Backpressure

[Lopez Mike]

Right now I have a non-compound engine with no vacuum pump. The exhaust pressure forces the condenser output into the hot well which is very low in the boat. Perhaps a six inch lift.

I plan on building a new hot well which will require the engine exhaust to work against perhaps as much as 2" (mercury) of head. The top of the hot well will be almost two feet above the bottom of the condenser. I will be adding as much as 18" to the height of the lift.

I would rather not add the complexity of a vacuum pump. 100-150 psi in to the engine. A low restriction exhaust feed water heater on the output.

Given the simple engine, how much a performance/economy penalty do you think I will be incurring?

[dhic001]

Sorry I can't answer your question, but I'd definitely go for the 'complexity' of the pump. The gain in performance by doing so, will far outweigh and disadvantages. I can tell if my vacuum had gone from Zeltic's foredeck, where I can't see or hear the engine, the boat physically slows down, and then speeds up again if it comes back. Add a pump, solve the hotwell issue and gain extra performance.
Daniel

[Lopez Mike]

Daniel,

That's interesting. I've had several people opine that carrying a vacuum on a single stage engine wasn't worth the trouble compared to with a compound where the LP cylinder would benefit from having vacuum. And yet, your experience says that it helps.

Drat! Now I will have to start thinking about the best way to deal with this. Either a reciprocating pump that doesn't care about direction of rotation or a rotating pump with the necessary array of check valves to make it suck either way.

Oh well. There is a pump design in Modern Steam Launches that is put together from brass tubing that I really like. And since I have learned from experience that eccentrics can be built around sealed ball bearings so I wouldn't be adding another lubrication point, I guess I'll start looking for the scraps of tubing to build one.

Isn't it fun to cause trouble for your friends? (grins)

[barts]

A single expansion engine with a conventional link is exhausting steam at perhaps 50% of boiler pressure. 18" of water isn't even 1 PSI of extra back pressure, so you won't notice any effects from the increased height. Running a vacuum will provide some additional power, minus whatever it takes to run the pump. However, the effect on a single is much less than that of a compound, because the area of the cylinder exposed to the vacuum is much smaller.

- Bart

[fredrosse]

Just ran a few numbers, assuming a 3 x 4 engine at 400 RPM. 120 PSIA mainsteam pressure, atmospheric exhayst gives 4.08 HP. Same condition with VACUUM exhaust gives 4.72 HP, a rather healthy increase with the extra 10 psi of vacuum.

[DetroiTug]

I've been wondering about this on the twin. The feedwater heater in the exhaust manifold has been bypassed, I'm thinking the steam condensing may have been lowering the manifold/exhaust piping back pressure. Won't know until I try it.

-Ron

[steamboatjack]

Folks
I am wondering just what you float your boats in? and what you fire them with?
If you are in fresh water, and fire with solid fuel then I suggest you throw the condenser and associated complication in the bin. Exhausting up the stack makes a huge difference to steam generation and more than offsets any benefit of a vacuum.
If the water is a bit dodgy you may find a feed tank more convenient.
The only advantage of condensing without an air pump is the quality of the feed water. Any arrangement in the exhaust line be it a condenser or feed heater will only INCREASE the back pressure, so will a blast nozzle but you are getting a benefit from this.
An exhaust feed heater on a single expansion engine can however be an overall advantage.
If you are not floating in an alpine lake, then increase your blow-down sequence to compensate.


Regards
Jack

[Lopez Mike]

Funny how we have assumptions without knowing. I had not thought of the possibility of steaming in non-salt water. There are a few lakes around here I guess. When I look out the door all I see is Puget Sound and island after island.

I was concerned about boiler pH until I was reminded that the treatment doesn't go on through the engine and up the stack. There is perhaps a ten percent increase in power with an air pump on a simple engine. Possibly worth the trouble since I need the condenser anyway. Individual choice.

My hot well isn't high enough to have its water level at the boiler water level. Since I don't use an injector, the hassle of bleeding displacement pumps would preclude regulating them that way.

After fifty years of working and fooling around with steam I still learn new stuff on here.