Refrigerated Facility Design Loads

Accurate refrigeration load calculations form the foundation for proper refrigerated facility design. The total refrigeration load consists of multiple components that must be systematically evaluated to determine equipment capacity requirements.

Load Calculation Methodology Overview

Refrigeration load calculations follow a systematic approach that accounts for all heat gains to the refrigerated space. The total design load represents the sum of simultaneous peak loads from all sources.

Total Refrigeration Load:

Q_total = Q_trans + Q_inf + Q_prod + Q_int + Q_equip + Q_lights + Q_people + Q_misc

Where:

• Q_trans = Transmission heat gain through envelope (Btu/hr or W)
• Q_inf = Infiltration air load (Btu/hr or W)
• Q_prod = Product cooling load (Btu/hr or W)
• Q_int = Internal equipment heat gain (Btu/hr or W)
• Q_equip = Process equipment loads (Btu/hr or W)
• Q_lights = Lighting heat gain (Btu/hr or W)
• Q_people = Occupancy load (Btu/hr or W)
• Q_misc = Safety factor and miscellaneous loads (Btu/hr or W)

Component Loads Summary

Transmission Loads

Transmission loads result from heat transfer through walls, floors, ceilings, and structural elements. This load operates continuously and represents a significant portion of total refrigeration requirements.

Transmission Load Equation:

Q_trans = U × A × ΔT

Where:

• U = Overall heat transfer coefficient (Btu/hr·ft²·°F or W/m²·K)
• A = Surface area (ft² or m²)
• ΔT = Temperature difference across envelope (°F or K)

Typical U-Values for Refrigerated Facilities:

For below-grade surfaces and slabs on grade, use modified calculation methods accounting for ground temperature and heat flow paths per ASHRAE Handbook—Refrigeration Chapter 24.

Infiltration Loads

Infiltration represents outdoor air entering the refrigerated space through door openings, structural leaks, and pressure differentials. This load includes both sensible and latent heat components.

Infiltration Load Components:

Q_inf = Q_sens + Q_lat

Sensible Load:

Q_sens = ṁ_air × c_p × (T_out - T_in)

Latent Load:

Q_lat = ṁ_air × h_fg × (W_out - W_in)

Where:

• ṁ_air = Mass flow rate of infiltrating air (lb/hr or kg/hr)
• c_p = Specific heat of air (0.24 Btu/lb·°F or 1.006 kJ/kg·K)
• h_fg = Latent heat of water vaporization (~1060 Btu/lb or 2465 kJ/kg)
• W = Humidity ratio (lb_water/lb_air or kg_water/kg_air)

Door Infiltration—Doorway Flow Factor Method:

Q_door = D_f × A_door × ρ_m × Δh × F_m

Where:

• D_f = Doorway flow factor (from ASHRAE tables)
• A_door = Door opening area (ft² or m²)
• ρ_m = Density of air mixture (lb/ft³ or kg/m³)
• Δh = Enthalpy difference (Btu/lb or kJ/kg)
• F_m = Door usage factor (0.4-1.0 depending on traffic)

Typical Door Infiltration Loads:

For freezer applications below 0°F (-18°C), vestibules or air curtains significantly reduce infiltration loads.

Product Cooling Loads

Product loads consist of heat removal required to cool incoming product from receiving temperature to storage temperature, plus heat of respiration for fresh produce.

Product Cooling Load:

Q_prod = ṁ_prod × c_p × ΔT / t_cool

Where:

• ṁ_prod = Product mass (lb or kg)
• c_p = Specific heat of product (Btu/lb·°F or kJ/kg·K)
• ΔT = Temperature reduction required (°F or K)
• t_cool = Cooling time period (hr)

For freezing applications, add latent heat:

Q_freeze = ṁ_prod × h_f / t_freeze

Where h_f = latent heat of fusion (typically 100-144 Btu/lb or 233-335 kJ/kg for water content)

Specific Heat Values for Common Products:

Respiration Heat for Fresh Produce:

Living fruits and vegetables generate metabolic heat that contributes to refrigeration load. Respiration rates vary significantly with temperature and must be included for produce storage.

Reference: ASHRAE Handbook—Refrigeration Chapter 19 for comprehensive produce data.

Internal Equipment Loads

Equipment operating within the refrigerated space generates heat that must be removed by the refrigeration system.

