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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PeteThePen1
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Condensing Circuit Logic

Post by PeteThePen1 » Mon Dec 24, 2018 1:57 pm

Hi Everybody

The Festive Season is upon us so it is time for us to do some thinking as well as feasting. For myself, I would like your thoughts on condensing circuits. The context is that I am nearly at the stage of installing a hot well and then plumbing things back together. My original plan has gone awry as the space I thought was available is not now that both boiler and engine are back in the boat. There is an alternative space on the opposite side of the boat providing I move the engine remote control levers (the engine sits ahead of the boiler).

What has worried me is whether I am understanding the condensing circuit correctly. This is a pictorial representation of my understanding:

Condensing_Concepts-small.jpg
Condensing_Concepts-small.jpg (17.39 KiB) Viewed 702 times

This is for a simpler condensing circuit with no air pump. The idea is that the back pressure from the engine exhaust should be enough to push the condensate up to the hot well. The little loop with a valve around the engine driven pump is to adjust the flow from the pump which could be a touch too much.

If that is conceptually correct I can press on with working out the pipe runs.

Best wishes to you all

Pete
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Re: Condensing Circuit Logic

Post by RGSP » Mon Dec 24, 2018 4:02 pm

Pete,
the back pressure on the engine will be around 1.2 psi for every 3 feet of lift, so certainly no need to worry about that. What could be a problem (and this is just me theorising [AND without a pint in my fist] ) is that the condenser could fill up with water, and therefore not have enough cooled area to condense the incoming steam. Someone else may have tried the set-up, and found a way round the potential problem - if indeed there is a problem. Mounting the condenser elements slightly higher than the hotwell might just be possible if the hotwell goes right down into the bilges.

Happy Christmas (I don't start saying that until Christmas Eve).

Dick
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Lopez Mike
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Re: Condensing Circuit Logic

Post by Lopez Mike » Tue Dec 25, 2018 1:26 am

I wouldn't plumb it that way. You would miss out on one of the very nice advantages of a hot well. Self regulation of the boiler water level.

The output of the feedwater pump should split in two directions. One into the boiler through the obligatory check valve and shutoff valve, and the other direction going through a seldom used valve and to a float valve in the hot well. Forget that regulating valve that you show bypassing the pump. The 'seldom used' valve can duplicate that function in the odd chance that the float valve should stick in the open position. Mine never has.

I find good float valves in farm supply outlets. They are used for animal watering tanks. My safety valve set point is around 140 psi and a common float valve has been in operation for years with no problems.

I don't know about other people's experience but I have had no problem with the engine output keeping the water out of the condenser. I have a keel condenser that is maybe 30cm. below the float valve.

My feed water pump has enough capacity to always have a significant flow though the float valve. In fact, I have it where I can keep an eye on it.

In case this is a new idea to anyone reading this, The theory is that since the total volume of water in the combined hot well and boiler is a constant except for leakage, safety valve operation and whistle use, regulating the hot well level automatically regulates the boiler level.

I find that when my system is reasonably leak free (all too seldom) and I'm half way attentive to the set point of my safety valve (again not often enough) and not spending half my time irritating everyone with the whistle, the water on my level gauge is so constant that I get careless about keeping an eye on it. Like many things, a mixed blessing.
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Re: Condensing Circuit Logic

Post by fredrosse » Tue Dec 25, 2018 9:45 pm

"the condenser could fill up with water, and therefore not have enough cooled area to condense the incoming steam. Someone else may have tried the set-up, and found a way round the potential problem - if indeed there is a problem."

If water is not flowing into the hotwell, it must collect in the condensing tubes of the keel cooler, then the heat transfer surface area is reduced (because steam has less surface area in contact with the sea water cooled condenser tube), less steam is condensed, and exhaust pressure increases. As previously stated, about 1 PSI pressure increase will force the water out of the condensing pipes and up into the hotwell. This is a self regulating condition, and exhaust steam will always build up enough pressure to force the water into the hotwell, even if the hotwell water level is a couple of feet above the condenser. This clears enough condensing surface area to allow the condenser to do its job.
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Re: Condensing Circuit Logic

Post by Lopez Mike » Wed Dec 26, 2018 12:05 am

A problem that I have not solved, or actually tried to solve, is that my keel condenser is about right for full throttle operation. At full snot the water coming into the hot well is awfully close to boiling. At cruise and below my hot well is, at best, tepid.

