Warning: This thread may contain nuts, 'real' engineering, sums and mixed units , as I am building the engines under the imperial regime but use metric for my physics.

There is nothing on my drawings regarding the injectors for my 4" scale traction engines, so I've been playing with some sums to see what size I need.

The engines are compound, double acting, and run at 170pisg, so the injector will need to work at that pressure and generate at least that pressure on the output. It is usual to add a bit, so let's say they'll need to generate 180psig.

Although the engine is a compound we only need to consider the HP cylinder to calculate steam consumption. The HP cylinder diameter is 2", stroke is 3.75" and assuming a cutoff of 80% and a maximum speed of 600rpm we can calculate the volume of steam used per minute. I get:

0.185m³/min

The specific volume of steam at 170psig is 0.1542m³/kg. Dividing by this we get the mass of steam per minute to be:

1.2kg/min

Obviously 1kg of steam is generated from 1kg of water and 1 litre of water weighs 1kg. So we need 1.2 litres of water per minute. In real money that's 2.1 pints, or and a drop. It would be helpful if the injector was capable of filling the boiler even with the engine running so let's say we need a 3 pint injector.

If I decide to build a test boiler to play with injectors it will be electrically powered. So how much power do we need to continuously run a 3 pint injector with input steam at 170psig?

Three pints equates to 1.7 litres and hence 1.7kg of water per minute. As a rule of thumb let's assume that the volume of water entering the boiler from the injector is made up of 11 parts water from the tank and 1 part water from the condensed steam at the injector input. So we need one twelfth of the mass to be steam, so 0.142kg/min. The total enthalphy of steam at 170psig is about 2786kJ/kg. So for the steam we will use in the injector per minute we need 396kJ/min, or 6596J/s, or 6.6kW. Seems a lot? But how long does your 3kW kettle take to heat 1 litre of water to 100ºC, let alone the 190ºC associated with steam at 170psig, and also convert the water to steam?

Andrew

Edited By Andrew Johnston on 16/11/2015 23:04:35

Edited By Andrew Johnston on 16/11/2015 23:04:51

Bill: I've been thinking about injector theory for some time now. However, at Forncett this year I had a good chat with Robert Bailey (aka 'steam_guy_willy' ) and he kindly showed me his electric powered boiler. Dunno why I didn't think of electrical heating; stupidity I suppose. Anyway, the discussion fired me up, as it should be easier to design/make/run an electric powered boiler than mucking about with liquid or gas firing. Especially as the kitchen has been designated as the test area.

From a practical point of view I am envisioning a simple cylindrical boiler with a couple of large stays, each containing a 1kW heating element. After looking at triacs for control of the heaters I then thought of using an off-the-shelf temperature controller. Much simpler, and as I do electronics for the day job, I don't really want to do it in my spare time. The boiler will need to run up to about 200psig and will only need to produce saturated steam, ie, no superheating needed. Given that I can close the loop on the heater control by measuring the temperature of the steam rather than the pressure, since there is a monotonic relationship between pressure and temperature for saturated steam. I can probably do away with a water gauge too; if the temperature rises too much due to a fault, or no water, then I can just turn off the heaters. A fellow member of this forum has given me a smack round the head and pointed out that temperature controllers are ten-a-penny on Ebay, rather than the expensive ones at Farnell/RS that I was looking at. You can get a controller, thermocouple and SSR (solid state relay) for less than £20. Bargain!

As a further thought, the only hot part of the injector is the steam inlet, and the only high pressure parts are the steam inlet and water outlet. So it could be possible to make the test injector body from a transparent plastic so that you can actually see what is going on in the internals.

You never know, something might even be on display at Forncett next year.

Andrew

Edited By Andrew Johnston on 17/11/2015 20:44:36

Interesting, I'll have a look at the water heaters. Although the cartridge heaters I was planning to use are encased in stainless steel they will not see boiler pressure, but will be embedded in large copper stays. The datasheet recommends a H7 fit and also use of their thermal goo, which RS don't sell. Typical, why sell the cartridge heaters if you don't sell the goo needed to use them? It's a bit like Farnell selling expensive high voltage. high current connector shells and crimps, but not the even more expensive crimp tool.

