building a steam condenser

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lighthousekeeper
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building a steam condenser

Post by lighthousekeeper » Wed Jul 01, 2015 4:10 pm

Hi Folks, just want to pick your guys brains on how to build a condenser, From what I understand the exhaust from engine goes thru the bottom of the hull into a copper pipe which is by the keel. now at what point does the black iron pipe from exhaust switch to copper? what kind of diameter do I need and how long of a piece does the copper pipe have to be? now once the steam turns into condensate how does it enter the hot well, just by the force of the steam ? also what size should the hot well be ? and were is it usually located ? Is their any other way of reusing the steam exhaust besides this method? thanks once again ! j.z.
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Re: building a steam condenser

Post by fredrosse » Thu Jul 02, 2015 3:27 pm

Steam condensing generally can take three forms on small launches:
1. "Injection Condenser", which is Direct Contact Steam Condensing. This system was used on almost all of the 19th century riverboats and excursion steamers that operated in fresh water. Exhaust steam enters a chamber (operating at vacuum conditions), about the size of the engine's steam cylinder. Cooling water ("Injection Water") from the river which is forced into the chamber by atmospheric pressure, is discharged as a spray into the exhaust steam space, condensing the steam. The condensed steam, along with the cooling water, is then pumped overboard by a large reciprocating pump. This pump, usually driven by direct connection to the main engine, also removes any air that leaks into the system, thus allowing high vacuum exhaust conditions. This system is only for operation in fresh water, and no condensate is recovered for re-use. The Injection pump is large, as it must pump both the condensate flow, plus the injection water, which is 20x to 80x the condensate flow. When the pump stops, injection water will continue, flooding the condenser, and possibly the engine cylinder, so a float valve to prohibit this is required.

2. The "Surface Condenser" uses a shell & tube heat exchanger for condensing exhaust steam. This arrangement is used for normal power plant practice, as well as historic large steamship practice. This type of system is also used for several steam launches. The heat exchanger here is generally named a "surface condenser", with exhaust steam entering the shell side of the condenser, and sea water is circulated thru the tubes. The sea water intake can be arranged with a "scoop" inlet, so the ship's forward motion forces the cooling water thru the tubes, although a small circulating pump is typical. Condensate is collected in a chamber attached to the bottom of the heat exchanger, and a combination water/air pump (named a "wet air pump") removes the condensate as well as air in-leakage.

In large plant practice, the storage chamber forming the bottom of the heat exchanger is named the "hotwell", and should store at least several minutes worth of water inventory for boiler feed. In small launch practice there is often no storage inventory within the heat exchanger, and the wet air pump continuously removes all condensed steam (condensate), feeding a small tank, which we steamboaters call the "hotwell". Sizing criteria is the same, with at least 10 minutes full steaming capacity the minimum recommended size. My "hotwell" holds one cubic foot of water, enough for an hour's full steaming. Makeup water feed, and chemical additives, as well as oil separation from the condensate often is arranged in the hotwell.

3. The "Keel Cooler" is most popular, a copper pipe running along one (or both) sides of the keel, with exhaust steam entering the beginning of the circuit, and condensation back to liquid water along this single pipe path. The sea water outside the copper pipe provides the cooling mechanism and works OK with the boat tied to the dock, and works much better with the boat underway with sea water circulating past the keel cooler. The endpoint of the copper pipe is the suction connection for the "Wet Air Pump". This endpoint should be at the lowest elevation so gravity forces the condensate toward the pump suction. Beyond that, the arrangement is the same as described above for the surface condenser wet air pump and the hotwell.

In fact, the "Keel Cooler" functions the same as the surface condenser, they are technically the same with respect to the condensing mechanism. An advantage is that there is no need for a cooling water circulating pump, a disadvantage is that the keel cooler may be prone to damage with boat groundings, or catching submerged string or lines, etc. The thru-hull connections on the keel cooler also have to be suitable for the high exhaust temperatures of the engine.

The size of all this stuff depends on the size and steaming rate of your engine. What details do you have here?
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Re: building a steam condenser

Post by Maltelec » Sun Jul 26, 2015 10:52 pm

In terms of size, you can never have it too large.

