How disciplined reasoning and first principles can untangle complex refrigeration faults.

I was reminded recently why engineering is such a fantastic career and being an engineer is the best job I can imagine. When you boil it right down, it is really all about solving problems for people. Some other professions can, I suppose, make the same claim: doctors for example. But they have to deal with “wetware” as the IT folks call us normal people and that can get a bit too icky for me.

Of course, finding the solution to a problem can sometimes be a frustrating process. There are several mantras that I cling to in these dark times. One of my favorites is “The problem is always completely baffling until you ‘get it,’ and then the solution is blindingly obvious.” It also helps to remember the words of Sherlock Holmes as told by Arthur Conan Doyle in “The Sign of the Four”: “when you have eliminated the impossible, whatever remains, however improbable, must be the truth.” Many other aphorisms also help, such as Occam’s Razor, which states that when faced with two possible explanations of what is happening, the simpler one is usually correct.

One of the most puzzling cases I ever dealt with concerned a spiral freezer in a food factory; the tale is retold here but some of the facts have been adjusted to keep the case anonymous. The freezer was in the middle of a large production area and was supposed to run at a suction condition of –40°F (–40°C) but back at the compressor it was –58°F (–50°C). The refrigerant was R-22 (it was a long time ago), so running at such a low back pressure caused lots of problems including loss of capacity.

The surge drum and pumps were in the machinery room together with the compressor and the suction line from the freezer to the machinery room ran up and down all through the roof space above the production area. The contractor who had installed the system analyzed the operating performance and concluded that the root cause of this symptom of low compressor suction pressure must be excessive pressure drop in the wet suction line caused by the many ups and downs of the pipe as it snaked through the space.

He managed to get hold of a second-hand surge drum and pump set and got permission from the factory owner to install it right next to the freezer with a very short wet suction line from the evaporator to this new surge drum. The original suction line could now run without any liquid in it, thus reducing the pressure losses caused by all the risers.

The original surge drum pumped liquid to the new surge drum and the new surge drum pumped to the adjacent evaporator. The plant was recommissioned and set to work. Back at the compressor the saturated suction temperature was –58°F (–50°C). The poor contractor concluded that his initial diagnosis had been wrong. He admitted he was stumped and he walked away.

I was asked to give a second opinion. I reviewed all of the original reports and concluded that the initial diagnosis was 100% correct; the plant had been suffering from a high pressure drop in the wet suction line due to the liquid risers. However, because the only available surge drum was second-hand, it was far too large for the freezer duty. It had been in service before and was quite dirty inside.

When the commissioning engineer tried to trim down the pump flow rate, the throttling valves would all choke up very quickly, so they were left wide open. As a result the R-22 flow rate in the evaporator was far too high. The liquid didn’t boil inside the coil and what little heat was gained appeared as flash gas in the return to the new drum.

What had happened was the original problem was correctly diagnosed and was cured by a solution that unfortunately introduced a second problem with exactly the same symptoms. The second solution was to clean up the new drum and fit smaller pumps. Suddenly, the plant burst into life and the compressor suction condition became –40°F (–40°C) as it should have been all along.