FLYING STEAM ENGINES - Generation the Steam
Conventional boilers
ln their simplest form a boiler is a round tank of water with a fire
underneath and in the case of my first engine (designed in 1967 by David
Parker) it is little more than that. What we can do with these little boilers is
to improve on the material normally used on model locomotives and to look at
just how big a safety factor we really need. Model locomotives sometimes have
a boiler shell that is twenty, thirty, even forty times stronger than it
needs to be! ! The modern aircraft which we happily board and in which we fly
away on our holiday works very safely on much lower factors of mechanical
strength than forty to one! It is really quite safe to use a factor of four
to one on a boiler of only 50 mm diameter which operates at perhaps 150 C.
David's 'Comet' engine (see photograph and drawings) has a quite generous
safety factor of 5:1 and the boiler is free of any long term danger due to
corrosion. The boiler is made of 0.5mm nickel silver sheet, silver soldered
with very simple water tubes of 6mm copper to increase its heating surface.
lt is spirit fired and l hydraulically tested mine to 3 times the working
pressure with no problems, it worked consistently for over five
years.
My experience with 'Comet' has led me to believe that it is
perfectly feasible to build a model speed boat engine along similar lines as
the airborne engine. Add reversing gear and a throttle valve and you could
build a fast planing steam boat that is quiet, controllable and above all
absolutely unique.. This is not silly thinking, the Comet's engine directly
coupled to a 12''x8'' propeller turns at 3200 RPM. and develops, a static
thrust of about 600 grams. The weight of this engine and boiler is little
more than a 540 size electric motor and a standard 7cell 1500 ma Ni/Cad
battery pack. The fastest steam powered model boats in the world are the
tethered flash steam hydroplanes which reach speeds of up to 170 kph this
brings me to a different kind of boiler that offers huge improvements in
performance for model aircraft, the flash or Monotube Steam
Generator.
Small monotube theory and practice
What is Flash Steam? There are two basic ways of generating high pressure
steam almost everyone is familiar with the locomotive boiler and the land and
sea born variants of the pressurised container, heated with either water or
fire tubes and an external source of heat. ln a flash boiler the heating
surface is a single or it may be a series of tubes into which the cold
headwater is pumped against the developed pressure. The heat source is
usually an oil, petrol, or gas flame directed against the tube. The tube is
normally coiled and housed in a thin light weight case of stainless steel and
the flame is directed through the centre of the coil. The development of high
pressure steam is virtually instantaneous, hence the term flash steam. The
little Groves 1936 engine that I have built goes from cold to full power in
8-10 seconds and the power is turned out all the time there is water and
fuel. Throttling works within fairly narrow limits just by turning the heat
supply down, the response is instant. The narrow band of power available can
be satisfactorily extended using a more elaborate control system. The Groves
engine is very simple and intended for free flight only where one power
setting is all that is needed. The steam so generated can be superheated to
the point of the engine's self distruction, where the lubricating oil is
turned into carbon before it reaches the cylinder walls. I am fascinated by
monotubes and in 2002 when the offer came I could not resist the temptation
to buy all 15 feet of Skylark with its oil fired Monotube steam source. Like
all experimenters I had to get it going ASAP in search of the first
modification or improvement I could introduce. I am quite sure the previous
owner would have been disappointed in me if I hadn't! There was one major
departure from normal monotube practice; it was not a monotube at all it was
a bi-tube, two parallel tubes wound alongside each other. (There is a
theoretical advantage in doing this which I will not go into at this
point.)
I have no complaint about Skylark, I launched her in the
Chichester Ship Canal in April 2003 and everything operated much as the
previous owner had said it should except, try as I might all I could get was
25psi and a slow walking pace. I was told that Skylark steamed well at
80-100psi and produced enough power to run at full hull speed which should be
say 3.5mph, a fairly good walking pace. That is, it did when it had a
monotube. I tried all I could to get that bi-tube to operate but it would not
play ball. The previous owner gave me his bi-tube pressure equalising valve
along with Skylark when I bought her and I experienced no sign of overheating
of one tube with the other tube running much cooler. I can look down the
funnel straight into the flame area and if anything is red hot I can see it
at once. My impression is it did divide the flow exactly as intended. All I
got was 25psi.
After a few weeks I gave up and made a new monotube from
the self same two 20 foot (6.1 M) lengths of 3/16" (4 mm) Kunifer (copper
alloy) brake pipe. They were joined with a screwed compression joint some way
up the chimney out the way of the direct heat of the flame. Result, 60 psi
whoopee progress! I do not think it was the bi-tube as a device that was the
problem, my firm belief is that by switching from bi-tube to monotube I had
DOUBLED the velocity of the feed water through the 3/16" (4 mm) Kunifer tube.
