Some time was spent measuring and drafting up the smokebox. The diameter of the chimney and length of the petticoat and height of the blast cap needed to be checked. The rule of thumb tapers for laying out the proportions of the above items were set out on CAD.
The Norman Spink Britannia drawings were also checked over to see what they were like. The drawings took some time and cross referencing to get an approximate height of the blast cap, needing to work backwards from the frame drawing, smokebox saddle drawing, and the smokebox layout. The main frames reference the bottom of the exhaust tee piece from a point in space on the centreline of cylinders. Naturally part of the frames had to be drawn up to get this point.
The tee piece was drawn, and the blast cap. The petticoat also, and the chimney. The proportion tapers 1-in-3, and 1-in-6 were laid out from the rim of blast nozzle opening. According to Martin Evans in "Model Steam Locomotive Construction" book, the tapers should intersect with the top of the chimney and the tangent point where the petticoat bell mouth meets the chimney bore.
The discoveries, after measuring up the smokebox and chimney we have here, revealed a larger diameter and shorter petticoat. The proportion tapers suggest that for a larger diameter chimney, the blast nozzle needs to be further down to "fill up" the chimney with steam, and the petticoat length will be proportional to the diameter.
Steam locos are a bunch of balances and compromises based on proportions and a few rules of thumb. What we don't want is a lack of vacuum in the smokebox from possibly sucking air down the chimney.
My theory with the Britannia is the firebox will well and truly produce enough steam based on surface area - it has a wide firebox, combustion chambers, and superheaters. Drawing the air fast enough through the grate so the fire is lively and doesn't clinker will be the challenge.
The blast pipe was lowered until the exhaust steam 1-in-3 taper point met the petticoat bell. Couldn't do much about the 1-in-6 taper not meeting the top of the chimney, without reverting to another design of blast cap. The blower jets were checked to make sure they weren't blowing outside the petticoat bell.
This brought up the next thing to check, do the exhaust steam and blower jets foul the main steam pipes? So a cross section was drawn to see what it looks like at that level. Currently OK, will need more room for a spark arrestor though.
Below is a drawing for the Tech Heads to see what we're on about!!
Happy steaming!
Nigel
The Norman Spink Britannia drawings were also checked over to see what they were like. The drawings took some time and cross referencing to get an approximate height of the blast cap, needing to work backwards from the frame drawing, smokebox saddle drawing, and the smokebox layout. The main frames reference the bottom of the exhaust tee piece from a point in space on the centreline of cylinders. Naturally part of the frames had to be drawn up to get this point.
The tee piece was drawn, and the blast cap. The petticoat also, and the chimney. The proportion tapers 1-in-3, and 1-in-6 were laid out from the rim of blast nozzle opening. According to Martin Evans in "Model Steam Locomotive Construction" book, the tapers should intersect with the top of the chimney and the tangent point where the petticoat bell mouth meets the chimney bore.
The discoveries, after measuring up the smokebox and chimney we have here, revealed a larger diameter and shorter petticoat. The proportion tapers suggest that for a larger diameter chimney, the blast nozzle needs to be further down to "fill up" the chimney with steam, and the petticoat length will be proportional to the diameter.
Steam locos are a bunch of balances and compromises based on proportions and a few rules of thumb. What we don't want is a lack of vacuum in the smokebox from possibly sucking air down the chimney.
My theory with the Britannia is the firebox will well and truly produce enough steam based on surface area - it has a wide firebox, combustion chambers, and superheaters. Drawing the air fast enough through the grate so the fire is lively and doesn't clinker will be the challenge.
The blast pipe was lowered until the exhaust steam 1-in-3 taper point met the petticoat bell. Couldn't do much about the 1-in-6 taper not meeting the top of the chimney, without reverting to another design of blast cap. The blower jets were checked to make sure they weren't blowing outside the petticoat bell.
This brought up the next thing to check, do the exhaust steam and blower jets foul the main steam pipes? So a cross section was drawn to see what it looks like at that level. Currently OK, will need more room for a spark arrestor though.
Below is a drawing for the Tech Heads to see what we're on about!!
Happy steaming!
Nigel
No comments:
Post a Comment
Leave us a comment!