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 (at -10/40°C about 7 times higher compared with that of R22)
• Extremely low refrigerant mass flow (approx. 14% compared to R22), 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 (volume change from liquid to vapour about 3 times greater than with R22).
• 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 the 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 (R717=1.31 / R 22=1.18), 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)

• Vacuum operation 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