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Transmission: for each surface U = 1/(Rsi + d/λ + Rse) with the surface resistances of EN ISO 6946 (walls 0.13 + 0.04, ceiling 0.17 + 0.04, floor on the ground 0.17 m²K/W) and Q = U·A·ΔT to the temperature on the other side. The door leaf is counted in the wall it sits in. Thermal bridges and sun are not included: use the safety factor or a higher outside temperature.
Air change through the door: the Gosney–Olama equation as given in ASHRAE Handbook—Refrigeration gives the heat flow through the fully open door from the enthalpy and density of the air on both sides; it is multiplied by the share of the day the door is open, the door flow factor (about 0.8) and (1 − effectiveness) of a strip or air curtain. Moist air properties from ASHRAE Handbook—Fundamentals ch. 1.
Products: Siebel's equations from the water content give the specific heat above and below freezing and the latent heat (ASHRAE Refrigeration ch. 9); the whole latent heat is taken at the initial freezing point. The heat is spread over the pull-down time. Fruit and vegetables add their respiration heat for the stored mass.
Internal loads: people 272 − 6·t W each for the hours they are inside (ASHRAE Refrigeration), lighting and other equipment with their hours, evaporator fans whenever the room is not defrosting, and the share of the defrost heat that stays in the room.
Design capacity = sum of the mean loads × (1 + safety factor) × 24 / running hours. Evaporating temperature = room air − ΔT, where ΔT follows the humidity wanted (about 4–5 K at 95 % RH up to 10 K at 75 %). Estimate only: the final load and the equipment are chosen by a qualified designer.
Add up the heat that enters per 24 hours: through walls, ceiling and floor, with the warm air when the door opens, from the products that are cooled or frozen, and from people, lights, fans and defrost. Divide by the hours the plant may run (often 16–18 h) and add a safety factor of about 10 %.
Coolers above about +2 °C with off-cycle defrost are often designed for 16 hours a day, freezer rooms with electric or hot-gas defrost for 18–20 hours. The rest of the day is margin for defrost, door openings and hot days.
Typical panels are 80–100 mm of polyurethane for coolers and 120–200 mm for freezer rooms. In the USA and Canada walk-ins must have at least R-25 (cooler) and R-32 (freezer) panels and an R-28 freezer floor. Thicker insulation also keeps the outside surface above the dew point.
Often the largest single load in a busy room. It depends on the door size, the temperature and humidity difference and how long the door is open. A strip curtain stops about 85 % of the air exchange, an air curtain about 70 %, as long as they are kept in good condition.
The smaller the difference between room air and evaporating temperature, the higher the humidity and the less the products dry out: about 4–5 K for fresh vegetables (95 % RH), 6–8 K for meat and general cold rooms, 8–10 K for packaged goods and freezer rooms.
No country prescribes a method for the refrigeration load. The app uses the heat-load components of ASHRAE Handbook—Refrigeration. It is an estimate; the final load, equipment and safety design (EN 378, ISO 5149, ASHRAE 15) are done by a qualified designer.
