Black Start, Chapter One: 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.
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.
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).
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.