Since the condenser is a simple six foot or so hunk of 5/8" copper pipe, I don't see any easy way to vary the cooling area. Maybe two pipes in parallel and a valve to block off one of them when doodling?

Or is this somehow self regulating as Fred says? But if so, it regulates for clearing the water but not in keeping my hot well very hot!

This cannot be a new problem. Thoughts?
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Re: Condensing Circuit Logic

Post by DetroiTug » Wed Dec 26, 2018 2:54 am

Mike,

Parallel pipes would be the route to go.

You could have a spring loaded cylinder (hydraulic or pneumatic spring loaded single acting ram) operated ball valve beyond the throttle that controls one half of the condenser. High pressure opens the valve and both zones of the condenser, low pressure closes the valve omitting half the condenser surface area.


-Ron
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Re: Condensing Circuit Logic

Post by Lopez Mike » Wed Dec 26, 2018 3:22 am

Interesting idea, Ron. Maybe too complicated.

How about having a temperature operated valve on the output of my existing condenser? One that opens more on low temps coming into the hot well.

I wonder of there is such a thing on any automotive heaters? The old wrecks of cars I drive use simple manual valves.
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Re: Condensing Circuit Logic

Post by PeteThePen1 » Wed Dec 26, 2018 12:11 pm

Hi Everybody

Many thanks for your wisdom everybody. Thanks especially to Mike for spotting that I had omitted the loop back to the float valve in the hot well. That idea had been in my head at some point as my box of 'bits to fit in due course' include a diminutive float valve, but the idea must have floated away by the time I got around to drawing the logic. So here is the corrected version:
Condensing Concepts 2 (400 x 283).jpg
Condensing Concepts 2 (400 x 283).jpg (11.66 KiB) Viewed 602 times
Thanks also for discussing the issue of flooded condenser pipes and the height to which back pressure will push the condensate.

I have plumbed the exhaust line in 22mm pipe and then used 15mm for the line out of the condenser towards the hot well. I should say 'will use' as the run that I have already installed will have to come out as I routed it for the hot well on the right of the boiler but it will now need to go on the left. My thinking was that the condensed water will be a lesser volume than the steam so a smaller pipe should be adequate. The outlet pipe goes to the bottom of the boss and is perforated so that a very small amount of water needs to collect in the condenser to cover the outlet. The condenser is two lengths of (I think) 22mm pipe with a cast bronze boss at each end and an overall length of something over 1m. (If Greg is still monitoring the site he may recall the exact details. Currently I am on location with relatives so the boat is not to hand for measurement).

Regards

Pete
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Re: Condensing Circuit Logic

Post by Lopez Mike » Wed Dec 26, 2018 1:23 pm

The output piping of the condenser can be surprisingly small. In my boat with a 3 x 4 single, I am using 1/4" inside diameter copper pipe and the flow velocities are still moderate. I'm not sure but I suspect that too large of a pipe at this point might be a problem.
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Re: Condensing Circuit Logic

Post by fredrosse » Tue Jan 01, 2019 3:47 am

In larger power plants a common efficiency goal is to keep the condensate temperature as close as possible to the saturation temperature corresponding to the condenser pressure, be it whatever prevailing vacuum. In large sea water cooled condensers, the tube bundle may be on the order of 20ft x 20ft, x 55 ft long, with thousands of cooled tubes. As steam condenses on the tubes, condensate then trickles downward, spilling over many relatively cool tubes, the water is sub-cooled several degrees below the saturation temperature. This subcooling is named "Condensate Depression".

Power plant condensers collect the condensate in holding trays, and trickle the condensate thru a steam heated space, exhaust steam accomplishes re-heating the water close to the saturation temperature of the condenser's steam, then the condensate falls into the condenser's "hotwell" by gravity, where it is pumped out, back into the power cycle. Good practice is to achieve condensate depression of about 1 F.

On steam launches with a keel condenser the hotwell is mounted above the condenser tube(s), and the wet vacuum pump (or slightly positive condenser pressure where there is no wet vacuum pump) pushes the condensate up into the hotwell. Depressed condensate temperature can be re-heated with engine exhaust steam. Use a small heat exchanger, say exhaust steam from the engine thru a 1-1/4 inch diameter pipe, with a small coil of 1/4 inch pipe (carrying condensate from the condenser water outlet to the hotwell) running inside the exhaust pipe. The engine exhaust steam will eliminate much of the condensate depression, increasing plant efficiency a small amount.

This re-heating should take place downstream of the wet vacuum pump, as all pumps often have trouble handling hot water.
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