What Furnace Volume do I need?

[op9]

I have to convert an existing LPG fired boiler to diesel and waste oil (mainly used lube oil from diesel engines) firing. The boiler is a firetube, but is a reverse flame furnace (ie a generally large diam. dead end furnace where the flame firing down the middle returns on itself, the gases flowing back around the inner periphery of the furnace and then through a convection pass. <br>I have had plenty of experience (mainly bad!!!) using waste oils and am aware of most of the pitfalls but here my question relates to this particular type of boiler furnace and how much furnace volume is required to enable an unrestricted flame to form and stay stable unaffected by the return flame or gas.<br>The existing NuWay (UK) package burner is firing at approx. 6000 MJ/ Hr. This is only an educated guess as there is no meter at present but hopefully will be installed before we are comitted. The present furnace backpressure whilst firing is about 12mbar. <br><br>The furnace dimensions are 795mm ID x 2440mm long before entering the convection pass tubes.<br><br>The oil burner I intend to use is an Oilon pressure jet made for firing waste oils with good diesel using 2 separate nozzles. Its usual published flame dimensions at<br>6000 MJ/ Hr is 700mm max diam. x 2500mm long. and it is capable of firing against 14mbar backpressure.<br><br>I was hoping somebody had had experience with firing into these or similar type of boilers (originally a Swiss &quot;Ygnis&quot; design) and could confirm whether the furnace appears adequately sized or have you had to somehow narrow the flame? Unfortunately the boiler manufacturer (local) is not very helpful OR doesn' t know.<br><br>The new burner manufacturer says it should be OK, but as the job is on an island a long way from home and there is a lot of additional auxillary systems like pumps, heaters, tanks and pipework to install as well, I need to convince myself it will work before proceeding.<br><br>I would really appreciate any definite information or helpful advice maybe on expected flue gas volumes in the furnace and associated pressure drops.<br><br>Thanks in hope???<br> <p>Rod Nissen<br><a href=mailto:nissen@powerup.com.au>nissen@powerup.com.au</a><br><a href= > </a><br>

 

[SOLIDDESIGNER]

I work in designing industrial ovens (both electric and gas). The burnner manufacture should specify a flame length for the burner for intened fuel. Then judge about 18&quot; more for safty. you should also have a purge blower installed and propper exhuast tubes. Also i would have an explosion hatch for safety. any furhter questions let me know.<br>
Jody Morgan

 

[dprice]

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Your reversing flame in the morrison tube is a curiousity to me. I am not familiar with this method here in the U.S.<br>
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The flame size you intend on putting into the pot is larger than I would recomend considering. You can derate the oil burner a bit as it will be more efficient than the LPG. This will result in a smaller flame.<br>
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The large size of the firing tube may be helpful and a hinderance to you. In the U.S. the firing tube (morrison tube) is one direction and normally about 12 - 24" diameter or 300 - 600 mm. I am familiar with one large diameter morrison tube and these units are quite old and need to be fired less strongly than the newer units. <br>
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You can get into trouble at times converting from gas to oil. When firing gas typically the flame is rather short, while firing oil the flame tends to be rather long. Also the oil burner imparts more heat into the radiant section of the boiler. The gas will liberate more in the convective pass. <br>
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With such a large Morrison tube you have the potential to create large thermal stresses, thus cause boiler failure. If possible, I would recommend derating the unit 10-20% when doing the conversion. This may be possible if you simply do not need all the steam produced by the original sizing.<br>
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You desire to measure the input is commendable and the correct thing to do. You might also consider measuring the output. In addition to the fuel change efficiency, the new burner may be more efficient than the old one and you may not need all that fuel. THis again will reduce the flame size you need.<br>
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I would also recommend an inspection of the unit prior to proceeding. I would look for irregularities in the waterside chemistry, and any indications the unit is corroded. I would also advise caution when starting up, careful inspection of the flame characteristics could avoid problems under operation.<br>
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[DenverEngineer]

I am also dprice.<br>
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I wanted to answer your questions a bit better. When converting to oil the pressure drop in the boiler will be lower than gas so you should be O.K.<br>
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The flue gas volumes should also drop slightly. <br>
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You can usually narrow a flame but the result is normally to increase the length. You are large on both accounts.<br>
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I would advise caution in considering the burner lengths and diameters reported. Most furnaces are not the turnaround type you indicate. Therefore the data should be for this flow pattern. You might require testing on tuning on site to get it to work. I envision some flow dyanamics there that may be challenging.<br>
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I am curious about the two nozzles on the burner. Are both firing silmutaneously?? Or is one for the diesel and the other for the waste oil. IF both are firing silmutaneously you may have a way to create an assymetrical flame in the chamber and allow a more desirable recirculation pattern. <br>
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Interesting fun you have there with these difficulties. Give me your e-mail address if you want to stay in touch on this issue.<br>

 

[op9]

Thanks for all the helpful advise. I am waiting for them to install the gas meters and measure existing burner firing rates.<br><br>The two nozzles are as you suspected: One for good diesel and the other for waste or diesel as applicable. The constant good diesel flame ensures a reliable ignition source for the often not so good waste. Ratio usually 1:1. So low fire is only diesel and high fire is diesel and waste simultaneously.<br><br>The reverse flame boiler or oil heater in this case is well known in Europe and here in Australia. I wouldn't say they were as popular as conventional dry back or wet back fire tube boilers. (Actually in Aust. the watertube boiler is the most common type built now from say 800Kw upward where they can be run unattended with additional safety controls, being deemed safe compared to firetubes with their large furnace tube and potential for low water &quot;meltdowns&quot;.)<br><br>The reverse flame is a very compact package with reduced water capacity and hence fast steam raising. Water treatment must be good to eliminate foaming and excessive surging under load changes. The original designers (the Swiss Ignis) claimed more efficient combustion due to the long flame path and so low excess oxygen capability.(Like roof firing a watertube). Personally I have found the combustion readings being not that much better, but you have to be careful with burner selection due to the potential for flame interference.<br><br>My e-mail is <A HREF="mailto:nissen@powerup.com.au">nissen@powerup.com.au</A> <p>Rod Nissen<br><a href=mailto:nissen@powerup.com.au>nissen@powerup.com.au</a><br><a href= > </a><br>