Boiler piped for reverse flow?

Mark Eatherton

I saw an AJAX boiler that was piped this way once. During extended heat calls, the boiler had a tendency to flash t steam because the operator adn high limits were still mounted on the top of the boiler. THe theory of counter flow is fairly obvious and commonly applied in HXer applications. If you choose to do this with a boiler, make certain that the controls are in the right place, otherwise you too may watch a heavy boiler attempt to do the low mass dance...

ME

Unknown

Boiler piped for reverse flow?

Has anyone seen a boiler piped like this? Return is going into the top of the boiler. Supply is coming out of the bottom to the circulators pumping away from the boiler.

This has me really scratching my head. Why would you want to pipe this way, in opposition to the natural gravity flow?

These 3 pass fire tube boilers were installed in 1981 to replace the original 1950 monsters which are still in the boiler room. If you look at the photo, you can see that the original was piped the same way. When replaced, the return connection to the top of the old boiler was cut and repiped to the new one.

The system actually works pretty well, but I am curious why they piped this way originally, and again when the boilers were replaced.

don_185

Thats odd

Are you sure the circ are not pumping into the boiler?

If piped like you say, I would have to assume that people are overheating and the gas bill has to be high.

I would have to think you would even have that crackling noise like one gets with a bad tip tube in a waterheater.

Unknown

No, the circulators are pumping away and the supply is taken from the bottom of the boiler as shown in the photo.

I found a diagram in the 1950's B&G engineering manual that shows exactly how the system was originally piped, which makes sense since it is a monoflow ststem. The design was pretty much taken from the book.

I still don't understand why you would pipe it this way though. The same manual shows different ways to connect boilers for even internal temp distribution and all show the supply taken from the top of the boiler. When you look at the sample piping diagrams however, they all show supply taken from the bottom when circulators are pumping away, but there is no explanation why.

It does seem to work pretty well though. Steady state combustion efficiency was measured at 83.5% with a 300F flue gas temp. Not bad for a 27year old gas boiler! Perhaps the fact that the cooler return water first contacts the third pass tubes with the lowest combustion gas temp adds to the efficiency?

radiconnection

My understanding (from reading in other forums..) is that it reduces thermal shock.. Make sense?

Charlie from wmass

This was sometimes used to supply zones from a steam boiler. The rads could not be more than 7 feet above the boiler water line.I would say it would reduce cavitation on large pumps as the mass of the water is above the inlet side of the pump.

Ross_7

G.E.boiler

In Starbuck's "Modern Heating Illustrated" from 1946, had a picture of a G.E boiler piped like this. Supposed to reduce thermal shock, I guess.

Ross

Jamie Hall

not only does it reduce

thermal shock, but it's good thermodynamic sense, too: the heat transfer rate of any heat exchanger (and that's what a hot water boiler is!) is determined by the temperature difference on the two sides of the plate or pipe or whatever. The way that's piped, you've got cooler water in contact with cooler stack gas and warmer water in contact with warmer gas -- splendid arrangement! Nice even heat exchange, and you can get the stack gas down to a lower temperature that way (in principle...) and hence get better efficiency.

Charlie from wmass

Ok so if this is true, WHy do we not pipe and oumo this way? I would gues it is a throw back to gravity and steam systems where out had to be the top? Also would the pump then have to over come the natural convection of the hot so would then need to be larger since it is in practice swimming up stream in the thermal currents?

Jamie Hall

I suspect

our usual piping is a holdover from gravity and steam systems -- we are a pretty conservative lot, after all! The pump would have to overcome the natural convection in the boiler, but that's really pretty minor in the overall scheme of things...

bob_46

Backwards

Triad and Edwards boilers used to come prepiped with the pump on the supply coming out of the bottom of the boiler. Two advantages that were apparent to me were pumping away and the whole boiler acted like a Rol-air-trol. Weil Mclain published a booklet called Normal Flow Versus Reverse Flow In Cast Iron Boilers. They don't recomend it with their boilers.

Unknown

TT Prestige

Has kinda the same arrangement except the return is in the bottom and the supply out the top but the burner is on top in a counterflow arrangement. Same thing but upside down.
This could be a new book for Dan. PUMPING AWAY AND BACKWARD. LOL
HAVE A MERRY CHRISTMAS

Unknown

Mark, you bring up a very good point. Typically the aquastat tappings are located for a normal flow situation and are certainly not optimal for the reverse flow case.

In this instance the safety and operating aquastats are placed at about about a 10 o'clock position from the vertical. They do seem to respond pretty well, probably due to the high pumping (about 250 GPM) there is a lot of flow and quite uniform temps throughout the boiler. Delta T is only about 6 to 7 degrees during firing, but it should be noted that these boilers are operated at minimum firing rate, about 50% of rated input. If the circulators shut down during a burn cycle, the normal convection circulation resumes and the aquastat setpoint is reached very quickly, within a minute or so.