Engineering task and calculation objective
This module calculates the thermophysical properties of ammonia according to the VDI Heat Atlas (VDI-Wärmeatlas, 12th edition 2019) — either for the single-phase state from temperature and pressure, or for the saturation state along the boiling line. Besides density, heat capacity, thermal conductivity, viscosity and Prandtl number, caloric quantities such as enthalpy, entropy and heat of evaporation are available, as well as the compressibility factor, speed of sound, surface tension and the critical-point data.
Calculating ammonia properties is everyday business in refrigeration engineering: as the natural refrigerant R717 with its high heat of evaporation, ammonia is widely used in industrial refrigeration plants, heat pumps and cascade systems. Beyond that, the property data are needed in the chemical industry (ammonia synthesis, fertilizer production), in absorption chillers, and increasingly for ammonia as a hydrogen carrier and fuel. The range of validity covers single-phase −50 °C to 300 °C at 1 bar to 500 bar, and the boiling state −75 °C to 130 °C — i.e. the entire range relevant to refrigeration up to close to the critical point.
Standard and calculation basis: VDI Wärmeatlas, 12. Auflage 2019
Calculation workflow
- Specify the type of state: The saturation-state query decides whether a single-phase state (gas or liquid) or the two-phase equilibrium state on the boiling line is calculated.
- Enter the state variables: In the single-phase case, temperature and pressure are specified independently; in the saturation case, one of the two quantities is sufficient — the other follows from the vapor pressure curve.
- Check the range of validity: The module verifies that the state point lies within the correlation limits (single-phase −50 °C to 300 °C, 1 bar to 500 bar; boiling −75 °C to 130 °C) and flags any violations.
- Calculate the properties: From the stored correlations of the VDI Heat Atlas, density, heat capacity, enthalpy, entropy, heat of evaporation, compressibility factor and speed of sound are determined, along with the transport properties thermal conductivity, viscosity, thermal diffusivity and Prandtl number — in the saturation case separately for boiling liquid and saturated vapor.
- Transfer the results to subsequent calculations: The property data feed directly into heat transfer, pressure drop and equipment calculations, for example into evaporator and condenser modules or the sizing of piping for refrigeration plants.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Pressure | p | Pa |
| Temperature | ϑ | °C |
| 1 | 1 | – |
| 1 | 1 | °C |
| 1 | 1 | kg/m³ |
| 1 | 1 | – |
| Saturation state? | 1) | kJ/kg |
| Saturation state? | 2) | kJ/(kg·K) |
| 1 | 1 | – |
| 1 | 1 | J/kg |
| 1 | 1 | J/(kg·K) |
| 1 | 1 | J/(kg·K) |
| 1 | 1 | 1/K |
| 1 | 1 | W/(m·K) |
| 1 | 1 | mPa·s |
| 1 | 1 | m²/s |
| 1 | 1 | m²/s |
| 1 | 1 | kg/m³ |
| 1 | 1 | – |
| 1 | 1 | J/kg |
| 2 | 2 | J/(kg·K) |
| 1 | 1 | J/(kg·K) |
| 1 | 1 | 1/K |
| 1 | 1 | W/(m·K) |
Calculation options
1
1 · Liquid · On the boiling curve · supercritical
Saturation state?
No · Yes
Saturation state?
No · Yes
2
1 · Liquid · On the boiling curve · supercritical
Frequently asked questions
Why is ammonia so widely used as refrigerant R717, and what does that mean for the property data?
Ammonia has an exceptionally high specific heat of evaporation (around 1,370 kJ/kg at −33 °C, several times that of synthetic refrigerants), good heat transfer characteristics, GWP = 0 and ODP = 0. As a result, plants manage with comparatively small mass flows. For design purposes this means that the heat of evaporation, vapor density and Prandtl number must be determined accurately as functions of the state, because they change strongly along the boiling line — which is exactly what this property module does.
Where are the critical point and triple point of ammonia, and why do they matter?
The critical point lies at about 132.25 °C and 113.3 bar, the triple point at −77.65 °C. Above the critical point there is no evaporation any more — the boiling-state calculation therefore ends at 130 °C, just below the critical point, where the properties of both phases converge and correlations become increasingly uncertain. Below the triple point no liquid phase exists.
What should I watch out for when using property data near the saturation line?
In single-phase mode, pay attention to which side of the vapor pressure curve the state point lies on: even small temperature or pressure deviations can switch the phase state, causing density and heat capacity to change abruptly. For states close to saturation, deliberately use the saturation mode and work with the separate values for liquid and vapor.
Does the module also cover safety-related characteristics of ammonia?
No, the module provides thermophysical property data. Safety-related characteristics — toxicity, flammability limits (about 15–34 vol% in air), material compatibility (no copper!) and classification as a hazardous substance — must be assessed separately according to the applicable codes (e.g. EN 378 for refrigeration systems).