Freezing Process

Overview

Ice cream freezing represents the critical process step that transforms a liquid mix into a frozen aerated product. The freezing process accomplishes three primary objectives: ice crystal formation and size control, air incorporation (overrun), and viscosity development for proper extrusion. The thermodynamic and mechanical conditions during freezing directly determine final product quality, texture, and stability.

The freezing operation occurs in two distinct stages: dynamic freezing in scraped surface freezers where 50-60% of the water content freezes, followed by static hardening where additional freezing occurs during storage. This section addresses the dynamic freezing stage and its refrigeration requirements.

Continuous Freezer Operation

Scraped Surface Freezer Design

Continuous freezers employ a horizontal cylindrical barrel with rotating dasher blades that scrape the freezing surface. The typical configuration consists of:

Barrel Construction:

• Stainless steel cylinder: 150-300 mm diameter, 1.5-3.0 m length
• Internal refrigerant jacket operating at -30 to -35°C evaporating temperature
• Heat transfer surface area: 0.7-2.5 m²
• Working pressure: 2.0-4.0 MPa to prevent boiling

Dasher Assembly:

• Rotating shaft with scraper blades: 200-400 rpm
• Blade clearance from barrel wall: 0.5-1.0 mm
• Drive motor power: 15-75 kW depending on capacity
• Scraping frequency: 4-8 passes per second per location

Process Flow:

1. Mix enters at 2-4°C from aging tank
2. Refrigerant evaporates in jacket at -30 to -35°C
3. Dasher blades continuously scrape frozen layer
4. Air injection at 100-500 kPa pressure
5. Product exits at draw temperature -5 to -7°C
6. Residence time: 30-90 seconds

Operating Temperature Parameters

The temperature profile through a continuous freezer determines ice crystal formation rates and final texture characteristics.

The temperature differential between refrigerant and product provides the driving force for heat transfer:

ΔT = T_refrigerant - T_product = (-32°C) - (-6°C) = 26°C (typical)

Production Capacity

Continuous freezer capacity depends on barrel geometry, refrigeration capacity, and product characteristics:

Capacity Calculation:

Q_product = ṁ × (h_inlet - h_outlet)

Where:

• Q_product = refrigeration load (kW)
• ṁ = mass flow rate of mix (kg/s)
• h_inlet = enthalpy of mix at inlet (kJ/kg)
• h_outlet = enthalpy of frozen product at draw (kJ/kg)

Typical Production Rates:

Batch Freezer Requirements

Operating Characteristics

Batch freezers serve smaller operations, artisanal production, and recipe development applications. Unlike continuous systems, batch freezers process discrete quantities in a stationary vessel.

Configuration:

• Vertical or horizontal barrel: 20-200 L capacity
• Stationary refrigerated jacket
• Rotating dasher and scraper assembly
• Batch cycle time: 8-15 minutes
• Manual or automated discharge

Operating Sequence:

1. Fill barrel with mix (60-70% of capacity)
2. Start dasher rotation (100-200 rpm)
3. Activate refrigeration (-30 to -35°C evaporating)
4. Inject air during freezing cycle
5. Monitor temperature and viscosity
6. Discharge when draw temperature reached
7. Clean barrel for next batch

Batch Freezer Refrigeration

The refrigeration load varies significantly during the batch cycle, creating control challenges.

Load Profile:

Capacity Calculation:

For a 100 L batch freezer producing premium ice cream:

Mix mass: 100 L × 1.10 kg/L = 110 kg Overrun: 80% Final product: 110 kg × 1.80 = 198 kg (including air)

Heat removal required:

• Sensible cooling 4°C to 0°C: 110 kg × 3.8 kJ/kg·K × 4 K = 1672 kJ
• Latent freezing (50% water frozen): 110 kg × 0.62 × 0.50 × 334 kJ/kg = 11,395 kJ
• Sensible cooling 0°C to -6°C: 110 kg × 2.5 kJ/kg·K × 6 K = 1650 kJ
• Total: 14,717 kJ over 10 minutes = 24.5 kW average

Peak load (during maximum freezing 2-6 min): 35-40 kW

Overrun and Air Incorporation

Overrun Definition and Control

Overrun quantifies the volume increase from air incorporation:

Overrun (%) = [(V_frozen - V_mix) / V_mix] × 100

Or by mass:

Overrun (%) = [(m_mix - m_frozen) / m_frozen] × 100

Product Categories:

Air Injection Systems

Air incorporation requires precise control to achieve target overrun consistently.

