The operating principle of a heat pump is based on the physical property that "the boiling point of a fluid increases with pressure". By lowering pressure, a medium can be evaporated while an increase of pressure will lead to condensation.
For large scale industrial applications, Ammonia is the most suitable refrigerant. It is mostly used in a pump system.
To understand the principle of operation of an mechanical heat pump, a log P-h dagram can be used. A log P-h diagram shows all state variables of the refrigerant. On the horizontal axis enthalpy (h) is shown. The vertical axis has a logaritmic scale and shows pressure. The other lines show:
The black line divides the graph according to the different states of Ammonia (NH3). On the left its fluid phase is shown. On the right side Ammonia is in its gaseous state. Below the black curve a combination of both gaseous and liquid Ammonia can be found. The area above the highest point is called the transkritical area.
The thermodynamic cycle
The log p-h diagram shows a cycle for a mechanical heat pump. The points represent the different components of the heat pump. In this diagram a heat pump with a condensation temperature of 80 °C and an evaporation temperature of 40 °C is taken as an example.
1-2 The compressor: With a compressor the pressure of the gaseous refrigerant is increased from 15 to 40 bar. Ideally the entropy (s) stays constant during this process. In reality the entropy will increase somewhat, because the electric energy needed to power the compressor is partly absorbed by the refrigerant. Due to that the temperature of NH3 gas will rise to 120 °C.
2-3 The condensor: The condensor delivers useful energy. In the condenser the superheated gas is cooled from 120 °C to 80 °C. Then condensation takes place at constant temperature of 80 °C, until all vapour has become liquid. The liquid flows to the expansion device.
3-4 Expansion device: Inside the expansion device the pressure is reduced from 40 to 15 bar. Due to the expansion a mixture of gaseous and fluid Ammonia is formed. This mixture flows to the liquid separator.
4-5/1 Liquid seperator: Inside the liquid seperator both fluid (5) and gaseous (1) Ammonia can be found. Its most important function is to seperate the liquid and vapour. The vapour flows to the compressor; the liquid is pumped over the evaporators.
5-6 Evaporator: The liquis ammonia at the bottom of the seperator is pumped over the evaporator(s). Inside the evaporator a part of the ammonia is evaporated at a constant temperature of 40 °C. The energy needed for evaporation is delivered by a source of waste heat. The mixture of liquid and vapour ammonia flows back to the seperator (6) and is separated again in liquid and vapour.