BOILER POWER

[wsmcycle]

Detroitug has said (correctly)>>power comes from the boiler. The engine is just a means of transforming the boiler's power in to real work.<<

This is a very important perspective to maintain in most discussions of "Engines and Boilers".
if by GPS measurement you could determine the RPM which produces the maximum velocity, it would go a long way in determining the best throttle position. I think the boiler is under powered in my boat. If I run at what "feels" like max speed, my boiler starts loosing ground. I have not been able by GPS to measure my max speed because of current in the river (Arkansas). I may be putting energy into beating the water. I should go to a lake and measure the Max speed and the RPM that produced it. Can the boiler maintain that RPM. To do that, I would get the boiler up to 100psi by reducing the throttle for a period of time. Set the throttle to the determined RPM using my bicycle tachometer. There are two thing i could learn from this. If the boiler can maintain the rpm, it is sufficiently sized for my boat. If it cannot maintain, I could reduce the throttle to the point it could maintain. Measuring that rpm I would have a measurable quantity (RPM) that represents the max steam flow (power) my boiler can produce.

[artemis]

A given design of boat has a maximum hull speed. Trying to go above this speed requires a substantial increase in engine horsepower (twice the horsewpower to gain a couple of mph in a 20' boat) and, therefore, a boiler able to put out a lot more steam. Find out the design hull speed, determine the propeller size and the HP needed to turn it, select your engine with those characteristics in mind and you'll be able to size your boiler appropriately. Anything else and you're in the realm of gueswork - and expensive "playing around". I suggest you get a copy of Weston Farmer's "From My Old Boatshop" and read the portions that discuss boat speed and propellers. Lots of good stuff in there that will save you beau coup bucks.

[PeteThePen1]

Hi Folks

On this topic, may I call upon your assembled expertise? I am probably in the area of "expensive playing around" that Ron mentions. I am building a version of the Lune Valley water tube boiler using the SBA designed pressure vessel. The question I am trying to answer is what is the heating surface area of this design. The sketch below gives the main dimensions. There are 29 copper tubes of 1/2" o/d (12.7mm).



My thoughts are that one would include 1/4 of the surface area of the mud drum, probably none of the steam drum as it is not in sight of the fire, 1/2 of all of the bottom of the steam generating tubes, but should one include the three layers above that cannot see the fire?

As ever, your thoughts would be most welcome.

Regards

Pete

[DetroiTug]

I'm thinking if it's in the combustion chamber it's calculated in to the heating surface.

-Ron

[DetroiTug]

I have not been able by GPS to measure my max speed because of current in the river

Run it both directions on the river, record the speed of each, add the two together, and then divide by two. That will provide an average speed.

-Ron

[gondolier88]

My thoughts are that one would include 1/4 of the surface area of the mud drum, probably none of the steam drum as it is not in sight of the fire, 1/2 of all of the bottom of the steam generating tubes, but should one include the three layers above that cannot see the fire?
Pete

Hi Pete,

Count any surface that has water on one side and flue gasses on the other- that would be all the mud drum, all the copper tubes, downcomers (although there won't be masive amounts of steam generation here, there will still be heat input with the water) and any surface of the top drum covered by water, don't forget to count feedwater heaters too.

Greg

[Dhutch]

Presumably, if the engine is condensing, or you can jury rig a method of condensing the exhaust you can measure the maximum evaporative rate of the boiler by firing hard, running the engine to match the firing, and and measuring the flowrate of the condensate produced?

If you feel its a little undersized, but your unsure what its currently doing, this might be a way of bench marking what you currently have. Something I mean to do with our boat one day.


Daniel

[Dhutch]

Presumably, if the engine is condensing, or you can jury rig a method of condensing the exhaust you can measure the maximum evaporative rate of the boiler by firing hard, running the engine to match the firing, and and measuring the flowrate of the condensate produced?

Can anyone comment on the validity of this suggested method?

Would be interested to compare what I can get out of our boiler, to what you are intended to be able to get out of it.

Daniel

[fredrosse]

The method you describe can indeed measure the boiler steaming capacity, however, as in many processes, "the devil is in the details".

I have tested boiler capacity by several methods, as outlined below.

1. Direct boiler output measure with sight glass

While running at maximum fire, eliminate feedwater flow, stop all steam flow for a few seconds, and mark the water level in the boiler, then ope up steam and run for a few minutes, then stop steam flow, and measure the water level again. The steam generation rate is equal to the change in boiler water mass divided by the time interval of the test.

This method works well if the boiler water volume vs. level indication is easily calculated, best with a VFT boiler that has no economizer coils. If the test can be conducted with the boiler stationary (boat on trailer or very calm water) the results are fairly accurate.

You need a large enough change in water level to minimize measurement accuracy. For example, if you measure a 1 inch change in level, and the sight glass is bobbing up and down 1/4 inch, then the accuracy of results is poor. On my boiler I can get about a 3 inch change in level with sight glass oscillations of less than 1/8 inch, fairly good accuracy results. Of course, time duration measurement is usually much more precise, and not much of an issue unless you are using a sand hourglass.

This test method provides the heat rate to bring the saturated water to saturated (or slightly superheated) steam. In continuous duty steaming, feedwater into the boiler is usually much cooler than the saturated steam temperature, so the available steaming rate while continuously underway will be somewhat less. A simple calculation will compensate for this, with continuous steaming being from 5% to 20% less than the test results.

[fredrosse]

2. Boiler output test by heating a tank of water.

The entire boiler output steam flow is directed into a tank filled with room temperature water. The steam instantly condenses, and heats the entire tank to a higher temperature over a measured period of time. A simple calculation determines the steaming rate for the boiler.

This test is best conducted with the main steam line directly feeding the cold water tank, but engine exhaust steam could also be used, and this would only introduce a minor error.

For this test to be ligitimate, the boiler must be operating under the same conditions of pressure, temperature, feedwater flow, and water level, throughout the test.

The test objective is to determine how much heat is flowing from the fire into the boiler water, while the test measure is actually how much heat is taken away from the boiler. If significant stored energy within the boiler is present, the test results can become meaningless.

For example, say your boiler is at 100 PSI steam conditions, and there is no fire at all. You direct the main steam into the cold water tank for 6 minutes, and heat 50 gallons of water from 70F to 100F. This gives about 12,500 BTU energy transfer in 1/10 hour, or an energy transfer rate of 125,000 BTU per hour. However the energy did not come from firing the boiler, but only from stored energy in the boiler, at the end of the test the boiler pressure would have gone down, as well as the water level.