Multiple Boiler System

JohnNY

Most multiple boiler systems' diagrams show pumps directing water into the boilers in a parallel circuit.
<A HREF="http://www.tekmarcontrols.com/listing/acrobat/a132-4.pdf">Like this.</A>

Is it wrong to pump away on an MBS, even just for consistency's sake?

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ALH_4

Vapor Pressure

It's probably better to pump into a boiler. One reason is because the return temperatures are lower than supply temperatures. The circulator is designed to operate at the higher temperature, but high temperatures tend to decrease the life of components. The other more important reason is if you pump into the boiler, the circ head adds to the pressure inside the boiler and reduces the possibility of cavitation on the suction side of the circulator.

JohnNY

I thought...

...the temperature issue was addressed years ago.

As for cavitation and head pressure inside the boiler:
I'm not sure those are directly related. If they were, wouldn't it always be an issue when pumping away?

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Brad White_185

I would say that

it makes sense to pump into a boiler which has a higher internal pressure drop, such as most ModCons do. To pump away from the boilers in this case would tend to cavitate because the boiler pressure drop probably represents 75% of your pump head. If not cavitate, at least the deeper pressure drop might upset internal pressure switches as was once explained to me.

With large-passage, low-pressure drop cast iron boilers, I cannot see that it makes any difference.

My $0.02 anyway!

Brad

Rich L.

Mod-con or cast iron

Brad's right on here. Most all mod-cons have higher pressure drops through them and therefor recommend pumping into them. Cast irons are normally set up pumping away from the boiler, after the expansion tank.

If you're asking about pumping away as in "Pumping Away" it's refering to pumping away from the point of no pressure change, referring to the expansion tank.

hot_rod

It doesn't

show in that drawing but the expansion tank and purger would be against the dual pumps in the loop. as such the system and boiler circs would be pumping away from the PONPC while pumping into the boilers.

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tbuck

pump/circulator

I think we it's important to remember the difference between a pump vs a circulator. If these are res size boilers with circulators the location will make marginal difference. If these are "big" boilers where there are actual pumps with some horsies driving them, were they installed as shown the maintanance guy would likly see some water on the floor from time to time (maybe every morning). From a cold start an actual pump installed on the return could push the releif valve open. If that opens only for a moment, building personel find it disturbing.
In the drawing our Friends to the North put together air seperation should be accomplished out in the primary (heat delivery) part of the system...where the occupants are. Air seperation can be attempted on the secondary (boiler) side but the air may not cooperate. Ask me why I know this...