Sir Frank Whittle, Father of the Gas Turbine

Discussions about gas turbines and their application to land-based power generation, gas pipeline and process plants should rightfully begin with British engineer Sir Frank Whittle.  The key word here is application. His predecessors were many, but Whittle should be credited for bringing ideas regarding the jet engine to fruition in industrial applications.
  In 1941, Sir Frank Whittle designed the first successful turbojet engine for air defense during World War II.  Dubbed the Gloster Meteor, it flew in defense over Great Britain.  Whittle improved his jet engine as the war progressed.  He shipped a prototype engine to General Electric in the United States in 1942.  GE built America’s first jet engine for military aviation applications the following year.
  Whittle came to the USA for the first time on a secret mission in the summer of 1942.  He met with officials from General Electric in Lynn, MA and Bell Aircraft Company in Buffalo, NY.  Later in 1942, he visited GE in Schenectady, NY, where a rudimentary propeller jet engine was under development.  Whittle’s comments and suggestions to American engineers proved invaluable in modifications and improvements that soon followed.
  One can argue that the development of jet engine might have been accelerated had World War II lasted longer.  However, the other side of that argument is that the application of turbo-technology to other industries became a post-war quest of American industry.  In the eyes of many engineers on both sides of “the pond,” this method of power production and propulsion could be used to drive land-based generators, compressors and other load devices, as well as to propel ships and aircraft in commercial applications.  All that was needed was funding and the imagination of the engineers involved, eager as they were to apply this innovative prime mover.

Fig. 1-1: Sir Frank Whittle and his multi-combustor jet turbine (circa 1941)

The multi-combustor, turbo-jet engine (hereafter called the gas turbine) has Frank Whittle proudly standing beside it in Fig. 1-1.  Notice that there are 10 combustion chambers (tube shaped) encircling the engine, with stainless steel nozzles to inject fuel into them at the front ends.  The chambers are interconnected by cross-fire tubes, as is common on most modern gas turbines.  The exhaust diffuser is in the center.  The reverse-flow concept of the hot gases is obvious from the photograph.  So are the transition pieces curling from the discharge of each combustor.
  “If necessity is the mother of invention,” as preached to engineering students by college professors, then the end of WW-II brought many needs to the front burner ready to be invented.  Jet engine technology needed to be harnessed and applied to other commercial endeavors.  As a prime mover, the gas turbine needed to find applications that could deliver power to other modes of transportation, electrical power delivery and natural gas pipelines prime movers.  However, as inventors soon found, not every idea has a viable application to industry, or a willingness of the public to accept them.  Engineers like Whittle would encounter doubters, the enemies of progressive thinkers.  Progress often depended upon inventors who could convince entrepreneurs and angel investors to take a chance on their ideas and innovations.  This presumes that negative forces are not overwhelmingly against such visionaries.  As explained in later chapters of this blog, GE engineers struggled to get funding in a fledgling gas turbine department in Schenectady, NY in the 1950s.
It is uncertain if Frank Whittle could have envisioned a modern gas turbine like the one shown in Fig. 1-2 below.  A single-fuel (natural gas) General Electric MS7001EA gas turbine (approximately 80 megawatt rating) is shown, with fuel line “pigtails” coming from the manifold on the left leading to each combustor. The chambers themselves are inside the combustion wrapper, which encircles the turbine, so only the covers are showing.

 

 Fig 1-2; Multi-combustor GE MS7001EA Gas Turbine inside Combustion Wrapper (circa 2000)

  Since the combustors are interconnected via cross-fire tubes, only one combustor needs to have a sparkplug (igniter) and another, a flame detector.  However, for redundancy and reliability, modern gas turbines typically have at least two of each, as shown in Fig. 1-3.

 Fig 1-3: Typical configuration of Multi-combustor Gas Turbine with Spark Plugs & Flame Detectors

Design of combustion systems, like those depicted herein, seems to be a “settled” issue.  Most manufacturers have decided that this is the design that makes the most sense.  It allows for temperature equalization and flow distribution to the first-stage turbine nozzle and rotating wheels with buckets (blades) that develop the output power.  Refer to Fig. 1-4 below.

 Fig 1-4: Cross-fire tubes between adjacent combustion chambers

Fig. 1-5 below should be studied for its completeness regarding the design of a typical modern combustion system for a GE MS7001EA gas turbine.  Notice that the combustors are “canted” in design to straighten the hot gas flow through the transition pieces toward the first-stage nozzle (not shown).  Also, this design shortens the length of the turbine and thus bearing spans.  The reverse flow of the air from the compressor discharge casing is also shown entering the combustor.

Fig 1-5: Typical Modern Combustion Chamber and Transition Piece Configuration

Other areas of development have also occurred over the past 70 years with gas turbine technology.  Advances in metallurgy, ceramic coatings and internal cooling designs have evolved over the past seven decades, to a point where efficiencies and higher internal firing temperatures have made the gas turbine a viable competitor to other forms of power generation.
  In conclusion, over sixty years ago an engineer from Britain named Frank Whittle envisioned, designed and built a multi-combustor, aero-derivative gas turbine engine for land-based applications. His innovative design in gas turbine technology has prevailed for the following six decades well into the 21st century.
Taken from Lucier Blog

The Brayton Cycle

The individual most commonly associated with the concept of the combustion (gas) turbine engine was an American named George Brayton (1830-1892).  He was an engineer with vision and ingenuity, who conceived the gas turbine thermodynamic cycle back in 1872, when he filed for a patent.  Discussions about gas turbines need to begin with Brayton.

