I probably should have figured this out through my reading in preparation for the throttle valve patent application or my reading to learn and understand how nuclear gas turbines could work. Unfortunately, I got the impression from the technical papers that helium gas turbines were already available or could be easily manufactured. The researchers who wrote the papers I read were far more interested in doing the computations to show how the system could work. As far as I remember they did not talk much about the difficulties of actually making it work. The handful of projects that had used helium as opposed to air or nitrogen for their closed cycle gas turbines were completed by manufacturers who had the resources to build prototype machines.
My revelation about the difficulty of designing and manufacturing a completely different kind of Brayton cycle compressor and turbine came about ten years before Hans Ulrich Frutschi published his excellent reference titled Closed-Cycle Gas Turbines: Operating Experience and Future Potential". If that book had been available, I might have made a completely different career decision. None of the papers I read about the 50 MWe Oberhausen II helium turbine included anything like the following comment from someone with intimate knowledge of its operation:
Because GHH had no gas turbine development staff of its own, the task was outsourced to an institute of a technical university. Although they had been working on this topic for years, the helium turbine, which was designed to produce 50 MW, only just managed 30 MW. The efficiency only reached 23%, instead of 34.5% as planned. Since this large deficit was the result of many small ones, no successful reconditioning was possible. (It would be necessary to design and build a new turbo machine.)Instead, I learned just how difficult, expensive and lengthy a process it would be to obtain a suitable helium turbine and compressor for the system I envisioned during an hour long discussion with a gas turbine expert at the University of South Florida. I cannot recall his name or how I found him, but he was a guy who had spent 20-30 years in an industrial gas turbine design and manufacturing career before he decided to spend the remainder of his career teaching.
. . .
This turbo set, which had a rather low output for a helium turbine, should have been designed for a much higher compressor and high pressure turbine rotational speed. The low speed of only 5500 rpm (adequate for air) resulted in very unfavorable hub to tip ratios for compressor and turbine, which led to poor polytropic efficiency levels in this machine. Also, the cycle pressure losses were excessive, especially the cooling and sealing mass flows, by a factor of 4.
I entered the meeting with the assumption that producing a helium cooled closed cycle gas turbine would be a fairly simple matter of assembling well proven, already manufactured "off the shelf" components. I left it with a much deeper understanding of the enormous differences in gas characteristics between helium and air, which was the working fluid that essentially all existing gas turbines use. I also learned just how much "art" and trial and error was involved in turbine and compressor design and construction, and how much money even experienced firms invest to develop a brand new design to the point where it could be manufactured to provide reliable service. I learned that nearly all "new" jet engines and industrial gas turbines are built by tweaking or modifying existing designs to make use of as much proven knowledge and as many proven parts as possible, but even then an engine manufacturer can spend hundreds of millions on relatively small machines and billions on larger ones designed for applications like passenger aircraft.
The only bright spot of the meeting came when I asked the professor what he would do if he wanted to build a closed cycle machine that operated on an inert gas to prevent corrosion and other unwanted reactions. He thought for just a moment and told me that it would be pretty simple to use compressors and turbines designed for air as the working fluid if the inert gas was nitrogen. After all, air is 80% nitrogen already and the thermodynamic characteristics of O2 and N2 are nearly identical. He told me there might need to be some small amount of O2 left in the system to prevent nitriding of the turbine blades, but he was not even sure that would be an issue with properly selected machinery.
I knew that N2 had been the cooling gas selected for at least one of the closed cycle gas turbine demonstration projects that I had researched - the Army's ML-1 - but I had shied away from that selection initially because there seemed to be such an overwhelming agreement in the papers that helium was a better choice. I also knew that the ML-1 had only operated for a few hundred hours, but I had not found any real details about why that was true. I left the meeting with a lot of chagrin - after all, I had taken a huge leap of faith based on my excitement in finding something "new" that others had overlooked. However, I also had found some hope that a different path could lead to a similar result.