Boiler Selection - Fire Tube vs. Water Tube 1

[Leverage]

I was wondering if anyone has any experience in selecting a fire tube boiler versus a water tube boiler. We are in the process of looking at changing out our water tubes for equivalent fire tubes. We are curious what the change in operating cost structure will look like. We are burning No. 6 Fuel Oil. The replacement boilers (possibly firetubes) will be designed to carry a 35,000 Lb/Hr Load at 100 PSI. Currently, our water tubes are oversized for our current downsizing. Any additional information, links, or experience in a similar situation would greatly help. Thanks in advance.

 

[boilerdoc]

Hi leverage,
should be no problems to find firetube boilers for your steam parameters, specially if you want to downsize. The advantage of firetubes are mainly in lower investment costs, and longer service life.
Because of the higher internal water content they are slower in load changes.
Hope this helps a little.

 

[rmw]

Water tube boilers will be easier to clean by standard methods, soot blowers, etc, than will fire tube boilers.

You start building up soot, and ash deposits in the fire tubes, and with no way to clean them, it can soon bring you down.

I have yet to see 6 oil that doesn't leave some nasty stuff behind, and I have a hard time visualizing how a bunch of that ash, etc, laying in the outlet ends of your fire tubes will be a good situation.

At least with a water tube, you can operate the soot blowers as needed.

I have had experience with firetubes burning 2 oil, (diesel)and they soot up badly enough. I shudder to think about 6 oil.

rmw

 

[eboelen]

The operating conditions you mention are well withing the firetube range tiday.
Burning #6 oil is not necessarily a problem: ask your vendors to propose zoned (air operated) sootblowers. Zoned, so that they can be used during the fire cycle and minimize the instantaneous air requirement, air operated since steam could result in making the deposit into a hard, difficult to remove crust.
Firetubes do have a large water content that slows down the response to load swings, on the other hand it helps to have a large amount of water at steaming temperature to deal with short peak loads (10 minutes or so) without the pressure dropping dramatically.
One coment about the sootblowers: the use of the sootblowers will result in fine ash leaving the stack, make sure that it doesn't get you in any trouble from an emissions standpoint.

Eric

 

[chicopee]

To replace a 35,000bs/hr WT boiler with an equivalent FT boiler will necessitate a very large boiler room for a very large boiler. I do not think that you can find a FT boiler with a steaming capacity over 16,000lbs/hr.

 

[MJCronin]

chicopee,

Johnston Boiler has been making large firetube units for many years. Fuels include; gas, No-2 oil, No-6 oil

The largest unit offered (509 series)is a three pass, 2500 BHP (PFTS200-3)size rated at 86,250 lbs/hr (at 212F)

Of course, a single large boiler may not be the best choice for this service.

At a duty of 35,000 lbs/hr, I would guess that two 500BHP units would be about right. More detailed evaluation is necessary

My thoughts only.....

MJC

 

[chicopee]

MJ Cronin--what is the drum diameter of the largest Johnston firetube boiler? I have inspected many firetube boilers and I have never encountered any FT boilers larger than 12000 lbm/hr. Economically speaking 16000-20000lbs/hr is the limit for FT boilers; over that range of size WT boilers are recommended.

 

[MJCronin]

chic,

http://www.johnstonboiler.com

MJC

 

[imok2]

For firetubes with soot deposits, there are machines that mechanically drive brushes or scrapers through the tubes, requiring almost no human effort. These machines are, in turn, connected to powerful vacuums that draw the loosened soot from the tubes simultaneously, leaving the tubes, boiler room, and operator completely clean.

Some firetube boilers develop scale deposits. This is particularly true with waste heat boilers and boilers fired on lesser quality fuels. Some of the worst waste heat boilers are found at hospitals where medical waste is burned in incinerators and a boiler is used to capture the subsequent heat generated. In many cases, a straight brushing action isn’t sufficient, and rotary equipment must be used.

Manually fed rotary systems are available that incorporate simultaneous vacuuming as well. Many of these systems were based on designs originally intended for condenser and chiller tube cleaning, and they work extremely well on scaled boiler tubes.

