Basic Steam Plant principle

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Anne from Little Britan
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Basic Steam Plant principle

Post by csonics » Wed Nov 18, 2009 6:35 pm

Post on behalf of Maltelec:

Maltelec
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Posted: Wed Sep 26, 2007 3:41 pm Post subject: Basic Steam Plant principle
Heres a copy of a reply which I have just types out for another forum which some of you may be interested in. It gives some basic detail on what to think about when designing a steam plant. It was in response to someone wanting to know about horizontal mill engines. The principle is easly adapted to boat plant design. Feel free to point out any mistakes:

Quote:
I want a small Horizontal mill type engine (model size, say 2ft long or thereabouts) which does actually work and will work for a couple of hours a day max. It will have light duties in that I want it to run an alternator to charge some 12V batteries.


Do you mean like this?

Image

Unfortunetly that isn't actually mine, I've just got it in my barn for a friend.

Excellant idea, but do you know how long it'll take to start up? An engine like that ^ about 3 foot long will require a boiler roughly 4 times the size (if you imagine 4 of them engines in a swuare shape) as a general rule of thumb. Give yourself about 30 minutes for the water to start to boil if using wood, about 20 min on coal, and thats if you can get the stuff to light.

House coal is no good, far too smokey and you will soot the boiler up in no time. Anthracite is the stuff, but you need a very good draft (achievable only by either puffing, forced draft or a very long chimney). Coke is excellant stuff for an engine like this, you simply pile it up and leave it. Not too hard to light either.

If it were me, I'd have a condensing engine in this case, probably with a fan-driven condenser. However this is extreamly complicated, and if you have a good fresh water supply, unneccesary. I would however have a feed heater as you want the water entering the boiler as hot as possible so the boiler has to do as least work as possible. Not forgetting that a feed pump (which must run constantly) will create a lower pressure, which can cause very hot water to flash, a feed heater should use the engine exhaust to heat the incomming water. The problem here is you then can't use the engine exhaust to blast up the chimney as it will have mostly condensed back to water - this can be reused but you then must have an oil filter system. It is very bad to have oil in a boiler.

A super heater will help on the ecconomy, and a large enough super heater would mean you can run the feed heater with less surface area, meaning the same heat will be passed into the feed water but the exhaust would remain saturated steam, and can then be used with a blast pipe.

Cylinder lubrication is essential with a super heater. It doesn't need much, a drop every 10 sec or so.

If you need to retain the water, you will need to seperate the oil like I have mentioned, and condense it back to water. To do this you would need a condenser, and ideally an air pump. The air pump pumps the condensate out of the condenser and into the hotwell. The hotwell is the "hot" water tank, on a boat it usually has a built-in oil seperator. This is best kept as hot as you can as it is the feed water.

The condenser simply removes the heat from the exhaust steam. You could even use a land rover radiator and simple fan, as long as it does the job. The condenser and engine exhaust also works on a vacuum generated by the air pump, usually nearing 30" Hg. This helps on the ecconomy much more, and gives you more powa.

The air pump is as the name sergests. I prefere them to be driven at something around 2.5:1 engine speed (you'd always use a random ratio like 2.4386 to prevent the air pump from putting the load on the same point every time). Think of it like an air compressor, only it sucks instead.

Feed water is another problem, probably the largest there is. You need to constantly feed water into the boiler at exactally the same rate as which the engine is using it. Most boilers have a reasonable capacity, so you can lave it 5 min, feed water in for 2 min (at a faster rate) and leave it for 5 min etc, but the pressure will be up and down like a yoyo. The absolute worst thing that can happen is if the water becomes too low. This is when the boilers explode.

Many forms of feed water control exist, but one of the simplest I have seen is a £2 item. You get a toilet system floaty valve thingy, stick it in the hotwell. This valve is T'd off from the boiler feed pipe (after the feed pump). Hotwell fills up, valve closes, water pressure increases until it is equal with the boiler and water is fed into the boiler. Hotwell level lowers (boiler fills up), valve opens, water is diverted back into the hotwell.

Feed pumps: the basics is they are a high-pressure water pump, usually engine driven but not always (it is common to have both an engine pump and a steam pump). They pump water from a source into the boiler.

