3 replies to this topic

[brazilianmecstudent]

Hello,

 

I am a Mechanical Engenieer brazilian student and I trying do design a project to my self as a way to improve my knowledge and I would like to know if someone could help me.

 

I am trying to desing a Thermoeletric system with a boiler, turbine, pumps, condensers and so on.

 

As a conceitual project, I designed a system using biogas.

 

I searched that the city I live produces 250.000 kg of trash per day, with 50% of it beeing organic, with 5.200 kg/h. I also searched that 1000 kg of organic trash produces 250 m³ of biogas and 1m³ of biogas generates 6 kwh. So it produces 7.800.000 J/s.

 

I saw a turbine that uses 1,275 MPa and 300ºC in the input and 6,25 KP in the output. So I think i need to heat the water at this preassure and temperature.

 

With 1,275 MPa, the saturated temperature is about 190ºC and the temperature in the boiler input is about 37ºC. So I used:

 

Q=MC(T1-T2), with 1 kg(of water) x 4184 x 153 = 640.152

Q = ML = 1 x 2.600.00 = 2.260.000

Q = 1 x 2010 x 110.= 221.100

 

to know the massic flow I did: 7.800.000 J/s / 3.121.252 J = 2.5 Kg/s = 9010 kg/h

 

Is that correct? Is this way to calculate correct?

 

Now Im trying to design the pipe. It has 250 m. With valves, conections, it has 304,1m. For the pressure loss, I have to know the coefficient of friction, To know it I have to know the Reynold number e for that I have to know the cinematic viscovisity, Is there a table or other way where I can find the the cinematic viscovisty of the superheated steam at 300º and 1,275 MPA? Is there another equation I can use to find the pressure loss in the pipe?

 

After, I will try to calculate the thermal loss in the pipe. Where I can find the convective coeficient of the superheated steam at 300º and 1,275 MPA?

 

Is there a number that I can use as the accurate eficience of the boiler (I know that the boiler have a efficience)?

 

Thanks a lot.

[Pilesar]

You offer a lot of details but I am not clear on what you are trying to do. If this is an imaginary concept only, it would probably be more efficient to generate steam nearer the turbine. This link will supply viscosity and some other useful properties for superheated steam:  https://www.spiraxsa...ed-steam-region

[breizh]

Hi,

Consider this link https://www4.eere.en..._tool/tutorials

You should find good pointers for your project.

Another calculator for steam properties 

https://www.tlv.com/...team-table.html

Good luck

Breizh

[Jorge Kley]

Hello.

Few comments:
- The way that you calculated the steam generation is ok, for general purposes.

- The final steam condition (approx. 12 bar and 300 °C) seems a bit off to me. Usually old power plants work on 50...60 bar and 450...460 °C range. New plants will work on 80...110 bar and 480...510 °C range.

- The boiler feeding is usually heated above 100 °C. Search for the "dearator" function to see what I'm talking about. 37 °C is too low. It not only will make your steam generation poor, but also would give lots of mechanical problems to the boiler, due to the dissolved gases present on water.

- There a significant pressure drop inside a superheated steam boiler. For your calculation, you should consider at least 10...15 bar of pressure drop between the drum and the superheater outlet. For your thermal calculation you should then use the energy to heat the steam at the saturation temperature at the superheated steam pressure + pressure drop.

- Pressure drop from boiler outlet to turbine generator house should be around 2 bar (not more than 4 bar usually).

- You should use ASME criteria for the pipeline design, since it is high pressure. It will impact on the pipe thickness, which will change the heat loss (for general purposes you should also use a thermal insulation).