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[southsteyne2]

Hi Guys as there are very few live steam boat models around my area I was hoping to get some comparison on steam consuption so just a brief idea of the engine and boiler ,scratch built twin cylinder slide valve 1/2 in bore 3/4 stroke runs very well (boiler 4 in dia x 7in long scotch marine return tubes gas fired full auto boiler control and  at 500 rpm no load uses 1ltr water per hour there are no steam leaks so just curious if this is reasonable usage or maybe excessive.
John

[AlexC]

Hi John,
 
This would be consistant for your engine/boiler running at a little under 80psi. with no losses... unlikely!!
 
If you can tell us the pressure you are set up for then it would help in making an informed assessment regarding your water usage.
 
best regards.
 
AlexC

[southsteyne2]

Thanks Alex boiler working pressure 35.40 psi safety valve set to 60psi
cheers

[AlexC]

Hi John,
 
Ok your engine having a 1/2" bore x 3/4" stroke means: - 
 
cylinder volume to be filled with steam = 0.1472625 cu.ins.
 
since you have 2 double acting cylinders this requires: -
 
4x 0.1472625 cu. ins per rev.
 
at 500 rpm this would mean 294.525 cu. ins steam per minute.
 
In1 hr this would be 17671.5 cu. ins steam required.
 
At 35.4psi 1 cu. ins of water will produce 514.5 cu.ins of steam.
 
So: - 17671.5/514.5 = 34.3469 cu. ins water per hour required to be converted to steam.
 
1pnt of water = 34.677 cu.ins.
 
so you need to convert 34.3469/34.677 = 0.99048 pints of water to steam.
 
1 ltr of water = 1.76 pints and you only need to convert 0.99048 pints (or approx 56% of your used water).
 
The remaining 44% being lost/wasted in some manner.
 
At the relatively low pressure of 35psi it is entirely possible that a large proportion of this loss is due to Condensing, especially if your pipe runs are long-ish and/or unlagged.
 
Unloaded running can also be a contributory factor since there will be little resistance within the cylinder, possibly causing a pressure drop... which will result in considerably more condensate being formed within the engine than when it is loaded.
 
Is your exhaust steam very wet?
 
If you repeat your test at the same pressure and rpm but with the engine driving a prop in water (try the bath tub when swmbo is not around ) then you may well find that your water consumption is a little lower... there will always be some loss... however... 44% is quite a lot... 15% - 20% would be more acceptable.
 
Hope this is of some help.
 
Best regards.
 
AlexC

[southsteyne2]

Thank you for your valuable input and will keep this formula for reference ,there is some water at start and very little after a few seconds running and I can live with a litre an hour as that is the capacity of the on board tank ,and as mentioned I just needed a comparison of similar plants
Thanks
John

[flashtwo]

Hi John,
 
The "Gas Burner Jet Heat Output" thread has been discussing steam usage over the last few days.
 
I'm using a monotube boiler set up with a Stuart D10 3/4in bore, 3/4in stroke double cylinder, double acting and a calibrated ram feed pump, so I know fairly acurately the feed water consumption versus pressure and engine RPM etc.
 
I've always been puzzled with the discrepancy between the theorectical steam consumption and the measured feed flows.
 
The conclusion that I've come to is that the steam is very wet with less than 25% of the feed actually being available as usefull steam at the cylinder, despite having new piston "O"-rings. If I can reduce the wetness of the steam I would reduce the feed requirements or, in the case of a conventional boiler increase the running time between refills.
 
Ian

[southsteyne2]

Thanks Ian perhaps you can give me some idea of the usage of your stuart d10 , I have a superheater on the back end of my boiler not sure if it helps so now looking at heating the feed water to maybe cut down on condensation any comments welcome
Cheers
John

[flashtwo]

Hi John,
 
The system I use pumps water from the pond for boiler feed, which is fine for most locations. If I'm doubtful, I put a 1 litre bottle of feed water in the boat and this gives me about 25mins steaming with the D10, though not at full revs of 550 but more like 350RPM.
 
I have an oil / condensate separator on the boat through which is a feed heater coil which raises the feed temperature from about 10degC from the pond to 96degC into the boiler. This may sound good, but doesn't improve the efficiency very much since most of the effort is required in adding latent heat to the water to turn it into steam. It is more useful in condensing the engine exhaust and separating the oil.
 
I think the wetness of the steam is more apparent with my system, because I accurately measure the feed flow to the monotube boiler for control purposes, and thence know the discrepancy between the theory and practice. It would be interesting to know from people with conventional boilers their system's actual feed consumption and possible steam wetness.
 
From earlier threads, if the steam is too dry, then the displacement oilers don't work so well because the steam doesn't condense so easily.
 
I will experimenting some more this season in order to increase the superheat, because the performance is enhanced so much. With a monotube boiler there is a fine line between stable steam temperature and temperatures too high for the piston  silicone "o"-rings. I've obtained some rather exprensive "Fluorosint" to make some higher temperature and harder wearing piston rings. Fluorosint is a type of PTFE which is chemically bonded to mica and is specifically used for piston rings that slide over steam ports.
 
I hope this info is of some use.
 
Ian

[flashtwo]

ps.
 
I obtained my fluorosint from M_Machine Metals on http://www.m-machine-metals.co.uk/mm/website/metals_menu.html
 
Ian.