In thermal power stations, why is a condensor needed?

Question

So as I understand it, to generate power in a thermal power station, water is heated to steam using e.g. coal and this steam is used to drive turbines that generate power. After the steam has passed the turbines, it is condensed back to water and the heat released in cooling towers. But why is this condensing stage needed? Why not just keep the water hot until it is re-heated? Wouldn't that save the amount of fuel needed to heat the water?

Answer 1

The main purpose of a condenser is to provide a very low pressure - or 'vacuum' as it is sometimes called - at the exhaust from the turbine. This allows the turbine to operate across a much larger pressure difference than would otherwise be the case. The steam plant then becomes more efficient.  The condensed steam - the 'condensate - can also be returned to the boiler for further re-cycling and this minimises the fouling due to the deposition of dissolved solids.

Answer 2

The condenser is called a 'Surface Condenser' which presents the maximum condensing area to the exhaust steam of the Turbine.

This extracts the greatest amount of energy from the steam to maximise the power output of the turbine. The steam, at this low pressure (vacuum) condenses at 65°C.

The condenser operates under high vacuum which occurs due to the condensing of the steam. (This causes a huge volume decrease in that the water, when converted to steam expands by 1,800 times. When condensing, it therefore occupies a volume 1,800 times LESS as water).

The water formed is pumped back to the Boilers for re-use.

The Cooling towers don't cool this water as it's treated water specifically used for steam generation.

A separate Cooling Water System is used for the surface condensers and other heat exchange (cooling) systems throughout the Power Station. It's THIS water that is cooled by the towers and is also recycled from the tower collecting basin to the Cooling Water distribution pumps.

EDIT: I've just remembered, the Surface Condenser condensate pumps, deliver the condensate through the Ejector Condensers to gain pre-heat prior to returning to the boilers.

Answer 3

The other guys missed the point.

Because if you didn't condense the steam (in the condenser) it would remain as steam, and steam takes a huge amount of space. And steam is useless in a boiler. A boiler needs water to operate, and hot water is better than cold water.

A steam system depends on the boiler to change hot water to steam, absorbing a lot of heat in the process. The steam is under pressure, and piped to the turbine where it is allowed to expand, driving the turbine. The condenser then changes the steam to hot water, which is reused immediately in the boiler.

The steam must either be: (and all 3 are in use)

1. condensed so it can be reused

2. allowed to escape to the atmosphere

3. used for nearby thermal heating purposes

For example, Boston has a power plant used to generate power for the subways. It's excess steam is pipped around the city and buildings use it for heating. If you go to the Boston Science Museum, there is an old very large steam engine from the 1890s that is fed steam from that power plant. The steam engine operates very slowly, maybe 2RPM, but it runs.

Answer 4

All heat engines depend on a temperature difference. The second law of thermodynamics implies that for upper temperature T1 and lower temperature T2, the maximum possible efficiency is

(T1 - T2)/T2

This has been known since the mid-19th century.

BTW, this applies to all kinds of heat engines, including the electricity generators of nuclear and fossil-fuel power stations.  James Watt's contribution was not, as legend would have it, using the expansive power of steam, but incorporating a separate condenser into earlier steam engines.

Answer 5

It is all a matter of controlling the management and storage, the amount of steam used to drive the turbines requires vast amounts of water per miute and to store this amount of hot water would require a massive area and would also cause even more health & safety issues.

Answer 6

To get the the large change in enthalpy across the turbine, the steam is at high temp. and pressure at the inlet and low temp. and pressure at the exit.  Note that the fluid exits the turbine as high-quality saturated mix.  To bring the fluid back to the high working pressure, a pump or compressor must be used.  Due to the differences in specific volumes of saturated water and steam, it is much more energy efficient to condense the saturated mix down to a saturated liquid and then use a pump to bring the fluid back to pressure.  By contrast, in order to use a compressor, the saturated mix would have to be heated to a saturated vapor, which would reduce the fuel required to super-heat the steam, but also would reduce the overall thermal efficiency due to the huge energy requirements of the compressor.  The energy required to use a pump is on the order of 1% of the net power output, while the energy required to use a compressor could be up to 50 or 75% of the net power output.  Also note that no temperature change occurs during a phase change, so a properly designed cooling tower should only facilitate a phase change, and not drop the temperature of the water too drastically.  So, no, you cannot build a good steam power plant without a condensor, because there is not a good, efficient way to bring the fluid back to the working pressure after the turbine.  If you could find an efficient way to pressurize a saturated mix, you would become a very rich person.  You should checkout feedwater heaters, though.  They're a technique used to raise the temperature condensate in order to reduce the fuel requirements.

