Temperature Humidity Control

Fundamental Requirements

Temperature and humidity control in egg storage facilities represents a critical balance between minimizing biological degradation and preventing condensation-induced contamination. Shell eggs remain metabolically active post-lay, with internal quality declining through enzymatic reactions, moisture migration, and gas exchange through the porous shell structure.

The principal mechanisms of quality loss include:

• Moisture Loss Through Shell: Porous shell structure permits water vapor transmission at rate proportional to vapor pressure differential
• Weight Loss Quality Grade Reduction: USDA grading standards penalize excessive moisture loss affecting air cell size
• Carbon Dioxide Loss pH Increase: CO₂ diffusion through shell elevates albumen pH from 7.6 to 9.2+
• Albumen Thinning Watery White: Ovomucin-lysozyme complex dissociation reduces thick albumen viscosity
• Yolk Membrane Weakening Breakage: Water migration from albumen weakens vitelline membrane
• Low Humidity Weight Loss Excessive: RH below 70% accelerates moisture loss beyond acceptable rates
• High Humidity Mold Growth Shell: RH above 85% promotes surface contamination and penetration

Design Temperature Requirements

Storage Temperature Ranges

Temperature selection balances preservation efficacy against condensation risk and energy consumption.

Temperature-Quality Relationships

The Arrhenius relationship describes quality degradation rate as function of storage temperature:

k = A × e^(-Ea/RT)

Where:

• k = reaction rate constant (quality loss rate)
• A = pre-exponential factor (collision frequency)
• Ea = activation energy (kcal/mol)
• R = universal gas constant (1.987 cal/mol·K)
• T = absolute temperature (K)

For each 18°F (10°C) reduction in storage temperature, quality degradation rate decreases by factor of 2-3, expressed as Q₁₀ coefficient:

Q₁₀ = k(T) / k(T-10°C)

Typical Q₁₀ values for egg quality parameters:

• Thick albumen height: Q₁₀ = 2.8
• Haugh unit decline: Q₁₀ = 2.5
• pH increase: Q₁₀ = 2.2
• Yolk index reduction: Q₁₀ = 2.0

Humidity Control Requirements

Target Humidity Range

Optimal relative humidity maintains egg quality while preventing surface moisture accumulation.

Target Range: 75-80% RH

Moisture Loss Rate Calculation

Weight loss rate through porous shell follows Fick’s first law of diffusion:

dm/dt = (K × A × ΔP) / L

Where:

• dm/dt = moisture loss rate (g/day)
• K = shell permeability coefficient (g·mm/day·cm²·mmHg)
• A = shell surface area (cm²)
• ΔP = water vapor pressure differential (mmHg)
• L = shell thickness (mm)

For standard large egg (58 g, 68 cm² surface area):

Shell permeability: K = 3.5-5.0 g·mm/day·cm²·mmHg

Vapor pressure differential calculation:

ΔP = Psat(Tegg) - Psat(Tair) × RH

Where Psat follows Magnus-Tetens approximation:

Psat(T) = 6.1078 × exp[(17.27 × T)/(T + 237.3)]

With T in °C, Psat in hPa (multiply by 0.7501 for mmHg).

Practical Moisture Loss Example

Storage conditions: 45°F (7.2°C), 75% RH Egg temperature: 46°F (7.8°C) initially

Psat(7.8°C) = 10.54 hPa = 7.91 mmHg Psat(7.2°C) = 10.23 hPa = 7.67 mmHg ΔP = 7.91 - (7.67 × 0.75) = 2.16 mmHg

Moisture loss rate: dm/dt = (4.2 × 68 × 2.16) / 0.35 = 1766 mg/day = 0.031 g/day

Daily weight loss: 0.031/58 = 0.053% per day (acceptable)

Psychrometric Analysis

Critical State Points

Egg storage facility psychrometric analysis identifies condensation risk and dehumidification requirements.

