How does face velocity drive coil pressure drop?

Airside pressure drop rises roughly with the square of face velocity for a given coil. Doubling velocity from 300 to 600 FPM increases the drop about four-fold, which is why staying near 400 to 500 FPM is the usual design sweet spot for cooling coils.

Why does it matter whether the coil is wet or dry?

On a cooling coil that is dehumidifying, condensate clings to the fins and adds resistance. A wet coil typically has 20 to 40 percent higher airside pressure drop than the same coil running dry, so this tool applies a wet-coil factor by default for evaporator coils.

What face velocity should I design for?

Cooling coils are normally sized for 400 to 500 FPM face velocity. Above about 550 FPM condensate can be blown off the coil and carried downstream, and noise and pressure drop climb fast. Below 300 FPM the coil and cabinet get oversized and expensive.

Where does this number go in my calculation?

The coil airside drop is one component of the total external static pressure the blower must overcome. Add it to filter, duct, register, damper and other accessory losses to get the total, then select a blower and speed that delivers your CFM at that static pressure.

Are these numbers exact for my specific coil?

No. They are representative curves for typical aluminum-fin, copper-tube coils at common fin spacing. For final design always use the manufacturer's published coil selection software, which accounts for exact fin density, tube pattern and circuiting. Use this tool for early sizing and sanity checks.

Evaporator / Condenser Coil Airside Pressure Drop Calculator

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The finned coil is usually the single largest airside resistance inside an air handler, and getting its pressure drop wrong throws off blower selection. This calculator estimates the coil airside drop in inches of water column from face velocity and coil row depth so you can build an accurate total external static pressure.

How it works

Airside pressure drop for a finned coil scales approximately with the square of face velocity, and grows with the number of rows. The tool uses an empirical coefficient per row depth fitted to typical manufacturer data:

face velocity V (FPM) = CFM / face area (ft²)
ΔP_dry (in WC)        = C(rows) · (V / 100)²
ΔP_wet                = ΔP_dry · wet factor (≈1.30 for a dehumidifying coil)

The coefficient C increases with row count (more fin surface to push air through). For cooling and evaporator coils the wet factor is applied because condensate on the fins raises resistance; condenser and heating coils run dry.

Worked example

A 4-row cooling coil with a face area of 6 ft² carrying 2,000 CFM:

Face velocity = 2,000 / 6 = 333 FPM
Dry drop (4 rows) = C₄ × (333/100)² ≈ 0.27 in WC
Wet drop = 0.27 × 1.30 ≈ 0.35 in WC

Push that same coil to 600 FPM and the dry drop rises by (600/333)² ≈ 3.25×, landing near 0.88 in WC dry and 1.14 in WC wet — a threefold penalty for doubling the velocity.

The wet-coil penalty explained

On a dehumidifying evaporator coil, moisture condenses on the fins and forms a thin water film. This film adds surface roughness and partially blocks inter-fin passages, increasing resistance by roughly 20–40% compared with a dry coil at the same velocity. The exact wet penalty depends on fin density, surface coating, and condensation rate. A factor of approximately 1.30 is a conservative estimate for aluminum-fin, copper-tube coils at typical cooling-season conditions.

Condenser coils and heating coils (hot-water or electric) run dry, so no wet factor applies.

Face velocity design range

The standard design window for cooling-coil face velocity is 400–500 FPM:

Below 300 FPM: Very low drop is good for the blower, but the coil and air-handler casing become oversized and costly. Dehumidification performance can also suffer because air spends too little time in contact with the cold surface at part load.
400–500 FPM: The practical sweet spot. Pressure drop stays manageable and the coil operates efficiently.
Above 550 FPM: Condensate droplets can be blown off the fin surface and carried into downstream ductwork — this is called carryover, and it is a code violation and a mold risk. Noise also increases sharply.

Where this number goes in your calculation

The coil airside pressure drop is one component of the total external static pressure (TESP) the air handler blower must develop. Add it to:

Supply and return filter pressure drops
Duct friction and fitting losses (from an equivalent-length calculation)
Register and grille pressure drops
Any mixing box, damper, or heat-recovery device losses

The sum is the TESP at design CFM. Select a blower and motor that deliver your required airflow at that static pressure from the manufacturer’s fan curve.