Electric Motors (Inside Refrigerated Space):

Q_motor = P_motor × 3.413 / η_motor

Where:

• P_motor = Motor nameplate power (kW)
• η_motor = Motor efficiency (typically 0.85-0.95)
• 3.413 = Conversion factor (Btu/hr per Watt)

Fork Lifts and Material Handling Equipment:

Usage Factor:

Apply a usage factor (F_use) based on actual operating hours:

Q_equip,avg = Q_equip,rated × F_use

Typical usage factors range from 0.3-0.7 depending on facility operations.

Lighting Loads

Lighting heat gain depends on lamp type, wattage, and operating schedule.

Lighting Load:

Q_lights = P_lights × 3.413 × F_use × F_bal

Where:

• P_lights = Total lighting power (W)
• F_use = Usage factor (fraction of time lights operate)
• F_bal = Ballast factor (1.0 for LED, 1.15-1.20 for fluorescent)

Typical Lighting Power Densities:

LED lighting reduces both energy consumption and heat gain compared to traditional high-intensity discharge (HID) lamps.

Occupancy Loads

Personnel working in refrigerated spaces generate sensible and latent heat. Load magnitude depends on activity level and protective clothing.

Heat Gain from People:

Q_people = N × (q_sens + q_lat)

Where:

• N = Number of occupants
• q_sens = Sensible heat gain per person (Btu/hr or W)
• q_lat = Latent heat gain per person (Btu/hr or W)

Heat Gain Values:

At lower temperatures, latent heat gain decreases significantly as water vapor condenses on clothing and protective gear before entering the space.

Safety Factors

Safety factors account for uncertainties in load calculations, future expansion, and unusual operating conditions. Apply safety factors to the sum of all calculated loads.

Recommended Safety Factors:

Higher safety factors apply when:

• Load calculations contain significant uncertainties
• Future expansion is anticipated
• Process equipment loads are not well-defined
• Facility operates near design conditions for extended periods
• Rapid pulldown capability is required

Lower safety factors apply when:

• Detailed load calculations are performed
• Equipment specifications are well-defined
• Conservative assumptions were used in base calculations
• Multiple refrigeration units provide inherent redundancy

Design Conditions

Outdoor Design Conditions

Select outdoor design temperatures based on ASHRAE Handbook—Fundamentals Chapter 14 climatic data. Use appropriate percentile values for the application.

Recommended Design Conditions:

The 0.4% design condition represents conditions exceeded 35 hours per year, while 1% represents 88 hours per year.

Indoor Design Conditions

Storage temperatures depend on product requirements and storage duration. Maintain temperature uniformity within ±2°F (±1°C) throughout the space.

Common Storage Temperatures:

Design Temperature Differentials

The temperature difference between outdoor and indoor conditions drives transmission and infiltration loads. Consider both summer and winter conditions.

Typical Design ΔT Values:

Load Diversity

Not all loads reach peak values simultaneously. Load diversity accounts for the temporal variation in different load components.

Diversity Factors

Typical Diversity Applications:

• Multiple rooms: When calculating central plant capacity for multiple rooms at different temperatures, apply diversity to account for non-simultaneous peaks
• Door openings: Multiple doors rarely operate at peak traffic simultaneously
• Product loading: Receiving operations typically occur during specific hours, not continuously
• Lighting: Not all lights operate simultaneously in large facilities
• Equipment: Material handling equipment operates on varying schedules

Diversity Factor Range:

F_div = 0.75 to 0.95 for total facility load (apply to sum of individual room loads)

Do not apply diversity factors to:

• Transmission loads (continuous)
• Individual room sizing calculations
• Safety-critical applications
• Rapid-pulldown requirements

Time-Dependent Loads

Some loads vary predictably throughout the day:

24-Hour Load Profile Considerations:

• Product loads: Peak during receiving hours (typically 6:00 AM - 2:00 PM)
• Occupancy: Varies with shift schedules
• Lighting: Follows occupancy patterns
• Equipment: Material handling equipment operates during active periods
• Transmission: Relatively constant, peaks during hottest outdoor conditions
• Infiltration: Related to door traffic patterns

Load Profile Application:

For central plant sizing, consider both peak instantaneous load and total heat removal over 24 hours. Thermal storage systems may leverage load diversity to reduce peak demand.

Peak vs Average Loads

Peak Load Determination

Peak refrigeration load represents the maximum simultaneous heat gain to the space. Size evaporators and compressors based on peak load plus safety factor.

