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| Pettijean explains it https://www.rypn.org/forums/viewtopic.php?f=1&t=3379 |
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| Author: | Dave [ Tue Jun 25, 2002 6:18 am ] |
| Post subject: | Pettijean explains it |
Engineer Bill Pettijean posted a beautiful explaination of spark arresting / fire problems on the ngdiscussion list today which should be required reading for those who are posting various solutions to the problem without a complete understanding of the technical problems. I don't understand the technical problems of computers to get it over here. Maybe he will post it here too? He also goes into the rationale behind the gas producer combustion system a bit with an overfire air proposal. Would it be a viable project to retrofit a CATS hog with a gas producer firebox since they all need heavy work anyhow and see how they compare to the ancient technology? Dave irondave@bellsouth.net |
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| Author: | Erik Ledbetter [ Tue Jun 25, 2002 8:09 am ] |
| Post subject: | CROSS-POST: Pettijean on spark arresting |
Cross-posted verbatim from the NG Discussion Forum... __________________________ Glen Butcher posted a line on stack misters. Here is my best shot at spark arresting. Before I start though, my heart goes out to all citizens in forested areas of the Southwest. I am very sensitive to wildfires because of my forestry background and I live in a heavily forested area. I hope the late summer rains come because that is the only thing that will stop these horrible firestorms. The other thing that is important is to support balanced forest management. If there is no logging these fire evolved forests will burn like this time and again. That's what they did before man came along and that's what they will always do unless the wood fibre is managed and removed enough to prevent massive fire storms that cannot be controlled. Back to spark arresting. First, cast iron brake shoes are a major fire starter. I think the locomotives still use cast iron brake shoes on the drivers -- this is a problem regardless of stack emissions. Second, remember all the exhaust steam goes up the stack too. The exhaust on a steam locomotive is pretty well "hydrated" without stack misters of any kind. Third, the stack velocities are nearly or exceed supersonic velocities. The enormous mass flow of flue gas and exhaust steam out of these puny stacks means particulate residence time is so short there is no time for cooling, much less absorption of moisture. Fourth, if these engines burn something like 100 lbs of coal per sq. ft. of grate per hour they will consume about 4,020 lbs per hour (40.2 sq. ft. of grate area) at about 11,000 btu per lb. This equals about 44 million btu per hour. Of this at least 20% goes up the stack unused at about 750 deg. f. So we have about 9 million btu of energy going up the stack in flue gas losses. The remaining 35 million btu of energy is mostly contained in the exhaust steam from the cylinders (except for about 7% that got converted into work). If we "mist" water into the exhaust stream it will give up about 1200 btu per lb to evaporate into steam and cool surrounding gases and particulates to approximately 212 deg. f. Now I am not going to go to the trouble of calculating the btu of energy contained in a flue gas temperature reduction from 750 deg. f to 212 deg. f., but let's say you are going to pull about 10% of the exhaust energy down to 212 deg. f. This will represent the energy contained in particulates big enough to ignite lineside fires. Then you have to remove about 4.4 million btu with the water mist. This means you need about 439 gallons of water per hour, or 7.3 gallons per minute if the heat exchange is 100% efficient, which it is not. Furthermore, the stack is exhausting something like 40,000 or 50,000 lbs of superheated exhaust steam per hour. You are sending in a mist at something like 3,600 lbs per hour, or 8% of the exhaust steam mass flow rate. From the foregoing it can be seen that stack misters are simply a feel good attempt that doesn't really have any beneficial effect, especially when ignition type cinders have a mass density that is too great to affect in all that exhaust "fog" at the high velocities and short residence times existing in hard working locomotive stacks. The fundamental problem rests with the Hesperus coal burned in these locomotives. It is friable, non-caking and breaks up readily in the fire. This creates an enormous amount of stack particulates, many of which are incandescent when they exit the stack (probably around 1400 deg. f.) Furthermore, these locomotives take all their combustion air underneath the grate even though probably 60% or 75% of the air needs to be mixed with coal gases ABOVE the grate and firebed. This situation means all the D&S and C&TS engines will always be serious fire starters. Trying to resolve the situation by detraining particulates in the stack region are mostly hopeless because efficient particulate removal devices all must operate on a seriously high pressure drop which messes up the drafting of the locomotive and adversely impacts its power output when working hard. A more balanced solution would be to NOT entrain all those particulates in the first place. Such a solution would put overfire air injectors in the fireboxes above the grates and over the brick arches. These overfire devices would be controlled by the firemen and they could use them to control the amount of smoke and the completeness of combustion. Most importantly, in light of today's fire hazards the introduction of significant air over the firebed would reduce the amount of air drawn violently through the firebed and thus the amount of ignition size particulates that would be ejected from the stack. This is not idle speculation. All this has been done before as early as the 1940's. Additionally, obtaining coal from the Somerset mines northeast of Montrose (on the other side of the mountain from Crested Butte) would put a more accurately sized semi-caking coal on the grate that would not break up as fast as the poorer grade Hesperus coal. C&TS tried this coal several years ago. They didn't like it, but didn't give it much of a test in my mind. I have occasionally suggested to both railroads that they seriously think about and engineer real solutions to their smoke and ignition source problems. If they had taken heed years ago they might not be in as much trouble as they are in now. Durango Herald article eledbetter@rypn.org |
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