State 1 - Outdoor Ambient:

• Summer design: 95°F DB, 75°F WB (50% RH)
• Winter design: 20°F DB, 18°F WB (80% RH)

State 2 - Mixed Air (with return air):

• Minimum outdoor air: 10-15% for odor control
• Mixed temperature calculated from mass balance

State 3 - After Cooling Coil:

• 38-42°F (apparatus dew point 36-40°F)
• Dehumidified to prevent storage space humidity rise

State 4 - Storage Space:

• 45-50°F DB, 75-80% RH (target condition)
• Dew point: 40-44°F

State 5 - Return Air:

• 46-51°F (slight temperature rise from egg respiration)
• Moisture gain from egg surface evaporation

Moisture Load Components

Total moisture addition to storage space:

W_total = W_eggs + W_product + W_people + W_infiltration

1. Egg Moisture Release:

For storage of N eggs: W_eggs = N × (dm/dt)

Example: 100,000 eggs at 0.031 g/day each W_eggs = 3.1 kg/day = 6.8 lb/day

2. Product Condensate (during loading):

Warm eggs entering cold storage generate significant condensate:

W_product = m_eggs × c_p × ΔT / h_fg

For 100,000 eggs (5,800 kg) cooled from 70°F to 45°F: W_product = 5800 × 0.8 × 13.9 / 1059 = 60.8 kg = 134 lb

3. Infiltration Load:

W_infiltration = V × n × ρ × Δω

Where:

• V = room volume (m³)
• n = air changes per hour (0.1-0.5 for cold storage)
• ρ = air density (kg/m³)
• Δω = humidity ratio difference (kg/kg)

Control System Design

Temperature Control Strategy

Primary Control Loop:

• Direct digital control (DDC) with PID algorithm
• Proportional band: 2-4°F
• Integral time: 4-8 minutes
• Derivative time: 0.5-1.0 minute
• Control signal: 4-20 mA to refrigeration capacity modulation

Refrigeration Capacity Modulation Methods:

Evaporator Control:

• Thermostatic expansion valve (TXV) or electronic expansion valve (EEV)
• Target superheat: 8-12°F for R-404A, R-448A, R-449A
• Evaporator coil temperature: 28-35°F (13-17°F TD from space)

Humidity Control Implementation

Passive Humidity Control:

Achieved through temperature control alone when space operates near saturation (high RH). Refrigeration dehumidifies during cooling cycle; moisture naturally evaporates from eggs to maintain equilibrium.

Advantages:

• No additional equipment required
• Energy efficient
• Simple control logic

Limitations:

• Limited to spaces with inherent moisture load
• Cannot actively add moisture if RH drops

Active Humidity Control - Humidification:

Required when refrigeration overcools and excessively dehumidifies, or during low outdoor humidity conditions.

Humidifier Selection for Egg Storage:

Steam humidification design:

Humidification load calculation: Q_humid = ṁ_air × (ω_target - ω_supply)

Steam required (lb/hr): ṁ_steam = Q_humid × 60 min/hr

Control sequence:

• Modulating steam control valve (4-20 mA signal)
• Humidity sensor interlock with high limit at 82% RH
• Minimum distance downstream of humidifier: 15 duct diameters
• Distribution tube with multiple injection points for uniformity

Active Dehumidification:

Necessary during high outdoor humidity or product loading operations.

Methods:

1. Overcool and Reheat (conventional)
2. Desiccant dehumidification (chemical or regenerative)
3. Heat pipe enhanced cooling (passive efficiency improvement)

Overcool-reheat psychrometrics:

• Cool to 35-38°F (below target dew point)
• Condense excess moisture on coil
• Reheat to 45-50°F supply temperature
• Energy penalty: 0.2-0.4 kW/lb moisture removed (cooling + reheat)

Sensor Placement and Specifications

Temperature Sensors

Type Selection:

Recommended: Platinum RTD Pt100/Pt1000

• Class A accuracy: ±(0.15 + 0.002|T|)°C
• 3-wire or 4-wire configuration to eliminate lead resistance error
• Transmitter output: 4-20 mA (spans 40-60°F typical)

Placement Strategy:

1. Primary Control Sensor (single-point):

   • Location: Geometric center of storage space
   • Height: 5-6 ft above floor (egg pallet level)
   • Protection: Aspirated radiation shield
   • Mount: Independent stand or suspended, not on wall

2. Averaging Sensors (multipoint):

   • 3-6 sensors distributed throughout space
   • Average signal sent to controller
   • Placement at 1/4, 1/2, 3/4 points along length
   • Detects stratification and hot spots

3. High/Low Limit Sensors:

   • Independent safety system
   • Hard-wired to alarm and equipment shutdown
   • Placement near ceiling (high) and floor (low)

4. Supply/Return Air Sensors:

   • Duct-mounted at evaporator inlet/outlet
   • Calculate heat removal and system performance
   • Verify proper refrigeration operation

Humidity Sensors

Type Selection:

Recommended: Capacitive Thin-Film Polymer

• Accuracy: ±2% RH (10-90% range)
• Temperature compensated
• Output: 4-20 mA or 0-10 VDC
• Operating range: -40 to 185°F
• Response time: <15 seconds (63% step change)

