Walk-In Cooler Heat Load
Box load = transmission (U x area x deltaT) + infiltration + product + internal, times a safety factor. U 0.05, 800 ft^2, 60 F, +3000/5000/1500, 1.10 -> 2,400 transmission, 13,090 Btu/hr (1.09 tons); a 6 in panel (U 0.03) cuts it to 12,034. Size the evaporator for an ~18 hr run. The box-load method and equipment ratings govern.
Formula and source
transmission = U x area x deltaT; total = (transmission + infiltration + product + internal) x safety; tons = total / 12000.
The walk-in cooler/freezer box heat-load method (transmission + infiltration + product + internal, with a safety factor), from ASHRAE Refrigeration, by name.
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