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 Post subject: Primary and secondary air inlet vent sizes
PostPosted: Sun Jan 07, 2018 3:50 am 
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Full Steam Ahead
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Joined: Fri May 13, 2016 2:42 am
Posts: 146
Location: Da Nang City Vietnam
Boat Name: Alphington
I am drawing up the furnace casing for my 3 drum boiler and need a little input.

My boiler will be burning Welsh dry steam coal, without forced draft.

Is there a common ratio that is used to calculate the vent areas for below grate and above grate relative to a given grate area?

Cheers, Lionel


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 Post subject: Re: Primary and secondary air inlet vent sizes
PostPosted: Sun Jan 07, 2018 12:33 pm 
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Steam on Deck
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Joined: Wed Nov 18, 2009 6:59 am
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Location: Neubrandenburg, Germany
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Hi Lionel,
Quote:
Is there a common ratio that is used to calculate the vent areas for below grate and above grate relative to a given grate area?

for the modern DR (Deutsche Reichsbahn)-Locos is the free grate area 40 - 42 % of total grate area. The air gaps are 14 mm wide and the crowns of the rod also. The sides of the rods should have 4 degrees.

Hope it is helpful.
Dietrich


Attachments:
File comment: Grate for the coalfired boiler for my steam loco.
The rods are 2 mm wide and 8 mm high.

Rost0208_kplt.jpg
Rost0208_kplt.jpg [ 42.69 KiB | Viewed 1009 times ]
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 Post subject: Re: Primary and secondary air inlet vent sizes
PostPosted: Sun Jan 07, 2018 1:35 pm 
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Full Steam Ahead
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Joined: Fri May 13, 2016 2:42 am
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Location: Da Nang City Vietnam
Boat Name: Alphington
Hi Dietrich ,


My ash pan below the grate is completely sealed and I need to fit a variable air vent, are you saying that the minimum open area of my air vent should also be about 42% of the grate area?

Do you know how large the vent for secondary air should be?

Cheers

Lionel


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 Post subject: Re: Primary and secondary air inlet vent sizes
PostPosted: Sun Jan 07, 2018 3:55 pm 
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Full Steam Ahead
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Joined: Mon Dec 14, 2015 2:12 pm
Posts: 163
Location: Very eastern England
Boat Name: Platypus, Shelduck
There are formulae for large industrial boilers, and you could probably find them via Google etc., but I think it would be a mistake to take them very seriously for our small launch boilers.

Proper Welsh steam coal is lovely stuff, but you still have to keep the firebed in good order, and feed it little and often. In these circumstances, the firebox door will be open quite a lot for inspection purposes as well as feeding, and will allow in plenty of secondary air, if not too much. It is common enough to see a circular "hit and miss" ventilator inserted into the said firebox door, but I confess I've never seen anyone use one even when fitted. What does get used is a position for the firebox door catch which keeps it open a half inch or so, like the ventilation catches seen on many casement windows.

Primary air arrangements are usually hidden away under the boiler, and possibly under the floor as well, and I don't think I can say very much about them other than the obvious: i.e. a substantial fraction of the grate area, like 50% will be fine as a maximum, and if you want to "stop the fire down" while having a pub lunch, an adjustable stop would be prudent, which allows the minimum area to approach zero in a controlled manner. Mind you, it's not uncommon to leave the primary air alone in these circumstances, and just hook the firebox door fully open: not an elegant solution, but it does work.


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 Post subject: Re: Primary and secondary air inlet vent sizes
PostPosted: Sun Jan 07, 2018 8:41 pm 
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Full Steam Ahead
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Joined: Thu Nov 07, 2013 9:16 pm
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Location: Northeast Ohio, USA
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Lionel Connell wrote:
My boiler will be burning Welsh dry steam coal, without forced draft.

You're going to ship welsh coal almost halfway around the world?? :?

-CB


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 Post subject: Re: Primary and secondary air inlet vent sizes
PostPosted: Mon Jan 08, 2018 1:00 am 
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Location: Phila PA USA
Boat Name: Margaret S.
If the openings in the grate are about 40% of the total grate area, then at least 50% of that open area will be blocked with coal and ash and clinker, so making the primary air damper about 25% of the total grate area would be fine.

With respect to CB's comment about shipping coal half way around the world, the Bridgeport Generating Station in Connecticut (400,000,000 watts) does just that. They burn Indonesian coal because it has only 0.07% Sulfur, so they don't need to put Sulfur Dioxide Scrubbers on their plant. At full load they burn about 1900 Tonnes of coal per hour.

Short Ton = 2000 Pounds
Long Ton = 2240 Pounds
Metric Tonn = 1000 kg = 2205 Pounds


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 Post subject: Re: Primary and secondary air inlet vent sizes
PostPosted: Mon Jan 08, 2018 1:21 am 
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fredrosse wrote:
With respect to CB's comment about shipping coal half way around the world, the Bridgeport Generating Station in Connecticut (400,000,000 watts) does just that. They burn Indonesian coal because it has only 0.07% Sulfur, so they don't need to put Sulfur Dioxide Scrubbers on their plant. At full load they burn about 1900 Tonnes of coal per hour.

Interesting. I wonder how much diesel oil that includes to ship it. That sounds pretty poor in actual total emissions. But I guess a lot of petroleum makes the same kinds of journeys.

-CB


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 Post subject: Re: Primary and secondary air inlet vent sizes
PostPosted: Tue Jan 09, 2018 2:06 am 
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Joined: Fri Feb 11, 2011 1:28 am
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Location: Strasburg, PA
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A rule of thumb for natural draft boilers is that the area of the stack is 25% of the grate area. The two launch boilers that I have experience with agree with that rule of thumb, and also have ash pan doors the same size, i.e. 25% of the grate area.

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 Post subject: Re: Primary and secondary air inlet vent sizes
PostPosted: Fri Jan 12, 2018 3:14 am 
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Full Steam Ahead
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Joined: Fri May 13, 2016 2:42 am
Posts: 146
Location: Da Nang City Vietnam
Boat Name: Alphington
Thanks all for your comments, I have taken them on board and I will put some CAD models of the final boiler design in my Alphington Launch page here. Today we finished the framework for my boat building jig, I will get some pictures up tonight. Just waiting for a quote from my CNC router man to cut the station moulds for the jig, hopefully the jig may be together and ready for the real work to begin by the end of next week.

Importing the coal?

Fortunately my company imports all kinds of raw material from all around the globe. We have our own importing license and complete the entire importing process through customs ourselves, so the paper cost of import is quite inexpensive. Also, where ever possible I combine my hobby shipments with my business shipments and often the shipping is almost free for my hobby stuff. But it does require that I plan my hobby ventures well in advance.

I imported the castings for the engine from UK, the electroless nickel plating system from USA, the P20 tool steel for my crank shaft came from Japan. Many of the engine seals and the hydraulic rings for the pistons came from Australia, as did the engine enamel, although that originates from the USA. For the boat, the white oak, comes from USA, the Mahogany from Fiji, the Teak from Burma, the Ash from Australia, I bought all of this timber in Australia and shipped it to Vietnam, the Paulownia for the planking I imported directly from China. All of the epoxy/Glass was bought in Australia but originates from USA, the paint varnish, and deck oil system originates from Australia. I will import all of the tubing, valves and probably also the gauges for the boiler from the UK. Also, all of my wood working machinery to make the boat I imported directly from China.

Now, if we add together the cost of all of the above together (which I won't do because it is too scary) , we will find that the cost to import the coal will fade into insignificance, besides, I will probably say that I burn all of the coal in my casting furnace and claim it as a cost of production.


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