Milk Processing

Milk processing refrigeration maintains precise temperature control throughout the receiving, pasteurization, storage, and distribution phases. Raw milk arrives at dairy processing facilities at 4-10°C and requires immediate cooling to 1-4°C within 2 hours of receipt to inhibit bacterial growth and preserve milk quality before processing operations begin.

Raw Milk Receiving and Cooling

Raw milk delivered from farms undergoes immediate cooling at the receiving dock to arrest microbial activity and enzymatic degradation. The refrigeration system must handle variable loads as milk tanker trucks arrive throughout the day with product temperatures ranging from 4°C to 10°C depending on farm cooling efficiency and transport duration.

Plate heat exchangers provide efficient cooling with chilled water at -1°C to 1°C circulating on the secondary side. The system design accounts for peak receiving periods when multiple tankers discharge simultaneously, requiring refrigeration capacity 2-3 times the average hourly load.

Raw milk silos maintain 1-4°C using glycol jacket cooling or internal plate coils. Tank agitators operate intermittently to prevent cream separation and maintain uniform temperature throughout the storage volume. Temperature sensors at multiple tank levels provide feedback to the refrigeration control system.

Pasteurization Heat Exchange

Pasteurization systems employ multi-stage plate heat exchangers with regeneration sections that recover heat from hot pasteurized milk to preheat incoming raw milk. The refrigeration system cools pasteurized milk from pasteurization temperature (typically 72°C for HTST or 138°C for UHT) down to 4°C before filling or storage.

Regeneration efficiency typically reaches 90-95%, reducing the refrigeration load by recovering sensible heat from the hot pasteurized milk stream. The cooling system operates continuously during production shifts, with glycol temperatures maintained at -2°C to -1°C to provide sufficient temperature differential for rapid cooling without freezing the milk.

Two-stage cooling separates the load between chilled water (cooling from 15°C to 6°C) and glycol solution (cooling from 6°C to 4°C). This arrangement improves refrigeration efficiency by operating the chiller at higher evaporating temperatures for the bulk of the sensible heat removal.

Pasteurized Milk Storage Tanks

Pasteurized milk storage requires precise temperature control at 1-4°C to maintain product quality and prevent bacterial regrowth after the pasteurization kill step. Storage tanks use jacketed cooling with glycol solution or direct expansion refrigerant circuits to maintain uniform temperatures.

Jacketed silos circulate -2°C glycol through the tank jacket at flow rates sufficient to remove heat infiltration through tank walls plus heat generated by agitator motors. A 50,000-liter silo with R-3.5 insulation in a 20°C room requires approximately 1.5-2.0 kW continuous cooling capacity to maintain 4°C milk temperature.

Temperature stratification prevention requires gentle agitation or recirculation to maintain uniform product temperature without incorporating air or damaging milk fat globules. Top-entry agitators operate at 20-40 rpm with intermittent cycling (15 minutes on, 30 minutes off) to minimize heat generation while preventing cream separation.

Processing Equipment Cooling Loads

Dairy processing equipment generates significant heat loads requiring dedicated refrigeration capacity beyond the direct product cooling requirements. Separator motors, homogenizer pumps, and packaging equipment contribute to the total facility cooling demand.

Homogenizers generate substantial heat through pressure energy dissipation as milk passes through the homogenizing valve at 2,000-2,500 psi (140-175 bar). The resulting temperature rise of 10-15°C requires additional cooling capacity in the post-pasteurization cooling section to bring product back to 4°C storage temperature.

Chilled Water and Glycol Systems

Centralized chilled water and glycol distribution systems serve multiple cooling loads throughout the dairy processing plant. Chilled water at 1-2°C supplies the first stage of product cooling, while glycol solution at -2°C to -1°C provides final cooling and storage tank temperature maintenance.

Propylene glycol serves as the secondary refrigerant in dairy applications due to food-safety considerations. A 28% propylene glycol solution provides freeze protection to -5°C with a density of 1.024 kg/L and specific heat of 3.9 kJ/kg·K at 0°C.

Multiple chillers operate in lead-lag configuration with capacity staging to match varying production loads throughout the day. Base load chillers run continuously during production shifts while trim chillers cycle on during peak receiving or pasteurization operations.

Temperature Control and Monitoring

Automated temperature monitoring systems track product temperatures at critical control points throughout the milk processing operation. Regulatory compliance requires continuous recording of pasteurization temperatures and cooling rates with alarm systems for deviations.

RTD (resistance temperature detector) sensors provide accurate temperature measurement with ±0.1°C precision at critical pasteurization control points. Storage tanks use thermistor sensors with ±0.5°C accuracy sufficient for monitoring purposes. All temperature sensors require calibration verification every 6 months against NIST-traceable reference standards.

Flow diversion valves automatically redirect milk flow if pasteurization temperature falls below the regulatory minimum, preventing unpasteurized product from entering the storage system. The refrigeration system must have sufficient capacity to cool diverted product during extended pasteurization upsets without exceeding storage tank temperature limits.

Cold Chain Management

Maintaining the cold chain from processing through distribution preserves milk quality and extends shelf life. Temperature excursions during storage, loading, and transport accelerate bacterial growth and reduce product shelf life exponentially.

Processing facilities employ dedicated loading docks with refrigerated holding areas maintained at 4°C to prevent temperature rise during truck loading operations. Insulated dock seals minimize ambient air infiltration during door opening cycles when forklifts transfer product pallets to refrigerated trailers.

Finished product storage coolers operate at 2-4°C with multiple evaporators providing uniform air distribution and minimal temperature variation throughout the storage volume. High-velocity ceiling-mounted unit coolers with wide air throw patterns prevent dead spots where product temperatures drift above setpoint.

Refrigeration System Design Criteria

Milk processing refrigeration systems require careful load analysis accounting for simultaneous receiving, pasteurization, and storage cooling demands. Design capacity must accommodate peak production periods plus safety factor for ambient condition variations and equipment degradation over time.

Ammonia refrigeration systems dominate large dairy processing facilities due to high efficiency, low refrigerant cost, and established safety protocols. Smaller operations may use HFC or HFO refrigerants in packaged chiller systems with remote condensers or evaporative condensers for heat rejection.

Two-stage ammonia systems with intermediate pressure vessels operate at -10°C evaporating temperature for glycol cooling and -2°C for chilled water production. Screw compressors provide efficient capacity modulation through slide valve control, matching refrigeration output to varying process loads without excessive cycling.

Energy Efficiency Considerations

Refrigeration energy consumption represents 30-40% of total dairy processing plant energy costs, making efficiency optimization critical for operating cost reduction. Heat recovery from refrigeration condensers supplies hot water for CIP (clean-in-place) operations, reducing both refrigeration heat rejection load and boiler fuel consumption.

Floating head pressure control reduces condensing temperature during cool ambient conditions, lowering compressor power consumption proportionally. A 5°C reduction in condensing temperature typically decreases compressor power by 8-12% depending on system configuration and refrigerant type.

Variable frequency drives on glycol circulation pumps reduce pumping energy during partial load conditions when fewer heat exchangers operate simultaneously. Pump speed modulation maintains minimum flow velocity through plate heat exchangers while reducing pressure drop and pump power consumption during low-demand periods.

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