When to use: Plot air states on an interactive psychrometric chart and simulate HVAC processes — heating, cooling, humidification, dehumidification, and air mixing. Watch the process lines animate between states and see all psychrometric properties update in real time.
This simulator plots air states on an interactive psychrometric chart and animates HVAC processes — sensible heating, cooling and dehumidification, humidification, and outdoor/return air mixing. Engineers use it to visualize air-side thermodynamics, calculate process enthalpy changes, and verify coil performance during design and commissioning.
The psychrometric chart defines the thermodynamic state of moist air at a given atmospheric pressure. Dry-bulb temperature (DBT) is plotted on the horizontal axis; humidity ratio (w, in g/kg or lb/lb of dry air) on the vertical axis. From these two properties, all others are derived. Wet-bulb temperature (WBT) is calculated using the Stull approximation: WBT ≈ DBT × arctan[0.152 × (RH + 8.31)^0.5] + arctan(DBT + RH) − arctan(RH − 1.68) + ...
Humidity ratio is calculated from saturation vapor pressure using the Magnus formula: pws = 0.61078 × exp(17.625 × T_C / (243.04 + T_C)) kPa. Dew point is derived from the inverse formula. Enthalpy (kJ/kg of dry air) combines sensible and latent components: h = 1.006 × T_C + w × (2501 + 1.86 × T_C).
AHU coil performance is analyzed on the psychrometric chart using the sensible heat ratio (SHR): SHR = sensible cooling / total cooling. A cooling coil process line from room conditions to the apparatus dew point (ADP) — the saturation curve intersection — defines both the leaving air temperature and humidity ratio. Lower SHR indicates more latent (dehumidification) work.
ASHRAE Standard 55 defines the psychrometric comfort zone — the region of DBT, humidity ratio, and airspeed within which at least 80% of occupants are satisfied. The operative temperature comfort range is approximately 67–82°F DBT depending on humidity and activity level. ASHRAE 55 also sets upper limits on humidity ratio (typically 0.012 lb/lb = 75% RH at 75°F).
ASHRAE Standard 62.1 references ASHRAE 55 for IAQ design. ASHRAE Fundamentals Handbook Chapter 1 provides the complete psychrometric equations and chart data. Equipment ratings per ARI Standard 210/240 and ARI 550/590 are based on psychrometric test conditions (e.g., 80°F DB / 67°F WB entering air for cooling coils).
The cooling process on the psychrometric chart follows a line from the room state toward the apparatus dew point (ADP) — the temperature at which moisture begins to condense on the coil surface. A lower ADP requires a colder chilled water supply temperature or more coil rows. The bypass factor determines how much air passes through the coil without contacting the surface.
Air mixing is critical for economizer calculations: when outdoor air (OA) and return air (RA) are mixed, the mixed air state lies on the straight line connecting OA and RA states on the psychrometric chart, at a point proportional to the OA fraction. This determines whether the mixed air is above or below the dew point, which affects condensation risk and cooling coil load.
Select a process type — heating, cooling, humidification, dehumidification, or air mixing. For heating and cooling, set the starting state (dry-bulb and RH) and the target state. The animated dot traces the process line on the chart, and the psychrometric properties update in real time.
For air mixing, set the outdoor air (OA) and return air (RA) states, then adjust the OA fraction slider to see the mixed air state shift along the line connecting both points. Use the Process Delta panel to read the change in dry-bulb temperature, humidity ratio, and enthalpy — these drive coil sizing calculations.
Dry-bulb temperature (DBT) is standard air temperature measured by a thermometer not affected by moisture. Wet-bulb temperature (WBT) is measured by a thermometer with a wet wick — as moisture evaporates, cooling occurs. The difference between DBT and WBT is the wet-bulb depression, which indicates relative humidity. At 100% RH, DBT = WBT. WBT cannot exceed DBT for air below saturation.
Enthalpy is the total heat content of moist air per unit mass of dry air, combining sensible heat (temperature-related) and latent heat (moisture-related). Coil capacity is calculated as Q = CFM × 4.5 × Δh, where Δh is the enthalpy difference in BTU/lb and 4.5 = 60 min/hr × 0.075 lb/ft³. Knowing enthalpy rather than just temperature is essential for accurately sizing dehumidifying cooling coils.
SHR = sensible cooling load / total cooling load. A room with high occupancy, moisture generation, or infiltration has a low SHR (say 0.6–0.7), meaning 30–40% of the cooling is latent (moisture removal). A room with mainly solar and equipment loads may have SHR of 0.9. The cooling coil must be selected to match the room SHR — if the coil's SHR is too high, the room will be too humid even if temperature is correct.
The ADP is the effective coil surface temperature — the dew point temperature at which moist air would be saturated if it fully contacted the coil surface. On the psychrometric chart, the process line from room air to the ADP determines leaving conditions. The bypass factor (BF) represents the fraction of air that bypasses the coil: leaving conditions = room × BF + ADP × (1 − BF). Lower BF means more rows or fins.
The conservation of mass and energy equations for moist air mixing are both linear. For two air streams with states 1 and 2 mixed in proportion x (fraction of stream 1): T_mix = x × T1 + (1−x) × T2, and w_mix = x × w1 + (1−x) × w2. Since both T and w vary linearly with x, the mixed state lies on the straight line connecting states 1 and 2 on the psychrometric chart.
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