Cold Storage:
Cold storage refers to a specialized facility designed to maintain perishable goods at specific temperatures and humidity levels to extend their shelf life.
1. Classification of Cold Storage
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Cool Storage: For fresh fruits and vegetables. Temperature range: 0°C to 10°C.
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Frozen Storage: For meat, fish, and ice cream. Temperature range: -18°C to -30°C.
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Controlled Atmosphere (CA) Storage: Regulates $O_2$, $CO_2$, and Nitrogen levels alongside temperature to slow down the ripening process.
2. Major Components & Cycle
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Compressor: Usually multi-cylinder reciprocating or screw types for high capacity.
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Condenser: Often evaporative or water-cooled for better efficiency in large plants.
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Evaporator: Typically ceiling-mounted unit coolers with forced air circulation.
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Expansion Valve: Thermostatic Expansion Valve (TEV) or Electronic Expansion Valve (EEV) for precise refrigerant flow.
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3. Insulation: The Vital Barrier
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Common Materials: Polyurethane Foam (PUF), Polystyrene (Thermocol), or Cork.
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Key Property: Low thermal conductivity ($K$-value) and high moisture resistance.
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Vapor Barrier: Installed on the warm side (exterior) of the insulation to prevent moisture migration and frost buildup inside the walls.
4. Technical Operations & Facts
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Refrigerants: * Ammonia ($NH_3$ / R-717): Most common for large industrial cold storage due to high latent heat.
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Freons (R-134a, R-404A): Used in smaller, commercial cold rooms.
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Defrosting Methods: Essential to remove frost from evaporator fins.
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Hot Gas Defrosting: High-pressure discharge gas is diverted to the evaporator.
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Electric Defrosting: Uses heater coils.
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Water Defrosting: Water sprayed over coils.
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Air Circulation: Ensure even temperature distribution; improper stacking blocks “throw” of cold air, creating hot spots.
5. Critical Design Factors
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Pull-down Load: The heat removed to bring fresh products from ambient temperature to storage temperature.
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Respiration Heat: Heat generated by living products (fruits/vegetables) during their metabolic process.
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Infiltration Load: Heat entering through door openings and cracks.
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Product Load: The heat contained in the goods entering the facility.
6. Important Terms for Exams
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Blast Freezing: Rapidly lowering product temperature (e.g., to $-30°C$ in hours) to prevent large ice crystal formation.
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Pre-cooling: Removing field heat immediately after harvest before entering long-term storage.
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Shrinkage: Weight loss in products due to moisture evaporation (controlled by maintaining high relative humidity, usually 85%–95%).
Safety & Maintenance
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Pressure Relief Valves: Must be installed on receivers and compressors.
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Oil Separator: Essential in low-temperature systems to ensure oil returns to the compressor crankcase.
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Purging: Removing non-condensable gases (like air) from the condenser to maintain system efficiency.