Gas Turbine: Full Load/Part Load Understanding

In summary, a gas turbine is an engine that is able to run at constant speed, and can produce extra torque when heat is added to the combustion chamber. The efficiency of a gas turbine decreases with increased load, but can be improved with increased power output.
  • #1
koolraj09
167
5
Hi all,
I am not able to physically understand the term "load" when we refer to Single Shaft Gas Turbine engines used for power generation purposes. What is causing the torque on the Gas Turbine (GT) to change? Again how the speed is maintained constant while the torque is varied? For a single shaft GT, shouldn't decreasing the load also decrease the speed, since we will be supplying lesser fuel?

Any intuitive explanation related to the this (working of GT for various loads), the explanation of torque-speed curve and how GT's behave in general, is what I want to understand.
Also why part load efficiency of GT's is poor? What causes this loss of efficiency?
I would be grateful for any explanations, resources or books you direct me at.
Thanks in advance!
 
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  • #2
koolraj09 said:
What is causing the torque on the Gas Turbine (GT) to change?

Any type of engine is a machine that transforms the energy from heat into mechanical energy, which is composed of torque and rpm. When it comes to «how it is done», the concept is to control the expansion of a gas after it is heated such that this expansion is pushing against a mechanical component (turbine, piston, etc.). That creates a torque acting on that component which value depends on the efficiency of the mechanism to convert the energy and also on the speed of that component (The faster it goes the less torque you can retrieve).

So to change the torque, you either change the amount of heat (how much gas is burn), you change the efficiency or you change the speed of the component.


koolraj09 said:
Again how the speed is maintained constant while the torque is varied? For a single shaft GT, shouldn't decreasing the load also decrease the speed, since we will be supplying lesser fuel?

When a turbine is at constant speed, there is only the friction force going against the turbine (from the bearings) and some others to run pumps and such, such that the engine can run. This represents the load at idle.

If you give enough heat (fuel combusted) to produce enough torque to go against this load, the engine will idle at constant speed.

If you give more heat, it will produce extra torque which will be transformed into acceleration of the moving parts (F=ma or T=Iα in rotation). Of course, if you give less heat, there will be a deceleration of the rotating components.

But you can increase the load by coupling an external torque to the shaft of the engine (a pump for example or a vehicle that needs to be accelerated). In that case, if you don't increase the heat in the combustion chamber, the engine will begin to decelerate. But if you increase it enough such that the torque converted by the engine equals the torque required by the load, then the speed of the engine will stay constant.

koolraj09 said:
Also why part load efficiency of GT's is poor? What causes this loss of efficiency?

Usually, to get the maximum efficiency when converting heat into torque and rpm, it demands an engine design that is dependent of the amount of heat, torque and rpm. A «perfect» engine would change shape as those quantities are varied. This is the idea with engines with variable compression ratio or variable timing, or gas turbine with variable geometry.

But if you can't modify the components as needed, you have to settle for a design that will be most efficient when needed (load and rpm -wise). At other loads and rpm (higher or lower), the efficiency is thus usually lower.
 
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  • #3
Hi Jack...Thanks for the clear explanation and sorry for the delay in reply..
I had one more doubt regarding IC Engines. Does the efficiency of ic engine increase or decrease with load? And how does it compare to Gas turbines? Does efficiency of gas turbine and ic engine follow the same trend with increase in load?
Thanks again..
 
  • #4
This is a BSFC map (one way of measuring engine efficiency) for a diesel engine:

TDI_BSFC_02.png

For a constant rpm, the fuel consumption gets lower (better efficiency) as load (or torque) is increased.

For a gasoline engine, maximum efficiency usually happens slightly before maximum load when at low rpm:

817717.jpg

For a gas turbine it is a little harder to find data online about fuel consumption, but I found this fuel efficiency map for a gas turbine used in trains. It shows that fuel efficiency increases with power output (but that doesn't assume constant rpm):

tb005.PNG

If we did a similar map based on the previous piston engine maps, we would get more of a U-shaped curve (With the previous diesel map, you have to follow the red line and then the top black line until you reach the maximum power; the intersection of the red and top lines being the best efficiency point).

