A Hotwell Filtration System

Steamboats come in two flavors: condensing and non-condensing. If you plan to build a steamboat with a condenser you'll have to find a way to separate the impurities from the condensate before it is returned to the boiler. Key to this is the hotwell, where condensed exhaust consisting of water, steam oil and other engine additives is made into pure water once more, by first passing it through a filter. If the filter doesn't capture 100% of the impurities, major problems can occur, ranging from poor performance to a catastrophic burn-through in a water tube within the pressure vessel.

In its youth one of the many things that prevented my steam launch Pegasus from attaining high speeds was foaming in the boiler. While at one B&W Steamboat Regatta back in the '80s I spent some considerable time looking at other hotwells to figure out what made a good one different from a bad one like mine.

The cleanest hotwell I saw there used shredded polypropylene as a filter medium. This material has two very special properties: it has an affinity for oil, but it won't hold on to water. This stuff is somewhat hard to find in small quantities, but an adequate substitute can be made from Oil Sorb, a product used to clean oily bilges in power boats. Oil Sorb is just the brand name I came across; others will do just as well, provided the stuff inside is genuine polypropylene. I found mine in the West Marine catalog but there are doubtless other sources.

Oil Sorb comes in plastic mesh "sausages" that look for all the world like elephant tampons. Simply take one apart, cut the sheet material into strips and then cut the strips into pieces an inch or so long. Once the stuff is in a form that will be useful, plop it loosely into a close-mesh wire basket (1/4" galvanized chicken wire bends nicely and it's rust-proof). The trick is to get the water to flow through, rather than over the filter material and the key to that is the mesh tube that is inserted into the middle of the filter material. The sheetmetal divider located towards the right side of the tank is an essential added precaution as well. To get the big picture, take a look at my sketches below.

This first sketch illustrates my original design, which worked poorly. Once it flowed out of the condenser, condensate was free to trickle over the filter media and percolate down the sides of the filter basket, bypassing the filter media almost entirely. I had only to run a finger along the walls of the hotwell to feel the oily film that had deposited there. That, plus the foaming in the sight glass and priming in the engine were evidence enough that the filter wasn't doing a proper job.

Side View In this original design I attempted a multi-step filtration scheme, using a variety of materials which I thought would get out every imagined form of contamination in successive steps. None of it worked, of course...

Modified Design: The condensate now has only one path it can take, the correct one down the wire mesh dispersion tube that is surrounded by Oil Sorb. Runoff can no longer occur.

Side View: Note also the divider at the right hand side of the tank. This is a piece of sheetmetal, held in the tank with a bead of RTV. As long as the water level within the tank is below the top of this barrier, any oil that might manage to escape the filter media from, say, unexpected rolling of the vessel (shit happens, don't ya know...) can only float harmlessly on top of the water on the filter-side of the barrier. Pure water passes through half a dozen 1/2" dia. holes that are punched into the bottom of the divider, so that clean water has a path beneath.

Here's a shot of the installation in my steamboat. The view is from the bow, looking aft. The condensate delivery tube is to the left and it drops condensate down the dispersion tube in the middle of the filter basket, which can be seen beneath it. The wire mesh basket's lip has been bent over, so that the basket just hangs on the edges of the hotwell; this makes removal and replacement a snap.
A hotwell can be fitted most anywhere it's convenient (I've seen them in bow cuddies, under stern seating and everywhere inbetween). In my case the best location was to port and just ahead of the pilot's station, where I could easily reach the feedwater bypass valve (easier to see in the next photo).
Side view The condensate delivery tube is in the foreground. The basket containing the Oil Sorb is just behind it. To the left is the feedwater bypass valve which connects to the float valve. The arm connecting the float to the float valve has a bend in it, so that it can range freely up and down in spite of the top of the sheet metal barrier that separates the filter from the pick-up end of the tank. A different location for the float valve might solve this problem, but what I've got works well enough. (The sloping bright line above the tank is a bellrope that runs from my whistle valve to the pilot's station.)
Here's the filter basket all by its lonesome. Yes, it's time to change the filter material. Normally the basket would be packed right up to its brim, but quite a bit of settling occurs once it has been in service for a few days' worth of steaming. The best plan is to make a spare filter basket and have it at the ready. The rather grubby 1/8" thick, perforated brass plate prevents breezes from flinging the clean, loose filter material all over the boat.

Return to Articles Index
Return to My Steamboat Pegasus