Refrigeration technology and refrigerant handling (F-gas)

A heat pump and a refrigeration system are technically the same machine: a refrigeration circuit that moves heat from a cold place to a warm one. In a refrigerator, heat is moved out of the cabinet; in a heat pump, heat is moved from outdoor air, ground or water into the house. Because the process uses a refrigerant under pressure, it is both technically demanding and heavily regulated — refrigerants are potent greenhouse gases, and the work is subject to certification requirements.

The refrigeration circuit exploits the fact that a liquid absorbs heat when it evaporates and releases heat when it condenses. By controlling the pressure, you can choose where evaporation and condensation occur — thus moving heat from cold to hot. Four components drive the cycle:

• 01Evaporator: the refrigerant evaporates at low pressure and low temperature and ABSORBS heat from the surroundings (outside air, ground, cooling room). Here it becomes cold.
• 02Compressor: draws in the cold vapour and compresses it so pressure and temperature rise sharply. This is where electrical work is added.
• 03Condenser: the hot, compressed vapour RELEASES its heat and condenses to liquid at high pressure. Here the useful heat is delivered (in a heat pump).
• 04Expansion valve: reduces the pressure on the liquid again, so it becomes cold and ready to evaporate — the circuit starts over.

Refrigeration technology's most important tool is the log p-h diagram (pressure-enthalpy diagram). It plots the refrigerant's state with pressure p (logarithmic) on the vertical axis and enthalpy h (heat content per kg) on the horizontal. Into the diagram is drawn a bell-shaped saturation curve: to the left of the bell the refrigerant is pure liquid to the right pure vapour and under the bell it is a mixture of liquid and vapour. The four cycle steps together form a rectangle in the diagram.

• 01Evaporation: horizontally to the right inside the dome at low pressure — the refrigerant absorbs heat and changes from liquid to vapour at constant evaporation pressure and temperature.
• 02Compression: upward along a line of (nearly) constant entropy — the compressor raises the pressure and the point moves up to the high pressure to the right of the dome (superheated vapour).
• 03Condensing: horizontally to the left at high pressure — the vapour cools, condenses and releases heat at constant condensing pressure and temperature.
• 04Expansion: downward vertically at (nearly) constant enthalpy through the expansion valve — the pressure drops back to the evaporation pressure, and the cycle restarts.

The diagram makes the invisible pressures and temperatures readable: the evaporation and condensation pressures are seen directly as the two horizontal lines, and the associated saturation temperatures are read where the lines intersect the bell curve. The length of the evaporator line on the enthalpy axis is the heat absorbed per kg, the condenser line is the heat released per kg, and the difference is the compressor work. Two deliberate additions are drawn with: superheat (the steam is heated a little extra after the evaporator, so liquid is never sucked into the compressor) and subcooling (the liquid is cooled slightly below the condensation temperature, so liquid is sure to be available in front of the expansion valve). Both are read as horizontal sections outside the bell.