How Does The Evaporator Coil Work in A Packaged Unit?

How does the evaporator coil work in a packaged unit?

The evaporator coil in a packaged unit is a core component of the refrigeration cycle, responsible for absorbing heat from the indoor air (or the air needing cooling) to achieve the cooling effect. Its operation is tightly linked to the refrigerant’s phase change (from liquid to gas) and the coordination of other system components (e.g., compressor, expansion valve).

Core Principle: Heat Absorption Driven by Refrigerant Phase Change

The evaporator coil leverages a fundamental physical property: when a liquid evaporates into a gas, it absorbs a large amount of heat from the surrounding environment (called "latent heat of vaporization"). In a packaged unit, the refrigerant (a specialized fluid with low boiling points, e.g., R-410A) flows through the evaporator coil, evaporates here, and "takes away" heat from the air passing over the coil—lowering the air temperature for cooling.

Step-by-Step Working Process

The evaporator coil does not operate in isolation; it is part of a closed refrigeration loop. Its specific workflow is as follows:

Step 1: Low-Pressure, Low-Temperature Liquid Refrigerant Enters the Coil

Before reaching the evaporator coil, the refrigerant undergoes pre-treatment by the expansion valve (or capillary tube):

The high-pressure, room-temperature liquid refrigerant from the condenser coil first flows into the expansion valve.

The expansion valve acts as a "throttle": it suddenly reduces the refrigerant’s pressure and temperature, converting it into a low-pressure, low-temperature liquid mist (similar to how spraying perfume cools your skin—pressure drop causes temperature drop).

This cold liquid refrigerant then enters the internal tubes of the evaporator coil.

Step 2: Air Passes Over the Coil (Heat Source Contact)

A blower/fan in the packaged unit forces the warm indoor air (or process air that needs cooling) to flow through the evaporator coil’s external fins:

The evaporator coil has a "tube-and-fin" structure: copper tubes (for refrigerant flow) are fitted with thin aluminum fins. This design maximizes the heat exchange area between the refrigerant (inside tubes) and the air (outside fins).

As warm air passes over the cold fins and tubes, heat from the air is transferred to the refrigerant inside the coil.

Step 3: Refrigerant Evaporates and Absorbs Heat

As the cold liquid refrigerant in the coil absorbs heat from the passing air:

It reaches its boiling point (which is very low, e.g., 4–10°C for air conditioners) and rapidly evaporates into a low-pressure, low-temperature gaseous refrigerant.

During this phase change, the refrigerant absorbs a large amount of latent heat—this is the key to cooling the air: the air loses heat and becomes cooler, then is blown back into the indoor space (or target area) as supply air.

Step 4: Gaseous Refrigerant Flows to the Compressor

After fully evaporating in the evaporator coil, the low-pressure gaseous refrigerant (now carrying the heat it absorbed) is sucked into the compressor—the "heart" of the refrigeration system.

The compressor compresses the low-pressure gas into a high-pressure, high-temperature gas, which is then sent to the condenser coil to release the absorbed heat (completing the refrigeration cycle).

Key Supporting Mechanisms for Efficiency

For the evaporator coil to work effectively, two additional mechanisms prevent performance degradation:

(1) Defrosting (Preventing Frost Buildup)

In high-humidity environments (e.g., summer or refrigerated storage), the cold evaporator coil surface (often below the dew point of the air) causes water vapor in the air to condense into liquid. If the coil temperature drops below 0°C (e.g., in heat pumps during winter heating or cold storage units), this condensation freezes into frost.

Problem: Frost forms a thermal insulator, reducing heat transfer between the air and refrigerant, and blocks airflow through the fins.

Solution: Packaged units use defrost systems (e.g., electric defrost heaters, hot gas defrost from the compressor) to melt frost periodically. The melted water drains away through a condensate pan and drain line (preventing water leakage).

(2) Airflow Control

The blower’s speed is often adjusted (via variable-speed motors in modern units) to match the cooling load:

If the indoor air is very warm (high load), the blower runs faster to push more air over the coil, increasing heat transfer and cooling capacity.

If the air is already cool (low load), the blower slows down to avoid overcooling and save energy.

Without proper airflow, the refrigerant may not fully evaporate (leading to "liquid slugging"—liquid refrigerant entering the compressor, which can damage it) or the coil may not cool the air efficiently.