boiler pressure and heat exchange

greg_7

Thanks for you responses. It makes alot of sense. Maybe I am far too interested in the arcane, but perhaps a more detailed techical heat engineering for dummies section would be neat.

greg_7

boiler pressure and heat exchange

Would heat transfer within a boiler be adversely affected by a boiler pressure above 12psi. Obviously pumping efficiency falls, but does the heat transfer slip as well?

Steamhead

Is this

in a hot-water or steam system?

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greg_7

Duh... I tried to make the question as simple and direct as possible. Oh well.

Hot water.

Steamhead

Shouldn't affect it

The only reason you need pressure at that level is to make sure the water fills the system to the top. Pressure does not affect circulator efficency- what goes up must come down, and the two balance each other out. Picture a Ferris wheel- the principle is the same.



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greg_7

I'm not concerned about the circulator...only about the effect of pressure on heat transfer from the boiler to the water, if sny.

Unknown

Yes.....somewhat

Greg,

What does change with pressure increase is the saturation temperature of the water at the walls of the boiler sections in contact with the 1100°F flue gasses. I can't remember the exact perameters, but as I recall, that temperature would be about 250°F at 30 psi and well in excess of 300°F at 80 psi. I have a chart somewhere with the exact numbers. Hope this helps.

Glenn Stanton

Burnham Corp.

greg_7

Aha! I knew it. Would it follow that a lower pressure, such as the 12 or so for the average homeowner, have a similar drop in saturation temp?

Do you design for an optimum or an industry standard.

Or is this a physics rule that is pretty much locked in place depending on the boiler's metal type and operating pressure?

It doesn't seem to be that great a difference anyway

Arthur

Pressure v temperature transfer.

If the temperature is the same it makes on difference on a Low Pressure hot water system. You still run the boiler at the same temperature (80 deg cent or 180 deg fah) it is the temperature difference (delta T) which affects heat transfer not the pressure as per Steamhead said.

Unknown

In most cases

the difference is minimal where the static fill of the system is 12 psi or even 20. Where it does make a difference though is in elevated pressure hydronic systems where static fill may be much higher. Most commercial boilers now come equipped with 50 psi relief valves and are actually rated for 80 psi. In this case, the saturated temperatures within the boiler can approach 300°F. Introduce into this environment return water cooler than 130°F and you may have some thermal stresses going on within the boiler sections. This is why we and most other manufacturers advise and stress the use of either a properly sized blend pump, modulating three way diverter valve and a return sensor to divert or bypass heated supply water to the return side to always keep the return side warmer. Hope this helps.

Glenn