1. Categories of Heat Load
Refrigeration systems are affected by multiple sources of heat gain:
Heat transmission through building envelope – conduction through walls, roof, floor.
Solar radiation – through roofs and windows (if applicable).
Air infiltration and ventilation – warm moist air entering through doors or leaks.
Product load – heat removed from goods being cooled or frozen.
Lighting and electrical equipment – heat released inside the refrigerated space.
Occupancy load – sensible heat from workers inside the room.
Process and auxiliary equipment – motors, conveyors, pumps, etc.
Additional allowances – defrosting, frequent door openings, and safety margin.
2. Common Formulas
Conduction (through walls/roof/floor):
Qcond=U⋅A⋅ΔTQ_{cond} = U \cdot A \cdot \Delta TQcond=U⋅A⋅ΔT
UUU: Overall heat transfer coefficient (W/m²·K)
AAA: Surface area (m²)
ΔT\Delta TΔT: Temperature difference (K)
Air Infiltration / Ventilation:
m˙=ACH⋅V/3600⋅ρ\dot m = ACH \cdot V / 3600 \cdot \rhom˙=ACH⋅V/3600⋅ρ Qinf=m˙⋅cp⋅ΔTQ_{inf} = \dot m \cdot c_p \cdot \Delta TQinf=m˙⋅cp⋅ΔT
ACHACHACH: Air change rate (1/h)
VVV: Room volume (m³)
ρ\rhoρ: Air density (kg/m³, approx. 1.2)
cpc_pcp: Specific heat of air (≈1.005 kJ/kg·K)
Product Cooling Load:
Qprod=m⋅cpprod⋅ΔT3600Q_{prod} = \frac{m \cdot c_p^{prod} \cdot \Delta T}{3600}Qprod=3600m⋅cpprod⋅ΔT
mmm: Product mass (kg)
cpprodc_p^{prod}cpprod: Specific heat of product (kJ/kg·K)
ΔT\Delta TΔT: Temperature drop (K)
Lighting and Equipment:
Simply add the rated power in kW.
Occupancy (People):
Sensible heat per person ≈ 70–100 W.
Total Load with Safety Margin:
Qtotal=∑QiQ_{total} = \sum Q_iQtotal=∑Qi Qdesign=Qtotal⋅(1+margin)Q_{design} = Q_{total} \cdot (1 + \text{margin})Qdesign=Qtotal⋅(1+margin)
Margin typically 5–15%.
3. Step-by-Step Example
Design conditions (example):
Room dimensions: 6.0 × 4.0 × 3.0 m → Volume = 72 m³
Inside temp: 2°C
Outside temp: 30°C
U-values: Walls 0.5, Roof 0.3, Floor 0.4 W/m²K
Air changes: 0.5 ACH
Product: 200 kg, cooled from 20°C to 2°C, cp=3.5c_p = 3.5cp=3.5 kJ/kg·K
Lighting: 200 W, Equipment: 500 W
Occupancy: 2 persons × 70 W sensible heat
Safety margin: 10%
Calculations:
Areas:
Walls: 2(L+W)H=60 m22(L+W)H = 60\ m²2(L+W)H=60 m2
Roof = Floor = 6×4=24 m26 × 4 = 24\ m²6×4=24 m2
ΔT:
ΔT=30−2=28 K\Delta T = 30 - 2 = 28\ KΔT=30−2=28 K
Conduction Loads:
Walls: 0.5×60×28=840W=0.840kW0.5 × 60 × 28 = 840 W = 0.840 kW0.5×60×28=840W=0.840kW
Roof: 0.3×24×28=201.6W=0.202kW0.3 × 24 × 28 = 201.6 W = 0.202 kW0.3×24×28=201.6W=0.202kW
Floor: 0.4×24×28=268.8W=0.269kW0.4 × 24 × 28 = 268.8 W = 0.269 kW0.4×24×28=268.8W=0.269kW
Infiltration:
m˙=0.5×72/3600×1.2=0.012 kg/s\dot m = 0.5 × 72/3600 × 1.2 = 0.012\ kg/sm˙=0.5×72/3600×1.2=0.012 kg/s
Qinf=0.012×1.005×28≈0.338 kWQ_{inf} = 0.012 × 1.005 × 28 ≈ 0.338\ kWQinf=0.012×1.005×28≈0.338 kW
Product Load:
Qprod=200×3.5×183600=3.5 kWQ_{prod} = \frac{200 × 3.5 × 18}{3600} = 3.5\ kWQprod=3600200×3.5×18=3.5 kW
Lighting & Equipment:
Lighting: 0.200 kW
Equipment: 0.500 kW
Occupancy:
2×70W=0.140kW2 × 70 W = 0.140 kW2×70W=0.140kW
Total Before Margin:
Qtotal=0.840+0.202+0.269+0.338+3.5+0.200+0.500+0.140=5.989 kWQ_{total} = 0.840 + 0.202 + 0.269 + 0.338 + 3.5 + 0.200 + 0.500 + 0.140 = 5.989\ kWQtotal=0.840+0.202+0.269+0.338+3.5+0.200+0.500+0.140=5.989 kW
With Margin (10%):
Qdesign=5.989×1.10=6.59 kWQ_{design} = 5.989 × 1.10 = 6.59\ kWQdesign=5.989×1.10=6.59 kW
Result: Required cooling capacity ≈ 6.6 kW.
4. Practical Tips
Always use consistent SI units and convert W to kW for reporting.
Select realistic U-values from construction materials.
Consider product-specific heat capacities (fruits, meat, seafood differ).
Add allowances for door openings, defrosting, and peak operation.
Use software tools for large-scale or multi-room projects.
Verify with field measurements after commissioning and adjust if needed.
Conclusion
Heat load calculation is the cornerstone of refrigeration design. By systematically considering conduction, infiltration, product cooling, internal loads, and adding a proper safety margin, engineers can size refrigeration equipment accurately. A well-calculated load ensures both energy efficiency and reliable performance for cold rooms, supermarkets, food processing, and industrial refrigeration applications.
