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In the beginning there was one-pipe steam, and one-pipe steam was pretty simple. The steam travelled up the pipe; the condensate fell down the pipe. Size and pitch the pipes properly, don't over-fire the boiler, keep the wet returns clean, and you were in pretty good shape. But as buildings grew larger, one-pipe steam became impractical because there was just too much condensate draining down against the steam. The Dead Men got around this for a while by using a system that sent the steam up to the attic through an express riser. From there, steam flowed downward toward the radiators, taking the condensate with it. That's called parallel flow, by the way, and it's a lot easier to deal with than counterflow. In the early days of heating, the rich complained because the one-pipe steam vents on their radiators often spit dirty water on their curtains and wallpaper. As a result of those complaints, the Dead Men invented what we today call Vapor heating. Vapor works with very low pressure (usually only about eight ounces or so) and two pipes at each radiator instead of one. The steam enters the top of the radiator through one pipe, and the condensate drains from the bottom of the radiator through the other pipe. The steam pushes the air through the radiator and shoves it out of the system through a big main vent, which is somewhere near the end of the dry return down in the boiler room. If that vent spit dirty water, the water went on the basement floor rather than on the curtains and that made the rich folks happy.
 When you work with two-pipe steam radiators you do have a challenge, though. These systems are set up like ladders and each radiator is like a rung on that ladder. One of the ladder's uprights is a supply main; the other is the return main. The Dead Men had to come up with a way of stopping the steam from scooting from one side of the ladder to the other. This was important because, for steam to move, they needed a point of high pressure and a point of low pressure. If the steam could find a shortcut through any of the radiators, it would pressurize the return side of the ladder and the air wouldn't be able to get out of the other radiators. The result would be no heat. So the Dead Men came up with all sorts of neat devices that would stop the steam from getting through the radiators. They used orifices, and little check valves, and tiny steel balls, and water seals, and whatnot. And then they came up with the idea of the thermostatic radiator trap and that idea stuck around for a long time. A thermostatic radiator trap is an automatic valve that responds to temperature. It's normally open. It lets the air get by. It closes to steam. It opens to condensate. It sets up the points of pressure and no pressure in the system. It fails after a number of years, and it usually fails in the open position. Steam traps trap steam, so there's little or no pressure on their outlet side. That gave the Dead Men another challenge because, with their one-pipe systems, they had been using the "leftover" steam pressure at the end of the main to help put the water back into the boiler. If they sized their pipes properly, all they had to do was allow for a vertical distance of 28 inches between the boiler water line and the bottom of the lowest steam main. They called this the "A" Dimension. The water would stack in that vertical space and its weight would combine with the leftover steam pressure. Those two forces were enough to overcome the pressure inside the boiler and get the condensate back where it belonged, which was inside the boiler.
Steam traps changed all that. With no pressure on the returns, the Dead Men had only the weight of the returning condensate to help them get the water back into the boiler. A column of water that's 28 inches high exerts a force of 1 psi. So on those two-pipe Vapor systems, they'd need at least 30 inches of vertical space between the boiler water line and the lowest return line for every pound of pressure inside the boiler (that extra two inches is for pipe friction losses). If the boiler ran at 2 psi, they'd need 60 inches of vertical height. I call this the "B" Dimension. If the boiler ran at 5 psi, they'd need 150 inches. The trouble was that those old basements weren't deep enough, and that's why they sized the piping for the lowest possible pressure drop.
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| Dan Holohan - [Intro] | [Email] | [Website] |
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