Placement Strategy:

1. Primary Control Sensor:

   • Location: Near temperature sensor, return air stream
   • Height: 5-6 ft above floor
   • Protection: Perforated PTFE filter (dust protection, airflow)
   • Avoid: Near humidifier discharge, doors, drains

2. Supply Air Monitoring:

   • Duct-mounted after cooling coil
   • Verify dehumidification performance
   • Calculate moisture removal rate

3. High Limit Sensor:

   • Independent safety interlock
   • Shuts down humidification at 82-85% RH
   • Activates alarm at 87% RH

Sensor Maintenance:

• Calibration frequency: Every 6-12 months
• Field verification against chilled mirror reference
• Replace polymer sensors every 3-5 years
• Clean filters monthly in dusty environments

Condensation Prevention During Loading

Load In Procedures

Introduction of warm eggs into refrigerated storage creates substantial condensation risk. Surface moisture promotes bacterial penetration through shell pores.

Temperature Differential Limits:

Safe loading temperature difference: ΔT_max = 20-25°F (egg to air)

Above this threshold, surface condensation forms rapidly.

Dew Point Analysis

Condensation occurs when egg surface temperature drops below ambient dew point.

Critical condition: T_surface ≤ T_dewpoint(air)

For storage at 45°F, 75% RH:

• Air dew point: 39°F
• Safe egg temperature at loading: >40°F
• Typical fresh egg temperature: 70-85°F (farm ambient)

Time to Reach Condensation:

Transient heat conduction through shell and internal egg mass:

T(t) = T_air + (T_initial - T_air) × exp(-t/τ)

Time constant: τ = (m × c_p) / (h × A)

Where:

• m = egg mass (58 g)
• c_p = specific heat (3.35 kJ/kg·K for whole egg)
• h = convective heat transfer coefficient (8-12 W/m²·K, still air)
• A = surface area (68 cm²)

For typical egg: τ = (0.058 × 3350) / (10 × 0.0068) = 2857 seconds = 48 minutes

Surface reaches dew point in approximately 15-20 minutes under still air conditions.

Staged Cooling Protocol

Method 1: Temperature Staging

Implement graduated cooling sequence:

1. Staging Area 1: 65-70°F, 60% RH (4-8 hours)
2. Staging Area 2: 55-60°F, 65% RH (4-8 hours)
3. Final Storage: 45-50°F, 75% RH

Method 2: Controlled Ramp Rate

Single space with temperature setpoint reset:

• Initial setpoint: 60°F at loading
• Ramp rate: 2-3°F per hour
• Final setpoint: 45°F reached in 5-8 hours
• Monitor surface condensation with dew point sensors

Method 3: High Air Velocity During Cooldown

Increased air circulation accelerates cooling, reduces condensation time:

• Boost fan speed to maximum during first 4 hours
• Air velocity at egg surface: 200-400 fpm
• Reduces time constant τ by factor of 3-4
• Return to normal velocity (50-100 fpm) after stabilization

Dehumidification During Loading

Temporary dehumidification during warm product introduction prevents RH spike:

1. Calculate moisture release:

   • Sensible cooling: Q_s = m × c_p × ΔT
   • Equivalent moisture: m_w = Q_s / h_fg (if released as vapor)

2. Activate supplemental dehumidification:

   • Pre-cool loading area 2-4°F below normal
   • Increase refrigeration capacity 20-30%
   • Continue enhanced dehumidification for 8-12 hours

3. Monitor dew point:

   • Track dew point depression (T_db - T_dp)
   • Maintain minimum 5°F depression during loading
   • Resume normal control when equilibrium reached

Equipment Selection Criteria

Refrigeration System Components

Evaporator Coil Selection:

Coil Capacity Sizing:

Total cooling load: Q_total = Q_transmission + Q_product + Q_infiltration + Q_equipment + Q_respiration

1. Transmission load: Q_trans = U × A × (T_ambient - T_storage)

2. Product cooling load: Q_product = m_daily × c_p × ΔT / operating_hours

3. Infiltration load: Q_infil = V × ACH × ρ × c_p × ΔT

4. Equipment load: Q_equip = P_lighting + P_fans + P_motors

5. Respiration load (eggs): Q_resp = m_eggs × q_respiration

   Where q_respiration = 0.5-0.8 BTU/lb·day at 45°F

Safety factor: Multiply calculated load by 1.15-1.25

Air Distribution Equipment

Fan Selection:

• Type: Axial or centrifugal depending on static pressure
• Air changes per hour: 15-30 ACH during cooling, 5-10 ACH in storage
• Discharge velocity: 500-800 fpm
• Sound level: <65 dBA at 10 ft distance
• Motor: Premium efficiency, inverter-duty rated
• Variable frequency drive (VFD) for capacity modulation

Ductwork (if applicable):

Most egg coolers use direct unit cooler discharge without ductwork. For ducted systems:

• Material: Galvanized steel or stainless steel
• Insulation: R-8 minimum, vapor barrier exterior
• Aspect ratio: <4:1 for rectangular ducts
• Velocity: 1000-1500 fpm in mains, <800 fpm at outlets

Control Hardware

DDC Controller Specifications:

• Processor: 32-bit or higher
• Memory: 2 MB minimum
• I/O capacity: 16+ points expandable
• Communication: BACnet, Modbus, or LonWorks
• Display: Local LCD with keypad
• Programming: Graphical or ladder logic
• Power: 24 VAC/VDC with battery backup

Operator Interface:

• HMI touchscreen: 10-15" color display
• Trend logging: 365 day minimum retention
• Alarm management: Email, SMS notification
• Remote access: Secure web-based interface
• Reporting: Automated daily/weekly summaries

Energy Efficiency Optimization

Operating Cost Analysis

Annual energy consumption for typical 100,000 egg storage facility:

Refrigeration energy: E_refrig = Q_avg × operating_hours / (COP × 3412 BTU/kWh)

Assuming:

• Average cooling load: 25 TR (300,000 BTU/hr)
• COP: 2.5 (typical for 10°F evaporator, 90°F condensing)
• Operating hours: 8000 hr/year

E_refrig = 300,000 × 8000 / (2.5 × 3412) = 281,690 kWh/year

Fan energy: E_fans = P_fans × operating_hours

Assuming:

• Evaporator fans: 5 HP total = 3.73 kW
• Operating hours: 8000 hr/year

E_fans = 3.73 × 8000 = 29,840 kWh/year

Total annual energy: 311,530 kWh At $0.10/kWh: $31,153/year

Efficiency Improvement Strategies

1. Temperature Setpoint Optimization:

Each 1°F increase in evaporator temperature improves COP by 2-3%.

Raising storage temperature from 45°F to 48°F (if acceptable for storage duration):

• Evaporator temperature: 35°F → 38°F
• COP improvement: 6-9%
• Annual savings: $1,870 - $2,804

2. Variable Speed Compressor Control:

Constant capacity systems cycle on/off; variable speed modulates smoothly.

Benefits:

• Eliminates start/stop losses (5-10% of energy)
• Reduces peak demand charges
• Extends equipment life
• Maintains tighter temperature control

Payback period: 2-4 years for systems >20 HP

3. Evaporator Fan VFD:

Fans run at full speed only during peak load. Affinity laws show:

Power ∝ (Speed)³

Reducing fan speed to 70% of maximum: Power = (0.7)³ = 0.343 = 66% reduction

Annual savings (8000 hr at 50% average speed): E_saved = 29,840 × [1 - (0.5)³] = 26,103 kWh Cost savings: $2,610/year

4. Free Cooling (Economizer):

When outdoor temperature < storage temperature + 5°F, introduce outdoor air directly:

Applicable hours per year: 500-2000 (climate dependent) Energy savings: 15-40% during economizer operation Simple payback: 1-3 years

5. Heat Recovery:

Capture refrigeration condenser heat for:

• Domestic hot water heating
• Glycol tempering for loading areas
• Snow melting or space heating (winter)

Recoverable heat = Q_cooling × (COP + 1) / COP

For 25 TR system with COP = 2.5: Q_recovered = 300,000 × 3.5 / 2.5 = 420,000 BTU/hr

6. Defrost Optimization:

Excessive defrost wastes energy and adds heat to space.

