Hi Pd's,
Derek asks: -
D_Gallagher_9.... we see the assumed ACSHB2 [horizontal boiler] side by side with an ACS ReGas gas tank
The previous temperature assessments of my [ACSVB2] similar...vertical boiler were confirmed & acknowledged with Sandy as 120>150 degrees C shell temperature
So .. how can this build excape the of the gas tank being subjected to the 100 odd degree Delta T>> [increase] [of the] boiler?
A very good question Derek
:post and one that deserves some consideration: - but I am not about to re-open the P1 V1 etc argument again......
In the particular installation you have specified the problem is not anything like as bad as you might think.
Whilst the surface of the boiler shell is, as you have stated, at between 120 deg and 150 deg C TEMPERATURE, the gas tank will not be directly exposed to this, since there is a direct air separation gap of some 1/2" or so between the two items, probably more if you consider the fact that they both has curved surfaces in opposition to each other..... the average spacing could be calculated for such curved surfaces, but let us suggest say 1" average.
The amount of HEAT transfer is directly proportional to the difference in TEMPERATURE, between the two objects, and the exposed SURFACE AREAs of the opposing objects, and is directly effected by the INSULATION of the media between the objects. In this case we have an AIR layer, forming and insulation band.
Consider also that the gas tank is only about 1/2 the length of the boiler, further reducing the exposed surface area.
The HEAT made available at the surface of the boiler by this TEMPERATURE would need to transfer to the gas tank in order to raise its temperature and this can only occur in one of three (3) ways, namely: -
CONDUCTION.
RADIATION.
CONVECTION.
Conduction can be largely ruled out, since AIR is a very poor CONDUCTOR of heat, and what little there would be would be effectively irrelevant due to the effects of CONVECTION.
For CONDUCTION to have any real significance, the gas tank outer shell, and the boiler outer shell would need to have some physical connection, i.e. be actually touching.
RADIATION.... there would certainly be some small amount of heat transfer due to this, however, it would not amount to a high level.
In the first place the temperature difference between the boiler shell and the surrounding atmosphere would not really be great enough for significant RADIATION transfer, and again, the effects of CONVECTION would, to a large degree, override the minimal transfer which would exist.
Secondly, heat radiation from a body is colour sensitive, being greatest from a dull black body and reducing as the colour changes to lighter tones.
The same applies to Radiation absorbtion, a dull black body will absorb heat more readily than will that of a lighter one.
Surface finish also has a significant effect on radiation absorbtion/emission with a shiny surface the effect is reduced considerably, since a lot of the heat would be reflected away again.
Daves boiler and gas tank are both light grey, and shiny, and will thus absorb/emit heat to a lesser degree than will those of a dull black colour.
CONVECTION.
This would certainly be the means of greatest heat transfer, but not directly towards the Gas tank.
Rather, the heat would rise vertically upwards.
Unless of course, you live south of the equator, where the reverse happens...... here the heat either goes downwards into the ground, making it very dry and arid.....or is absorbed into the modellers body, where it is used to expound awkward questions..... :nah
:music
Air molecules, directly in contact with the boiler shell, would initially heat up and in doing so will expand, thus becoming lighter.
These would rise upwards and be replaced, from below, by cooler molecules.... the cycle would thus continue.
In Dave Gallaghers case, since he is in the Northern hemisphere, and his model is also an open launch, the convection would be largely between the boiler shell and the atmosphere, with the heat freely escaping into the latter, to be replaced with much cooler air from below.
In this particular instance, the amount of heat transferred, via all methods would probably not not amount to more than 10% at best, probably a lot less.
Which would mean the gas tank would not rise in temperature by much more than 10 - 15 degrees above ambient. A safe amount.
The other side of the equation.... the rate at which Butane and Propane lose heat whilst being extracted from the gas tank would be more than enough to probably eliminate any rise in tank temperature in the first place.
The above is true for an open launch, however, the same can not be said for an enclosed system, such as PS DECOY, or any other steam driven model with the equipment stowed beneath a weather deck.
In such cases..... CONDUCTION would now become (indirectly) higher, due to increased temperatures in the atmospheric condition surrounding the entire installation.. and which will be in DIRECT contact with the outer surface of the GAS tank.
Radiation would have a slightly higher effect due to the reduction in cool convection air.
CONVECTION would now play a much bigger role, in that it would quickly heat up the entire surrounding atmosphere (now trapped beneath the weather deck), which in turn means the air surrounding the gas tank would be at a higher temperature, thus causing it to increase in temperature due to direct contact CONDUCTION.
Here again, though, the ability of gasses such as Butane and Propane to CONDUCT heat, and thus absorb it are quite low, (Butane has a conductive coefficient of 0.7 whilst that of Propane is 0.8 whilst air is 0.21), copper on the other hand has a coefficient of some 2570.7.
The natural cooling effect of the gas, whilst being drawn from the tank for combustion, will again help keep this in check.
In these sort of installations it is sensible to arrange for the gas tank to be mounted a greater distance from the boiler, and preferably be located directly beneath a suitable sized air vent, where the inrush of external atmospheric (much cooler) air can help keep thing under control.
Such air inlets are vital, in any case, to allow for sufficient oxygen to enter the system, to keep the burners working properly, and to allow for the escape of heated air and any combustion gasses that may escape the flue system.
So there we have it chaps, I Hope that is all understandable. if not, then all questions on a postcard to BANDIT the dog. :twisted: :twisted: just kidding John..... then again!!!
Best regards.
Sandy.
:vacat :computer
Main Menu
UserBox
Search
