Condensing unit sizing basics for 43 °C ambient

Start with the heat load

The condensing unit must remove the heat load at design conditions, divided by the hours you plan it to run. A heat load sheet gives this in kW. Our heat load example for a small chiller came to about 2.06 kW at a 43 °C ambient, allowing 18 running hours and a 10% margin.

Set the evaporating temperature

The refrigerant in the evaporator must be colder than the room air for heat to flow into it. Designers choose a temperature difference between the room air and the evaporating refrigerant to suit the product. For chilled rooms, differences of roughly 6 to 10 K are common in design practice, so a +2 °C room might evaporate around −6 to −8 °C; freezer rooms evaporate correspondingly lower. A smaller difference keeps humidity higher, which suits fresh produce, but needs a larger evaporator.

Every degree lower in evaporating temperature reduces the unit's capacity, so freezer duty needs a much larger unit than chiller duty for the same load.

Allow for the condensing temperature at 43 °C

An air-cooled condenser rejects heat into the surrounding air, so its condensing temperature sits some way above ambient; selections commonly assume something like 10 to 15 K above. At 43 °C ambient that means condensing in the mid-50s °C, and higher still if the unit is in the sun or breathing its own exhaust.

Higher condensing temperature reduces capacity and raises power draw. Check that the compressor's operating envelope and the high-pressure safety settings allow the condition you expect, so the unit does not trip on the hottest days.

Worked example: comparing two units

Assumptions: required capacity 2.06 kW at −8 °C evaporating and 43 °C ambient. Capacities below are hypothetical figures of the kind found in a maker's capacity table, used to show the method.

What the example shows

Judged at the mild condition, both units look large enough. At 43 °C, only unit X meets the load. Unit Y would run continuously in summer, the room would drift warm in the afternoon, and the compressor would work at its hottest. Always ask for capacity at your evaporating temperature and ambient, not just a nominal or HP figure.

Site checks that protect capacity

• Place the unit in shade with free airflow; a sun-baked roof can be far hotter than the shade temperature in a weather report.
• Keep units apart so one does not breathe another's discharge air.
• Choose coil fin spacing and coatings for dusty or coastal sites, and plan regular cleaning.
• Size the evaporator to match the condensing unit at the same design condition; a mismatched pair underperforms.
• Keep pipe runs reasonable and correctly sized; long or undersized suction lines cut delivered capacity.