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[DBS88]

Good evening, hopefully you have all survived the recent extreme weather? Whilst keeping out of the sun I have begun to think about boiler designs? Is the Centre Flue fire tube boiler the best design for use in model boats?
I have a number of centre flue fire tube boilers which are very good, for example the Cheddar Proteus Boiler and the MacSteam Boiler, however, I have recently begun to wonder if they are the best design for use in model boats? Ignoring costs for now, is there a better design in terms of power and efficiency?
So lets assume its for a scale model boat (not a high performance speed machine) of upto 1.5 metres in length, with a pump to constantly supply the boiler with water. If you could have a boiler of any design to supply steam in these circumstances, what would the ideal design of boiler be?

[SteamboatPhil]

If it is a scale displacement type hull I would go for a centre flue water tube boiler, some boilers can be over complicated ( have a look at K N Harris book ) but I always go for simple design (well thats me  )

[DBS88]

Phil your insights are always appreciated and valued. The designs in the KN Harris book also make me wonder why the centre flue is so popular, I assume its easy to make and pretty good at steaming, although not so efficient in transferring the heat to the water - a lot of heat just going straight up the chimney. I like the look of the Scotch Marine type boiler (design 16). I had and regret selling a Horizontal Self Contained Cornish Boiler (Design 3) with water preheater and steam dryer, see photo, very little heat left the chimney when compared to a centre flue boiler.
So assuming the skills and the opportunity exists to make a different and more efficient design marine boiler for use in an average scale displacement model boat hull which design would you most like?

[Geoff]

An interesting question. I have two Cheddar model Puffin horizontal boilers which both have their benefits:


The first is a MK 1 so return flue. In this you have a center flue with cross water tubes the whole being placed low down and return fire tubes in the top half. I've never had any issues in raising steam and it uses a blow lamp gas burner.


The down side is that you can't light it from the funnel, even if you had a ceramic burner and because of the design the actual pressure vessel is quite modest in size. Nevertheless in my Battleship (53 inches long) I get a good 25 to 30 mins sailing using the standard Puffin engine. It can also be tricky to light the burner.





The Puffin MK 2 boiler is exactly the same size and uses a center flue and then straight up the stack. The good news is that with a ceramic burner you can light it from the top of the stack and it flashes back and the actual pressure vessel is larger so it contains about 40% more water. I've never had any issues with it raising steam. In my model destroyer (65" long) I get about 25 mins run time. This is because I used a different engine larger than the Puffin to ensure adequate speed so despite the larger water capacity the duration is slightly less because its producing and using steam at a faster rate.


Given the scales we are typically working at I'm not sure there is an optimum design and other factors would come into play, such as the quality of the boiler insulation and burner to really measure much difference.


My gut feeling is that the return flue is better as there is physically more heating area to conduct the heat to the water. This is one of the main differences with real maritime boilers - heating area, and fuel consumption.


At our scale there is really little to choose.


Cheers


Geoff

[SteamboatPhil]

Geoff has a good point with regard to heating surface and insulation (which is the most important part) However the easy part of a centre flue boiler (rather than return) is an easier construction (although if ever a cross tube goes the boiler becomes part of your retirement scrap metal pile  ) The most important part is the super heater coils, so on a centre flue, at the end a vertical loop of around 3" then turn to the vertical loop of about the same, this is based on a mid sized boiler.


Just a few thoughts.....


P
(will try and find a picture )

[SteamboatPhil]

An idea.

[SteamboatPhil]

Ohh come on, you must be impressed I found it...

[DBS88]

Geoff thank you, ease of lighting the boiler is important and as you say this can be a problem with the return flue. I have a Cheddar CMB6 Return flue boiler that has never been used, so will be careful when I come to firing up.


For completeness here is a diagram of the self contained Cornish boiler.