Answer 7

The difference between the heat of steam per unit weight at the inlet to the turbine and the heat of steam per unit weight at the outlet to the turbine represents the heat which is converted to mechanical power. Therefore, the more the conversion of heat per pound or kilogram of steam to mechanical power in the turbine, the better is its efficiency. By condensing the exhaust steam of a turbine at a pressure below atmospheric pressure, the steam pressure drop between the inlet and exhaust of the turbine is increased, which increases the amount of heat available for conversion to mechanical power

Answer 8

I know for nuclear power stations, the water needs to be cooled to a certain temp to enter back into the stream, river etc.  For them it is an environmental thing.  If the water enters the river above a normal temp it changes the ecosystem of that environment.

I'm not sure if it exactly answers your question, since this is nuclear power and the water is used to cool the reactor down.

Answer 9

The main purpose of a condenser is to recover the water used to make the steam and "close the loop"...   A condenser really isn't necessary to operate steam equipment (reciprocating or turbine could exhaust steam to the atmosphere), but  the use of a condenser will greatly increase efficiency.   Consider a steam locomotive which ran the rails of the old west without a condenser("Open Loop"). You just had to refill the locomotive with water periodically to replace what was lost in the exhausted steam.

By using a condenser the steam cycle loop is closed allowing a great increase in efficiency by the recycling of water and recovery of heat that would otherwise be lost.

However by using a condenser the overal plant efficiency is greatly improved.

1.) The condensing of low temp steam in a condenser causes a rapid collapse in its volume resulting in a vacuum. (air ejectors are used to create the initial vacuum and remove non condensable gasses )   Having a vacuum into which steam is exahusted into from the turbine is like having another 14.7 psi of steam pressure to work with.   Its like pushing with steam at the inlet and sucking out the steam  vs just pushing. (The vacuum removes back pressure from the exhaust)

2.) High pressure boilers need very pure water to prevent the formation of scale and sludge. Both of which can severely damage a boiler.   Constantly boiling water off and adding fresh to replace it (Open Loop) will cause the disolved minerals present in the feed/makeup water to concentrate in the boiler.

By Closing the Loop water is recycled with a minimum need to add additional makeup water. This minimizes the addition of disolved minerals to the boiler to a level that can be managed  and helps to prevent scaling of the boiler tubes.    This prevents damage to the boiler and saves money by avoiding expensive repairs and down time.

3.) Cool condensate is used to recover heat from other parts of the steam cycle which would otherwise be wasted.  The water is gradually warmed back up to nearly the temperature in the boiler before it is pumped back into the boiler.

Water as it leaves the hot well of the main or auxilliary condenser is first reheated by Air Ejector condensers, then typically a Gland Ehaust Condenser, 1st stage heater, Direct Contact Deaerating Water heater, and finally an "Economizer" . The Economizer uses exhaust gases from the boiler fires to recover heat that would otherwise go up the stack.

In essence you have a counter flow heat exchange happening along points in the steamcycle.  High temp steam flowing from the boiler to the condenser (after doing work via the turbine) in one direction . Condensate (water) from the condenser heated up as it makes its way back to the boiler.

4.) I mentioned pure water is necessary in the boiler. Making water pure for use in a boiler costs money.   So closing the steam cycle loop and recycling the water saves having to purify or make boiler water.    On a ship at sea the water is condensed from sea water , on land fresh water is treated in ion exchange systems to remove contamination.    Chemicals are added to scavenge oxygen and adjust PH in order to control corrosion.   So even small steam leaks represent a loss of economy not only in terms of thermal losses but in the need to replace volaile O2 scavenger chemical and replace feed water. Steam ships have to make boiler feed water from sea water, a process that takes energy and represents cost interms of fuel,equipment and manpower. Shore side plants may benefit from local supplies of freshwater but still need to further purify and chemically treat boiler make up water.