Demand defrost control:

• Monitors coil pressure drop or temperature
• Initiates defrost only when needed
• Reduces defrost frequency by 30-50%
• Annual savings: 3-8% of refrigeration energy

Monitoring and Alarm Systems

Critical Parameters

Continuous Monitoring Requirements:

Data Logging and Trending

Storage Requirements:

Regulatory compliance (USDA, FDA):

• Logging interval: 15 minutes maximum
• Retention period: 12 months minimum
• Data integrity: Tamper-evident, audit trail
• Backup: Redundant storage, off-site replication

Trending Analysis:

Monitor trends for predictive maintenance:

1. Refrigeration efficiency degradation:

   • Track kW/TR over time
   • Increasing ratio indicates fouling, refrigerant loss, or mechanical wear

2. Temperature stability:

   • Calculate standard deviation of space temperature
   • Increasing variance indicates control issues

3. Defrost cycle duration:

   • Normal duration: 15-30 minutes
   • Increasing duration indicates coil icing or defrost failure

Alarm Notification Protocol

Multi-tier Alarm Response:

Tier 1 - Warning (Non-critical):

• Local audible/visual annunciation
• Log event to database
• Email notification to facility operators
• No immediate action required

Tier 2 - Alarm (Approaching critical):

• All Tier 1 notifications
• SMS text message to on-call personnel
• Automated phone call (optional)
• Response required within 30-60 minutes

Tier 3 - Critical Alarm:

• All Tier 2 notifications
• Immediate phone call to multiple contacts
• Escalation after 5 minutes non-response
• Emergency shutdown procedures may auto-execute

Alarm Documentation:

Each alarm event record must include:

• Timestamp (start and acknowledgment)
• Parameter value at alarm
• Duration of alarm condition
• Personnel who acknowledged/resolved
• Corrective action taken
• Root cause analysis (for recurring alarms)

ASHRAE Design Guidelines

Referenced Standards

ASHRAE Handbook - Refrigeration (Chapter 35: Refrigerated Eggs and Egg Products):

Key recommendations:

• Storage temperature: 45-50°F for 30 days maximum
• Relative humidity: 75-80%
• Air velocity: 50-100 fpm in storage
• Egg respiration heat: 0.012 BTU/lb·day at 45°F

ASHRAE Standard 15: Safety Standard for Refrigeration Systems

Requirements for egg storage facilities:

• Machinery room ventilation: 0.5 cfm/ft² or 20 ACH
• Refrigerant detection and alarm
• Emergency shutoff controls
• Pressure relief venting

ASHRAE Standard 62.1: Ventilation for Acceptable Indoor Air Quality

Not directly applicable to egg storage, but relevant for adjacent processing areas:

• Minimum outdoor air: 0.06 cfm/ft² for storage warehouses
• Increased rates for occupied processing areas

ASHRAE Guideline 0: The Commissioning Process

Commissioning activities for new egg storage facilities:

• Verify design intent documentation
• Functional performance testing of control sequences
• Seasonal testing of capacity and efficiency
• Operator training and documentation turnover

Psychrometric Design Process

Step 1: Define Indoor Design Conditions

• Temperature: 45-50°F (select based on storage duration)
• Relative humidity: 75-80% RH
• Calculate humidity ratio and enthalpy from psychrometric chart

Step 2: Determine Outdoor Design Conditions

• Summer: 0.4% DB/MWB from ASHRAE climatic data
• Winter: 99.6% DB/MWB from ASHRAE climatic data

Step 3: Calculate Load Components

• Transmission loads (walls, roof, floor)
• Product loads (sensible and latent)
• Infiltration loads (air changes method)
• Internal loads (lighting, equipment, people)

Step 4: Determine Supply Air Conditions

• Sensible heat ratio (SHR) = Q_sensible / Q_total
• Plot room condition and SHR line on psychrometric chart
• Select supply air state on SHR line
• Calculate required airflow: ṁ_air = Q_sensible / (c_p × ΔT)

Step 5: Size Refrigeration Equipment

• Coil capacity from cooling load + fan heat
• Select coil with appropriate TD (10-15°F)
• Verify dehumidification capacity matches latent load

Step 6: Verify Humidity Control

• Calculate moisture removal: Δω = (ω_return - ω_supply)
• Compare to facility moisture gain
• Size humidifier if supply air over-dehumidifies

Conclusion

Temperature and humidity control in egg storage facilities demands integrated design addressing thermodynamic, psychrometric, and biological considerations. Proper implementation maintains USDA grade standards while optimizing energy efficiency. Critical success factors include accurate load calculations, appropriate equipment selection, precise sensor placement, and robust monitoring systems. ASHRAE design guidelines provide the engineering foundation, while operational discipline ensures consistent product quality and regulatory compliance.

Advanced control strategies such as variable capacity refrigeration, demand defrost, and predictive maintenance algorithms offer significant operational improvements over legacy systems. Energy efficiency measures provide attractive payback periods while reducing environmental impact. Comprehensive alarm and monitoring systems protect product integrity and support HACCP compliance in regulated facilities.