Cooling-coil bypass factor

Estimate temperature-based bypass and contact factors using a known apparatus dew point.

How this calculation works

This temperature-based check describes how far the leaving air remains from an assumed effective coil temperature. Supply the entering dry bulb, leaving dry bulb and apparatus dew point from a suitable coil model or design source. The apparatus dew point is an input, not a value inferred by this calculator. Subtract it from the entering and leaving temperatures, then divide the leaving difference by the entering difference. A smaller bypass factor means the leaving temperature is closer to the apparatus dew point under this model. The complementary contact factor is displayed as a percentage. Leaving temperature must lie between the apparatus dew point and entering temperature. These factors describe a simplified thermal model; they do not measure physical air leakage or establish latent capacity or a coil selection.

Inputs and units

Entering dry bulb (°C)
Leaving dry bulb (°C)
Apparatus dew point (°C): Effective coil-surface temperature supplied by the designer or coil model.

Method and formula

BF = (Tleave−Tadp)/(Tenter−Tadp); contact factor = 1−BF.

Worked example

Example inputs

Entering dry bulb: 27 °C
Leaving dry bulb: 15 °C
Apparatus dew point: 12 °C

Calculation steps

Entering-to-ADP difference = 27 − 12 = 15 K; leaving-to-ADP difference = 15 − 12 = 3 K.
Bypass factor = 3 / 15 = 0.2 fraction. Contact factor = (1 − 0.2) × 100 = 80%.

Example results

Bypass factor: 0.2 fraction
Contact factor: 80 %

Assumptions

A single effective apparatus dew point represents the coil.
Temperature ratio approximates the air-side bypass model.

Limitations

Does not determine ADP, latent capacity, physical air leakage, or coil selection.
Use enthalpy-based manufacturer methods when sensible heat ratios or conditions change.
Preliminary educational check; verify inputs and equipment data before design or operation.

Sources

Texas A&M: Psychrometrics: Printed page 47, equation 1.34: temperature-based coil bypass factor