Information on Surface condensers

A special section just for steam engines and boilers, as without these you may as well fit a sail.
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artemis
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Re: Information on Surface condensers

Post by artemis » Tue Jan 27, 2015 8:20 pm

Back at the end of the last century I owned a sweet, 26', ex-Navy motor whaleboat named Artemis. I live in Portland, OR, at the junction of the Columbia and Willamette Rivers - where the river temperature can be at 76F in the summer and below freezing in the winter. The first year I had her out I had problems maintaining vacuum even though there was plenty of keel condenser. A year or more brought the realization (I was born and lived in the Puget Sound area of Washington until I moved to Portland in 1980) that I was thinking "in Puget Sound" where overboard temperatures are 51F + or - 1F year around. What was OK there was not OK in Portland with the wide temperature differential according to the season.

I finally figured out that the really only good way to adjust to widely variable condensing water temperature was to regulate the flow. So an inboard, shell condenser was the way to go. I had some experience with this sort of thing during the two or so years that I was an adult engineering advisor on Sea Scout steamer Oceanid and felt that I should be able to put something together. That's what books are for!

I was looking for "how much cooling/heat exchange surface did I need?". Turns out that the old rule of thumb, posited in Steamboats and Modern Steam Launches, of 1 sq.ft. per HP for an inboard shell condenser and 1/2 sq.ft. per HP for an overboard keel condenser is reasonably accurate. I read the books to confirm it. The engine I had in Artemis was a 3" + 6" x 4.5" engine made by the builder and capable of 10HP. A good friend - who worked on hydraulic equipment a few years back - had an old-stock Borg oil cooler of 9.8 sq.ft. This condenser was provided cooling water from overside by a centrifugal baitwell pump with a 3/4" suction that could produce enough water (10 lbs. of water per 1 lb. of steam) to condense the engine exhaust to 146F when the overside water was 76F in September. And required less than 3A @ 12vdc. I already had 12vdc onboard for running lights, fathometer, and electric bilge pumps with a steam driven gen set that worked great.

A side benefit is Artemis steered much better without that 9' of 1-1/4" copper pipe stuck on the outside of the hull.

I found the numbers quoted above to be: 1). what I calculated from the information available to me in pre-internet days was spot on; 2). very easy to regulate via a globe valve on the cooling water discharge side of the condenser
Ron Fossum
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Lopez Mike
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Re: Information on Surface condensers

Post by Lopez Mike » Tue Jan 27, 2015 8:49 pm

Something to be said for being able to regulate the flow with a valve. Wish I could come up with an ingenious way to control how much keel condenser area I'm using depending on temps and load.

Personally I wouldn't be able to share my boat with an electric pump. Right now with the engine hooked up a bit and turning maybe 250-300, all is peaceful and quiet.
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Re: Information on Surface condensers

Post by artemis » Tue Jan 27, 2015 11:38 pm

Lopez Mike wrote:Something to be said for being able to regulate the flow with a valve. Wish I could come up with an ingenious way to control how much keel condenser area I'm using depending on temps and load.
Yeah, and since it's a centrifugal pump as I decrease the flow through the condenser by creating back pressure in the pump discharge, the power draw is less. :D Wonder if there's a power rebate available for that? :lol:
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Re: Information on Surface condensers

Post by barts » Wed Jan 28, 2015 12:54 am

I've been wondering about another approach - use a long condenser tube from a through-hull fitting, but bring the condensate back through the same tube via a small (say 3/8" or 1/2") tube inside the larger condenser tube.

You could size the condenser tube to be as long as needed in the warmest water you're going to float in, and the suction return will be
nearly the same temperature as the exhaust was from the engine. Obviously, this will only work on engines that exhaust at say 175 F or below, but still :).

If one is nervous, one could bring the condensate into the boat, through the air pump, and then back out through the same kind of contained tube and simply have a counterflow feedwater heater inside your condenser.

- Bart
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Bart Smaalders http://smaalders.net/barts Lopez Island, WA
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Re: Information on Surface condensers

Post by Lopez Mike » Wed Jan 28, 2015 1:09 am

Well, that IS cool. There must be somethings basically wrong with it. Hmm.

On my boat, the lowest point of the keel condenser is at the rear where my water exit tube dips into the condensate. And there is no condensate/vacuum pump.

So your idea is that I would have a double wall tube. The exhaust is introduced into the annular space at the rear and condenses at it flows forward. Then the condensate enters the inner tube at the front end of this assembly and flows back down to the rear end and is forced up to the hot well by the engine exhaust pressure as at present.

Maybe reverse the whole thing front to rear to make sure that only condensate exits to the hot well.

The idea being that the inner tube is a condensate re-heater. Or am I not getting it (as usual).

All sorts of niggly construction issues like soldering on some thingys to the inner tube to keep it centered.

Why hasn't this been done before? Or has it? Or have crude power plants like mine no been considered worth such sophistication?
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Re: Information on Surface condensers

Post by fredrosse » Wed Jan 28, 2015 2:46 am

Steam surface condensers in power plant (as well as large marine steam plants) practice are designed to discharge condensate no more than 1F below the steam saturation temperature within the condenser steam space. The degrees below saturation is called "condensate depression", and any significant depression indicates poor condenser performance. With that arrangement and condensate depression nearly zero, there is no chance of the exhaust steam heating up the condensate.

