Proportions, etc from other sources

I'm looking over some old documents........the newest is from 2000, and is a draft for oil conversion for the Garratt at Ffestiniog. This appears to be the second design proposal, as the writer compares his to a previous one I don't have. The first proposal was for one centrally mounted burner, but this writer finds it difficult to achieve adequate spotfiring ability with one burner large enough to generate adequate steam on the road, so he proposes one central burner for pilot /spotfire use, and two others beside it on either side for on the road firing. All are of equal capacity. "Turn down ratio" for each burner is 4:1or 5:1.......so the system in its entirety provides 12:1 or 15:1 ratio of turndown.....whatever that means. I'm thinking British burners must not be as variable as Yank burners are. There is difficulty with sizing burners since he didn't have figures for steaming rate or boiler efficiency, so he juggled some numbers and came up with a flow rate of 350 kg/h or on overfire 420.......and based on other things he's read he assumes an "excess air value" of 60% in sizing air inlets. Firebox vacuum is estimated at 25MM water. So, excess air of 60% based on burning 350 kg/hr with 20% tertiary air. All this is from the summary page........

Getting further into the design, the row of 3 burners is mounted at a 30 degree angle from horizontal inside the front of the firebox above the level of the mud ring, aimed towards a flash wall below the door. Primary air is through the burners themselves, secondary is anybody's guess(?) but tertiary from a second opening(?) in the firepan between the front of the burners and the door sheet. I think we lost a source for secondary air or a tertiary opening someplace........maybe I'm being dense or missing something.

There's a lot more here, but this is the basic plan. It's extremely complete down to the point of including pictures of what valve handles look like. there's also a 1999 paper included that proposes an R&D program for doing pretty much what we're doing, but with a less open and more defined set of goals. I'll scan and post this soon.

Dave

Upon further reading, it seems the "excess air" is air over and above that amount calculated as necessary from a stoichometric perspective.

Now I'm looking at a technical bulletin which i suspect is from Laidlaw Drew, dated 1962. It's also very interesting, but more practical and less theoretical. They base air inlet area on tube opening area, and decree it must not exceed 66% of tube effective area through boiler tubes, and find that firebox air inlet areas of 40-50% of tube area are satisfactory. They also limit firepan air opening area to encourage turbulence and fuel / air mixing........which they call "pan tube" area. They base initial design on a pan tube area of 65% of boiler tube area, followed by a pan tube area calculation of 10 square inches for every square foot of grate area. If this second calculation shows a result that's 40-50% of boiler tube area, it probably will steam well unless the boiler is simply designed with insufficient grate area.........

Other considerations: a grate of 20 square feet or less should not use pan tubes of 4" diameter, which is the maximum recommended for larger grates. Must inlet air resistance is at the outside end of the pan tube so a bell-mouthed opening is recommended. Pan tubes should be aimed outward radially from the center of the burner.

There follows several pages of formulas for calculating draft, funnel sizes, funnel efficiencies, and boiler air flow resistances. I'll scan and post these also, along with a nice page with a drawing and formula from God's Wonderful Railway on front end proportioning.

Dave