Feedwater pre-heating with a hotwell and vacuum pump

[fredrosse]

“How do boats that are not running a condensing system deal with the oxygen issue ? If they are pulling right thru the hull and pumping it to the boiler there is no way for the "de-oxygenate"..............not sure if that is a word............the feedwater.”

There are several ways to “deaerate” feedwater, the most basic way is to heat the water to saturation (212F, 100C at atmospheric pressure), which drives off virtually all of the non-condensable gasses, including Oxygen. Heating to about 150F (at atmospheric pressure) will drive out most of the Oxygen, but considerable amounts will remain. Vacuum Deaerators, again operating at a pressure close to saturated conditions will also remove virtually all Oxygen. In power plant practice, steam heated deaerators are typically installed in the feedwater system and continuously heat all feedwater to saturation before it enters the boiler. This however requires a feedwater pump that can handle the hot water.

I heat my feedwater to the boiling point (with a steam sparger in the tank) and drive off the Oxygen in all my feedwater tanks, which will usually allow a days boating without use of lake water. The tanks then cool down to ambient temperature, but the Oxygen is not present. A Nitrogen blanket would keep all Oxygen away, but I do not use this, as the Oxygen that creeps back into the water thru the tank vent is minor for most conditions I encounter.

Another method used in the power industry is to inject Oxygen scavenger chemicals into the feedwater. I understand that similar chemicals are available for small steam plants, however I have no direct experience with this in steamboat service.

Most steamboaters just live with the Oxygen and its corrosive effects. Cleaning and proper boiler layup practices are much more important, and usually sufficient, since our steamboats generally are in layup conditions more than 99% of their lives.

[fredrosse]

"The condensate is delivered to the hotwell via the vacuum pump on the inbound end of the condenser. "

The vacuum pump suction should always be on the outlet side of the condenser. The exhaust steam must pass thru the condensing tubes before the vacuum pump suction, so that the pump will not see exhaust steam, but only non-condensable gasses and liquid water condensate.

[fredrosse]

"As condensate moves in to the hotwell the float rises until it actuates the valve the directs the flow into the boiler.

I was hopeful that if I filled the system to the appropriate level on the water glass and in the hotwell, and understood the volume of water that would have to be "consumed" by the system to lower the level in the drum to the point that replenishment is needed that I could set the float valve appropriately to "automatically" replenish."

The hotwell float valve opens a bypass to reduce flow to the boiler in the system shown. Makeup water to the hotwell must be controlled by another float valve, or manually. Perhaps I misunderstand your statement?

[barts]

What I do with the Otter to introduce make up water is to switch the pump suction from the hot well to the makeup tank. This causes the hotwell to start filling immediately; if I forget to switch it back the hotwell spilling over on my leg reminds me . If I used a vacuum pump I'd likely just use a valve to suck additional water from the tank into the condenser inlet.

I've considered the issues with de-oxgenating feed water, but it seems unneeded for a boiler that is fired perhaps 50 hours/year if I'm lucky.
Since it doesn't freeze here, I just layup w/ a full boiler, tightly closed, treated with TSP.

- Bart

[preaton]

If I used a vacuum pump I'd likely just use a valve to suck additional water from the tank into the condenser inlet.

John York in his Ben Ma Shree II (spelling?) used a sight drip feed just this way - connected between makeup tank and condenser. The drip feed has a needle valve incorporated. Obviously the sight feed needs some modification so it does leak air into the system. He used it to counteract predictable losses such as steam atomizing burner etc. This was written up in Live Steam in the 80s

Paul

[JonRiley56]

Fred,

How much feedwater are you carrying ? I am looking at the hotwell as an "automatic boiler replensihment system". Am I wrong in that ? Is it just a way to capture the condensate and keep it warm ? Is adding water to the boiler a completely seperate exercise ?

jon

[barts]

The float valve actually serves to keep the hotwell level constant... the operators needs to see that the boiler water level has dropped due to steam leaks (largest is usually the whistle) and then manually admits more water into the system. Note that we generally don't use automatic boiler water level control.

- Bart