Interactive Mollier i-x Chart
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State points
| Point | t [°C] | φ [%] | x [g/kg] | h [kJ/kg] | t_r [°C] | t_m [°C] | ρ [kg/m³] | |
|---|---|---|---|---|---|---|---|---|
A | -5,0 | 80,0 | 1,98 | -0,10 | -7,6 | -5,9 | 1,315 | |
B | 22,0 | 12,2 | 1,98 | 27,16 | -7,6 | 9,2 | 1,195 |
t [°C]
φ [%]
Processes
A → B
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RegisterMollier i-x chart online — psychrometric calculator for HVAC
Interactive Mollier i-x chart for analyzing humid air properties and designing thermodynamic processes in ventilation, air conditioning and heat recovery systems. The calculator derives the full set of state-point parameters — temperature t [°C], relative humidity φ [%], humidity ratio x [g/kg], specific enthalpy h [kJ/kg], dew point t_d [°C], wet-bulb temperature t_w [°C] and density ρ [kg/m³] — from any two input values.
The tool supports all standard air processes used when designing AHUs: heating, dry cooling, cooling with dehumidification on a coil (ADP), adiabatic humidification, steam humidification and stream mixing. For each process the calculator returns thermal capacity [kW], water or steam consumption [kg/h] and the resulting end point on the chart. Export the project to PDF with a chart drawing and point table.
How to use the Mollier calculator in 3 steps
Add a state point: click anywhere on the chart (creates a point at the clicked t, x) or enter temperature t and relative humidity φ in the form on the right. The point appears in the table with the full parameter set (x, h, t_dew, t_wet, ρ). You can also drag points — all values update live.
Define a process: in the “Processes” panel pick the type (heating, dry cooling, cooling coil, adiabatic/steam humidification, mixing), choose the start point and target parameter (e.g. end temperature or ADP for a coil). Enter the dry-air mass flow in kg/h. The calculator computes the end point and draws the process vector on the chart.
Read results and save the project: the process table lists thermal capacity in kW, condensate flow (coil) or supply water/steam (humidifiers) in kg/h, plus warnings (dropping below dew point, fog risk in adiabatic humidification). Export the whole project to PDF with chart, point table and process list — ready as a design attachment.
What the Mollier calculator computes
Based on the points and processes you enter, the calculator returns:
- State point properties — for each point the full set: t, φ, x, h, t_dew, t_wet, ρ. Values come from standard psychrometric equations (Magnus-Tetens for saturation vapor pressure, ideal-gas equation of state) at atmospheric pressure 101.325 kPa.
- Thermodynamic processes — process vector on the chart, process type (auto-classified when you connect two points), end point and ΔH, Δx increments.
- Thermal and water capacity — for the given dry-air mass flow: heating/cooling coil power in kW (Q = ṁ·Δh), condensate rate on the cooling coil in kg/h, water/steam consumption of humidifiers.
- Design warnings — messages about unintended dropping below the dew point, fog risk in adiabatic humidification (exceeding φ = 100%), unreachable end point for the chosen coil (ADP above stream dew point).
Humid air parameters — what they mean
Every point on the Mollier chart is a set of interrelated thermodynamic quantities. Two independent parameters fully define the state of air — the calculator fills in the rest:
Dry-bulb temperature t [°C]
Temperature measured with a regular thermometer, independent of humidity. Vertical chart axis. Typical HVAC design values for Central Europe: winter t_w = −20 °C, summer t_s = 32 °C.
Relative humidity φ [%]
Ratio of actual water vapor pressure to saturation pressure at the same temperature. Orange curves on the chart, 10 % to 100 %. Comfort range for residential spaces: 40–60 %. Above 70 % promotes mold; below 30 % irritates mucous membranes.
Humidity ratio x [g/kg of dry air]
Mass of water vapor per 1 kg of dry air (constant reference mass — why x is used in ventilation balances). Skewed chart axis. Heating and dry cooling do not change x — a vertical move along x = const on the chart.
Specific enthalpy h [kJ/kg]
Total thermal energy of 1 kg of humid air (sum of sensible heat and latent heat of water vapor). Skewed lines on the chart. Key quantity for sizing heating and cooling coils: Q = ṁ · Δh, where ṁ is the dry-air mass flow in kg/s.
