Houston Heating, Cooling, and Electrical

What Is The Purpose Of An Evaporator Coil?

The evaporator coil is a major component in the refrigeration cycle, responsible for absorbing heat from a surrounding medium and cooling the air. This is the part of the air conditioner that most people think of first because it’s the part of the system that feels cold to the touch. It’s also the part of the system that ices over when there are issues with airflow, so the evaporator comes up a lot in HVAC conversations.

How does an evaporator coil work?

An evaporator consists of many evaporator coils, and each one is filled with refrigerant. It’s this refrigerant that does the work in getting heat out of the home, as it absorbs it from the surrounding air. With many coils in tandem absorbing heat, along with fins to increase the surface area, the evaporator is a dense network of cold tubes.

As the hot air passes over the cold coils, heat is pulled out, and this also causes water vapor to condense out of the air and onto the coils. This occurs because as the air’s temperature drops, its ability to hold moisture also drops, producing an effect that’s also seen on a cold soda can during a hot day. That’s how your evaporator keeps you cool and dry.

How does the refrigerant absorb heat?

There’s only one way to move heat from one place to another – using a temperature gradient. This means that heat always travels from a hot medium to a cooler medium. In this case, it’s heat moving from hot air to the cold refrigerant. The refrigerant can hold a lot of heat since there’s a big difference in temperature between the refrigerant and hot air.

The question, then, is how do air conditioners decrease the refrigerant’s temperature?

Under Pressure

A traditional split-system air conditioner works by pressurizing and depressurizing refrigerant during different stages of the refrigeration cycle. This is more science, but as pressure increases, so does temperature, and vice versa. Imagine a bunch of molecules bouncing around inside an otherwise empty chamber. It’s the movement of those molecules that register as heat to us.

Now, if you decrease the chamber’s volume (like with a compressor), the molecules have less space to move around. The molecules start bouncing off of each other and off of the chamber walls more, which increases the pressure. As pressure increases, so does temperature, so by reducing the chamber’s volume, you’re increasing the temperature. This is how an air conditioner’s compressor works.

This compressed, heated refrigerant is in liquid form due to the intense pressure, but right before it gets to the evaporator coils, it passes through a metering device. The metering device decreases the refrigerant’s pressure just as it enters the evaporator, and this is what causes the drop in evaporator coil temperature.

The cooling power of an evaporator, though, is twofold. The refrigerant’s low temperature is one factor, and a primary one, but what also absorbs heat is the phase change the refrigerant undergoes as it passes through the evaporator. As soon as the refrigerant enters the evaporator, it is phase changing from a liquid to a gas due to the drop in pressure. This phase change requires a lot of energy to take place, and the evaporator grabs this energy out of the air in the form of heat and absorbs it. It’s not necessary for an HVAC system to leverage this phase change, but it greatly improves the system’s performance.

How often does an evaporator need maintenance?

The evaporator does a lot of heavy lifting, so it needs to be checked often to prevent failures. Some of this maintenance is basic and can be done by anyone willing to put in the work. For example, it’s best to check the evaporator for any icing at least monthly, and to defrost the evaporator if necessary. Frequent cleaning is also recommended to prevent airflow issues.

An experienced HVAC technician should be brought in to handle further maintenance, which includes a closer inspection of the evaporator and its components. Some of the things the technician will do during an evaporator inspection include:

  • A deep evaporator coil cleaning – HVAC technicians have commercial-grade cleaners and foaming agents on hand to deep clean the coils and remove any dirt trapped in the fins.
  • Drain pan and line inspection – Some problems originate at the drain pan, where overflowing water can produce leaks or mold growth. The technician will check the pan for foreign material and verify that the drain line is sloping properly away from the evaporator.
  • Fan motor and blade inspection – The technician will lubricate all motor and fan parts that require it, and will verify that the fan is in good condition.
  • Defrost heater inspection – The defrost heaters are responsible for keeping the evaporator coils from icing over, but they need to be checked periodically to ensure they are timed properly.
  • Electrical connection inspection – The system’s electrical connections should be inspected regularly to prevent damage to the unit. HVAC systems deal with a lot of wear and vibration, so tightening the connections will ensure they aren’t compromised by this movement.

The evaporator coil is where the cooling happens in an air conditioner, where heat is taken out of the air and directed outside. It’s responsible for keeping your home comfortable and should be regularly inspected and maintained. This will ensure your evaporator coils can keep doing their job efficiently and safely.


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