This matter of water velocity has been discussed in the book "Experimental
Flash Steam", it is so relevant to these notes that I have included the
results in my table, I quote verbatim from the book:-.
Quote from Experimental Flash Steam, by Benson and Rayman
my copy being published by Model Aeronautical Press Ltd. The Experiments
themselves were carried out by Mr Edgar T Westbury one of the most well known
writers and designers of model engineering projects in UK. He is known the
world over, the world of model engineering that is; (not a very big world!).
Page 59.
“Tests carried out on three copper boilers each 11
Feet in length and 3/16" (4 mm), 1/4" (6.3 mm) and 5/16" (7.9 mm) diameters
respectively and with a wall thickness of 0.03" (0.7 mm). Each coil was wound
upon a circular tapered form, 2 1/4" (57 mm) inside diameter down to 1 1/2"
(38 mm) diameter and spaced 1/8" ( 3.2 mm) apart. The casing left 1/8" (3.2
mm) gap at the largest coil. Water was fed from a water pump driven by an
electric motor. Each boiler was fired by the same air-gas blowpipe 1 7/8" (48
mm) diameter, and various evaporation tests were conducted with a spring
loaded outlet valve set to blow at 500psi.
In each test the 3/16" dia. boiler gave the best results
and on a maximum evaporation test managed 27 cu. Inches per minute with the
gas blowpipe flat out and the steam highly superheated. This represents about
1 lb per minute and seems a remarkable figure for only 11 feet of tube and
the fact that this boiler had the lowest heating surface. In every test the
5/16" boiler gave the worst results. On repeating the experiments using
thicker walled tube the relative results were confirmed but evaporation
increased by about 12%! End of quote.
E.T. Westbury's test results on 3/16" tube is included in
the table which follows. SO what is going on? 60lbs of highly superheated
steam per hour from about 1/2 sq. ft heating surface! It must be said at once
that this sort of hard driven performance is VERY wasteful of heat using only
a few hundred degrees Centigrade from the flame. A Gas-Air torch typically
burns at about 2200º C at the flame cone. A thin piece of steel wire at
the outlet to my tiny boilers glows red, about 800-900 C. We do not want red
hot exhaust on a steam boat and I certainly do not get it on
Skylark.
Why is velocity so obviously critical in the performance of
monotubes? The previous owner of Skylark has a theory which I believe to be
correct. As the water flows and heats at some variable point along the tube
it begins to form tiny steam bubbles on the inside wall. These must tend to
stick to the surface just as they do to the bottom of a saucepan when you
boil water in it. The steam bubbles seem obstinately glued to the metal. In
order for a monotube to operate efficiently the water must flow fast enough
to scour bubbles away IMMEADIATLY they form. The conductivity of any vapour
is thousands of times worse than water and maybe 100,000 times worse than
copper alloys like Kunifer. If as I believe the dramatic increase in
performance is due to the increased water velocity then I thought maybe I can
juggle a few numbers and come up with a very interesting, if empirical figure
that represents a likely MINIMUM velocity to aim for when designing very
small monotubes like Skylark's Monotube and the really tiny tubes I have in
Tiddler's Monotube.
Another result of this scouring is that scale and oxides do
not form on the inside of the tube. Indeed if you cut open a well used
section of a monotube you will see that the inside surface looks as if it had
been lightly etched.
I could have directly compared the results
of the bi-tube and monotube and left it at that but in addition to Westbury's
3/16" diameter tube experiment, a further source of data is available in an
article published in the Model Engineer magazine in 1992.
This was written by Bob Kirtley covering in great detail the construction of his world record
breaking hydroplane Pisces II which raised the Class B Steam record from
about 80 mph to 104 mph in one step. I have seen her go and it is a joy to
behold, the noise is like no other, music to my ears and any other
Monotubist's.
What I did was to compare directly the water velocities of the four separate
cases and tabulate them as follows. The different values for Monotube area
(Ma) for Pisces and Westbury are because of the differing tube wall
thickness.
