Building a heat exchanger into the hull

One per engine. Sorry for the confusion.

 

1.5" seems a bit tall, and you will more likely than not have a problem with laminar flow and the hot water rising to the top of your tank, and you'll be just cooling the "cold" hot water. However if you can cause the flow to become disturbed, you will have a more "evenly distrubuted" temperature through out the flow of your heat exchanger.

I don't think I understand the mounting location of this, maybe a drawing illustrating your heat exchanger, mounting area, and water levels would be helpful. Or I could just be extremely dense and not catching on to what you're saying.

Well, if you have your pump flow rate and the inlet and outlet IDs of your hose to your heat exchanger (or if you don't just make up some ideal numbers) I can do a rough hand calc to see if that number would work. It would also help if I knew how cold you want the water after it has passed through the heat exchanger and the outlet ID of the water pump in your system.

After thinking about this for a second, I can just send you an excel spreadsheet after I work out the equations so you can change things around in your cooling system and know how much surface area you will need in order to cool that water.

 

Hi,
I found some formulas for diesel:
Energy in fuel=100%, Cooling loss=25%, Exhaust loss 28%, Other 2%, Indicated effect= 50% (friction 3-6% and output power 44-47%).
If cooling loss is 25% of fuel energy and engine efficiency is 44%, your cooling loss will be 57% (0.25/0.44=0.57) of rated power. For a diesel engine rated 152 kW (207 hp SAE)...

P(cool) = 0.57 * 152 = 86,6 kW (117.8 hp)

rgds
Olle

 

Am I reading this right you want to use the hull shell itself as part of the exchanger? Also the boat is like 20 Feet aluminum construction !!

I would seriously consider a exchanger unit that is "piped in" unless you would like a sweat box.

 

Not to throw any other wenches in the works, but what do you plan to do with the exhause? I guess you can run a dry exhaust, but in most cases at least part of the exhaust system is raw water cooled.

 

A few things to think about with your cooling system. I work in the commercial marine field. Skin coolers are common on workboats and are most often made by welding “C” channel to the hull and circulating the coolant from the engine through them. They have sea water on 3 sides and make very effective cooling systems. Often they are several runs of this channel side by side to affect the needed amount of contact area. It sound like you want to have yours recessed inside the hull instead of out. I have seen this to and it works but not as well. You will need more length to your cooling system to compensate for the heat only being exchanged on one side. Also it will create heat in the bilge area of you boat. This may or may not be a problem. Location of the battery and fuel tank need to be considered as well as what will be located on the deck above the cooling system. For example the drink cooler or live well would be a pore choice to place over your cooler.

Next you need to think about the water pump and is it suited to the task. To much velocity and the coolant will make the loop to fast to cool it. Also you may need to design in some baffles to slow done the water as well as to mix it. You will need an exspanchan tank higher than the engine. Also be prepared to deal with trapped air in the system. Skin coolers are known for this problem. Also location on the hull is important. Make sure that your choice will be completely immersed in the water at all times and is not likely to develop an air pockets at high speeds.

You also need to think about the exhaust. A dry exhaust is not a pleasant thing on a small boat. The water helps to cool it and to muffle the noise. You said you are running a jet pump so I will assume that you do not have a gear that will require cooling. But does the engine have an after cooler on it? If so you will need to address this issue also. There should be an oil cooler for the engine. On most truck applications this is done with air. Have you addressed this issue or is the oil cooler in the coolant circuit? And as if I haven’t pointed out enough you need to think about the type of aluminum you are using for you components. Some coolants have high PH and can do harm to many grades of aluminum. Second aluminum is always a problem on boats when it comes to electrolyses. Make sure you use plenty of anodes on and inside your coolers.

As far as size this is a fairly simple problem. You need to know how much heat your engine makes “Heat rejection in BTU’s” per min. This can be found in the specifications from the manufacture of the engine. After that you need enough surface area to exchange the amount over what you want the engine to run at “operating temperature” remember typical there is only a 10 to 15 degree difference in temperature of the cooling water gong in and coming out. I will stop short of laying out the math I don’t want to be the one to cause you to “roast” you engine but I will give you a number where help can be had.

