Mixed Air Enthalpy
& Bypass Factor
Audit outdoor and return air mixing ratios and cooling coil bypass effectiveness coefficients.
Psychrometric Mixing Thermodynamics & Coil Bypass Factor Physics Overview
Mechanical air handling systems often pull a portion of their supply air from the hot, humid outdoor atmosphere (Fresh Air Intake) and combine it with cooler, drier indoor air returning from the structure (Return Air). The thermodynamics of this mixing process is linear: the final Mixed Air enthalpy is simply the weighted average of both streams based on their volumetric ratios. However, the efficiency of the cooling coil is not absolute. No cooling coil has 100% heat transfer efficiency; a certain volume of air always travels through the finned structure without ever contacting a cooling surface—this is the **Coil Bypass Factor**. A high bypass factor means “short-circuiting” air is delivering unconditioned humidity into the building, significantly damaging the sensible heat ratio (SHR) and preventing proper dehumidification.
Frequently Asked Questions
A: Latent moisture removal only happens when air strikes a coil surface that is below the dew point. If 30% of the air bypasses the coil due to wide fin spacing or surface fouling, that air enters the room exactly as moist as it was entering the unit. This moisture is never extracted, forcing the indoor relative humidity to creep up, creating a “clammy” environment despite the air temperature technically feeling cool.
A: Bypass factor is heavily dependent on face velocity (the speed at which air moves through the coil). If air moves too fast, contact time is too short for moisture to condense. By using a VFD to slow down the blower motor when the cooling demand is low, the air “dwell time” on the coil increases significantly. This extra contact duration lowers the bypass factor, allowing the coil to be much more effective at scrubbing humidity out of the air stream.