Your argument makes a lot of sense. I like the onboard air/ pressurized tank idea. Would you use a pressure vessel or self designed custom tank? Aluminum, steel or stainless (I can make all three). Stainless would be best to avoid corrosion issues inside the tank. It would be low pressure but even 10 psi is a lot over an area of any size so there would be a few internal bulkheads etc. So a fill/ outlet port, drain, two couplers for the coil, a hole for a valve stem and a fitting for a pressure gauge. Would you insulate the tank? Or the lines if the tank was say... in the bed of a 45? I suppose it would depend on performance of the coil, you've got all day to heat the water.
Rambling again I am
All questions I asked myself...
Stainless steel, and rated for the pressure. 4 to 6 gallon. The company I'd have make it is here in IA and they do stainless welding all the time. I'd have a port plus plate at each end. Into the plates I'd have the fittings mounted. That way if I want to change things I can just modify the plates. It also allows for cleaning out. Full insulation of at least 1", more where I can fit it. No plans for baffles as the tank will be round and mounted vertically.
Air pressurization will be done with a regulator and hose plumed into a port on the top plate of the tank. I'll need a check valve, tee, 2 ball valves, filter and regulator in that line. Order in the line from compressor tank to water tank: ball valve, regulator, filter, check valve, tee, tank, with the other leg of the tee having the other ball valve on it for venting pressure. The check valve is to prevent back flow when the compressor's tank is drawn down heavily by something else. An overpressure valve will be in the top plate of the tank. I'll run a hose from it down and out of the truck. I may also put the pressure venting valve in the top of the tank. The tank fill will be via garden hose connector with a ball valve.
Yes tank pressure will be relatively constant with this setup and will need to be released for filling.
Engine coolant lines to and from the tank would be insulated as best possible.
I just realized as I was writing this up it should be possible to use a 3 or 5 gallon premix cannisters like used on the soda machines. Many brewing places handle them. The outlet has a 1/4" pipe to the bottom of the cannister. Into the fill clean out port you could plumb the heat exchanger. The inlet could have the pressurizing lines hooked to it.
5 Gallon new $118,
Used $38
Likely you could get a SS welder to weld additional ports onto the top of it if you wanted.
I'm not set on doing a tank myself as Isotherm's are within reason price wise. I've just explored it as a possibility.