Walk-In Cooler Outage Holdover and Dry Ice

ScopeThe refrigeration tech is asked two things: how long until the product is out of temperature, and how much dry ice would hold it. This calculator answers both with an energy balance. The closed-door heat gain is the box transmission, U x area x (ambient - box), plus a small closed-door infiltration allowance (10% by default), or the gain entered from the Walk-In Cooler Heat Load calculator; product, people, and lighting loads stop when the box is shut and dark. The product's heat capacity between its starting temperature and the allowable limit is the reserve, and the reserve divided by the gain is the holdover. Dry ice absorbs about 246 Btu per pound as it sublimates. An 8 x 10 x 8 ft walk-in with 4 in panels (U 0.04) in a 90 degF kitchen, held at 38 degF, has 448 sq ft and a 1,025 Btu/h gain; 3,000 lb of produce (cp 0.9) at 35 degF with the FDA Food Code's 41 degF limit holds 16,200 Btu, about 15.8 h with the door shut. Cancelling the leak takes 4.2 lb of dry ice an hour, 100 lb a day, and a 72-hour outage needs 234 lb net of the product's reserve -- a number worth knowing before the storm. The USDA figure of 50 lb for two days in a full 18 cu ft freezer implies a 256 Btu/h leak, reported as a check. It does not model door openings, stratification, frozen product, or pharmaceutical storage, and whether product is safe is the health authority's call. Dry ice gives off carbon dioxide; ventilate. The FDA Food Code as adopted, the USDA guidance, and the local health department govern.

Formula or tableQ = U x A x (T_ambient - T_box) x (1 + infiltration allowance), or the entered closed-door gain; E = product lb x cp x (T_allowable - T_start); holdover = E / Q; dry ice = Q / 246 lb/h, and for an outage of T hours max(0, Q x T - E) / 246 lb; USDA check 50 x 246 / 48 = 256 Btu/h.

Source and editionAn energy balance by name, with the FDA Food Code Section 3-501.16 cold-holding limit (41 degF) and the USDA FSIS power-outage guidance (about 4 hours for an unopened refrigerator; a full freezer about 48 hours, half full about 24; 50 lb of dry ice holds a full 18 cu ft freezer about 2 days) cited as the limit and the check, and dry ice's heat of sublimation, about 246 Btu/lb, as physical data.

Free accessThe FDA Food Code is free at fda.gov and the USDA FSIS power-outage guidance is free at fsis.usda.gov; the arithmetic runs on the user's own box, product, and temperatures.

What governsEstimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.

API field namesentered_heat_gain_btuh, box_length_ft, box_width_ft, box_height_ft, panel_u_btuh_per_sqft_degf, ambient_degf, box_degf, infiltration_pct, product_lb, product_cp_btu_per_lb_degf, product_start_degf, allowable_degf, outage_hours, surface_sqft, transmission_btuh, heat_gain_btuh, reserve_btu, holdover_hours, dry_ice_lb_per_hour, dry_ice_lb_per_day, outage_dry_ice_lb, usda_check_leak_btuh

• Only the envelope leaks with the door shut product, people, and lighting loads stop in a shut, dark boxenergy balance
• Cold-holding limit 41 degFFDA Food Code Section 3-501.16
• Dry ice absorbs about 246 Btu per pound heat of sublimation of solid carbon dioxidephysical data; USDA FSIS power-outage guidance as the check