Furthermore, comparing diesel and gas turbine under part load:

The next graph shows the change of thermal efficiency of 5000 hp class diesel and gas turbine locomotive as a function of engine output power. This class of gas turbine is classified as middle size and it's part load efficiency is relatively good but still worse in comparison to diesel locomotive especially at low horsepower.

gr20019.png

And what is even worse, gas turbine consumes much fuel than other reciprocating engines at idle. As is mentioned above, compressor continuously must spin at high speed to produce effective compressed air to maintain engine idling,  When gas turbine is idling, turbine spins at 60% speed or more. For example, 1000 hp class gas turbine should spin at over 10000 rpm, and if the engine is kept idling for one hour it will consume over 40kg of fuel, this amount will be four times or more than that of diesel.

This is not so serious in high speed rail application where long time high speed cruising is common and high cruising power is required. But in ordinary rail application coasting time is much longer than powering time resulting in bad fuel economy. In bad conditions, fuel consumption may be twice as much as that of diesel train. In american heavy duty freight train, it is estimated that 5000 hp class gas turbine operation will increase fuel consumption 25% against diesel operation.

Another aspect of high fuel consumption is that low speed operation of gas turbine worsen the fuel efficiency. It occurs even in two shaft gas turbine in spite of it's high torque at low speed. Turbine is designed to bring out it's best performance at specific speed. This is called as "designed point". When turbine is spinning at this speed and then axial load increases, rotation speed will decrease and balance speed because reduction of turbine speed increases its torque. In this process, amount of fuel consumed does not change. This is the distinguishing difference between turbine and reciprocating engine. In reciprocating engine, same amount of fuel is consumed at each explosion and then fuel consumption is proportional to the rotation speed of the engine. But gas turbine is continuous combustion engine and amount of fuel injection is not influenced by the engine speed. If turbine torque doubles at half speed, there is no problem but as turbine efficiency worsens at rotation speed out of designed point, turbine torque does not double but approximately 1.5 times. This means 25% loss occurs at this speed.

 
  • #5
hello every one
This is Adnan am master researcher in UKM Malaysia i have some confuse with 2 tests:
1- if i start the engine with 3500rpm without load then apply load by eddy current dynamo meter to reduce the speed by interval 500rpm every times until reach to 1000rpm and record all engine parameters power ,torque and efficiency all these tests with constant throttle body 18%
2-if i start with 1000rpm and constant load like 20Nm then increase the speed interval 500rpm by increase the throttle body until we reach to 3500rpm and every times we recorder all engine parameters power,torque and efficiency, constant load and different throttle body

so how i can compare between two test? in another words i mean for the first test when i increase the load the engine speed will reduce and the efficiency will increase while the efficiency will increase when i increase the engine speed for second test.
Thanks
 
  • #6
If you are running a generator off grid you'll be in speed control trying to keep the generator supply at 50hz (either 3000 or 1500 RPM depending on the alternator design) this will translate into a GT spool speed of several thousand RPM, there being a gearbox between the GT and alternator. Suppose you were running at half load with a throttle position x, if the load increased the shaft speed would start to drop and the governor would open the fuel valve to keep the speed to give 50hz. If your running on a grid the speed will be set by grid frequency and you'll use speed droop in the governor to control load.
 
  • #7
adnanfares34 said:
so how i can compare between two test?

What do you want to compare?

If you do the first test for different throttle positions and the second test with different constant loads, eventually you will get the same data from both tests.
 

1. What is the difference between full load and part load operation for a gas turbine?

Full load operation refers to the maximum power output of a gas turbine, while part load operation refers to operating the turbine at a lower power output. This can be due to varying demands for power or to maintain efficiency at lower power levels.

2. How is the efficiency of a gas turbine affected by full load and part load operation?

The efficiency of a gas turbine is generally higher at full load operation compared to part load operation. This is because at full load, the turbine is operating at its designed capacity and can achieve higher gas temperatures and pressures, resulting in better efficiency.

3. What factors determine the full load and part load capabilities of a gas turbine?

The full load and part load capabilities of a gas turbine are determined by its design and size, as well as the type and quality of fuel being used. Other factors, such as ambient temperature and altitude, can also affect the performance of the turbine.

4. How does a gas turbine maintain stability during part load operation?

To maintain stability during part load operation, a gas turbine may use techniques such as variable geometry, variable inlet guide vanes, or bypass valves. These techniques help to maintain proper airflow and fuel-to-air ratio for efficient operation at lower power levels.

5. Can frequent switching between full load and part load operation affect the lifespan of a gas turbine?

Frequent switching between full load and part load operation can affect the lifespan of a gas turbine due to the thermal stresses that occur during start-up and shutdown. However, modern gas turbines are designed to handle frequent start-ups and shutdowns, and proper maintenance can help prolong the lifespan of the turbine.

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