Building a heat exchanger into the hull

Greetings

I am building a 20 ft aluminum boat/submarine. It will be powered by a Ford 7.3 Diesel and cooled with a close system. The hull is 5000 series 3/16 inch aluminum and I plan to build the heat exchanger into the hull just forward of the engine. My question is how many square inches of surface area are required in order to provide adequate cooling for warm water environments? Just looking at the huge radiator that came with the engine is was thinking about two loops that would cover a total of 220 square inches or 2 panels 50 inches long and 22 inches wide. Does anyone know where to find some formulas or data to better estimate the area? Please feel free to make an educated guess too?

Thanks a bunch
Doug Jackson
www.submarineboat.com

 

Consult a standard mechanical engineering text on heat transfer, or look in the appropriate section of Mark's handbook. Everything you need will be in there.

 

Thank you CDBarry. "Mark's handbook" sounds much more appealing than a mechanical engineering text but I am having trouble locating the book you are referring to. Do you have the complete title or Mark's last name?

Thanks again
Doug

 

I take it you still have that huge radiator right? Measure the total distance of the pipe that winds back and worth. Then find the inner diameter of that pipe. Then it's just pi*diameter to find the circumference, and multiple your circumference by the total distance of the pipe to find the surface area needed to cool.

Be aware that this is the amount of surface area needed to cool that engine with air at a certain velocity through the material of your heat exchanger. Also in this process the air is getting all around the pipe.

However using this number, and considering the factors above I'd keep these things in mind: Is my heat exchanger material more or less thermal conductive than my radiator? Are you cooling one side or both sides of your heat exchanger? Is my heat exchanger placed in an area where it may see stagnate water at any time during boat operation? Is my heat exchanger material thin enough that heat will transfer away from your warm water to the cold body of water outside of the boat?

 

No ..sold it off with the rest of the truck. But your other questions are what had me thinking too. The flow size is not a problem, I just need to make sure the cross section of my heat exchanger is no less than 5 square inches using your suggestion of Pi r^2 and the inside diameter of the return hose. The next part is trickier. I am cooling through 1 side when on the surface, remember this is a submarine and the heat exchanger in inside of what is essentially the main ballast tank. The hull is 3/16 inch aluminum, and since it's a jet pump drive you can be assured that if the engine is running then water is moving around the exposed surface. I know aluminum is a fantastic heat conductor, even better than steel. I have the burns to prove it But do I need more surface area than an air cooled radiator or less?

Thanks for your assistance.
Doug

 

Water conducts heat way better than air, so you won't need nearly as much area. Consider this, my Chevy 350 in my truck has a 4 row radiator measuring 38"x20". My Crusader 350s in my boat (270 HP, 70 more than the truck) have heat exchangers measuring 22" long (approx.) by 3 1/2" diameter.

 

This would not be a good idea. If you limit your cross-sectional area of your heat exchanger by this and make it any size, you will have an inefficient heat exchanger. Rather you should be designing to have a pipe run in to a HUGE cross-sectional area for your heat exchanger, or have it run through a similiar sized LONG tube that is being cooled. The idea here is that heat transfers when it is in contact with another material for a duration of time. In other words you want to make the flow rate through your heat exchanger as low as possible.

If I understand how you have your system set up correctly, you have the aluminum hull, your hot water that needs to be cooled, then a bulkhead or some other surface, and this is around your entire submersible? Or is it like above but only through certain panels?

Either way, you will only be cooling on one surface of your hot water that is touching against the hull, no matter if you are above or below water.

The question I purposed was aimmed, at the possibility of idling or being anchored in place and having the engine running. Essentially cases where your boat is not moving, therefore you will be heating the immediate water next to your boat and not moving away from it.

Yes aluminum is a good heat conductor, however copper is a better one, and possibly the cupro-nickel would be applicable if you really wanted to improve your heat transfer abilities. However aluminum should work just fine.

That is a very good question. Knowing more about the radiator, specifically the thickness of the tube in it would have helped. Technically, you may not need as much surface area as an air-cooled radiator due to water's superior cooling properties.

However, how do you quantify how much less you need? Well if you really wanted to design this properly, it would take looking up in some heat-transfer and fluid dynamics books to correctly model the situation and get you the exact surface area that you need.

If you don't want to do that amount of work or just want a "good-enough" estimate on how much surface area you need. I would suggest doing it one of three ways. In all cases, remember you want to keep the height of your heat exchanger down to reduce the amount of water that doesn't get cooled.

