Next generation aeroderivative-II

The early models of the Whittle jets were sent to the US towards the end of the war. GE and Westinghouse were selected to develop US jets for our military needs. These companies had the background in rotating steam turbines and seemed to be the ones to develop the new jets. Pratt and Whitney soon saw what would happen to the reciprocating engine business and joined GE and W designing and building jets. The rest is history. In England, Frank Whittle formed a research and development company, and Rolls Royce took over the jet engine manufacturing business. Other smaller companies sprung up on both sides of the Atlantic to develop and make jet engines.

Double gas turbine development paths

The industrial gas turbine developments have taken two distinct paths. Out of the Brown Boveri early units came the so-called Heavy Duty (HD) units, and out of the Oham/Whittle jets came our industrial aero derivatives. The aero jet engines took the developmental lead because of the large governmental funding and advancements were passed down the HD groups. Out of the jets sprung GE and Westinghouse as the original US HD manufacturers using steam turbine technology.

The HD units were like giant oak trees with strong and robust limbs and large leaves whereas the aeros were more like elm trees with more slender branches and smaller, more delicate leaves. The evolution of these two distinct machine development roads has been well documented by Septimus van der Linden and others. From the Brown Boveri HDs came the jets, and from the jets then came improvements in the HD units. There exists today both the HD Oaks and the aero Elms, each having their roots firmly planted in the ground and growing together side by side for electric power generation and mechanical drive applications.

Alstom Reheat (RH) Gas Turbines

A spin off of the HD units has been the present-day Alstom GT 24 and 26 RH gas turbines. Actually, the early jets applied after burners where a large amount of additional fuel was burned to produce a significant increase in short time thrust. This concept was applied to HD units by ABB (outgrowth of Brown Boveri) where extra fuel was burned part way down the expansion path. The all-welded stiff single shaft rotor made it possible to lengthen the bearing span to give space for the extra compressor stages and the RH combustor. The first GT 24 and 26s had their cooling problems which were overcome by externally cooling the cooling air and then using the cooled air to do the blade and RH combustor cooling. The first unit had a compression ratio of about 30 and a TIT of around 2000

^o

F. The exhaust temperature was about 1200

^o

F which was most favorable for the combined cycle with 2400 psig and 1000/1000

^o

F steam conditions.

The ABB RH gas turbines applied a single stage impulse type turbine downstream of the first EV type combustor to cut the pressure in half to about 15 atmospheres, to produce power and to reduce the gas temperature. The gas was then reheated by a ramp/vortex generator type combustor where the temperature rise was only half that of the first combustor. The RH gas turbine was very successful and many of these units are in operation today around the world.

Since Pratt & Whitney introduced the geared turbofan aircraft engine, the question has been what is the next generation aeroderivative? Ivan Rice explores that question in this series.

Next generation aeroderivative-I