35-45% typical)
• Product loading density
• Product piece geometry
• Pressure differential across belt

Evaporator and Fan Configuration

Belt freezers use ceiling-mounted or side-mounted evaporator coils with axial or centrifugal fans directing air downward through the product.

Common configurations:

Fin spacing on evaporator coils: 4-6 mm to accommodate frost accumulation while maintaining adequate air flow between defrost cycles.

Air Recirculation Ratio

Belt freezers operate with partial air recirculation to balance energy efficiency against product quality requirements.

Recirculation parameters:

• Fresh air intake: 10-20% of total air flow
• Recirculated air: 80-90% of total air flow
• Exhaust air rate: Equal to fresh air intake
• Air turnover rate: 60-100 air changes per hour

Higher fresh air percentages benefit high-moisture products by removing water vapor, while lower percentages improve energy efficiency for dry products.

Belt Materials and Design

Mesh Belt Construction

Belt materials must withstand cryogenic temperatures, resist moisture-related degradation, and provide adequate open area for air penetration.

Material specifications:

Belt Width and Speed

Belt dimensions directly impact system capacity and freezing performance.

Standard belt widths:

• 600 mm: Laboratory and pilot systems
• 1000 mm: Small production lines (200-500 kg/hr)
• 1500 mm: Medium production (500-1200 kg/hr)
• 2000 mm: Large production (1200-3000 kg/hr)
• 2400 mm: High-capacity systems (3000+ kg/hr)

Belt speed range: 0.5-10 m/min with variable frequency drive control

Belt speed adjustment provides precise control over product residence time, compensating for:

• Product size variations
• Inlet temperature changes
• Desired final product temperature
• Production rate fluctuations

Product Loading Density

Optimal product loading maximizes capacity while maintaining piece separation and adequate air penetration.

Loading specifications:

• Layer thickness: 20-60 mm (single product layer)
• Product coverage: 70-85% of belt surface
• Piece spacing: 3-8 mm between pieces
• Loading density: 8-25 kg/m² belt area

Automated feed distribution systems ensure uniform product spreading, preventing thin spots (inadequate capacity utilization) and thick spots (insufficient freezing).

Freezing Time Calculations

Plank Freezing Time Equation

For products that can be approximated as infinite slabs, the Plank equation provides freezing time estimation:

t_f = (ρL/T_m - T_a) × (Pa/h + Ra²/k)

Where:

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

Heat Transfer Coefficient

The surface heat transfer coefficient depends on air velocity and product geometry:

h = C × v^n

Where:

• h = heat transfer coefficient (W/m²·K)
• C = empirical constant (typical range 10-15)
• v = air velocity (m/s)
• n = exponent (typically 0.6-0.8)

Typical h values for belt freezers:

Residence Time Calculation

Required belt length calculation:

L_belt = v_belt × t_f

Where:

• L_belt = required belt length (m)
• v_belt = belt velocity (m/s)
• t_f = freezing time (s)

Example calculation:

Product: 10mm diced vegetables Freezing time required: 6 minutes (360 seconds) Belt speed: 2.5 m/min (0.0417 m/s) Required belt length: 0.0417 × 360 = 15 m

Add 20% safety factor for variability: 18 m minimum belt length

Performance Specifications

Capacity Factors

System capacity depends on multiple interrelated factors:

Capacity equation:

Q = W × v × ρ_loading × 60

Where:

• Q = capacity (kg/hr)
• W = belt width (m)
• v = belt speed (m/min)
• ρ_loading = product loading density (kg/m²)

Energy Consumption

Belt freezer energy consumption includes refrigeration load and auxiliary power.

Energy components:

Total specific energy: 0.35-0.65 kWh/kg frozen product

Energy efficiency improves with:

• Higher product loading density
• Lower product inlet temperature
• Optimized air recirculation ratio
• Efficient defrost scheduling
• Variable speed drive controls

Product Applications

Ideal Products for Belt Freezers

Belt freezers excel at processing:

Vegetables:

• Diced products: 6-20 mm cubes
• Sliced products: 3-10 mm thickness
• Green beans, corn kernels, peas
• Loading density: 12-18 kg/m²

Fruits:

• Berries: strawberries, blueberries, raspberries
• Diced fruits: mango, pineapple, melon
• Loading density: 8-14 kg/m²

Seafood:

• Shrimp: all sizes
• Scallops: individual pieces
• Fish portions: uniform thickness
• Loading density: 10-16 kg/m²

Prepared foods:

• Meatballs, nuggets
• Pasta pieces
• Formed products
• Loading density: 12-20 kg/m²

Product Quality Considerations

Belt freezers provide superior quality through:

• Rapid freezing minimizes ice crystal size
• Individual piece freezing prevents clumping
• Uniform air exposure ensures consistent quality
• Gentle handling reduces mechanical damage
• Minimal dehydration compared to cryogenic systems

System Integration

Upstream Equipment

Product conditioning before freezing impacts performance:

• Pre-chilling to 0-4°C reduces freezing load by 25-35%
• Moisture removal via air knives or vibrating screens
• Size grading ensures uniform freezing times
• Distribution systems spread product uniformly

Downstream Equipment

Post-freezing handling maintains product quality:

• Transfer conveyors: insulated, temperature-controlled
• Glazing systems: for seafood moisture protection
• Packaging systems: automated filling and sealing
• Cold storage: -18°C to -25°C holding

Maintenance Requirements

Routine Maintenance

Daily tasks:

• Belt tracking inspection and adjustment
• Product distribution verification
• Temperature monitoring and recording
• Visual inspection for frost accumulation

Weekly tasks:

• Belt cleaning and sanitization
• Evaporator defrost cycle verification
• Fan bearing lubrication inspection
• Air flow pattern observation

Monthly tasks:

• Refrigerant level and pressure checks
• Electrical connection inspection
• Belt wear assessment
• Drive system alignment

Defrost Systems

Belt freezers require periodic defrost to maintain air flow and heat transfer efficiency.

Defrost methods:

Defrost scheduling depends on product moisture content, ambient humidity, and operating temperature.

Design Selection Criteria

System Sizing

Select belt freezer configuration based on:

1. Production capacity requirements (kg/hr)
2. Product characteristics (size, shape, moisture)
3. Available floor space and building height
4. Upstream and downstream integration points
5. Energy cost and efficiency priorities
6. Maintenance access requirements

Comparative Analysis

Belt freezers offer optimal performance for products requiring gentle handling, individual piece freezing, and production flexibility across varying product types and sizes.