Refrigeration Load Calculations for Cold Storage & Food Processing

Refrigeration Load Calculations for Cold Storage & Food Processing

Accurate refrigeration load calculations determine equipment capacity, operating costs, and temperature control performance. This guide covers all heat gain components for cold storage, food processing, and commercial refrigeration applications, with calculation methodology per ASHRAE Refrigeration Handbook.

Total Refrigeration Load Components

Total heat gain:

$$Q_{total} = Q_{trans} + Q_{prod} + Q_{resp} + Q_{inf} + Q_{equip} + Q_{people} + Q_{other}$$

Where:

• $Q_{trans}$ = transmission load through walls, floor, ceiling
• $Q_{prod}$ = product load (cooling from initial to final temperature)
• $Q_{resp}$ = respiration heat from living produce
• $Q_{inf}$ = infiltration load (air exchange, door openings)
• $Q_{equip}$ = equipment heat (fans, lights, forklifts)
• $Q_{people}$ = occupant heat gain
• $Q_{other}$ = miscellaneous loads (defrost, piping heat gain)

Transmission Load

Wall, Ceiling, Floor Heat Gain

Heat transfer through envelope:

$$Q_{trans} = U \cdot A \cdot \Delta T$$

Where:

• $U$ = overall heat transfer coefficient (Btu/hr·ft²·°F)
• $A$ = surface area (ft²)
• $\Delta T$ = temperature difference, outdoor - indoor (°F)

U-value calculation:

$$U = \frac{1}{R_{total}} = \frac{1}{R_{out} + R_{wall} + R_{insulation} + R_{in}}$$

Typical U-values for insulated cold storage:

Design temperature differences:

Solar Radiation Load

Roof solar gain (additional to transmission):

$$Q_{solar} = A_{roof} \cdot CLTD$$

Where CLTD = Cooling Load Temperature Difference (includes solar effect)

Typical CLTD for dark roof in summer: 30-50°F (add to ambient ΔT)

Mitigation:

• White reflective roof coating (reduces solar gain 40-60%)
• Insulation above roof deck
• Ventilated air space

Product Load

Sensible Cooling Above Freezing

Heat removal to cool product from initial to final temperature:

$$Q_{prod,sensible} = \frac{m \cdot c_p \cdot (T_i - T_f)}{t}$$

Where:

• $m$ = product mass (lb)
• $c_p$ = specific heat above freezing (Btu/lb·°F)
• $T_i$ = initial product temperature (°F)
• $T_f$ = final product temperature (°F)
• $t$ = cooling time (hr)

Typical specific heats (above freezing):

Approximation: $c_p \approx 0.20 + 0.008 \times %_{water}$

Freezing Load

Heat removal during freezing:

$$Q_{freezing} = m \cdot h_{latent}$$

Where $h_{latent}$ = latent heat of fusion (Btu/lb)

Latent heat approximation:

$$h_{latent} = 144 \times %_{water}/100$$

Example: Beef (75% water): $h_{latent} = 144 \times 0.75 = 108$ Btu/lb

Sensible Cooling Below Freezing

Heat removal after freezing:

$$Q_{prod,frozen} = \frac{m \cdot c_{p,frozen} \cdot (T_f - T_{storage})}{t}$$

Where $c_{p,frozen}$ = specific heat below freezing (typically 0.35-0.50 Btu/lb·°F)

Total freezing load:

$$Q_{prod,total} = Q_{above} + Q_{freezing} + Q_{below}$$

Respiration Load

Fresh fruits and vegetables generate heat through respiration:

$$Q_{resp} = m \cdot q_{resp}$$

Where:

• $m$ = product mass (lb or ton)
• $q_{resp}$ = specific respiration heat (Btu/day·ton or Btu/day·lb)

Respiration heat (at storage temperature):

Important: Respiration increases exponentially with temperature (Q₁₀ ≈ 2-3)

Rate at higher temperature:

$$q_{resp,T2} = q_{resp,T1} \times Q_{10}^{(T_2-T_1)/10}$$

Where $Q_{10}$ = temperature coefficient (typically 2.0-2.5)

Infiltration Load

Air Exchange Load

Heat gain from outdoor air entering space:

$$Q_{inf} = V \cdot \rho \cdot c_p \cdot (T_{out} - T_{in}) \cdot N_{ACH}$$

Or using simplified form:

$$Q_{inf,sensible} = 1.08 \cdot CFM \cdot \Delta T$$

$$Q_{inf,latent} = 4840 \cdot CFM \cdot \Delta W$$

Where:

• $CFM$ = infiltration airflow rate
• $\Delta T$ = temperature difference (°F)
• $\Delta W$ = humidity ratio difference (lb/lb)

Infiltration air changes per day:

Infiltration through door openings (Gosney-Olama equation):

$$CFM_{door} = K \cdot A_{door} \cdot \rho_{avg} \cdot \sqrt{H \cdot (1 - \frac{\rho_{in}}{\rho_{out}})} \cdot F_{usage}$$

Where:

• $K$ = flow coefficient (0.6-0.8)
• $A_{door}$ = door opening area (ft²)
• $H$ = door height (ft)
• $\rho_{in}, \rho_{out}$ = air density inside/outside (lb/ft³)
• $F_{usage}$ = usage factor (door open fraction of time)

Mitigation:

• Air curtains (reduce infiltration 60-80%)
• Strip curtains (reduce 50-70%)
• Vestibules/airlocks
• High-speed doors (minimize open time)

Equipment and People Loads

Internal Equipment

Typical heat gains:

Forklift load calculation:

$$Q_{forklift} = N \cdot P \cdot U \cdot 2,545$$

Where:

• $N$ = number of forklifts
• $P$ = motor power (hp)
• $U$ = usage factor (0.2-0.4 typical)

People Load

Heat gain per person:

Occupancy diversity: Average number, not peak (use time-weighted average)

Safety Factors and Design Margins

Refrigeration load safety factors:

1. Equipment capacity: Add 10-20% to calculated load

   • Uncertainty in usage patterns: 10%
   • Future expansion: 10-20%
   • Equipment degradation over time: 5-10%

2. Pull-down load: Additional capacity for initial cool-down

   • Empty room: 2-3 × steady-state transmission load
   • With product: Calculate pull-down time requirement

3. Peak vs. average load:

   • Design for peak load, not annual average
   • Consider seasonal variations (higher summer transmission)

Avoid over-sizing:

• Excessive capacity → short cycling → poor humidity control
• Target: equipment runs 16-20 hr/day at design conditions

Complete Load Calculation Example

Related Technical Guides:

• Material Thermal Properties
• Food Storage Requirements
• Vapor Compression Refrigeration
• Compressor Selection & Performance
• Cold Storage Facility Design

References:

• ASHRAE Refrigeration Handbook, Chapter 24: Refrigeration Load Calculations
• ASHRAE Refrigeration Handbook, Chapter 19: Thermal Properties of Foods
• ASHRAE Fundamentals Handbook, Chapter 14: Climatic Design Information
• Stoecker, W.F., “Industrial Refrigeration Handbook”
• USDA: The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks