In most power plants, the heat from nuclear energy (or heat from the combustion of fossil fuels, such as coal or natural gas) actually boil the water, so that steam is used to drive the turbines, not hot water. The steam is produced under high pressure, so the temperature of the steam is considerably higher than the normal boiling point of water at one atmosphere. In some plants the steam is right at its boiling point (saturated steam), and in some plants the steam is superheated.
The exception to this is hydroelectric plants, in which flowing cold water from a dam drives the turbines.
Even without high pressure, the temperature of steam can be considerably higher than the boiling point of water. The boiling point is the minimum temperature for steam, not the maximum.
Good point. However, in my defense, all of my personal experience is with naval nuclear plants, in which the steam that is produced is saturated. The only way to produce saturated steam at very high temperatures is for the steam to be under high pressure.
Conversely, the water in the primary loop is kept liquid at comparable temperatures by maintaining the primary loop at a considerably higher pressure than that of the secondary loop.
I’m not so sure about that… Take a sealed but expandable container (say, a bottle with a balloon over the opening, or a cylinder with an unloaded piston) and fill it completely with water. Heat the contents of the bottle to an arbitrary temperature, while allowing the container to expand freely. You’ve now got saturated steam at high temperature and atmospheric pressure.
I wonder if this is a problem of terminology. You can of course heat the steam to an arbitrarily high temperature, but if it’s above the saturation temperature (boiling point), then it’s no longer saturated, but superheated.
Note that by “very high temperatures,” I mean temperatures greatly in excess of 100 deg C (212 deg F). In fact, naval steam plants routinely produce steam at temperatures in excess of 400 deg F.
With respect to your example, Chronos, if the arbitrary temperature exceeds 100 deg C at 1 standard atmosphere, and the system is adiabatic and allowed to reach equilibrium, then all of the liquid water will flash to steam at a temperature greater than the boiling point, resulting in superheated steam.
By definition, saturated steam is steam in equilibrium with liquid water (i.e. at the boiling point of water). For any temperature greater than the normal (atmospheric) boiling point, by necessity the pressure must be greater than atmospheric to have saturated steam.
Ah, I was presuming that it meant that the mixture of gases was holding as much steam as it possibly could, a situation for which pure steam with no admixture of other gases surely qualifies. But for equilibrium with liquid water, yeah, you’ll need pressure for that.
A water tube power boiler is generally one of two types either a drum boiler or a once through boiler.
On a drum boiler, saturated dry steam leaves the steam drumb and enters the superheater tubes and heated above the saturation point.
On a once through boiler water enters the boiler and passes through a series of tubes until all the water in turned into steam, then it continues through the tubes and is heated above the stauration point.
I don’t get this bit. We have a number of AC generators from different sources feeding into the same grid. This might include domestic power as well as power plants such as hydro, coal, nuclear and solar.
How is it then that the different AC sources don’t cancel each other out? The phases of each of these AC generators would have to be perfectly synchronised to become additive. Otherwise the wave form would be all over the shop.
The generators are in synchronized. If a generator connected to the buss out of synic then it will either pull into synic, trip out, or trip out the system.
When bring a generator on line it is brought up to near system speed, then the generator is brought into synic with the system. When the speed produces 60.0 cycles and is in synic then the breaker is closed. At this point the generator is not outputting power to the grid. The govenor load setting is increased. Rather than increase the speed the generator will begin to output power to the grid. One generator will not incrrease the cycles of the whole system.
If you have a two or three generator when an addition generator is added at the same time the load on one generator is incrreased the others have to be decrreased or the cycles will increase.
The steam produced in all of the naval nuclear plants that I am familiar with is saturated steam, not superheated. The advantage of this is that the temperature of the steam generator tubes is moderated, being maintained right at the characteristic boiling point of water for the operating pressure. Conversely, there is no upper temperature limit for superheated steam. If the operating temperature were to get too hot, you increase the chance of steam generator tube failure, which would result in the release of radioactive primary coolant.
No reference for naval nuclear plants (obviously), but see here:
But how are generators across the whole grid synchronised? Is there a synchronising circuit in each unit that does the job. For example, if a domestic power generator feeds power back into the grid, does that unit have some sort of synchronising device built in?
Whenever you bring a new generator into the system, you have to sync it up with the rest of the grid. One of the ways to do this is with phase lights. These are dirt simple. You just wire up lights in between your generator and the grid. Your generator is making a sine wave, and the grid is making a sine wave. If the two sine waves are in phase, then there won’t be any voltage difference between your generator and the grid, and the light won’t light. If the generator and the grid aren’t in phase, though, there will be a voltage difference, and the light will light up.
If your generator and the grid aren’t close in frequency, the phase light is just going to flicker. Once you get your generator close to the grid frequency, the phase difference doesn’t change so quickly and the flickering slows down until the light is going bright and dim and bright and dim relatively slowly (maybe once every couple of seconds). You just wait until the light goes out and throw the switch.
A synchroscope is a fancy meter that shows you the phase difference on a dial, but otherwise it’s the same as the lights. The little dial is going to spin around like mad until you get the generator close to the grid frequency, then you just wait for the dial to get to the zero position and throw the switch.
Once the generator is online with the others, the AC line itself will keep them in sync. If your generator tries to slow down, the phase difference between its since wave and the grid will cause current to flow into it and speed it up like a motor. If it tries to go faster, its AC sine wave will be advanced from the grid causing more current to flow from your generator to the grid, making it harder to spin your generator. The increased mechanical load will cause it to slow down. Since it effectively can’t speed up or slow down, your generator remains locked to the grid frequency.
Because there isn’t any synchronizing mechanism other than the AC line itself, changing the frequency of the entire grid gets a bit tricky, as was mentioned on the previous page of posts.
As was stated above, you have to synic the generator to the system before bringing on line. Once on line it should not slip out of synic, if it does the plant either trips or the system trips.
Imanage 10 people holding tightly onto a pole and running at a constant speed. If an eleventh perso were to run up to the group and grab the pole he will have to match the groups speed. If he comes in to fast or to slow he will trip, loose connection with the group or if he holds on tight he can cause the group to trip.