Engineering Guide
The psychrometric chart is the standard HVAC engineering tool for reading the thermodynamic state of moist air on a single graph. It visualises the relationship between temperature, relative humidity, absolute humidity, dew point and enthalpy; it makes humidification, dehumidification, cooling, heating and mixing processes readable as graphical paths. This guide is a practical reading manual for engineers who want to use the chart in the field.
The psychrometric chart is a 2D graphical tool that visualises every thermodynamic property of moist air at a given pressure. The horizontal axis is typically dry-bulb temperature (°C) and the vertical axis is absolute humidity (g/kg or kg/kg). The curves and lines on the chart let you read, for any single air state, the relative humidity (%RH), wet-bulb temperature, dew point, enthalpy and specific volume.
The value of the chart for HVAC engineering is not just easy reading, it is the ability to draw physical processes (humidification, heating, cooling, mixing) as visible paths. An arrow from one point to another shows exactly what change is taking place. The capacity number read off the chart matches the formula-based result; the visual reading speeds up understanding and error detection.
Reading a psychrometric chart correctly requires familiarity with seven curve groups.
| Curve / Axis | Direction | Symbol / Unit | Description |
|---|---|---|---|
| Dry-bulb temperature | Horizontal (bottom) | T_db, °C | Standard thermometer reading |
| Humidity ratio (absolute humidity) | Vertical (right) | x, g/kg | Mass of water vapour per kg of dry air |
| Relative humidity curves | Curves below saturation | RH, % | 10%, 20%, ..., 100% (saturation curve) |
| Wet-bulb lines | Sloping from saturation to the left | T_wb, °C | Constant wet-bulb temperature lines |
| Dew point | Horizontal from saturation | T_dp, °C | Temperature where condensation starts |
| Enthalpy lines | Sloping outside saturation | h, kJ/kg | Total energy content |
| Specific volume | Sloping left to right | v, m³/kg | Volume per kg of air |
The saturation curve (100% RH) is the most critical reference, air cannot exist above this curve; reaching it triggers condensation. Relative-humidity curves run below saturation as proportional copies of it; the vertical gap between them widens as temperature rises. Constant wet-bulb lines and constant enthalpy lines are practically very close (psychrometric identity) and are treated as the same line, especially for adiabatic processes.
To locate an air state on the chart, two independent properties are sufficient. The most common pair is dry-bulb temperature + relative humidity; but temperature + dew point, temperature + wet-bulb, or absolute humidity + temperature also work.
For instance, to locate 22 °C / 50% RH: mark 22 °C on the horizontal axis, go up vertically; the intersection with the 50% RH curve is point A. From this point a horizontal line to the right axis reads off the absolute humidity (≈8.2 g/kg). From the same point, a horizontal line to the saturation curve gives the dew-point temperature (≈11 °C). Following the sloping enthalpy line gives the total energy content (≈43 kJ/kg).
Humidification is the addition of moisture, so on the chart the air state moves upward from its starting point. The direction is set by the type of humidification.
Steam humidification (for example a resistive steam humidifier or a steam-to-steam system fed by plant steam) injects steam generated at about 100 °C, so the process is practically vertical; dry-bulb temperature changes by only a few tenths of a degree while absolute humidity rises significantly. This makes steam humidification ideal for applications with strict indoor temperature specifications (printing, museums, precision manufacturing).
Adiabatic humidification (atomising, wetted media) introduces water at ambient temperature; the latent heat needed for evaporation is drawn from the air itself. So the process follows the constant-enthalpy (wet-bulb) line up and to the left, absolute humidity rises while temperature falls. In summer this provides an evaporative cooling effect; but if the indoor temperature must be held, the heat drawn from the air has to be returned by reheat.
In winter, conditioning outdoor air for use takes two steps: first heat, then humidify. On the chart these two steps are read clearly and drawn together.
Example: outdoor air arrives at 5 °C / 30% RH (≈1.6 g/kg). It is first run through a heating coil and brought to 22 °C. In this step, absolute humidity does not change (no water is added), temperature rises, and relative humidity drops dramatically, the new state is 22 °C / 10% RH (still 1.6 g/kg). On the chart this is a horizontal arrow to the right. In the second step the humidifier brings the absolute humidity to target (e.g. 40% RH at 22 °C ≈ 6.6 g/kg); steam draws a vertical-up line, adiabatic draws a sloping line up and to the left.
| Step | Process | T (°C) | RH (%) | x (g/kg) | Direction on Chart |
|---|---|---|---|---|---|
| 1. Outdoor air | Inlet | 5 | 30 | 1.6 | Starting point |
| 2. After heating | Constant x | 22 | ~10 | 1.6 | Horizontal right |
| 3. After steam humidification | x rises, T constant | ≈22 | 40 | 6.6 | Vertical up |
| 3-alt. After adiabatic humidification | x rises, T falls | ≈10 | ≈90 | 6.6 | Constant enthalpy (up-left) |
The chart's strength is not limited to humidification, cooling + dehumidification can be read on the same chart. In summer outdoor air is hot and humid (e.g. 32 °C / 65% RH ≈ 19.5 g/kg, dew point ≈ 24.6 °C). Air passing a cooling coil first cools at constant humidity ratio, moving left on the chart; once it reaches the dew point, condensation begins and the humidity ratio falls. On a real coil this path is drawn as an approximately straight line from the inlet state towards the coil surface temperature.
