Crimedog

Alright folks, sorry for the length, and I hate to have to post this one up but I am pretty much stumped on a thermodynamics problem for school. Basically our campus (Minnesota State University - Mankato) tried to employ a cogeneration system about 15 years ago. They use boilers to make steam to heat all the buildings and water, etc. The steam comes off the boilers at 150 psi and has to be cut back to 10 psi before it enters the buildings. This is done by a pressure relief valve that essentially lets more steam through when the building side pressure drops.

To take advantage of this pressure drop they tried to install a cogen turbine to get some electricity, which requires 40,000 lb/hr to make 434 kW of electricity. The inlet was at 150 psi right off the boiler and the outlet was 50 psi. Any excess steam was bypassed and throttled down to 50 psi to later combine with the cogen steam. The problem was that when they brought the system online the pipes didn't allow enough flow at the lower 50 psi pressure and the buildings got starved for heat.

Now for the problem... we are supposed to come up with a solution to this, it just has to make logical sense, economics aren't super important. The only kicker is we HAVE to keep the relief valves at the buildings and can't change the diameter of the building supply pipes.

Is there something out there that would allow the 50 psi turbine steam and 150 psi bypass steam combine and form 100 psi to the relief valves so they could operate properly? I was thinking of basically a turbine and compressor so the 150 psi steam turns the turbine and compresses the 50 psi steam on the turbine side, thus equalling out the two and just combine them after the contraption? Not sure how that would work though...

Anybody have any ideas? I've been mulling it over for a week and can't find anything that seems to make sense!

BC847

I'm no engineer but perhaps one could drop the condensing pressure and temp at the building end to increase the heat extracted. Similar to chilling the exhaust of a prime mover's (steam turbine) such that the condensate chest runs in a vacuum.

Perhaps even reheat the turbine's exhaust (increase the superheat).


Just a thought . . .. .

CamperAndy

Change the regulator (relief) delta pressure setting. It would open more and longer to allow for the needed amount of hot water (condensate) to flow into the buildings.

BTW your kW and MW numbers do not match up. A 600 Megawatt generator site is a very large facility, way larger then what a University would try to manage.

YZ3500F

Find a millwright im a millwright apprentice in the twin city's and i know allot of guys who work with turbines but i have never worked on one so i cant really help but I do know that there are a few other millwrights on this site and maybe one of them could help

YZ3500F

I thought about your turbine compressor idea and i have head about gas turbines being used to fire steam turbines using the exhaust to heat the steam for the steam turbine. But i was thinking wouldn't it be possible to bump the pressure out of the boiler and therefor bumping the pressure in the stream line prior to the turbine and then making the exhaust exiting the turbine under higher pressure and supplying the 50psi that you would need idk if this can be done but its a thought

Crimedog

Thanks for the ideas guys, it'll definitely give me something to keep thinking over. Best thing I've come up with so far is running a couple of feedwater heaters. I did the calc's for a reheat and came up with a zero efficiency! Might have to make some adjustments...

The 600 megawatts is not the size of any generator, that's the handle of a gentleman on here that works with big time turbines and I've heard him talk about some thermo stuff. I just used that to try and get his attention

DaveB.inVa

Pretty sweet your college has a cogen setup!

Can you give out any details concerning your distribution system? What voltage do you generate at (I guess 480V), what kinda switchgear does it connect to, is your system ungrounded, high resistance grounded or solid grounded, how do you tie to the utility? Single line diagrams are always welcome!

I work at a large chemical company and we run quite a few generators although we don't cogen. Its all very interesting to me, I work only on the electrical side of things though and probably should learn more about the steam power side.

Crimedog

DaveB,
I really don't know how they had their switchgear... The system actually was brought online back in the mid 90's and ended up a failure within a short period of time because of the lack of flow. Basically a $500,000 investment down the drain! It's just sitting in the basement of one of the buildings right now, waiting to be parted out.

I've never thought about how they tie into the utility, I'm sure there's some sensative electronics to feed it in, or maybe they planned to just cut one building out of utility and feed it only with the cogen.

The steampower side is pretty interesting, it's incredible how much steam and fuel they go through in a day around here... millions of dollars per year!

DaveB.inVa

[QUOTE=Crimedog;2003896
I've never thought about how they tie into the utility, I'm sure there's some sensative electronics to feed it in, or maybe they planned to just cut one building out of utility and feed it only with the cogen.
[/QUOTE]


I'd say they left the generator tied in to the utility. There are lots of advantages of being tied to a large source such as them. You get to take advantage of the utilities short circuit capabilities, frequency regulation and stability. It probably doesn't have much on the way of electronics beyond standard relaying and such though.

I was interested in it because I've never seen one that small before and am curious just how they went about the tie and distribution.

Sounds like a fun project though! Good luck on it!