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How to Use the Evaporator Sizing Calculator
Pick walk-in cooler, walk-in freezer, air conditioning or process, then enter the total room load in BTU/hr. If you do not have a load figure yet, enter room dimensions and an outside design temperature for a transmission-only estimate. Choose a safety factor, 10 to 25%, unless your load calculation already includes a margin.
The TD is the box temperature minus the SST, and it is the main variable in coil selection because manufacturers rate capacity at a stated TD. Also select the refrigerant to see the suction pressure at your SST. The SST must be below the box temperature.
If the compressor is allowed to run 18 of 24 hours, the coil must deliver 24 divided by 18, or 1.33 times, the average hourly load. The remaining hours cover defrost and off-time.
The results give the required capacity in BTU/hr and tons, the capacity per degree of TD, and the equivalent rating at the TD used in your catalogue. Round up to the next catalogue size and confirm it against the manufacturer's tables.
Evaporator sizing for Canadian refrigeration systems
The evaporator coil absorbs heat from the refrigerated space, and its size sets how well the box holds temperature, how humid it stays, how often it frosts and how long the compressor runs. An undersized coil leaves the compressor running without reaching setpoint. An oversized coil costs more and, at a small TD, removes less moisture from the air. Canadian boxes also swing between summer transmission loads and winter door infiltration, so size for the peak summer load and then check that defrost can handle the cold-weather frost.
Temperature difference and catalogue ratings
TD is the box temperature minus the saturated suction temperature, and manufacturers state coil capacity at a particular TD. Unit cooler ratings for factory-made, forced-circulation free-delivery coolers using a volatile refrigerant, fed by direct expansion or liquid overfeed, are defined in AHRI Standard 420 (I-P) and 421 (SI). A catalogue number is therefore only comparable at its stated TD. The calculator scales your requirement to the rating TD you choose, treating capacity as proportional to TD: 13,800 BTU/hr at 10°F TD is 1,380 BTU/hr per degree. That is a fair approximation for small TD changes, and the final choice belongs to the manufacturer's capacity tables. Confirm the SST from your gauge reading with the saturation temperature calculator.
TD and humidity
A larger TD means a colder coil, and a coil colder than the room air's dew point condenses moisture out of it. A small TD keeps the surface warmer and the room more humid, so produce and other high-humidity storage is usually designed at 8 to 10°F and general packaged goods at the higher end of the cooler range, up to 14°F in this calculator. Freezers are less sensitive to humidity and run 10 to 20°F. The apparatus dew point calculator shows where the coil surface sits against the air it is cooling. The results flag a TD outside the usual range for your application and say whether the coil will be larger or run drier as a result.
Refrigerant and suction pressure
The refrigerant does not change the heat the coil has to absorb, but it fixes the suction pressure that corresponds to your SST. The calculator uses saturation fits to CoolProp data for R-410A, R-32, R-454B, R-22, R-404A and R-290, from -58 to 86°F. R-404A at 25°F SST gives 62.1 psig, and R-410A at 45°F, typical of comfort cooling, gives 130.3 psig. Gauge readings are measured relative to atmospheric pressure, and an SST below the refrigerant's boiling point at atmosphere shows as vacuum. Use the PT chart calculator to look up other temperatures.
Runtime, safety factor and compressor match
A compressor allowed to run 18 hours a day must remove the day's heat in those 18 hours, so the coil needs 24 divided by 18, or 1.33 times, the average hourly load. The safety factor then multiplies the load first. At 15% and 18 hours the combined factor is 1.15 times 1.33, or about 1.53 times the entered load. If your load calculation already carries a margin, choose a low safety factor to avoid counting it twice. Match the compressor to the same figure with the compressor sizing calculator so the two are sized for the same duty.
Transmission load estimate
If you enter room dimensions, an outside design temperature and an insulation R-value, the calculator estimates conduction through all six surfaces as area times temperature difference divided by R. A 12 by 10 by 8 ft box has 592 ft² of surface, and with 95°F outside, a 35°F box and R-32 panels that is about 1,110 BTU/hr. This is conduction only, and a floor on grade sees a cooler outside than the air does. Use the U-value calculator to convert a wall assembly to the R-value you enter. Solar gain, infiltration, product and equipment loads sit on top of it, and the refrigeration load calculator handles the full total.
Reducing the load before you size
Every BTU/hr removed from the load shrinks the coil, compressor and condenser. Natural Resources Canada's walk-in refrigeration guidance lists better wall, ceiling and floor insulation, door gaskets, vestibules and air or strip curtains, and evaporator fan control when the compressor is off. Door infiltration is the load that sizing estimates tend to miss, so treating it first is the cheapest way to cut the required coil size.
Frequently Asked Questions
Start with the total room heat load in BTU/hr, including transmission, infiltration, product and internal loads. Add a safety factor, then correct for runtime by multiplying by 24 divided by the hours per day the compressor is allowed to run. Subtract the saturated suction temperature from the box temperature to get the TD, then divide the required capacity by that TD to get the capacity needed per degree. For a 9,000 BTU/hr load with a 15% margin, 18 hours of runtime and a 10°F TD, the coil must deliver 13,800 BTU/hr, which is 1,380 BTU/hr per °F of TD. Select the next catalogue size above that and confirm it in the manufacturer's capacity tables.
This calculator treats 8 to 14°F as the usual range for a walk-in cooler and 10 to 20°F for a walk-in freezer. Produce and other high-humidity storage sits at the low end, around 8 to 10°F, because a small TD keeps the coil warmer and removes less moisture from the room air. Packaged goods tolerate a larger TD, which allows a smaller coil. Treat these as design ranges and not limits, and use the equipment manufacturer's recommendation when it differs. Near an exterior wall in a Canadian climate, size for the peak summer transmission load and check that the defrost system copes with winter door infiltration.
An oversized coil delivers the same load at a smaller TD. That keeps the coil surface warmer, so less moisture is removed from the air and the room stays more humid, which suits produce and is why those rooms are designed with larger, low-TD coils. In a packaged-goods cooler where some dehumidification is wanted, the effect is less useful. The firm costs of oversizing are a higher purchase price and a bigger coil that may not fit under the ceiling in a retrofit. An oversized coil also lets the box pull down quickly and then cycle, so check that the compressor is not being asked to short-cycle.
Frost insulates the fins and tubes and narrows the air passages, so heat transfer and airflow both fall as frost builds between defrosts. A coil delivers its full rated capacity right after a defrost and less as the next one approaches. How much it loses depends on the room humidity, the door traffic and the defrost schedule, so there is no single figure. This is why defrost and coil sizing are linked decisions. A coil sized with a margin above the calculated peak load can hold the setpoint late in the cycle, and the safety factor in this calculator is where that margin goes.