Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length.

[UKJonathan]

Hi again
               I have looked through the forum in the hopes of finding out about ways to try stop the gas tank freezing hence losing pressure to no avail. I have seen in some pics some have positioned the tanks really near the boiler heating unit, while I have heard that some run a coil from the steam outlet near the tank. How does other overcome this problem of pressure drop?
jonathan.

[Bunkerbarge]

I've used a copper heat shunt to transfer heat from the seperator to the gas tank.  I haven't tried it for any great length of time yet but bench testing looks promising.  The increased pressure in the gas tank is controlled by the pressure regulating valve to the gas control valve but if this proves to be excessive in operation then the heat shunt will have to be reduced.  I may actually cut it into two pieces to give me variable amounts of heat transferred depending on the ambient conditions.  Such as on a cold day fit the whole shunt, but on a hot day it may only require a small one.

It is better described here:

http://www.modelboatmayhem.co.uk/forum/index.php?topic=6094.0

[derekwarner]

Jonathan asks...... "How does other overcome this problem of pressure drop?"

It is the combined gas equation [Boyles & Charles Law]..........P1xV1/T1=P2xV2/T2.......thermal warming is required   with >+ 3000 Mayhem members ......I am sure one of the"wordly" body of members  GAS engineers + so much hot air ...will help  - Derek   

[Underpressure]

I was told many years ago to open the gas valve just enough to maintain the required boiler pressure while running.

It is the transfer of the gas to the burner that brings the tank temperature down, so by keeping the flow of gas to the minimum it helps reduce the cooling affect. An attenuator or electronic valve will do the same thing, but they were rare and expensive when I was last messing about with (steam) boats.

Neil

[kiwimodeller]

Many years ago when working in an equipment hire place I learned that if we hired out a small LPG heater for warming up factories I had to send with it one LPG bottle but if I sent out the large heater I had to send out two bottles linked in parallel. The reason was that the big heater drew off gas fast enough to freeze up a single bottle. Twin bottles worked because the surface area of liquid from which the gas was drawn was far greater than the surface area of one tank and therefore the drawoff per square inch was far less. I have since proved in boats that a wide flat gas tank works fine where a narrow tank of the same total capacity did not. It is not always easy to find room for a "flat" tank but if you can it is unlikely that you will need to heat the tank. I have also reverted to a Primus "roarer" burner as used by plumbers rather than a ceramic burner as it is capable of putting out far more heat. I can then run the burner fairly gently and this helps to reduce the draw. Hope this helps, Ian.

[Bunkerbarge]

From what I remember of school physics it is all to do with the higher energy molecules being the ones that are released from the liquid and form the gas during evaporation.  This means that the average quantity of energy (heat) contained within the liquid falls and therefore the temperature drops.

Anything that slows down the evaporation rate will reduce the effect and give the liquid chance to absorb heat from it's surroundings again so that is why throttling in the tank outlet valve could help but only if you are reducing the gas that is being burned.  Consequently you are affecting the heat going into the boiler and therefore the rate at which you can generate steam.  Unfortunately you actually want the evaporation rate the burner demands so you may be stuck between the two conflicting requirements.

Some options are:

1) Control the gas to the burner so you only use what you want to maintain boiler pressure so an attenuator or an electronic gas control valve are very useful.
2) Add heat to the gas tank by either having it exposed to the sun, assuming there is any, fitting a steam coil from the exhaust steam line, which will generate more condensate which has to be removed before it exhausts or using a heat shunt, which transfers heat via conduction from a hotter source such as the seperator tank.  Note there are some organisations that forbid the heating of gas tanks and obviously great care must be taken when doing this.  Obviously you cannot have a safety valve so there is a danger of the tank rupturing if you go too far.
3) Raise initial steam pressure from an external tank so you do not cool the gas tank before it even gets on the water and bring the boiler back to pressure when you top it up fromusing the same external gas source so once again you are not over cooling the tank unecessarliy.

[mrsgoggins]

Bunkerbarge, you didn't mention taking liquid from the tank to the burner via a vapouriser. I know it's a little more complex, requirng gas/liquid change over after warming up but cooling of the tank is minimal with little risk of overheating.