Given an ideal air pump, which can create an absolute vacuum, then the limiting factor for the vacuum it can create is dependant on the temperature of the water entering it.

E.g. if your condensate was 60C, the vacuum generated cannot be greater than 24 inch Hg as the water condensate will boil into steam.

There is however a reasonable limit; At around 25C and below, there is very little gain and a theoretical 29 inch Hg can be obtained, the absolute limit being 29.9 inch Hg, dependant on the weather and altitude.

So the general rule of thumb is to have a condenser large enough so that in the worst conditions you have a return temperature of 25C or less. That way any vacuum below 29 inch Hg must be down to the air pump or other limitation.

Note: I will use the term "Vacuum" to indicate a pressure lower than atmosphere. A Higher Vacuum means a lower pressure.

As for the size of pipe:

http://www.1728.org/flowrate.htm

Velocity is tricky with a condenser as it is constantly slowing down until all the steam has condensed. And of course, steam has a much greater volume than water.

Here we need to know the amount of steam used by the engine. For instance a 15HP triple may use 600 lb/hour of water. This is 270 kg/hour = 0.075kg a second. However because we are running at 29 inch Hg, this steam has expanded.

At -0.5 Bar, an estimated pressure on the LP exhaust (Vacuum) saturated steam occupies 3.4 meters cubed/kg, which is constantly expanding. However this enters a pipe which is at 29 inHg. This causes the steam to expand to fill the space in the exhaust pipe above the water. This will also push the water into the air pump, the exhaust being at a higher pressure than the condensate. At this point the larger the space above the condensate water the better, as any bottle neck will restrict the engine's exhaust steam from escaping.

This steam will quickly enter the cool part of the condenser, occupying the space left by the air pump removing the condensate. This steam its self will condense, lowering the pressure in the pipe and allowing more steam to occupy that space, which its self will condense. As long as the condensate water is removed the steam will continue to condense and keep the vacuum pressure up.

The engine exhaust above the hull of the boat is really a receiver in this case. Taking the exhaust steam out of the engine and holding it until it can condense. For this reason it cannot be large enough. However in the example of a 15HP Triple we have found a 2" BSP pipe 18" long works fine. This is 56.7mm internal diameter.

This pipe enters into a copper pipe under the hull of the boat. You have to imagine that at this point the exhaust is still steam greatly expanded because it is at 29 inHg. The input to the condenser must be able to take a large volume of steam. I suppose ideally you want the 2" exhaust to reduce logarithmically along its length until it reaches the air pump.

Realistically we see that the large exhaust pipe above the hull can accommodate the excess steam and feed it slowly into the smaller (22mm diameter) condenser pipe fitting, remembering that the exhaust comes in bursts.

From this point on, all we need is for the steam/condensate to reduce in temperature until it reaches our ideal 25C. Any further cooling is a bonus for the air pump, but a drag on the hull.

At 0.075kg a second steam usage out the boiler, we will see 0.075kg a second being pumped out of the air pump. This just happens to be 75ml a second, or 1 Litre every 13 seconds, which feels about right.

We don't want the air pump to be forcing the water out, we want to give it an easy time. If we used a 22mm pipe into the hotwell, the velocity would be:

velocity = 20mm/second. Sounds about right for a larger engine.

Of course if you have a steam plant half this size, a 15mm pipe will give you roughly the same. This is under the worst sustainable conditions, if you were to take more than the boilers capacity then you will push out more water, until you run out of steam. With a 600 lb/hour boiler at full power you will also be constantly shovelling coal.
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Re: building a steam condenser

Post by Lopez Mike » Mon Jul 27, 2015 7:34 pm

You have heard from the experts. I have no quibble with their information, data and calculations. For a fairly sophisticated power plant, it's all good advice. There are, however, a number of us out here steaming along with somewhat simpler arrangements.

The advantages of having a vacuum pump to empty the condenser on a small simple (single expansion) engine are rather small and the tradeoffs of theoretical efficiency v.s. simplicity are worth examining.

My craft is fairly near the limits of simplicity. My friends are less kind using phrases like crude and primordial.

My engine is a single cylinder 3 x 4 and it exhausts into a simple keel condenser (3/4" copper six feet long) and the condensate is moved up to the hot well by the exhaust pressure of the engine. The elevation is less than two feet so we are talking about perhaps 1 p.s.i. of back pressure on the engine.

The advice that you can never have too much condenser capacity is true when you are trying to get the highest (lowest?) vacuum in order to help the low pressure cylinder of a compound. In the case of a power plant with no condensate pump, too much condenser area makes for a cold hot well. Mine is just right for full throttle operation but in normal use (piddling about) my hot well is rather tepid. Around blood heat perhaps

This means that I am often pumping rather cool water into the boiler and there is some possibility of oxygen in the feed water though I do keep a bit of T.S.P. in the system to scavenge any oxidants. I probably need a feed water heater.

Probably the only detail I might add to the previous construction advice would be to use a separate tube at the output of a keel condenser arranged such that it picks up the condensate from the lowest point of the condenser.

It is very easy to become so obsessed with efficiency issues that you loose sight of many other joys of steaming. More gadgets and valves are fun but not everything.
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Re: building a steam condenser

Post by Maltelec » Mon Jul 27, 2015 10:23 pm

Running with a condenser but without an air pump to create vacuum is not common in the UK, but it does add a fair bit more complexity.

Another thing to think about is a hot-hotwell.

If your hotwell is pretty hot, the vacuum generated by the feed pump by the inertia of the water and any pressure drop through check valves may be enough to flash the water into steam inside the feed pump. This can also release any air in the water, and you end up trying to pump a little bit of water with steam and air.

This effect can be reduced by larger pipes and slower feed pumps, and a cooler hotwell.
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Re: building a steam condenser

Post by S. Weaver » Fri Jul 31, 2015 4:22 pm

Thank you, Simon.
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Re: building a steam condenser

Post by Lopez Mike » Fri Jul 31, 2015 4:27 pm

Perhaps I misunderstand but how does eliminating the vacuum pump add complexity?

Seems like just the opposite.
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Re: building a steam condenser

Post by dhic001 » Sat Aug 01, 2015 6:00 am

I don't understand why anyone would think having an air pump adds complexity to the system, its just one more pump and a couple of connections. Sure it may be a bit of a process building it, and fitting it, but once its done, it just goes without any attention. The water from the condenser has to go back to the tank somehow, if a pump is in between helping it, it doesn't seem too much of a complication.

Personally, having run with and without and airpump, I wouldn't consider being without it. I once ran Zeltic without her airpump, and the engine ran horribly. it banged and crashed and sounded like it was going to fall apart. The boat didn't perform all that well either. I refitted the air pump, and then engine became lovely, quiet, smooth and the boat went well. I can only assume the extra work of pushing the water to the hotwell under exhaust pressure put far too much load on everything. As soon as it gained the vacuum from the pump it went back into equilibrium.

Oh, and if for some reason the air pump acts up, I can tell from anywhere on the boat if underway, because it slows down immediately!

My 5 cents worth,

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Re: building a steam condenser

Post by Lopez Mike » Sat Aug 01, 2015 6:13 am

Is your engine a compound? Running vacuum makes good sense in that case. But with a single expansion engine that bit of reduced back pressure is hardly worth the effort.

20" equals 10 psi. Important to a low pressure cylinder that is often only seeing a receiver pressure not much higher than that. But my single cylinder engine is looking at over a hundred PSI most of the time. An extra ten pounds isn't worth the extra iron mongery to me. I just heave in another slab of fir bark and crank up the dilithium crystal.
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Re: building a steam condenser

Post by dhic001 » Sat Aug 01, 2015 10:38 pm

Nope, its a single, 4 by 5. Have you got a steam chest pressure gauge? I put one on mind and was amazed at actually how little pressure the engine sees, considerably less than boiler pressure. As I say, mine ran terribly without the air pump, yet is wonderful with it, and faster for no extra work on the part of the fireman. What height is your hotwell at, mine is quite high, so that may account for some of the extra work, pushing the condensate uphill. The higher hotwell of course helps the feed pump when the water is hot because the pump isn't sucking at all.
Worth the small amount of extra ironmongery in my book.
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