Boiler Details
Parameter |
Skylark Bi Tube
3/16” Dia. |
Skylark Monotube
3/16” Dia. |
Pisces II Monotube
3/16” Dia. |
Westbury Test Monotube
3/16” Dia. |
Pump Ram
Diameter (D) |
0.953cm |
0.953cm |
0.653cm |
N/A |
Pump
Stroke (S) |
1.80cm |
1.80cm |
1.27 |
N/A |
Stroke/
minimum (Sm) |
300 |
300cm |
2500 |
N/A |
Volume/
minimum (Vm) |
384cc |
384cc |
1002cc |
440cc
see text |
Monotube
area (Ma) |
0.141cm2 |
0.075cm2 |
0.103cm2 |
0.081cm2 |
H2O Velocity (WV)
=Vm/Ma |
384/0.141
2723cm/min |
384/0.0705
5446cm/min |
1002/0.103
9728cm/min |
440/0.081
5432cm/min |
W V
Metres/min) |
27.23 |
54.46 |
97.28 |
54.32 |
It was with smug satisfaction that I noticed the close tally between Skylark
and the Westbury test data. To other Monotubists everywhere please give me
the current particulars of your system so we can acquire a data bank for
future use by others. All the tabled parameters plus pressures, temperatures,
fuels, burners anything you can think of that may help others who will follow
on after us.
I don't pretend that my observation and
experience is a scientific study but it may prove more concrete than any
other data that I have seen to date. It is further born out by the fact that
if I slowly reduce the heat input setting on Skylark there comes a point at
about 50 psi when the pressure drops very quickly from 50 to 25 psi; without
a commensurate reduction in fuel flow. With the available data I would
suggest that anyone contemplating a small monotube design for normal cruising
speeds and pressures a WATER INPUT VELOCITY of about 50 metres per minute
(0.8 Metres per second) should be a safe minimum to aim for at the systems
normal 'cruising' power.
This study, whilst interesting may be of
very limited use much outside of the tube diameters in the table, I would
hazard a guess that all would be well up to 3/8" (9 mm) bore tube. I have
done some calculations on turbulant flow at the temperatures and pressures at
which Skylarks monotube normally cruises and it seems to be that turbulent
flow will occur at all the water flow rates we are likely to work at and it
is unlikely to fall outside that rule of Thumb up to maybe 3/8" (9 mm) bore.
More is needed in this arena to prove anything. It may just be that in full
size practice perhaps the known point of turbulence is the lower end of any
particular monotube's efficient and useful working range. Do we have a
proffessional Monotubist out there ready to help us?
Pisces has a water velocity twice that in
Skylark and produces at least eight to ten times the power. I know this
because Class B boats have similar rules be they Steam or IC Powered, the IC
boats are usually fitted with 30cc racing two stroke glow motors which are
known to be capable of 5 HP. A point I must make here Bob Kirtley's engine
has a displacement of 13cc and yet has a performance comparable with 30cc IC
engines!. A good steam Hydro is not that much slower than an IC boat and yet
is generally 2-4 lbs heavier. I doubt if Skylark needs more than half a horse
power to drive her as she goes at the moment.
This brings me to the question of tube
length. The longer the tube, the more heating area = greater efficiency. Yes,
but there are limitations and in small sizes pumping effort is a very
significant such limitation. I want to try a feedwater heater section above
the monotube in the smoke stack and because this area is unlikly to have
boiling water in it, feedwater velocity is of much less significance. This
then permits the use of bigger bore tube for the feedwater heating section of
the system which will help with reducing pumping effort. I appreciate that
there is NO WAY of predicting where along the tube the water actually boils
however since the exhaust temperature at the chimney top is only about 300 C.
and boiling point at 100 psi is about 150 C. it is far less likely to boil
water at that point than right in the fire. THE RELATIVE MERITS, FLASH OR
CONVENTIONAL BOILER? Conventional boiler merits. 1. The containment of a mass
of heated water makes power control as simple as a carburettor on an IC
engine. 2. The complications of pumps and pump control on a flash plant make
the conventional boiler more reliable especially in the smallest sizes. 3.
Pressure control in a heated pressurised tank, using a safety valve is
essential for safety and very useful when throttling with a simple valve. 4.
The pressure controlled tank of heated water is a store of instantly
available energy in its latent heat. This latent heat is the source of the
destructive power of a conventional boiler explosion. Disadvantages. 1 .
Requires regular pressure testing. 2. ls heavier than any self respecting
flash unit of the same power. 3. Limited in its capacity to safely contain
very high pressures and temperatures 4. Difficult to fly inverted. 5.
Generally a lower thermal efficiency than a flash plant.
At some future
time I will also go into details like how to automatically control steam
temperature (it is perhaps more important than the pressure). The likely
limits to power up to the 3/8 bore to which the above table probably applies.
Lubrication at the upper limits of temperature, the use of solid lubricants
and posh oils. How to estimate a value for the real pressure inside the
monotube, if indeed you want to know it.
That is how I
see small bore monotube design and is as far as I have taken it to date
January 2007. More will follow as I have notes, pictures and drawings to make
up perhaps a further 20 pages but the pay is not good and the workshop
beckons at every idle moment but there will be additions to these pages as
time permits.
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