Condensation Loop Length and Pipe Diameter

[JonRiley56]

Howdy all,

I have read a bit on condensation loops but am still not entirely clear on what the requirement is for copper pipe diameter and also the length. At this point I am looking at having a little over 30 feet of pipe in the circuit with about 20 of it being under water.

My intent at this point is to use 3/4 " copper. I wam running 60 sq ft water tube boiler but only running a 5 hp single engine. I also do not intend to run a vacuum pump, but will rely on the exhaust pressure to push the condensate through.


Does this sound like a resonable approach ?

jon

[fredrosse]

Assuming 40 PPH per horsepower, your engine will be exhausting 200 PPH (Pounds Per Hour) at atmospheric pressure. A 3/4 inch diameter pipe would have a velocity of over 400 feet per second, way too high. 1-1/4 inch copper pipe would be ok.

"I also do not intend to run a vacuum pump, but will rely on the exhaust pressure to push the condensate through."

Just a bit of technical nerd stuff here: even with a high vacuum and a vacuum pump, the exhaust pressure (although it is a very low pressure) is the only thing that pushes the condensate through. In engineering fundamentals, structural engineers are taught "you can't push on a rope" (a rope or cable cannot support compressive stress), and fluids engineers are taught, "you can't pull on a fluid" (fluids cannot support tension stress).

[JonRiley56]

Thanks Fred !

Does the 1 1/4 apply to the entire loop ? I have a couple of convenient holes in the transom that I was going to use a copper bulkhead fitting on. At their current size they would accomodate the 3/4 through the hull. If I run 1 1/4 to the transom. neck down to go through and then run 1 1/4 for the loop will I be OK or do I need to run 1 1/4 through the transom as well ?

jon

[barts]

http://www.engineeringtoolbox.com/steam ... _1129.html

Indicates that 30' of 3/4" pipe would have about 7 psi of pressure drop assuming no condensation... so you'd have less
drop than that...

- Bart

[JonRiley56]

Thanks Bart,

So I could use 3/4" bulkheads as my thuhulls ? I am wondering how I am going to handle 1 1/4" pipe............ sounds a lot more intimidating thatn 3/4 for some reason..........I was kind of counting on being able to bend the stuff with a conduit bender to try and run along the shape of my hull down to the skeg.

jon

[barts]

I used 3/4" heavy copper tube soldered w/ Sil-Fos... it's worked well trailering for 15 years or so. On your engine you'll end up with more back pressure since it's somewhat larger, but the volume drops very quickly to virtually nothing as the steam condenses. Remember you can always use two or more 3/4" runs in parallel if you like....

- Bart

[Lopez Mike]

Maybe I'm misunderstanding Fred but I believe he is telling you that the velocity in the condensing (underwater) part would be rather high at first. Many keel condensers use multiple parallel runs to slow down the flow. Easier to bend smaller pipes too. And, as Bart notes, the volume drops rapidly as the condensation happens along the way. My unit is 3/4" in and 3/8" out!

There is, of course, no serious penalty for using larger pipe from the engine to the condenser. Just don't burn your leg on it.

There is a point where you can over cool the condensate and waste energy. If you can stick your finger in your hot well without discomfort, perhaps you are over doing it.

I've seen some much smaller units doing a decent job. Also depends on the lake/sea water temp. I'm floating around in 40 degree water all the time.

Maybe provide some disconnect points so that you can try different configurations without tearing the world apart?

Also, be sure that your condenser output is taken from the lowest point in the system.

[fredrosse]

With a single 3/4 inch copper exhaust tube, the velocity of 400 feet per second is before condensation begins, and once exhaust steam is in the water cooled part of the condenser, the velocity drops rapidly. The larger flow area is required in the run from the engine into the first few feet of submerged tubing. After that, half the flow area would be OK.

A single 3/4 inch exhaust line and thru-hull connection could work, but you will be adding significant pressure drop from the engine exhaust to the condenser, That will cost you about 1 horsepower loss in a nominal 5 horsepower engine.

As Bart mentions, with the exhaust steam flow split into two parallel 3/4 inch paths, the horsepower penalty would only be about 1/4 horsepower, a much more acceptable result. Running two parallel 3/4 inch copper exhaust lines and keel condenser tubes is a reasonable compromise if the 1-1/4 tubing is too troublesome.

20 feet of submerged 3/4 inch copper tube (7/8 inch actual outside diameter) will give about 5 square feet heat transfer surface area, in the right vicinity for your 5 HP engine. The performance of the keel condenser is dependent on the sea water temperature, the cleanliness of the condenser tubing (don’t paint it!), and the velocity of cooling water flowing around the condenser tubes.

[artemis]

Hey everyboday When we're talking heat transfer, the greatest heat transfer occurrs at the outer surface. The least is at the center. Thus, it is the surface area in square feet, metres, etc. that matters, not the volume. A 1" diameter pipe will have the same surface area as TWO 1/2" pipes, but it will take FOUR 1/2" pipes to equal the volume. In the same sense it will take longer for the heat at the center of a 1" diameter pipe to make it to the outside surface than if it is 1/2" in diameter. Additionally, as the steam condenses to water, the heat transfer from the water will increase, creating a partial vacuum. The size of the air/condensate pump is governed not by the amount of water to be removed, but from the volume of entrained air (hence the term "air pump") in the exhaust steam.

The best air pump I've ever used is the US Navy (or Bailey). A good description is given in an article in Steamboat's and Modern Steam Launches by Dave Fogg (he even gives a good description "how to" using brass and bronze scraps). The one I had on Artemis was built up in this way and, if the condenser was properly sized consistently pulled 24" vaccum.

[steamboatjack]

folks,
I can vouch for the “Dave Fogg” type pump, its the bees knee's just like a uniflow engine in reverse, inlet through the centre ports and discharging at each end. No clearance volume as the water pushes open the delivery valves. Just one point, they are better with a long stroke to bore ratio which makes a long pump, I suppose you could make a single acting version but have never tried it.
Many air pumps suffer from air trapped in the clearance volume above the bucket (piston) hence it is usually best to have a single acting vertical pump, in the case of the Fogg pump this is not so critical.

Regards
Jack