UP IN SMOKE

[wsmcycle]

There is a trade off between high rate of flue discharge gasses and heat transfer time in the boiler. With high flow rate, the increased combustion rate of the fuel and the higher temperature of the exhaust gases through the boiler (greater temperature differential) increase heat transfer. However, this results in high temperature exhaust gases out the stack also and that is indicative poor heat transfer because it is waste.
It is a catch 22. I "fire-er-up" (my boat)using Propane alone. When I put a pot over the top of the flue, the boiler makes pressure much faster. This is because the heat of combustion is transferred more completely. No heat "flows" out the flue. (the losses are simply radiation) I understand that I am able to cover the flue because the propane burn rate is not dependent upon increased, turbulent air flow.
I suppose this dilemma has puzzled steam men through the ages. Where is the balance. I thought it would be an interesting topic.
For instance, what if you choked the volumetric flow periodically. Could the flow pulsations increase the fuel combustion rate while increasing (maintaining) the time which the gasses spend in the boiler. I send my engine exhaust up the flue. I could put a flap (like on an old tractor) that would blow open with each engine pulse.
I like to travel just below hull speed and I would like to hassle-less with boiler maintenance. I believe there are three horse powers involved in steaming along.
1) the fuel burn rate is an energy rate which can be identified in horse power

2) the boiler production is more obviously an energy rate (horse power)

3) engine output horse power. ( this one could be further broken down to usable motivation and unusable beating of the water.

I want the burn-rate to control the boiler-production and in turn control the engine-output (to usable motivation)

[farmerden]

My boat can amble along at 6.3 knots using everything the boiler has.But to travel at 4.5 knots fuel consumption is way down.The boiler pressure can be maintained at around 140psi instead of 100psi flat out.This increased boiler pressure allows the compound engine to perform much better.Sometimes, as the pic shows I can go really fast, but I think that is cheating somewhat as I had the tide flowing with me so 8.2 knots felt pretty good and if I had to go the other way,well
I think to choke the burn down would increase soot build-up and inhibit good heat transfer.I watch my stack and try and achieve an almost smokeless burn.Any residual heat left going up the chimney could be used with the placement of an economizer coil in the smoke stack.That would also reduce the size of the chimney.Steam is still a grand experiment!

[wsmcycle]

Farmerden
At this pressure, are you throttling down the steam? I guess if you were not throttling back, the pressure would continue to rise. My cycle in this situation would be to cut the inlet air down. Then my pressure would go back down to 50 and I will open the inlet to try and rev the fire up. Back and forth...... If I had a draft balancer on the flue, I could maintain a nearly constant draft.
I do not have an economizer at this time but I will install one very soon. As you observed, this will choke down the flue some.
Thanks

[Lopez Mike]

I suppose this is as good a time and place to ask.

Most of my steam plant experience has been with locomotives, both full size and scale.The general rule is this: Unless you are limited by wheel spin or excessive speed, the throttle is left open and the power is controlled with cut off. It is thought wasteful to throttle the steam with the losses and 'wire feeding' and such that ensue.

Now all of the steam boats I've been on including mine have perhaps one notch back from the maximum settling. ( don't know what percentage of cut off that is. I guess I had better get out the tools and look into this!)

So it seems to me that in Den's case, the engine cannot tell whether the pressure has been regulated by lowering the boiler pressure or by partly closing the throttle. And yet, his engine likes it better to use the throttle.

I'm wondering of the better running of his engine is due to incidental super heating caused by the expansion and reheating at the throttle? It then occurs to me that it would be more efficient to use an economizer and run the lower boiler pressure and leave the throttle open.

So why don't we have more range of adjustment on our reversing levers? On a big ship where the engine runs pretty much at it's designed output from day to day, it makes more sense. Or does it have to do with the effect of driving a load that drops off as the engine rpm drops. Unlike a locomotive, we don't have to deal with many hills, either up or down. Head winds qualify, I suppose.