In industrial use the two processes typically work together: cooling + dehumidification in summer, heating + humidification in winter. Both cycles inside the same AHU are planned on the psychrometric chart; setpoint shifts at seasonal transitions can be verified instantly.
Once the chart shows how the two humidification types differ, the comparison table below guides selection.
| Criterion | Steam Humidification | Adiabatic Humidification |
|---|---|---|
| Working principle | Water injected as steam (~100 °C) | Water atomised/evaporated (ambient T) |
| Temperature impact | Negligible (T constant) | Significant drop (~2.5 °C / g/kg) |
| Humidity impact | Absolute humidity rises | Absolute humidity rises (same Δx) |
| Direction on chart | Vertical up (T constant, x rises) | Constant enthalpy (up-left, T falls) |
| Energy consumption | ~0.75 kWh / kg (electric steam) | Low pump energy; if indoor temperature is held, the ≈0.68 kWh/kg drawn from the air is returned by reheat |
| Hygiene / sterility | High (steam is generated by boiling) | Depends on water (RO/UV needed) |
| Setpoint band | Tight (down to ±1% RH for resistive units; Neptronic data) | Wider (typically ±3–5% RH) |
| Typical use | Printing, museum, hospital, pharma, precision | Textile, warehouse, data centre, hybrid cooling |
Humidification capacity is computed from the Δx delta read between two points on the chart. After the start and target points are marked, their absolute humidity values are read on the vertical axis and subtracted.
Example: starting at 22 °C / 30% RH (x ≈ 4.9 g/kg) and going to 22 °C / 50% RH (x ≈ 8.2 g/kg) requires Δx ≈ 3.3 g/kg. At 10,000 m³/h airflow the capacity is: ṁ = ρ × V̇ × Δx = 1.2 × 10,000 × 0.0033 ≈ 40 kg/h. Adding a 20% service margin gives a selection capacity of about 48 kg/h. The same answer is obtained from the formula or by reading the chart.
| Step | Action on the Chart | Output |
|---|---|---|
| 1 | Mark the starting point (T_in, RH_current) | x_current |
| 2 | Mark the target point (T_in, RH_target) | x_target |
| 3 | Read the vertical distance (humidity-ratio difference) between them | Δx (g/kg) |
| 4 | Apply ρ × V̇ × Δx | ṁ_steam (kg/h) |
| 5 | Add a 15–25% (typically 20%) service margin | Selection capacity |
Common misreadings of the chart in the field translate directly into wrong engineering decisions. The list below summarises the most frequent interpretation errors seen in the field.
| Mistake | Outcome | Correct Approach |
|---|---|---|
| Treating RH delta as mass directly | Wrong capacity | Convert to absolute humidity first, then take Δx |
| Confusing dew point with wet-bulb | Cooling coil mis-sized | Read each property separately |
| Design point above the saturation curve | Non-physical, condensation occurs | Limit duct supply RH to 85–90% with a high-limit humidistat |
| Drawing adiabatic vertical | Temperature drop ignored | Draw along constant enthalpy (up-left) |
| Treating steam as constant-enthalpy | Steam energy ignored | Draw vertical (T constant) |
| Skipping the mixed-air state | Outdoor-air impact missed | Mark intermediate point from mix ratio |
| Using summer design for winter capacity | Insufficient at peak | Separate design points per season |
NKT – Nem Kontrol Teknolojileri uses the psychrometric chart in device selection and supply not only as a reading tool but as a visual checkpoint for the selection decision. Showing the start and target points on the chart clarifies technical communication with the customer and prevents misunderstandings.
The psychrometric calculator on the NKT website (absolute humidity, dew point, enthalpy) uses the same equations and produces results consistent with the chart. When temperature and RH measured at an existing site are plotted on the chart, the actual operating point can be compared with the design target and the device selection checked against that difference.
NKT selects and supplies the Neptronic humidifier family that suits the application (resistive SKE4, steam-to-steam SKS4 and high-pressure atomisation SKH). In static-electricity-sensitive ESD production areas, museums and archives, and tight-spec print halls, correct device selection is the basis of operational continuity.
The psychrometric chart is the visual language of humidification system design. Knowing how to locate an air state on the chart, how humidification processes are drawn (steam vertical, adiabatic constant-enthalpy), and how heating and cooling steps integrate, is core engineering literacy for correct device selection and capacity sizing.
Reading capacity off the chart is the visual verification of the formula calculation; both methods give the same number, but the chart speeds up understanding and error detection. Using chart-based visualisation together with numerical calculation makes the device-selection decision transparent and strengthens technical communication with the customer.