BTU per pound? Scaling from a small to a large production coffee roaster?

I received an email from a customer this morning with a good question about BTUs:

“I am seeking a roaster that can do somewhere in the range of 20-25 pound batches at 12 or so minutes. we are on an 8 pound roaster now and we can get 8pounds to FC at between 8-9min and finish up before 13min.

Our current 3kg USRC roaster was advertised as an 8 pound roaster but came with something in the range of 15-20k BTU burner and it would take 18-25 minutes to get 8 pounds done. We upgraded the burner and it is is now producing somewhere between 35-40k BUTs. Before this roaster we had a Mill City 1kg and we did 1500g batches wonderfully. That roaster is rated at 16k BTUs. Between these two roasters about 5000 per pound has been consistent. Now as I consider my upgrade, I was first looking at your 6kg and 10kg roasters however I notice the BTUs for those roasters is 54,400 and 67,200. Although the max batch is stated as 15.8 lbs and 26.4 lbs I am concerned there will not be enough energy from the burner to roast that size batch in the time frame we like to roast.”

This was my reply:

Hi I–.
 
BTU spec for our roasters is based on the CSA certification testing at 5.5″ water column natural gas pressure. The roasters are designed to allow more than triple that gas pressure. The 10kg roaster generates 158000 BTUs at 14″ natural gas.*
 
Beyond that, mass matters. We build much heavier machines that absorb and release enormous quantities of heat. There’s a huge difference in performance between the “input BTU”** spec and the way a machine is built to transfer heat to seeds. Suffice to say, it’s not as easy as it looks. Construction matters a lot.*** You’ll probably never get close to using the entire 14” WC on a 10kg roaster.
 
Lastly, small machines roast differently from large machines. There is a limit to how much heat any individual seed can stand before passing over to heat defect. This means that, all things being equal and to avoid heat defect, bigger batches must be roasted marginally slower than larger batches. As a practical matter, a 10kg roaster will successfully run much closer to a 1kg roast profile than a 20kg. Coffee cup quality can be maintained and even improved in successively larger machines, but it requires professional sensory analysis, roast profile planning, and advanced machine control skills. 
 
The good news is your business is growing and you’re scaling. The bad news is you’re not done learning and there is no book or video that will guide you through the work it takes to figure this stuff out on your machine and with your coffee. A good class from people that roast for a living might help, but after that it comes down to doing your best batch after batch and blind cupping the results. About 1,000,000 pounds into this process, you will probably have figured a couple of things out and you’ll likely be pretty darn good at it.
 
Lastly, I know there’s a lot of heuristic chatter about BTU’s, thermocouples, declining ROR, crash-n-flick, etc, etc. All of this stuff is valid as far as it goes, but a shocking number of people spouting this stuff don’t roast very well and their coffee quality shows it. Worse, the people hollering the loudest about this stuff do not production roast at all, but you’d have to hold a gun to their head to get them to admit that.
 
Do yourself a favor and very carefully consider the source. Better yet, order some of their coffee. Whether it’s of a quality you aspire to or over or under roasted dreck, you’ll have a reasonable basis with which to weight their advice.
 
Call me if you have any questions.
 
Steve

* Calculation of BTUs for all MCR roasters at CSA and non-CSA gas pressure:

**one BTU is defined as the amount of energy necessary to raise one pound of water one degree Fahrenheit. Input BTUs are calculated from the volume of gas passing through any given size orifice at any given gas pressure for a period of one hour. One cubic foot if natural gas contains approximately 1030 BTU. So a single .7mm orifice fed burner passes roughly 45 cubic feet of natural gas equaling about 4600 BTUs. Please bear in mind that these numbers are not representative of gas consumption as your roaster is operated at far less than “max” gas over the course of a roast profile.

***Re. construction: To take full advantage of our larger burner sets, relative to industry standard, our roasters have thicker double walls and greater diameter/capacity drums with more surface area to allow maximum efficiency contact time from seed to drum without automatically crossing over into conductive heat defect. Maximum efficiency of convective heat transfer is achieved with more powerful and more precisely controlled high performance centrifugal roaster exhaust fans. Lastly, aggressively roast profiled full batch sizes are more quickly cooled with a combination of our 3x capacity cooling tray capacities, self adjusting stirring arm assemblies, and oversized cooling exhaust fans.