Slush Ice Systems

Technical Overview

Slush ice systems represent an advanced refrigeration technology that generates a pumpable mixture of ice crystals suspended in chilled seawater or brine solution. The resulting ice slurry, commonly termed liquid ice or flow ice, maintains temperatures between -1°C and 0°C, providing rapid and uniform cooling for freshly caught fish. The three-phase mixture consists of ice crystals (solid phase), chilled liquid (liquid phase), and entrained air (gas phase), creating optimal heat transfer characteristics superior to conventional block or flake ice.

The thermodynamic advantage stems from the latent heat of fusion available from the ice fraction combined with sensible cooling from the liquid carrier. This dual cooling mechanism enables extraction of approximately 334 kJ/kg from ice melting plus additional sensible heat removal from the liquid phase, achieving cooling rates 3 to 5 times faster than traditional icing methods.

Slush Ice Generation Technology

Direct Expansion Scraped Surface Systems

Refrigerant evaporates at -8°C to -12°C inside a cylindrical heat exchanger while a rotating scraper blade continuously removes ice crystals forming on the internal surface. The scraper rotational speed typically ranges from 20 to 40 rpm, preventing excessive ice buildup that would insulate the heat transfer surface. Refrigerant-side heat transfer coefficients reach 800 to 1200 W/m²·K while water-side coefficients achieve 1500 to 2500 W/m²·K due to the scraping action.

The evaporator operates under flooded conditions with ammonia or HFC refrigerants, maintaining consistent ice crystal nucleation. Scraped surface generators produce ice fractions from 10% to 40% by weight, with crystal sizes ranging from 0.5 to 3 mm diameter. Power consumption for the scraper drive motor adds 0.5 to 1.5 kW per ton of ice production to the total system energy demand.

Vacuum Ice Generation

Water sprayed into a vacuum chamber at absolute pressures of 600 to 650 Pa undergoes flash evaporation, with the evaporative cooling driving ice crystal formation. Approximately 13% to 15% of the water mass evaporates to freeze the remaining liquid into ice crystals. The vacuum pump continuously removes water vapor, which condenses on refrigerant-cooled heat exchanger coils operating at -5°C to -10°C.

This process generates exceptionally uniform ice crystals with diameters of 0.3 to 1.0 mm, creating a highly pumpable slurry. Vacuum ice systems consume 90 to 110 kWh per ton of ice produced, with vacuum pump energy representing 15% to 25% of total power input. The hermetically sealed chamber prevents contamination and allows precise control of ice fraction.

Supercooling Methods

Seawater or brine solution subcools to -1°C to -2°C without ice formation in a heat exchanger, then passes through a nucleation device (orifice plate, static mixer, or ultrasonic generator) that triggers instantaneous crystallization. The degree of supercooling determines the ice fraction, with each 1°C of supercooling yielding approximately 12% to 15% ice by weight based on the latent heat relationship.

Supercooling systems require precise temperature control within ±0.2°C and elimination of nucleation sites in the cooling heat exchanger. Stainless steel plate heat exchangers with turbulent flow patterns (Re > 4000) maintain supercooled conditions while achieving overall heat transfer coefficients of 2500 to 3500 W/m²·K.

Ice Fraction and Consistency Control

Ice Mass Fraction Optimization

The ice fraction directly affects both cooling capacity and pumpability. Optimal ice fractions for fish chilling applications range from 25% to 35% by weight, providing:

• Sufficient latent heat capacity for rapid temperature reduction
• Acceptable fluid rheology for pumping and distribution
• Complete fish coverage without excessive weight
• Economical refrigeration energy consumption

Ice fraction calculation follows the enthalpy balance:

IF = (h_in - h_out) / h_f

Where IF represents ice fraction, h_in denotes inlet water enthalpy, h_out signifies outlet slurry enthalpy, and h_f equals the latent heat of fusion (334 kJ/kg).