Air Supply Requirements:

• Filtered compressed air: 100-500 kPa
• Flow rate: 5-15% of mix volume flow
• Dew point: -40°C minimum (moisture removal)
• Oil-free compressor required
• Sterile filtration: 0.2 μm absolute

Control Methods:

1. Fixed orifice metering:

   • Simple, low cost
   • Overrun varies with back pressure
   • Acceptable for narrow product range

2. Mass flow controller:

   • Electronic measurement and control
   • Maintains constant air mass regardless of pressure
   • Required for consistent overrun across products

3. Ratio control:

   • Air flow proportional to mix flow
   • Automatic adjustment for production rate changes
   • Highest accuracy and consistency

Air Distribution:

• Injection point: upstream of dasher assembly
• Bubble size at injection: 1-5 mm
• Bubble size after dasher: 20-80 μm (target: 40 μm)
• Bubble count: 10⁸-10⁹ per mL

Impact on Refrigeration Load

Air incorporation affects refrigeration requirements through multiple mechanisms:

Heat of compression: When air is compressed from atmospheric pressure to injection pressure, compression heat must be removed.

Q_compression = (ṁ_air / ρ_air) × P_injection × [(γ-1)/γ] / η_compression

For typical conditions:

• Air flow: 10 L/min at 300 kPa
• Heat addition: 0.5-1.5 kW (typically negligible)

Heat transfer coefficient enhancement: Air bubbles increase turbulence and enhance heat transfer:

h_aerated = h_base × (1 + 0.015 × Overrun%)

For 80% overrun: 12% increase in heat transfer coefficient

Thermal capacity reduction: Air has negligible thermal capacity, reducing the mass requiring cooling:

C_p,effective = C_p,mix × [m_mix / (m_mix + m_air)]

This reduction partially offsets the refrigeration load but requires increased heat transfer surface residence time.

Draw Temperature Specifications

Temperature Targets

Draw temperature represents the product temperature exiting the freezer. This critical parameter determines product consistency, pumpability, and ice crystal size distribution.

Target Ranges by Product:

Psychrometric Considerations

The relationship between temperature and ice phase depends on mix composition, particularly sugar content which depresses the freezing point.

Ice Formation Curve:

The fraction of water frozen at any temperature below initial freezing:

I = (T_f - T) / (T_f - T_w)

Where:

• I = mass fraction of water frozen
• T_f = initial freezing point (°C)
• T = actual temperature (°C)
• T_w = temperature where all freezable water is frozen (°C)

For typical ice cream mix:

• T_f = -2.5°C
• T_w = -55°C (theoretical)
• At T = -6°C: I = (-2.5 - (-6)) / (-2.5 - (-55)) = 0.067 (50% of total water)

Freezing Point Depression:

ΔT_f = K_f × m × i

Where:

• ΔT_f = freezing point depression (°C)
• K_f = cryoscopic constant for water (1.86 °C·kg/mol)
• m = molality of solutes (mol/kg)
• i = van’t Hoff factor

Higher sugar content → Lower freezing point → Less ice at given temperature

Temperature Control Strategies

Maintaining consistent draw temperature requires active control of refrigerant conditions and product flow rate.

Control Variables:

1. Refrigerant evaporating temperature: -28 to -35°C range
2. Mix feed rate: ±10% adjustment range
3. Dasher speed: typically fixed, ±5% fine tuning
4. Air injection rate: proportional to mix flow

Measurement:

• RTD sensor in product discharge: ±0.2°C accuracy
• PID controller with 2-5 second update rate
• Control action: modulate refrigerant expansion valve
• Secondary control: adjust mix feed pump speed

Draw Temperature Impact:

Too warm (-4 to -5°C):

• Insufficient viscosity for extrusion
• Product slumps after filling
• Large ice crystals form during hardening
• Texture defects

Too cold (-7 to -8°C):

• Excessive pump work required
• Dasher motor overload
• Air incorporation difficulty
• Stress cracking in product

Refrigerant Systems for Ice Cream Freezing

Ammonia (R-717) Systems

Ammonia dominates large-scale ice cream production due to superior thermodynamic properties and environmental sustainability.

