Help me size a tank for thermo siphon!

[Randybee1]

I have a small 2 cylinder Kubota diesel that is a thermo siphon system. These things were used a lot on boats and/or semi's ( aka lorry's) It has an  attached 6.5 kw gen head. I'll probably just use this for occasional back up power. Worse case situation I might run it 2-3 days straight. With that in mind, how big a coolant tank should I use? I've only run it for 5 minutes at a time while a "fix" all the stuff the PO did to it. I'm going to hook up a 5 gallon bucket this weekend and run it for a few hours to get some temp measurements.  Thanks, Randy B

[AdeV]

Lister specced a 120gal (124gal US) tank for their 12/2 engine (in temperate climates - 170gal in tropical), and since I guess your little Kubota is rated at 15-20hp, I reckon you'll need at least that size for a 2-3 day run.

Maybe you should look at either a rad cooled option - or, if you need/want the big tank of hot water, add a radiator as a "heat overflow", so when the tank's up to full temp, the radiator will dump any excess heat.

[XYZER]

Thermo siphon with a radiator would keep things under control and consume less space at the same time.....oh and less antifreeze.

[Randybee1]

Thanks guys. So about this radiator.. Just hook up a radiator as if it were a thermosyphon? Leave the rad cap off so pressure does not build up? There is no provision for a thermostat.... Randy B

[Ronmar]

+1 on the radiator or heat exchanger over drum cooling.  Most of a drums cooling capacity comes from evaporation when they get warm, so they have to be large...  For a radiator, all the same basic rules apply in thermosiphon.  Hot out of the engine needs to go upward immediatly to the top port on the radiator.  That should be the highest point in the system and must remain submerged.  The radiator needs to have the coolant tubes vertical, and preferably, the oultet of the radiator runs downward to the coolant return on the engine.  You can add a thermostat pretty easilly in the hose running upward from the head.  I have a diagram somewhere I can post later.  You could also control temps using a thermally controlled electric fan on the radiator, but a thermostat in the coolant line does a better job of maintaining a constant temp IMO... 

[Randybee1]

I was actually thinking I might just hook up a fan. The problem with adding a thermostat is my coolant lines are only 5/8 diameter. Its a little 14 hp engine. Not sure why they are so small!  Randy B

[Crofter]

You will have to deal with expansion somehow. That will depend upon the volume of the particular radiators tanks and the placement of the inlet openings. The key issue here is that there must always be liquid above all parts of the thermo siphon loop or it will stop circulating. Most modern automotive radiators are cross flow instead of vertical and have very minimal tank volume. Quite commonly they have no filler connection so a separate filler and expansion tank would be needed.

Thermo siphon is a fairly weak flow compared to pumped circulation so it is safer to go for a fairly large sized unit and mount it fairly high to keep flows vertical as much as possible in your hoses.

I would say that an outlet thermostat is essential to get engine coolant temperature up as soon as possible. Unless you have a very large rad you will most likely have to fan it during long runs or hot summer temperatures. As Ronmar mentions, a temperature sensing switch is easy to fasten to the return hose. I used a furnace fan limit switch and a 110V house fan that you can find at a yard sale.

[Crofter]

I was actually thinking I might just hook up a fan. The problem with adding a thermostat is my coolant lines are only 5/8 diameter. Its a little 14 hp engine. Not sure why they are so small!  Randy B

You could use an adaptor to jump up immediately to a fitting (like a 1 1/4 union) that would house an automotive thermostat. There are some very tiny ones designed for outboard motors but I think they would induce more restriction than I would like to see in a simple thermosiphon system.

A 14 horsepower engine is also putting out about 14 HP heat eqivalent into the cooling system at full load so that has to be dealt with. If the coolant inlets to the engine are that small and cannot be worked around to gain more flow potential you may have a problem that only a circulating pump could cure. Restriction and line loss is the enemy of thermosiphon.

[LowGear]

I'm thinking motorcycle wrecking yard or craigslist.  Everything you need, maybe even a pump for those 5/8s lines.  Radiator with expansion tank, lines, thermostat, temp switch and fan. 

One of my secret dreams (don't tell anyone) was to use a Model T brass radiator.  A born natural.

Casey

[vdubnut62]

Guys, about that that thermostat thing with thermo-siphon, has anyone investigated the idea that a T-stat is necessary?
I mean, it does make sense to me, but International Harvester built Cub tractors for years using a 10hp water cooled 4cyl
flat head engine that utilized thermo-siphon and no provision for a T-stat. The manual states that one is not needed since flow
would not occur until the engine was at operating temperature. I wonder what that temperature was?
I'm not trying to start a fuss, but I am genuinely interested if anyone has done any research.
Ron

[Randybee1]

I think the idea behind adding a T-stat is to bring the engine temp up quicker which would improve efficiency!

Looks like I will be heading to the junkyard. There's one about 2 hours away that has 2 acres of used mowers, jet skis ,snowmobiles, as well as 35 acres of trucks/van etc and the prices are dirt cheap!