Crystal Size Distribution

Ice crystal diameter influences heat transfer rate, pumpability, and mechanical impact on fish tissue. Target crystal size distributions maintain:

• Mean diameter: 1.0 to 2.5 mm
• Size variance: Less than 40% coefficient of variation
• Maximum crystal size: Under 5 mm to prevent tissue damage
• Minimum crystal size: Above 0.3 mm to avoid agglomeration

Smaller crystals provide higher surface area for heat transfer (surface area increases inversely with diameter) but exhibit greater pumping resistance and tendency to fuse. Larger crystals pump easily but reduce contact area and may cause bruising on delicate fish species.

Heat Transfer Advantages

Enhanced Contact Cooling

Slush ice conforms to irregular fish surfaces, establishing intimate contact over 95% to 98% of the exterior area compared to 40% to 60% for conventional ice. The liquid carrier eliminates air gaps that act as thermal insulators, reducing the effective thermal resistance from fish surface to cooling medium by a factor of 3 to 5.

Convective heat transfer coefficients at the fish-slurry interface range from 200 to 400 W/m²·K, substantially exceeding the 20 to 50 W/m²·K achieved with block ice in air. The Nusselt number correlation for slush ice cooling follows:

Nu = 0.35 * Re^0.6 * Pr^0.33 * (IF)^0.25

Where Reynolds number (Re) characterizes flow conditions, Prandtl number (Pr) represents fluid properties, and IF denotes ice fraction.

Temperature Uniformity

The fluid nature of slush ice enables circulation throughout storage volumes, maintaining temperature uniformity within ±0.3°C compared to ±2°C to ±4°C gradients common in static ice storage. Recirculation rates of 1 to 3 volume changes per hour eliminate warm zones and ensure consistent cooling across entire fish lots.

Temperature measurement at multiple locations within storage tanks confirms:

• Core-to-surface temperature differential: Less than 0.5°C
• Top-to-bottom gradient: Under 0.4°C
• Time to thermal equilibrium: 30 to 60 minutes for 1000 kg fish mass

System Components and Design

Slush Ice Generators

Primary generation equipment selection depends on production capacity, ice fraction requirements, and available refrigeration infrastructure.

Storage and Distribution Tanks

Insulated storage vessels maintain slush ice at -0.5°C to 0°C with minimal melting. Tank design specifications include:

• Insulation thickness: 150 to 250 mm polyurethane foam (k = 0.022 W/m·K)
• Internal agitation: Slow-speed mixers at 5 to 15 rpm prevent settling
• Heat gain limit: Less than 5 W/m² of tank surface area
• Volume sizing: 1.2 to 1.5 times daily ice production for buffer capacity
• Material: 316 stainless steel for seawater compatibility

Vertical cylindrical tanks with conical bottoms facilitate complete drainage and minimize dead zones. Horizontal tanks provide better surface-to-volume ratios for large installations but require multiple discharge points.

Pumping and Piping Systems

Slush ice exhibits non-Newtonian fluid behavior, with apparent viscosity increasing exponentially with ice fraction above 30%. Pump selection criteria include:

Centrifugal Pumps:

• Suitable for ice fractions below 25%
• Open impeller design with vane clearances exceeding 8 mm
• Head generation: 10 to 40 m at 50 to 200 m³/h
• Efficiency: 45% to 65% with slush ice

Progressive Cavity Pumps:

• Effective for ice fractions from 25% to 45%
• Gentle pumping action minimizes crystal fracture
• Pressure capability: Up to 8 bar
• Efficiency: 55% to 70% with high ice fractions

Piping systems require:

• Minimum diameter: 80 mm for ice fractions above 20%
• Velocity range: 1.5 to 3.0 m/s to maintain suspension
• Pressure drop: 20 to 60 kPa per 100 m depending on ice fraction
• Material: Schedule 40 stainless steel or HDPE for seawater service

Refrigeration Plant Integration

Slush ice generators integrate with ammonia, CO2, or HFC refrigeration systems operating at evaporating temperatures from -10°C to -5°C. System configurations include:

Direct Expansion:

• Refrigerant evaporates in generator heat exchanger
• Thermostatic or electronic expansion valve control
• Compressor capacity: 1.2 to 1.4 times ice generation load
• Evaporator pressure drop: 20 to 40 kPa

Pumped Liquid Overfeed:

• Low-pressure receiver supplies liquid refrigerant at 3:1 to 5:1 circulation ratio
• Separator vessel returns vapor to compressor suction
• Improved heat transfer coefficients by 20% to 30%
• Suitable for large systems above 50 ton/day capacity

