Years ago, I flew to Denver to visit a guy who had heard that I like steam heating a lot but he still thought I was stupid. We’d never met but we’d talked on the phone and he was convinced I was just plain wrong. Nice way to begin a relationship, isn’t it?
He had bought a bunch of thermostatic radiator steam traps from a company in Chicago, installed them in an apartment building in Denver and thought they weren’t working. There were bigger problems in this building but he was convinced that he couldn’t use Chicago traps in Denver.
“Water boils at 202 degrees Fahrenheit here,” he said on the phone. “I need Denver traps.”
“But there’s no such thing as a Chicago trap or a Denver trap,” I said.
“Well, where the heck were these traps made?”
“Chicago,” I said.
“That’s why they won’t work here,” he said.
“But these traps know where they are. They get off the plane, look around and adjust.”
“What are you talking about?”
“I can prove it,” I said, and he bought me a plane ticket.
He picked me up at the airport and we drove straight to the job. Along the way, I told him how a thermostatic radiator trap starts out normally open.
“I know that,” he said.
“They let air pass by ahead of the steam. The air leaves through either the main vents or the condensate pump’s receiver vent. When the steam arrives at the trap, it causes the thermostatic bellows to close. That stops the steam, and with nowhere to go, the steam gives up its latent heat to the cold radiator and turns into water.”
“And then the trap reopens to let the water drain out,” he added, a bit annoyed that I was taking him through such basic stuff.
“Yes,” I said. “But how does the trap’s thermostatic bellows open and close?”
“What do you mean? It’s a thermostat, right?”
“Not really,” I said. “It is sensitive to temperature, but the temperature it senses varies with the system pressure. The bellows are distended at first. The manufacturer pours in a mixture of alcohol and water that has a boiling point of 180 degrees at sea level. At this point, the element is still open to the atmosphere.”
“Okay,” he said.
“They next heat the elements to a point where the mixture flashes right off, and while it’s flashing, the technician seals the element and removes the heat.”
“Oh, I get it,” he said. “So when the stuff inside the bellows cools, the bellows collapses in on itself.”
“Right. The vapor is shrinking and no air can get in so the bellows has to collapse, but when the steam comes along, it heats the mixture and makes it flash to vapor again. The bellows swells and closes the trap. Pretty simple, isn’t it?”
“Yeah, it is,” he admitted, “but what does this have to do with the trap knowing what city it’s in?”
“Well, when will the trap reopen?”
“I don’t know. I guess when the condensate cools the alcohol/water mixture.”
“Yes, and by about twenty degrees,” I said. “In a sea-level system operating at, say, two-psi, the traps will reopen when the condensate drops to about 200 degrees because two-psi steam is 219 degrees. But suppose you raise the system pressure to 15 psi? The temperature of steam at 15 psi is 250 degrees. Under that pressure, the trap will reopen and release condensate at about 230 degrees. But how does the trap know what’s going on?”
“You got me,” he said.
“It knows because it’s not as dumb as it looks.”
“Neither are you apparently,” he said.
“I couldn’t possibly be,” I said. “Now here’s the key to the whole thing: The system pressure squeezes the element and raises the boiling point of the alcohol/water mixture along the same curve that boiler water has to follow. The higher the pressure is, the higher the boiling point will be. That’s how the trap knows when to open.”
“Okay, I get it. But what does this have to do with Denver?”
“Well, Denver is under a lower atmospheric pressure than Chicago or New York, right?”
“That’s what I’ve been telling you all along.”
“So when the trap gets off the plane, it senses that lower atmospheric pressure, distends its element and automatically lowers the boiling point of the alcohol/water mixture inside the element. In other words, in Denver, it’s able to sense steam at 202 degrees. It knows.”
“So how come my system’s not working?” he asked as we reached the building and pulled around to the boiler room door.
“Well, we’re about to find that out, aren’t we?”
And we did. But that’s a story for another day.