First a couple of things regarding water. It is compressable, if you look at the great web site that Doc Goggles provided go to the "Bulk Modulus" of water and it gives a pretty good example of how much force it takes to compress water (and also other fluids). Every thing is compressable, even steel, it just takes more force. Second pressure in a fluid system is developed when there is resistance to flow, which is why Dynoroom sees pressure without having any water temperature. Third: Air in the water: Being concerned that you need to be able to have the air in the water come out in the water tank, if you have the system pressurized to above 15 psi is a mott point, once the system is at 15 psi any air that is desolved in the water is trapped. Because of the very low viscosity of water it does not hold air in solution well so this is probably not worth even worrying about. If you are concerned with the fact that you may have air pockets in the engine then you don't have a big enough water pump. Yes the expansion of the water due to heat makes pressure, but a properly designed cooling system will use a pump that can develop 30 to 40 psi of pressure when it tries to pump the proper amount of water through the engine (10 gallons per minute per 100 hp is a good rule to sizing the pump). So now the pumps inlet sees the pressure in the tank and then that pressure is added to the pressure developed by the pump forcing the water thru the restrictions of the engines cooling system. So the engine looks like a big orifice to the water pump with the pressure on one side being the "orifice" resistance to water flow and the pressure on the other side is the pressure in the water tank which also happens to be the pumps inlet pressure. You cannot have to big of a water pump, and you cannot pump the water to fast thur the engine, all of the BS that is out there about if you pump it to fast it does not have enought time to adsorb the heat is just that! pure and simple BS!!! A water pump that is pumping 100 gpm at 40 psi and is only 50% efficient only requires 5 horse power to turn, which is a good reason to avoid electric water pumps, they will not make this type of pressure.
Regarding the 30x16x12 rectangular tank that Krusty wantst to build, your material selection is good but you will, you must! internally brace the large 30x16 sides of the tank or it WILL deform to a nice somewhat round shape. Using internal stiffeners or welding a series of alum rods in various places from one side to the other will work, and the more the better. I would also to some on the 12 inch sides just to be safe.
Rex