Phils points about ease of construction will no doubt be reflected in a manufacturer's pricing, more complex equals more cost. Its also good that we are identifying the important elements, insulation and steam drying. Have included some photos of a MHB Der Seekadet Boiler while is a return flue boiler with a steam dryer in the top. The return flue to the chimney is only half the length of the boiler, although the space continues the full length and contains the steam drying coils that Phil describes.


Its a healthy discussion and looking at the different designs so far is interesting

[KNO3]

Hello, I also have the Cheddar Puffin Mk 1 return flue boiler and the Puffin Mk 2 centre flue boiler with water tubes. While both are the same outer size, the centre flue holds more water, thus allows a longer running time without refilling.
The Mk1 return flue boiler's exhaust is noticeably cooler than in centre flue boilers, which means it is more efficient regarding converting the burner heat into steam.

My conclusion is also that the centre flue boiler with water tubes represents a good compromise, with good steaming ability, water capacity and simplicity of construction.
The steaming capability can be further improved by adding one more flue and boiler, there are a few boilers out there with this construction by commercial makers.

[DBS88]

Another option we can explore is boiler designs with twin fire tubes, a number of manufacturers past and present offer twin fire tube boilers. So with a 4 to 5 inch dia boiler, are there benefits to be gained from having a twin fire tube arrangement?
Apart from adding to the complication in manufacture and cost are there disadvantages from running with twin fire tubes?

[Geoff]

Another item to consider is super-heaters for dry steam and whilst in theory this is fine if overdone the steam will not properly work the displacement oilers we typically uses as they use condensed steam to force oil into the steam line.


Cheers


Geoff

[Geoff]

Whilst I'm not aware of any issues with twin burners one concern I would have would be what happens if one goes out and the gas keeps coming from the other!


I would speculate that twin burners provide a larger heating surface and hence boiling capacity albeit unless you have a water pump then the duration would be less for any given overall boiler size.


Again my general opinion would be that at the modest scales we use there would probably not be too much difference.


A lot depends upon the complexity you are comfortable with. I tend to follow the KISS principle and just have a boiler filled with water and sail until it nearly runs out. I also have the ability to put the burner out remotely on my models just in case you get stuck in the middle of the pond on debris.


You can't beat steam, the smell of hot oil, steam and burned fingers! The bit that was hot last week is just as hot this week but we still have to check!


Cheers


Geoff

[rhavrane]

Bonjour,

As I have several boilers of different types, I think I prefer horizontal single flue boilers for three reasons :
   - For the same size, they content more water so more autonomy (I don't have pumps in all my boats) and are lighter,
     (example, this huge boiler https://www.youtube.com/watch?v=DKpip1n4eBE contains only a total of 70 cl !)
   - They are quite easier to light on, for me it's a pain to light the second burner on without smashing the fitrst one.

About superheating, I dont' like it also for two reasons :
   - For our working pressures, between 30 and 60 PSI, it does not change anything or I would be happy to have a demonstration of the benefit,
   - Wet steam is lubricant and protects my machines even if my displacement oilers become empty.

About safety, like Geoff, I install a RC shut off gas valve on (almost) all my boats and furthermore, I also have gas regulators/attenuators.

[KBIO]

Hello Phil !
Ohh come on, you must be impressed I found it...  


  For sure that we are impressed ! Have another one and "Let's twist again !" 
Sorry! I get back to where I come from .... 




I would be happy to have a demonstration of the benefit,
Maybe time to open a book Rhavrane !

[rhavrane]

Bonjour Dave,

If you wish using a double flue boiler, this one, a Scott, is almost easy to light on : https://www.youtube.com/watch?v=jZHZp-5Y-xk

Yves, if you have a book explaning the benefits of overheating a 2 bar - 30 PSI steam, please, share it with us and I will be happy to read it.

Unfortunately, I have no possibility to make any experimentation by myself as I like it beause I don't have the equipment to do it .

