Apple Storage Refrigeration Systems

Overview

Apple storage refrigeration systems represent specialized HVAC applications requiring precise environmental control to maintain fruit quality over storage periods ranging from 1 to 12 months. The primary objectives are temperature maintenance, humidity control, gas composition management, and airflow distribution to minimize respiration rates, water loss, and physiological disorders while preserving marketable quality.

Storage facilities typically operate at temperatures between -1°C and 4°C (30°F to 39°F) with relative humidity maintained at 90-95% to minimize transpiration losses. Advanced storage employs controlled atmosphere (CA) or modified atmosphere (MA) techniques with reduced O₂ and elevated CO₂ concentrations to further suppress respiration and extend storage life.

Storage Temperature Requirements by Variety

Temperature management is the most critical factor affecting apple storage life. Each cultivar exhibits specific temperature sensitivities related to chilling injury susceptibility and respiration rate characteristics.

Temperature Specifications

Temperature Control Precision Requirements

Storage rooms require temperature control within ±0.5°C of setpoint to prevent:

• Freezing injury: Ice crystal formation causes cellular rupture, resulting in watersoaking and tissue breakdown upon thawing
• Accelerated senescence: Temperatures above optimum increase respiration rate exponentially (Q₁₀ = 2-3)
• Uneven ripening: Temperature gradients within storage create non-uniform maturation

Temperature uniformity specifications:

• Vertical temperature gradient: ≤1°C from floor to ceiling
• Horizontal temperature variation: ≤0.5°C across storage room
• Temperature recovery time: Return to setpoint within 2 hours after door opening

Respiration Rate Temperature Dependence

Apple respiration rate follows the van ’t Hoff equation:

Q₁₀ = (R₂/R₁)^[10/(T₂-T₁)]

Where:

• Q₁₀ = temperature coefficient (typically 2.0-3.0 for apples)
• R₁, R₂ = respiration rates at temperatures T₁ and T₂
• T = temperature in °C

For Golden Delicious at 0°C: RR = 3-5 mg CO₂/kg·h For Golden Delicious at 10°C: RR = 15-25 mg CO₂/kg·h

This 5-7× increase in respiration rate at 10°C versus 0°C demonstrates the critical importance of rapid cooling and consistent temperature maintenance.

Humidity Control Requirements

Water loss through transpiration causes weight loss, shriveling, and loss of crispness. The transpiration rate depends on vapor pressure deficit (VPD) between fruit surface and surrounding air.

Target Relative Humidity

Optimal RH: 90-95% Acceptable range: 88-98% Critical minimum: 85% (increased shrivel risk) Maximum: 98% (mold and decay risk)

Transpiration Rate Calculation

The moisture loss rate follows:

dW/dt = k · A · (P_fruit - P_air)

Where:

• dW/dt = transpiration rate (kg/h)
• k = mass transfer coefficient (depends on air velocity)
• A = fruit surface area (m²)
• P_fruit = vapor pressure at fruit surface (kPa)
• P_air = vapor pressure in surrounding air (kPa)

For typical storage conditions:

• Weight loss rate: 0.2-0.5% per month at 90-95% RH
• Weight loss rate: 1.0-2.0% per month at 80-85% RH

Humidity Control Methods

Refrigeration system considerations:

1. Evaporator Design:

   • Large evaporator surface area minimizes TD (temperature difference)
   • Low TD = reduced dehumidification
   • Recommended TD: 2-4°C for apple storage (versus 8-12°C for comfort cooling)
   • Increased fin spacing (10-12 mm) to reduce frost accumulation

2. Defrost Strategy:

   • Off-cycle defrost preferred (minimal heat input)
   • Hot gas defrost for low-temperature rooms
   • Defrost frequency: Based on frost thickness measurement
   • Target: ≤6 defrosts per day to minimize humidity fluctuation

