The way a turbine's efficiency is defined is a bit different from most other efficiencies in that the actual steam uasage is compared with the usage in a perfectly isentropic expansion, i.e.
E = (h1 - h2) / ( h1 - h2S)
The terms are as defined in Sheiko's first post and h2S is the enthalpy at the outlet pressure after an isentropic expansion
I understand that E is the isentropic efficiency.
Does this mean that we can always consider the expansion process through the real turbine as an isentropic one?
The efficiencies you have back calculated seem very low. I would expect them to be >50% and more likely in the 70-80% range, depending on the sizes. You say the 25-30% numbers you found are in line with what you have seen in the literature, but I can only think that those must be based on some other definition of efficiency. If you define E = (h1 - h2) / h1 You would probably get those sort of numbers, but that is not the way I have seen it done - although it does give you a better indication of what fraction of the energy in the steam you have converted to shaft power.
Attached is the specification where I have found the formula shown in my first post (by the way, seems that the formula is incorrect right?).
Efficiencies are defined as the turbine efficiency and range from 20% to 70%, depending on the power rating of the turbine. In my case, the steam turbines are rated < 40 kW, so I have considered 20-25% as a sensible range.
For example, one data sheet display:
- inlet pressure = 25 bara
- inlet temperature = 340°C
- outlet pressure = 3.5 bara
- max power = 36.8 kW
- max specific consumption = 37.8 kg/h/kW
Edited by sheiko, 16 March 2012 - 11:20 AM.