Wet Coil Analysis: Understanding Dehumidification in Cooling Coils

When a cooling coil operates with its surface temperature below the dew point of the entering air, moisture condenses on the coil surface. This "wet coil" condition is fundamental to air conditioning, as it provides the dehumidification necessary for occupant comfort. Analyzing wet coil performance requires understanding the simultaneous heat and mass transfer processes that occur.

Dry vs. Wet Coil Operation

A dry coil operates when the coil surface temperature is above the dew point of the air. In this case, only sensible heat transfer occurs, and the analysis is straightforward using the standard UA-LMTD or NTU-effectiveness methods.

A wet coil operates when the surface temperature drops below the air dew point. Both sensible heat transfer (temperature reduction) and latent heat transfer (moisture removal) occur simultaneously. The total heat transfer includes:

Q_total = Q_sensible + Q_latent

Where Q_latent = ṁ_air × h_fg × (W_in - W_out), with h_fg being the latent heat of vaporization of water (approximately 2,501 kJ/kg at 0°C) and W being the humidity ratio.

The Enthalpy Potential Method (Threlkeld)

The most rigorous approach to wet coil analysis is the enthalpy potential method, developed by Threlkeld (1970). This method recognizes that the driving force for combined heat and mass transfer is the difference in enthalpy between the air and the saturated air at the coil surface temperature.

The key equation is:

dQ = U_o × dA × (h_air - h_s,w) / c_p,m

Where:

This approach is more accurate than treating sensible and latent heat transfer separately because it properly accounts for the coupling between temperature and humidity changes.

The Sensible Heat Ratio (SHR)

The SHR is defined as the ratio of sensible heat transfer to total heat transfer:

SHR = Q_sensible / Q_total

Typical SHR values for HVAC applications:

The SHR depends on the entering air conditions, coil surface temperature, and coil geometry. Lower coil surface temperatures and more rows generally produce lower SHR values (more dehumidification).

Apparatus Dew Point (ADP)

The apparatus dew point is the theoretical temperature at which the coil would bring the air if it had infinite surface area. It represents the saturation point on the psychrometric chart where the condition line intersects the saturation curve.

The bypass factor (BF) represents the fraction of air that passes through the coil without being affected:

BF = (T_leaving - T_ADP) / (T_entering - T_ADP)

Typical bypass factors range from 0.02 to 0.15, depending on the number of rows and fin density. More rows and higher fin density reduce the bypass factor.

Wet Surface Enhancement

An interesting phenomenon in wet coil operation is that the condensate film on the fin surface can actually enhance heat transfer compared to dry operation. The thin water film increases the effective thermal conductivity at the surface and promotes better air-side heat transfer. Research has shown enhancement factors of 1.1 to 1.2 for wet coils compared to dry coils.

However, heavy condensation can also increase air-side pressure drop by 20-50% compared to dry operation, as water droplets partially block the air passages between fins.

Practical Implications for Coil Design

Fin spacing: Wet coils require wider fin spacing (lower FPI) than dry coils to allow condensate drainage and prevent water bridging between fins. A maximum of 14 FPI is recommended for cooling coils.

Fin surface treatment: Hydrophilic coatings help condensate form a thin film rather than droplets, improving drainage and reducing pressure drop.

Coil orientation: Cooling coils should be oriented so that condensate drains by gravity. The coil should be tilted slightly (1-2°) toward the drain pan.

Drain pan design: Adequate drain pan sizing and slope are essential to prevent standing water, which can promote biological growth and reduce air quality.

Computational Approach

Modern coil selection software implements the enthalpy potential method using a segmented approach, dividing the coil into small elements and solving the coupled heat and mass transfer equations iteratively. This approach accounts for:

The ExCoil engine uses this segmented approach with validated correlations to predict wet coil performance, including the transition between dry and wet operation that occurs when the surface temperature crosses the dew point.

Conclusion

Wet coil analysis is essential for accurate cooling coil sizing in HVAC applications. The enthalpy potential method provides the most rigorous framework for predicting combined sensible and latent heat transfer. Understanding these principles helps engineers design coils that deliver the required dehumidification while maintaining energy efficiency and proper condensate management.