Dear Friends: Your April cover article ("Fahrenheit 3,600") certainly caught my attention. I would like to comment on a few issues.

The Siemens SGT-8000H gas turbine depicted does not have an efficiency of 60 percent; that would be the expected combined-cycle power plant efficiency.

Professor Lee Langston does muddy the waters a little bit concerning higher firing temperatures related to aero engines and industrial engines. In aero engines, weight-to-thrust ratio is very important and increased firing temperatures at 3,600°F or higher increases the specific output, essential for high performance.

The Joint Strike Fighter test platform in Florida.

High temperature for industrial engines does not necessarily equate to higher efficiency. More important than increasing firing temperature are component efficiencies and parasitic losses, such as air extraction for component cooling and clearances controlling leakage. The efficient use of cooling air (parasitic load) means more working fluid for the turbine expander. However, there is still leakage, and improved active clearance control will bring additional future improvements. All these advanced engines have exhaust gas temperature over 1,000°F, which drives the bottoming cycle energy recovery, allowing 60 percent CC performance.

I am not against improving turbine inlet temperature. We simply need to keep it in perspective. We see Mitsubishi Heavy Industries pushing the 3,000°F envelope for the H in their R&D facilities. The OEMs need to improve durability and reliability of the advanced engines before rushing off to yet another level of firing temperature or they must consider more complex cycles with moderate firing temperature, such as reheat or intercooling, to achieve higher overall plant efficiency.

Incidentally, I like his "springbok and zebra patrolled" nuclear power station at Koeberg, Cape Town, site of the 165 MW pebble-bed modular reactor and closed-cycle helium gas turbine scheduled for 2013 operation.

Editor's note: The writer is Past Chair of the International Gas Turbine Institute's Electric Power Committee. A reply by Lee Langston follows.

Lee S. Langston
Storrs, Conn.

To the Editor: I appreciate Mr. van der Linden's commentary and critique of my April 2007 article on the state of the gas turbine industry.

His point on the thermal efficiency of the SGT-8000H gas turbine is well taken. In the article's text, it is noted that this new Siemens H machine combined-cycle efficiency is slated to be over 60 percent, but the SGT-8000H cutaway diagram caption incorrectly lists the gas turbine itself at 60 percent.

Also, in other comments, Mr. van der Linden points out the danger to turbine durability and reliability of simply increasing turbine inlet temperatures in an industrial gas turbine. In the article, I cite an example of 3,000°F, which could be the peak turbine inlet temperature in a commercial jet engine during aircraft takeoff. When flight cruise conditions are reached, this temperature typically drops by 300°F to 400°F to, say, 2,600°F, a value that would not be exceeded during most of the flight. However, industrial gas turbines, especially those used for electrical power generation, typically must operate at their rated peak inlet turbine temperatures for thousands of hours. This makes long-term turbine airfoil high-temperature durability a major OEM (and customer) consideration, as Mr. van der Linden points out.

JSF engine will develop 40,000 lbs. of thrust.

The first gas turbine used to produce electric power, built by Brown Boveri and installed at Neuchatel, Switzerland, in 1939, had a turbine inlet temperature just over 1,000°F. We now have industrial gas turbine temperatures as high as 2,700°F and, as Mr. van der Linden writes, the envelope is being pushed to 3,000°F by R&D at MHI. Without this historic steady increase in firing temperatures, today's 60 percent combined-cycle plants wouldn't be possible. Certainly, the 3,600°F class JSF engine gives proof that the gas turbine community expertise in film cooling technology should one day make possible long-lived industrial machines that operate reliably and efficiently, in the 3,000°F range.

Editor's note: The writer is professor emeritus of mechanical engineering at the University of Connecticut.

To the Editor: I read with interest the short article titled "The Urge to Explore" by Buzz Aldrin and Wyn Wachhorst (November 2004). One statement in that article was that the night launch of Apollo 17 was visible for 500 miles.

James Gamble and I were less than 100 feet from the summit of 6,593-foot Mt. LeConte in the Great Smoky Mountains National Park when the Saturn 5 lifted off. We had a radio and were listening to the launch information. The spacecraft was either 7 miles up and 9 miles downrange or 9 miles up and 7 miles downrange when we saw the orange glow of the first stage.

Only a few seconds after we spotted the orange light, the first stage separated with a flash and the blue hydrogen flame briefly appeared before disappearing behind some high clouds.

So, this wonderful marvel of technology and power was seen about 650 air miles away in Tennessee. I think NASA has documentation of this distance sighting record for an Apollo moon launch buried in their archives.

To the Editor: Frank Kreith and Ron West's well-written article about future automobile development ("The Road Not Yet Taken," April, page 24) is thorough within its narrow confines, but portrays us engineers as self-centered gearheads.

A more realistic approach would be to acknowledge that the use of an automobile is already heavily subsidized by the general public, motorists and non-motorists alike, by an amount estimated by different people as between $5,000 and $20,000 per vehicle per year.

The result is exactly the same as if the government decided to give us free ice cream. We would all become dependent on ice cream; we would become fat, unhealthy, and overtaxed, and a huge lobby would arise to ensure that dairy farmers and manufacturers, along with the consumers, could keep getting fat on our taxes.

To compound the totally skewed economics by devising federal programs aimed at arriving at plug-in hybrids and the like would mean that the lucky owners of these vehicles would almost totally avoid paying the small share of the cost of driving that comes from the gas tax. And while the drivers would be using a pollution-free vehicle, the poor people who live downwind of electric power plants would suffer from greater emissions.

Kreith does advocate a carbon tax, but even that could result in gross inequities. A policy that would institute strong incentives and fairness for everyone, from rich to poor, in accomplishing the fastest viable transition to a sustainable future is described in the Web site davidgordonwilson.net. If this policy were adopted, the U.S. would again become a beacon to the world, and miracles would be seen in the land. Engineers: Become champions for a long and delightful future.

Editor's note: The writer is professor emeritus of mechanical engineering at the Massachusetts Institute of Technology.

To the Editor: The article in the March issue about a gas turbine project at MIT ("The Little Engine," page 30) was fascinating, both for what it said and what it didn't say.

A gas turbine engine, as I used to understand it, consists of a turbine, combustor, compressor, and a regulating system. The article showed an intriguing miniature turbine and a space labeled combustor.

There was no compressor shown. Perhaps the intent is to prepressurize the fuel plus some carrier air. It would be interesting to know. Also, how is this machine regulated? Perhaps the fuel flow is regulated by some system not shown.

The article did say that all the parts of the system have been demonstrated individually, and now they have to be combined. I will be very interested in an article describing the complete, assembled system.

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