Wet, Dry and Super heated steam
Posted: Mon Dec 21, 2009 7:45 pm
Just when you thought steam power was a diddle, someone asks you; Are you using wet or dry steam?
There are simple answers and complicated answers. The answer below gives a more simple answer to a very complicated question, and so misses out on some of the details. Look below for the more technical answer.
Wet Steam
As you should know, (pure) water boils at a set temperature, dependant entirely on the pressure it is at. Looking at basic steam tables will tell you the temperature of the boiling water compared to its pressure. E.g. at 150psi the temperature is approx 173 Celsius.
However as soon as the steam leaves the water, it will begin to cool down, while still being under the same pressure. As the steam flows down the pipe to the engine, it may still be at 150psi but its temperature will have dropped. This means the steam is no longer in a state where it wants to stay as steam.
Thus it will begin to condense back into water. You therefore get steam with little droplets of water going down the steam pipe - which is what is called Wet steam.
Some advantages are that the water helps to lubricate the engine, and you don't have to much about with steam dryers or excessive cylinder lubrication - particularly useful if condensing.
Dry Steam
This is generally regarded as steam which is at, or just, or just below above the boiling point of water, but containing no little water droplets.
To obtain this state, the steam is either dried in a mini super heater (see Super Heated Steam), which turns any water droplets back into steam without adding excessive heat, or the steam is sent round a steam cleaner which removes the water droplets, often via centrifugal force (if you believe such a force exists).
Dry steam doesn't stay dry for long. The main advantage is you don't have any water entering the engine. However you need to start thinking about cylinder lubrication. However because no super heat is applied, the steam will turn into wet steam reasonably quickly.
The drying of steam also occurs at any pressure drop. Should the steam at 150psi and 160 Celsius pass through the main throttle valve and drops to 50psi the other side, the steam and water droplets will instantly expand to the lower pressure, however the temperature drop is not linear. The temperature will drop less than the pressure, so the new equilibrium of steam and pressure could well see the steam being hotter than the boiling point of water at the 50psi. This would cause any water droplets to flash back to steam, thus automatically drying the steam.
Super Heated Steam
This is the steam which all the big boys go for. A Super Heater is quite simply another pipe outside of the main pressure vessel, sometimes before the main boiler tubes, sometimes after, and sometimes going through them.
The aim is to heat the steam up excessively over the temperature of which the water would boil in the given pressure.
For instance, 150psi of steam at 173 Celsius would be right on the border between wet and dry steam. Run this through a super heater and you may get the steam up to 300 Celsius, the only limit is when the super heater explodes.
You then have dry steam at a significantly higher temperature but still at the same pressure. This has massive advantages of the amount of power you can get out of the steam.
E.g. it takes 4186 Joules of energy to heat 1kg of water by 1 Celsius.
1kg of steam passes through an engine, engine drops the temperature (through expanding the steam) from 150 C to 80 C. This will give you 293kJ of energy.
1kg of super heated steam passes through an engine, engine drops the temperature (through expanding the steam) from 350 C to 95 C. This gives you 1,067kJ of energy.
All that extra energy from heat which would have otherwise been waisted.
However super heaters come with drawbacks. Cylinder lubrication is vital. In order to get the expansion of the steam (and thus the drop in pressure / drop in temperature), condensing is essential. The engine must be specifically designed for this purpose.
Of course a simple single engine would benefit from mild super heat, though the border between a steam dryer and a super heater is very grey.
Super heaters are also vulnerable parts, having no water to keep them cool they run very hot. Some people insist on a safety valve after the super heater, some insist on one before the super heater. The best practice in my opinion is 2 safety valves, the one after the super heater blows off 1st to pass steam through the super heater to help cool it down. A large-bore pipe direct out of the boiler is not something you want to break very often.
There are simple answers and complicated answers. The answer below gives a more simple answer to a very complicated question, and so misses out on some of the details. Look below for the more technical answer.
Wet Steam
As you should know, (pure) water boils at a set temperature, dependant entirely on the pressure it is at. Looking at basic steam tables will tell you the temperature of the boiling water compared to its pressure. E.g. at 150psi the temperature is approx 173 Celsius.
However as soon as the steam leaves the water, it will begin to cool down, while still being under the same pressure. As the steam flows down the pipe to the engine, it may still be at 150psi but its temperature will have dropped. This means the steam is no longer in a state where it wants to stay as steam.
Thus it will begin to condense back into water. You therefore get steam with little droplets of water going down the steam pipe - which is what is called Wet steam.
Some advantages are that the water helps to lubricate the engine, and you don't have to much about with steam dryers or excessive cylinder lubrication - particularly useful if condensing.
Dry Steam
This is generally regarded as steam which is at, or just, or just below above the boiling point of water, but containing no little water droplets.
To obtain this state, the steam is either dried in a mini super heater (see Super Heated Steam), which turns any water droplets back into steam without adding excessive heat, or the steam is sent round a steam cleaner which removes the water droplets, often via centrifugal force (if you believe such a force exists).
Dry steam doesn't stay dry for long. The main advantage is you don't have any water entering the engine. However you need to start thinking about cylinder lubrication. However because no super heat is applied, the steam will turn into wet steam reasonably quickly.
The drying of steam also occurs at any pressure drop. Should the steam at 150psi and 160 Celsius pass through the main throttle valve and drops to 50psi the other side, the steam and water droplets will instantly expand to the lower pressure, however the temperature drop is not linear. The temperature will drop less than the pressure, so the new equilibrium of steam and pressure could well see the steam being hotter than the boiling point of water at the 50psi. This would cause any water droplets to flash back to steam, thus automatically drying the steam.
Super Heated Steam
This is the steam which all the big boys go for. A Super Heater is quite simply another pipe outside of the main pressure vessel, sometimes before the main boiler tubes, sometimes after, and sometimes going through them.
The aim is to heat the steam up excessively over the temperature of which the water would boil in the given pressure.
For instance, 150psi of steam at 173 Celsius would be right on the border between wet and dry steam. Run this through a super heater and you may get the steam up to 300 Celsius, the only limit is when the super heater explodes.
You then have dry steam at a significantly higher temperature but still at the same pressure. This has massive advantages of the amount of power you can get out of the steam.
E.g. it takes 4186 Joules of energy to heat 1kg of water by 1 Celsius.
1kg of steam passes through an engine, engine drops the temperature (through expanding the steam) from 150 C to 80 C. This will give you 293kJ of energy.
1kg of super heated steam passes through an engine, engine drops the temperature (through expanding the steam) from 350 C to 95 C. This gives you 1,067kJ of energy.
All that extra energy from heat which would have otherwise been waisted.
However super heaters come with drawbacks. Cylinder lubrication is vital. In order to get the expansion of the steam (and thus the drop in pressure / drop in temperature), condensing is essential. The engine must be specifically designed for this purpose.
Of course a simple single engine would benefit from mild super heat, though the border between a steam dryer and a super heater is very grey.
Super heaters are also vulnerable parts, having no water to keep them cool they run very hot. Some people insist on a safety valve after the super heater, some insist on one before the super heater. The best practice in my opinion is 2 safety valves, the one after the super heater blows off 1st to pass steam through the super heater to help cool it down. A large-bore pipe direct out of the boiler is not something you want to break very often.