Let's use a bit of imagination for a minute.

Imagine you have two totally identical 3-ton systems. One of them is completely normal, and the other has no fins at all on the evaporator coil.

They both have the same charge, airflow, and compressor capacity. What will be different in terms of readings and performance of the one with no fins? Think about it yourself before moving on.

The fins attached to the tubing increase the contact time and turbulence of the air over the lower-temperature coil surface.

This increased air-to-metal contact time and surface area allow more heat to leave the air and enter the refrigerant. The pressure and temperature of the refrigerant in the evaporator coil (assuming it is changing state from liquid to vapor) is a function of the amount of refrigerant being moved through the coil (by the compressor) and the amount of heat entering the refrigerant from the air.

If the evaporator coil had no fins, we would see these results (compared to the normal system with fins):

• Lower suction & coil pressure because less heat is entering the coil
• Lower coil (saturated suction) temperature because of the lower pressure
• Lower delta T because the air has less contact time on the metal as it rushes right past
• The compressor moves less refrigerant because the suction vapor is less dense, and the compression ratio increases
• The system is less efficient because the compressor is moving fewer BTUs per watt due to the higher compression ratios
• Very little humidity is removed; even though the coil is very cold, there is less contact between the metal and the air moving over it

Eventually, the suction pressure/temperature would stabilize once the amount of heat being picked up in the coil and the reduced compressor capacity reached an equilibrium. By that time, the tubing would likely be freezing over, and the system would be VERY inefficient.

A coil with no fins has a high BYPASS FACTOR and a low CONTACT FACTOR.

As a technician, you will likely never calculate bypass or contact factors, but you will see their impacts all the time, like in the extreme thought experiment above. It is easier to wrap your head around the impact of the bypass factor by considering the extreme edges of the design, with a coil with no fins being an extreme example of high bypass and a massively oversized evaporator with very dense and deep fins being the opposite side of the spectrum.

If you have more surface contact between air and metal, and when the air moves at a lower velocity (slower), the air will impart more heat to the coil and get closer to the coil temperature.

If you have less surface contact (like no fins) and the air is moving at a higher velocity (faster), the air will impart less heat into the evaporator coil and will stay warmer when compared to the coil temperature.

Here's the part that can make this tricky to imagine: You'll realize that changing the bypass factor impacts the difference between the air temperature and coil temperature simultaneously, AND it changes the coil temperature itself. That's because the coil temperature is partially dictated by the amount of heat entering the coil FROM the air.

Look back up at the bullet list of impacts that occur with a coil that has no fins. Now, flip the results for an oversized evaporator coil (assuming the airflow remains constant):

• Higher suction & coil pressure because more heat is entering the coil
• Higher coil (saturated suction) temperature because of the higher pressure
• Higher delta T because the air has more contact time on the metal as it moves more slowly over the larger surface
• The compressor moves more refrigerant because the suction vapor is denser, and the compression ratio decreases
• The system is more efficient because the compressor is moving more BTUs per watt due to the lower compression ratios
• More humidity is removed because even though the coil is warmer, there is more contact between the metal and the air moving over it***

Now, this last point about the humidity has a caveat; the coil still needs to be well below the dew-point, so this correlation between increased coil size and increased dehumidification isn't absolute. When you couple a larger coil with LOWER airflow, you can get the best of both worlds by having lower bypass factors without driving the coil temperature up too far, resulting in improved efficiency and latent capacity.

I'm not saying you should go out and put larger coils on everything, but as a design consideration, you may want to take a good look at manufacturer expanded performance data when choosing a coil match. You may find an upsize gives you better performance, even if you dial back the airflow a bit.

I know this may be a little bit of a brain mush concept, but keep going back to the impact of the evaporator with no fins to help set your mind straight. It certainly has helped me.

—Bryan