Pump Location 1

[tbedford]

Through many years of visiting different plants I have noticed that sometimes the hot water is pumped away from the boiler. Somtimes the water is pumped into the boiler.

Which system is correct and why??

thanks, Tom

 

[MiketheEngineer]

My guess is that pumping in would be more efficient. But I know not!!

 

[vpl]

I would think it would depend on what is being done with the hot water ...

If you think about it, a "boiler" implies that heat is being added. This implies the outgoing product is going to be hotter than what comes in. In the end, though, it really depends on what you mean by "hot water" and what is being done with the "hot water."

For example, in a pressurized water nuclear reactor, the water is hot when it comes in (about 275°C (530°F)) and even hotter when it comes out (around 315°C (600°F), all temperatures approximate and not representative of any particular reactor design.)

In a boiling water nuclear reactor, the water is slightly hotter when it goes in (285°C (550°F) same disclaimer, but leaves as steam.



Patricia Lougheed

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[TBP]

If you pump away from the boiler, you'll have far fewer problems with dissolved air removal. The air separator & expansion tank should be locater near the boiler outlet, just ahead of the circ pump.

The closer that water is to the boiling point, the more dissolved air tend to pop out of the system. And that place is just after the boiler, and just ahead of the pump suction.

 

[racookpe1978]

For a conventional (simple coal-fired) plant, the water leaves the condensate pump at relatively low pressure and low temperature, goes through the main feed pump gaining tremendous pressure but little temperature and enters the boiler under those conditions. In the boiler, it loses a little bit of pressure (through flow loss in the thousands of feet of small bore pipes) but gains tremendous heat energy - leaving the superheater at a slightly lower pressure but much greater energy.

(All of the above ignores various feedwater and condensate and reheater arrangements.)

You are, in general, most accurate thinking of a boiler as the place where heat energy is added to the steam system. Pump location in the flow doesn't define the heat exchangers or boiler.

 

[rmw]

The question is curious. Most of my experience has been that water is pumped into a boiler and then the same mass flow leaves as steam. What little water that leaves the boiler in the form of water (blowdown) leaves under the motive pressure of the boiler steam (created by the pump that sent the water there in the first place.)

I've seen water pumped out of a boiler but only just to pump it back in at another point within the boiler that it couldn't get to without pumping (natural convection creates most of the water flow within a boiler).

Your question doesn't seem to fit that scenario. Can you elaborate?

Now, one side bar. If the boiler you mention is a hot water "boiler" (not a boiler at all) then all the above is off. The "boiler" adds the heat raising the water's temperature and then the heated water is transmitted to the use point with a pump.

Maybe it would help if you better described the "plants" you visited to make the observations.

rmw

 

[TBP]

rmw - The term "hot water boiler" is very common in the HVAC world.

tbeford would most likely have been better off asking his question in the HVAC/R forum on this site, but his choice is not illogical if he deemed it to be a thermodynamics issue. But the equipment location & sequence in these systems is strictly for the most effective removal of dissolved air.

Bell & Gossett has literature that explains all of this much better than I can.

 

[tbedford]

thanks to rmw and TPV...

when I think back most of the installations have been hot water boilers.

I need to also note that the placement of the expansion tank wrt the pump suction is most effective at the point of least pressure change.

Also thanks to TPV because the Bell and Gossett site is a very informative library.

Indeed this post may have been more appropriate in the HVAC forum...
t

 

[MechEngNCPE]

I always pump into a hot water boiler, but have also been curious as to why.

 

[btrueblood]

IMOP, like all things engineering, it is a trade-off:

Pump upstream of boiler: pump seals will last longer, and cavitation damage will be more easily avoided, if pumps don't see the highest temperature water.

Pump downstream of boiler: putting the pump downstream keeps the boiler design simpler. Design of the boiler is impacted if the maximum system pressure and maximum temperature are occurring simultaneously, as would happen with pump upstream of boiler. This can result in thicker walls on the water tubes, and reduced heat transfer efficiency.

 

[TBP]

The pump seal issue has been long-since dealt with. It was an issue post WWII, but not for the last 25 years.

The VAST majority of commercial HVAC boilers are installed in mechanical penthouses, so they see very low pressures. Many have relief valves set for 30#.

The mechanical arrangement is designed for effective air removal. If there's air bubbles in the heating loop, it sounds like it's full of BB's. Not much of an issue in industrial systems, but in a hotel room...

The Bell & Gossett literature explains all.

Ignore it at your peril...

 

[rmw]

I will readily admit I don't lurk in the HVAC or building heat world much if at all. Most of what I deal with you could throw a how water boiler through the doors of. Thanks to TBP for the reference and a star to boot.

rmw