3. Humidification Systems:

   • Ultrasonic humidifiers: 1-5 μm droplet size
   • High-pressure spray nozzles: 10-20 μm droplets
   • Capacity: 1-3 kg/h per 100 tonnes of apples
   • Control: Modulating based on RH sensor feedback

4. Air Circulation Management:

   • High air velocity increases transpiration (higher k value)
   • Target air velocity over fruit: 0.15-0.30 m/s
   • Minimum for temperature uniformity: 0.10 m/s
   • Maximum to prevent desiccation: 0.50 m/s

Psychrometric Analysis

At -1°C (30.2°F) storage temperature:

• Saturation vapor pressure: 0.5628 kPa
• At 95% RH: Vapor pressure = 0.5347 kPa
• At 90% RH: Vapor pressure = 0.5065 kPa
• VPD at 95% RH: 0.0281 kPa
• VPD at 90% RH: 0.0563 kPa (2× higher, doubling transpiration rate)

Ethylene Management

Ethylene (C₂H₄) is the primary ripening hormone produced by apples. Concentrations as low as 0.1 ppm accelerate ripening, reduce storage life, and trigger physiological disorders.

Ethylene Production Rates

Control Strategies

1. Ventilation:

• Air exchange rate: 1-2 room volumes per day for regular atmosphere storage
• Outside air introduction during cold nights (when available at appropriate temperature)
• Calculation of ethylene accumulation:

C = (P × M)/(V × λ)

Where:

• C = ethylene concentration (μL/L or ppm)
• P = production rate (μL/kg·h)
• M = mass of apples (kg)
• V = room volume (L)
• λ = air exchange rate (room volumes/h)

2. Controlled Atmosphere (CA) Storage:

• Reduced O₂ (1-3%) suppresses ethylene production by 50-80%
• Elevated CO₂ (1-3%) further reduces production
• Total ethylene production in CA: 10-30% of regular atmosphere levels

3. Ethylene Scrubbing:

• Potassium permanganate (KMnO₄) oxidation
• Catalytic oxidation at 180-200°C
• UV photocatalytic oxidation (TiO₂ catalyst)
• Ozone treatment (0.3-0.5 ppm)
• Capacity requirement: 50-200 m³/h per 100 tonnes

4. 1-MCP Treatment (SmartFresh):

• 1-methylcyclopropene blocks ethylene receptors
• Application: 0.625-1.0 μL/L for 24 hours at 0-10°C
• Reduces ethylene sensitivity for 4-8 months
• Applied within 7 days of harvest for maximum effectiveness

Air Circulation Patterns

Proper air distribution ensures temperature uniformity, adequate oxygen supply for respiration, and removal of metabolic heat and gases.

Airflow Design Requirements

Air circulation rate: 30-60 room volumes per hour Typical value: 40 changes/hour for bin storage Box storage: 50-80 changes/hour (higher resistance)

Circulation Patterns

1. Horizontal Airflow (Parallel Flow):

• Air flows horizontally through stacked bins
• Supply plenum on one end, return plenum on opposite end
• Uniform velocity requires tapered plenum design
• Pressure drop: 25-75 Pa across product

2. Vertical Airflow (Perpendicular Flow):

• Air supplied from ceiling, returns at floor level
• Better temperature uniformity in tall rooms
• Requires perforated flooring or floor channels
• Pressure drop: 30-100 Pa depending on stack height

3. Around-Bin Circulation:

• Air flows around bins rather than through them
• Lower pressure drop: 10-25 Pa
• Requires wider aisle spacing (reduced storage density)
• Better for tightly packed bins

Pressure Drop Calculations

For airflow through bin-stacked apples:

ΔP = (f × L × ρ × V²)/(2 × D_h) + K × (ρ × V²/2)

Where:

• ΔP = pressure drop (Pa)
• f = friction factor (0.08-0.15 for bulk apples)
• L = flow path length (m)
• ρ = air density (kg/m³)
• V = superficial air velocity (m/s)
• D_h = hydraulic diameter (m)
• K = entrance/exit loss coefficient