System Design Parameters:

Thermodynamic Performance:

At -32°C evaporating, +32°C condensing:

• COP (Coefficient of Performance): 2.8-3.2
• Volumetric capacity: 1950 kJ/m³
• Compression ratio: 7.5:1
• Specific work: 275 kJ/kg

System Components:

• Flooded shell-and-tube evaporator with ammonia on shell side
• Secondary glycol or CO₂ circuit to freezer jackets
• Screw or reciprocating compressors
• Evaporative or air-cooled condensers
• Thermosyphon oil cooling
• Engine room ventilation per IIAR-2

Safety Considerations:

• Refrigerant charge: minimize through efficient evaporators
• Machinery room: separate from production area
• Emergency ventilation: 30 air changes per hour
• Ammonia detection: 25 ppm alarm, 150 ppm evacuation
• Personnel training: IIAR requirements

R-404A and R-407A Systems

Synthetic refrigerants suit smaller operations and areas where ammonia is restricted.

R-404A Properties:

System Characteristics:

• Direct expansion to freezer jacket (no secondary loop)
• Smaller refrigerant charge than ammonia systems
• Higher operating pressures require robust components
• Scroll, reciprocating, or small screw compressors
• Air-cooled or water-cooled condensers
• Electronic expansion valves for precise control

Phase-Out Considerations:

R-404A faces regulatory pressure due to high GWP. Alternatives:

• R-407A: Lower GWP (2107), temperature glide
• R-448A: GWP 1387, drop-in replacement
• R-449A: GWP 1397, similar performance
• CO₂ cascade: GWP 1, higher efficiency

Carbon Dioxide (R-744) Cascade Systems

CO₂ cascade systems employ ammonia or HFC for high-stage, CO₂ for low-stage at freezing temperatures.

System Configuration:

• Low-stage: CO₂ at -35°C evaporating
• Cascade condenser: CO₂ condenses at -8 to -12°C
• High-stage: NH₃ or HFC evaporates at -12°C
• High-stage condenses at ambient +30 to +35°C

Advantages:

• CO₂ in production area (safe, non-toxic)
• Excellent heat transfer properties
• High efficiency at low temperatures
• Minimal environmental impact (GWP = 1)

Thermodynamic Performance:

Low stage (CO₂):

• Evaporating: -35°C at 630 kPa
• Condensing: -10°C at 2450 kPa
• COP: 3.5-4.0 (low stage only)

Overall system COP: 2.5-2.8 (comparable to ammonia)

Heat Transfer in Scraped Surface Freezers

Heat Transfer Mechanisms

The scraped surface freezer involves complex heat transfer combining conduction, forced convection, and phase change.

Overall Heat Transfer:

Q = U × A × ΔTLM

Where:

• Q = heat transfer rate (W)
• U = overall heat transfer coefficient (W/m²·K)
• A = heat transfer surface area (m²)
• ΔTLM = log mean temperature difference (K)

Thermal Resistance Network:

1/U = 1/h_refrigerant + t_wall/k_wall + 1/h_product

Where:

• h_refrigerant = refrigerant-side coefficient: 2000-3000 W/m²·K (boiling)
• t_wall = wall thickness: 3-6 mm
• k_wall = thermal conductivity of stainless steel: 15 W/m·K
• h_product = product-side coefficient: 800-2000 W/m²·K (scraped)

Typical U-values:

Product-Side Heat Transfer

The product-side coefficient depends on scraper action, flow velocity, and product rheology.

Correlation for Scraped Surface:

Nu = 0.023 × Re^0.8 × Pr^0.33 × (1 + 2.5 × N_scrape)

Where:

• Nu = Nusselt number = h·D/k
• Re = Reynolds number = ρ·v·D/μ
• Pr = Prandtl number = μ·Cp/k
• N_scrape = scraping frequency parameter

Impact of Scraping:

• Removes insulating frozen layer (1-3 mm thickness)
• Enhances turbulence near wall
• Reduces effective thermal resistance by 3-5×
• Critical for maintaining high heat transfer rates

Temperature Profile in Product:

Radial temperature gradient exists from wall to centerline:

T(r) = T_wall + (T_center - T_wall) × (r/R)^n

Where n depends on flow characteristics (n = 1.5-2.5 for ice cream)

For 150 mm barrel:

• Wall temperature: -25°C
• Center temperature: +2°C
• Average bulk temperature: -6°C

Enhancement Techniques

Dasher Blade Design:

• Blade angle: 30-45° to rotation plane
• Blade width: 20-40 mm
• Material: food-grade plastic or coated metal
• Replacement frequency: 500-1000 hours operation

Surface Treatments:

• Electropolished stainless: Ra < 0.4 μm
• Hydrophobic coatings: reduce ice adhesion
• Pattern texturing: enhance turbulence

Operational Optimization:

• Maintain minimum scraping frequency
• Control ice layer thickness < 2 mm
• Optimize dasher speed vs. production rate
• Monitor power consumption as indicator

Refrigeration Load Calculations

Comprehensive Load Analysis

Accurate refrigeration load calculation ensures proper equipment sizing and efficient operation.

