Receiving Raw Milk

Raw milk receiving represents the critical first stage in dairy processing where proper refrigeration system design directly impacts product quality, regulatory compliance, and processing efficiency. The receiving system must rapidly cool incoming milk from farm temperatures while maintaining continuous temperature monitoring and microbiological quality control.

Receiving Bay Environmental Requirements

The receiving area requires controlled environmental conditions to support equipment operation and prevent contamination.

Temperature Control

The receiving bay HVAC system must provide:

• Ventilation rate: 10-15 air changes per hour minimum
• Positive pressurization: 0.02-0.03 in. w.g. (5-7.5 Pa) relative to exterior
• Filtered air supply: MERV 8 minimum, MERV 13 preferred
• Emergency ventilation capability for refrigerant release scenarios

Structural Integration

Refrigeration equipment placement requires:

Loading dock configuration:

• Multiple tanker positions with independent unloading capability
• Overhead door vestibules with air curtains (3000-4000 fpm discharge velocity)
• Insulated dock seals to minimize infiltration
• Floor drainage pitched to collection points (minimum 1/4 in. per foot slope)

Equipment zones:

• Heated areas for pumps and instrumentation (maintain above 10°C)
• Refrigerated equipment rooms maintained at 15-18°C
• Separation between glycol systems and ammonia refrigeration zones

Tanker Unloading System Design

Milk Tanker Specifications

Farm milk arrives in insulated tankers designed to maintain temperature during transport:

Temperature rise during transport:

ΔT = (U × A × t × ΔT_ambient) / (m × c_p)

Where:

• U = Overall heat transfer coefficient (0.15-0.25 Btu/hr·ft²·°F)
• A = Tank surface area (ft²)
• t = Transport time (hours)
• ΔT_ambient = Temperature difference between ambient and milk
• m = Milk mass (lb)
• c_p = Specific heat of milk (0.93 Btu/lb·°F)

For a 6,500-gallon tanker in 30°C ambient conditions traveling 2 hours:

• Expected temperature rise: 1-2°C
• Acceptable arrival temperature: ≤10°C (PMO Grade A requirement)
• Critical rejection temperature: >10°C sustained

Unloading Equipment

Sanitary pumps:

• Type: Centrifugal or positive displacement (lobe, flexible impeller)
• Flow rate: 150-300 gpm (570-1,140 L/min) typical
• Discharge pressure: 30-50 psi (207-345 kPa)
• Materials: 316 stainless steel wetted parts
• Sanitary connections: Tri-clamp or DIN 11851
• Motor enclosure: TEFC (Totally Enclosed Fan Cooled), wash-down rated

Flow measurement:

• Magnetic flowmeters (preferred): ±0.25% accuracy
• Turbine meters: ±0.5% accuracy
• Coriolis meters (premium): ±0.1% accuracy, simultaneous density measurement
• Calibration frequency: Monthly verification, annual certified calibration

Temperature monitoring:

• RTD sensors (Pt100 or Pt1000): ±0.1°C accuracy, 3-wire configuration
• Installation: Sanitary thermowell with heat transfer compound
• Response time: T90 < 10 seconds in flowing milk
• Recording interval: Continuous with 15-second averaging

Unloading Protocol

Pre-acceptance verification:

1. Visual inspection of tanker cleanliness
2. Seal integrity verification
3. Temperature measurement at multiple tank locations
4. Sample collection for rapid testing (smell, appearance, temperature)
5. Antibiotic screening (typically 5-7 minute rapid test)

Unloading sequence:

1. Connect sanitary hoses with proper tri-clamp seals
2. Verify CIP (Clean-In-Place) cycle completion on receiving lines
3. Open tanker outlet valve
4. Start receiving pump at controlled flow rate
5. Monitor inlet temperature continuously
6. Sample collection during unloading (composite sample)
7. Complete unloading and flush lines
8. Document volume, temperature, and acceptance data

Unloading time:

• Target rate: 200-250 gpm (760-950 L/min)
• 6,500-gallon tanker: 26-33 minutes
• Maximum rate limited by downstream cooling capacity

Immediate Cooling Requirements

Raw milk must be cooled rapidly to inhibit bacterial growth and maintain quality. The FDA Pasteurized Milk Ordinance (PMO) establishes the regulatory framework.