Thoughts? (Before I add a zillion notches in my reversing gear quadrant!)

[fredrosse]

”I understand that I am able to cover the flue because the propane burn rate is not dependent upon increased, turbulent air flow. “

Trying to understand your statement, how does the air get into the propane flame, and where do your exhaust gasses get out when you have shutoff the stack?

[gondolier88]

Hi,

Unless I have misunderstood, you are asking how to extract the most amount of energy from your fuel without trading off heat transfer efficiency?

You are correct in saying there are three quantifiable energy measurements you can take- but these are not seperate entities-
energy out = energy in.

As I understand it, if you restrict the flue at the top you find you can increase steaming capabilitties through a slower flue gas velocity and the resulting longer heat transfer time, then why not fit an adjustable flue damper- flue restrictors are standard fit on many modern gas appliances for the very same reason?

As you are propane firing you can afford to fit complex off-the-shelf heat exchangers with multiple tubes and tight fins- you could kill two birds with one stone and restrict the flue with a heat exchanger- slowing down the gasses in the boiler enough to improve steaming, and then use the heat exchanger as a feedwater heater (with a safety relief on the feed line!).

Also you can fit baffles in the combustion space that increases turbulance and linger time of the gasses. One option that has been used at model size boilers is to fit a stainless steel gauze above the burner that glows red hot under service, this converts the heat from the gasses into radiant heat- essentially becoming a replacement coal bed.

Greg

[farmerden]

If I slow the engine down [use less steam] then the boiler pressure increases.Then I can adjust fuel consumption as well.When running a compound at lower pressures ,the LP cylinder does not get much pressure and goes for the ride ,so to speak.At 140 psi I have 30-40psi at the LP side and you can feel the power difference-the engine just sounds better! My boiler has no more steam left at flat out speeds. The boiler and engine are in sync with each other but I would prefer to have a bigger boiler than I need then I could be a little more gentle on the fuel consumption.And of course get a bigger whistle!

[Lopez Mike]

So is that 140 psi at the boiler or after the throttle? Can your engine tell the difference between having the boiler at 140 and slowing down the engine with the throttle versus letting the boiler pressure drop to slow the engine down with the throttle wide open?

Kinda like the line from the hill billy encountering his first thermos bottle. "You put in hot. It comes out hot. You put in cold. It comes out cold. How do it know?' (grin)

[barts]

If we're firing with propane, we should admit only enough air to burn all the propane present; this keeps the flue gases as hot as possible and maximizes heat transfer. Placing a pot over the stack should make the fire go out; if not where is the exhaust going? You either have leaks or are admitting excess air.

Excess air is often required when firing wood to prevent pulsating burning; oil can have the same problem if the boiler is cold.

- Bart

[artemis]

If I slow the engine down [use less steam] then the boiler pressure increases.Then I can adjust fuel consumption as well.When running a compound at lower pressures ,the LP cylinder does not get much pressure and goes for the ride ,so to speak.At 140 psi I have 30-40psi at the LP side and you can feel the power difference-the engine just sounds better! My boiler has no more steam left at flat out speeds. The boiler and engine are in sync with each other but I would prefer to have a bigger boiler than I need then I could be a little more gentle on the fuel consumption.And of course get a bigger whistle!

Compound engines were not developed until the 1870s when boilers could finally handle over 100 psig. Most compounds are designed for around 150 - 175 psig. The pressure should be proportional to the ratio between the cylinder head areas: if the engine is 3" bore for the HP and 6" for the LP then the ratio of the AREA between the HP and the LP is 4 to 1. So if your pressure at the throttle is 155 PSIA, your vacuum is 5 PSIA, then your steam at the LP should be 37.5 PSIA. Cutoff for most compounds is set at 60% to 65% of HP stroke (don't bother with cutoff on the LP as it will not affect the amount of steam consumed). Den and Mike: for a nice explanation of this, contact Keith Sterenberg. Variations in the ratio between the piston areas determined valving and all kinds of things.