Total Load Components:

Q_total = Q_product + Q_motor + Q_air + Q_transmission + Q_defrost

Product Cooling and Freezing Load

Sensible Cooling (above freezing):

Q_sensible = ṁ × C_p,mix × (T_inlet - T_freezing)

Typical values:

• ṁ = 0.5 kg/s (1800 L/hr at 1.1 kg/L)
• C_p,mix = 3.8 kJ/kg·K
• T_inlet = 4°C
• T_freezing = -2.5°C
• Q_sensible = 0.5 × 3.8 × 6.5 = 12.4 kW

Latent Freezing Load:

Q_latent = ṁ × X_water × f_frozen × L_fusion

Where:

• X_water = mass fraction water in mix: 0.60-0.65
• f_frozen = fraction of water frozen: 0.50-0.55
• L_fusion = latent heat of fusion: 334 kJ/kg

Q_latent = 0.5 × 0.62 × 0.52 × 334 = 53.8 kW

Sensible Cooling (frozen product):

Q_sensible2 = ṁ × C_p,frozen × (T_freezing - T_draw)

C_p,frozen = 2.3-2.7 kJ/kg·K (varies with ice content)

Q_sensible2 = 0.5 × 2.5 × 3.5 = 4.4 kW

Total Product Load: 12.4 + 53.8 + 4.4 = 70.6 kW

Mechanical Energy Input

Dasher work converts entirely to heat within the product:

Q_motor = P_motor × η_transmission / η_cooling

Where:

• P_motor = motor power draw: 30 kW (typical for 1800 L/hr)
• η_transmission = mechanical efficiency: 0.85
• η_cooling = fraction appearing as heat in product: 0.90

Q_motor = 30 × 0.85 / 0.90 = 28.3 kW

This represents 28.3/70.6 = 40% of product cooling load, a significant contribution.

Air Compression Heat

Q_air = ṁ_air × C_p,air × ΔT_compression

For 80% overrun at 1800 L/hr production:

• Air flow: 1440 L/hr = 0.40 L/s = 0.48 g/s
• Compression heating: 300 kPa, ~40°C rise
• Q_air = 0.00048 × 1.005 × 40 = 0.02 kW (negligible)

Transmission Losses

Heat ingress through insulation and process connections:

Q_transmission = U_insulation × A_surface × (T_ambient - T_refrigerant)

For typical freezer:

• Surface area: 8 m²
• U-value with 75 mm insulation: 0.25 W/m²·K
• ΔT = 20 - (-32) = 52 K
• Q_transmission = 0.25 × 8 × 52 = 0.10 kW (negligible during operation)

Total Refrigeration Requirement

Q_total = 70.6 + 28.3 + 0.02 + 0.10 = 99.0 kW

Safety Factor: Apply 10-15% safety factor for:

• Mix composition variation
• Ambient temperature swings
• System efficiency degradation
• Future capacity expansion

Design Load: 99.0 × 1.15 = 114 kW

Compressor Selection:

At -32°C evaporating, +32°C condensing:

• Required refrigeration: 114 kW
• System COP: 2.8 (ammonia)
• Compressor power: 114 / 2.8 = 40.7 kW
• Select: 45 kW compressor with capacity control

Equipment Specifications

Freezer Barrel Assembly

Materials:

• Barrel: 316L stainless steel, 3-6 mm wall
• Jacket: 304 or 316 stainless, welded construction
• Insulation: 50-100 mm polyurethane foam, density 40 kg/m³
• Exterior shell: Stainless or painted steel

Mechanical Specifications:

• Barrel internal volume: 15-45 liters (residence volume)
• Heat transfer surface: 0.7-2.5 m²
• Design pressure: 3.0 MPa (barrel), 2.0 MPa (jacket)
• Maximum speed: 500 rpm
• Bearing type: sealed food-grade

Dasher Drive System

Motor Specifications:

