How a Ceiling Cassette Air Conditioning Unit Evaporator Coil Works

How a Ceiling Cassette Air Conditioning Unit Evaporator Coil Works

The evaporator coil of a ceiling cassette air conditioning unit works as the "heat-absorbing core" of the cooling system, transferring heat from indoor air to the refrigerant to lower room temperature.

Preparatory Stage: Refrigerant Inlet

Low-temperature (typically 5–10°C), low-pressure liquid refrigerant is delivered to the evaporator coil through the system’s capillary tube or expansion valve.

Before entering the coil, a distributor (a small, branch-like component) splits the liquid refrigerant evenly into each copper tube of the coil. This ensures every part of the coil participates in heat exchange, avoiding inefficient "cold spots."

Core Process: Heat Absorption & Refrigerant Phase Change

The ceiling cassette unit’s built-in fan draws warm indoor air downward through the return air grille. This air is forced to flow through the evaporator coil’s dense fins.

As warm air passes over the cold coil (copper tubes + aluminum fins), heat from the air is rapidly transferred to the liquid refrigerant inside the tubes.

The absorbed heat causes the liquid refrigerant to evaporate (phase change): it turns from a low-pressure liquid into a low-pressure gas. This phase change is key—it absorbs far more heat than a simple temperature rise, enabling rapid cooling of the air.

Secondary Effect: Dehumidification

While absorbing heat, the coil’s low surface temperature (below the air’s dew point) causes water vapor in the warm air to condense into liquid droplets.

These droplets attach to the coil’s fins, then drip down into a condensate pan installed below the evaporator. A small drain pipe connected to the pan carries the water out of the room (e.g., to a building’s drainage system), reducing indoor humidity.

Final Stage: Refrigerant & Air Outlet

The now-heated, low-pressure gaseous refrigerant exits the evaporator coil and is sucked into the outdoor unit’s compressor. It then enters the next cycle (compression → condensation) to release the absorbed heat outside.

The cooled, dehumidified air passes through the coil and is blown back into the room via the ceiling cassette’s four-way air outlets, lowering the indoor temperature to the set value.

Key Design Features Supporting Its Work

To optimize efficiency, the evaporator coil of ceiling cassette units has two critical designs:

Finned Structure: Thin aluminum fins are tightly attached to the copper tubes. This design expands the heat exchange area by 10–20 times compared to bare copper tubes, accelerating heat transfer between air and refrigerant.

Horizontal Layout: Adapted to the ceiling-mounted cassette form, the coil is placed horizontally. This matches the unit’s "top-suction, four-way discharge" air flow path, ensuring air flows evenly across the entire coil surface.