PMO Temperature Requirements

Grade A raw milk for pasteurization:

Bacterial growth relationship to temperature:

Psychrotrophic bacteria doubling time:

• At 7°C: 12-18 hours
• At 4°C: 24-48 hours
• At 2°C: 48-96 hours

Target storage temperature of 2-4°C significantly extends shelf life and maintains quality.

Cooling System Design Philosophy

Two-stage cooling approach:

1. Primary cooling (plate heat exchanger): 10°C → 4°C using well water or chilled glycol
2. Secondary cooling (silo tank refrigeration): Maintain 2-4°C during storage

This staged approach optimizes energy efficiency and equipment sizing.

Plate Cooler Design

Plate heat exchangers (PHE) provide the most efficient method for rapid milk cooling at the receiving stage.

Plate Heat Exchanger Configuration

Physical design:

• Type: Gasketed plate and frame
• Plate material: 316 stainless steel
• Gasket material: EPDM (ethylene propylene diene monomer) or NBR (nitrile) food-grade
• Plate pattern: Herringbone or chevron corrugations
• Plate thickness: 0.5-0.7 mm
• Plate spacing: 2-5 mm typical

Flow arrangement:

• Configuration: Counterflow for maximum heat transfer efficiency
• Passes: Multiple pass design (2-4 passes per side typical)
• Pressure drop: 5-15 psi (35-100 kPa) per side
• Velocity: 0.3-0.6 m/s in milk channels

Heat Transfer Calculations

Overall heat transfer coefficient for milk cooling:

U = 1 / (1/h_milk + t_plate/k_plate + 1/h_coolant + R_fouling)

Typical values:

• h_milk = 3,000-5,000 W/m²·K (convective heat transfer coefficient, milk side)
• h_coolant = 4,000-7,000 W/m²·K (water or glycol side)
• k_plate = 16 W/m·K (316 stainless steel)
• R_fouling = 0.0001-0.0002 m²·K/W (milk side fouling factor)

Overall U value: 2,500-4,000 W/m²·K (440-705 Btu/hr·ft²·°F)

Cooling Duty Calculation

Heat removal required to cool milk from 10°C to 4°C:

Q = ṁ × c_p × ΔT

Where:

• ṁ = Mass flow rate of milk (kg/s)
• c_p = Specific heat of milk = 3.93 kJ/kg·K (0.94 Btu/lb·°F)
• ΔT = Temperature change = 6 K

Example calculation for 200 gpm (757 L/min) milk flow:

ṁ = 757 L/min × 1.032 kg/L ÷ 60 s/min = 13.0 kg/s

Q = 13.0 kg/s × 3.93 kJ/kg·K × 6 K = 306 kW (87.5 tons refrigeration)

Plate Heat Exchanger Sizing

Required heat transfer area:

A = Q / (U × LMTD)

Log Mean Temperature Difference (LMTD) for counterflow:

LMTD = (ΔT₁ - ΔT₂) / ln(ΔT₁/ΔT₂)

For milk cooling from 10°C to 4°C using glycol at -1°C entering and 3°C leaving:

• ΔT₁ = 10°C - 3°C = 7°C (hot end)
• ΔT₂ = 4°C - (-1°C) = 5°C (cold end)

LMTD = (7 - 5) / ln(7/5) = 5.94 K

Required area: A = 306,000 W / (3,000 W/m²·K × 5.94 K) = 17.2 m² (185 ft²)

Typical plate sizes:

• Small plates: 0.03-0.1 m² per plate
• Medium plates: 0.1-0.3 m² per plate
• Large plates: 0.3-0.6 m² per plate

For this application: approximately 60-170 plates depending on plate size selection.

