Thermodynamic properties of R717
Ammonia has predominantly favourable thermodynamic characteristics and a low energy requirement. Disadvantages include the high operating temperatures during compression (discharge gas temperature).
- Very high evaporation enthalpy
- Extremely low refrigerant mass flow, but this makes injection control more difficult at low capacities and/or dry expansion.
- Very low vapour and liquid density
- Outstanding heat transfer values, partly due to intensive boiling.
- Resulting selection criteria or measures:
- Relatively small dimensions for pipes, refrigerant pumps, control devices
- Low pressure drops
- High oil separator efficiency
- Minimum refrigerant charge possible, especially with compact systems (optimised liquid coolers with up to approx. 30 kW refrigerating capacity per kg R717)
- Especially high coefficients of performance at medium or high temperature conditions
- Specific adaptation of the heat exchanger and the expansion devices to the mass and volume flow characteristics required (Oils and their influence on system design)
- The "dry evaporation" places high demands on the refrigerant distribution, especially in case of strongly subcooled liquid (e.g. 2-stage economiser)
- Take suitable measures to protect against wet operation and liquid slugging (suction accumulator, expansion devices with stable control behaviour)
- High adiabatic exponent, resulting in high discharge gas and oil temperatures limiting the application range of the compressor (Compressors).
- Relatively small enthalpy difference in the superheated vapour phase, leads to significant temperature and volume changes in case of heat input.
- Resulting design criteria:
- Limitations in the application range of the compressor, low temperature applications for reciprocating compressors are only possible with 2-stage compression or, for screw compressors, with oil cooling.
- Heat exchangers must be designed for small differences in temperature (low pressure ratio) and low suction gas superheat
- Heat exchangers must preferably be in flooded or pumped design
- Install short and well isolated suction gas lines (for low pressure drops)
- Operation in vacuum range already occurs at an evaporation temperature lower than -33.4°C.
- Resulting measures:
- Use high quality shaft seals, seals and packing glands
- Install an automatic venting system
Symbol | R717 | R22 | R404A | R410A | R134a | ||
|---|---|---|---|---|---|---|---|
M | Molecular mass | g/mol | 17.03 | 86.5 | 97.6 | 72.6 | 102 |
K | Adiabatic exponent at 20°C and 1013 mbar | cp/cv | 1.31 | 1.18 | 1.12 | 1.17 | 1.11 |
tn | Normal boiling point | °C | -33.4 | -40.8 | -46.2 | -51.4 | -26.1 |
ρ | Density of the liquid at 40°C | kg/dm3 | 0.58 | 1.13 | 0.97 | 0.98 | 1.15 |
p | Vapour pressure -10°C Vapour pressure +40°C | bar bar | 2.9 15.5 | 3.54 15.3 | 4.34 18.2 | 5.72 24.1 | 2.01 10.2 |
tcr | Critical temperature | °C | 132 | 96.1 | 72 | 71.3 | 101.1 |
pcr | Critical pressure | bar | 113.5 | 49.9 | 37.3 | 49 | 40.7 |
AEL* | Toxicity | ppm | 50 | 1000 | 1000 | 1000 | 1000 |
ODP | Ozone depletion potential | 0 | 0.055 | 0 | 0 | 0 | |
GWP | Global warming potential | 0 | 1810 | 3922 | 2088 | 1430 |
Global warming potential according to IPCC IV (time horizon 100 years), also basis for the EU F-Gas Regulation 517/2014.
* Acceptable exposure limit