The best advice I can offer is to give the people over at Fernstruma call. Even if you don’t want to buy a grid cooler from them I will bet they can give you a hand at sizing this thing..
R. W. Fernstrum & Company
1716-T Eleventh Ave.
Menominee, Minnesota
U.S.A., 49858
Tel: 906-863-5553
Fax: 906-863-5634

 

Try and error engineering?

for the boat I am building, I was thinking about keeping the original radiator, wrapping it into an stainless steel enclosure (container), making this one sealed so nothing can get in or out that container. You wil have to put in an expansion container as wel, just the same as the old radiator. The radiator keeps his closed cooling system with the frechwater or coolant mixture, a same mixture can be put in the first container. The first container you build will have to come into a second container, sealed as wel and in this one you pump some sea-lake-river water to cool the 1st container and radiator down.
The whole idea is to make an heat exchanger cooling the water down without getting any sea water to any part of the engine.

The whole thing get's a bit bigger than the original radiator an inch or two side to side.

In the sealed container you might need to force the flow, so the water doesn't just get in and out, it needs to do some travel around.

If this idea could suite your app. I send you a sketch of it. I am sure this will do the job in my app, but I don't know if it will for yours

Will have to do some stainless steel welding, get the old radiator off from a junk-yard and build up the whole thing, would cost you under (Belgium EUR 200) USD244.

Greetz,

Daniel Peeters

 

flow and area

consider this - that 5 ton air conditioner at your house has an outdoor condensing unit. you know there is a lot of heat exchange area there. a five ton freon-to-water is about the size of a gallon milk jug.

the higher the flow, the more efficient the transfer. not an issue because the flow it determined by the engine.

first, figure out how much cooling capacity you need. somebody, somewhere, knows how many Btu's that radiator will move. this is a function of water temp, flow, and area (assuming some fixed outside temp.). get that number and you are ready to go to the next step. alternatively, measure the water temp entering and exiting to get the temp difference and measure the flow. that will also tell you how many Btu's you need to loose.

the equation for heat transfer is Btu=U*A*(tempin-tempout)

you know the Btu you need from the radiator specs or from measuring. the tempin is the themostat setting (180), and the temp out is sea water temp. "U" is the reciproical of the resistance and will be less than 0.05 for 1/4" aluminum plate. then solve the equation for "A", the area in square feet, and that is the hull area.

this is a quick and dirty steady-state situation. it ignores turbulent flow and mixing, etc.

remember, surface area is the key ingedient. keep the exchanger dimensions flat, with much more hull contact than depth of cooling water over the exchanger area.

it wouldn't be too hard to test a prototype using a truck, parked at the water's edge, and re-route its radiator hoses through your prototype and watch the trucks temp guage.

i think you will be surprised how efficient the water to water heat exchanger in your boat is, compared to the water to air heat exchanger in your truck.

 

Thank You

My many thanks to all who contributed. So many things in life just look like simple problems. I needed a replacement for the radiator. My Inboard Motor Installations book that mentioned coolers could be build into the hull. I have and aluminum hull. So I build a thin channel like box into the hull and connect it into the water coolant circuit of the engine, and it's done. Right! I just need an idea about how many square inches of surface area need to be in contact with the water. So I post my simple question to the www.boatdesign.net forum, and now learn about BTU's, heat transfer coefficients, and laminar flow. I just wanted to know if 220 square inches sounded about right, and now I am looking at mechanical engineering text and remembering why I chose to data processing as a career; NO MATH!

In very layman terms here is what I think I know:

A) The cross section of the channel needs to be no more restrictive than the hose. Since the hose is 2 1/4" diameter that means 3.15 x (2.25/2)^2 using area = pi*R squared or about 4 square inches

B) According to "TSITL": Since only the wetted side of the hull and the channel are cooled the channel needs to be relatively flat in order to keep the hot water from rising to the top of the channel away from the hull and the cooling effect of the outside water. And 1 1/2 is likely too much. Perhaps 1/2 inch is better, which means in order to get the 4 square inches of cross section needed the channel would need to be no less than 8 inches wide.

