Evaporator 101

The evaporator is the heat absorber, and it’s the component that many of us associate with HVAC; A/C units don’t make cold air, but they remove heat from the air. A/C evaporators are typically cold, with the coil generally having a temperature of 40 degrees Fahrenheit (with some refrigeration evaporators being as cold as -40 degrees). The temperature is relatively low so that heat can move into the refrigerant with the evaporator.

Many evaporators have fins that increase the surface area, which gives air more contact time and enables heat transfer into the refrigerant within the coil. Generally, the refrigerant will enter the evaporator from the bottom and exit through the top as a gas; from there, it goes down the suction line.

Before the evaporator coil, a metering device drops the pressure of the liquid refrigerant. That’s what allows the evaporator to be relatively cool (compared to the relatively warm liquid in the condenser bottom and liquid line). The metering device also flashes off some of the liquid refrigerant and makes it become a vapor.

When the refrigerant absorbs heat inside the evaporator, there is a rapid change from the liquid state to the vapor state, which is called boiling. This boiling isn’t actually hot; refrigerants have very low boiling points compared to water (which is what many of us have observed boiling). As such, we even have to manipulate refrigerant pressures to get them to boil at a temperature as high as 40 degrees. During the phase change from liquid to vapor, the refrigerant is at saturation and maintains a constant temperature until it has completely boiled.

We generally want to keep A/C evaporator coils above 32 degrees Fahrenheit (0 degrees Celsius), as water can freeze on the coil. Frost and ice buildup on the coil reduce the efficacy of heat transfer. Freezers need to have far colder evaporator coils, as heat needs to be able to move into the refrigerant even at a low temperature. Those coils will freeze, but refrigeration systems also rely on defrost strategies to melt ice off the coil.

Our goals are to control the temperature and feed most of the coil with boiling liquid refrigerant. The last (top) part of the coil is where superheating happens; the refrigerant has become 100% vapor, and the temperature begins to rise. We can measure the superheat to determine how far we’re feeding that coil with liquid refrigerant. Reaching our target superheat would indicate that we’re feeding the coil with the ideal amount of boiling liquid refrigerant.

We also need to make sure we’re moving the correct medium (usually air, sometimes water) across the coil at the correct rate. For example, a dirty air filter can prevent the right amount of air from moving over a coil. If there’s not enough air, then there’s not enough heat, so the pressure will drop (and may lead to a frozen coil).