With typical "Keel Condensers" as used on steam launches, the condensate collects on the bottom of the sea water cooled condenser pipe, which causes significant condensate depression very often. Bart's idea to reheat the condensate back up to exhaust temperature with the small concentric pipe should work OK, provided there is thermal isolation between the larger Keel Condenser pipe and the smaller concentric condensate pipe. Also any subcooled condensate within the annulus space has to be kept away from the smaller condensate return pipe. Another method to accomplish the same result is to install a seperate condensate heater external to the condenser, with exhaust steam passing thru this exchanger.

With a typical inboard surface condenser the condensate depression problem is typically much less. The condensate forms on the surfaces of the condenser tubes, then falls thru the condenser steam space, and is heated nearly to saturatuon temperature. Condensate that finally falls into the lower condenser shell is not subcooled as it would be in the Keel condenser, because the surface condenser's shell is held very close to exhaust steam temperature, and is not cooled in any significant way.
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Re: Information on Surface condensers

Post by Lopez Mike » Wed Jan 28, 2015 3:57 am

Umm. It's late. I think I will sleep on this. Thanks Bart and Fred. I think.

I will not be back from Baja to tinker with things until Spring so I have plenty of time to sort all this out.
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Re: Information on Surface condensers

Post by S. Weaver » Wed Jan 28, 2015 3:16 pm

fredrosse wrote:Steam surface condensers in power plant (as well as large marine steam plants) practice are designed to discharge condensate no more than 1F below the steam saturation temperature within the condenser steam space. The degrees below saturation is called "condensate depression", and any significant depression indicates poor condenser performance. With that arrangement and condensate depression nearly zero, there is no chance of the exhaust steam heating up the condensate.
Fred, this first paragraph is intriguing, and I don't pretend that I am necessarily following it. Could you expound on this using some LP exhaust examples? Thanks.
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Re: Information on Surface condensers

Post by fredrosse » Wed Jan 28, 2015 5:29 pm

In any steam cycle, the condenser represents the lowest temperature in the cycle, and the objective is to condense all of the exhaust steam into liquid condensate. From that point on, the condensate must be again heated up to saturation temperature of the boiler before boiling begins.

If we have a 100 PSIG steam plant, saturated steam temp is 338F. Running a condenser at 20 inches Mercury Vacuum, saturated condensate is at 161F. If there is only one degree F condensate depression, the condensate is removed from the condenser at 160F, and is eventually pumped into the boiler. Before boiling can begin, the condensate has to be heated up to 338F, a temperature rise of 338 - 160 = 178F temperature rise. This can be done with feedwater heaters, and/or economizer, and final heating within the boiler proper.

If the same plant over-cools the condensate, say down to 130F (condensate depression of 31F), then before boiling can begin, the condensate has to be heated up to 338F, a temperature rise of 338 - 130 = 208F temperature rise. The amount of heat required has increased by 17%, and this heat requires extra heat transfer surface area. This can be done with an exhaust steam feedwater heater (as Bart suggests with the surface area inside the keel condenser), or a separate exhaust steam feedwater heater. Either of these devices can work OK, and the exhaust steam heating source is essentially "free heat", so no penalty on the plant's efficiency. With a properly functioning condenser, and virtually no condensate depression, then condensate re-heating is not required, and any extra heat transfer device is not needed.

If the condensate temperature is depressed and there is no re-heating to saturation with exhaust steam, then plant efficiency will suffer. In this case an economizer needs extra surface area, plus extra heat from the hot flue gas exhaust, and some final heating within the boiler proper, consuming some extra fuel because the temperature rise to get the condensate up to the point of boiling is somewhat larger than if condensate depression did not exist.

So much for the thermodynamics of condensate depression, now another relevant point: pumping of the condensate.

Pumps to handle saturated condensate have difficulties, especially reciprocating pumps. If the condensate is close to boiling temperature, and the reciprocating pump takes a suction stroke, then some of the liquid can flash to steam, and the pump looses suction. Here condensate depression helps the pump to function OK because flashing of condensate is much less likely.

In large plant practice, the condensate pumps are located with their suctions 15 to 20 feet below the condenser hotwell, so gravity gives some extra "head" at the pump suction. In marine installations where this much of elevation difference is not available, special low speed centrifugal pumps provide the initial boost in pressure, and are mounted as low as practical.

For steam launches with absolutely no room for low pump mounting (not possible to mount the pump below the keel condenser), some condensate depression is necessary to have the pump workable.

With a keel condenser the ideal arrangement is to use an exhaust steam heat exchanger downstream of the condensate pump (wet vacuum pump typically) to recover as much "free exhaust steam heat" as possible.

A surface condenser located inside the hull can reduce condensate depression, and often the condensate pump (or wet vacuum pump) can be mounted low enough (below the shell of the steam surface condenser) to function properly.
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Re: Information on Surface condensers

Post by malcolmd » Wed Jan 28, 2015 7:03 pm

I am really glad I asked this question, as it has provided a valuable education... Thanks to all the responders and to Fred for a clear explanation of a number of complex issues....I am certainly going for the inboard solution, and can see the need for some more thermocouples!!!
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