The Speed of Steam

DaPluma

Why does steam travel faster under less pressure?

David Efflandt

lower pressure = move volume, less head loss

Lower pressure steam has more volume and less density. So it expands out of the boiler quicker, and has less inertia, so it is less obstructed by changes in direction (less head loss).

Unknown

Also, compression.

Let's assume that you need 10,000 BTUH at the end of the main. Regardless of pressure, the load is constant.

At low pressure, that 10,000 btuh package isn't compressed much, and takes up most of the space in that main.

If you run and run and run the boiler and compress that steam until a 10,000 btuh package only takes up a small bit of the pipe, and you have several "10,000 BTU packets" in that same pipe, now.

Since you still condense the same number of BTUHs, these small, compressed packets will each do the same work as the whole pipeful of low pressure stuff did.

Said another way, 10,000 BYUH can be the whole pipeful, and at lower fuel costs, or about a quarter pipeful, at much more fuel.

Now, if the whole low pressure pipe empties in an hour, and on the other hand, the high pressure pipe only moves a quarter of it's steam to heat the same 10,000 load in an hour, the low pressure stuff is moving 4 times as fast in the same space.

Yet, only 10,000 BTUs per hour come off the pipe in either case. It's not about how much steam pressure you make, it's about how much steam VOLUME you use.

Higher pressure allows smaller pipes to be designed into a system, but these old heating systems don't need to be designed again. The pipes were sized for low pressure losses, typically 2 ounces loss in 100 feet of pipe.

Hhalf a pound of steam will fill 400 feet of main, above that you would split the load and run more mains.

Noel