4 replies to this topic

[rls363]

Hello

I am a 4th year chemical engineering student and am doing my design project.

I am designing a boiler to cogenerate steam for a steam turbine and a paper process that requires 15kg/s steam (14.5kg/s steam@5bar and 0.5kg/s steam@10bar) . The heat is recovered from gas turbine exhaust.

My original design was to be a boiler with superheater - generating steam at 99 bar with 140K superheat

Therefore producing circa 9MW (15kg/s x 600kJ/kg). 600kJ/kg is taken from the Mollier diagram when 99bar723K stm is expanded isentropically to 5bar423K stm. Please see diagram attached.

BUT a professor in Chemical Engineering recommended generating ONLY SATURATED STEAM, thus making the design of my boiler simpler (shell and tube heat exchanger).

I am now very confused as I can't find the ideal conditions for a saturated steam turbine, nor the maximum MW rating. This is because of my inexperience with the Mollier Diagram and lack of understanding.

Please could you help me?

Thankyou in advance
RLS363

Attached Files

•  Energy_Balance_over_Steam_Turbine_Cheresource.doc   136.5KB   71 downloads

[Art Montemayor]

You state: "I am now very confused as I can't find the ideal conditions for a saturated steam turbine, nor the maximum MW rating. This is because of my inexperience with the Mollier Diagram and lack of understanding. "

A saturated steam system is, in my opinion, simpler than the superheated version you were on before. Therefore, if you have done all your Thermodynamics homework and understand the theory and the problem, you should not have any confusion. Something is basically wrong here. If you feel you are inexperienced with the Mollier Diagram, then you haven't done your Thermo homework! And that probably is what is causing you problems.

We all here stand ready to help you in this interesting project, but you must be prepared to discuss the Mollier - and other thermodynamic databases and/or principles. If you are not prepared to do that, you will have nighmares with what we will be trying to explain to you.

You see, we don't teach Thermodynamics here. We apply it and discuss its applications in the real world. You and your teacher are responsible for you learning Thermo - and that includes the Mollier Diagram, among many other things. So we have to have your committment to apply yourself to this problem, because we can't (and won't) do it for you. We will help and guide you, but we need your learned attention and questions.

Await your reply.

[donthurtme]

saturated steam turbines exist i guess, but think of those turbine blades spinning and hitting water particles as they form. That is why steam turbines use superheated steam and typically keep it superheated when it leaves.

[Art Montemayor]

Hurt & RLS363:

Saturated steam turbines do exist and, I suspect, they far outnumber superheated steam turbines in number – but not in total installed horsepower.

I’ve operated numerous steam turbines on saturated steam and they have operated very well and very reliably. I’m also thinking about “those turbine blades spinning and hitting water particles as they form”, but I don’t have any fears since in my turbines we always ensured that the entering steam was saturated and not a “wet mixture”, or bad quality steam. (Saturated steam is 100% steam vapor - with no liquid particles in it. The quality of a steam flow is the % by weight which is steam vapor.) For that purpose an engineer employs a good steam system design that incorporates adequate and well-installed pipe insulation, sloped headers with good drain points, sufficient steam traps and header drains, and condensate separators.

I have commented on the subject of applying saturated steam turbines to pumps and other rotating equipment on this Forum before and a “Search” through the Forum would bring up that information. I consider this as valuable, basic information for any engineer to have in his/her hip pocket when it comes to process design and/or working in process plants. Steam energy is not outdated. It is alive, efficient, and very practical to apply in many process circumstances. If interested, go to:

http://www.dresser-rand.com/steam/calc/main.asp

for a quick method to estimate the resulting hp generated with steam energy.

Some Steam turbines use superheated steam for a variety of reasons:

• To ensure that the entering steam is 100% in quality, without resorting to any other auxiliary equipment;
• To pack in more energy into the fluid in order to extract more useful work per unit of mass flow upon expansion through a turbine;
• To allow easy use of a back pressure in the exhaust steam in order to use the product as a good heating fluid source in a processing plant complex.

Contrary to what Hurt states, the exhaust steam may not be “typically superheated” as it exits the turbine. Some very efficient turbines are classed as “condensing” turbines and their exhaust, as the name implies, is in the partial vacuum range.

[rls363]

Dear Art & Hurt

Thankyou for your responses and particularly to Mr Montemayor

Since my first message I have been lucky enough to see a steam turbine expert and he was able to explain the thermodynamics of steam turbines!

With use of steam tables and the Mollier diagram we were able to prove that Saturated Steam could be used, but clearly less energy would be produced.

In industry they have agreed that 12-15% wetness is the limit that saturated steam can have on leaving the turbine. After calculation, expanding 20kg/s saturated steam (assumed 5% wet) from 30 bar to 5 bar was proved possible, as the steam leaving was about 14.5% wet.

This would produce about 5.2 MW.

My aim to find this out was selfish as it meant I would be able to design a simple Shell and Tube Heat Exchanger Waste Heat Boiler.

I have gone against this decision. My process is a Gas Turbine-Steam Turbine cycle and superheating will greatly increase the efficiency of the process. With 99bar 450C superheat the steam turbine can generate about 12 MW.

Thankyou again for your responses. There is a thread below already but I might return with Waste Heat Boiler queries!

Rls363

ps the wetness is certainly not ideal BUT if below 15% it would suffice for a design and you could get away with it. in practise the turbine would need higher maintenance. im pretty sure many turbines in nuclear stations use just saturated.