Now comes the real tricky thing, calculating things.

How much water do you use?

Take the engine I have designed (cos I have all the calculations written down). It is a Vertically-opposed piston twin single-acting uniflow. 100mm bore, 80mm stroke per piston (thats like 100mm bore, 160mm stroke).

At 11 bar, 1kg of water occupies 0.1774m[sup]3[/sup] - Taken from a steam chart book.

My engine, 2513.28 cubic centimeters ... 0.014167kg of water per stroke

x 400 (rpm) = 5.67 kg of water per minute.

Times by 2ish because you ALWAYS want a pump more capable than what you need = 10kg of water a min.

So for my (rather large) engine, I will need a pump to deliver 10 litres of water per minute.

Air pump: This should be 10-20% of the capacity of the engine. On a single stage engine like ^, 10% would probably be fine. On a tripple expansion engine where you expand the steam to its limits, 20% (of the larger cylinder) is oftern better. However, with superheated steam, the expanded steam is oftern still superheated, and so more work will needed to be removed by the condenser and a larger air pump would be beneficial.

Right, now the basic stuff is out of the way we can get onto the real technical things.

The boiler:

The boiler does the work, the engine only converts energy. If you have a boiler which is too small for the ammount of energy you need, you won't get the power you want, or you find you have to run the boiler at max all the time. This is not good as if you suddenly require a lot of power, the boiler won't have any spare capacity. It doesn't matter what engine you have, an engine twice the size will give the same power given the same steam from a boiler.

Water tube, fire tube or mono tube?

Water tube boilers have water in the tubes. They oftern compose of 2 or 3 large drums, one at the top, 1-2 at the bottom, and a network of tubes between the two. The top drum holds the steam, and is known as the steam drum. The bottom drum(s) hold the mud, silt and anything else which somehow gets in there, and is called the mud drum. You feed water into the mud drum(s), and you must also blow-down the boiler to clear them.

Water tube boilers are quick to steam up, quick for instant power, but are also quick to loose the power. This is due to their lower water capacity. With an automatic water system, this is not a problem.

Water tube boilers are oftern made of steel drums and copper tubes. These tubes can be expanded into the drums, or even held in by special tube-holding thingies. There is no right/wrong way, but some are better suited for some situations. I don't see anything wrong with expanded tubes, they are easy to rip out when they need changing. Solid copper boilers are ok upto about 150psi working pressure, and these tend to be solver-soldered together, meaning they are almost impossible to take appart. A vertical water tube has 1 drum, and tubes comming out the side in a vertiaty of patterns and going in at the bottom. The LIFU design is prehaps the best design of them all with a tube comming out the side, doing 3 coils at an angle and going back in.

Fire tube boiler, like most loco's use (a loco boiler being a classic design) have fire in the tubes and water on the outside. They are generally much larger for the same output as a water tube, however the greatly increased water volume has the benefit of being very stable boilers, able to use its larger volume to maintain its working pressure even with a sudden demand. They are slow to start up, as there is a lot of water and steel to heat, and the tubes are very prone to sooting up on poor coal, but they are generallt considered to be the best choice for industrial use.

Their design is commonly a loco type, having a fire box at one end surrounded by a water jacket, and tubes directing the fire to a smoke box at the other end, all surrounded by water. This does not leave much space for a super heater, and loco's would oftern have the super heater within the fire tubes themselvs. A vertical type of fire tube boiler has a large drum (imagine a beer barrel) with tubes going vertically down.

The chimney is prehaps the most important part of any boiler. The chimney is what gives the fire its deaft, and the better the draft, the more it fires. The chimney wants to be as large as the maximum air going into the boiler or a bit more. It also wants to be as high as physically possible, not to make the smoke go over your head, but to cause a strong draft. If you are using a blast pipe, you can have a short chimney, but these can make a lot of noise if a lot of draft is required from it (i.e. a steam train noise - they have very short smoke stacks).

You would probably want an exiter on the chimney. This is a device which uses steam to make steam. You need to make the chimney into a venturi shape, where it starts off big, quickly goes small and then tapers to big again by the top. You then put a small nozel just below the small diameter part (read a book to calculate the best place) and when steam is driven through the nozel the venturi effect causes a very strong draft. This is excellany for helping to start up, but it can use a suprising ammount of steam - same principle as a blast pipe.

