Mixing and Coil Process: Method

Mixing and Coil Processes

An air handler mixes return air with the outdoor air the ventilation code demands and passes the mixture through a coil. Both steps are mass balances on dry air, water vapor and energy.

Mixing

The streams mix without heat exchange with the surroundings, so the mixed state has the mass-weighted humidity ratio and enthalpy of the two streams, weighted by their dry-air mass flows (the volume flows divided by the specific volumes, which differ because the streams are at different temperatures). On the psychrometric chart the mixed state lies on the straight line between the two streams, dividing it in the inverse ratio of their masses. The mixed dry bulb is recovered from the enthalpy relation.

Cooling coil

The coil removes the enthalpy difference between the mixed and leaving states times the mass flow. The sensible part is the dry-bulb change at the humid specific heat, the latent part is the rest and corresponds to the condensed water, ṁ(W_m − W_l). Air leaves a wet coil near saturation, 85 to 95 percent relative humidity, with a dry bulb a few degrees above the coil's apparatus dew point; the leaving state entered here should be consistent with that. The sensible heat ratio, sensible over total, is what the room's load line must match for the supply air to hold the room condition.

Heating coil

A heating coil adds sensible heat only; the humidity ratio stays constant and the state moves horizontally on the chart.

Temperatures are entered in the units of the selected system because their scales carry offsets. Fan heat, coil bypass factor and the room process are not included.