Short #56 – Bypass Factor

By Bryan Orr

In this short podcast episode, Bryan covers the bypass factor in airflow and psychrometrics. He also explains why it matters to techs.

The bypass factor is a ratio: you take the difference between the evaporator coil temperature and outlet air temperature as compared to the inlet. In other words, you find the difference between your TD and delta T.

When air moves over the coil, only some of it contacts the coil's surface. Therefore, only some molecules will become the same temperature as the coil. Other air molecules will bypass the coil, which typically happens when coils have a lower surface area. Evaporators have fins to increase the coil surface area, which helps those air molecules bump into the coil and transfer their heat. Without those fins, your performance will suffer; the saturated suction temperature will drop, and your temperature split will be lower, indicating a higher bypass factor. We want that air to have MORE contact time with the coil; therefore, we want a LOWER bypass factor.

However, when we account for total enthalpy change across the evaporator coil, we also have to look at the latent content; that topic can get complicated and theoretical very quickly, so we avoid that discussion for the sake of simplicity.

The bypass factor also accounts for contact time, which is the amount of time needed for the air molecules to transfer their heat to the refrigerant. You can reduce air velocity to increase the contact time, which is the inverse of the bypass factor.

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