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Mixing and Coil Process

The two processes that make up an air handler: adiabatic mixing of a return-air stream with an outdoor-air stream, and the coil that takes the mixture to its leaving state. The mixed state follows from the mass-weighted humidity ratios and enthalpies; the coil load is the enthalpy change times the dry-air mass flow, split into sensible and latent parts with the sensible heat ratio and the condensate rate for a cooling coil (or the sensible heating load for a heating coil, whose leaving humidity ratio equals the entering one). The states are drawn on a psychrometric chart. Temperatures are plain numbers in °F (USCS) or °C (SI).

  • Mixed-air state from two streams by their dry-air mass flows

  • Coil total, sensible and latent loads, sensible heat ratio, condensate rate, apparatus dew point check

  • Psychrometric chart with the return, outdoor, mixed and leaving states and the process lines

Mixing and Coil Process

Unit System: /

Return Air (temperatures °F for USCS, °C for SI)




Outdoor Air




Coil and Altitude





Mixing and Coil Processes | Method page
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Mixing and Coil Process

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(Wm − Wl). 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.



References

American Society of Heating, Refrigerating and Air-Conditioning Engineers (2021). ASHRAE Handbook: Fundamentals. ASHRAE, Peachtree Corners, GA.

McQuiston, F. C., Parker, J. D., and Spitler, J. D. (2005). Heating, Ventilating, and Air Conditioning: Analysis and Design, 6th ed. John Wiley and Sons, Hoboken, NJ.

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