The cooling cycle process in h,log p diagram — superheating, subcooling and efficiency

When you learn to dimension troubleshoot and optimize refrigeration systems you eventually end up with the same tool: the h log p diagram. It's a diagram where the refrigerant's state — pressure temperature and energy content (enthalpy) — can be read and where the whole cycle process can be drawn as a square. The competency aims for the education mention it directly: you must be able to diagram cycle processes in h log p diagrams and derive physical constants and specific outputs for a given refrigerant from them. The diagram isn't school material you leave behind after basic course — it's a working tool you use the rest of your career.

§The diagram's two axes

On the vertical axis, pressure is read on a logarithmic scale (hence 'log p'), and on the horizontal axis, enthalpy is read — that is, how much energy the refrigerant contains per kilogram. In the middle of the diagram is a bell-shaped curve: to the left of the curve, the refrigerant is liquid, to the right it is vapor, and under the bell itself it is a mixture of liquid and vapor. Each curve in the diagram belongs to a specific refrigerant because different substances boil and condense at different pressures and temperatures.

§The Four States in the Circuit

The circuit process itself is drawn as four line segments following the four main components in the system. From the evaporator the refrigerant is sucked into the compressor as a vapour at low pressure. In the compressor it is compressed and both pressure and temperature rise markedly — shown in the diagram as a vertical jump to a higher pressure level. In the condenser the refrigerant releases heat to the surroundings at nearly constant pressure until it has become liquid. Through the expansion valve the pressure drops abruptly without the enthalpy changing — this is a vertical line down towards the low-pressure side. Finally the refrigerant absorbs heat in the evaporator and evaporates again at low pressure and the cycle is complete.

• 01Evaporator: the refrigerant absorbs heat and evaporates at low pressure.
• 02Compressor: the refrigerant is compressed — pressure and temperature rise.
• 03Capacitor: the refrigerant releases heat and becomes liquid at high pressure.
• 04Expansion valve: pressure drops sharply without the energy content changing.

§Overheating — your most important control parameter

Overheating is the difference between the temperature the refrigerant actually has when it leaves the evaporator and the temperature it would boil at the current pressure (saturation temperature). If the overheating is too small, you risk that there is still liquid refrigerant going into the compressor, which can damage it — liquid refrigerant cannot be compressed like vapour. If the overheating is too great, the evaporator is not fully utilised, and the system becomes less efficient. Therefore, overheating is something you measure and adjust every time you service or commission a system with an expansion valve that operates on this very principle.

§Subcooling—the corresponding picture on the condenser side.

Subcooling is the corresponding value on the high-pressure side: the difference between the condenser's saturation temperature at the measured pressure and the actual temperature of the liquid refrigerant just before it reaches the expansion valve. Sufficient subcooling ensures that there is really 100 % liquid at the expansion valve—if there is vapor mixed in the expansion valve loses its capacity and the system performs less than it should. Too little subcooling can point to too little refrigerant or a condenser that does not give off heat efficiently enough; too much subcooling is often seen if too much refrigerant has been filled into the system.

§COP — circuit efficiency

When you've drawn the circle and read the enthalpies at the four corners you can calculate the system's efficiency often expressed as COP (Coefficient of Performance): the ratio between the cooling or heating output the system delivers and the energy the compressor uses to deliver it. The higher the COP the more output you get per euro of electricity. COP is a snapshot at specific operating conditions and that's precisely why the education's competency aims emphasize you can calculate energy transport and cooling output for both simple and special cooling processes — so you can assess whether a system really runs optimally or wastes energy.

“The one who can read a psychrometric chart can see what is happening inside a system without taking apart a single screw.”