Engineering tools · Free to use
Moist-Air Sensible and Moisture Enthalpy Split
Choose a virtual intermediate state to split the air-side enthalpy change into a temperature term at fixed humidity ratio and a moisture enthalpy term at fixed temperature. The latter is sometimes used as a latent approximation; it is not separately measured phase-change latent heat or complete equipment duty.
Inputs
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Results
Positive means the air-side enthalpy increases; negative means it decreases. Both terms sum to the air-side total for the selected reference state.
Method and reference
State 1 is inlet and state 2 outlet. For moisture first, x=(T₁,w₂): the temperature term is ṁda(h₂−hx), the moisture term ṁda(hx−h₁). For temperature first, x=(T₂,w₁): the temperature term is ṁda(hx−h₁), the moisture term ṁda(h₂−hx).
The PsychroLib SI approximation is h=1.006T+w(2501+1.86T), with h in kJ/kg dry air, T in °C, w in kg water vapour/kg dry air. Dry air and liquid water are assigned zero enthalpy at 0 °C; water vapour at 0 °C is approximately 2501 kJ/kg relative to that liquid-water reference.
Applicability and limits
Steady state at common absolute pressure with equal dry-air flow; inlet, outlet and intermediate states are unsupersaturated moist air without droplets or ice crystals. Saturation checks follow the PsychroLib phase convention: vapour pressure over ice at or below 0.01 °C and over liquid water above 0.01 °C; supercooled droplets, condensation and frost formation are not simulated. Temperature range −20 to 50 °C, absolute pressure 50–120 kPa and non-negative humidity ratio. The path is a mathematical decomposition, not the sequence of the actual process; the terms may differ by path while their sum remains the same.
The constant-temperature moisture term uses a water-vapour enthalpy reference and may approximate a latent component; it is not the latent heat of phase change hfg and does not necessarily equal actual equipment latent load. Enthalpy carried by injected water, steam or condensate and heat loss must be considered separately. It cannot determine whether condensation occurs or directly establish coil duty.
Example and sources
1 kg/s, 30 °C and 12 g/kg → 15 °C and 8 g/kg at 101.325 kPa. Moisture-first gives intermediate 30 °C and 8 g/kg: temperature term −15.3132 kW, moisture term −10.2272 kW, total −25.5404 kW.
MIT OpenCourseWare: moist air, virtual paths and drain-water energy balance; PsychroLib 2.5.0, Copyright © 2018–2020 PsychroLib Contributors, MIT License.
For understanding air-side enthalpy change under the selected reference and path; equipment sizing requires a complete mass and energy balance.