The amount of heat input to the vapouriser, if my calcs are right, is of the order of 10 watts for my set-up and I am about to test a vapouriser secured to the boiler end plate by a modified blanking plug. Heat input will be by conduction from the boiler.

Would appreciate your views.

Keith

[Bunkerbarge]

No, you're right because I'm not sure in my own mind yet as to whether it is the way to go.  I fully agree that it significantly reduces the cooling effect in the tank, because basically you are letting the evaporation take place at the nozzle instead of the tank so the cooling effect is instantly removed by the heat of the burner.

I have seen one of these systems and found it very interesting however, as is the case with so many of these things, you resolve one challenge and create another.  In this case I haven't seen or heard of a system that controls the flow of liquid to the burner so the benefit of controlling the boiler pressure and therefore gas conservation is lost.  We are then back to having to use the steam to prevent the safety valve lifting etc..etc..  At least when you burn the gas you can control its use to conserve it both by regulating the boiler pressure and how you use the model.  One thing I also didn't mention was the fact that you don't have to go hammering around the lake at full belt all the time using huge amounts of gas and cooling the tank.  You actually have the option to go slower and even stop for a while to allow the gas tank to warm up again.  

I'd be interested to hear of any system that controls the flow of liquid to the burner, based on a signal from the boiler pressure because then we will have something useful.  Until then I think the disadvantages outweight the advantages.

[mrsgoggins]

Bunkerbarge, thanks for your comments.

At the moment I am, with success, using a combined gas regulator/boiler pressure attenuator to Malcom Beak's design which I intend to feed with gas from the boiler mounted vapouriser. There is a reasonable length of pipe between the vapouriser and regulator so the latter will not be handling hot gas.

Initial, rather crude trials, look hopefull.

I will post any interesting developments.

[malcolmbeak]

Keith
I'll be very interested in knowing how well this works. I had wondered if I should feed hot/warm gas into the attemuator considering it has a "rubber" diaphragm. I did consider making one out of very thin brass and attempting to press some concemtric ripples into it to allow sufficient movement, but so far haven't got roung to it yet.
Do you start the burner up using liquid, or start off with gas then switch to liquid when things have warmed up a bit - the same way that Peter Arnot does?
Malcolm

[Bunkerbarge]

Interesting stuff gents, I'm looking forward to seeing what may come of this.

[ian kennedy]

Hi Chaps,

If you can get it try thin berilium copper sheet for your attenuator diaphragm, it won't stretch or deform like the neoprene rubber types and is very flexible for a metal.

John Hemmens is re working his attenuators using this material , and it works well after i have done trials with it for him most of this year.

Ian

[Circlip]

And handle Berillium VERY carefully, It's a highly toxic material.  Check the safety sheets.

  regards  Ian.

[steamboatmodel]

And handle Berillium VERY carefully, It's a highly toxic material.  Check the safety sheets.

  regards  Ian.

Berillium copper was used in model racing props in the 60s and 70s, I don't know if it still is, but the nasty thing is if you get a cut it will almost certainly get infected and be very hard to heal. I knew one fellow who lost two fingers to infection from it and it was touch and go for a long time as to whether he would loose his whole hand.
Regards,
Gerald.

[mrsgoggins]

Malcom, I intend to make sure the gas is cool by the time it reaches the regulator, by adding a cooling loop to the pipework if necessary. I will start on gas and change to liquid once steam is raised. I hope to do some meaningful tests in the next week or so.

Keith

[malcolmbeak]

Great stuff Keith. I look forward to hearing the results. I would guess that there will be no need to wait until you actually raise steam, by that time you could already have lowered the tank temp quite a bit. Obviously some interesting experimentation will be going on. I always find I end up with a bit of the "suck it and see" approach.
Malcolm

[Ivor Bittle]

I use electrically heated gas tanks. They work well. The details are on my web site in minature steam. Access it using
www.ivorbittle.co.uk but not using Firefox. Or just search ivor bittle.

