TJ-067 - Converting from Propane to Natural Gas

Discuss roast levels and profiles for espresso, equipment for roasting coffee.

#1: Post by Dregs »

I recently converted my North TJ-067 from liquid propane (LP) to natural gas (NG). With the growing interest in 1 lb - 1 kg roasters, I thought some description of the process might be helpful to others.

A word of caution: I am far from an expert at anything involving LP or NG. Whatever limited knowledge I have I got from the internet. Gas can be very dangerous (as can electricity) so please do your own research, follow manufacturers' directions, use common sense and abundant precaution, and don't blame me if something goes wrong.

Before making any changes to the TJ-067, I timed how long the roaster took to heat up from room temperature to 360F using LP with maximum gas flow, minimum fan setting and empty drum - just over 6 minutes. That would be my benchmark to know whether the conversion resulted in heating capacity with NG equivalent to what it was with LP.

From what I've read, NG that is piped into the house is almost pure methane. Methane burns at a lower temperature and has roughly 40% of the energy content for a given volume compared to propane. The pressure in home gas lines, usually between 3.5 to 7 inches of water column, is lower than the pressure supplied by the regulator (round metal thing) on an LP hose, which is standardized at 11" WC. The conversion from LP to NG requires that an appropriately larger volume of gas be allowed to reach the burners. The TJ-067 doesn't have any kind of built-in regulator. Maximum gas flow is a function of the size of the orifice (hole) in the brass nozzle housed in each burner and the pressure of the supply. The nozzles screw into the gas manifold and the burners screw onto the nozzles. I hooked up the roaster to the NG line before making any changes to the nozzles to determine line pressure in my house and measured 5.2" WC on the gauge.

The TJ-067 arrived factory equipped with nozzles for LP. The nozzles for NG were sent in a separate baggy. The TJ-067 didn't come with any instructions for the conversion (yeah, Chinese roaster); osanco, aka Steve Green of Mill City Roasters, wrote a blog entry with lots of helpful information that I used to guide the process. Using the shank end of some tiny drill bits ($5/set on Amazon), I measured the orifice in the propane nozzles to be between a #73 and #72 bit (larger numbers = small sizes). The orifice in the NG nozzles was sized between a #71 and #70 bit. Using Steve's handy chart, I calculated right off the bat that the orifices in the NG nozzles were way undersized (intentionally, I think, to allow for adjustment) even if the pressure was the maximum found in home systems. Again using the chart, I estimated the NG orifice size needed for BTUs equivalent to the LP nozzles to fall between a #64 and #63 bit at the pressure I had available.

I was concerned that putting a larger volume of gas through the burners would cause them to burn inefficiently, so I didn't just drill out the orifices in the NG nozzles to their likely final size. Instead, I went in steps a few drill bit sizes at a time to see how the flame looked and how the roaster performed. At each step, the flame was larger than before, but not so large as to appear unsafe, and still burning blue with infrequent streaks of orange. So I kept going. Drilling the orifices by hand is easy using a set of tiny bits and a pin vise to hold the bit, no power tools necessary or advisable. At the original orifice size in the NG nozzles, the flame was barely visible and the roaster would never get to my 360F benchmark. With a #68 bit size orifice, time to 360F was 9+ minutes. With a #64 bit size orifice, heating time was about 20 seconds longer than my 6 minute benchmark. I've done a half dozen roasts with batch sizes between 10 oz - 2 lbs and didn't notice any performance difference, but the TJ-067 never breaks a sweat at those batch sizes. I decided to hold there and do more testing of the roaster with maximum batch sizes before deciding whether to go to the next larger orifice. The first photo below show the burners using the original LP; the second photo shows the burners converted to NG.

Here is a photo of a couple of brass nozzles and a #68 drill bit.

The pressure gauge that came with the roaster has a range of 0 - 20" WC. Since maximum pressure in my NG line is just over 5" WC, I swapped the gauge for one with a 0 - 10" WC range.

A few words about safety: I never leave the roaster unattended while roasting. When not roasting, I turn off the gas with a shutoff valve in the pipe coming out of the wall. I also unplug the electric connection when not in use. I shop-vac the chaff out of the cyclone before every roasting session. I have a 5lb CO2 fire extinguisher within arms reach of the roaster and two 10lb ABC extinguishes along the path to exit. I bought a handheld propane/methane detector on e-bay ($90) that is well-made and very sensitive. I used it to test the conversion and recheck periodically (soapy bubbles work, but not as well, and this meter is way cool!). I have a carbon monoxide alarm plugged in to a wall outlet close by.


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#2: Post by JK »

My Gauge reads from 0-5" KPA
My highest setting I use is 3.5" KPA with 3" KPA much more common..
Most 1KG roasts I can do at a max of 2.5 KPA..
I only use 3- 3.5" if I get behind my numbers...
I'm on a Mission from God!

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#3: Post by Boldjava »

I recently converted my North TJ-067 from liquid propane (LP) to natural gas (NG). With the growing interest in 1 lb - 1 kg roasters, I thought some description of the process might be helpful to others...
Barry, clear writing and informative. Even I could follow it. Learned some things, thanks. Loved the dime and the accompanying humor.
LMWDP #339