• Type: Variable frequency drive (VFD) controlled
• Power: 15-75 kW depending on capacity
• Speed range: 0-400 rpm
• Torque: 200-800 N·m at full load
• Efficiency: 92-95% (premium efficiency)
• Enclosure: NEMA 4X (washdown duty)

Gearbox:

• Ratio: 3:1 to 8:1
• Type: Helical or planetary
• Lubrication: Food-grade synthetic oil
• Sealing: Triple-lip seals, positive pressure

Refrigeration Components

Expansion Device:

• Electronic expansion valve (EEV) for precise control
• Capacity range: 10-100% modulation
• Response time: < 2 seconds
• Superheat control: maintain 5-8 K
• Fail-safe: close on power loss

Evaporator/Jacket:

• Type: Flooded or direct expansion
• Refrigerant charge: 5-15 kg (DX), 25-50 kg (flooded)
• Pressure drop: < 20 kPa
• Defrost: Hot gas or warm glycol every 8-12 hours

Control System:

• PLC-based controller
• HMI touchscreen interface
• Recipe storage: 50-200 recipes
• Data logging: temperature, pressure, flow, power
• Integration: Plant SCADA/MES systems

Energy Efficiency Optimization

Coefficient of Performance

System COP quantifies energy efficiency:

COP_system = Q_refrigeration / W_total

Where W_total includes compressor, pumps, fans, and controls.

Target COP Values:

Energy Recovery Opportunities

Heat Recovery from Compression:

• Compressor discharge heat: 120-140 kW for 40 kW compressor
• Hot gas temperature: 85-100°C
• Applications: Pasteurization preheat, cleaning water heating
• Heat recovery efficiency: 60-75%
• Annual energy savings: 200,000-400,000 kWh

Cold Recovery:

• Product sensible cooling: prechilled by exiting cold product
• Plate heat exchanger: 0.5-1.0 kW savings per 1000 L/hr
• Reduced refrigeration load: 3-5%

Process Optimization

Draw Temperature Control:

Precise control minimizes energy waste:

• Each 1°C colder than necessary: +8-10% energy increase
• Optimize to minimum acceptable: typically -5.5 to -6.0°C
• Real-time adjustment based on product formulation

Overrun Optimization:

Higher overrun reduces specific energy (per liter):

• Energy per kg mix: constant
• Energy per liter product: decreases with overrun
• At 80% overrun: 44% less energy per liter vs. no air

But quality constraints limit maximum overrun.

Production Scheduling:

Continuous operation superior to batch:

• Eliminate repeated cooling and warming cycles
• Reduce startup surge loads
• Maintain stable refrigeration load
• Target: > 18 hours/day continuous run

Predictive Maintenance:

Equipment condition affects efficiency:

• Worn dasher blades: 15-25% efficiency loss
• Fouled heat transfer surface: 10-20% loss
• Refrigerant undercharge: 5-15% loss
• Monitor: power consumption per unit production

Advanced Technologies

Variable Speed Drives:

• Compressor VFD: match load to demand
• Condenser fan VFD: optimize based on ambient
• Mix pump VFD: precise flow control
• Combined savings: 15-30% electrical energy

Smart Controls:

• Adaptive superheat control
• Floating condensing pressure optimization
• Demand-based defrost timing
• Production recipe optimization

Low-GWP Refrigerants:

• Transition plan from R-404A to low-GWP alternatives
• Consider ammonia for new large installations
• CO₂ cascade for medium operations
• Natural refrigerants: zero direct emissions

Performance Metrics

Key Performance Indicators:

Energy Benchmarking:

Best-in-class facilities achieve:

• Total energy: 50-55 kWh per 1000 L finished product
• Refrigeration: 30-35 kWh per 1000 L
• Mechanical (dasher): 18-22 kWh per 1000 L
• Controls and auxiliaries: 2-3 kWh per 1000 L

Continuous monitoring and optimization targeting these benchmarks ensures competitive operation and minimal environmental impact.

Conclusion

Ice cream freezing requires precise control of thermodynamic conditions, mechanical action, and air incorporation to produce consistent, high-quality products. Refrigeration system design must address high heat loads at low temperatures while maintaining energy efficiency. Understanding the fundamental heat transfer mechanisms, load calculations, and equipment specifications enables HVAC professionals to design, operate, and optimize these critical systems effectively.

The evolution toward low-GWP refrigerants and enhanced energy recovery presents both challenges and opportunities for the industry. Successful implementations balance food safety requirements, product quality objectives, regulatory compliance, and operational economics through comprehensive engineering analysis and systematic optimization.