Refrigeration Load Calculator

How to Use the Refrigeration Load Calculator

How a walk-in cooler or freezer load adds up

The load on a walk-in is a sum of heat paths, and each one has its own cause and its own fix. Some are fixed once the box is built, such as the panels. Others depend on how the room is used, such as the door and the product. The calculator keeps them separate so you can see which one drives the number and which one you can still change. All the results are an average over 24 hours, then converted to the capacity the compressor must deliver while it runs.

Heat through the walls, ceiling and floor

A panel's U-value is one divided by the total resistance. The calculator adds 0.15 m²K/W for the inside and outside air films to the panel's own resistance, which is its thickness divided by the insulation's conductivity (0.024 W/mK for polyurethane foam, 0.030 for XPS and 0.037 for EPS). A 100 mm polyurethane panel works out to 0.232 W/m²K, and a 150 mm panel to 0.156. In the cooler example that thicker panel cuts the capacity needed from 1,752 W to 1,578 W. The U-value calculator does the same layer-by-layer sum for a wall with several materials. Walls and ceiling see the full outside-to-room difference, 30 K for the cooler and 50 K for the freezer. The floor sees the ground temperature instead, so the freezer slab loses 112 W even though the ground is 10°C.

Door infiltration and the open-time factor

The calculator uses the Gosney and Olama equation, the form given in the ASHRAE refrigeration chapter. It takes the door area, the enthalpy difference between outside and room air, the room air density, a density-ratio factor and the door height, and it gives the heat flow while the door stands fully open. That figure is multiplied by the fraction of the hour the door is open, then by a doorway flow factor of 1.1 below 11 K of temperature difference and 0.8 above it, and last by one minus the effectiveness of any strip curtain. Elsayed's 1998 study of infiltration in cold rooms tested the equation against a model of flow in a confined room and found it overpredicts steady-state infiltration, so treat the result as a cautious figure. In the cooler example the open door admits 49.6 kW, the door is open 8.3% of the time and the curtain passes 496 W, or 42% of the load. Raise the openings to 25 an hour and the capacity needed climbs to 2,298 W.

Product load above and below freezing

Product heat is the mass received each day times the heat that must come out of it. Above freezing that is a specific heat times the temperature drop. A product that freezes in the room also gives up latent heat, then cools further as a solid. The calculator uses Siebel's relations, which estimate specific heat from the water fraction: 0.837 + 3.349 times that fraction above freezing, 0.837 + 1.256 times it below, and 334 times it for the latent heat in kJ/kg. Fresh meat is taken as 70% water freezing at -1.7°C. In the freezer example, 200 kg of meat arriving at 5°C and leaving at -20°C takes 15.9 kWh a day, which is 663 W averaged over the day and 26% of the freezer's load. The cooler example cools only 120 kg of produce from 12°C, so product is a small part of that load. If frozen goods arrive already at room temperature, leave the quantity at zero or enter their real entry temperature.

Why the moisture in the air matters

Outside air at 32°C and 50% relative humidity carries 70.5 kJ/kg of heat, against 11.8 kJ/kg for room air at 2°C and 90%. Most of that difference is water vapour, so a humid day makes the door much worse than a dry day at the same temperature. The page works out enthalpy and density from the Hyland-Wexler saturation pressure, the same formulation ASHRAE uses, at sea-level pressure. For a closer look at one air state, the enthalpy calculator shows how temperature and humidity set the heat content. Use the summer design condition for your site and a room humidity of 85 to 95% for a cooler.

People, lights, fans and defrost heaters

A person working in the room adds 272 minus 6 times the room temperature in °C, in watts, which is 260 W at 2°C and 392 W at -20°C, since people give off more heat when the room is colder. It is multiplied by the hours spent inside and divided by 24. Lighting is its wattage times its hours, and evaporator fan motors are counted as running all day because they run whether or not the compressor does. Electric defrost heaters add their wattage times the minutes a day they run, which is 50 W for the 1,200 W heater run for an hour in the freezer example. Defrost heat is counted in full as a load here, although only a part of it reaches the room air in practice, so the figure leans high.

From daily load to equipment capacity

The safety factor is a percentage of the subtotal. Emerson's refrigeration load guide says 5 to 10% is often added, and it also selects compressors for 16 to 18 hours of running a day on coolers and 18 hours on low-temperature systems with normal defrost, which is a second margin. The calculator divides the 24-hour load by the run hours and multiplies by 24. That gives 1,752 W (5,978 BTU/h, 0.50 tons) for the cooler at 16 hours and 3,397 W (11,589 BTU/h, 0.97 tons) for the freezer at 18 hours. These numbers are the capacity needed at the design evaporating temperature, not the nameplate of a unit. Match them against the published capacity at that suction temperature and the condensing conditions of your site. A strip curtain matters most in the cooler, where without it the capacity needed would be 6,393 W, but the effect only holds while the strips are whole and hang to the floor.