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How to Use the Pull-Down Load Calculator
Type the room length, width and height, the starting room temperature, the target setpoint and the outside temperature. The setpoint must be colder than the start.
Enter the mass of shelving, racking and the evaporator, then the product mass, its starting temperature and its type. Choose Custom to set your own water content and freezing point.
Enter the rated capacity at design evaporating temperature, the capacity loss at setpoint, and the heat from evaporator fans and lights. Add a target time to get the capacity needed.
Check the estimated pull-down time, the capacity needed for your target, and whether the system can still hold setpoint after its capacity has dropped.
How pull-down time is estimated
A pull-down estimate answers two questions: how long a given system takes to bring a room and its contents to setpoint, and how much capacity is needed to reach a required time. The calculator adds up the heat stored in the room air, the shelving and racking, and the product. It then divides that energy by the net capacity the system delivers on average, which is the rated capacity less the average capacity loss, the transmission gain and the internal heat from fans and lights.
Pull-down load is not the steady-state load
Most of the work in the first hours goes into removing stored heat from mass, and the compressor is limited by its own capacity, not by the building. Transmission gain behaves the other way. It grows as the room gets colder, because the temperature difference to the outside gets larger. The calculator takes transmission at the mean of the start and end room temperatures, then checks the load at setpoint against the capacity left at the end of pull-down. For the full steady-state load, including infiltration, door openings and product respiration, use the refrigeration load calculator.
Freezing product and the latent heat plateau
When the setpoint is below the product freezing point, the water in the product has to freeze, and that takes about 334 kJ for every kilogram of water. In the blast freezer example, latent heat is 70% of all the energy removed, and the product holds near its freezing point while the coil works at full capacity. To see how large a phase change is next to a temperature change for the same mass, run the numbers through the latent heat calculator. The pull-down tool assumes all the water freezes at the freezing point, which slightly overstates the latent load for products that freeze gradually.
Thermal mass: what actually has to be cooled
Room air has very little thermal mass. The 72 m³ blast freezer holds 86 kg of air, which is 87 kJ/K, so cooling the air is a rounding error next to 500 kg of meat. Structure and product carry the load. The product estimate uses specific heats scaled from the water content of the product, about 0.837 + 3.349 times the water fraction above freezing and 0.837 + 1.256 times the water fraction when frozen, in kJ/kg·K. Water data underlying those figures is published in the NIST Chemistry WebBook entry for water. The structure field uses 0.5 kJ/kg·K, the value for steel. Concrete is closer to 0.9, so for a slab or masonry walls enter about 1.8 times the real mass.
Panel insulation and transmission gain
The U-value field sets the transmission gain. A polyurethane panel has a conductivity near 0.025 W/m·K, so a 100 mm panel works out to about 0.25 W/m²K and a 150 mm panel to about 0.17. Joints, doors and thermal bridges raise the whole-wall value. To build a U-value from the layers of a wall, floor or ceiling, use the U-value calculator. The tool applies the same U-value and the same outside temperature to every surface, so treat the floor as a conservative simplification when it sits on warmer ground.
Pull-down records and food safety
A logged pull-down gives you a baseline for the room: the start time, the time to reach setpoint and the product temperature at the end. The Canadian Food Inspection Agency explains preventive controls in its handbook for food businesses under the Safe Food for Canadians Regulations. The handbook does not set cooling or freezing times, so the time and temperature limits for a given product come from your own preventive control plan.
What this estimate leaves out
The result is a capacity-limited time. It does not model how fast heat leaves the product, which depends on case or pallet size, packaging and air velocity, and it does not include defrost cycles, door openings, infiltration or the heat capacity of panels and floors. Measured pull-downs run longer than the calculated time, so use the result to compare equipment and test the target, then confirm the coil selection with the manufacturer.
Frequently Asked Questions
It depends on the stored heat in the room and product and on the net capacity. The Example: Walk-in Cooler button uses a 5 m by 3.5 m by 2.4 m room with 150 kg of shelving and 300 kg of beverages at 20°C, cooled from 25°C to 2°C by a 3.5 kW unit with a 20% capacity loss at setpoint and 0.3 kW of fan heat. The calculator removes 24,591 kJ (6.83 kWh) in 2h 42m. That is a capacity-limited minimum, because product inside cases lags the air. An empty room is governed by the mass of the panels, floor and shelving, so load product after the room reaches temperature.
Add up the heat to remove, divide by the target time, add transmission and internal heat, then divide by the average capacity factor. The Example: Blast Freezer button uses a 6 m by 4 m by 3 m room, 500 kg of meat cooled from 4°C to -35°C, 300 kg of racking, 3 kW of fan heat and a 35% capacity loss at setpoint. It removes 173,725 kJ (48.26 kWh), and 70% of that is latent heat. An 8-hour pull-down needs 11.78 kW of rated capacity. The 20 kW unit in the example reaches setpoint in 3h 46m on paper. Real freezing time also depends on air velocity and product thickness, so confirm the coil with the equipment supplier.
Capacity falls as the evaporating temperature falls, and a freezing product holds near its freezing point until the latent heat is removed. The calculator models the capacity loss as a straight line from the rated value to the value at setpoint, so the average loss is half the loss you enter. In the blast freezer example, a 35% loss at setpoint is a 17.5% average loss, which turns 20 kW into 16.5 kW on average. Use the manufacturer capacity table at your design evaporating temperature to set the derate.
Yes. Transmission gain grows as the room gets colder, and capacity is lowest at setpoint. The calculator compares the load at setpoint with the capacity left at the end of pull-down. In the blast freezer example the load is 4.19 kW against 13 kW of capacity, a hold margin of 8.81 kW. If the margin is negative the system cannot hold setpoint even with an unlimited pull-down time.