On 11-12-07 06:11 PM, Joshua Cude wrote:
On Wed, Dec 7, 2011 at 4:38 PM, David Roberson <[email protected] <mailto:[email protected]>> wrote:The pressure must be established within the boiler so I guess the hotter steam does not make its way back to the boiler. Isit likely that some form of check valve is used at the throttle? If that were possible, then higher pressure could be applied tothe cylinders due to the super heater.It's not necessary to use higher pressure to superheat steam. In fact, the point is that the temperature of the steam is above the boiling point at the local pressure. Otherwise, it's saturated.
The point in superheating in a locomotive wasn't to increase the pressure, it was to increase the *volume*. (In fact, the whole point in turning the water into steam to start with was simply to increase the volume. The *pressure* couldn't very well exceed the pressure of the water being delivered to the boiler from the tender, and that would have been determined by a pump. Exceed that pressure and your boiler will eventually run dry.)
In fact, the non-super-heated and the superheated steam in the locomotive were at the s*ame pressure*. Actually, come to think of it, since the superheated steam was "downstream" from the cooler steam, and some amount of backpressure must result from forcing gas through pipes, the superheated steam must have been at a slightly *lower* pressure than the cooler steam.
Somewhat similarly, when you drive a fuel-injected car, the fuel is injected into the cylinders at high pressure, and the point in burning it is to increase its *volume*. The pressure in the cylinder head isn't going to exceed the fuel pressure in the injectors! (Granted, the *air* is introduced at much lower pressure, and the pressure in the cylinder head goes up rapidly during the burn, but it's still not going to go high enough to force fuel back into the injectors. Note that this observation doesn't apply to carbureted cars, where _all_ fluids are introduced into the cylinder at low pressure.)