Simplified empirical relationship for apples in bins:

ΔP = C × Q^n

Where:

• C = coefficient (150-250 Pa·s^n/m^3n)
• Q = airflow rate per unit area (m³/s·m²)
• n = exponent (1.6-1.8)

Fan Selection

Required fan characteristics:

• Type: Forward-curved centrifugal (low pressure, high volume)
• Static pressure: 100-200 Pa for typical installations
• Power: 0.8-1.5 kW per 100 tonnes of apples
• Speed control: VFD for load matching and energy efficiency

Storage Room Design

Room Dimensions and Layout

Ceiling height: 5-7 m for bin storage (allows 5-6 high stacking) Floor loading: 15-25 kPa (includes refrigeration equipment, bins, product) Aisle width: 2.5-3.5 m for forklift operation Storage density: 250-350 kg/m² of floor area

Insulation Requirements

Thermal envelope design based on steady-state heat gain minimization:

Insulation materials:

• Polyurethane foam: R = 5.6-6.5 m²·K/W per inch (λ = 0.022-0.026 W/m·K)
• Polystyrene (EPS): R = 4.0 m²·K/W per inch (λ = 0.033 W/m·K)
• Polyisocyanurate: R = 6.5-7.0 m²·K/W per inch (λ = 0.020-0.023 W/m·K)

Vapor Barrier

Critical to prevent moisture migration into insulation:

• Material: 6 mil polyethylene or aluminum foil laminate
• Location: Warm side of insulation
• Permeance: ≤0.06 perm (3.4 ng/Pa·s·m²)
• Joints: Sealed with vapor-tight tape

Door Design

Specifications:

• Insulated sliding or hinged doors
• U-value: ≤0.25 W/m²·K
• Air infiltration barrier: Strip curtains or vestibule entry
• Width: 2.5-3.0 m for forklift access
• Pressure relief port: Prevents pressure differential during door operation

Refrigeration Load Calculations

Total refrigeration load consists of multiple components that must be calculated and summed:

Component Loads

1. Transmission Load (Q_trans):

Q_trans = U × A × ΔT

Where:

• U = overall heat transfer coefficient (W/m²·K)
• A = surface area (m²)
• ΔT = temperature difference (K)

Example: 1000 m² wall, U = 0.20 W/m²·K, ΔT = 30 K Q_trans = 0.20 × 1000 × 30 = 6,000 W = 6.0 kW

2. Product Load (Q_product):

Consists of two components:

a. Sensible heat removal (cooling):

Q_sensible = m × c_p × ΔT / t

Where:

• m = mass of apples (kg)
• c_p = specific heat (3.59 kJ/kg·K for apples at 85% moisture)
• ΔT = temperature reduction (K)
• t = pulldown time (s)

Example: 100,000 kg apples, 20°C to 0°C, 24 hours Q_sensible = (100,000 × 3.59 × 20) / (24 × 3600) = 83.2 kW

b. Respiration heat:

Q_respiration = m × RR × H_CO2

Where:

• m = mass of apples (kg)
• RR = respiration rate (kg CO₂/kg·s)
• H_CO2 = heat of respiration per kg CO₂ produced (≈ 16,000 kJ/kg CO₂)

Example: 100,000 kg apples, RR = 4 mg CO₂/kg·h at 0°C Q_respiration = 100,000 × (4 × 10⁻⁶ kg/kg·h) × (16,000 kJ/kg) / 3600 s/h Q_respiration = 1.78 kW

3. Air Infiltration Load (Q_infiltration):

Q_infiltration = V × ρ × c_p × ΔT × N × F + V × ρ × Δw × h_fg × N × F

Where:

• V = room volume (m³)
• ρ = air density (kg/m³)
• c_p = specific heat of air (1.006 kJ/kg·K)
• ΔT = temperature difference (K)
• N = air changes per door opening
• F = door opening frequency (times/day)
• Δw = humidity ratio difference (kg water/kg dry air)
• h_fg = latent heat of vaporization (2501 kJ/kg)

Typical values: N = 0.3-0.5 air changes per door opening, F = 10-30 times/day

4. Fan and Motor Heat (Q_fan):

Q_fan = P_motor / η

Where:

• P_motor = motor input power (W)
• η = motor efficiency (0.85-0.95 for premium efficiency motors)

5. Lighting Load (Q_lights):

Q_lights = W × A × U

Where:

• W = lighting power density (5-10 W/m²)
• A = floor area (m²)
• U = usage factor (0.3-0.5 for storage facilities)

6. Personnel Load (Q_people):

• Sensible heat: 100-150 W/person (light activity in cold room)
• Latent heat: 50-75 W/person
• Occupancy factor based on work schedule

7. Fork Truck Load (Q_forklift):

• Electric forklift: 2-4 kW per truck
• Usage factor: 0.2-0.4 (intermittent operation)

8. Safety Factor:

Apply 10-20% safety factor to account for:

• Calculation uncertainties
• Future expansion
• Degraded performance over time
• Extreme weather conditions

Total Load Calculation

Q_total = Q_trans + Q_product + Q_respiration + Q_infiltration + Q_fan + Q_lights + Q_people + Q_forklift

Apply safety factor: Q_design = Q_total × 1.15

Typical Load Distribution

For a typical apple storage facility during steady-state operation:

Note: During pulldown, product load dominates (60-75% of total).

Cooling Curve Considerations

The cooling rate of apples depends on fruit size, air velocity, initial temperature, and cooling medium temperature.

Heat Transfer Analysis

The cooling process follows Newton’s law of cooling:

T(t) = T_∞ + (T_0 - T_∞) × e^(-t/τ)

Where:

• T(t) = product temperature at time t (°C)
• T_∞ = cooling medium temperature (°C)
• T_0 = initial product temperature (°C)
• τ = time constant (h)

The time constant τ depends on:

τ = (m × c_p) / (h × A)

Where:

• m = fruit mass (kg)
• c_p = specific heat (kJ/kg·K)
• h = convective heat transfer coefficient (W/m²·K)
• A = surface area (m²)

Seven-Eighths Cooling Time

Time to cool from initial temperature to within 1/8 of temperature difference:

t_7/8 = 2.08 × τ

This represents practical cooling endpoint (e.g., from 20°C to 2.5°C when final target is 0°C).

Cooling Rate Factors

1. Air Velocity Effect on h:

For forced convection over apples:

Nu = 0.615 × Re^0.466

Where:

• Nu = Nusselt number = h × D / k
• Re = Reynolds number = V × D / ν
• D = characteristic dimension (apple diameter ≈ 0.075 m)
• k = thermal conductivity of air (W/m·K)
• V = air velocity (m/s)
• ν = kinematic viscosity (m²/s)

Typical h values:

• V = 0.5 m/s: h ≈ 15 W/m²·K
• V = 1.0 m/s: h ≈ 22 W/m²·K
• V = 2.0 m/s: h ≈ 32 W/m²·K

2. Fruit Size Effect:

Larger apples cool more slowly due to higher mass-to-surface-area ratio.

(Assumes initial temp 20°C, air temp -1°C, air velocity 1.0 m/s)

Recommended Cooling Protocols

1. Rapid Initial Cooling:

• First 24 hours: Maximum refrigeration capacity
• Target: Reduce pulp temperature to <10°C
• Prevents excessive respiration and quality loss

2. Gradual Final Cooling:

• Days 2-7: Controlled approach to final temperature
• Prevents condensation on fruit surface
• Minimizes risk of freezing injury

3. Temperature Monitoring:

• Pulp temperature measurement (not air temperature)
• Multiple locations within load
• Target uniformity: ±1°C across entire room

Storage Duration by Variety

Maximum storage duration depends on variety characteristics, harvest maturity, and storage atmosphere composition.