I've no idea what LSM recommend, although I don't recall seeing anything on the drawings. However I do have a priori knowledge. A couple of years ago Terry Baxter ('injectorman' on TT) kindly lent me his bending rolls for my rear wheel rims, so I asked him what injector I needed, and he immediately said 3 pints. Nice to know why he said that!

I didn't know that domestic plumbing was rated to 5 bar. I wonder if there is some sort of regulation in the system as I know that the 4" or so mains water distribution pipes run up to 250psi (17 bar). Some years ago I did all the electronics for a small ultrasonic leak detector that was put into the mains pipe and went with the flow, so to speak, listening for the ultrasonic noise that is indicative of a leak. In order to narrow down the location of a leak the device listened for a series of ground based radios above the pipe and tagged the data each time it heard one. Now the pipes were 1" wall thickness cast iron, so how did the radio work through that? As the textbooks say I'll leave it as an exercise for the reader, for the time being.

I've been looking at the injector steam nozzle this evening. As far as I can see it is essentially a convergent/divergent nozzle similar to those used for impulse steam turbines. Steam turbines are pretty ubiquitous, so there is plenty of literature regarding them and the design of the nozzles. One thing puzzles me though. Most of the diagrams show a relatively short convergent section and a much longer divergent section, which is the opposite of that shown for all the model injectors I've seen. Why? Could be a number of reasons:

1. I've got it wrong and the design of the injector steam cone is not the same as an impulse steam turbine nozzle

2. That's how the first model injectors were made, and everybody has subsequently stayed with it because it works

3. Some other reason I haven't thought of

I've made a note of the next Forncett steam up day in the diary.

Andrew

Cylinder diameter is 2", fixed by the design

Stroke is 3.75", fixed by the design

Cutoff is 80%, this is arbitrary, but is what I will be aiming for when I re-design the valve gear

Maximum engine speed is 600rpm, probably a bit high even for a 4" scale engine, but one has to put a peg in the ground somewhere, and it's what I used when re-designing the water pump

We only need to consider the HP cylinder since the LP cylinder steam comes from the HP cylinder, not from the boiler. Of course we're ignoring use of the simpling valve, but that is only used very short term.

The volume of a cylinder is:

V = pi x radius squared x length

So for one stroke (TDC to BDC or vice versa) the swept volume is:

pi x 1² x 3.75 = 11.78 in³

But with a cutoff of 80% steam only enters the cylinder for 80% of the stroke so the volume we're concerned with is:

11.78 x 0.8 = 9.423 in³

The engine is double acting, so steam fills the swept volume twice per revolution, so:

9.423 x 2 = 18.85 in³

Multiply by 600 (rpm) to get the swept volume per minute:

18.85 x 600 = 11309.73 in³/min

I'm too idle to look up the conversion factor from in³ to m³, so I multiply by 25.4³ and divide by 10^9 to get cubic metres:

11309.73 x 25.4³ / 10^9 = 0.185m³/min

Of course there are all sorts of ifs and buts that can be made against my assumptions, but you have to start somewhere, and the calculations are only a marker to size the injector.

Andrew

So 30 litres per hour is 0.5l/min or 0.88pts/min; rather less than I am calculating. I am being pessimistic using 80% cutoff, and optimistic with a speed of 600rpm. However, as stated we want the injector to fill the boiler so some over-capacity is no bad thing. Unlike a water pump there is no penalty if the injector is not being used.

One litre of water weighs 1kg. If we assume that for each 12 units of water going into the boiler 11 come from the feedwater and 1 comes from the steam we need 0.083kg of steam for each litre of water injected.

We also need to take into account the behaviour of the injector as the input steam pressure decreases. A quick scan of the literature shows that the output velocity of the steam from the steam cone doesn't vary much with input pressure. However, the mass does, so there is less mass of steam going into the combining cone. If we want to stick with the 1 to 11 ratio that implies less feedwater and hence less water overall going into the boiler with reduced input pressure. No idea if that is correct; my off-the-cuff ideas done while typing have a habit of going pear-shaped because of something I didn't see.

Andrew