Fire bar expansion question

[PeteThePen1]

Hi Folks

A dead simple question for you (I hope), but I cannot seem to find the answer anywhere.

The fire bars in my boiler casing are to be supported by two lengths of 1" angle iron creating a "shelf" on each side of the fire box. The gap between the vertical faces of the angle iron is meant to be 306mm. My first thought was that one could then fit fire bars, made from strip, that were 300mm long. However, remembering expansion, how much shorter than that should they be so as not to exert lateral pressure on the supports for the angle iron when the fire is burning well?

Regards

Pete

[Mike Rometer]

As long as the ends fit on the shelf, and when shuffled as far one way as they will go, and they are still long enough to remain on the shelf at the other end, then they can't fall out, but can be removed in a hurry should that become necessary.

So if you've a 1" shelf at either end, the bars will be fine at about 1 1/2" longer than the gap between the shelves.

Perhaps something else to think about is whether you intend to be able to remove the bars through the firedoor or through the ashpan.

[fredrosse]

In the power piping industry, there is a "Rule of Thumb" for expansion of steel pipe, "3/4 inch expansion in a pipe 100 feet long, with 100F temperature change. Applying this rule to a temperature change of, say 1200F (bringing the bars to near red heat) would give 9 inches expansion in 100 feet, 0.9 inches in 10 feet, and 0.09 inches in 1 foot, which closely corresponds to your 306mm width.

These numbers are for steel, and other metals expand differently. Cast Iron, Stainless Steel, and Copper alloys expand more, but are all less than 2x the steel value. Assuming 200% of the steel expansion, we have 0.18 inches expansion for the 1200F temperature change. If you allow about 5mm for expansion, binding will not occur.

As stated previously, you could probably allow much more clearance, 6mm or even 16mm with no hardship, assuming your support angles are big enough.

[dhic001]

Zeltic runs with cast "fire plates", basically flat plates with a series of tapered holes in them. The grate is circular, about 16 inches in diameter, and the plates are in three sections. The last set of plates suffered some overheating at times, and expanded and went out of shape. What I didn't realise was how much they had expanded! When I build the new firebox, I didn't have new castings to hand, or the patterns for the plates, so I made the firebox to suit the old plates, allowing for slightly smaller fitting new plates when the time came. The plates sit on a rolled steel ring which protects the castable refractory shelf that the end of the plates are meant to sit on. The old ones well and truely sat on the castable refractory, so I was most suprised to find the new plates sit only on the steel ring. I was bemused by this, so lifted the plates out of the box again and sat them on the old wrecked plates (by this stage they had got totally bent and had cracked into pieces). I found that the old plates had expanded almost an inch in diameter! I'm very glad that I didn't have a horrible wetleg boiler, they could so easily have done serious damage if they'd been a tight fit in a wetleg.

Daniel

[PeteThePen1]

Many thanks to everyone for your comments and suggestions.

What would I do without the forum and your support?

Regards to all.

Pete