Calculator Inputs
Example Data Table
| Dry-bulb (°C) | Relative Humidity (%) | Pressure (kPa) | Humidity Ratio w (kg/kg) | Enthalpy h (kJ/kg) |
|---|---|---|---|---|
| 25 | 50 | 101.325 | 0.009881 | 50.322 |
| 30 | 70 | 101.325 | 0.018795 | 78.236 |
| 15 | 30 | 101.325 | 0.003156 | 23.072 |
Formula Used
1) Saturation vapor pressure
The calculator computes saturation vapor pressure Pws from temperature using widely used ASHRAE correlations (water above 0 °C, ice below 0 °C).
2) Water vapor partial pressure
Pv = RH × Pws (RH as a fraction from 0 to 1).
3) Humidity ratio
w = 0.621945 × Pv / (P − Pv)
Here, P is total pressure. Units must be consistent.
4) Enthalpy of moist air
h = 1.006T + w(2501 + 1.86T)
Where T is in °C, w is kg/kg dry air, and h is kJ/kg dry air.
How to Use This Calculator
- Choose an input method: Relative Humidity or Humidity Ratio.
- Enter dry-bulb temperature and select its unit.
- Enter barometric pressure and select the correct unit.
- If using RH, enter relative humidity from 0 to 100%.
- If using w, enter humidity ratio in kg/kg dry air.
- Click Calculate to view enthalpy and intermediates.
- Use the download buttons to export CSV or PDF.
Why Moist-Air Enthalpy Matters
Moist-air enthalpy combines sensible heat and latent heat. It is the backbone of cooling-load, dehumidification, and energy‑recovery calculations. In HVAC practice, engineers track enthalpy differences across coils to estimate kW or tons of refrigeration. A small enthalpy error can misstate equipment capacity significantly.
What This Calculator Outputs
The calculator reports humidity ratio, vapor partial pressure, and specific enthalpy per kilogram of dry air. Enthalpy is shown in kJ/kg dry air, which matches common psychrometric charts. Intermediate values help with troubleshooting: if humidity ratio looks high while RH seems moderate, pressure or temperature inputs may be inconsistent.
Core Enthalpy Relationship
For typical building conditions, the moist-air enthalpy equation is h = 1.006T + w(2501 + 1.86T). The first term is dry‑air sensible enthalpy. The second term adds the latent contribution of water vapor plus its sensible portion. Temperature is in °C, and w is kg water per kg dry air.
Typical HVAC Operating Ranges
Many comfort applications sit between 20–26°C and 30–60% RH at near‑sea‑level pressure. At 24°C and 50% RH, a representative result is w ≈ 0.0093 kg/kg and h ≈ 47.8 kJ/kg. Hot, humid outdoor air can be far higher; at 32°C and 70% RH, h can reach about 86 kJ/kg.
Impact of Humidity Ratio on Load
Humidity ratio strongly drives latent load. Around 30°C, each 0.001 kg/kg (1 g/kg) increase in w adds roughly 2.6 kJ/kg to enthalpy. That means ventilation air moisture swings can dominate coil duty even when dry‑bulb temperature is steady. Tracking w alongside T is essential for humid climates.
Influence of Barometric Pressure
Pressure affects how RH converts to vapor pressure and humidity ratio. At higher altitude, the same RH and temperature yield a different w because the dry‑air partial pressure changes. If you have a local station pressure, enter it rather than assuming sea level. This improves agreement with measured humidity sensors.
Worked Example with Numbers
Suppose T = 30°C and w = 0.010 kg/kg. Then h = 1.006·30 + 0.010(2501 + 1.86·30) ≈ 30.18 + 25.57 = 55.75 kJ/kg. If w rises to 0.016 kg/kg at the same T, enthalpy increases to roughly 71.1 kJ/kg, highlighting latent sensitivity.
Common Uses in Design and Audit
Use enthalpy differences to estimate coil load: Q̇ ≈ ṁ_da(h_in − h_out). In audits, compare outdoor and return-air enthalpies to decide when economizers are beneficial. In process air‑conditioning, enthalpy helps set targets for desiccant or heat‑recovery wheels. Consistent units and dry‑air basis keep results comparable.
Frequently Asked Questions
1) What does “kJ/kg dry air” mean?
It means enthalpy is referenced to one kilogram of dry air, with water vapor treated as an added component. Psychrometric charts and HVAC load equations commonly use this dry-air basis for consistent mass flow calculations.
2) Should I enter relative humidity or humidity ratio?
Use relative humidity when you have RH readings or want chart-like inputs. Use humidity ratio when w is known from measurements, a chart, or a humidity sensor that directly reports mixing ratio. Both paths compute the same enthalpy.
3) Why does barometric pressure affect the humidity ratio from RH?
RH is a ratio based on saturation vapor pressure, but converting to humidity ratio also depends on dry-air partial pressure. Lower ambient pressure changes the same RH into a different w, which shifts enthalpy and coil-load estimates.
4) What temperature and pressure units can I use here?
You can enter temperature in °C or °F, and pressure in Pa, kPa, MPa, bar, atm, psi, or mmHg. The calculator converts everything internally before computing humidity ratio and enthalpy.
5) What are typical enthalpy values for indoor comfort air?
Indoor comfort air often falls near 35–55 kJ/kg dry air, depending on temperature and moisture. For example, 24°C and 50% RH is about 47.8 kJ/kg at sea-level pressure.
6) Can I use the calculator for heating and humidification?
Yes. Enthalpy applies to any moist-air state, including winter heating and humidification. If humidity ratio increases while temperature rises, enthalpy increases from both sensible and latent contributions.
7) Why might my results differ from another psychrometric tool?
Differences usually come from the saturation vapor pressure correlation, rounding, or whether pressure is sea-level or station pressure. Ensure consistent units, the same pressure basis, and the same input method when comparing results.