Short answer: It sure does, but not in the way you may think.
Steam traps trap steam but that’s not all they do. They also pretend to be air vents whenever the system starts. And I say “pretend” because the traps usually aren’t venting the air from the system. They’re just passing it downstream to a real air vent that will toss it out. This is an important job because every steam system ever built is open to the atmosphere and filled with air on start-up. Steam is lighter than air and won’t mix with air, so for the steam to move, the air has to get out of its way. If the air can’t get out, the steam can’t get in, and that means cold radiators and unhappy people.
So the steam traps all smile as the air moves through them. If you look closely, you’ll see those smiles at the outlets of the two-pipe radiators, and near the ends of the steam mains. That’s where the traps do their work.
Once the steam shoves the air through the traps, the steam is free to give up all those latent-heat Btu it carried from the boiler to the cold metal of the radiators. The steam turns back into liquid water, which we call condensate, and then flows by gravity into the return lines and makes its way back to the boiler. It’s a simple system with very few moving parts.
The steam that travels through the steam mains gives up less heat because the mains are insulated (or at least they’re supposed to be insulated). Once the steam reaches the end of a main, a float-and-thermostatic trap holds out its hand to stop it and keep it out of the condensate-return piping. All these traps, on the radiators and near the ends of the steam mains, are acting like traffic cops. They’re also acting sort of like the balancing valves in a hot-water system. They only allow through what’s supposed to go through.
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