In the HVACR world, it is not impossible that the designers of a walk-in cooler/freezer to mistakenly chose the wrong FPI (fins per inch) selection for the application you are servicing. Here is a table to reference when you are looking at possible causes of an iced-up coil. With fin spacing too close, frost can easily bridge the gap and quickly reduce air flow through the evaporator.

The proper selection of fins per inch plays a role in maintaining proper humidity levels in the conditioned space as well. The bottom line is this: don’t always assume that the application engineer can’t make a mistake in the selection process. Always look at this factor when troubleshooting your HVACR evaporator problems.

EXCERPT FROM Technical Manual Series available at the ONLINE STORE at RSES.ORG, your HVACR training authority.