Walk-In Door Infiltration Load and Frost

ScopeCold dense air pours out along the floor while warm humid air flows in above, and the exchange runs continuously for as long as the door stands open. THE DOOR OPEN FACTOR IS JUST THE FRACTION OF THE HOUR THE DOOR IS OPEN, and it is where a busy kitchen diverges from a design assumption -- sixty openings an hour at twenty seconds each is a third of the hour, and a door propped open during a delivery is a hundred percent of it. On a box of ordinary size the door load at that duty commonly rivals or exceeds the product and transmission loads combined, which is why it is worth calculating rather than assuming. A LARGE SHARE OF THE FREEZER DOOR LOAD IS LATENT -- commonly a third to a half of it by enthalpy, and the share is computed here rather than asserted, because it depends entirely on the two conditions -- and that share is the part with consequences beyond the compressor. The moisture carried in with the warm air does not simply add heat: it condenses and freezes on the evaporator coil, which blocks airflow, which raises the defrost frequency, and every defrost cycle puts heat back into the box that the compressor then has to remove again. The frost accumulation is reported here in pounds per day for that reason -- it is the number that explains a coil that ices between defrosts and a box that cannot hold temperature on a hot day. DOORWAY PROTECTION IS THE CHEAPEST INTERVENTION ON THE LIST. New strip curtains pass about five percent of the unprotected load and worn ones fifteen to twenty (ASHRAE), an air curtain thirty or more, so the avoided load is most of it -- and cutting infiltration by that much cuts the frost accumulation by roughly the same, which lengthens the interval between defrosts and compounds the saving. The word new is doing work: strips that are torn, missing, curled, or hung too short pass far more than even the worn figure, and a curtain nobody has looked at in two years is not the curtain in the calculation. A steady-state estimate from a doorway airflow the reader takes from the ASHRAE doorway flow relations or the manufacturer's data for the door size and temperature difference, with the enthalpy and moisture differences read off a psychrometric chart at the two conditions. It does not compute that doorway airflow, which depends on the door height, the density ratio, and the flow regime; it does not model the transient at each opening, the time the air curtain takes to re-establish, or the mixing that a heavily trafficked doorway produces. It does not size the refrigeration system, which needs the transmission, product, lighting, motor, personnel and defrost loads alongside this one, and it does not evaluate the defrost strategy, the coil, or the anti-sweat and floor heat that a freezer door needs. ASHRAE's refrigeration handbook, the box and door manufacturer's data, and the refrigeration designer govern.

Formula or tabledoor open factor = openings per hour x seconds open / 3,600; effective airflow = the fully-open doorway flow x that factor x the doorway protection factor; load = airflow x 60 x air density x the enthalpy difference; latent load = the same mass flow x the moisture difference x 1,061 Btu/lb.

Source and editionThe walk-in doorway infiltration relations by name. The fully-open doorway flow comes from the ASHRAE doorway flow relations or the manufacturer's data for the door size and temperature difference, and the enthalpy and moisture differences from a psychrometric chart at the two conditions. Protection factors (the share that still gets through) run about 0.05 for new strip curtains, rising to 0.15 to 0.2 as they wear (ASHRAE Refrigeration ch. 24: effectiveness 0.95 new, 0.8 to 0.85 in service), and 0.3 or more for an air curtain; ASHRAE also applies a doorway flow factor of 0.8 above an 11 C (20 F) difference, which the full-open airflow entered here should carry. It does not size the refrigeration system. ASHRAE's refrigeration handbook, the box and door manufacturer's data, and the refrigeration designer govern.

Free accessOne fraction and two multiplications on a doorway flow and a psychrometric difference the reader supplies; no ASHRAE table is reproduced.

What governsEstimate. AHJ and licensed professional govern.

API field namesdoor_width_ft, door_height_ft, full_open_cfm, openings_per_hour, seconds_open_each, protection_factor, enthalpy_difference_btu_lb, moisture_difference_lb_lb, air_density_lb_ft3, door_area_ft2, door_open_factor, effective_cfm, mass_flow_lb_hr, total_load_btuh, latent_load_btuh, latent_share_pct, frost_lb_day

• The doorway airflow is entered it depends on door height, density ratio and flow regimeASHRAE's refrigeration handbook
• Steady state, not per-opening transient the air curtain re-establishment and doorway mixing are not modelledthe door manufacturer's data
• Not a refrigeration load calculation transmission, product, lighting, motor, personnel and defrost loads sit alongside this onethe refrigeration designer