Peak Load Calculation:

Q_peak = (Q_trans,max + Q_inf,max + Q_prod,peak + Q_int,peak + Q_lights,peak + Q_people,peak) × SF

Where SF = safety factor (1.10 to 1.30)

Peak loads typically occur when:

• Outdoor temperature is at design maximum
• Maximum product receiving occurs
• Full personnel complement is working
• All lights and equipment operate
• Doors experience high traffic

Average Load Determination

Average load represents typical operating conditions over a 24-hour period. Use average loads for:

• Energy consumption calculations
• Operating cost estimates
• Annual refrigerant consumption
• Heat recovery system sizing

Average Load Estimation:

Q_avg = Q_trans,avg + Q_inf,avg + Q_prod,avg + Q_int,avg + Q_lights,avg + Q_people,avg

Average loads typically range from 40-60% of peak design loads depending on facility operation.

Load Factor:

LF = Q_avg / Q_peak

Typical load factors:

• Distribution centers: 0.45-0.55
• Food processing: 0.55-0.70
• Cold storage only: 0.40-0.50

System Sizing Considerations

Equipment Capacity Selection

Refrigeration equipment capacity must accommodate peak loads while operating efficiently at part-load conditions.

Compressor Capacity:

Select compressor capacity based on:

Q_comp,required = Q_peak / COP_system

Where COP_system accounts for:

• Compressor efficiency
• Condenser performance
• Evaporator performance
• Piping and component losses
• Defrost loads (if applicable)

Multiple Compressor Staging:

For improved part-load efficiency and reliability:

• Use 2-4 compressors per system
• Stage capacity in 25-50% increments
• Provide minimum 50% capacity with largest unit off
• Consider variable-speed compressors for modulation

Evaporator Capacity

Select evaporators with sufficient capacity at design temperature difference (TD) between refrigerant and air.

Evaporator Sizing:

Q_evap,rated = Q_peak / (1 - f_defrost)

Where:

• f_defrost = Fraction of time in defrost (0.08-0.12 for low-temp applications)
• TD typically ranges from 8-15°F (4-8K) for coolers, 10-18°F (6-10K) for freezers

Evaporator Selection Criteria:

Lower TD values provide:

• Better humidity control
• More uniform temperatures
• Higher equipment cost
• Improved product quality

Higher TD values provide:

• Lower equipment cost
• Reduced refrigerant charge
• Lower humidity (more dehumidification)
• Increased defrost frequency

Condenser Capacity

Size condensers to reject both refrigeration load and compressor work at design conditions.

Heat Rejection:

Q_condenser = Q_evap + W_comp = Q_evap × (1 + 1/COP)

For air-cooled condensers:

• Size at 95°F (35°C) ambient minimum for most locations
• Consider 105-115°F (41-46°C) for hot climates
• Allow 120-125% capacity margin for fouling and degradation

For evaporative condensers:

• Size at design wet-bulb temperature
• Provide 110-115% capacity margin
• Consider water treatment requirements

System Redundancy

Provide redundant capacity for critical applications:

Redundancy Strategies:

• N+1 Configuration: Provide one additional compressor beyond minimum required
• 50% Rule: Size equipment so facility operates with any single largest unit offline
• Separate Systems: Divide facility into multiple independent refrigeration circuits
• Emergency Backup: Provide portable refrigeration connection points

Redundancy by Application:

Capacity Verification

Verify final equipment selection accounts for:

1. Load calculation accuracy: Review all assumptions and confirm input data
2. Safety factor: Ensure appropriate margin for uncertainties
3. Future expansion: Consider planned facility growth
4. Part-load performance: Confirm efficient operation at average loads
5. Extreme conditions: Evaluate performance during unusual events
6. Defrost impact: Account for capacity loss during defrost cycles
7. Refrigerant charge: Confirm adequate charge for all operating conditions

Final Capacity Check:

Q_installed ≥ Q_peak × SF × F_future

Where:

• Q_installed = Total installed refrigeration capacity
• F_future = Future expansion factor (1.0-1.25)

Document all load calculations, assumptions, and equipment selections for future reference and system troubleshooting.

Calculation Documentation

Maintain comprehensive documentation including:

• Design basis and assumptions
• Climatic data sources
• Building construction details
• Product specifications and throughput
• Equipment schedules and operating profiles
• Safety factors applied
• Diversity factors used
• Equipment selection rationale
• Alternative scenarios evaluated

Reference ASHRAE Handbook—Refrigeration Chapters 13 (Load Calculations) and 24 (Refrigerated Facility Design) for detailed calculation procedures and additional guidance.

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