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Need Advice on Propane Heater Conversion

I've been converting a 150,000 BTU natural gas salamander heater to propane. I finally got everything together Friday, and turned it on. Basically I have produced an afterburner. Way too much gas flow -- sounds like an F-16. BIG flame and it cycles on the overheat safety, not the thermostat.

OK, so I didn't calculate the resized orifices correctly. The jet block consists of a cylindrical brass piece with ten radial #40 drill holes. I machined a new one with #50 holes, based on the cross sectional area of the holes and the relative heat content of natural gas vs. propane (because I left the NG regulator inside, the pressure supplying this jet block is the same as it was for NG). I figured that this would be a good first attempt.

Clearly not even close to the right size. I can deal with that, but there is an interesting conundrum here. In trying to tame the flame, I throttled it down with the propane tank valve, and got down to a more reasonable flame size. For safety, I've put a CO detector in the garage during these experiments, and with the BIG flame, it detects no CO. However, with the throttled flame, the CO level rises fairly rapidly, and there is obviously something in the air that strongly irritates the nose and throat.

I would have thought that the opposite would happen. The air flow is fixed by the electric blower, so with too much gas there should be incomplete combustion. There is quite a bit of yellow tips on the flames with the full gas flow, which seems consistent with this hypothesis. However, with less gas, there should be more complete combustion. There are fewer yellow flames when I throttle back the gas flow, but I begin to get a lot of CO, and that irritating byproduct.

Is my logic wrong here? I'm confused. With the gas throttled back, there is very little yellow, the heater cycles on the thermostat, not the safety cutout, and all seems fine except that it's trying to kill me. Thoughts from you experts?

Reply to
Bob Chilcoat
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I wouldn't assume the NG regulator is OK for propane, given the difference in inlet pressure.

Reply to
ATP*

Sounds like fun! Good first approximation approach on the orifice size, except that for small nozzles the flow is not linear with area. It involves voodoo and math that made my head hurt.

Regarding the incomplete combustion, my WAG is that with low pressure the gas mixture is too lean to keep all of itself at combustion temperature. You might try throttling the air intake; although a waste gate would be better it is a lot of fooling around for a trial.

Reply to
Fred R

The ones I have seen that are propane look and sound like an afterburner Stock. Big loud swirling blue flame. Maybe you do have it right. Loud enough that it is hard to talk around if not impossible. But they make heat really fast so it only has to be on for a minute or two.

Reply to
Mike

I did the same thing with some naturally aspirated NG heaters. Take out the orifice and carry it to a propane supplier. Tell them what the heat output rating for the device is (mine were 25K and 30K). Have him look up the proper orifice size for that heat output with LP fuel gas and drill a replacement orifice to the appropriate size. Worked great for me. What you have now is basically a weed burner in a salamander body. I have a 30K-55K LP heater that was purchased in that configuration. I also have the 500K weed burner. You can tell when the heater is on since it has a fan, but it's nowhere near as loud as the weed burner at full bottle pressure.

Reply to
Thomas Kendrick

The NG regulator is a final reduction with a 1/2" inlet pressure and a lower outlet pressure (haven't measured it). Since the pressure from the propane regulator at the tank is 11", and 1/2 psi is 14", the NG regulator should just drop the propane pressure to whatever the normal NG pressure is to the orifice block. So I need to figure out the equivalent orifice size for LP at the same pressure as the NG. If I took out the NG regulator, I could just use the proper LP jet (which is supplied at 11"), which I can get from the manufacturer, or machine one with the same size holes. I'm trying to save the hassle of putting in a length of tubing where the NG regulator is (between the orifice supply tube and the solenoid valve). OTOH, it might be a lot easier to do that in the long run, but perhaps not as interesting. Drilling ten radial holes in my shop-built orifice block involved using the indexing head that I've only used once before. Kinda fun (HSM content).

Reply to
Bob Chilcoat

Don,

I think you've identified why throttling back the gas pressure produces that irritating contaminant. I hadn't thought about NO2 (although is should have, I've been involved with N2O for years in my anesthesia work -- related oxide of nitrogen). It makes perfect sense that with less gas and a fixed airflow, the temperature should rise. I guess there's an optimal temperature range that minimizes CO and NO2 production. Now I have to figure out what that is. Thanks.

Reply to
Bob Chilcoat

Maybe there's a reason why the manufacturers have it set up without the NG regulator. I think there are a lot of potentially complicated combustion engineering issues here and the stakes are very high.

Reply to
ATP*

Right, Bob. Combustion products NOx and CO are like radio buttons: reduce one, raise the other. Some industrial burner controls use oxygen sensors in the flue gas to control fuel-air mix to meet standards for NOx and CO emissions. I think auto engines do that as well, now that they have engine control computers. O2 content of flue gas seems to be a useful indicator of combustion condx. It's an indirect and inferential measurement, but it seems to work. Oxy sensors are cheap, and they're robust in a fluegas environment.

I'm no expert in the field but you should be able to find data from ASHRAE. If the process was oil combustion I am sure you could get data from Dr. Tom Butcher at Brookhaven Nat'l Labs. You might try pinging Dr. Ulrich Bonne at Honeywell Labs in Plymouth, MN. Ulrich probably knows as much about gas combustion as anyone on the planet. snipped-for-privacy@honeywell.com

I don't know if he'd respond or not, but he's a good guy so he might. Tell him Dr. Foreman referred you to him. That might just evoke a very informative response because he knows damned well I'm not a PhD and he purely hated it when I was assigned to conduct a critical tech review of his work once upon a time.....

Reply to
Don Foreman

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