Condensing Circuit Logic

A special section just for steam engines and boilers, as without these you may as well fit a sail.
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Lopez Mike
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Re: Condensing Circuit Logic

Post by Lopez Mike » Tue Jan 01, 2019 7:37 am

Right now my hot well temperatures are so near ambient that it's not worth the trouble but I wonder whether when the temperatures climb higher that insulating the hot well might be worth the trouble. Likewise, would insulating the condenser to hot well piping be worth the bother.

Also, I suspect that my hot well volume is much greater than needed. I have not thought out the tradeoffs involved. Once there is room for the float to operate, why go larger?
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Re: Condensing Circuit Logic

Post by Mike Rometer » Tue Jan 01, 2019 9:57 am

Larger (but not too large) would mean less time worrying whether there is enough to cover losses, perhaps?
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Lopez Mike
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Re: Condensing Circuit Logic

Post by Lopez Mike » Tue Jan 01, 2019 3:52 pm

True. One of the nice things about my VFT boiler is the time lag between my stupidity and a catastrophe.
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Re: Condensing Circuit Logic

Post by RGSP » Tue Jan 01, 2019 5:06 pm

My boat has a really tiny hotwell - nearer 1 pint than 2, without a float valve, and a 3 drum water-tube boiler. It does have a regulation mechanism which does almost the same thing as a float valve, in that when the level in the hot well drops, the feed pump takes in some air, and pumps out less water. It self primes and clears, and reliably pumps full volume again as soon as the intake sees full liquid water. It works very well when steaming steadily, with a 1/4 turn manual valve to switch the feed pump intake from the hotwell to a largish reserve feed water tank. The whole caboodle is just by the helmsman's right ankle, and is easy to see and adjust.

To be honest I think a larger hotwell and float valve may be a slightly nicer option, but mine is not bad and does work. With the feed-pump taking water from the reserve tank, it takes about 5 minutes while cruising for the boiler sight glass to go from bottom to top. If the boiler is anywhere near the top of the sight glass when cold, the combined effect of water expansion and bubbles puts extra effective volume into the (low volume) circuit, and the hotwell overflows via a pipe into the bilge for a few minutes. I think that loss of circuit water is the main snag of the small hotwell volume, but if the volume was larger, the boiler might be over-filled inadvertently, even without pumping from the feedwater tank.

The tiny hotwell certainly comes up to temperature rapidly, though I also have a feed-water heater bolted to the front boiler casing plate (under the insulation) on the high pressure side of the feed pump. Goodness knows whether the feed heater has a measurable effect on efficiency, but a normal day cruising at 6 mph uses around 12 litres of kerosene.
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Re: Condensing Circuit Logic

Post by barts » Sun Jan 06, 2019 8:11 pm

The primary reason to keep the hotwell temperatures high in larger plants is to reduce dissolved oxygen, which is bad for boilers. Subcooling the condensate is inevitable with our small plants running at varying power levels, and given the fact that we're going to shut down soon anyway makes the dissolved oxygen concerns pretty moot. The exhaust steam has plenty of energy to reheat the condensate back to nearly exhaust temperature - the vast majority of the energy in the exhaust steam comes from the vapor->liquid transition. Say we drop the condensate from 200 F to 60F (easy with cold water and light loads). To reheat a pound of condensate back to 200F would take 140 BTU - but the condensate released nearly 1000 BTU when it when from steam to water. So don't worry about a cold hotwell - just enjoy the good feed pump performance, and use a feed water heater to scavenge some of those BTU before they get lost in whatever you're floating in......

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Lopez Mike
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Re: Condensing Circuit Logic

Post by Lopez Mike » Sun Jan 06, 2019 11:19 pm

Reassuring advice. Especially when it agrees with my overriding drive which is sloth. Now all I need to do is build and install that feed water heater.
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Re: Condensing Circuit Logic

Post by DetroiTug » Sun Jan 06, 2019 11:50 pm

Quote: "Now all I need to do is build and install that feed water heater"

:lol: :lol: :lol:

(For those that might not understand the humor, he's been intending to install a feedwater heater for about 10 years now. )

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Lopez Mike
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Re: Condensing Circuit Logic

Post by Lopez Mike » Mon Jan 07, 2019 12:28 am

Hey! I haven't had this scow for ten years yet!

But at the rate I'm completing projects it well may be ten years.

What's that old hack? Better to proceed hopefully than not to procede at all. Or something like that.
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