If the orifices on my 5 kg roaster were #55 for propane, what would I have to drill it out to for natural gas?

I have 12.2" WC now with natural gas. I had 14" water column before with propane.

And a little coffee pR()n

Would you dare do that DIY as professional roaster in the US legal environment? (just asking, also since you MAY also have to adapt the air inlet)
As drilling existing holes is a bit hit or miss I'd try find natural gas nozzles, probably from the manufacturer.

Marcelnl wrote:Would you dare do that DIY as professional roaster in the US legal environment?.

Yes

I already opened them up from 55 to 49 last night. Big improvement. I have documentation from the service tech from the original owner that it could be as much as 44 to achieve the same relative btu output.

The manufacturer is in turkey and does not supply orifices, only complete burners.

This roaster ran on natural gas before I got it. I converted it to propane. I wish I could remember where I got them, but since I'm going larger instead of smaller now, it's easy to just ream out the hole a bit.

I take it your using number drills for accuracy?

Yep

My dilemma is that all the typical conversion charts compare 11"wc LP to 6 or 7"wc natural gas, which is the way they're normally supplied.

https://www.ssl.ca/wp-content/themes/si ... -Chart.pdf

But due to my new SF75, I had the gas company convert me to high pressure NG @ 12.2"wc instead of the usual 6.

I know there is a density difference as well, but I'm trying to figure out what orifice size I need to maximize BTU output and get me as close to my old setup as possible.

I was getting about 14"wc propane in my garage. I'm getting 12.2"wc of natural gas now. But when I did a trial roast yesterday I found that my gas settings needed to be significantly higher to hit the same milestones.

Also, with my needle-type control valve I was getting to 12"wc in 270* knob rotation. Now I get to 5" wc at about the same knob setting with either fuel, but I need to turn it all the way around 3-4 times in order to hit 12"wc with NG.

One drill size at a time from here on in. If you screw up I suppose it could be plugged and re-done with some JB Weld.

What's the thread pitch on the orfices. Thinking out loud there probably copper, like a main jet on a Mikuni Carb. A substitute may be found elsewhere.

That's part of the problem in buying new orifices. They all seem to be sold by mfg'r instead of thread spec. And my roaster was made in turkey and they don't supply orifices, only the complete burner. I can't seem to get a thread size/pitch.

And since they are brass and pretty soft, the opening can be peened closed if necessary. At least that's what I read.

Consensus is it's a size 44 bit for my application. Seems awfully big.

Alan, if you have the maximum BTU/hour input rating on the nameplate ,then you can fire up the burner to max (no other gas appliances operating while you do this measurement) and then clock your gas meter. One cubic foot of natural gas = 1000 BTU's (this can vary slightly so you might want to check with your natural gas supplier for their actual value). Watch the cubic foot dial on the gas meter for 10 minutes and multiply by 6 to get your BTU/hour then compare to the nameplate rating. It would have been nice if you could have done this before you did your first drilling to give you a feel for the change per drill size.
(Edit after reading Marcel's comment below.)
Marcel is correct, the flow changes with regard to orifice size are not linear for a given pressure, so bear this in mind when deciding to go to the next drill size or not.

It's been a while since I made calculations on gas flow, yet I seem to recall that when opening an orifice flow increase is non linear.
It may be possible to calculate total gas flow from the BTU rating of the roaster, divide that by the number of nozzles and come up with the diameter for each if you have the gas pressure and specific density of the very gas you are using at hand. Someone with a tad more recent gas flow learning may be able to help you, it's been 40 years since I last did that.

Drilled the jets to #44. Massive improvement. Had to turn down the regulator spring. Flame is nice. I set the valve dial to where it used to be at 5"wc for propane and adjusted the regulator to match. Trying a roast and we'll see what happens.