Underfired boiler

Jamie Hall

Probably a stupid question, but -- I thought I'd best ask the pros before I put my foot in it too far.



I've encountered a situation where some radiation had been removed from a building.  The boiler was not changed -- but the burner and nozzle were.  The present firing rate (1.35 gph) matches the radiation requirements moderately well.  However... the boiler is a six section H. B. Smith, which, according to its nameplate, had a firing range of 3.2 to 3.6 gph.



Would I be correct in thinking that this much underfiring could cause all sorts of problems with heat not getting into the system particularly fast (well, let's put it another way -- glacial slowness!) in spite of apparently adequate venting and all, and possibly excessive fuel consumption?  Never mind possible problems on the combustion side of the boiler...



Help!

BobC

Too much?

That's a pretty drastic down firing, did they do anything with the firebox?



My Burnham V75 is rated at 1.65GPH and I've got it running at 1.1 right now and I'm not sure I haven't gone past the point of economical operation. It heats the house fine on a zero degree day but the consumption is iffy.



Bob

Dave in QCA

What is the rating of the burner?

Jamie, it is not unusual for large (very large) fire tube boilers to have a burner capable of a 10:1 turndown.  It is my understanding, that when you are completely controlling the air, as you do in a power burner, that the only gasses going through the boiler are the very hot combustion products, and fire.  The smaller the firing rate, the greater the ratio becomes between the surface area of the boiler and the firing rate, and therefore more efficient.  However, it is also my understanding, that a turndown of anything greater than 10:1, (maybe anything in the area of 10:1) starts to become innefficient, because of the thermal losses of the boiler to its surrounding space.  i.e., you cant run a 320 HP boiler with a stick match.    So, a power burner boiler is very well suited to a turndown, because there is no secondary air to delute the combustion products.  The issue then becomes, is the burner capable of firing cleanly and completely at the adjusted firing rate.  From your hint, it sounds like you may be having probelms with proper combustion.  And if that is the case, it of course would be very innefficient.  It would be better to be oversized, then to be downfired with a crappy fire.  The burner manufacturer's manual should give the min and max firing rate of the burner.

Dave

Mike Kusiak_2

Themal mass

While the burner and boiler may be capable of a high turndown ratio, the thermal mass of the boiler and contained water remains the same at any firing rate. So even though the burner and boiler may run cleanly and efficiently at a low firing rate, it may take significantly longer to bring up to temperature and actually produce steam. This may be what Jamie is referring to when he mentions the "glacially slow" response time of the system.



Typically with the high turndown burners, the system is set up with a proportional comtroller, so the boiler can run at maximum firing rate to generate steam quickly to fill the system and then modulate down to a low firing rate to maintain the desired steady state pressure. Trying to just operate a big boiler at a firing rate less than half its rating could result in very slow initial steam generation which is what Jamie may be noticing.



Maybe a dual fire burner with vaporstat control would make sense in this situation?

Charlie from wmass

Do you know the pump pressure?

Jamie if the pump pressure is above 100 psi you can check the nozzle charts to get the fire rate. Weil McLain is ok with a 50% down fire, I actually spoke with tech support last month concerning this type of a situation. The difference is the Smith has a lot more cast iron mass than the Weil McLain does though.

Jamie Hall

Going to work on this chap

to get him to reinstall a bigger (like 2.0 gph) nozzle in this boiler, to see if that helps some.  Unfortunately, this is all going on by e-mail... I just have this image of a huge kettle of chile sitting on top of a can of Sterno, and going absolutely nowhere!  It's a system which was rather large (hence the big boiler) and half the radiation was disconnected -- and the burner sized down to match (rather closely) the remaining radiation.  But the boiler wasn't changed... oh well.  I'll keep you all posted!

Jamie Hall

It worked.

The 2.0 gph nozzle is big enough to let that poor old boiler really make some steam.  Not super great, but at least the system is running better...



Makes one wonder if anyone has done a study on a particular boiler, to generate some general information on just how firing rate effects steaming ability and efficiency...



Thank you all!

Dave in QCA

Fantastic!

Jamie, I think there are some studies out there that you might find by googling.



My recollection is that extreme turndown ratios run at a lesser efficiency.  The advantage comes in situations where there is a continuous, but fluctuating demand for steam.  The studes I recall indicate that while the boiler runs at a lower efficiency at a very deep turndown, it is more efficient than the losses that would occur with a lesser turndown, and the resulting on/off cycles.  This is because of the losses that occur in the post firing and prefiring purge cycles that basically blow cold air through the boiler.   The study that I recall reading was on a large, (greater than 100 HP) Scotch Marine type fire tube boiler.