DetroiTug's Tug is going together

[stevey_frac]

She's looking beautiful.

Perhaps once she's all done, I can charter a short trip on her?

Detroit is not too far away!

[DetroiTug]

Hi Steve,

Sure thing, we are planning another trip up the Detroit river past Windsor sometime in the next few months. I'll send you a message on here and let you know when, you're more than welcome to come along. Probably be a weekday, less traffic.

-Ron

[DetroiTug]

Economizer in the smoke hood. 20 feet of sch40 1/4" pipe.



-Ron

[Lopez Mike]

Is there a reason that you didn't make the whole thing out of 1/4" or 3/8" copper? My first thought was that maybe the heat would soften the copper but the connections to the steel are copper. Seems like the better heat transfer might be worth it since the flue gasses have probably cooled down a bunch.

I am so hot to build one of these for my boiler. That and an exhaust feed water heater would help a fair amount in holding off the impending deforestation of the San Juan islands.

[farmerden]

The saga continues! And the work is super as usual. It's been a real trip to watch the boat being built on this forum.And now chapter 2 or 3 -the fine tuning! Is the coil welded to the cross pieces? I haven't built mine yet and a worry was the vibration cracking a pipe. You seem to have solved that.And the curve between welds will allow for expansion and contraction .Thanks again for the time you took from your boat to enlighten us all! Den n

[DetroiTug]

Mike,

I originally had one tied in made out of 15 feet of 3/8" copper, it worked very well for preheating the feedwater, however it was very noisy. I had it tied in very tight with steel heavy gauge fence wire, which it's really supposed to be copper wire. When the wire heats, it lengthens and the coil is then loose. I could hear every stroke of the feedpump in the old coil. And of course hammering like that it was just a matter of time until it beat hole in it.

So I made the plan last fall as I was getting some coils wound for an upcoming steamcar project, to have a pancake wound for the tug. Just weld it in the supports, it shouldn't shake anymore, but as I was piping it today, I did notice the similarity between it and a clock gong, we shall see. This coil is a lot larger than the old one. Steel transfers heat 10 times slower than copper. It may be an even swap, won't know until I try it.

I bypassed the feedwater heater in the exhaust manifold. I'm thinking the economizer should be ample. If I don't have the valves on the exhaust set just right, I get rain out the stack. That problem should be gone this year too. Didn't bother me, but the folks on the after deck seemed to not like it. Especially with the oil mixed in.

I can say unequivocally, that the economizer is a must have. I'd never put another system together without one. It's essentially like coupling a small monotube boiler to a VFT.

Two things on the economizer, it is wise to have valving so it can be bypassed if it fails, and in that piping make sure and install a small relief valve.

Den, Thanks, I enjoy sharing the project, it keeps me moving on it as well.

It's been a lot of work, often times when I didn't feel like it, but I can see light at the end of the tunnel now. I'm thinking by the end of June, I will be finished with the major work, then it's trinket time.

-Ron

[fredrosse]

My day job for the last 43 years has been designing heat transfer equipment, and stuff like that.

The use of copper vs. steel produces very little difference in the economizer or boiler arena. Yes, copper does have a thermal conductivity that is several times the conductivity of steel, but with flue gas heating water, the dominant resistance to heat transfer is the convection coefficient between the hot flue gas and the tube metal surface. This convection coefficient is independent of the tube material conductivity.

I ran some rough numbers for the economizer here:

With 20 feet of 1/4 inch schedule 40 steel pipe (0.540 OD, 0.088 wall), 200F entering water, 600F entering flue gas, 222PPH water is heated to 245F. The overall heat transfer coefficient here is 11.26 BTU/hr-ft2-F

With 20 feet of 3/8 Type L copper tube (0.500 OD, 0.040 wall), 200F entering water, 600F entering flue gas, 222PPH water is heated to 242F. The overall heat transfer coefficient here is 11.30 BTU/hr-ft2-F

One may find it surprising that the steel pipe here carries more heat to the feedwater, with such a low metal conductivity, and more than twice the metal thickness compared to the copper tube. The reason for this is that the 1/4 inch steel pipe is 0.540 inch OD, vs. 0.500 OD for the copper, That gives about 8% more heat transfer surface area for the steel tube, and that more than overrides the advantage of high copper conductivity and thin tube wall thickness for the copper tube.

There are many applications where the tube metal conductivity is much more relevant, but for typical fired boilers the tube material has little impact on heat transfer performance.

[fredrosse]

For economizer service, the steel pipe is more likley to survive if water flow is stopped. Ordinary steels are all good for 650F or higher, and copper alloys are only permitted up to 406F.

[DetroiTug]

Fred,

Thanks for that info. Should be better than that then, as the "3/8" copper" was OD instead of ID.

On the Copper vs Steel/absence of water. That was another reason I went with the steel coil. The steam car folk espouse steel boiler components frequently in conversation. With their small boiler volumes, it happens that the boilers are ran dry occasionally for various reasons, the copper will typically fail and the steel will survive. The copper will "scorch" as they refer to it.

-Ron

[Lopez Mike]

Well, Fred, you just saved me a bunch of fussing around. I had a wonderful but labor intensive design all worked out with copper tubing brazed to support arms. Now I just have to cobble together a bender for 1/4" sched. 40 pipe. This presupposes that the danged pipe comes in 20 foot lengths so that I don't have to make a connection in the middle of the coil.