1) Find the radiator that matches your engine online, and find technical information or get someone to measure it for you so you can find out specifically how much surface area it has on the inside to cool and design your heat exchanger to have the same.
Pros: Should be a sufficient estimate to get you started. Concrete number that you can do calculations from. If the hull's surface area is not greater than the area you calculated for the radiator you will know that you won't be able to package your heat-exchanger without making the boat bigger.
Cons: May/may not address the stagnation water heating issue depending on location. If hull surface area is equal to the heat exchanger surface area then when you are above water, you will be poorly cooling your water (may even heat it depending on how your water flow pattern works).

2) If you know the surface area of the boat behind the propellar and have a box with similiar surface area thickness as your hull, you can fill the box with hot water (180 deg farinheit I believe is the common temperature, may want to test 200 for safety concerns) to various depths (1/4 increments to an inch). Place the box of hot water in to a large body (prefferably a lake or ocean) with a thermometer in the hot water body. Record the time and temperature of the water till it cools. If the time that water takes to cool to an acceptable temperature is equal to or less than the time it takes for that same volume of water to be pumped in and out of your heat exchanger, then that amount of surface area would work for you.
Pros: Great experiment if you love doing experiments more than doing calculations. Could eliminate your stagnation water problem as long as when you are idling, the properler still spins.
Cons: Need to know the flow rate of your pump as well as the flow rate through your system, specificly your heat exchanger. This is a very hit and miss approach and can be very time consuming if you don't already have materials on hand. Having a neutral gear would essentially ruin the point of this idea.

3. Throw scientific method and proofs out the window, just make the heat exchanger to go all the way around your entire hull through a thin section (1/4" - 1" thick).
Pros: Maximized surface area for cooling.
Cons: Water entry and exit from heat exchanger needs to be well thought out, so the water doesn't just go straight from enterance to exit instead of through out the entire surface area. Could potentially heat the water when the boat surfaces. Could lead to making a lot of small weldments in a maze-like fashion in order to ensure water travels across entire surface area. May not have enough surface area in the boat to cool the water.

Oh, and final word on material thickness, 3/16" will probably work, however that 3/16" has to be heated before the water of the lake or ocean you're on ever gets heated. For computer processor water-coooled heat exchangers the thickness of some of the best are only a few microns seperating the water from the processor. However for this application that hull also has to be able to take damage so making your material thinner may not be practical.

Hope this helps.

 

Walter Keel Coolers

I hate to be simplistic or take the fun out of anyone's project, but...

In the time it would take to read this thread, you could call Walter Keel Coolers, 201-656-5654 or visit http://www.waltergear.com/kc.htm

Get a professional on the phone and order the right thing. If it were my time and money I'd spend it paying an expert rather than on becoming an expert myself.

djs

 

Good stuff. Thanks Skipper. Thats about 225 sq inches for the surface area for each exchanger. You have 2?

 

No, I agree. I want a big cross section, never any smaller than the cross section of the return hose.

My exchanger will be a large flat box only 1 1/2 inches tall and partitioned so the coolant must flow the intire length. The bottom side will be the 3/16 hull and the other side will be 1/8 aluminum sheet. When surfaced only the bottom will be wetted surface, when or partially submerged both sides are wet the RPM's will be low.

Well yes, but my jet pump is direct drive and the exchanger wraps around the intake port, so if the engine is running there will be movement of water even if its tied to a dock.

All good to know.

Now you talkng And Yes, it's all helpful to understanding the problem and no doubt each approach you outlined would work too, but I am old and lazy on top of dim. So do you think 220 sq inches is enought exposed surface area? It's not the space shuttle, and I only have $600 in the engine

 

Thanks djs. Even if I wasn't so cheap, it's just too much fun to do it myself and the other problem is that I need to be able to run into things, like the bottom of the lake without causing serious damage so keel coolers are out. But I did see exchangers mounted inside a potective box, but I still figure that the hull already makes half of the exchanger.

 

Marks Handbook of Mechanical Engineering, published by McGraw-Hill in various editions. Most good sized libraries have one in the reference section.

 

Mark is the last name, BTW

 

Thanks Again CD. I'll go see if Tulsa's library is good sized.

 

A too large heat exchanger won't hurt. The thermostat takes care of temperature control. Also, if the boat has antifouling paint, the heat transfer will be less. The paint manufacturers have tables for it. Many commercial metal boats have keel coolers