Hunslet draughting issues

[David Powell]

HI All. I am James'Dad, David.I am NOT a qualified steam engineer/ operator. . However, I have been involved with and watching steamers from a very early age. When I was a child we lived in Buckley North Wales. The local brickworks still used 3 Sentinel S4 waggons for brick deliveries. Our house overlooked a road junction on a hill. The Sentinels almost invariably approached the junction blowing off and stormed off up the road far faster than any petrol or diesel lorries of the era. Some years later I owned, learned to drive on, and fired a similar tractor version. I sometimes had difficulties keeping a full head of steam with inferior coal. On asking the brickwork.s drivers about this they told me their " Secret" to burning poor coal, it was not, as we had thought, fitting smaller blast nozzles and putting up with less engine performance but was welding two pieces of wire ( welding rods with the flux knocked off) about 1/8" diameter across the large size exhaust nozzle in the form of a cross > With that done they were able to burn poor quality coal and still get around quickly so as to be able to spend lots of time in the local and not so local pubs!!!!. From that I have become interested in blast nozzles, chokes, multi jet arrangements etc. I cannot do the advanced maths easily but hope I can see the concepts. I have been able to make ALL my models steam adequately, but some arrangements work" out of the box' but others need "careful refined tinkering" to get good results. Lets not argue but collaborate and see if we can make things simple and make all models steam well. Regards David Powell.

[Fred_V]

Thanks for the reply David. I wonder if the welding rod created some restriction and increased the blast velocity.

[Fender]

I expect the welding rods created more turbulence in the exhaust.

[Doug_Edwards]

On asking the brickwork.s drivers about this they told me their " Secret" to burning poor coal, it was not, as we had thought, fitting smaller blast nozzles and putting up with less engine performance but was welding two pieces of wire ( welding rods with the flux knocked off) about 1/8" diameter across the large size exhaust nozzle in the form of a cross.

David,

Thanks for your comments in your post. I especially enjoyed the comment on the "cross" put over the blast nozzle. The PRR did this in the teens (iirc) and it became a standard for the locos at the time. They had fitted locos with their version of this style of cross and tested the results on their test bed. The improved results lead to it becoming a standard on their RR. As I remember, the cross section of the "cross" is a triangular shape with the point down.

Thanks for sharing that the Sentinals did the same thing and that it improved their ability with poor coal.

Regards,

Doug

[Fred_V]

Steve's thread on "Coal issues" just gave me a thought about grates. I looked at my cast iron grates and they have 1/4" bars and less than 1/4" spaces. I was also firing with a deep coal bed. I would think that could cause the ash to not reach the ash pan but go out the stack. This is what was in the smokebox.

[BryceGTX]

I would appreciate any thoughts anyone has on this.

Seems prudent to understand what is wrong with the design prior to any changes. It seems most of the designs that were replace by a Lempor or other designs were given careful testing and modification. Yet the original design was often not given the same careful attention.

So lets first understand what is wrong with the design and propose some solutions based on well known solutions that worked.

First lets look at the original design and assure ourselves that it does not violate the most basic principal. That is, the steam blast of 10 degrees does not exit out the top of the stack.

Since that is the result of the 1:6 and 1:3 design that works so well, lets look at it first in that light.

I have drawn the front end according to the dimensions given in this thread. Is clear that the 1:6 (10 degrees) does not exit out the top of the stack. Further, its clear the 1:3 does impinges in the lower part of the stack. So the stack-blast angles suggest a good design.

[BryceGTX]

In this drawing I closer show the calculations and the position of the two cones. The 1/7th calculation show that the blast nozzle is not too small.

[BryceGTX]

However, now comes the problem. Its clear that the top fire tubes show higher velocity than the lower tubes. This is a well known problem with a well known solution.

It becomes a much more significant problem when one considers the pulsation of the steam blast. This pulsation causes extreme pulsation in the fire tubes. This high velocity pulsation means that heat transfer from hot gases in the fire tube to the fire tube becomes much less efficient as the gas velocity increases.

This a particular problem when two hundred years of researchers say there is no problem with pulsations.

The end result is too much coal is burned to create the steam. Hence high coal usage.

[BryceGTX]

The solution is quite simple and was successfully implemented in 100s of thousands of locomotives. Quite prevalent in US Locomotive designs.

This picture shows the effects of the baffle in the front end. The baffle tends to equalize the velocity of the hot gases in the fire tubes. However, just as important it slows down the velocity during operation at low cutoff.

It slows the velocity by increasing the path length of the hot gases in the smoke box. The pulsating pressures in the smoke box tend to reduce the further from the blast you get. It also creates a plenum in the front end which reduces pulsating effects.

[BryceGTX]

The next thing that is missing from the design that is shown to be most effective is the addition of the flare at the base of the stack. Virtually all research shows this flare to be effective.

[BryceGTX]

If you prefer to add a taper to the stack, it must be added where the 10 degree steam blast hits the inside of the stack. And it must be no more than 10 degrees, because that is the minimum steam blast angle.

[BryceGTX]

Once again, proper exhaust design is critically important. Just as important as the stack design itself. The two exhaust passages should converge into a larger passage and feed the blast nozzle. These dimensions are for a full size locomotive and only given to show shape.