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Gas Turbine: Full Load/Part Load Understanding

  1. Aug 16, 2014 #1
    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!!
     
  2. jcsd
  3. Aug 16, 2014 #2

    jack action

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    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.


    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.

    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.
     
  4. Aug 21, 2014 #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..
     
  5. Aug 21, 2014 #4

    jack action

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    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:

     
  6. Apr 24, 2015 #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
     
  7. Apr 25, 2015 #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.
     
  8. Apr 25, 2015 #7

    jack action

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    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.
     
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