All Steam Is Not Created Equal

Published: July 5, 2018 - by Dan Holohan

Categories: History Lesson, Troubled Heating Systems

the lost art of steam heating

Wet steam? Dry steam?

Take a pound of water (that's about a pint), and heat it from 32° F to 212° F. That will cost you 180 Btu because one Btu will raise one pound of water one degree on Gabriel Fahrenheit's scale. But you still don't have steam. That's just very hot water. To get the steam magic to happen, to get those water molecules so excited that they leap into space, you need to add an additional 970 Btu for each pound of water. Do that and the water will instantly expand 1,700 times and charge toward the radiators.

But is that good steam? You know, the sort that will heat the radiators and make the people smile? Is the steam dry? Or is it wet?

Did you know there were two types?

Back in the day, we were all about the dry stuff. Steam boilers were tall and had had large steam chests, the hydronic auditoriums where the escaping steam could gather, leaving the liquid water behind. Those boilers also had wide sections, which allowed the steam to be polite as it rose through the liquid water toward the steam chest. Hey, no shoving allowed!

And since the sections where wide, the exit holes could also be wide. That was great because those wide doorways to the supply pipes kept the steam's exit velocity down. If the steam goes too fast it will suck the liquid water out of the boiler and that means wet steam, which is steam that contains more than 2% liquid water. It's tough to heat a building with this sloppy stuff because the steam wants to condense in the liquid water its dragging along rather than in the radiators. You wind up burning lots of fuel and the building still won't heat well.

The boiler manufacturers used to test for dry steam, and they wouldn't give a boiler their seal of approval if it couldn't produce steam with less than 2% liquid water. But then came the movement to make boilers with greater combustion efficiencies, and the only way they could do that with steam boilers was to make them smaller. That led to narrower sections, cramped steam chests, choked exit holes, higher steam velocities, and wet steam.

So during the '80s, the manufacturers stopped testing for dry steam, which was good for them because you can't fail a test you're not taking. So there.

In place of the dry-steam test they gave us really good near-boiler piping instructions. It then became the installer's responsibility to pipe the boiler in such a way that the near-boiler piping would separate the liquid water from the steam as it left the boiler.

So pipe that replacement boiler like a professional and it will heat the building. Or cut corners and wrestle with water hammer, uneven heating, high fuel bills, and customers who won't want to pay you. It's good to have a choice, right? Make the right one.

Dry humor

You can be laughing all the way to the bank with dry steam if you go a bit beyond the boiler manufacturer's instructions by doing this:

  • Use more risers out of the boiler to the header. If the manufacturers says use one, use two. You'll cut the steam's exit velocity in half and get drier steam. Manufacturers compete with each other, and if one says their steam boiler will work well with one riser to the header, the competitor will be reluctant to recommend two risers for their boiler, even though we all know that physics rules. Use as many risers as you can and the steam will slow and get dry.
  • Increase the size of the header. The larger the header is, the lower the steam velocity will be, and that's good for both you and your customer. The steam will leave the boiler very quickly; that's a given these days. A larger-than-required header will slow it and give the carry-over water a chance to flow back into the boiler rather than up into the system piping.
  • Use a drop-header. This is where you come out of your boiler with risers as high as you can before turning through two elbows to drop into your horizontal steam header at a lower level. The rise out of the boiler will leave much of the water behind. Drop-headers are a Dead Men trick and they work wonders when it comes to quieting the system and lowering the fuel bills.

Superheat!

There's a third type of steam we call superheated, and no, it doesn't wear a cape. This is the steam that runs turbines. It's under very high pressure and it's hotter than it's supposed to be. They make that happen by running normal steam through a superheater, where it picks up more heat than it should be holding. It becomes absolutely, positively dry. That makes the turbine very happy. Imagine a very large turbine spinning at, say, 3,600 rpm. Now toss some water in there and see what happens. Whoa!

Superheated steam can cool as it passes through the turbine without turning to water. We don't use superheated steam for heating because saturated steam, which is always in equilibrium with liquid water at that same temperature, has a higher useful heat content. You can see all of this on a Saturated Steam Table, should the spirit move you.

Did you know?

  • We don't use bucket traps near steam pressure-reducing valve stations, because there will be some momentary superheated steam on the low-pressure side of the PRV. That excess heat can enter the traps prime and cause it to boil away. And when that happens, the trap will stop trapping.
  • Those who work in power plants where there is high-pressure steam will often carry a broom in front of themselves while walking the pipes. These plants are very noisy and you can't see high-pressure steam leaks. You can see the broom's bristles disappear, though. Which means you should stop walking NOW, or suffer the broom's fate.

Want to learn more about steam-heating systems? Check out The Lost Art of Steam Heating Revisited.