Regular Atmosphere (RA) Storage

Controlled Atmosphere (CA) Storage

CA storage extends duration by 50-150% compared to RA storage:

Storage Disorder Prevention

Superficial scald:

• Brown pigmentation on fruit skin
• Prevention: DPA treatment (1000-2000 ppm) or 1-MCP
• CA storage with low O₂ (1-2%) reduces incidence

Internal breakdown:

• Brown discoloration of flesh
• Caused by CO₂ injury or senescence
• Prevention: Avoid excessive CO₂, maintain proper O₂

Core browning:

• Oxidative damage to core tissue
• More common at temperatures >2°C
• Prevention: Rapid cooling, low O₂ CA storage

Bitter pit:

• Calcium deficiency disorder
• Prevents: Pre-harvest calcium sprays
• Storage: Maintain optimal temperature, avoid water stress

Quality Monitoring

Systematic monitoring ensures product quality maintenance throughout storage period.

Physical Parameters

1. Weight Loss:

• Measurement frequency: Weekly
• Acceptable rate: <0.5% per month
• Method: Sample box weighing (same boxes each time)

2. Firmness:

• Measurement: Penetrometer (11 mm probe)
• Frequency: Biweekly
• Target retention: >80% of harvest firmness
• Critical minimum: 5.4 kg force (53 N)

3. Color Development:

• Instrument: Colorimeter (Lab* scale)
• Frequency: Monthly
• Track chlorophyll degradation and anthocyanin development

4. Soluble Solids Content (SSC):

• Measurement: Refractometer (°Brix)
• Frequency: Monthly
• Slight increase expected due to water loss

5. Titratable Acidity (TA):

• Measurement: Titration (% malic acid)
• Frequency: Monthly
• Declines during storage (respiration)

Atmospheric Parameters (CA Storage)

1. Oxygen Concentration:

• Measurement: Electrochemical or zirconia sensor
• Monitoring: Continuous with alarm
• Control precision: ±0.2% of setpoint
• Calibration: Weekly with standard gas

2. Carbon Dioxide Concentration:

• Measurement: NDIR sensor
• Monitoring: Continuous with alarm
• Control precision: ±0.3% of setpoint
• Scrubbing: Activated carbon or lime water

3. Ethylene Concentration:

• Measurement: Electrochemical or gas chromatography
• Frequency: Weekly or continuous
• Action level: >1 ppm (initiate scrubbing)

4. Nitrogen Concentration:

• Calculation: 100% - O₂ - CO₂
• Enrichment source: PSA nitrogen generator or liquid N₂

Temperature Distribution Mapping

Conduct thermal mapping quarterly:

1. Install 15-20 temperature sensors throughout room
2. Record temperatures every 5 minutes for 48 hours
3. Analyze for hot/cold spots
4. Maximum acceptable deviation: ±1.0°C from setpoint
5. Adjust air distribution or insulation as needed

Disorder Assessment

Monthly inspection sample (100 apples minimum):

• Scald incidence: % of fruit affected
• Decay incidence: % with fungal growth
• Internal breakdown: Destructive sampling (10-20 fruits)
• Overall marketability: % meeting grade standards

ASHRAE and USDA Guidelines

ASHRAE Standards

ASHRAE Handbook - Refrigeration (Chapter 35: Deciduous Tree and Vine Fruits):

Key design parameters:

• Storage temperature: 30-32°F (-1 to 0°C) for most varieties
• Relative humidity: 90-95%
• Freezing point: 29.3-30.5°F (-1.5 to -0.8°C) depending on variety
• Specific heat above freezing: 3.59 kJ/kg·K (0.86 Btu/lb·°F)
• Specific heat below freezing: 1.76 kJ/kg·K (0.42 Btu/lb·°F)
• Latent heat of fusion: 249 kJ/kg (107 Btu/lb)
• Respiration heat at 32°F (0°C): 0.35-1.1 W/tonne (1.1-3.5 Btu/ton·day)

