1) Identify Design Conditions
Room setpoint (Tr): e.g., −18 °C (frozen), 0…+5 °C (chilled).
Ambient conditions: design dry-bulb (and wet-bulb for water-cooled/evaporative).
Operating pattern: pull-down needs vs. steady storage, door usage, shifts.
Refrigerant & system type: influences Te/Tc selection and efficiency.
Typical temperature approach:
Evaporating temperature (Te) ≈ room setpoint minus 6…10 K (freezers may use 8…12 K).
Condensing temperature (Tc) ≈ ambient + 10…15 K (air-cooled) or approach to cooling water (water-cooled/evaporative).
2) Calculate the Heat Load (Qtotal)
Total load is the sum of:
A. Transmission (through walls/ceiling/floor)
Qtrans=∑(U×A×ΔT)Q_{\text{trans}}=\sum (U \times A \times \Delta T)Qtrans=∑(U×A×ΔT)
U: overall heat transfer coefficient (W·m⁻²·K⁻¹) from construction.
A: area (m²).
ΔT: ambient (or ground) minus room temperature (K).
B. Infiltration/Ventilation
Air exchange via door openings or intentional ventilation. Estimate via ACH (air changes per hour), doorway traffic, or use manufacturer/industry guides. Freezers often need air curtains or PVC strip curtains to limit this load.
C. Internal Gains
People (~75–100 W sensible/person in cold gear), lighting (W/m²), fan motors, defrost heat (add as average), forklifts or other equipment.
D. Product Load
Sensible: cooling product from entry temperature to target.
Latent: freezing water content and sub-cooling below freezing point (for frozen goods).
Respiration: for fresh produce (heat released by respiration).
Use daily throughput, specific heat, freezing point, and latent heat values to compute average kW over the duty cycle.
E. Safety & Miscellaneous
Add 10–20% to cover uncertainties, coil frost, aging, and fouling-especially for hot climates and heavy door traffic.
3) Convert to Required Refrigeration Capacity
Sum all components to get Qtotal (kW) at design.
Choose Te and Tc as above; consult compressor/condensing-unit catalogs for capacity at (Te, Tc) (capacity changes significantly with Te/Tc!).
Consider run hours per day. If the system should meet the average daily load with ≤18 h runtime, divide the daily load (kWh/day) by 18 h to set the average kW needed; ensure peak loads can be handled (pull-down periods).
Apply your safety margin; select the next standard model.
4) Worked Example (Frozen Room)
Room: 6 m × 5 m × 3.5 m → 105 m³ at −18 °C.
Ambient: 35 °C (hot climate), air-cooled condenser.
Assumptions: U-walls/ceiling = 0.25, U-floor = 0.30 W·m⁻²·K⁻¹; ground 20 °C; moderate door use with strip curtain; one person occasional; lighting 5 W/m²; fans 0.3 kW; defrost averaged in internal gains; no heavy daily pull-down (storage mode).
A. Transmission
Walls: area 77 m² → 0.25×77×(35−(−18)) ≈ 1.02 kW
Ceiling: 30 m² → 0.25×30×53 ≈ 0.40 kW
Floor: 30 m² → 0.30×30×(20−(−18)) ≈ 0.34 kW
Qtrans ≈ 1.76 kW
B. Infiltration (strip curtain, moderate traffic): ~0.8 kW (rule-of-thumb 7–9 W/m³ for freezers with managed doors)
C. Internal Gains
People 0.08 kW (intermittent), lighting 0.15 kW, fans 0.30 kW, defrost average 0.15 kW → ~0.68 kW
Subtotal: 1.76 + 0.80 + 0.68 ≈ 3.24 kW
Add 15% safety → ~3.7 kW required at design Te/Tc.
Select Te/Tc:
Room −18 °C → choose Te ≈ −26 °C (8 K TD).
Ambient 35 °C → Tc ≈ 48–50 °C (13–15 K lift above ambient).
Now pick a condensing unit whose rated capacity at Te = −26 °C and Tc ≈ +50 °C is ≥ 3.7 kW. If catalog ratings are given at other points (e.g., Te = −25 °C, Tc = +45 °C), apply manufacturer correction factors or select the next larger size. If frequent door openings or pull-down of warm product is expected, upsize one model or add hot-gas bypass/variable capacity to manage cycling.
(Rule of thumb mapping for quick quoting: 1 TR = 3.517 kW. The above load ≈ 1.05 TR.)
5) Practical Tips
Doors: Use air curtains/PVC strips; consider vestibules for large traffic.
Coils: Choose proper TD and fin spacing (freezers often 6–8 mm to reduce frost).
Defrost strategy: Electric vs. hot-gas; account for average heat return.
Condenser choice: In hot regions, consider larger air-cooled coils or water-cooled/evaporative options to reduce Tc and save energy.
Documentation: List all assumptions (U-values, door pattern, Te/Tc). It speeds approval and reduces disputes.
6) Deliverables for Clients (what to include in your offer)
Calculated Qtotal and design Te/Tc.
Selected condensing unit model, refrigerant, capacity at Te/Tc, COP/EER.
Evaporator model (TD, air flow), defrost method.
Electrical data, sound level, and options (controllers, VFD fans, oil separator, coatings).