Coolant Flow Requirements

Glycol flow rate for balanced design:

ṁ_glycol = Q / (c_p,glycol × ΔT_glycol)

For 30% propylene glycol solution:

• c_p = 3.75 kJ/kg·K
• ΔT = 4 K (from -1°C to 3°C)

ṁ_glycol = 306 kW / (3.75 kJ/kg·K × 4 K) = 20.4 kg/s = 324 gpm

Glycol-to-milk flow ratio: 1.6:1 (typical range 1.4:1 to 1.8:1)

Glycol System Requirements

Propylene glycol solutions provide non-toxic secondary coolant for food processing applications.

Glycol Concentration Selection

Standard selection: 30% propylene glycol for -2°C to 4°C supply temperature

Glycol Chiller System

Primary refrigeration system:

• Refrigerant: Ammonia (R-717) in industrial installations, or R-404A/R-507 in smaller facilities
• Evaporator type: Flooded shell-and-tube or brazed plate
• Glycol supply temperature: -2°C to 0°C
• Glycol return temperature: 3°C to 5°C
• Temperature control: ±0.5°C stability required

Glycol circulation:

• Pump type: Centrifugal, bronze or stainless steel construction
• Flow rate: 1.2 × design flow (20% excess capacity)
• Pressure: 40-80 psi (275-550 kPa) discharge
• Variable frequency drive (VFD) control for load following

Buffer tank:

• Capacity: 15-30 minutes of cooling demand
• Insulation: 3-4 inches polyurethane foam (R-20 to R-25)
• Internal baffles to prevent stratification
• Level, temperature, and concentration monitoring

Glycol System Heat Load

Total cooling load includes:

1. Milk sensible cooling: 306 kW (calculated above)
2. Pump heat addition: 1-2% of cooling load
3. Heat gain from piping: 3-5% of cooling load
4. Safety factor: 10-15%

Total glycol chiller capacity: 345-390 kW (98-111 tons)

Glycol Distribution Network

Supply piping:

• Material: Schedule 40 carbon steel (black iron) or stainless steel
• Size: Based on 2-4 ft/s (0.6-1.2 m/s) velocity
• For 324 gpm: 4-inch nominal pipe size
• Insulation: 2-3 inches closed-cell elastomeric foam
• Vapor barrier: All-service jacket (ASJ) or PVC fitting covers

Control valves:

• Type: Modulating control valve at PHE glycol inlet
• Sizing: Cv based on pressure drop and flow requirements
• Actuation: Pneumatic or electric actuator with 4-20 mA control
• Fail position: Fail-open to protect equipment

Temperature control strategy:

• Primary: Modulate glycol flow to maintain milk outlet temperature
• Secondary: Stage chiller capacity based on glycol return temperature
• Alarm: High milk temperature at 6°C, critical at 7°C

Silo Storage Tank Cooling

After plate cooling, milk enters insulated storage silos where it must be maintained at 2-4°C until processing.

Silo Tank Design

Construction:

• Material: 304 or 316 stainless steel, sanitary finish (Ra ≤ 0.8 μm)
• Capacity: 10,000-100,000 gallons (38-380 m³) typical
• Orientation: Vertical cylindrical with dished heads
• Insulation: 3-4 inches polyurethane foam spray-applied
• Cladding: Aluminum or stainless steel weather protection

Internal components:

• Cooling coils or dimple jackets for refrigeration
• Agitator to prevent cream separation and maintain temperature uniformity
• CIP spray balls for automated cleaning
• Multiple temperature sensors at different heights

Cooling Jacket Design

Dimple jacket configuration (preferred for large tanks):

• Pattern: Staggered or aligned dimple arrays
• Dimple depth: 0.5-1.0 inch (13-25 mm)
• Spacing: 2-4 inches (50-100 mm) center-to-center
• Coverage: 60-80% of cylindrical surface area
• Glycol channels created by spot-welded outer jacket

Heat transfer through dimple jacket:

U_jacket = 1 / (1/h_milk + t_wall/k_steel + 1/h_glycol + R_fouling)

Typical values:

• h_milk = 200-400 W/m²·K (with agitation)
• h_glycol = 1,000-2,000 W/m²·K (turbulent flow in dimples)
• t_wall = 3 mm (jacket and tank wall combined)
• k_steel = 16 W/m·K

Overall U value: 150-300 W/m²·K (26-53 Btu/hr·ft²·°F)

Silo Cooling Load Calculation

Heat gains to storage tank:

1. Incoming warm milk:

   • Sensible load from 4°C to 2°C storage temperature
   • Intermittent load during receiving hours

2. Heat of agitation: Q_agitation = P_motor × η_mechanical × duty_cycle

   For 5 HP agitator operating 50% duty cycle: Q_agitation = 5 HP × 0.746 kW/HP × 0.50 = 1.87 kW

3. Heat gain through insulation: Q_insulation = U_overall × A_surface × (T_ambient - T_milk)

   For 50,000-gallon silo (D = 12 ft, H = 24 ft) with R-20 insulation at 20°C ambient:

   • A_surface ≈ 1,200 ft² (111 m²)
   • U_overall = 0.05 Btu/hr·ft²·°F (0.28 W/m²·K)
   • Q_insulation = 0.28 × 111 × (20-2) = 559 W

4. Ambient heat gain through roof and connections:

   • Estimate 10-20% additional heat gain
   • Q_ambient ≈ 60-110 W

Continuous cooling requirement: 2.5-3.0 kW per 50,000-gallon silo

Glycol Flow for Silo Cooling

For steady-state cooling with 30% glycol solution:

ṁ_glycol = Q / (c_p × ΔT)

Using -2°C supply and 2°C return (ΔT = 4 K):

ṁ_glycol = 3,000 W / (3,750 J/kg·K × 4 K) = 0.20 kg/s = 3.2 gpm per tank

Dimple jacket flow velocity:

• Target: 2-4 ft/s (0.6-1.2 m/s) for good heat transfer
• Turbulent flow regime: Reynolds number > 4,000
• Distribution manifold design to ensure uniform flow across jacket

Agitation System

Purpose:

• Prevent cream separation (fat stratification occurs within 2-4 hours without agitation)
• Maintain temperature uniformity throughout silo
• Ensure representative sampling

Agitator specifications:

• Type: Top-entering, close-clearance side-wall scraper or propeller
• Speed: 40-60 rpm typical
• Motor power: 3-10 HP depending on tank size
• Blade tip speed: 1.5-2.5 m/s
• Mixing time: Complete turnover in 15-20 minutes

Agitation cycle:

• Intermittent operation: 5-10 minutes per hour during storage
• Continuous operation: 30 minutes before and during tank-out
• Temperature-triggered: Activate if temperature gradient > 1°C detected

Quality Testing Requirements

Comprehensive testing ensures milk meets safety and quality standards before acceptance and processing.

Incoming Milk Testing

Temperature verification:

• Method: Calibrated digital thermometer or RTD sensor
• Locations: Multiple points in tanker (top, middle, bottom)
• Acceptance criteria: All readings ≤10°C
• Documentation: Record maximum temperature observed

Organoleptic evaluation:

• Visual appearance: No discoloration, foreign matter, or separation
• Odor: Fresh, sweet smell; no off-odors
• Preliminary enzyme tests: Phosphatase test for pasteurization verification

Rapid Screening Tests

Antibiotic residue testing:

• Method: Enzyme-linked immunosorbent assay (ELISA) or receptor-based rapid tests
• Time: 5-7 minutes
• Target antibiotics: Beta-lactams, tetracyclines, sulfonamides, aminoglycosides
• Action level: Reject if any detectable residue above FDA tolerance

Bacteria content (platform testing):

• Method: Direct microscopic count or rapid ATP bioluminescence
• Standard Plate Count (SPC) requirement: <100,000 CFU/mL for Grade A
• Preliminary Incubation (PI) count: <200,000 CFU/mL
• Result time: 5-10 minutes (preliminary), 48 hours (confirmed plate count)

Somatic cell count (SCC):

• Method: Flow cytometry or viscosity-based rapid test
• Regulatory limit: <750,000 cells/mL (Grade A individual producer)
• Quality target: <400,000 cells/mL for premium milk
• Significance: Indicator of udder health and milk quality

Laboratory Analysis

Composition analysis:

• Fat content: 3.0-4.5% typical (Babcock or infrared method)
• Protein content: 2.9-3.5% typical (Kjeldahl or infrared)
• Lactose: 4.5-5.0%
• Solids-not-fat (SNF): 8.5-9.0%
• Total solids: 11.5-13.5%

Freezing point determination:

• Normal range: -0.525°C to -0.535°C (Hortvet scale)
• Added water detection: Freezing point > -0.525°C indicates dilution
• Method: Thermistor cryoscope

Microbiological testing (scheduled):

• Standard Plate Count (SPC): Aerobic bacteria enumeration
• Coliform count: Sanitation indicator
• Psychrotrophic count: Cold-storage bacteria
• Thermoduric count: Heat-resistant bacteria (post-pasteurization concern)

Continuous Monitoring

Automated systems:

• Inline temperature recording with 15-second intervals
• Flow rate and totalizer for volume verification
• Automatic sample collection during receiving (composite sampler)
• Data logging integrated with plant SCADA system

Alarm conditions:

• High temperature: >6°C in receiving line
• Low flow: Equipment malfunction indication
• High pressure differential: Filter or heat exchanger fouling
• Glycol system failure: Low supply temperature or flow

PMO Temperature Compliance

The FDA Pasteurized Milk Ordinance (PMO) establishes comprehensive temperature control requirements throughout milk handling.

Regulatory Framework

PMO temperature standards (Grade A milk):

Recording requirements:

• Chart recorder or electronic data logger
• Accuracy: ±1°F (±0.6°C)
• Calibration: Monthly verification against certified thermometer
• Chart retention: Minimum 3 months
• Electronic records: Continuous data capture with backup

Temperature Monitoring System Design

Sensor placement:

• Receiving line: Immediately post-pump
• Plate cooler outlet: Verify cooling performance
• Each silo tank: Minimum 2 sensors (top and bottom third)
• Glycol supply and return: Monitor cooling system performance

Data acquisition:

• PLC or SCADA-based system
• Sampling rate: 5-15 seconds
• Recording interval: 15 seconds to 1 minute
• Trending and alarm functions
• Network backup and remote access capability

Calibration protocol:

• Frequency: Monthly verification, quarterly certified calibration
• Method: Comparison to NIST-traceable reference thermometer
• Ice point verification: 0°C ± 0.2°C
• Adjustment: Recalibrate or replace sensors outside ±0.5°C tolerance

Documentation and Record-Keeping

Required records:

• Individual tanker receiving logs (source, volume, temperature, test results)
• Continuous temperature charts or electronic records for all storage tanks
• Calibration records for all temperature measuring devices
• Quality test results and acceptance decisions
• Cleaning and sanitizing logs for all milk-contact surfaces

Retention periods:

• Temperature records: 3 months minimum (PMO), 6-12 months recommended
• Laboratory test results: 1 year minimum
• Calibration records: 2 years
• Regulatory inspection reports: Permanently

Traceability:

• Lot identification from source farm through finished product
• Electronic tracking systems integrate receiving data with production scheduling
• Rapid product recall capability based on receiving records

