Confused person's steam locomotive questions

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  • Member since
    April, 2003
  • 302,134 posts
Confused person's steam locomotive questions
Posted by Anonymous on Saturday, January 24, 2004 4:59 PM
I have recently heard numerous references to "tubes" and "flues" in the context of a locomotive boiler. The way I learned it, hot gasses from the firebox are taken through to the smoke box in the larger tubes, called either "fire tubes" or "boiler tubes." The "wet" steam then collects in the steam dome, where it is taken to the "superheater tubes," which are smaller and fit inside of the larger "boiler tubes." This superheated steam is then sent to the cylinders. Is my understanding flawed, or if it is correct, what is the actual superheater, then? Also, what are the "tubes" and "flues," and are they the same thing as the "boiler tubes" and "superheater tubes" as I understand them? I realize these are a lot of questions, but any help you can give me would be most appreciated.

Most gratefully and sincerely,
  • Member since
    January, 2002
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Posted by M636C on Sunday, January 25, 2004 7:22 AM

Your understanding is basically correct.

Hot gases go through all the tubes from the firebox to the smokebox, sucked through by the exhaust steam from the cylinders acting as a pump, reducing pressure in the smokebox.

The small tubes are called "tubes", and in a locomotive without a superheater, all the tubes are like that.

The large tubes are called "flues", and the superheater "elements" sit inside them.
There are basically two types of superheater, the Type A and the Type E.

The older Type A has a bigger flue, and each "element" reverses twice, so that there are four "pipes" in each flue so the steam goes down and back twice.

In the Type E the steam only goes down and back once, there are only two pipes making up the element in each flue, which is smaller in diameter. There are more flues and elements, so the total superheater is bigger.

The easy way to think about it is that the flues and tubes have the water on the outside, and the gases on the inside. The superheater elements have the steam on the inside and the hot gases on the outside.

The throttle is usually on the boiler side of the superheater. Steam in contact with the water in the boiler reaches "saturation" and can't absorb any more energy. The steam in the superheater elements is no longer in contact with water, and can be heated further. This reduces condensation in the cylinders, and increases power by about 10% while improving fuel economy by up to 20%.

The superheater elements are connected to the boiler by the "header", a big steel casting in the shape of a box (more or less), which is divided in two inside. One side is connected to the boiler, usually through the throttle valve. Each element is connected to both sides of the header, and the steam goes from the boiler side through the element, is heated and returns to the other side of the header which is connected to the cylinders.

The "superheater" consists of the header and the elements together. The superheater can be removed and replaced as a unit, but usually one element at a time is changed if it needs repair.

The boiler is designed to operate with the superheater in place, and will not operate correctly without it, as the gas flow through the flues without the elements in place would be too fast, and heat would not be transferred into the water correctly.

Boilers without superheaters have only small tubes and no flues.

I hope this helps! If not, I'll try again!

  • Member since
    April, 2003
  • 302,134 posts
Posted by Anonymous on Sunday, January 25, 2004 8:22 PM
Dear Peter,
Thank you very much for clearing up that matter. I no longer have a looming cloud of doubt as to whether I am making a fool of myself when I discuss tubes and flues![:D]

Most gratefully,


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