The single biggest improvement that we did to the 2-4-0 was to add a stainless steel arch to the firebox to increase the path length of the burning gasses from the coal. It really helped out her steaming capabilities and turned her from an ok steamer to a great steamer. Here is how/why we did it. Firebox Arch When my dad and I first started running our Railroad Supply 0-4-0, later modified in 2001 to a 2-4-0, in the spring of 1996 we had trouble keeping up steam. The safety valve was set for 130 psi, but many times we would drop down to 60 psi after running for ½-3/4 of a mile. Once we learned how to fire the engine, we were able to keep the pressure up…for a while anyways. All three of the engines we built are coal burners with our 10-wheeler able to convert to propane in about 45 minutes. The 0-4-0 was the first engine we (actually mainly my dad, since I was 6 when he started it) built and was our first coal burner. My dad and bought a little Shay that burned kerosene in a pot burner and that is how we first got started. It was slow, needed some work but it got us by until we completed the 0-4-0. During the last Tri-State Locomotive club meet at Bill/Paul Fitt’s track, it was mentioned that we might want to install an arch in the firebox to help with combustion and steaming ability. In photo 1) you can see the brick arch that is installed on the Strasburg (former Canadian National) Railroad #89. The brick is supported by 3 arch tubes that run from the throat sheet below the tubes to the door sheet about the firebox door. Very few model steam engines have arch tubes so we had to figure out a way to support the in the firebox without having the support be destroyed by the heat and not have it ruin the steaming capacity. It was not until 2000/2001 that we had figured out how to do it. The arch is up against the throat sheet and extends out to the side sheets so only a minimal amount of air can pass by without being deflected by the arch. The arch is below the tubes so it protects the tubes from being exposed to the direct heat and flames of the fire thereby prolonging flue life. The arch will also help to heat up any air coming into the firebox through a hole in the fire and minimizing the temperature shock on the flues when a hole develops in the fire. Also since the arch is exposed to the heat from the fire, it will also keep the heat when the firebox temperature drops and will help heat up the gasses to get a better combustion. When fresh coal is added to the fire, the temperature inside the firebox drops until the green coal is brought up to temperature where it can burn. The more coal that is added, the longer it takes for the temperature to rise and the longer it will take for the boiler pressure to rise; this is why filling the box up is not as efficient as firing lighter and a little more often. The hydrocarbon gasses may start to be driven out of the coal around 400 degrees F, with most of them being driven off by 900 F. However the temperature needs to be around 1,800 F in order for the hydrocarbons to break up into free carbon and free hydrogen and burn. If the temperature is less than 1,800 F, the hydrocarbons pass through the flues (where the temperature drops quickly) and up the stack as unburned, wasted fuel. The arch will not let the gasses in the front of the firebox pass directly into the flues. The cooler gasses will hit the hot arch (which acts as a heat bank) and the hot arch will help raise the temperature of the gasses so they can burn as well as have the gasses pass over the fire before heading through the flues. This helps to cut down on the smoke and wasted fuel. The arch length is determined by how high the arch can go. The area between the top of the arch and the crown sheet needs to be 115%-185% (depending on which document you look) of the net flue area. For models that do not have arch tubes that determine the height, the arch usually extends 1/3-1/2 way back from the throat sheet. The formula for finding the net flue area is: R= radius of the tube (1/2 the diameter) π = 3.1416 Net Flue Area = R^2 * π * number of flues Area above Arch = 1.2 * Net flue area = Firebox width * height above arch Now applying this to the 1:8 scale engines proved to be a little more interesting. In 1996 I first started out with a frame made of mild steel to support the arch. The frames legs rested on the grate supports so it took up a little bit of grate area. That did not help, and the frame was too cumbersome and took up too much room in the firebox and the steaming qualities were decreased. In addition to that, the mild steel cannot take the heat and would burn out very quickly. I used mild steel as we had it and this way I found out without buying stainless that this way of holding the arch would not work. Around 2001 or so I thought about welding a holder/frame for the arch to the rear tube sheet. This would hold the arch 0.5”-0.75” below the first row of flues and would not effect the grate area nor would it reduce the firebox volume by much (unlike the frame I had tried before.) The frame (see drawings) is made with 316 stainless steel (if you cannot find 316 S.S. then 304 works well too) and is welded together so that the arch will slip into the slots. The number of slots and size depends on the size of your firebox. The height of the frame should be around ¾”, any shorter and it is easy for the arch to work out of the frame during use. The pieces of the arch are made from ¼” thick 316/304 S.S. and have a “tail” that is 1” long that will set into the frame. The width of the arch is such that it can fit through your firebox door and the length of the arch is about 1/3rd the length of the firebox. For bending the arch and figuring out how high the arch should be at the end, you need to calculate your net flue area. Then multiply that by 1.2, and then divide by the width of the firebox. This will tell you how much height should be above your arch. If it is less that this distance, the fire can be choked off as not enough air will be able to pass about the arch. The arch can also be made out of thinner metal if it is easier to find. We also found that we needed to weld a ¼” round bar to the arch at the bend on the rear/fire side of the arch to rest against the frame. This helped to prevent the arch from falling on the fire during use. Picture 2 shows the arch through the firebox door on our Mikado and you can see that the arch frame is almost totally in the fire. Picture 3 and 4 shows the arch with the fire and after the fire was dumped after 4-5 hours of running. The 2 pieces of the arch are not to the same height after 7+ years of service, but we do not see any negative effect from that. We welded the frame together outside of the firebox and when we welded the frame to the rear flue sheet, we filled the boiler with water to act as a heat sink to make sure we did not over heat the sheet nor the tubes. The arches are easy to remove through the firebox door. We drilled a hole in the end of each piece so our poker stick, for the fire, would fit into it and be able to lift the arch in and out. In addition to cutting down on the build up of carbon (we would have to brush out the flues on the 2-4-0 every 2.5 or so hours before the arch. With the arch we can run around 5 hours and the flues will be dirty, but not really limiting the steaming capacity) on our flues, the arches also prevent the hot coals from accidentally entering the tubes and has also cut down on the number of hot cinders that come out of the stack and the amount of ash in the smokebox after each run. Pictures 5 and 6 show the 2-4-0 (built from a RRSC 0-4-0) 2-8-2 (ex RRSC) and 4-6-0 (modified from Allen Models castings) and all of our engines now have arches. The 2-8-2 and 4-6-0 have 2 piece arches and the 2-4-0, due to the smaller firebox door, has a 3 piece arch. The arches are easy to remove through the firebox door. We drilled a hole in the end of each piece so our poker stick, for the fire, would fit into it and be able to lift the arch in and out. We have been very satisfied with how our engines are running with having an arch in the firebox.
Posted on: Wed, 13 Aug 2014 00:22:27 +0000
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