ASHRAE Standard 15: Safety Standard for Refrigeration Systems

• Refrigerant leak detection requirements
• Machinery room ventilation
• Pressure relief requirements

ASHRAE Standard 34: Designation and Safety Classification of Refrigerants

• Refrigerant selection for food storage applications
• Toxicity and flammability considerations

USDA Guidelines

USDA Agricultural Handbook 66: The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks

Apple-specific recommendations:

• Harvest maturity indices (starch index, firmness)
• Variety-specific storage conditions
• Compatibility in mixed storage
• Disorder identification and prevention

USDA Agricultural Marketing Service (AMS):

• U.S. Standards for Grades of Apples
• Quality tolerances for stored fruit
• Inspection procedures

FDA Food Safety Modernization Act (FSMA)

Relevant requirements for cold storage facilities:

• Temperature monitoring and documentation
• Sanitation standard operating procedures (SSOPs)
• Preventive controls for biological hazards
• Record retention (2 years minimum)

Industry Best Practices

International Controlled Atmosphere Storage Guidelines:

• Dynamic CA (DCA) protocols
• Rapid CA establishment procedures
• Low oxygen stress testing
• Emergency procedures for atmosphere loss

Postharvest Technology Center (UC Davis):

• Cultivar-specific storage recommendations
• Disorder management protocols
• Emerging technology guidance

System Design Example

Design parameters for 1000-tonne apple storage facility:

Storage specifications:

• Capacity: 1,000,000 kg apples
• Variety: Mixed (primarily Granny Smith, Fuji, Gala)
• Storage type: Controlled atmosphere (CA)
• Temperature: 0°C (32°F)
• Relative humidity: 92%
• Atmosphere: 2% O₂, 1% CO₂, 97% N₂

Load calculations:

1. Transmission load (steady-state):

   • Wall area: 2,400 m², U = 0.18 W/m²·K, ΔT = 30 K → 13.0 kW
   • Roof area: 1,200 m², U = 0.15 W/m²·K, ΔT = 35 K → 6.3 kW
   • Floor area: 1,200 m², U = 0.30 W/m²·K, ΔT = 20 K → 7.2 kW
   • Subtotal: 26.5 kW

2. Respiration load:

   • Average rate: 3.5 mg CO₂/kg·h at 0°C
   • Q = 1,000,000 kg × 3.5×10⁻⁶ kg/kg·h × 16,000 kJ/kg / 3600 s/h
   • Subtotal: 15.6 kW

3. Fan load:

   • Evaporator fans: 12 kW
   • Circulation fans: 8 kW
   • Subtotal: 20.0 kW

4. Infiltration load (20 door openings/day):

   • Sensible: 6.5 kW
   • Latent: 4.2 kW
   • Subtotal: 10.7 kW

5. Other loads (lights, people, equipment): 5.2 kW

6. Total steady-state load: 78.0 kW

7. Safety factor (15%): 11.7 kW

8. Design steady-state capacity: 89.7 kW (25.5 tonnes refrigeration)

Pulldown capacity:

• Product sensible heat: 100,000 kg/day × 3.59 kJ/kg·K × 20 K / 86,400 s = 83.2 kW
• Total pulldown load: 83.2 + 78.0 = 161.2 kW
• Design pulldown capacity: 185 kW (52.6 tonnes refrigeration)

Refrigeration system selection:

• Two-stage ammonia system
• Three evaporators (62 kW each) for redundancy
• Screw compressor with capacity control
• Evaporative condenser
• Thermosiphon liquid recirculation
• Evaporator TD: 3°C (SST = -3°C)

This comprehensive design ensures optimal storage conditions for extended apple preservation while maintaining energy efficiency and product quality.