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How to Use the Defrost Calculator
Choose electric, hot gas or off-cycle, then pick the space type and enter the coil face area. Coil mass is optional and comes from the data sheet; if you leave it blank the calculator assumes 15 kg per square metre of face area. Evaporating temperature must be below 0°C.
Door openings per day set the frost estimate. For electric defrost enter the heater wattage and drain pan heater wattage from the nameplates. For hot gas you can enter the heat rate to the coil, or leave it blank for an estimate. Heat effectiveness is the share of the heat that reaches the frost and coil.
Enter the cycles per day, the longest defrost you will allow, the termination temperature and your electricity rate. The calculator gives the frost per cycle, the heat needed, the melt time and the annual energy and cost.
If the melt time exceeds the allowed time, the results show the heater wattage needed to finish inside it. Change the method, the cycle count or the assumptions and calculate again to compare options.
Defrost design for Canadian refrigeration systems
Any coil that runs below 0°C collects frost, because the moisture in the air that reaches it freezes on the fins. A thin layer is harmless. As it thickens it insulates the coil and narrows the air passages, so the coil delivers less capacity, the fans move less air and the compressor runs longer. A defrost system clears that layer on a schedule, using as little energy and as little disturbance to the stored product as it can.
Where the frost comes from
Frost is water that came into the room as vapour. Door openings, warm product, gasket leaks and unsealed penetrations all admit air, and the moisture in that air ends up on the coldest surface, almost always the evaporator. How much arrives depends on how much water each kilogram of incoming air carries, which is why a freezer frosts faster in a humid July than in a dry January. The humidity ratio calculator shows the water content of the air entering your door for any temperature and humidity.
This calculator estimates the daily frost mass as a space factor (0.4 to 1.2 kg per square metre of coil face for the five space types) times the face area times (1 + door openings / 40). That is a screening estimate scaled to coil size, not a measurement. If a coil frosts much more or less than the result suggests, weigh the meltwater from a cycle and use the measured figure to judge the schedule.
How the heat per cycle is added up
Each cycle has four heat terms. Warming the ice from coil temperature to 0°C takes 2.1 kJ/kg·K. Melting it takes 334 kJ/kg, which is almost always the largest frost term. Warming the meltwater to the termination temperature takes 4.18 kJ/kg·K. The coil metal also has to be warmed through the same temperature rise, using 0.65 kJ/kg·K for a mix of aluminium fins and copper tube.
In the example (1.2 m² freezer coil, 18 kg, -25°C, 20 door openings, 3 cycles a day, 10°C termination) the frost is 0.48 kg per cycle and the total heat is about 615 kJ, or 0.171 kWh. The coil metal accounts for 410 kJ of that. Smaller frost loads still need most of the heat, because the metal has to be warmed regardless. The calculated time is the heat-up and melt time. Real cycles run longer, since drain time and fan delay come after the heat stops.
Electric defrost
Resistance heaters in the fin pack heat the coil from the evaporating temperature to the termination setpoint, melting the frost into the drain pan, while a separate heater keeps the pan and drain line from freezing. It is simple, reliable and easy to control, and it is the usual choice in walk-in freezers. The cost is that every kilowatt-hour of heater energy is also heat the refrigeration system has to remove again after the cycle. The calculator charges the coil heaters and drain heater for the whole melt time, and it uses the heat effectiveness input, 60% by default, because some of the heater output warms air and structure instead of the frost.
Heater banks draw real current. A 1,500 W bank on a 208 V supply draws about 7.2 A, and the drain heater adds to it. The amperage calculator converts heater watts to amps for the wire and breaker selection.
Hot gas defrost
Hot gas defrost sends discharge gas from the compressor through the coil, so the heat comes from the compressor and the rest of the system instead of from heater elements. That recycles heat that would otherwise be rejected at the condenser, which is why it often costs less energy than electric defrost. The price is more piping, more valves and a defrost circuit that has to be designed for the system. The calculator assumes the electricity attributed to a hot gas cycle is 65% of the heat delivered, a 35% saving, and you can change that percentage. Actual savings depend on the system design and the heat available. The hot gas heat rate defaults to 3.5 kW per square metre of face area if you do not enter one. The compressor sizing calculator helps check that the compressor can carry the extra load while a circuit defrosts.
Off-cycle defrost
Off-cycle defrost stops refrigerant flow to the coil and lets the evaporator fans keep running, so room air melts the frost. No heater energy is used. It only works when the room air is above 0°C, so it suits walk-in coolers and medium-temperature cases and is not possible in a freezer, where the room air is colder than the ice. The calculator estimates the melt rate at 35 W per square metre of coil face for each kelvin the room is above 0°C, and takes the room as 3°C for coolers and display cases. The coil can only warm to room temperature, so the calculator uses the lower of the termination setting and the room temperature as the end point.
Cycle count and termination
The calculator defaults to 3 cycles a day for a freezer. For comparison, the U.S. Department of Energy set a default of 2.5 defrosts per day in the walk-in cooler and freezer test procedure, as described in its 2014 final rule. Use 2 to 4 for a freezer and 1 to 2 for a cooler as a starting point, then adjust by looking at the coil. End each defrost on coil temperature, typically 7 to 13°C, and keep a time limit only as the backup if the sensor fails.
Product temperature during defrost
Defrost heat warms the air around the coil, so product stored near it can drift upward during each cycle. For drug products this matters, because Health Canada defines a temperature excursion as a variance outside the labelled storage conditions and expects excursions to be investigated, as set out in guidance document GUI-0069. Keep sensitive product away from the coil, schedule cycles for quiet periods and avoid a termination setting higher than the coil needs.
Frequently Asked Questions
Heavy frost between cycles means either more moisture is entering the space or the defrost is not finishing. The usual moisture sources are a worn or misaligned door gasket, a door propped open during loading, and warm product. The usual causes of an unfinished defrost are a termination sensor placed where it reads warm too early, a heater element that has failed, or a drain pan or drain line that has frozen so meltwater refreezes on the coil. If the frost is uneven, heavy on one side or along the bottom, check the airflow. A slipping fan blade or a blocked return path leaves some of the coil starved of air and the frost pattern shows it.
Put the termination sensor on the warmest part of the coil, usually near the middle of the fin pack and away from the heater elements, and set it to end the defrost at 7 to 13 degrees Celsius. Too low and ice can remain at the coil edges. Too high and the cycle wastes energy and pushes more heat into the room. Temperature should be the primary signal, with a time limit as a backup for a failed sensor, which is why the calculator asks for both. Set the time limit longer than the calculated melt time so a normal cycle always ends on temperature, but short enough that a stuck sensor cannot overheat the space.
Two to four electric defrost cycles a day covers most walk-in freezers, and 3 is a common starting point for moderate door traffic. Busy rooms and humid seasons may need more, and a well-sealed storage freezer with few openings may manage on 2. Shorter, more frequent cycles keep the frost layer thin, so each one melts less ice, but every cycle also adds heat to the room. Judge the schedule by the coil: if it is clear at the end of each cycle and not heavily iced just before the next one, the count is about right. Revisit it when the season or the door traffic changes.
Yes. Defrost heat warms the air around the coil, and product stored close to it can rise in temperature even though the system pulls the room back down afterwards. This matters most for products with narrow temperature limits, such as pharmaceuticals, where a brief rise can count as an excursion that has to be investigated. Schedule defrosts for quiet periods, end each one on temperature as soon as the coil is clear, and keep sensitive product away from the coil and heater elements. Shorter cycles also shorten the time the coil is warm, so a method that finishes quickly reduces the exposure.