Supporting > Boilers
Routing steam engine exhaust up boiler chimney.
crueby:
Great discussion and information, I think I've been bitten by this problem in the past - On my Lombard the exhaust pipes are fairly small, and it never seems to get the same power on steam as it does on compressed air at same input pressure. Makes sense now, if I understand correctly, that the steam exhaust is continuing to expand in the pipes, causing back pressure to the cylinder more than air does?
Gas_mantle:
Hi all, many thanks to those who replied :)
Firstly in response to the questions asked, the exhaust pipework I've been using is 5/32 OD, it is just a temporary affair rigged up using the coiled copper tube available from the usual model suppliers. Secondly I take the point MJM made about establishing it is actually the pipe bore that is too small and not just the outlet hole in the chimney - I ran a quick test with the engine exhausting via a short tube of the same dia as the pipework and still the engine was constricted.
At the moment the pipework is as I say temporary but now that I want to run a more permanent feed I need to go for something bigger so I thought about trying 1/4 OD - admittedly that doesn't sound a big improvement on 5/32 but I think the area of the bore will perhaps be about 2x as much.
As I want the updraught for coal firing I guess it makes sense to have the outlet nozzle as small as possible without choking the engine - I think for that I'm very much in guesswork land so the idea of interchangeable nozzles seems to be the way forward. I can't be sure what the existing nozzle dia is until I dismantle things but I soldered a brass nipple on the 5/32 pipe so the hole can't be more than 2mm dia?
Clearly the nozzle needs to be considerbly larger but I just hope increasing the bore doesn't adversly affect the chimney updraught.
I am able to power the Stuart 1 at healthy speeds on coal without the exhaust venting through the chimney but I need to open the steam blower slightly and waste steam in the process - hopefully with appropriate exhausting the engine can do the work of the steam blower :)
Peter :)
MJM460:
Interesting to see the observation that engines seem to go better on air than steam raised once again. It was one of the very early topics on the Thermodynamics thread, though never explained to my satisfaction let alone anyone else’s.
It is very clear that steam pressure in the cylinder and air pressure will have the same effect while the valve is open, and once the valve closes, and expansion starts, the difference is tiny. So that leaves the observed engine performance not explained.
I suspect there are two factors which lead to the observed difference in performance.
First, the real difference in thermodynamic properties of steam and air is that at normal atmospheric pressure pure steam will condense. This condensing starts as soon as the steam leaves the boiler where heat is being added, especially in the pipes which in a model are probably not normally well enough insulated. It also occurs in the cylinder, again rarely well insulated, but complicated by the fluctuation in temperature between supply and exhaust temperatures, which means there is always heat loss even with steam jacketed or other construction to reduce it. This condensing leads to loss of volume and pressure. But it is not enough to explain the total picture if there is enough heat input to the boiler to maintain the pressure right through to the valve chest while steam flows to the engine.
There is no chance of air condensing at normal atmospheric temperature, though it might cool (and hence reduce in volume a bit) if it is still warm from compression when it leaves the volume tank.
The big difference between using a compressor and using a boiler to power a steam engine is the nature of energy input to the working fluid. A steam boiler in a model is usually designed to be enough to run the engine with not too much to spare. The heating surface limits the actual heat into the water, some heat is always lost up the chimney, and the steam space is often less than a litre, so the amount of steam taken in by each stroke of the engine does affect the momentary pressure. The burner might be say 500 watts for my small meths burners, to several kW for a decent size blow lamp, gas burner or coal fire. A good portion of this energy goes up the stack, and even more goes into evaporating the water into steam. And that latent heat is all lost in the exhaust without being available to produce useful work. Normally less than 10% of the energy is available to produce work in the engine, so perhaps 50 to 500 watts. In fact, in the efficiency competition results I have seen, even the best 5 in gauge locomotives rarely exceed 5% efficiency.
I suspect that nobody with a boiler able to absorb 500 watts after latent heat is allowed for, ever complains about the power output.
But consider the air compressor. Even a little air brush compressor will have a few hundred watts of energy input at the motor. A workshop shop compressor might be be 2 to 5kW. This is comparable with those big burners and coal fires, but none of that energy is used to evaporate water, to be subsequently rejected in the exhaust, so the greatest proportion of the energy is available to produce work in the engine.
There are the inevitable losses in every process involving conversion of heat to work, but those losses are significantly smaller in the air compressor cycle than in the steam cycle, due to the absence of the evaporation/condensation part of the cycle.
So perhaps the reason that steam engines seem to do better running on air is that even on the smallest air compressor installation, there is much more of the input energy available at the engine to do work. Mind you there is no winning in this, it would be better to drive the load directly with the electric motor thanto use the motor to drive a compressor to drive an engine to drive the load apart from some rather special circumstances where it is done.
I think if you tried to pump a minimal volume tank with a good bike pump, and had to keep the pressure up by hand, you would soon decide steam was not such a bad option.
Enough thermodynamics, I will go back under my rock and watch with interest to see how Peter’s steam plant experiments progress. There is a lot of learning going on there for us all.
MJM460
Ian S C:
For the exhaust put the pipe into the chimney and take it right too the top, for a blower take a smaller pipe direct from the boiler via a valve to the base of the chimney. On one of my little boilers the pipe actually goes up the outside of the chimney. The main problem is that until the engine is warmed up you can get quite a fountain of water, so I have it angled so that the water goes overboard(it was in a boat). Cured the problem, went to a hot air motor.
Ian S C
MJM460:
Hi Peter, to address your specific issue, the size of the exhaust port is normally a good guide to what is likely to be required for an exhaust pipe on an engine doing a lot of work, though this port is a little restricted by the challenges of cylinder design, so the pipe is normally at least one size larger outside the engine.
John Mills has posted the sizes for his no 1 engine and I assume yours are the same or similar. You might get away with smaller tube when the engine is just running unloaded, but essentially if the throttle is nearly closed, the engine will be running very slowly if at all. It does not matter if that throttle is on the boiler outlet, the engine inlet or the engine exhaust, it restricts the steam flow through the engine. If you move on to getting the engine to do anything like the work it is capable of, or at least the maximum possible with your boiler, you will need a larger exhaust.
If the boiler seems to suffer due to the size of the engine exhaust protruding into the stack, or simply only needs a small flow to assist draft, it might be worth experimenting with splitting the engine exhaust using a tee fitting, with a small tube to the chimney and the remainder direct to atmosphere with just enough restriction to force the flow required for draft assistance. A butterfly valve, as has been described as a governor valve in other threads, would allow you to experiment with the flow split. Or a screw on nozzle so you could easily experiment with different hole sizes.
You have not mentioned the inlet pipe size. As the inlet pressure is higher a size or more smaller than the exhaust is usual. For a high pressure engine, several sizes smaller. Again the inlet port on the engine gives a guide, and again the pipe outside the engine would normally be a size or two larger than the engine inlet port.
Great to see you still experimenting, you will eventually see what works. There is a huge amount of learning in all this effort.
MJM460
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version