[Safetyvalve]

Hi,
I was very interested to read in this thread about the use of Copper Berillyum in modeling.  We have used it in the aircraft industry for many years (hydraulic pump spline drives etc.) and, because of its health hazards, not only is it identified in our maintenance manuals as a hazard, equipment containing Berillyum is now marked with a hazard warning identification on the outside.  Obviously the health hazard is mainly respiratory but we also wear disposable gloves that are disposed of as hazardous waste after touching these splined shafts.  I know this is how our company treats Berillyum and it might be overkill - but better safe than sorry.
Dave

[oiler]

Just be careful how much you warm the gas. I work in a welding/medical bulk plant for oxygen,argon,helium,carbon dioxide,hydrogen,nitrogen,nitrous oxide, propane, methane and acetylene. When we fill gas cylinders we often use heat exchangers to convert the liquid to gas. We use electric heaters to convert CO2 or carbon dioxide to gas for gas mixes( beer gas or shielding mix for welding). The gas can get to very high pressures, upwards of 2400psi.

On the propane side as one could imagine we do not use any kind of heat exchanger because it can be dangerous and propane is sold exclusive in a liquid state. Safeties on a typical propane tank is around 400psi and the tank is tested to about 950psi. In our hobby I have not seen any kind of a pressure safety valve on the camping stove tanks or one's built for our use.

There has been some who tried to use hot water or other methods to heat their tender gas tanks for G scale steam locomotives and have exploded the gas tank. My recommendation would be be very careful not to use any heat on the tank or lines but instead just coil the gas line by the boiler so it cannot transfer too much heat toward the tank.  At a temperature of 70 degrees, an 80 percent-full propane tank has a vapor pressure of 124 pounds per square inch. If you use small copper or flexible tubing it could fail under these pressures. Butane expands lower than propane but still if too much heat is applied it could lead to failure.

I would recommend possibly using a small CPU heatsink and wrapping the gas tubbing around it to create a passive heat exchanger. We use passive heat exchangers in the gas industry for CO2 liquid vessels. http://www.taylorwharton.com/assets/base/images/products/External-Heat-Exchangers.jpg

Link to a possible CPU heatsink for this purpose.
http://www.tomshardware.com/gallery/,0101-116309-0----jpg-.html


propane/butane safety
http://www.rversonline.org/02Propane.html

[gondolier88]

Fantastic advice!

Greg

[flashtwo]

Hi,

Over the last couple of weeks I've been running my gas fired steam boat in ambient temperatures of about 11C instead of the summer temperatures of 21C.

I have a servo controlled gas valve which regulates the boiler steam outlet temperature ( it is a flash boiler so I don't control the pressure as such).

Despite the colder conditions, I am still getting a reasonable performance - the gas cylinder is getting cold and the cylinder pressure is dropping from about 2bar to 0.6bar, but the control gas valve is compensating for the conditions.

It is worth keeping a second cylinder in a warmer place and swop over after a while and let the first cylinder warm up.

The cylinder is not far from the boiler, but no direct heating is applied. In the summer I have to open an air vent to stop the cylinder zone from going above 50C!


Ian G.

[Bunkerbarge]

I think it's important to note that whatever experiments we try with heating gas tanks a pressure gauge is cruicial to enable us to see what is going on.  Gentle application of heat is the best way and careful monitoring of the tank pressure at all times is essential.

[Bee]

Has anyone considered using the lake water with a good heat exchanger to the tank. This would ensure a constant temperature during the run with no danger of overheating. Even though in winter it would be low (too low for butane) the advantage would be minimal variation during the run so the burner could be set to compensate.

[MOG8]

I haven't tested it yet, but I'm accepting bids: will it work?
The attached sketch shows a copper coil inside a steam separator.
Coil and top of the separatore are removable.
There is a tapped hole with plug to drain the condensed steam with a syringe.
The 2 meter coil is where the gas passes and it is supposed to be heated by the engine exhaust.
Has anyone tried this?
This is my latest attempt to avoid gas-tank chilling problems.
This set up is supposed to work with a ceramic burner around which I cannot wind a gas heating coil.
I accept bids and advices.

[MOG8]

sorry, I cannot upload a sketch and an image