Brayton conceived an engine that compressed atmospheric air to a high pressure.  In his concept turbine, the compressed air would then be mixed with a fuel (most commonly natural gas or #2 distillate oil) and ignited in one or more combustion chambers.  The excess air (that is, air not needed in the combustion process) would then be used to dilute and reduce the high-temperature combustion gases to a more moderate level, without significantly reducing the pressure leaving the combustors.  This would be known as combustion at constant pressure.

In Fig.1 below, air from the atmosphere adjacent to the turbine is drawn in and compressed, as shown from point 1 to point 2. Notice that the volume decreases as the pressure rises.  Heat is then added between points 2 and 3 on the graph.  However, the pressure remains essentially constant, as represented by the horizontal line on this pressure-volume (P-V) diagram.


Take a few minutes to study all aspects of the graph below.  Pressure is on the vertical axis (ordinate); Air Volume is on the horrizontal axis (abscissa).  Notice how volume decreases as pressure increases along the up slope from Point 1 to Point 2.  Trace the line from the Start Point 1 around to Point 4. Imagine how the pressure and volume change along the route.  Notice where heat is added to the compressed air.

Fig. 1-1 Pressure Volume Diagram for Brayton Cycle

Thereafter, the hot gases expand through stationary nozzle segments that direct the flow to impinge on the turbine blade surfaces (a.k.a. buckets) and develop torque (power).  According to Brayton, power will be developed by the gases applying impulse forces on the turbine rotor blades. Additional power results from reaction forces of the hot gases accelerating away from the turbine blades.  These TWO forces develop rotational power to turn turbine wheel(s).

An extension shaft from the turbine wheels would then be connected to an electric generator or other load device to do useful work.  Brayton envisioned that approximately 2/3 of the power developed by the gas turbine would be required to drive the turbine’s own axial-flow compressor and such required auxiliaries as fuel, oil, hydraulic and water pumps.  Finally, the exhaust gases would then be sent to a diffuser (to reduce the flow velocity) and out to the atmosphere through a stack enclosure.

The Brayton Cycle is considered to be an open system, since the exhaust gases are expelled back to the atmosphere from whence they originated.  Please refer to Fig. 2 below.

          Fig. 1-2 Gas Turbine and the Brayton Cycle

The stick diagram (Fig.1-2 above) and the associated pressure-volume diagram (Fig. 1) clearly show the gas turbine in its most rudimentary form.

The four numbered corner points show following modes:

• Points 1 to 2: Compression (air drawn from atmosphere and compressed)
• Points 2 to 3:  Combustion (combustion at essentially constant pressure)
• Points 3 to 4: Expansion (expansion across turbine section)
• Points 4 to 1: Exhaust (exhausting hot gases back to atmosphere)

The Brayton Cycle, in its simplest form, is not particularly complicated.  However, it took almost 60 years before working engines were developed.  This was due, in large part, to the fact that Brayton’s idea was one whose time had not yet come. Technology lagged behind his concepts because the need was not yet beckoning for such a device as a gas turbine.

The axial-flow compressor requires work to compress the air (W1-2) as shown in Figure 1-1.  Energy, in the form of fuel (natural gas or #2 distillate oil are the most popular), is injected into the combustor(s) shown as Q2-3. The output work developed between W3-3’ is required to power its own compressor and auxiliaries. The remaining power (W3’-4) is used to drive a load device (generator or load compressor).  The gases going to the atmosphere are hot, but this is often wasted energy (Q4-1), unless heat recovery equipment is employed.

Figure 1-3 below shows gas turbine operation for three different ambient conditions: an ISO day (compressor inlet temperature of 59 ˚F day, which is 15˚C) is represented by the sloped line in the middle.  To the left is the characteristic control line for a MAXIMUM ambient day (assume something like 100 ˚F at the compressor inlet).  The third line shows a loading curve for MINIMUM ambient day (assume 32 ˚F at the inlet).

Fig. 1-3 Base and Peak Load Operation for 3 Ambient Days

Loading the gas turbine from No Load to Rated Load for the ISO day, the fuel flow and exhaust temperature would track along the center line until the BASE load limit is reached.

• If PEAK load is then selected, the curve would track higher to intercept the upper line.
• On a MAXIMUM ambient day (hot), the governor control would track along the line on the left until BASE or PEAK load was intercepted, as desired.
• Similarly, a MINIMUM ambient day (cold) is reflected in the governor tracking along the right-side line to BASE or PEAK load.  Notice that a different BASE load level is achieved depending upon the ambient day of operation.  For instance, suppose the outside temperature at the compressor inlet is 32 ˚F, more power would be developed than on an ISO (59 ˚F) day.
• Much more power would be developed on a 32 ˚F day than on a MAXIMUM (say 100 ˚F) day, but fuel costs will increase too.

There are some minor efficiency gains on colder days, but for the most part this additional power is developed as a consequence of more fuel being burned in the combustors.  This raises the pressure acting on the turbine blades (buckets).  It costs the gas turbine operator more in fuel for the additional power generated.  However, the cost per kilowatt generated decreases.

George Brayton never lived to see his concept engine, the gas turbine, become a reality.  If he lived today, the F-class gas turbines that develop upwards of to 200 megawatts would likely bring a grin to his face some 14 decades later.

Taken From Blackstart