Posted By iceclimber on 14 Apr 2012 06:14 AM
Whats a "steam dryer"?
When enough calories are added to liquid water the temperature will reach the boiling point. Then you have to add LOTS MORE heat to get it to convert to a gas. That is called the Heat of Vaporization. But the temperature of the gas form of water does not change with all that added heat. When not contained in a pressure vessel, the temperature will be 212 degrees (F) (100 Celsius). BOTH the liquid water AND the Gaseous Steam will be at that same temperature.
But the gaseous form is now insulated from the heat source by the water and starts to lose calories and will revert to the liquid state (condense).
The condensed water is held in suspension in the gaseous form as very tiny droplets... maybe even only 2 or 3 molecules of water attached to each other. When they come in contact with more molecules that have lost heat energy, they will join and eventually become too heavy to remain in suspension and will essentially become "rain drops" and fall back to the bottom (or into the water that is being heated). That liquid water in suspension is NOT COMPRESSIBLE! That is a very important thing to note.
The steam around the liquid form of joined molecules is compressible and is attempting to expand, but in a boiler it is contained (Hopefully!) by the pressure vessel and you get pressure that can do work. So you pass that "wet" steam (gas with liquid suspended in it) through a valve (throttle) and on to the valve gear and cylinder/piston to use that pressure to move the piston. But the steam is continuing to condense to water and losing pressure. And that liquid form of the water, being as that it is not compressible, has no pressure in and of itself and is thus not capable of doing any work to move the piston. It is just water flowing in the pipes and taking up space and using energy being pushed around, energy that would be better used to move the piston to do the work desired of the engine.
Ideally all the pipes and the valves and the cylinders should be heated to above the temperature at which the steam was formed. To raise the heat energy in the steam and keep more molecules in the gaseous state.
There are actually two ways to separate the liquid form of water from the gaseous form (or "dry" it).
One way is to spin the "wet" steam in a centrifuge. No, not the kind that an astronaut trains in, but a container where the "wet" steam enters from the side and is deflected by some baffles to cause it to spin in a circular motion. The liquid water will want to travel in a straighter line than the gaseous form and thus move to the outside of the container and the gaseous form will tend to deflect more and can be extracted out via a pipe near the center of the container. Thus you get "dry" steam out of the "Steam Dryer" and the liquid form condenses on the outer walls and drains out via slots in the bottom of the container.
The other way is to pass the "wet" steam through a pipe that is close to the heat source where it picks up additional calories (heat) and the liquid droplets again revert to a gas. It this pipe is just in the smoke box then not much heat is added and it is called a "Steam Dryer". If the pipe passes close to the fire, like the pipe is threaded into the flue of the boiler or actually into the firebox, then LOTS of heat is added and it is called a "Super Heater".
Now, as to that compressing water with air... consider this thought experiment:
You have a container made of titanium steel and it has 10 ft thick walls around a void in the middle that is about 1 cubic ft in volume. There are two openings. One is a valve out the side and one is for a piston that can be inserted from the top. You fill the void with water, close the valve and insert a piston. Being sure that there is no air in the system (NONE!). Now, you put a 100 pound weight on the piston to push it down into the void in the container. The container is so thick and strong that it does not stretch and cannot deform in any way. Then weld the piston into the position so it cannot move in any direction; either out or further in. Since it is welded in place, you could take the weight off of it and it won't release the pressure that has been applied to it.
Now open the valve in the side of the container. What happens?
I will tell you that NO water will come out! If there was no air in the system to compress and the container did not stretch and the piston cannot move from its position, then the water has no pressure and will NOT squirt out. Water is not compressible.
When a boiler of water is compressed by attaching a compressed air source, the boiler will expand like a rubber balloon, just not as much as a rubber balloon. The water that squirts out is because the boiler shell is contracting to its original size, not because the water has any pressure in it.
The very real DANGER of using compressed air to pressurize an UNKNOWN quality pressure vessal is that the vessel COULD burst. IF the source of air is capable of supplying an unlimited quantity of air then there is a very real danger of an EXPLOSION. Not because the water has any energy in it, but because the compressed air pushing on it has energy in it and it will push the water our the rupture and there will be an equal and opposite reaction to the moving water, which will move the container the other direction in direct proportion to the mass and speed of the content of the vessel.
Another experiment you can think about (or actually do part of it!):
Fill a balloon with water and then attach a pressure gauge to the balloon and measure the pressure. The normal cheap balloon will probably have no more than a pound or two. Now stick the balloon with a pin and watch where the water goes. Does it just fall to the ground in a big splash or does it violently travel around the room in all directions?
Now fill the balloon with air and measure the pressure. It should also be just a pound or two. Now stick a pin in the balloon. What happens? Does it make a LOUD NOISE? Does it toss parts of the balloon all over the place?
Now, imagine putting 60 pounds of pressure in a "balloon" that is full of water. Pop it! What happens? I'll tell you that the water will fall to the floor in a splash just like the cheap balloon with 1 or 2 pounds of pressure.
Now imagine doing that with air in the "balloon" and what kind of noise it would make! "Pop" just will not describe the sound with any sort of accuracy. Do you really think it would be wise to have your own body parts in the vicinity? How far from this "balloon" would you want your antique curved glass china cabinet?
Did you ever have one of those toy rockets that you fill 1/2 way with water and then attach it to a pump to inject air into it? Then you aim it UP (and not at your sibling! and NOT IN THE HOUSE!... and of course you did heed the warning to not pump more than 10 to 15 strokes... yeah right!) and pull a release catch that releases the rocket from the pump. The compressed air pushes the water out the bottom and the rocket goes "hundreds" (yeah, right!) of feet in the air!
If you do not put water in it, but just pump it with air, it might get a few inches off the pump nozzle. If you fill the rocket all the way with water, then it is lots more difficult to pump air into it. You CAN get some air into it and it gets absorbed into the water, or the rocket shell stretches to accommodate the increased volume, but it is very difficult to work the pump. When you release the catch the vessel walls will relax and shove some water out the bottom and the air that is contained in the water will come out and push some water out, but the rocket flight will be dismal and when it lands there will still be water in the rocket because there was not enough quantity of pressure to push it all out. The only quantity was the stretched container (the body of the rocket) and the small amount of air that got in and compressed due to the body stretching to make a larger volume of the container.