Dew point temperature t_d [°C]
Temperature at which the current humidity ratio x reaches saturation (φ = 100 %). Critical for assessing condensation risk on building envelopes, pipes and cooling coil surfaces. If t_surface < t_dew — water vapor condenses on it.
Wet-bulb temperature t_w [°C]
Temperature a thermometer wrapped in wet gauze reaches under adiabatic humidification to saturation. Used in sizing evaporative coolers and adiabatic humidifiers — this is the minimum temperature achievable without external cooling.
Humid air density ρ [kg/m³]
Mass of 1 m³ of humid air at given conditions. Affects the conversion between volumetric flow (m³/h) and mass flow (kg/h) — important for energy balances. Standard value for t = 20 °C, φ = 50 %: ρ ≈ 1.20 kg/m³; for outdoor air in winter (−20 °C): ρ ≈ 1.39 kg/m³.
Air processes on the Mollier chart
The calculator supports 6 process types — in a real AHU they chain together: filtration → heat recovery → heating coil → cooling coil → humidifier → fan. Each process has its characteristic path on the chart:
Heating (heating coil) — vertical move up
Air absorbs sensible heat with no change in humidity ratio (x = const). Temperature and enthalpy rise, relative humidity φ drops (dry heating!). Coil capacity: Q = ṁ · c_p · Δt ≈ ṁ · Δh. Typical use: secondary heating coil in an AHU, radiators, heat-pump coil in heating mode.
Dry cooling — vertical move down (above t_dew)
Cooling at x = const, when the heat-exchanger surface temperature stays above the air dew point. No condensation; all the capacity is sensible heat. Typical for cooling already dehumidified air or at small temperature differences (e.g. heat-recovery-only cooling in shoulder seasons).
Cooling coil with dehumidification (ADP) — straight line to ADP
When the coil surface temperature (ADP — apparatus dew point) is below the stream dew point, water vapor condenses on the fins. The real process path is a straight line from the inlet directly to ADP on the saturation curve (φ = 100 %); the air does not first hit saturation and then slide along it. The actual outlet point is determined by the bypass factor (BF) — the fraction of air that bypasses ADP and mixes with the stream cooled to ADP. The calculator returns total capacity (sensible + latent) and condensate flow.
Adiabatic humidification (water) — along an isenthalp (h ≈ const)
Water is sprayed into the air stream. It evaporates by drawing heat from the air — temperature falls, humidity rises, enthalpy stays nearly constant (ideally h = const). Used for “natural” summer pre-cooling (adiabatic cooling). Limit: you cannot cross the saturation line — fog appears.
Steam humidification — x rises with a small t increase
Water vapor with high enthalpy (≈ 2675 kJ/kg at 100 °C) is injected into the stream. The humidity ratio x rises dominantly; temperature increases noticeably (steam at ~100 °C is much hotter than supply air at 20–25 °C), although that rise is modest compared with the latent enthalpy gain. The process direction on the chart depends on the steam enthalpy. Used in hospitals, server rooms, museums — anywhere precise humidity control is required without the biological contamination risk of adiabatic humidification.
Mixing two streams — a point on line A-B
Joining two air streams with different parameters (e.g. return + outdoor air). The mix point lies on the straight line between A and B, weighted by dry-air mass flow ratio (lever rule): x_mix = (ṁ_A·x_A + ṁ_B·x_B) / (ṁ_A + ṁ_B), analogously for h.
Practical design tips
When designing an AHU, always draw the entire process chain on one chart — this catches inconsistencies (e.g. a secondary heating coil sized “cold” instead of after the cooling coil). For a cooling coil with dehumidification, remember that real capacity depends on the bypass factor (BF) — the fraction of air that bypasses the ADP. Typical BF for a 4-row coil: 0.15–0.25; 6-row: 0.05–0.15. Lower BF = better dehumidification but higher pressure drop. For adiabatic humidification, check that the end point stays below φ ≈ 90 % — above that, fog forms and duct corrosion risk rises. In secondary heating coil sizing, remember winter conditions: for Central Europe (t_w = −20 °C) and ~30 % room humidity after humidification, the secondary heater typically reaches 30–50 % of the primary heater capacity. Room-level control target: relative humidity should stay above 30 % in winter (comfort) and below 60 % in summer (hygiene).
Related ventilation calculators
The Mollier chart is the starting tool for AHU design — complete it with calculators for sizing individual components: