Thermosiphon Cooling


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The following comes from a recent topicon SmokStak, which can befoundon the Internet at: smokstak.cgi.

I'm using a screen-cooling tank on a 17 HP Ruston engine, running slow and not under load. I tried a belted pump, but the engine would not warm up. I have the bottom of the tank higher than the water level in the block of the engine. After it comes out of the engine, the water has to go up about 38 inches to the screen.

Is 38 inches too much of a rise from the top of the cylinder to flow to the screen-cooling tank? - Terry

To thermosiphon, the top pipe must be below the water level to make a complete loop. A screen-cooled tank won't allow thermosiphoning. Could you install a valve before the pump to throttle the flow? - Bill

You can use any kind of tank for your cooling system, and if you use a pump you can certainly use a screen cooler. If you do not use a pump you have a thermosiphon system and the water will circulate by convection as it gets warm. If you want a screen cooler you must use a pump - thermosiphon will not 'pump' water up to a screen. You mentioned you used a pump and your engine didn't get warm. You were circulating water from your cooling tank and the engine had to heat up all that water. I had a 15 HP Reid and it would run for four or five hours before it would heat all of the 40 gallons in my tank. Give it time! - Norm

Aren't some screen-cooled Moguls thermosiphons, yet they have a screen tank? The water barely trickles out if you run them slow, but until the engine is pretty warm nothing comes out! - Laura

Some of the smaller Moguls are screen-cooled, without a pump. Until the engine reaches boiling nothing happens, but when the water starts to boil it bubbles up the pipe and runs down the screen. This makes for quick warm-up and keeps the engine at a good temperature for burning kerosene. Notice the screen tank is mounted high (see above photo) so the cylinder is filled with water all the time. Can you see any pump on this one? - Ken

A screen-cooled 4 HP IHC Mogul, snapped by Patrick Livingstone at the September 2002 Sydney (Australia) Antique Machinery Club Rally.

A friend of mine has a 6 HP, and it also does not have a water pump. This is how they left the factory and it works fine. The base of the tank is higher than the cylinder so there is always water in there. Once the engine warms up it pushes a lot of water over the screen and it keeps the engine nice and warm for running on kerosene. Australs and Black-stones have inline taps in the water line to limit the flow of water. This is common to thermosiphon tank-cooled engines and engines fitted with water pumps and cooling towers. It allows for adjustment in the flow of water, which is a big help in keeping hot-bulb engines warm. - Patrick

When I set up my Model YH Fairbanks, I put the pipe back into the tank below the water level. The water starts circulating as soon as there is a temperature difference. If the water gets low the water in the engine will get up to boiling and bubble up the pipe and drip into the tank. Either way, as long as the cylinder is covered with water there is no problem with overheating. Many old farm tractors and cars did not have a water pump. If the water level is below the cylinder, as in the Famous and Titan engines, then a pump is required. - Ken

I had the bottom of my tank higher than the cylinder -maybe I didn't let the engine warm up enough. I've dropped the tank 6 inches and installed a pump, and I'm going to try a valve under the tank to restrict water flow to the pump. My concern is the pump might not hold its prime. If it doesn't work the tank might still be high enough for thermosiphoning without the pump. Thanks for all the comments and suggestions. I'm sure I'm not done experimenting yet! -Terry

For cooling our Bessemer engines, we found it better to place a 'T' after the pump. One line goes to the engine and the other line goes to the top of the tank for a bypass. A valve in eachline makes it easy to control the flow through the engine until it warms up. - Kent

In my thermosiphon-cooled (open system, no pump) 10 HP IHC Mogul, the water tank/cooling screen is considerably higher than the cylinder it cools. There is no real thermosiphoning going on, in the sense of a molecular heat transfer as obtained in a closed-loop system where heated water rises from the top of the cylinder and cool water from the tank enters the cylinder from the bottom.

In the Mogul's case, when the engine is running and under load it gets hot. In fact, it can reach boiling temperature rather quickly, which is great for a kerosene-burning engine. As the water begins to boil it creates steam and forces itself (steam and hot water) up the pipe and out of the holes in the return pipe on top of the screen, where it cools as it falls back into the tank. As this happens, it creates a partial vacuum in the cylinder proper, which allows a critically balanced, inline horizontal check valve from the bottom of the cooling to partially/momentarily open (from the weight of the water in the cooling tank pushing on it), allowing cooler water to enter the water jacket around the cylinder. As soon as the cooler water enters the water jacket, it cools the cylinder enough so that it stops steaming, the vacuum is broken and the check valve closes. As the engine heats up again, it goes through the same cycle.

On the Famous they 'usually' had a water pump circulating the coolant through the engine and up and across the screen. - Paul

So it's more of a 'percolating' effect rather than thermosiphoning with the Mogul? - Laura

'Percolator Cooler,' I love it! That's exactly what it is, not a thermosiphon system. Enjoyed this thread, and learned something I didn't know about percolator cooling systems. - Mike

You guys making fun of my percolator? We'll see who's laughing on that cold engine show night. I'll have my coffee cup dipping into my screen cooled 'percolator' - maybe even put some instant coffee in it, giving 'International Coffee' a whole new meaning! I bet nobody has ever had home-brewed coffee that's 'percolated' through the innards of an old gas engine! - Laura

An old gentleman who worked in the oil fields told me that if the cooling system was closed, meaning that the top pipe entered the tank below the water level, it was called 'thermosiphon,' and if the system used boiling water and steam to push the water up and over the top into the tank it was called 'slugging.' I've used both, and the first method takes much longer to heat up. - Howard

SmokStak is an engine conversation bulletin board with over 46,000 messages on file and is part of the Old Engine series of Web sites that started in 1995 as 'Harry's Old Engine.' Harry Matthews is a retired electronic engineer and gas engine collector from Oswego, N.Y., now residing in Sarasota, Fla.