Understanding Heat Exchangers and Air Filters

You’ve probably seen a heat exchanger in your car, computer, fridge, and your house. The ductwork in your walls and ceiling contains many more.

A heat exchanger transfers thermal energy from one fluid to another without allowing them to mix. Most follow either a coil or plate design. Most heat transfer occurs by convection and conduction. Radiation is also involved but to a lesser degree.

Evaporator Coils

Your HVAC system depends on a complex series of physical factors to manage your indoor air temperature. These include the degree of contact between moving air and your system’s evaporator coil and compressor. When the coil’s surface ices up, it restricts the airflow that cools your space.

Your system’s evaporator coil sits in the air handler, where it works with a blower and other components to circulate cooled or heated air throughout your home or business. The evaporator coil holds the chilled refrigerant that’s been circulating through the compressor, which has changed it from a high-pressure gas to a low-pressure liquid.

The air that reaches the evaporator coil passes through an air filter, which improves indoor air quality by trapping particles like dust, pollen, and pet dander. The air also passes through dampers, adjustable plates that can open and close to control the flow of conditioned air into your ductwork. Dirty or blocked air filters restrict the airflow, affecting the evaporator coil and other coils.

Condenser Coils

A heat exchanger transfers thermal energy between two systems of fluids. In the case of your air conditioner, the evaporator coil acquires heat from the air in your home and the condenser coil releases it into the outdoor environment.

The condenser coil is found housed in the outdoor AC unit box and resembles a series of copper or aluminum tubes with thin, aluminum fins surrounding them. If you were to remove the cover and peer inside, this is what you would see.

Unlike the evaporator coil, this type of heat exchanger is a shell and tube unit. The fluids (refrigerant and water) never mix but transfer thermal energy through the tube walls.

You can also find helical-coil heat exchangers, which are less common but work on similar principles. These are a little more complicated than shell and tube units, and they often have components in multiple phases (solids, liquids and gases). They require more regular cleaning to avoid mechanical problems.

Heat Exchangers

Using one fluid to transfer heat to another is the job of heat exchangers, which are found in forced-air furnaces in homes and commercial spaces. These devices transfer thermal energy between two fluids while keeping them physically separated, allowing the higher-temperature fluid to move toward the lower-temperature fluid’s temperature.

There are many different kinds of heat exchangers, but most fall into one of two design types: coil or plate. Coil heat exchangers use pipes that run back and forth a number of times to transfer the heat. The pipe wall transfers heat to the fluid inside via convection, while the fluid outside carries it away through conduction.

In shell and tube heat exchangers, a bundle of tubes is surrounded by a shell, and the hot or cold fluid flows through the tubes to absorb and disperse heat. Baffles are used to increase the turbulence of the flow and promote efficient heat transfer. This type of heat exchanger is used to process solutions with high viscosity, such as food processing or pharmaceuticals.

Air Filters

Although they are not heat exchangers in themselves, air filters play an important role in your HVAC system’s overall functionality. They collect and neutralize many pollutants that could otherwise contaminate your home’s air, such as dust, pet dander, pollen, bacteria, mold spores, and other particulates.

The way in which your air filter does this work is through convection, rather than by physical contact between two different materials. This means the particles never come into direct contact with each other.

A dirty air filter makes it harder for your furnace to release the air into your home, so it needs to run longer and at a higher energy usage rate to warm or cool your home. This increases your electricity bills and causes a greater impact on our environment. Changing your filter on a regular basis—especially in households with pets—is essential to keeping this important piece of equipment functioning optimally. Second Nature can help you determine the best filter change frequency for your home’s unique needs.