How walk-in refrigeration load is calculated
A refrigeration system must remove all the heat that enters or is produced inside the box over 24 hours, but it only runs part of the day, because coolers need off-cycle time to defrost their coils and freezers need defrost periods. The daily heat load is therefore divided by the expected run time (typically 16 hours for coolers and 18 hours for freezers) to get the hourly capacity of the condensing unit and evaporator. The calculator follows the same four-part method used in refrigeration engineering handbooks and manufacturer load-calculation guides.
1. Transmission load
Heat conducts through walls, ceiling and floor from the warmer surroundings. For each surface, heat gain = area × U-value × temperature difference × 24 h. Polyurethane walk-in panels insulate at roughly R-7 per inch, so a 4 in panel is about R-28 and a 5 in panel about R-35. Outdoor boxes get an extra temperature allowance for sun on the roof and walls. Uninsulated concrete floors are calculated against a ground temperature of about 55 °F (13 °C).
2. Air infiltration (air changes)
Every time the door opens, warm, humid air replaces cold air. The calculator uses typical air changes per 24 hours for the box volume (smaller boxes have more changes per day), multiplied by your usage level. The heat in each cubic foot of incoming air is calculated from the actual enthalpy difference between the surrounding air and the room air, including the moisture that freezes onto the coil as frost.
3. Product load
Warm product brought in each day has to be cooled to room temperature. For coolers this is daily weight × specific heat above freezing × temperature drop. For freezers the product also releases its latent heat of fusion as it freezes and is then cooled further using the specific heat below freezing. The calculator assumes the product is pulled down within 24 hours.
4. Internal loads, fans and safety
People give off about 900 BTU/h in a cooler and 1,300–1,400 BTU/h in a freezer. Lights and other equipment add 3.412 BTU/h per watt while they are on. Evaporator fan motors run inside the box and add roughly 5–10% to the load, and a safety factor of about 10% covers unknowns such as door gaskets, extra stocking days and aging equipment.
Typical design conditions
| Application | Room temperature | Typical run time | Common panel |
|---|---|---|---|
| Produce / flower cooler | 34–50 °F (1–10 °C) | 16 h | 4 in |
| Restaurant meat & dairy cooler | 33–38 °F (1–3 °C) | 16 h | 4 in |
| Beverage / beer cooler | 34–40 °F (1–4 °C) | 16 h | 4 in |
| Storage freezer | −10 to 0 °F (−23 to −18 °C) | 18 h | 4–5 in, insulated floor |
| Ice cream / low-temp freezer | −20 to −10 °F (−29 to −23 °C) | 18 h | 5–6 in, insulated floor |
From BTU/h to equipment
Condensing units and evaporators are rated in BTU/h at a specific saturated suction temperature (SST) and ambient temperature. A unit rated 10,000 BTU/h at a 25 °F SST for a cooler may deliver only about half that at the −20 °F SST a freezer needs. That is why the horsepower shown above is only a rough guide (around 9,000 BTU/h per HP for coolers and 5,500 BTU/h per HP for freezers). Always choose equipment from the manufacturer’s capacity tables at your actual room temperature, ambient temperature and refrigerant, and match the evaporator to the condensing unit.
Ways to reduce the load
- Fit strip curtains or an air curtain at the door: infiltration is often the largest single load in busy boxes.
- Keep the box out of direct sun and away from cooking equipment, or shade an outdoor box.
- Pre-cool hot product before it goes into the walk-in, and never store cooked food at full temperature.
- Use LED lighting and turn lights off with a door switch or timer.
- Check door gaskets, heaters and closers regularly; a door that doesn’t seal can double the infiltration load.