Ammonia Evaporators in Spiral Freezer Machines

Ammonia Evaporators in Spiral Freezer Machines

The core advantage of spiral freezers lies in extending the residence time of food within the freezing chamber via a “spiral conveyor belt,” while simultaneously achieving “Individual Quick Freezing (IQF)” using low-temperature airflow supplied by ammonia cooling fans. The synergistic process is as follows:

Ammonia refrigeration cycle provides cooling capacity: Ammonia refrigerant evaporates within the refrigeration system's evaporator (i.e., the heat exchange coil of the ammonia air cooler), absorbing significant heat and lowering the air cooler coil temperature to -35°C to -45°C (adjusted according to freezing requirements).

Forced air circulation by the chiller: A high-power axial fan (or centrifugal fan) integrated into the ammonia chiller draws air from the freezing chamber. This air passes over the low-temperature coils, cooling to -30°C to -40°C, and is then uniformly blown at a specific velocity (typically 3–5 m/s) onto the surface of food products on the spiral conveyor mesh.

Synchronized spiral conveying and freezing: Food items (e.g., shrimp, scallops, squid, and other seafood) ascend/descend slowly along the spiral mesh belt, continuously exposed to the low-temperature airflow from the ammonia chiller. Surface moisture rapidly crystallizes, while core temperatures drop from ambient (or 4°C refrigerated) to below -18°C within 30–90 minutes, completing the freezing process;

Airflow and Conveying Coordination: The ammonia air blower's airflow direction is typically designed as a “combination of co-current and counter-current”—co-current at the belt inlet (rapid cooling) and counter-current in the middle section (enhanced heat exchange)—ensuring uniform food freezing and preventing localized icing or thawing.

The design of the ammonia air blower must be deeply matched with the spiral freezer's chamber dimensions, freezing process, and food characteristics, focusing on the following parameters:

Cooling Capacity Calculation: Precise Matching of Freezing Load

Cooling demand must be calculated based on the food's “freezing volume, initial temperature, final temperature, and latent heat of freezing.” Reference formula:

Q = m × (c1 × Δt1 + r + c2 × Δt2)

Q: Total cooling demand (kW); m: Hourly food freezing volume (kg/h) ; c1: Specific heat capacity at ambient temperature (kJ/kg·℃); Δt1: Temperature difference from initial to freezing point (℃); r: Freezing latent heat (kJ/kg, e.g., seafood approx. 270–330 kJ/kg); c2: Specific heat capacity below freezing point (kJ/kg·℃); Δt2: Temperature difference from freezing point to final temperature (℃).

Example: For hourly freezing of 1000kg shrimp (initial temperature 4℃, final temperature -18℃, freezing point -2℃), calculation yields Q≈120kW. Select an ammonia air-cooled condenser with cooling capacity ≥120kW (accounting for 10%–15% margin).

Structural Design: Adapt spiral chamber and airflow circulation

Coil Layout: Spiral freezer chambers are typically “cylindrical” or “square.” Ammonia chiller coils must be designed as “ring coils” (wrapping around spiral mesh belts) or “side-mounted multi-group coils” to ensure airflow covers the full belt width without dead zones.

Fan Selection: Prioritize “low-noise, high-static-pressure” axial fans with air velocity controlled at 3–5 m/s (excessively low velocity slows freezing, while excessively high velocity may cause food dehydration—e.g., seafood moisture loss must be controlled below 1%);

Material Selection: Coils made of 304 stainless steel (corrosion-resistant, preventing ammonia reaction with salt in seafood environments); fan blades made of engineering plastic (lightweight, reducing vibration).

Defrosting System: Prevents coil frost buildup that impairs efficiency.

During spiral freezer operation, moisture in the air causes frost formation on the ammonia cooling coil surface. This requires either hot gas defrosting (using high-temperature ammonia vapor from the refrigeration system circulated backward through the coils) or electric heating defrosting (as an auxiliary method). Design considerations:

Defrost Cycle: Every 4–6 hours (adjust based on food humidity; e.g., shorter cycles for frozen shellfish with faster frost buildup).

Drainage Design: Melted water after defrosting must be rapidly drained via “inclined coils + drain valves” to prevent ice buildup and cavity blockage.