Steam Generation Question

[JonRiley56]

All,

I need some help. I have gotten my system set up reasonably well now.
I have an issue with an air leak in the Mercruiser PS cooler that I am using
As a feedwater per-heater but I will sort that out.

My issue is steam volume. My boiler is more than big enough to supply my 3x4
Single. I have a 60 sq ft water tube that should be about twice the size I need.

I am seeing a steady decline in pressure as I run, even if I start at over 100 psi.
As I think about it I am wondering If I am running the water level in the boiler to low.
I have been holding the level well in the lower portion of the glass, rarely going higher than
Abou 30% of the gauge. The way my glass sits, that means I probably only have the
Water level at about 25% in the steam drum.

Is the volume of steam maintained/produced related to the mass of water in the steam drum ?
If so, having a greater volume if water in the steam drum may
Take care of my problem.

Any help will be appreciated.

Jon

[artemis]

...I have an issue with an air leak in the Mercruiser PS cooler that I am using
As a feedwater per-heater but I will sort that out.

Obey the maxim "KEEP IT SIMPLE STUPID". Temporarily bypass the "cooler/feedwater pre heater

... My issue is steam volume. My boiler is more than big enough to supply my 3x4
Single. I have a 60 sq ft water tube that should be about twice the size I need.

Yes, lots of heating surface. But is the fire big enough!

I am seeing a steady decline in pressure as I run, even if I start at over 100 psi.
As I think about it I am wondering If I am running the water level in the boiler to low.
I have been holding the level well in the lower portion of the glass, rarely going higher than
Abou 30% of the gauge. The way my glass sits, that means I probably only have the
Water level at about 25% in the steam drum.

Is the volume of steam maintained/produced related to the mass of water in the steam drum ?
If so, having a greater volume if water in the steam drum may take care of my problem.

The water should NEVER be below 1/2 glass. The amount of steam produced increases with the quantity of WATER exposed to heat and the amount of heat. Steam exposed to heat does not make more steam, just hotter steam. Getting water TO the boiling point takes much less heat than boiling the water. For example: to bring water TO to the boiling point at atmospheric pressure requires 180 BTU; to cause it actually boil requires an additional 970 BTU - and this varies with pressure.

The air leak is probably also causing a problem as entrained air (doesn't take much) creates a boundary layer between the water and the heating surface of the boiler. Air is NOT a good conductor of heat!

[Oilking]

Jon,
Is your boiler a VFT, or a horizontal drum water tube? On a water tube as long as the level is high enough to keep the tubes filled it shouldn't degrade the steaming rate, but I'd keep it at half a glass. On a VFT the water level has a great influence on the amount of water exposed to the heated surface. for example on the little VFT I'm working on, one inch of water level is about one sqft. of heated surface, and if the water dosn't touch the tube it isn't going to make steam.

To add a little to Ron's comments, a BTU is the energy required to raise one pound of water one degree F, so it takes 180 BTU's to raise one LB. of water to 212 deg F, sarting at 32 deg F, and an additioal 970 BTU's to convert it to steam at atmospheric pressure. One gallon is 8.346 Lb. So starting with 50deg F water we get the following:

(162 BTU+970 BTU)8.346LB/Gal = 9447.7 BTU's to convert 1 gallon of water to steam at atmosphere starting at 50 deg F.
In a perfect world that would be about 0.07Gal. of diesel fuel (~135,000 BTU/Gal Diesel).

Im' with Ron, by-pass the heater and see what happens?

Good Luck
Dave

[JonRiley56]

Hey Guys,

Thanks for the responses. I did bypass the preheater and the waer hammer went away. I still have a problem with air because my return/recirc line to the hotwell is lower than my inline filter. I will re-plumb to take care of that. One reason I was trying to run the pre-heat was because I thought that I was knocking down the pressure with the cool feedwater.

My boiler is a water tube set up. Am I to understand that I will have more consistent steam generation if I keep the steam drum at least half full ?

Thanks

jon

[fredrosse]

Jon has a large Roberts type watertube boiler, so virtually all of the steam is generated in the tubes running from the two lower drums to the top center water drum. Keeping the gauge glass 1/3 full, or 1/2 full should have virtually no impact on the steam generation rate. As long as there is adequate water in the top drum of the boiler, the steam generation rate will basically depend on the size of the fire, and to a lesser degree on the temperature of feedwater to the boiler.

If you keep up enough feedwater flow to maintain a constant boiler water level, and steam pressure is dropping, then the steam generation rate is lower than the engine's steam consumption rate. Pressure can be kept up by either generating more steam flow with more fire, or reducing engine steam consumption by throttling the engine (or linking up the engine valve gear).

Pre-heating the feedwater will allow an increase in steam generation, roughly 100F increase in feedwater temperature will allow about 10% increase in steam generation rate. This assumes you are pre-heating the feedwater with steam engine exhaust steam.

[JonRiley56]

Thanks Fred !!

Help my understand "Linking Up". I am not sure that I get it...................... I was getting a nice temoerature rise off my pre-heater, not sure of the actual temp rise, but the water was too hot to comfortably touch in my hot weel when I was running through the pre-heater.

jon

[barts]

If steam production is inadequate, your engine is taking too much or your boiler isn't producing enough. Weighing the condensate will give you a quick way of estimating the BTU that are being transferred; the fuel consumption will let you know how many BTU are being generated.

A 3x4 will likely need about 1.5 - 2 gallons/hour of diesel to run at full output.

- Bart

[Lopez Mike]

Need some clarifications.

The water going through your exhaust heat exchanger. Is it going from there directly into the boiler or is it going into the hot well?

If it's going directly into the boiler, I'm not clear why it should affect the hot well temperature.

I know of one person here in the N.W who heats his hot well. I've always thought that the whole point of the feed water heater was to heat up the water just before it entered the boiler.

You speak of some water hammer. It's not clear how a leak in the heat exchanger would cause that. It would seem to me that any air in the water after the pump would soften any hammering. If the heat exchanger had a leak it would allow pressurized feed water to enter the exhaust stream at that point and and then flow through your condenser and back to the boiler.

Sorry about all of the questions. It not the first time nor will it be the last where I don't get it.

Mike

[DetroiTug]

Jon,

Does this boiler have stainless steel coils? If so, that may be part of the problem. Stainless steel has a very low thermal conductivity rate. You may just need to fire it harder. Or possibly add an economizer of copper. I had to add 20' of copper to my steel economizer as it was not doing that great of a job after extended testing. It works great for short pump cycles, but once it cools down, the heat transfer is obviously not great enough to adequately heat the water. I could literally hold my hand on the outlet, about 110 degrees.

Here is the conductivity rates for different metals.



Ron

[barts]

metal conductivity is a very small part of the overall thermal resistance; you'll find the film coefficient predominates.

Note that it takes a while to learn how to fire a new boiler effectively; excess air is a common problem, and robs much efficiency.

The reason to heat a hotwell is that it lowers dissolved oxygen due to the rise in hotwell temp. Note that this means the condenser is too big... but given the widely vary water temperature trailerable boats may encounter, better too big than too small!

- Bart