[KNO3]

Hello, I also have the Cheddar Puffin Mk 1 return flue boiler and the Puffin Mk 2 centre flue boiler with water tubes. While both are the same outer size, the centre flue holds more water, thus allows a longer running time without refilling.
The Mk1 return flue boiler's exhaust is noticeably cooler than in centre flue boilers, which means it is more efficient regarding converting the burner heat into steam.

My conclusion is also that the centre flue boiler with water tubes represents a good compromise, with good steaming ability, water capacity and simplicity of construction.
The steaming capability can be further improved by adding one more flue and boiler, there are a few boilers out there with this construction by commercial makers.

What I meant to say is "can be further improved by adding one more flue and burner", sorry for the mistake.

[Circlip]

Why do so many misinterpret that putting a piece of pipe in the boiler flame from the steam take off as 'Super heating'? At best, in the 'Low pressures' involved, it's steam drying. Flash steam is a different steam kettle.


  Regards  Ian.

[SteamboatPhil]

I do sort of agree Ian, however for my use I am not using low pressure, my engines run at around 120 psi and by super heating (very old school) my boat will do (and has been timed) just over 30mph, the other advantages of super heating is you get dry steam, but make sure you have a good steam oil going in 
Now that is with a Scott type boiler, my flash steam plant I managed 75 mph.........the record is over 125 mph, (Now not by me so I gave up at that point).
Now even on a simple oscillating engine (such as a mamod ..ekkk) let it run and hold a lighter under the steam pipe and watch  the improvement   

[/size][size=78%]Just a personal [/size]view [size=78%] [/size][size=78%]

[rhavrane]

Bonjour Phil,

30 mph (48 km/h) ! Very impressive, I would be happy to reach even a quarter of this speed, but with a conventional boiler at 120 PSI.

I would greatly appreciate if you would share with us a video of your boats 

Besides this, I am also interest ed in a demonstration of the benefit of superheating a 2 bar - 30 PSI  steam

[DBS88]

Thankyou for the suggestions and healthy discussion around steam dryers/super heaters - I love Phil’s very simple and effective experiment with a Mamod engine and his photo of a copper coil used to raise the temperature of the steam. I have thought about this and note that boilers from Stuart, Cheddar, Saito and Hemmens do have loops of copper pipe used to increase the steams temp so thought it was either a fashionable trend or it works so out of curiosity I did some research. The reasons for heating the steam further appear to be twofold, first to deal condensation forming in the cylinders and secondly to increase the energy in the steam, therefore improving its ability to perform work in turning the engine.
From the work of K. N Harris he suggests that for Model steam engines, “the value of superheating in running model engines can hardly be overstated.”  For brass engines raising the temp by no more than 50F, for gunmetal 100F and Cast Iron as high as you can reasonably attain.
If you are drying the steam it’s essential to ensure a good supply of steam oil to lubricate the valves. At lower levels of steam drying the normal displacement Oiler’s still work, but at higher levels a mechanical oiler becomes necessary. It also appears as the temperature increases piston valves are better suited to higher levels of super heat than are slide valves.
The next thing that came to light is the design of the pipe work to be used for this purpose - the dia of the pipe should be 50% bigger than the steam pipe, the bends used should be gentle and the steam inlet end of the pipe should be further away from the heat than the outlet end (contra flow principle). Lastly the steam control valve should be on the hot end of the pipe.
My conclusion drawn is there is an advantage to be gained from a moderate degree of heating the steam further so I will add a steam drying to the design list for an improved boiler design.
Perhaps the next thing to consider is whether or not there is a benefit to be gained from the inclusion of a boiler feedwater system?

[DBS88]

While continuing to try and learn more about steam drying and superheating, it’s advantages and disadvantages I came across an informative video so here is a link https://youtu.be/WzK0PJ6soxs
Hopefully the video explains things and leaves us with a better understanding of the mysteries of steam at the low pressures we use it at

[CliffW]

Another factor to consider is water volume...the beauty of a single center flue design (with cross tubes for heating surface area) is that water volume is somewhat maximized vs double flue or more complicated designs.