Low-Pressure Steam vs. High-Pressure Steam

Some of what I’m about to tell you may be tough to swallow, but it’s all true. If you pay close attention, you’ll understand once and for all why low pressure is good.

Let’s start with a simple fact: low-pressure steam travels faster than high-pressure steam.

Surprised? It’s true. If the load is the same in both cases, the low-pressure steam will move through the pipes much more quickly than the high-pressure steam. The reason is simple: High-pressure steam fits into a smaller space than low-pressure steam. The pressure squeezes it. You wind up with less volume. So to transfer the same load, the steam doesn’t have to move as quickly through the pipes.

That means when you crank up the Pressuretrol you’re not making the steam move faster, you’re actually making it move slower!

If you look at a steam-velocity chart, the kind engineers use to size piping, you’ll see that this is true. For instance, let’s say we wanted to move 200,000 Btus out of a boiler into a three-inch main. The velocity chart tells us that at 0 psig the steam will be moving at 30.44 feet per second once the air is removed from the system. That’s about 20 miles per hour. Pretty fast, eh?

But when we raise the Pressuretrol setting to 5 psi, the same load slows to a velocity of 22.5 feet per second, or about 15 miles per hour. Surprised?

Bring the pressure up to 10 psi and the steam will hit the brakes again. This time it slows to 18 feet per second. That’s only about 12 miles per hour.

Can you see it in your mind’s eye? At 10 psi we’ve squeezed the 200,000 Btu load into a tighter package. It can get the job done without moving as fast. That’s why they use such small pipes in industrial steam systems. The steam is under maybe 100 pounds of pressure. You can get a lot of steam through a small pipe at 100 psig. Ever see the size of the steam lines down at your local dry cleaners? Small, aren’t they? It can get the job done without moving as fast.

Think about another gas: propane. How do they manage to get so much of it in one of those little barbeque tanks? They compress it. It’s a gas, just like steam. Forget about hot water and start thinking about gasses. That’s what you’re working with now: steam.

Crank it down!

Want to learn more? Check out The Lost Art of Steam Heating.