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Evaporator sizing for Canadian refrigeration systems
The evaporator coil is the heat exchanger that absorbs heat from the refrigerated space. Getting the size right determines box temperature stability, humidity control, defrost frequency, and compressor runtime. An undersized evaporator forces the compressor to run continuously and never pulls the box to setpoint. An oversized evaporator removes too much moisture from the air, drying out produce and increasing frosting on the coil.
In Canada, refrigeration systems deal with wide seasonal swings. A walk-in cooler near an exterior wall in a Manitoba warehouse faces very different transmission loads between January and July. Always size the evaporator for the peak summer load, then verify the defrost system can handle the winter condensation and frost load when warm, moist air infiltrates during door openings.
Temperature difference and coil selection
TD (Temperature Difference) is the difference between the entering air temperature and the saturated suction temperature. It's the primary variable in evaporator coil selection because manufacturers rate coil capacity at a specific TD. A coil rated at 10,000 BTU/hr at 10°F TD produces less capacity at 8°F TD and more at 12°F TD.
For fresh produce requiring high humidity, keep TD at 8-10°F. This minimizes the temperature difference between the coil surface and the room air, reducing moisture removal. For general coolers storing packaged goods at 35-38°F, a TD of 10-12°F is standard. For freezers operating at 0°F to -10°F, TD of 10-12°F is typical. Use the saturation temperature calculator to confirm your SST from your suction pressure, then verify actual TD in the field with the superheat calculator.
Daily runtime and compressor sizing
Evaporators are sized for the heat load the compressor must handle during its runtime hours. If the system runs 18 hours per day, the compressor needs to remove the full 24-hour heat load in 18 hours — so the compressor and evaporator capacity must be sized at 24/18 = 1.33 times the average hourly load. The remaining 6 hours are for defrost cycles and off-time. Use the compressor sizing calculator alongside this tool to match compressor capacity to evaporator load.
Transmission load estimation
If you enter room dimensions, this calculator estimates the transmission load through the insulated walls, ceiling, and floor using your insulation R-value and the temperature difference between inside and outside. This is a simplified estimate — a full load calculation for a Canadian commercial refrigeration project should account for solar gain on south and west walls, door infiltration, product pull-down load, occupancy, and lighting. For complete load calculations, see the load calculations category.
Frequently Asked Questions
Start with the total room heat load in BTU/hr: transmission through walls and ceiling, infiltration, product load, and internal heat sources like lighting and motors. Select a TD — 10°F is standard for medium-temperature coolers. Divide the design load (including a 15% safety factor) by the manufacturer's coil capacity per unit TD at your operating SST. Apply a runtime correction if the compressor runs less than 24 hours per day. Always select the next standard coil size up. Verify after installation with the superheat calculator to confirm correct evaporating temperature.
For fresh produce requiring high humidity (85-95% RH), use TD of 8-10°F to minimize moisture removal. For general coolers with packaged product, 10-12°F TD is standard. For freezers, 10-12°F TD is typical. In Canadian climates near exterior walls, add extra capacity for summer transmission load and verify the defrost system handles winter frost accumulation from door infiltration. Use the saturation temperature calculator to confirm your actual operating SST from measured suction pressure.