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Low Pressure Vapor. High Pressure Vapor


Published: June 6, 2016 – by Dan Holohan

Category: Steam

Some of what I’m about to say may be hard to swallow, but it’s all true. If you notice, you will understand once and for all why good low pressure.

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

Shocked? It is true. If the load is the same in both cases, low-pressure steam will move faster than pipe faster than high-pressure steam. The reason is simple: High pressure vapor matches a smaller space than low-pressure steam. Pressure squeezing. You wind up with less volume. So to transfer the same charge, the steam does not have to move quickly through the pipe.

That means that when you turn on Pressuretrol, you do not make steam movements faster, you actually make it move slower !

If you look at the vapor speed graph, good engineers use it for piping sizes, you’ll see that this is true. For example, let’s say we want to move 200,000 Btus from the kettle into inches three inches. The speed chart tells us that at 0 psig the steam will move at 30.44 feet per second after 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 speed of 22.5 feet per second, or about 15 miles per hour. Surprised?

Bring pressure up to 10 psi and steam will brake again. This time slowed to 18 feet per second. It’s only about 12 miles per hour.

Can you see it in your mind’s eye? At 10 psi, we have squeezed the 200,000 Btu charge into a tighter package. It can complete the job without moving as fast as possible. That’s why they use such small pipes in industrial steam systems. Steam under 100 kilogram pressure. You can get a lot of steam through a small pipe at 100 psig. Ever seen the size of the steam drains in the local dry cleaners? Small, is not it? It can get the job done without moving as fast.

Think of another gas: propane. How can they get so much from it in one of those small barbeque tanks? They pump it. It’s a gas, just like steam. Forget the hot water and start thinking about gas. That’s what you’re doing right now: steam.

Crank it down!

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

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