Walk-in cooler and freezer BTU calculator

A refrigeration load is all the heat that gets into the box in a day. The ASHRAE Refrigeration Handbook splits it into a few sources, and this calculator works out each one:

• Transmission: heat through the walls, ceiling and floor panels.
• Infiltration: warm, moist room air that rolls in each time the door opens.
• Product: heat taken out of what you put in, and the heat to freeze it in a freezer.
• Internal: people working inside, lights, and the evaporator fan motors.

Add them for 24 hours, add a safety factor, and divide by the hours the system is allowed to run. That last step is the one people miss. A cooler's coil frosts and has to defrost, so the system cannot run around the clock. Doing a day of work in 16 hours means the equipment has to be 50% bigger than the hourly average.

Required Btu/h = (daily load × (1 + safety factor)) ÷ running hours

Many quick calculators multiply box volume by a temperature difference and stop there. That ignores the door, the product and the people, which on a busy restaurant cooler are most of the load.

Heat flows through each panel at a rate set by its insulation and the temperature difference across it:

Btu per day = (1 ÷ R) × panel area × (room temperature − box temperature) × 24

Federal rules (10 CFR 431.306) have required walk-in panels of at least R-25 for coolers and R-32 for freezers since 2009, with R-28 freezer floors. Those are the calculator's starting values.

A box outside in the sun, or against a hot kitchen line, sees a bigger difference. Use the hottest temperature the room around the box reaches on a busy summer day.

A cooler set on a concrete slab without floor panels also gains heat from the slab. This calculator does not count that, so add floor panels or allow extra. Freezers need insulated floors, or the ground under them freezes and heaves.

When the door opens, cold heavy air spills out along the floor and warm room air flows in over the top. ASHRAE's door equation (Gosney and Olama) works out that flow from the door size and the difference in air density and heat content between the room and the box. The calculator then scales it by:

• How long the door is open: openings per day × seconds each, as a share of 24 hours.
• A doorway flow factor of 0.8, because traffic in the doorway blocks some of the flow.
• A strip curtain, if there is one, which ASHRAE puts at 85% to 95% effective when new. The calculator uses 85%.

On the sample cooler (3 × 7 ft door, 60 openings of 15 seconds a day, 85°F and 50% humidity outside), the door is open about 1% of the day and still brings in 27,068 Btu a day, more than the panels.

Moisture is a big part of door air. Humid air carries far more heat per pound than dry air at the same temperature, and the moisture ends up as frost on the coil. That is why a humid back room or a box opening to the outdoors needs more capacity and more defrost.

Everything that goes in warm has to be cooled. The heat to remove depends on the weight, how much the temperature drops, and the product's specific heat. Foods are mostly water, so the ASHRAE Handbook uses Siebel's equations, based on water content:

• Specific heat above freezing: 0.8 × water fraction + 0.2 Btu per lb per °F
• Specific heat below freezing: 0.3 × water fraction + 0.2
• Heat to freeze: 143.4 × water fraction Btu per lb

Cooling 500 lb of meat a day from 45°F to 35°F takes 500 × 0.76 × 10 = 3,800 Btu. Freezing is a different story: taking 100 lb of meat from 35°F to -10°F takes 0.76 × 7 + 100.4 + 0.41 × 38 = about 121 Btu per lb, or 12,128 Btu. Most of that is the freezing itself. A freezer that receives fresh product is doing the work of a blast freezer and needs to be sized for it, or the product should be frozen elsewhere first.

If product has to come down faster than 24 hours, set the pull-down time and the calculator scales the load up to match.

The same box at -10°F needs about 1.6 times the refrigeration of the same box at 35°F before any product goes in, and far more once fresh product has to be frozen. Here is why, using the calculator's sample box (10 × 8 × 8 ft, 85°F room):

• Panels. The difference across the walls goes from 50°F to 95°F. Thicker R-32 panels claw some of it back, but transmission still rises by about half.
• Door air. Colder box air is denser, so it spills out the door faster and pulls more warm air in, and each pound of room air has more heat to give up. On the sample box the door load more than doubles.
• Product. Anything that comes in unfrozen has to give up its latent heat to freeze, about 100 Btu per lb for meat. That one step is far more than cooling it 10°F.
• People. They give off more heat in the cold: about 412 W each at -10°F against 262 W at 35°F.
• Defrost. The freezer cannot rest its coil to defrost, so it runs fewer hours, usually figured at 18, and the heat from defrost heaters has to be removed afterward.

Freezer equipment also loses capacity faster as the box gets colder, because the compressor is pumping thinner, colder vapor. Always read the condensing unit's table at the box temperature you are designing for, never a nominal horsepower.

People give off more heat in a colder box, because the body works harder to stay warm. ASHRAE gives it as 272 − 6 × the box temperature in °C, in watts: about 262 W per person at 35°F and 412 W at -10°F.

Lights and evaporator fan motors turn all their power into heat inside the box: watts × 3.412 × hours. The fans run all day, so they add up. On the sample cooler, 150 W of fans adds 12,283 Btu a day, 16% of the load. Use the nameplate watts from the evaporator you plan to install.

Safety factor. A 10% allowance covers what a calculation cannot see: a door left open, a hot delivery, panels that have aged.

Running hours. Coolers at 35°F and above usually defrost by simply letting the compressor rest, so 16 hours of running is common. Freezers need heaters or hot gas to defrost and are usually figured at 18 hours.

74,889 Btu ÷ 16 hours = 4,681 Btu/h. Pick a condensing unit and evaporator rated for at least that at your box temperature and the outdoor temperature the condenser sees, from the maker's capacity tables. Ratings drop as the condenser air gets hotter, so a rooftop unit in Phoenix needs more nameplate than the same box in Seattle.

Then quote it from Koira with the condensing unit, evaporator, line set, controls and the strip curtain option on separate lines.