Gaines Wall and Firebox Size

[L-105]

In a typical deep behind the drivers firebox, which has a deep throat sheet and the grate well below the bottom of the back tube sheet, the inside firebox dimensions align with the grate area. This contrasts with shallow, over-the-drivers, type fireboxes which from what I have come across, typically utilised a Gaines wall behind the throat sheet, with the position of the Gaines wall varying in different designs, some being situated right at the throat sheet whilst others were several feet behind it.

 

I understand the purpose of the Gaines wall, but for shallow fireboxes which extend over the drivers, the grate area is often not representative of the actual size of the firebox proper. If you consider the inside firebox dimensions excluding the effect of a Gaines wall to get a true reflection of size (not including combustion chamber), then the firebox sizes found on various classes of locomotive would look as follows:

 

1. NP Z-5   Length: 266 1/8”   Width: 114 1/4”                        Area: 211.1 sq ft

 

2. NP Z-6/Z-7/Z-8   Length: 246 1/8”   Width: 114 1/4”         Area: 195.3 sq ft

 

3. D&RGW L-105   Length: 224”   Width: 108 1/4”                 Area: 168.4 sq ft

 

4. D&H J-95   Length: 213 1/32”   Width: 108 3/16”              Area: 160 sq ft

 

5. UP Big Boy   Length: 235 1/32”   Width: 96 3/16”              Area: 157 sq ft

 

6. B&O EM-1   Length: 228”   Width: 96”                                 Area: 152 sq ft

 

7. DM&IR M-3/M-4   Length: 210 1/8”   Width: 102 1/4”     Area: 149.2 sq ft

 

8. SP AC-9   Length: 205 21/32”   Width: 102 1/4”                 Area: 146 sq ft

 

9. UP 4664   Length: 187 1/32”   Width: 108 3/16”                Area: 140.5 sq ft

 

10. C&O H-7   Length: 204 1/8”   Width: 96 1/4”                    Area: 136.4 sq ft

 

The Delaware and Hudson J-95 is particularly good example, its firebox being considerably larger than its 108 sq ft of grate area suggests. Likewise the B&O EM-1 actually has a bigger firebox than either the DM&IR M-3/M-4 and SP AC-9 2-8-8-4s. The later UP 4-6-6-4s, after the CSA-1 and CSA-2 batches, had the Gaines wall removed increasing the grate area to 132 sq ft from 108 sq ft, although it doesn’t add up to 140 sq ft so what alternative structure existed just behind the throat sheet I don’t know. As for never built designs, I’d be interested to know what the firebox dimensions would have been for the N&W Y7, 1930 ATSF 2-8-10-2, and 1934 GN 2-8-8-4 proposals, and whether their generous grate areas were simply a result of having no Gaines wall or a genuinely huge firebox.

 

Another point is that whilst a Gaines wall reduces the available grate area, it doesn't reduce firebox volume. Since oil fired engines don’t need grates, could it be that the inside firebox dimensions are a better measure of performance, at least for oil firing, rather than going by grate area?  

[Overmod]

Remember that the object of good firing is to optimize heat uptake by the radiant surface in the 'firebox and chamber' area, without quenching any carbon to soot before it can fully react with oxygen in a reducing environment, and the object of good firebox design is to keep the combustion plume long and stable to accomplish that full combustion even at high rate.

The grate only needs to be large enough to sustain a fire producing combustion gas at the rate the boiler can absorb it.  Burning fuel across a wider grate can not only waste fuel but make the boiler more difficult to fire (and perhaps increase emissions).

Think of the firepan as a rough equivalent of 'grate area' for an oil-fired locomotive.  In fact the control dimension for plume is the 'throw' of the burner from the throat sheet back, in most burners, but the effect of limiting fuel rate is comparable.

The Gaines wall keeps flame from impinging on the lower tubes and tubesheet, and soot from accumulating there as a result -- or so is the idea.  It limits the physical size of grate without limiting the length of the radiant legs or circulation, which can be important too.  The thing that has to be watched is the differential expansion in the inner wrapper due to the very different heat uptake in various sections of the gas path -- Flannery staybolts do a great many things.

[Paul Milenkovic]

It appears from Cole Ratios and all of that, there was still a notion that you could make a grate too large, that it would waste too much coal in standby conditions under steam in relation to the coal saving from reducing the air velocity that can lift particles off the grate at high firing rates.

I believe Baldwin's Ralph Johnson suggested in his book that with proper air dampers, having the grate "too big" is not the problem once thought.