 

[curve3104]

Leverage,

Depending on the size of your boiler room, firetubes would be fine to replace your watertubes. Most firetubes are designed to have 5 sq. feet of heating surface per boiler horsepower. These can take up a great deal of space in a hurry.
As mentioned before in this thread, cleaning a watertube is much easier than a firetube. Especially if you are burning #6.
You should contact a boiler manufacturer to discuss your project and they could be of great assistance. Good firetube manufacturers include Hurst, Johnston (even though they build huge bombs, i.e. 2500 hp firetubes), Cleaver Brooks, and English Boiler.

 

[Leverage]

Guys,
I am looking for information more along the lines of efficiency and cost savings. If anyone has any experience in making the switch, what savings were realized from everything regarding operations to water treatment. I understand real estate is a concern, and we feel that we have a good handle on this. However, I am looking to apply the numbers for justification of the project. I want to make sure that we left nothing out. Thanks for all of your info to date.

 

[TBP]

Most plants run without any proper steam metering, so there won't much in the way of real-world boiler efficiency comparisons available. Water treatment costs will be pretty much the same - the advantage of fire tubes is that with the water all around the tubes, it's almost impossible (although it HAS been done...) to block the water circulation entirely. It's relatively easy to plug a single water tube, and have it burn out. And this is just for scaling/fouling - fire tubes will corrode just as nicely as water tubes. However, with the pretreatment and chemical programs available today, this should not really figure into the selection process. A huge portion of the water treatment cost is not determined in the boiler room anyway - it's very dependent upon how much condensate is actually returned to the boiler. If you have a plant in which all, or almost all of the steam that leaves comes back to the boiler as condensate, then the chemical costs will be very minimal, regardless of what's going on with the boiler itself.

From a performance and operating cost point of view, I don't think that it will make any difference which design you select. In any event, if there's no individual fuel metering, and proper steam metering on your existing boilers, and/or you aren't planning on doing this for the replacements, there's simply no way to calculate or compare the boiler efficiency of one design over the other.

 

[eboelen]

I was personally involved in the design of large firetubes and would like to interject a few comments here since I feel that some unjustified remarks are made.
A large firetube is designed for 5 sqft per horsepower "because of perception in the domestic market", for no other reason, nobody in the rest of the world builds firetubes with 5 sqft per HP heating surface. You'll find some European designs that have as little as 3.5 sqft. This indeed makes it larger than a comparable watertube but on the other hand also lowers the thermal load per sqft of heating surface resulting in a longer lifetime of the firetube. At the same time, this lower thermal load in a way reduces a little the quality standards for the chemical treatment. Water that will work fine in your firetube may result in scale formation in a watertube.
Tests have been done and it was proven that a boiler with around 4.5 sqft per HP was actually more efficient than a unit with 5 sqft per HP. The cleaning of a firetube in my opinion is easier than of the watertube. It can be done without getting inside the unit, which is not the case of the watertube. Special tools, some automated, are on the market to clean tubes but be aware that tube failures have been attributed to the use (proper or improper) of some of these automated aggressive tools.
Secondly, I don't like to use the word "bomb" when talking of a boiler at all. These units are built to ASME standards and with ample safety margin. If the unit is properly maintained, water treatment is done right, and it is used within the parameters imposed by the vendor, then there is no reason why that pressure vessel should fail. All boiler failures have been caused by lack of maintenance, improper water treatment or improper use of the unit. There have been cases where improper installation resulted in a failure but luckily very few and always caused by contractor error.
I also agree with TBP that it is impossible to predict any savings if you don't have operational data from your old boilers. The only thing that can be said with certainty is that today your boilers run at a reduced firing rate which, and i assume here the burners have also quite a few years behind them, is going to result in a much higher excess air ratio than the new, properly sized boilers. You will pick up overall efficiency in that area. The new firetubes will also have almost a zero tramp air factor which may not be, or probably is not the case with the older watertubes, so there is additional savings there.
And finally, since this a new project you want to make sure that new, state of the art, controls are used to keep the boiler operating at maximum efficiency. That alone, compared to the old (often manual) controls and adjsutements, will be your biggest savings. You need to make sure a burner management system with O2 control is definitely included.
Let me know if you need more.

 

[Combustor1]

Leverage,

I posted a similar post over a year ago and did not get many satisfactory answers. We are currently converting 4 small WT to 3 FT - let you know in a few months.

I have heard that FT do not last as long and are not good for swings in load. My choice was for WT.

Good Luck


Combuster