why steam condensation in power plants

[faslan]

ok in conventional coal power plants we lost large amount of heat (35%) when steam condensation to water, why don't we just send stream back in to furnace without condense in to water more fuel save

Edited December 11, 2014 by faslan

[Endy0816]

Converting the water into steam is what is driving the turbine's rotation. To put it another way you would have at least a 100% loss without it.

 

More accurately what you need is cold water entering the cycle, not necessarily a Condenser. Reason why we do use a Condenser is because water from nature has all kinds of contaminants, which would cause buildup on the piping and damage to the turbine.

[Danijel Gorupec]

Hi faslan... You spend less energy if you pump small volume of water into a high-pressure boiler than if you pump large volume of steam into the same high-pressure boiler.

 

Also note that a limited-size thermal power plant cannot be 100% efficient - not even in theory. Someone can therefore argue that a thermal power plant is rather efficient.

[billvon]

ok in conventional coal power plants we lost large amount of heat (35%) when steam condensation to water, why don't we just send stream back in to furnace without condense in to water more fuel save

 

Because you would have to pump the steam back in under pressure to the boiler. It would take more energy than you get out of the turbine to do that. Condensing the water back to a liquid allows you to pump it back in with much less energy used, resulting in a net gain in energy.

[John Cuthber]

One way to look at it is that the turbine is driven by the fact that water expands nearly 2000 fold on boiling.

[swansont]

Isn't the condenser the difference between the Newcomen and Watt steam engines? (vague recollection from one of the James Burke series) The latter was much more efficient because it did a much better job of condensing the steam, drawing more work out of the system.

[Endy0816]

Your memory hasn't let you down:

Newcomen's engine was only replaced when James Watt improved it in 1769 to avoid this problem (Watt had been asked to repair a model of a Newcomen engine by Glasgow University. A model exaggerated the scale problem of the Newcomen engine). In the Watt steam engine, condensation took place in a separate container, attached to the steam cylinder via a pipe. When a valve on the pipe was opened, the vacuum in the condenser would, in turn, evacuate that part of the cylinder below the piston. This eliminated the cooling of the main cylinder, and dramatically reduced fuel use. It also enabled the development of a double-acting cylinder, with upwards and downwards power strokes more suited to transmitting power to a wheel.

http://en.wikipedia.org/wiki/Newcomen_atmospheric_engine#Successor

@OP: If you want to look up about the Carnot Cycle, that might shine more light on the subject.

[Danijel Gorupec]

 

Because you would have to pump the steam back in under pressure to the boiler. It would take more energy than you get out of the turbine to do that. Condensing the water back to a liquid allows you to pump it back in with much less energy used, resulting in a net gain in energy.

For the sake of clarity I must say that it is not necessary to condense steam back to liquid to generate net energy gain. It is enough to remove some heat from the steam (so that its volume decrease) - however condensing it into liquid is certainly the charming idea.

[Enthalpy]

In addition to

- Condensation not necessary, railway engines don't condensate

- Clean water protects the boiler against scaling, and a closed cycle keeps water clean

- Pumping liquid water back is better than vapour

- Condensing at <<100°C hence <<1atm improves the efficiency

 

I wish to add that most steam engines let some vapour condensate during the expansion, not only at the condenser.

- This is unavoidable where the source temperature is bad, that is at water-cooled nuclear reactors. Pressure must be high for good expansion, so the initial vapour is nearly saturated, and some condenses very early. Nukes have a vapour dryer and a re-heater between the high and low pressure turbine stages.

- This early condensation improves the motor's efficiency, as opposed to what most textbooks claim. It makes good use of some heat used to evaporate the water. Without it, simple figures tell that nuclear power plants couldn't possibly achieve their observed efficiency.

- But the droplets' high-speed impact wears the blades more quickly. They need an adequate alloy and must be replaced more often.

 

One may also note that gas- and oil-fired power plants have a combined cycle, where the gas burns in a gas turbine whose exhaust heat powers a steam turbine - plus many heat exchangers, pre-heaters and so on meanwhile. This achieves well over 40% conversion. This is more difficult with coal which can't burn within a gas turbine, but because coal is cheap, plants exist that first transform coal (using water) into a liquid or gas that can burn in a gas turbine.

 

One may also wonder if water, needing much heat to evaporate, is the best choice. The answer is "yes" at usual flame temperatures, especially thanks to all the added exchangers and pre-heaters. At low temperature, say to exploit the temperature difference between the Ocean's surface and deep water, other fluids like carbon dioxide are better. The non-obvious comparison results from complete cycle computations including technical losses.

 

The condenser is a very difficult part of a power plant. It must treat huge volumes of vapour (pressure as small a possible), drop as little pressure as possible, drop as little temperature as possible, but transfer a huge heat power. Where no river suffices to cool the plant, the cooling tower is the other very difficult part.