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 on 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 drop in parallel below saturation. 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 (≈8.3 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 (≈42 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 (electrode, resistive, steam-to-steam) injects steam that is already at a high temperature (~100 °C average), so the process is practically vertical, dry-bulb temperature changes very little 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 gives free cooling; but if the indoor temperature is fixed, compensation reheat is needed.
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 = 6.7 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 | 6.7 | Vertical up |
| 3-alt. After adiabatic humidification | x rises, T falls | ~9 | ~95 | 6.7 | 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 g/kg); when run over a cooling coil it first drops in temperature, then upon reaching dew point condensation begins and absolute humidity falls. On the chart this looks like a path that follows the saturation curve.
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 | ~0.03–0.1 kWh / kg |
| Hygiene / sterility | High (steam is sterile) | Depends on water (RO/UV needed) |
| Setpoint band | ±1–5% RH | ±5–10% 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 = 4.98 g/kg) and going to 22 °C / 50% RH (x = 8.33 g/kg) requires Δx = 3.35 g/kg. At 10,000 m³/h airflow the capacity is: ṁ = ρ × V̇ × Δx = 1.2 × 10,000 × 0.00335 = 40.2 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 horizontal distance between them | Δx (g/kg) |
| 4 | Apply ρ × V̇ × Δx | ṁ_steam (kg/h) |
| 5 | Add 10–20% margin | Selection capacity |
Common misreadings of the chart on the field translate directly into wrong engineering decisions. The list below summarises the most frequent interpretation errors observed across NKT projects.
| 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 | Keep design point below 95% RH |
| 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 Humidity Control Technologies uses the psychrometric chart not only as a reading tool but as a visualised checkpoint for engineering decisions on every project. The standard NKT project report shows starting and target points marked on the chart; this visualisation clarifies communication with the customer and prevents misunderstandings.
The psychrometric calculators available on the NKT portal (absolute-humidity converter, dew-point calculator, capacity calculator) use the same equations and produce results that match the chart. On existing facilities, pre-commissioning field measurements mark the actual operating point on the chart; deviations from the design point are then traced systematically (filter loading, damper position, water quality).
With the Neptronic humidifier range, NKT ensures the humidification side operates consistently and predictably on the chart. In static-electricity-sensitive ESD production areas, lithium-battery dry rooms and tight-spec print halls, this engineering approach 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. The NKT approach pairs chart-based visualisation with numerical calculation on every project; this method strengthens both the transparency of engineering decisions and the technical communication with the customer.