I got back to the project for a while over the last two days. I got the burner mounted and the oil feed lines plumbed in and even did a test run yesterday.
I'm reusing the oil burner system and oil/nozzle preheat systems from the old boiler. The test roun went okay for the most part. The nozzle was way too big for this boiler though. 1.75 GPH running at ~ 160 PSI so that had to be changed out. Lots of chuffing puffing and soot production on last night run even with the pump turned down to it bottom end limit of ~80 PSI nozzle pressure.
Today I took the preheater assy out and cleaned it up and switched it over to a 1.2 GPH nozzle. It ran much better but ws still just a bit high on the fuel delivery and was still making soot plus puffing bakc a bit so I went down to a 1 GPH nozzle.
After that change I ran it for ~5 hours straight at ~120 PSI nozzle pressure without problems heating both the house and the shed.
Here's what the oil/nozzle preheater unit looks like taken apart.
And here it is assembled.
I don't know why it went blurry and dark in the next shot of the bottom.
Basically it's a piece of 1 .125" x 4" solid copper bar stock with a 3/8" threaded passage going from end to end in line with the brass nozzle holder that itself is braze welded to it. The 3/8" passage is threaded to increase its surface area plus there is a 3/8" steel rod that fits inside the bore to keep the oil flowing along the threads for maximum heat transfer. Without it the cold oil going in will tend to develop a laminar flow effect where it won't transfer the heat from the copper into the oil evenly.
That passage is drilled offset in the bar so that a cartridge heater can be installed parallel to it. Initially I had a 3/8" 500 watt cartridge heater in this when I made it but it burned out and I only had a 200 watt one to replace it so I added an external jacket type 350 watt heater to the outside to make up the difference. That's the grey jacket held in place with the two hose clamps.
Up at the front just under the nozzle holder is a thermocouple that reads the temperature for the PID controller unit that regulates the heating.
The way it works is used oil is too thick to spray in a fine mist like fuel oil or diesel fuel will on a normal oil burner furnace so it has to be heated up to get it thin enough to spray into a similar fine atomizing mist. To do that raw heavy oil needs to be preheated to 300 - 400F depending on its viscosity. I run mine set at a 350 F but as with today where it's - 15 F and below if the oil is not heated up enough in the supply lines before it gets to the heater assy the 550 watts of heaters top out at ~320 - 330 F and never cycle off. As thick as this load of oil is I would like to have it heating closer to 400F but with the temperatures we are having I would need to be running around 750 watts to reliably manage it.
Still at a the 320F preheat it's atomizing well enough to burn clean but the flame is noticeably sputtery from the oil spray not being atomized fine enough.
So there's the secret to how you convert a old fuel oil furnace burner unit to run old used oil as fuel.