Using rhythm as a metaphor for transient behavior in refrigeration and control.

I’ve been thinking a lot about different forms of compression and the ways they could be used to advantage in refrigeration systems. I don’t recommend you follow me down this rabbit-hole; it is confusing and misleading. You may be wondering what the rhythms of the fox-trot have to do with compressors. There are four thermodynamic processes for compressing a gas; three are slow and one is quick-quick.

The quick process is the one that we all use every day and take for granted. Known as “isentropic,” or sometimes “adiabatic,” compression, it is characterized by a lack of heat transfer during compression. Adiabatic means no transfer of heat and isentropic means no change of entropy, so all isentropic processes are adiabatic but not all adiabatic processes are isentropic. Almost all the compressors used in vapor compression refrigeration systems, from single piston machines, through scrolls and screws to huge centrifugal machines are of this type. However, the high speed introduces frictional losses and other inefficiencies so there has been a resurgence of interest in the other processes in recent years.

This usually means using isothermal compression, where the pressure increases but the temperature doesn’t change. This is achieved by allowing heat to flow out of the compression chamber (which takes time) while the volume is being reduced and the pressure raised. An oil-injected screw compressor looks like it could be isothermal since the heat of compression is removed through the oil cooling circuit. However, the additional power required to compress the injected oil (or the injected liquid if liquid-injected oil cooling is used) offsets the advantage of operating at a lower discharge temperature and the power input required is best calculated by ignoring the oil cooling and then adjusting the discharge temperature after the power has been calculated. True isothermal compression would be achieved by compressing the gas inside a special heat exchanger and allowing time for the heat to transfer. This is likely to be bulky, expensive and slow.

Other slow processes are even odder. Isochoric compression occurs when the pressure rises, achieved by heat input, but the volume doesn’t change, so no work is done on the gas. This sounds far-fetched, but it is very common in static situations like when a closed cylinder bursts after being heated under extreme high pressure. It is more difficult to visualize what an isochoric flow process would look like, but that is one of the four steps in a Stirling engine. Gas is passed through a heat exchanger without the volume changing. As heat is added the pressure goes up and the volume remains the same.

Isochoric compression seems normal in comparison to the fourth of the quartet of processes; isobaric compression. This means compression without a change of pressure. It seems, at first sight, to be crazy talk to speak of compression at constant pressure, but again there are numerous examples of static processes using this process. This is what makes hot air balloons float; the pressure inside and outside the canopy is the same but the density inside is less (in other words the specific volume is greater than the cold air that it displaces) and so the balloon is buoyant. An isobaric flow process is even harder to envisage, but could lead to a step change in efficiency if it can be achieved economically.

Fred was impressed by Ginger’s grasp of thermodynamic processes.