Evaporator Defrost Heat and Cycle Time

They are not the same size: melting the ice costs 144 BTU per pound against about 21 to warm that same ice from a freezer coil's temperature up to 32 F, so the latent term dominates -- but the coil warm-up term is the one that gets forgotten, and on a large coil with heavy fin stock it is real. Defrost efficiency captures everything that is NOT melting frost: heat going into the box instead of the coil, into the drain pan, and out through the insulation, and on electric defrost it is commonly only 60% to 80%. The last relation closes the loop, because frost accumulates at the coil's moisture removal rate, which is set by the box's latent load -- door openings, product respiration, infiltration. That determines how much frost is on the coil when defrost initiates, which determines how long defrost takes, so a box with heavy traffic needs more defrosts and each one is longer, and every minute of defrost is a minute of heat going into a freezer. A freezer coil at -10 F with 20 lb of frost, 60 lb of coil warmed 60 F, a 3 kW heater and 80% efficiency needs 4,575 BTU and runs 26.8 minutes, which lines up with the 20 to 30 minute terminations most controllers are set to. But halve the frost by fixing a door gasket and it falls to 14.7 minutes; double it and it climbs to 51.0. A fixed termination time is right for exactly one frost load, which is the argument for demand defrost. A sizing estimate; the manufacturer's defrost data and a measured coil temperature at termination govern.

sensible_btu = frost_lb x 0.5 x (32 - coil_temp_f); latent_btu = frost_lb x 144; coil_warmup_btu = coil_mass x specific_heat x rise; total_btu = (sensible + latent + coil warm-up) / defrost_efficiency; defrost_min = total_btu / heater_btuh x 60.

Defrost heat from the sensible, latent, and coil warm-up terms -- 144 BTU/lb to melt ice, about 0.5 BTU/lb-F to warm it to 32 F -- divided by the defrost efficiency, by name; public thermodynamics with the efficiency entered rather than bundled. The equipment manufacturer's defrost data and a measured coil temperature at termination govern.

The latent heat of fusion and the specific heat of ice are public physical constants; the frost load, coil mass, heater rating, and efficiency are the system's own values.

Estimate. AHJ and licensed professional govern.

Field names used by the API: frost_lb, coil_temp_f, coil_mass_lb, coil_specific_heat, coil_temp_rise_f, heater_btuh, defrost_efficiency, sensible_btu, latent_btu, coil_warmup_btu, total_btu, defrost_min

• Latent dominates 144 BTU/lb to melt against about 0.5 BTU/lb-F to warm the ice to 32 Fphysical constants
• Defrost efficiency commonly 60% to 80% on electric defrost; the rest is heat into the boxrefrigeration practice
• Frost load drives time a fixed termination time is correct for exactly one frost load -- the argument for demand defrostrefrigeration practice