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Posted by Boiler Guy on March 07, 2004 at 10:15:21:
In Reply to: Re: Fire tube Boiler Applications posted by steamplants on March 04, 2004 at 20:07:20:
The DA - firetube analogy is all wrong for a couple of reasons:
1) ENERGY - The DA is operating at 5 PSIG opposed to a typical firetube at 125 to 150 psig. Should the pressure vessel rupture it will cause damage but at 5 psig no where even remotely near as significant as a firetube of the same physical size and water volume.
2) OPERATION - A DA is an unfired pressure vessel. The worst case senario for the DA is overpressurazation, if that happens a safety valve lifts.
The firetube has a 2,500 to 3,000 deg F furnace that must always be water-cooled or risk immediate and catastrophic failure. Firetubes are exposed to low water risk, scale created local hot spots, cracking from differential expansion and cycling stresses, etc.
3) CONSTRUCTION - A DA is a tank with a shell and heads, an ideal construction for a pressure vessel. A firetube on the other hand must hold back enourmous force with flat tubesheets subject to a number of different stresses. Not an ideal construction method. Watertube boiler construction is another advantage of a watertube over a firetube but we are talking DA's vs FTs' now, that's for another post.
Life cycle will truely be a factor of operation and maintenance. Way too many variables but assuming both a firetube and watertube operated and maintained correctly there should be no difference between the two. A watertube will be more expensive to retube than a firetube but this is a function that shouldn't occur for 30 years after purchased new. If anything I'd expect a firetube to require a more frequent retubing than a watertube making that a wash.
As for fuel costs and efficiency, assuming each boiler to have an economizer both the watertube and the firetube will have the same fuel-to-steam efficiency. Assuming both have feedwater in at same temp, both operate at same excess air and FGR rate, and both have same flue gas exit temperature, the efficiency will be exactly the same. True, without economizers the firetube will have about a 1% to 1.5% edge, but on boilers the capacity we are talking there will almost certainly be economizers. There are no long term fuel costs advantages to one over the other.
As if I already didn't like big firetubes to start: the more we debate, the more opposed to these ultra-large beasts I become. Firetubes have their place up to 800 HP and that is it.