C) "Corpus Skipper" has a 270 HP engine cooled with an external heat exchanger or "keel cooler" that is 22" long and 3 1/2" in diameter, which would make it about 225 square inches of cooled surface area. That means my 1/2" tall by x 8" wide channel would need to be 225/8 or about 29 inches long.

D) I need to keep the area of the heat exchanger in the water. Not a problem, it's on the *** end of a 5,500 pound boat. And the water needs to flow moving even when the boat is stationary. Also not a problem since the heat exchanger area will be next to the water intake for the jet pump, so if the engine is running, the water is moving.

E) "CaptPPan" asked "...what do you plan to do with the exhause?" Yea, right! That will be dumping a lot of extra heat into the cooling system too because I plan on building water cooled aluminum exhaust manifolds that will be plumbed for there in-flow and into the block for their return. From "Gonzo": "A too large heat exchanger won't hurt." Sounds like good advice. "Suede" says that the exhaust loss is 28% and cooling loss is 57% so if I doubled the square inches of cooled area it would more than compensate for cooling all of the exhaust loss, even if that were possible. So now it's 450 sq inches and a channel 1/2" tall x 10" wide by 45" long will do the trick. Raw water will be introduced into the exhaust but that in only the help keep the hull temperature down.

F) The bilge area will get hot. But that's ok because its a long way from the cabin on the bow.

G) From "FlybabyJim": "Next you need to think about the water pump and is it suited to the task. To much velocity and the coolant will make the loop to fast to cool it. Also you may need to design in some baffles to slow done the water as well as to mix it." Hmmm, the pump is stock so it should do fine, and the 1/2 height should help with the mixing, but adding baffles could be done as long as it did not restrict the width to less than 8 inches. Might be good for stiffing the channel too so it would not be damaged when stood on.

H) "FlybabyJim" also suggest finding a coolant that is safe on aluminum.

I) "FlybabyJim" also eludes to gear box cooling, and yes there is a gear box to get the diesel rpm up to where the jet pump need them, but that will get cooled by a raw water set up if it is required.

...and thanks to so many others that posted advice to the forum. My deepest apologies to those whose math and engineering skills are so far beyond mine.

If you would like to see more about my project I envite you to visit my web site: www.submarineboat.com All suggestions are always welcomed. God knows I need the help

Thanks again
Doug Jackson

 

Yes but the boat is also a submarine and the cabin is on the bow, 16 feet from the engine and heat exchanger. Have a look at www.submarineboat.com. The idea is to have a submarine that can go fast on the surface, but having looked at www.cheetahcat.com fast is very relative I'm putting a boat under water; you should put wings on yours.

 

There is a straight forward idea, but I sold the radiator with the rest of the truck body, and I'd like something lighter anyway. I am already seriously over weight. I will add the reservoir and radiator cap to my list. Thanks

 

I believe you will need approx 1/3 square foot per horsepower. This should cool in most all reasonable water temp. Tom

 

On commercial boats I have seen the hull cooling surface was a smooth built with internal web flanges for bolts.

The top plate bolted on inside and had a welded passage or maze that ran the water back & forth over the exterior surface.

Was quite efficent as no hull drag (or coat) of std keel cooler , although to get sufficent area it was large.

Figure the AREA of all the surfaces on a std keel cooler , and you should be in the ballpark.

The vessels I have seen were displacement speed (at best!) and what should be added for airation near the cooling surface on a plaining boat is open to question.

The system should be sized for he added heat of a water cooled exhaust manifold , as that makes the engine space cooler , and safer.

Antifouling was used but not necessary the hot water soon cooks off any riders.

FAST FRED

 

FastFred: That sounds perfect. Adding more area will not be a problem either. Thank you.

 

C channels could both serve as stringers and skin coolers! Seems like a heat exchanger might be easier.