Optimizing utility consumption is crucial for sustainable refrigeration practices.

In last month’s column I wrote about the need to develop the skill of discernment when faced with new, complicated, fantastical claims. I have always found that being able to formulate analogies to explain the unfathomable is a helpful technique. In particular, relating an obscure concept to something much more common in everyday life can create a link from the known to the unknown and blow away the fog of incomprehensibility.

When I was in my senior year of high school, my science project was to construct an experiment that measured the transient response of a mass on a spring to changes in a driving force and to show that this followed the same mathematics as an electrical tuned circuit. I learned that there were actually two useful analogies; one where the mechanical mass is analogous to capacitance and the other where it is analogous to inductance.

Later, when I had newly started work as a graduate engineer, my mentor was Dr. Jimmy Brown of the University of Strathclyde who was a non-exec director at Star Refrigeration. He was equally skilled in mechanics and electronics and would regularly reply to a mechanical question with an electronic answer (and vice versa). My high school science project was very useful in keeping up with his examples.

Jimmy’s familiarity with both disciplines stemmed from his early career investigating various physical phenomena related to refrigeration, including heat transfer through complex structures and the behavior of reciprocating compressor valves. In those days, before the introduction of cheap computers and the proliferation of computational techniques in fluid dynamics and finite element modeling, it was quite common to investigate mechanical events by constructing networks of electrical components and studying the resulting variations in voltage and of current flow through the net.

This seemed to lead to a far better understanding of the underlying mechanisms at work in the real world through study of the analog one. There’s a risk nowadays that better visualization leads to poorer comprehension.

Another fluid flow analogy that has always intrigued me is the comparison of gas flow in pipes with traffic on a busy highway. When there is a constriction downstream the individual particles get closer together, the density goes up and the velocity goes down, but the mass flow rate past the restriction remains constant. Once past the constraint the density drops and the velocity increases.

It is also possible to observe how a small flow entering the mainstream from the side can cause a major disruption to the flow. In gas systems this is the mechanical equivalent of a transistor. On the roads it is the disruption caused by traffic entering from an on ramp or a side street.

When there is a sudden halt to the flow of traffic, for example an accident, a shock wave of deceleration, like water hammer, travels back up the road as traffic comes to a stop. It has been observed that this wave travels at a speed of 12 miles per hour and persists long after the original incident has cleared.

Just think how often you have been held in slow moving traffic for ages, only to find when the road clears that there is no apparent reason for the delay.

However, this analogy can be stretched too far; there is no equivalent in fluid mechanics to the effect of “rubber-necking” on the roads, where a blockage on one side leads to a tailback in the opposite direction.

Perhaps highway engineers have missed a trick here; screening each side of the highway from the other would halve the number of tailbacks by insulating each stream from the effect of the other. What will driverless cars do to this analogy?