Randy B

[admin]

a heater core would be probably just about right,

pick one from a van or a large sedan, and get it with the fan

the hose barbs will be 5/8 and 3/4 generally and if you look around you might find one
with a suitable housing so that you could divert the heat to something useful like heating
a room or two?

a thermistatically controlled fan would be appropriate

big trucks have 195 degree tstat, but they run at ~205 degree's and modulate the engine temp
with the thermastatically controlled engine driven fan

that way under load and at normal operating temps the tstat stays fully open and the flow is constant
through the engine and radiator, the fan keeps things ~200-205 degree' F  in most applications.

some of the low flow systems have 1" radiator hoses, going to and from the radiator, reverse flow and low flow
they are used to cool 400hp, which has amazed me no end since they first came out over 20 years ago.

for up to probably 20hp, a small bosch electric water pump as used by vw, mercedes, audi and others for an
auxilliary heater pump is more than adequate for a water pump, they use 5/8 hose barbs and work very well
with a thermostatically controlled/temp switch

it allows my s195 changfa to run with the 195 stat wide open and cycle the temps from ~205-214 degree's F
in a closed system with 7lb cap, with 50/50 antifreeze.  that is at full load producing over 7kwe, the fan cycles
on and off just fine, somewhere around 45 seconds on and 2 minutes off at max engine output.

the radiator and fan are from a sub 2 liter nissan sentra mid 80's with the matching electric fan and thermoswitch
control from a wrecked car.  its just about a perfect match, for this system, and could easily handle 20 or more hp.

bob g

[Randybee1]

Thanks guys. I think I'll start small with a heater core with a fan. If that doesn't work, I'll up size to a small radiator. This is just a little project I'm having fun with. I got the engine cheap, the guy said the gen head did not work. I spent zero dollars so far .. got the engine running and the gen head works real nice . Just gotta get the cooling system up to snuff and get a muffler on it!

Randy B

[Ronmar]

5/8" lines are going to be a problem with thermosiphon.  As Crofter mentioned, line restriction is the enemy of thermosiphon.  There is such a small force powering the process that any restriction can make a huge difference in performance.  If I was going to do it, I would follow the suggestion of upsizing the pipe as sooon as it left the engine to the 1 1/4"-1 1/2" size of the typical small radiator port.  I would also put the radiator directly above the engine(bottom radiator tank above cylinder head) to help create the best possible flow.  With coolant ports that small, and the potential for 13 or so HP of load, I would not be surprised if you need a circ pump... 

As for heater cores to cool an engine, most auto heater cores are not all that large.  Unless it is one sized to heat the interior of a buss, it is not going to pass much heat, and probably not enough to cool your engine.  I would guess conservatively that you are going to have to pass at least 40-50KBTU/HR out of the cooling system at load... I also don't think it will thermosiphon as most heater cores I have experience with have the ports on the same end and circulate coolant up one half and back along the other half of the core.  It's small internal volume will also impede thermosiphon...

Without a thermostat, you will get all kinds of running temps, depending on how your cooling system is configured.  One thing I noticed when I did my fuel consumption tests, was that I burn't less fuel at full load when the engine was fully warm, then when it was not fully warmed up.  A cool running engine will also grow more carbon IMO.  Here is a pic of an easy way to add a thermostat.  You would of course want the bulb end of the thermostat as close to the cylinder head as possible.

         

[Crofter]

Guys, about that that thermostat thing with thermo-siphon, has anyone investigated the idea that a T-stat is necessary?
I mean, it does make sense to me, but International Harvester built Cub tractors for years using a 10hp water cooled 4cyl
flat head engine that utilized thermo-siphon and no provision for a T-stat. The manual states that one is not needed since flow
would not occur until the engine was at operating temperature. I wonder what that temperature was?
I'm not trying to start a fuss, but I am genuinely interested if anyone has done any research.
Ron

Ron there is some truth in the theory that flow will increase with demand but with no thermostat there is nothing to prevent flow starting as soon as there is ANY heat in the cylinder. If the coolant immediately replacing it is -35 F you would be lucky to even get the top of the cylinder up to hand warming temp. Not so terribly bad on a gas engine but on a diesel you would have a pretty wet and smelly exhaust. It does not seem intuitive but engine cylinder and ring wear is much higher at 140 F than at boiling temperature.

The fact that thermo siphon is affected by temperature differential also comes into play if your radiator heat dissipation is insufficient to cause a fairly large drop in temperature: as soon as the temperature comes close to equalizing in the down side of the loop the pressure differential goes toward zero and circulation reduces when you need it most. You then have instant change to an evaporative cooling process!

During the second world war, and I guess the depression years before it, you could not get anything but methyl alcohol anti-freeze and thermostats had to be 165 F or less to keep from boiling the good part away. I have a notion that Dad made a bit of his own antifreeze at times,