Firing the boiler:

Coal, gas, oil, wood, random burnable things, it doesn't make too much difference. The only thing is the shape of the flame. A coal fire is what ever shape you make it, square, rectangle, round etc. which is usually determined by the boiler type. However gas and oil are commonly round flames. Vertical boilers suite this flame type much better. The LIFU design was specifically designed for an oil fired burner, LIFU stands for (IIRC) Liquid Injection Fuel Unit. LIFU designed almost fully automated steam boat plants, requiring only 1 valve to control the boiler pressure. Combined with automatic water level, automatic pressure control, you could very, very easily make a steam boat which has start, stop, ahead/astern and speed controls, and you wouldn't have to touch anything else (except maybe the steering wheel).

The size of boiler is always the hardest. As I said at the start, you want one roughly 4 times the size of the engine. This is a very basic rule of thumb, and a LOT has to be taken into consideration. The first being the heating surface of the boiler. The bigger the better. Not forgetting that you need return tubes for the cooler water to cycle back down to the bottom of the boiler. The faster you can get the water circulating round, the better it will fire. You must remember about boiling water, creates bubbles, these bubbles want to get to the top (where the steam is) and if they can't get up the tube fast enough when the water is wanting to get back down, the boiler won't produce steam like it should.

The next big thing is the grate size. This is how much fire you can get in. Again, larger the better, but on something like a loco type boiler you can find it is quite small. The more fire you have the better it will steam. You also need the draft to cope with the fire, so no tiny pipe for a chimney.

The engine:

Now, you've got your eyes set on a horizontal mill engine. This is good, they are typically a slow reving long-stroke engine (unless of course you see the 1000 IHP mill enigne around Manchester when its run properly, it don't go slow!).

Mill engines of 2' long type are usually as basic as they come. Slide valve, 2 main bearings, simple crosshead desing etc. The large (1000 IHP size) oftern use coreless valve gear and other random types where the steam in goes through a different port to the steam out, thus saving energy as the steam out is colder after it has expanded, which cools down the steam going in. You want as much steam in the engine as possible, and steam expands as it cools (and it cools as it expands). So hotter the better.

Unoflow engines are an easy way round, they work like a 2-stroke infernal combustion engine where the steam enters at one end and exhausts out the centre of the cylinder. That is how the engine I have designed works, but they are annoying to start due to their high compression and they must have an air pump to work efficiently. High ammounts of super heat also helps them a lot.

Slide valve engines are good in one respect as the valves wear in instead of wear out.

There are many reversing links you can choose from, but why bother? You only want the engine to turn one way so ignore all of that revering stuff and just use a simple mechanism. It just adds unneeded loads and friction to the engine.

The flywheel is something which is quite important on a mill engine. As the load is usually quite high on them, they use a large flywheel to prevent the engine from resonating too much, where the engine slows at TDC and BDC and speeds up again in mid-stroke. This will always happen, its impossible to remove, but a large enough flywheel will smooth it out so it can't be noticed.

You want the engine to run as fast as fesable. Power = torque x speed, and a steam engine has most torque when its stalled (in mid stroke), so there is an upper limit, but given the ports on the valve(s) are suitable, it should rev quite a bit before the steam isn't able to pass through fast enough. Being a long stroke engine, it is designed to run at slower than normal speeds anyway. I would imagine the picture ^ one would be about optimal at 600 rpm.

Lubricating:

Like all engines, you need to oil them. Having gun metal bearings, they are designed to run with a total loss oiling system. You throw oil at it every day, trying to aim it into the oilers. The important ones are the crank pin bearings and the top end bearing, the main bearins comming 3rd. Oftern an engine will have a cup with a pipe in the middle. These are made to take a specil drip-fed system where you take a bit of cotton, tie it in a special way to some copper wire and have one end in the pipe and the other end in the cup of oil surrounding it. This then allows a drip of oil into the bearing every so oftern. You simply remove the cottoned wire when you're done with the engine and replace it in the morning.

