| Load Component | Watts | BTU/h | % of Total |
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How to Use the Freezer Load Calculator
Length, width and height set the panel area. Enter the freezer setpoint, the summer ambient temperature and its relative humidity, and the floor or ground temperature. The panel U-value is in W/m²K, and a lower number means better insulation.
Pick the product, then enter the mass, the entry temperature and the water content. Water content drives the latent load, and the specific heat values fill in from it. Product that arrives already frozen adds no latent load.
Walk-in mode spreads the daily product energy over 24 hours and scales the equipment duty by the compressor run hours. Blast mode removes each batch over the freeze time you enter and sizes the equipment for that period.
Enter door openings per hour and the open time of each, the fan and lighting power, the defrost heater and its schedule, and a safety factor. The results give the load by component in watts and BTU/h, the equipment duty and an indicative annual energy cost.
Freezer load calculation: what makes it different
A freezer load differs from a cooler load because product can cross its freezing point inside the room. That crossing releases the latent heat of fusion of the water in the product, 334 kJ per kilogram of water, and it usually outweighs the sensible cooling on either side of it. The calculator also separates two kinds of freezer: a walk-in that takes daily intake and holds it, and a blast freezer that freezes a batch against the clock.
Product freezing load
Product energy has three parts: cooling from the entry temperature to the freezing point, freezing the water, and cooling the frozen product to the setpoint. For 300 kg of poultry a day at 73% water, entering at 4°C and held at -18°C, the product energy is 87,817 kJ a day. Latent heat is 73,146 kJ of that, and averaged over 24 hours the product load is 1,016 W. The calculator counts latent heat only when the product arrives above its freezing point, because frozen product entering at -15°C has no water left to freeze. It assumes all the water freezes, which slightly overstates the latent load, because some water stays unfrozen at storage temperatures.
Blast freezers and batch freezing time
A blast freezer removes a whole batch's energy in hours, so averaging it over 24 hours would understate the equipment needed while the batch is freezing. In blast mode the product load is the batch energy divided by the freeze time you enter, and the equipment duty is the total during that period with no run-hour correction. For 1,500 kg of poultry over 12 hours the product load is 11,197 W, and the total design load is 19,044 W. Freeze time depends on product thickness, packaging and air velocity, so enter the figure from your process and use the pull-down load calculator to check a specific batch.
Panel insulation and transmission
Transmission is U times area times temperature difference, summed over the walls, ceiling and floor. In the walk-in example the panels are U 0.18 W/m²K, which is R-32, the minimum wall, ceiling and door insulation that the US DOE rule for walk-in freezers sets, with R-28 for the floor. That gives 592 W through the envelope. U is the insulation conductivity divided by its thickness, so at a conductivity of 0.023 W/m·K, 100 mm of polyurethane gives 0.23 W/m²K and 150 mm gives 0.15 W/m²K. Use the manufacturer's panel data for the real value.
Door infiltration
Each time a freezer door opens, warm moist air flows in at the top and cold air spills out at the bottom. The calculator uses the Gosney and Olama model, which ASHRAE's research project 1915 describes as the recognised method for infiltration loads. The open-door load is 0.221 times the door area, the enthalpy difference, the cold-air density, a density term, the square root of g times door height and a density factor, with a doorway flow factor of 0.8. In the walk-in example it is 70.7 kW while the door stands open. Five openings an hour of 20 seconds keep it open 2.8% of the time, so the average infiltration load is 1,963 W. Strip curtains, air curtains and vestibules cut it, and the load scales with open time, which is why it is an input.
Floors and ground temperature
The floor of a slab-on-grade freezer faces the ground and not the outdoor air, so the calculator takes its own temperature, which defaults to 10°C. A floor sitting on warm ground in summer has a smaller temperature difference than the walls, and a raised floor over outdoor air should use the ambient value instead. Freezer floors also need protection against ground freezing, usually a heated or ventilated sub-slab. The U-value calculator converts a floor or wall build-up into the U-value the calculator needs.
Defrost heat and frost load
The calculator adds all the defrost heater's energy as load, daily heater watt-hours divided by 24. That is conservative, because some of the heat leaves with the melt water, but it is the usual simple basis. In the walk-in example, three 30-minute cycles at 1,500 W add 94 W. The moisture in infiltrating air becomes the frost the heater has to melt, and the humidity ratio calculator shows how much water a given ambient condition brings in. Use the defrost calculator to check that the schedule suits the frost load.
Frequently Asked Questions
Transmission scales directly with the panel U-value and the temperature difference, so at 30°C ambient and -35°C inside, every square metre of panel loses U times 65 W. A panel at U 0.18 W/m²K, equal to R-32 and the US DOE minimum for walk-in freezer walls, loses about 11.7 W per square metre. A panel at U 0.12 loses about 7.8 W, so thicker panels cut transmission in proportion, and the saving is larger the more hours the room runs. The U-value comes from the panel thickness and its insulation conductivity, so confirm it from the manufacturer's data. Freezer floors also need protection against ground freezing, usually a heated or ventilated sub-slab, so check the local building code and the panel supplier's floor details.
Continuous running without reaching setpoint means the load is above the system's capacity at that moment. The usual causes are more product than the system was sized for, door gaskets or strip curtains that leak, a frozen defrost drain, a coil blocked with frost, and a refrigerant charge that has leaked away over months. Check the charge and the suction superheat first, because an undercharged system loses capacity, then look at the coil for frost bridging between the fins. Finally confirm that the defrost termination ends the defrost before the coil refreezes. If the checks pass, compare the actual door openings and product intake with the figures in this calculator, since infiltration and product load are the two that drift most.
A storage freezer mostly holds product that is already frozen, so the work is conduction through the panels and door infiltration. A blast freezer takes fresh product from above freezing to below it, which means removing the sensible heat, the latent heat of fusion and the sensible heat below freezing, and it does that over a freeze time of hours and not spread across the day. In the blast example, 1,500 kg of poultry removed over 12 hours is 11,197 W, which is 59% of the total design load. The same product spread over 24 hours would be half that, so the freeze time you enter changes the equipment size directly.
Above its freezing point a food holds most of its water as liquid, which has a specific heat of about 4.18 kJ/kg·K. Below it, much of that water is ice at about 2.1 kJ/kg·K, so each degree of cooling removes less energy. The calculator estimates both values from water content with the Siebel equations, for example 3.28 and 1.75 kJ/kg·K at 73% water, and you can overwrite them with measured values. Using one average value across the whole range misses the latent heat of fusion and the change in specific heat, and it understates the load. The calculator therefore treats the three steps separately: cooling to the freezing point, freezing, and cooling to the setpoint.