Hot wells

A special section just for steam engines and boilers, as without these you may as well fit a sail.
ccdewitt
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Hot wells

Post by ccdewitt » Thu Sep 24, 2020 12:06 am

A question: I am building a launch with a 4" x 5" simple, condensing, air pump, operating at 125 psi with no superheat. Anticipating operation not to exceed 400 rpm maximum but generally lower as I am not in a hurry. What is a reasonable hot well volume? Thanks in advance.
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barts
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Re: Hot wells

Post by barts » Thu Sep 24, 2020 12:41 am

If you're using a float value so you have automatic bypass control and you pump water into the boiler for make-up, the necessary volume is really room enough for your float and bypass valve. Smaller hotwells absorb less air.

If you add make up to the hot well manually, you'll want enough volume so that you can add enough water w/o overflowing the hot well.

If you plan on a manually controlled bypass (only do this if your boiler is large so you have time to react, and even then a float is better), you'll want plenty of volume (say 2x boiler volume of high water - low water )since your control will be more erratic. Sucking air into the feed pump because you got distracted and forgot to open the bypass is a PITA.

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Re: Hot wells

Post by TahoeSteam » Thu Sep 24, 2020 3:20 am

Maybe 1/2 or 1x the volume of the boiler? Good question. I'm not sure there is a hard and fast rule for that... Maybe the "Bible" (Steamboats and modern steam launches) or one of Audel's writings would have a suggestion? I would think tall and narrow would reduce possibility of oxygen entrainment over low and wide, no matter what the volume was. Add a cover or smother the water's surface with something (oilsorb pads?) to further reduce that risk.
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Lopez Mike
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Re: Hot wells

Post by Lopez Mike » Sun Sep 27, 2020 4:42 am

On my little VFT-40 my hot well holds about couple of gallons max. It seems quite large enough. Relatively tall and narrow. If I forget to open the boiler feed isolation valve (about once a year!) it takes several minutes for the hot well to fill up the remaining three or four inches to overflowing and tell me to wake up. By then the water is well down on the glass and the rush to correct things is co-ordinated with my very red face. I keep forgetting to make a check list.

I have considered a safety valve on the output of the feed water pump with the relief port plumbed to a nozzle aimed at the helm position.

I cannot imagine operating my boat without the hot well float. Perhaps even unsafe to do so.
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ccdewitt
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Re: Hot wells

Post by ccdewitt » Mon Sep 28, 2020 2:47 am

Thanks for the comments
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Re: Hot wells

Post by Kelly Anderson » Sat Oct 03, 2020 10:13 pm

I would say it is more a case of how much room is available for it in the boat. More capacity is great, but not at the expense of taking up space better used for something else.

My 23' x 6' launch has a hot well that measures 7" x 12" x 23" because that was the convenient space for it. It is equipped with an automatic float valve, and is set up for paper towel filters.

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Last edited by Kelly Anderson on Mon Oct 05, 2020 3:19 am, edited 5 times in total.
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Re: Hot wells

Post by dampfspieler » Sun Oct 04, 2020 2:38 pm

Hello Kelly,

a very attractive job. Can you offer some more details like inner and pipe layout, type of filters and seller of them?

Best Dietrich
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Re: Hot wells

Post by Kelly Anderson » Mon Oct 05, 2020 3:29 am

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The filters are common Bounty brand household paper towels, held in place with rubber bands.

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The plumbing from compartment to compartment is copper tubing. The openings can be seen in the bottom of the hotwell of my previous launch. The o-ring on the filter slides into the copper tube to seal one compartment from the next.
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Re: Hot wells

Post by fredrosse » Tue Oct 06, 2020 5:27 am

Consider the two relevant hotwell tank vs. boiler free surface areas.

One issue that should be considered is the hotwell tank air/water interface area compared to the boiler steam/water interface area. When steaming along with the feed pump bypassed, the water level in the boiler will go down, with a corresponding rise in the hotwell level. If the two water surface areas are the same, then approximately a level change in the boiler will result in a similar level change in the hotwell.

If the hotwell tank free surface area is much larger than the boiler free surface area, then a very small change in the hotwell level will result, and the boiler water level could get seriously low before the hotwell float valve would properly respond. Generally this would be more of a problem with water tube boilers, especially an Olfeldt type, where a small boiler inventory mass change can rapidly deteriorate the boiler water level.

This issue is somewhat independent of the boiler and hotwell water inventory volumes. The hotwell storage inventory should hold enough water for several minutes steaming, in USA large power plant practice this is 3 to 5 minutes usable hotwell storage volume at full steaming rate.
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Lopez Mike
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Re: Hot wells

Post by Lopez Mike » Wed Oct 07, 2020 1:59 am

Fred,

Thanks for the clear explanation. I've gone through this many times with interested spectators as well as boat owners. Your way of explaining it will help.

Another way to rationalize the hot well float operation is to say that the sum of the water in the boiler and hot well is a constant subtracted from by whistle operation, leaks and the like and added to by replenishing from reserve supplies. That said, it is then clearer that when you regulate the level in the hot well you are necessarily regulating the level in the boiler. The levels move in opposite directions of course.

It's great fun to manually regulate the level on the bench when testing and fussing about but I regard it as bad seamanship to have your attention diverted from navigation when in motion. Few of us employ a power plant engineer to deal with such things.

Like an autopilot for long periods of cruising in a pleasure boat, once you try it you'll understand. You'll never go back.
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