Energy Consumption Considerations

Refrigeration Energy Requirements

Theoretical energy to produce slush ice at -0.5°C from seawater at 15°C:

Q_total = Q_sensible + Q_latent
Q_sensible = m * c_p * ΔT = 1000 kg * 3.93 kJ/kg·K * 15.5 K = 60,915 kJ
Q_latent = m_ice * h_f = (300 kg) * 334 kJ/kg = 100,200 kJ
Q_total = 161,115 kJ per 1000 kg slush ice (30% ice fraction)

Compressor work at -8°C evaporating and +35°C condensing (COP = 3.2):

W_comp = Q_total / COP = 161,115 kJ / 3.2 = 50,348 kJ = 14.0 kWh per ton

Actual system energy consumption including auxiliary equipment:

Energy Efficiency Optimization

Strategies to minimize energy consumption:

1. Condensing Temperature Reduction: Each 1°C decrease in condensing temperature improves COP by 2% to 3%, reducing compressor energy by 15 to 20 kWh per ton ice with seawater condensers.

2. Heat Recovery: Condenser heat at 35°C to 40°C preheats process water or provides low-grade heating, recovering 30% to 40% of rejected heat.

3. Variable Speed Control: VFD-controlled compressors and pumps match capacity to demand, reducing energy consumption by 15% to 25% at partial loads.

4. Optimal Ice Fraction: Operating at minimum ice fraction meeting cooling requirements reduces refrigeration load proportionally.

Applications in Fish Handling

Vessel-Based Systems

Fishing vessels install slush ice systems for at-sea chilling, preserving catch quality during multi-day voyages. Shipboard applications require:

• Compact generators fitting engine room constraints
• Motion-tolerant tank designs with baffles preventing sloshing
• Corrosion-resistant materials for marine environment
• Capacity matching catch rates of 5 to 50 tons per day

Fish immersed in slush ice immediately after harvesting achieve core temperatures below 2°C within 30 to 90 minutes compared to 4 to 8 hours with block ice. This rapid chilling inhibits bacterial growth and enzymatic degradation, extending shelf life by 3 to 5 days.

Processing Plant Reception

Shore-based facilities receive fish in slush ice and maintain the cold chain through processing:

• Reception tanks with 10,000 to 100,000 kg capacity
• Continuous slush ice addition replacing melted ice
• Fish conveying via slush ice pumping to processing lines
• Temperature monitoring with automated recording

The fluid transport capability eliminates mechanical handling that damages delicate species such as sardines, anchovies, and herring. Pumping rates of 5 to 20 tons per hour move fish through pipes with gentle acceleration.

Storage Applications

Slush ice storage maintains fish quality for 5 to 12 days depending on species and initial freshness:

Quality Preservation Benefits

Slush ice systems provide measurable quality improvements:

• Microbial Control: Total viable count remains below 10⁵ CFU/g for 8 to 10 days versus 5 to 6 days with flake ice
• Drip Loss Reduction: Weight loss during storage decreases from 2-4% (block ice) to 0.5-1.5% (slush ice)
• Texture Retention: Compression testing shows 20% to 35% better firmness scores after 7 days storage
• Color Preservation: Reflectance spectroscopy indicates reduced browning and oxidation
• Odor Development: Sensory panel scores remain acceptable 2 to 4 days longer than conventional icing

The protective liquid environment cushions fish from mechanical damage, reducing bruising and scale loss by 40% to 60% compared to bulk ice storage where fish weight compresses lower layers.

Specialized Applications

Live Fish Transport: Slush ice at 10% to 15% ice fraction chills transport water while maintaining dissolved oxygen above 6 mg/L for live fish or shellfish transit over 6 to 24 hours.

Tuna Freezing Preparation: Pre-chilling tuna to -0.5°C in slush ice before blast freezing reduces core temperature differential, minimizing ice crystal formation and quality degradation during the freezing process.

Shellfish Depuration: Slush ice maintains optimal temperatures in shellfish purification systems, controlling bacterial activity while preserving product viability through 24 to 48 hour cleaning cycles.