Equipment Specifications

Receiving Pumps

Sanitary centrifugal pump (typical receiving duty):

Positive displacement pump (alternative for long distance or high head):

• Type: Rotary lobe or flexible impeller
• Flow: 150-200 gpm (567-757 L/min)
• Pressure: 50-100 psi (345-690 kPa)
• Advantages: Gentle product handling, self-priming
• Disadvantages: Higher cost, more maintenance

Plate Heat Exchanger

Gasketed PHE for milk cooling (200 gpm capacity):

Performance verification:

• Milk outlet temperature: 4°C ± 0.5°C at design flow
• Approach temperature: 1-2°C (milk outlet to glycol inlet)
• Fouling factor: Monitor pressure drop increase over time
• Cleaning frequency: CIP every 4-8 hours of operation typical

Glycol Chiller

Ammonia/glycol chiller package (100-ton capacity):

System protection:

• Low glycol temperature cutout: -5°C to prevent freezing
• Flow switch: Proof of glycol circulation before compressor start
• Differential pressure switch: Tube-side fouling detection
• Emergency ventilation: Activated on ammonia detection

Silo Storage Tanks

Vertical silo tank (50,000-gallon capacity):

Installation requirements:

• Foundation: Reinforced concrete pad with anchor bolts
• Seismic restraint: Per local building code requirements
• Access platforms: For top-mounted equipment maintenance
• Leak containment: Secondary containment or sloped floor to drain

Instrumentation and Controls

Temperature measurement:

• Sensor type: RTD (Pt100 or Pt1000), 3-wire or 4-wire
• Accuracy: Class A (±0.15°C at 0°C) or better
• Installation: Sanitary thermowell, tri-clamp mount
• Transmitter: 4-20 mA output, HART protocol

Flow measurement:

• Technology: Magnetic flowmeter (primary choice)
• Accuracy: ±0.25% of rate
• Repeatability: ±0.05% of rate
• Output: 4-20 mA + pulse output for totalizing
• Display: Local digital readout with totalizer

Level measurement:

• Technology: Guided wave radar or ultrasonic
• Accuracy: ±2 mm typical
• Range: 0-30 feet (0-9 m) typical for silos
• Output: 4-20 mA, HART protocol
• Safety: High level alarm and emergency shutoff

Control system architecture:

• PLC: Allen-Bradley, Siemens, or equivalent industrial PLC
• HMI: Touch-screen operator interface, 12-15 inch minimum
• Network: Ethernet/IP or Profinet communication
• Integration: SCADA system for plant-wide monitoring
• Alarm management: Prioritized alarms with escalation
• Data historian: Long-term storage and trending

Cleaning and Sanitation

All milk-contact equipment requires rigorous cleaning and sanitizing between uses.

CIP (Clean-In-Place) System

CIP cycle for receiving system:

1. Pre-rinse: Ambient water, 5-10 minutes, flush product residue
2. Caustic wash: 1-2% sodium hydroxide at 75-80°C, 10-15 minutes
3. Intermediate rinse: Ambient water, 5 minutes
4. Acid wash: 1-1.5% nitric acid at 60-70°C, 10 minutes (periodic)
5. Final rinse: Potable water until neutral pH and conductivity
6. Sanitize: Before use, typically peroxyacetic acid or hot water (88-93°C)

CIP solution flow rates:

• Minimum velocity: 5 ft/s (1.5 m/s) in piping
• PHE cleaning: Forward and reverse flow at design flow rate
• Tank spray devices: 0.5-1.0 gpm per inch of diameter at 30-50 psi

Chemical concentration monitoring:

• Conductivity for caustic concentration
• pH measurement for acid neutralization
• Sanitizer concentration test strips or inline sensors

This comprehensive receiving system design ensures regulatory compliance, product quality protection, and operational efficiency in dairy processing facilities.