The main steam line would need oiling unless you're using wet steam. I would not reccomend a displacement lubricator from personal experiance, but I won't stop you trying. On my dads boat we use a mecanical forced lubricator which works very well. It simply pumps a bit of oil into the main steam pipe onve every 15 sec.

Piston rings:

Very important. Most models and toy engines don't use piston rings, or at best a completely useless one. I'd highly recomend using Cluplex piston rings, which are cast iron like a keyring. 2 of them on each piston and it'll be as water tight as a ducks.... The difference they can make is amazing.

Govoners:

You have a problem in that your load is not constant. This means that if the load suddenly goes, the engine will race, which wouldn't so it much good. You must have a govoner on the engine to prevent this. Steamboats don't need them as their load is mostly constant, but alwaus there.

Bearings:

Bigger the better. The bigger they are, the less force is applied to the same area and the longer they last. Gunmetal is the stuff to use, which is in between brass and bronze. It comes in many forms.

Pipe work:

Appart from not running the boiler dry, the pipe work is prehaps the next major thing. Imagine the blow-down pipe blew off the bottom of the boiler. You'd have 5 gallons of water at 180C being forced out at a very great rate. This water instantly flashes to steam and the force is so great it will probably move the boiler along the ground.

Superheaters should be steel to deal with the fact that you are heating a pipe without water in it. The saftey valve should be at the engine-end of the super heater so when it blows, steam is drawn through the super heater to help cool it down. On a larger setup (like a plant driving a 10 IHP engine or more, sometimes less) you'd have 2 saftey valves, one set a smigdin lower on the end of the super heater and one on the boiler.

The pipe to the engine can be copper, but always be wary of the pipes being moved while the engine is running. All pipes will fatigue but copper is perticuarly bad when its been heat treated like it is when its used as a steam pipe.

My dad uses steel pipes to the engine, but it is a long run. Steel is difficult to work with, copper is far easier to bend etc.

For high pressure small jobs you need thick-walled refrideration pipe, not every day plumbing pipe. And use only high-quality fittings, absolutely NO 8-sided nuts!!!!!!! T'is a fail on a steamboat!

Olives are good, as are soldered nipples. Screw-joints must be sealed with PTFE tape or Loctite pipe thread sealant, or anything suitable for 200C+ of steam.

Flanges are also good, but they don't allow us to use asbesdos any more :'( We've got the super heater on flanges.

Fittings:

Gauge glasses are important. I'd highly recommend the flat-glass type which is a VVV shape on the back. Very thing glass, very well protected and very easy to see. The light is reflected by the water but dispersed when its dry, so there is a very strong light/dark constast (as long as you stick a U shape behind it). The glass is about 10mm, its never going to break!

The other type is commonly a fragile glass tube protected by 3 glass walls. Dodgy is my term for them. Very common in the steam age. I've seen one crack at full pressure, amazingly it didn't blow alltogether - if the bottom saftey system fails you have 5 gallons of very very hot water in your crotch!

Valves should all be rated to the working pressure and temperature. The best ball valves are stainless ones, which bolt together at each side. DO NOT use B&Q ones!!!!! These are for mains water, not steam!!!

Most bass fittings are perfectally acceptable, as are bronze. Absolutely no di-cast ones!

Pressure gauges should be tested before use and calibrated if neccessary. We're not dealing with toys here, so we don't treat them like one. You'll need a boiler gauge (200psi if your wp is 150psi), a feed pressure one (same again usually), and a vacuum gauge if you use an air pump - upto 32" Hg. You oftern get a special gauge which does 15psi to 32" Hg which tells you if the air pump has failed without trying to force the gauge past the pin.

On the feed pump gauge you must use a reducer (like a valve mostly closed) to protect the valve from the water hammer. It will destroy a gauge in a few minutes! This is where the water pressure shoots right up as it hydraulics before it is pushed into the boiler. We have a 500psi gauge on the feed pipe, it used to peak at 350psi before it broke.

All gauges (except the vacuum gauge) must have a valve on it incase the gauge breaks and the steam needs to be shut off.

Thats all I can think of at the monent, if I think of any more I'll add it on.
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I've got the vehicle , just need the boat.
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