Wet, Dry and Super heated steam

Want to know a general knowledge question? Or think you can answer some simple questions? Have a look here.
Post Reply
User avatar
Maltelec
Master of the Forum
Master of the Forum
Posts: 266
Joined: Sat Nov 14, 2009 7:01 pm
Boat Name: No Boat Yet
Location: Cumbria, UK
Contact:

Wet, Dry and Super heated steam

Post by Maltelec » 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.
I've got the vehicle, just need the boat.
User avatar
Maltelec
Master of the Forum
Master of the Forum
Posts: 266
Joined: Sat Nov 14, 2009 7:01 pm
Boat Name: No Boat Yet
Location: Cumbria, UK
Contact:

Re: Wet, Dry and Super heated steam

Post by Maltelec » Sat Jul 23, 2011 7:30 am

Here is a more detailed answer from Fredrosse:

Wet, Dry, and Superheated Steam

The boiling temperature of water is linked directly to the pressure to which the water is exposed. Higher pressures result in higher boiling temperatures, as defined by the table included with this post. The “boiling point” temperature is said to be the “saturation temperature”, which is the temperature (particular to the surrounding pressure) where both liquid water and gaseous steam can simultaneously exist.

In this table of saturation temperatures, pressure is either expressed as “Gauge Pressure (for example, Pounds per Square Inch Gauge, PSIG), which is the pressure above atmospheric pressure, or “Absolute Pressure” (for example, Pounds per Square Inch Absolute, PSIA). Normal atmospheric pressure IS 14.7 PSIA, or 0 PSIG, with a “boiling point” of 212 F, or 100 C.

PSIG PSIA Temp, F Temp, C

0 14.7 212 100

15 29.7 250 121

50 64.7 298 148

100 114.7 338 170

150 164.7 366 185

When liquid water is heated, it is brought up to the “boiling Point” temperature by the energy source, and then the temperature does not increase as more heat is added, instead the liquid water experiences a phase change and begins to change into a gas. Of course, this gas is what we call steam, and as it issues from the surface of the boiling water, it is at the same temperature as the boiling water, and it is said to be “saturated steam”. The energy level of steam at saturated conditions is several times the energy level of water, even though both are at the same temperature.

If no water droplets are carried in suspension with the steam, this steam is said to be “Dry Saturated Steam”. This is 100% gaseous steam, with no entrained water droplets, and is referred to as “100% quality steam”.

If dry saturated steam issues from a boiler (more on this later) it then travels along the steam piping, and (for a simple steam plant), always there is some heat lost from the piping. When this energy is taken away from the steam, the temperature of the steam does not reduce, instead some of the steam turns back into liquid water. This is a condition where we have “wet steam”, which is a mixture of both saturated water and saturated gaseous steam flowing in the pipe, both at the saturation temperature.

If the gaseous steam exiting the boiler is passed through pipes that are gaining heat energy rather than losing it, then the gaseous dry steam will begin to increase in temperature, and we have what is called “superheated steam”, meaning the steam is at a temperature above the saturation temperature. Liquid water cannot exist at a temperature above the saturation temperature, therefore superheated steam is, by definition, also “dry steam”, however it carries more energy than steam that is merely dry saturated steam.

Use of superheated steam tends to increase the efficiency of steam engines, and a heating coil of tubing within the boiler casing is often used to bring this additional energy to the steam flow, and of course this heating coil is called a “superheater”.

While superheated steam can provide better engine efficiency, this advantage also results in problems, especially for marine steam plants. Superheated steam is a very poor lubricant for the slide valves and steam cylinder of the engine. With slightly wet steam, the liquid water droplets can provide some measure of lubrication between the rubbing surfaces, and many engines rely on this wet steam for satisfactory internal engine lubrication. With inlet steam that is significantly superheated, the engine may require injection of lubricating oil into the steam flow, and removing this oil from the steam is generally necessary, and difficult.

Often the steam that issues from a boiler is not entirely “dry”, as some liquid water is carried away with the steam flow as it exits the boiler. Boilers that issue a few percent of liquid water along with the steam are not uncommon. This condition can occur because of impurities such as oil on the surface of the boiling water, excessive impurities in the boiler water, or carrying too high a water level in the boiler. Indeed some poorly designed boilers produce wet output steam even with all the other mentioned conditions in proper order.
I've got the vehicle, just need the boat.
Post Reply