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What I've learned about steam pressure

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Author
Dan Holohan
Published
July 16, 2009
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It had stood on this corner just off Wall Street in Manhattan for nearly a century. It stood witness to the comings and goings of countless people who never gave much thought to its many windows and its 25 stories that were served by one-pipe-steam radiators. The building just sat there, another granite fixture, a part of New York City. During the winter, many of its windows yawned open, letting loose into the wind of lower-Manhattan the heat produced by those hundreds of heavy cast-iron radiators. In other parts of the building, people were perfectly comfortable. And then there were those who were always cold. And that's why we were there.

We'd gone through the building, surveying the pipes and radiation, trying to pry into the minds of the Dead Men who had designed this system. They had left us plenty of clues in the corridors and crawl spaces. And now, the building engineer, the consultant, the manufacturer's rep, and I looked at each other across a mahogany conference table. What should we do about the balance problems in this building?

"I think thermostatic radiator valves are the solution," the rep said. "We'll place them between the air vents and the radiators in the offices that are overheating."

"And to get the steam around a building this size," the consultant added, "I think we'll need to run the boiler at about seven-psig, cut-out pressure."

The building engineer swiveled his chair toward the consultant. "Why the heck would you want to do that?" he asked.

"Because of the size of the building," the consultant said, gesturing toward the ceiling. "It's twenty-five stories tall!"

"I know how tall it is," the building engineer said. "But I've been running this system on twelve ounces of pressure for years. Why would I want to raise the pressure to seven psig now?"

The consultant stared at the building engineer for a moment, and then insisted, "You can't heat a building this size with twelve ounces of pressure."

"But I've been here for seventeen years," the building engineer shot back. "And that's exactly what we've been doing. We've never raised the pressure above twelve ounces. We run the boiler on a vaporstat. Didn't I show you the vaporstat when we were in the boiler room?"

The consultant shook his head. "I don't see how that can be," he said.

"Well, it is!"

"In my experience, that doesn't make a bit of sense."

"What about my experience?"

And this went on for quite a while. You really had to be there to appreciate it.

A few things I've learned about steam.

I've learned that the required steam-heating pressure depends on pipe size and load, not building size. The Dead Men developed pipe-sizing charts that measured pressure drop in ounces per 100 feet. From what I've read in old books, they did this for economic and safety reasons. Low-pressure steam wasn't as dangerous as high-pressure steam, and it cost less to produce. Cost and safety were important issues back in a coal-fired era when boilers were exploding nearly every day.

A space-heating steam system is very different from a commercial- or high-pressure steam system. With space heating, all you have to do is make the radiators 215 degrees on the coldest day of the year. The Dead Men went so far as to create the term EDR (Equivalent Direct Radiation), which locked them into that 215-degree maximum temperature for space heating. It continues to lock us in today - if we're paying attention.

With space heating, thanks to the Dead Men, you don't need much pressure to overcome the friction the steam faces as it flows through the pipes. To the steam, those oversized mains look like a superhighway.

And that's why the building engineer could heat that 25-story building with just 12 ounces of pressure. To run the system at seven psig would have made no sense at all.

I've learned there's a difference between steam pressure and steam load. This used to confuse me, and it really held me back at first when I'd troubleshoot an old system. You can have a boiler with a heating load of, say, 1,000,000 BTUH. If you run that boiler at 12 ounces or seven-psig pressure, you're still only going to get 1,000,000 BTUH out of that boiler. Will a radiator with seven-psig pressure be hotter than a radiator with 3/4-psig pressure? You bet it will because it's violating the spirit of the term EDR.

Your radiators may get hotter at the higher pressure, but that doesn't mean you're getting any more heat out of your boiler. It just means that you ran your burner too long. All you'll get for your efforts is an overheated building and open windows.

I've learned that the piping pressure drop relates to the steam load, not the steam pressure. As you increase the load by either overfiring or oversizing the boiler, the pressure drop across the system will increase. But as you raise the steam pressure, the pressure drop across the system will actually decrease. Not by much, but it will decrease. This is because of something else that I learned, and this one threw me for a loop! You ready? Here goes:

High pressure steam moves slower than low-pressure steam.

Isn't that wild? You can see this so clearly on the steam-velocity charts. For instance, let's say you wanted to move 200,000 BTU/Hr. out of a boiler into a three-inch main. The charts show that at 0-psig pressure, the steam will be moving along nicely at 30.44 feet per second. Raise the pressure to 5-psig and the steam slows to 22.5 feet per second. Bring the pressure up to 10-psig pressure and the steam moves at just 18 feet per second.

You know why this happens? Because both the load and the pipe size are fixed. When you increase the steam pressure, you compress the steam. Since the load is the same, the steam moves more slowly.

I think about this whenever someone tells me they raised the pressuretrol setting because they wanted the steam to get to the radiators faster. I always tell them to go back and crank it down. "That doesn't make sense," they'll say. But it's true.

I've learned that high-pressure steam can lock some or all of the air vents closed. And when that happens, you'll never get your radiators hot.

This is where the pressuretrol comes in. The pressuretrol operates the steam system on a curve. Cut-in, cut-out, cut-in, cut-out. It does this so the air vents can open and close. If you keep the cut-in pressure too high, some or all of the air vents won't open after the first cycle. Lower the pressure on that space-heating system and watch how much better it heats.

I've also learned that when the pressure is too high, people open the windows. They do this because they can. It's the only control they have. It's their double-hung zone valve. Want to cut the fuel bills? Crank it down!

On that corner just off Wall Street, people opened their windows because their radiators stayed hot for too long. The building owner installed thermostatic radiator valves to solve that problem. A thermostatic radiator valve can work wonders in a steam-heated building. It won't make a cold radiator warm, but it will keep just about any radiator from overheating.

And then there were the people who were too cold. We found their solution in the air vents. Many of their main vents had filled with rust and sediment. Where there is air, there can be no steam so we replaced the clogged vents. We saw a difference right away.

When the air vents clog, the urge to crank up the pressure is almost irresistible. Don't do it. Instead, think like a Dead Man.