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Cooling Coil Face Velocity Calculator

Check how fast air moves across a cooling coil, and how much coil face you need to stay under a velocity limit.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Unit system

Use your project’s design criterion. Example: NIH requires cooling coils for its laboratory and animal-facility air handlers to be sized for no more than 400 fpm (480 fpm with future growth) to prevent moisture carryover.

Result

Within your limit

Face velocity
400fpm
Share of your limit
100%

400 fpm across the coil — within your 400 fpm limit.

Coil face area
5ft²
Face area needed at your limit
5ft²
Most airflow at your limit
2,000CFM
Show the working
  1. Face area = finned length × finned height
    30 × 24 ÷ 144 = 5 ft²
  2. Face velocity = CFM ÷ face area
    2,000 ÷ 5 = 400 fpm
  3. Area needed = CFM ÷ limit
    2,000 ÷ 400 = 5 ft²

Why face velocity matters

Face velocity is airflow ÷ finned face area. Above a certain speed, condensate on a cooling coil can be blown off the fins into the air handler and ductwork (moisture carryover), and the coil’s pressure drop rises. Lower velocity also leaves more time to remove moisture.

Choosing a limit

The limit belongs to the coil design and your project’s criteria. As one published example, the National Institutes of Health sizes cooling coils in its laboratory and animal-facility air handlers for no more than 400 fpm at present design, and 480 fpm with future growth, to prevent carryover. Coil makers state their own limits.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Airflow through the coil
2,000 CFM
Finned length (width) of the coil face
30 in
Finned height of the coil face
24 in
Face velocity limit
400 fpm

Result

Face velocity
400 fpm
Share of your limit
100 %
Coil face area
5 ft²
Face area needed at your limit
5 ft²
Most airflow at your limit
2,000 CFM

400 fpm across the coil — within your 400 fpm limit.

Assumptions

  • Face velocity = airflow ÷ finned face area (not the cabinet opening).

Limitations

  • The right limit depends on the coil design, fin spacing and drain pan; follow the coil maker’s and your project’s criteria. The NIH value is an example for one building type.

Frequently asked questions

How do I calculate coil face velocity?

Divide the airflow by the finned area of the coil face in square feet. 2,000 CFM through a 30 × 24-inch coil (5 ft²) is 400 fpm.

What happens if face velocity is too high?

Condensate can be carried off the coil into the unit and ducts, causing water damage and microbial growth, and the coil’s pressure drop increases.

Does this apply to heating coils?

The arithmetic is the same, but carryover only concerns wet (cooling) coils. Heating coils have their own pressure-drop and velocity guidance from the maker.

Sources

  1. Cooling Coil and Drain Pan Requirements for Air Handling Units (News to Use, August 2015) — National Institutes of Health, Office of Research Facilities. Proper coil face velocity and drainage prevent moisture carryover; NIH sizes cooling coils for a nominal face velocity not exceeding 2.0 m/s (400 fpm) for present design and 2.5 m/s (480 fpm) for future growth.

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