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How Does The Condenser Coil Work in A Packaged Unit?
After the evaporator, the low-pressure, low-temperature gaseous refrigerant is sucked into the compressor (the "power source" of the system). The compressor squeezes this gas, significantly increasing its pressure (e.g., 2000–3000 kPa for air conditioners) and temperature (e.g., 50–80°C, much higher than outdoor air temperature).
This superheated, high-pressure gaseous refrigerant is then pumped into the internal tubes of the condenser coil (located in the "outdoor side" of the packaged unit).
A dedicated condenser fan (mounted near the coil) forces outdoor air to flow rapidly over the coil’s external fins.
The condenser coil uses a "tube-and-fin" structure (same as the evaporator, but with optimized fin design): copper tubes (for refrigerant flow) are fitted with thin aluminum fins. This structure maximizes the heat exchange area between the hot refrigerant (inside tubes) and the cool outdoor air (outside fins).
Since the refrigerant’s temperature (50–80°C) is much higher than the outdoor air (e.g., 25–35°C in summer), heat transfers from the refrigerant to the air through the tube and fin walls.
The coil has a "shell-and-tube" or "tube-in-tube" structure: the hot refrigerant flows through the inner tubes, while a separate stream of cooling water (from a cooling tower or water source) flows around the outer tubes.
Heat transfers directly from the hot refrigerant tubes to the cooling water, which then carries the heat away (e.g., to a cooling tower, where the water is cooled by evaporation before being recycled).
It first cools down to its saturation temperature (the temperature at which it starts to condense, e.g., 40–50°C for R-410A refrigerant at high pressure).
Next, the refrigerant undergoes phase change: it condenses into a high-pressure liquid (still at saturation temperature) while releasing a large amount of latent heat. This is the "core heat-releasing step"—most of the heat absorbed by the evaporator is released here.
By the end of the coil, the liquid refrigerant is often "subcooled" (cooled slightly below saturation temperature, e.g., 35–45°C) to ensure it remains fully liquid when entering the expansion valve (preventing vapor from interfering with the throttling process).
When outdoor temperatures are high (e.g., 35°C+), the fan runs faster to push more cool air over the coil, accelerating heat release and preventing refrigerant temperatures from rising too high (which would reduce compressor efficiency).
When outdoor temperatures are low (e.g., 15–20°C), the fan slows down to avoid over-cooling the refrigerant (which could cause excessive pressure drops) and save energy.
Without proper airflow, the refrigerant may not fully condense (leading to "vapor carryover"—uncondensed gas entering the expansion valve, reducing cooling capacity) or the compressor may overheat (due to high discharge pressure).
For air-cooled coils: Dust and debris on fins reduce heat transfer between refrigerant and air. This forces the compressor to work harder (higher discharge pressure) to condense the refrigerant, increasing energy consumption and risk of compressor failure.
For water-cooled coils: Mineral deposits (e.g., calcium, magnesium) from cooling water build up inside tubes, reducing heat transfer to water.
Solution: Regular cleaning (e.g., pressure washing air-cooled fins with water, chemical descaling for water-cooled tubes) is required to maintain coil efficiency.
Packaged units are equipped with a pressure relief valve (mounted on the condenser coil or compressor discharge line). If pressure exceeds a safe threshold, the valve opens to release excess refrigerant, protecting the coil, compressor, and pipes from rupture.
