Types of Steam Turbines

15 Jul
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All steam turbines can be classified into two categories; extraction (condensing) steam turbine and non-condensing steam turbine also known as back pressure steam turbines.  Both of these steam turbine types have their own operating schemes and benefits which are described here in detail.

Extraction Steam Turbine

Figure 1: Extraction (condensing) steam turbine

The extraction turbine contains two outlets as shown in figure 1. The first outlet extracts the steam with intermediate pressure for the feeding of the heating process while the second outlet extracts the remaining steam with low-pressure steam for the condensation. The extraction of heat from the first outlet can be stopped to generate more output. Steam control valves at this outlet make this steam very flexible and allow adjusting the output as per demand.  The steam from the second outlet goes to the condensation chamber where cooling water brings the temperature of the steam down. The condensed water then goes back to the boiler for the regeneration of the electricity of power, therefore, it is also known as the regenerative steam turbine. The scheme of extraction turbine with cogeneration system is shown in figure 1. This turbine has following benefits and disadvantages.

Advantages:

  • This type of steam turbine can be used to generate a high amount of electricity.
  • It is a flexible turbine with the ability to regulate output as per changing need.

Disadvantages:

  • It is costly turbine with lots of auxiliary components
  • Heat rejection in the condensation process reduces the overall efficiency of the system.
  • It is usually used on industrial level and requires complex configuration

Figure 1: Extraction (condensing) steam turbine with cogeneration system

Back Process Steam Turbine

The non-condensing steam turbine uses high-pressure steam for the rotation of blades. This steam then leaves the turbine at the atmospheric pressure or lower pressure. The pressure of outlet steam depends on in the load, therefore, this turbine is also known as the back-pressure steam turbine. This low-pressure steam uses for processing and no steam is used for condensation. The schematic diagram of the back process steam turbine with cogeneration system is shown in figure 2. There are lots of benefits of this steam turbine but at the same time it has few disadvantages which are listed below.

Advantages:

  • The configuration of this steam turbine is very simple
  • It is relatively inexpensive as compared to extraction steam turbine
  • It requires very less or no cooling water
  • Its efficiency is higher as it does not reject heat in the condensation process

Disadvantages:

  • The biggest disadvantage of this type of steam turbine is that it is highly inflexible. The output of this turbine can’t be regulated as it does not allow changing the pressure and temperature of steam in the turbine, therefore, it works best with the constant load.
  • The thermal load of this turbine defines the flow of steam mass which makes it difficult to change the output value. Other methods to regulate output reduce the efficiency of the overall system.

Figure 2: Non-Condensing (Back-Pressure) Steam Turbine

29 thoughts on “Types of Steam Turbines

  1. Dear

    i want some knowledge about performance of steam turbine if operated at slight higher back pressure (1.2 kg/sq Cm) than the design (0.9 kg/sq Cm). is there any mechanical demage harm to steam turbine?

    • Dear Sana,
      more the pressure of the exhaust steam , more power you compromised.
      i.e. the power generated at 0.9ksc (exhaust pressure) is 1MW ( for eg), and if we make the exhaust pressure as 1.2ksc, this means we lost the power which we could have generated from the pressure drop of 0.3ksc (1.2-0.9).
      NITISH SHARMA

  2. Sir i need some details, how much velocity required for rotating for this turbine anda head leangth and which types of generator used

  3. My partner and I absolutely love your blog and find many of your post’s to be just what I’m looking for. Would you offer guest writers to write content for you personally? I wouldn’t mind publishing a post or elaborating on some of the subjects you write regarding here. Again, awesome website!

  4. i am dharmendra working as an Sr. mechanical engineer in patil group . i am interested in power generation please give the details of all turbine 7 his working principal

    thank

  5. hi,

    i had a few queries with respect to the CHP process:

    1) With extraction, fuel consumption will be higher as compared to power only system. Is my assumption correct?

    2) If steam is not being extracted, then can the same amount of fuel be used to generate a higher amount of power or is it a sunk cost and power generation is limited to rated output irrespective of whether steam is being extracted or not?

    thanks
    sameer

    • dear sameer,

      yes with the extraction fuel consumption is higher as compared to fully condensing mode bcs in the full extraction mode we have utilized the all energy of steam means atmospheric pressure to condensing pressure (0.17 bar) which gives 15-20 % extra power.

    • Dear Sameer,
      1. The power from extraction turbine is generated as an advantage of the generated steam, though our prime use of steam is for processes. i.e. for any process if we need to generate steam ,it is advantageous to increase the capital and operation cost to generate steam of higher pressure and temperature as needed in the process. through this we can generate power as a by-product at very optimized cost.
      for eg. – if you need to construct 3 rooms for your need in your house, and suddenly you realise that you can make extra 2 rooms for optimized increased construction cost for tenants and will make income out of it .
      2. The generated power is directly proportional to the amount and pressure and temperature of the steam , but limitation is the metallurgy of the blades and casing of turbine and the rating of alternator.This means we cannot go beyond the rated capacity of turbine and alternator due to metallurgy limitations.

      Hope this will help you a bit
      NITISH SHARMA

      • we need extraction steam for processes. If we purchase higher extraction capacity steam turbine in case we need more extraction in the future, how efficiency is affected when we use the turbine at low extraction. For example;

        steam turbine which have 160 ton/h extraction capacity, works at 130 ton/h extraction and 50 MW
        steam turbine which have 220 ton/h extraction capacity, works at 130 ton/h extraction and 50 MW

        which is more efficient in this case?

  6. Hello Sir I personally thanking you that there is lot useful stuff and basic of turbine, I am a student of Master in Engineering and want to know more about steam turbine as I want to further work on it. Actually I want to know that how much pressure and temperature required to run steam turbine and is it possible to make 2-3 Kw capacity turbine ??? Sir if you know contact me as early as possible because in this week i have to finalize my topic and have to work on it.

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  8. I am diwakar singh I working in nahar group in power plan sir please give me details of formula use in calculation of 1steam velocity, 2 turbine heat rate ,3 turbine efficiency

  9. I am unable to understand the exact difference between the back pressure and extraction turbine.

    Is it correct to call an extraction type of turbine a back pressure turbine if we are not condensing steam.

    Thanks for the reply in advance.

    • hi…..I m Mani,studying diploma in mechanical engineering.
      Is any turbine like impulse-reaction turbine.. If any what’s its working principle

  10. Dear Sir 1
    I going to by condensed turbine , with capacity 25 MW h , we need to extract only 35 ton low pressure steam 3 bar and the rest will be condensate water . How much steam consumption needed for MW ? ( steam used 41 bar 370 C)
    Best regards
    Mohamed mill manager for Edfu pulp and paper mill

  11. Pingback: Steam Turbine Efficiency - Turbines Info

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