My Thanks Go Out To: - solidity, who wrote the original LUA script to simulate boiler heatup, which served as the base for mine - MilConDoin, who provided me with a couple formulas I was missing and pointed me towards the script in the first place !Attention! This article was written during the 1.4.x era. Due to changes implemented in later versions of Railcraft and/or other mods, some fuel value numbers, MJ production numbers and/or editorializing statements presented herein may not be correct anymore in all cases. However, the main analysis is calculated directly in Heat Units (HU), and the derived conclusions remain independent of changes up to Railcraft 184.108.40.206. Due to a complete boiler rework in Railcraft 220.127.116.11, this analysis no longer applies to that version and beyond. The Purpose Of This Post I'm by far not the first one to tackle the subject of steam boiler math (especially since I've been procrastinating on writing this up for so long, lazy as I am). There's spreadsheets aplenty and even online calculators that tell you exactly how long any boiler will take to heat up, how much fuel it will consume during that period, and how much fuel it will continue to consume once at maximum heat. There is, however, one topic that is often overlooked, or even misunderstood and done wrong (particularly by those who are just looking to use boilers, not to do math on them). That topic is fuel efficiency. Now, we live in an age of infinite fuel loops. Powering a max-size boiler 24/7 is trivial; many people have never built anything but max size boilers, even if these things are completely oversized for their needs and most of their output is quite often wasted unused. So what, it's gonna be running around the clock anyways, and all that matters is that the output is available when you need it. Why, then, would anyone even bother with the (actually quite complicated) issue of boiler fuel efficiency at all? Because I'd like to clear up those misunderstands that are being handed about for truths. A lot of this really goes down to imprecision in language; a forum poster who once read somewhere that "high pressure boilers are just as efficient as low pressure boilers after heatup" will happily repeat this information to other users, without stopping a moment to ask himself what the person who originally made that statement actually meant to say, and whether the correct words were used. The truth of the matter is that after heatup, a high pressure boiler will create just as much steam per unit of fuel as a low pressure one; but it will never be as efficient overall as a low pressure boiler. This might seem like a trivial distinction - and some might not even understand where the difference is in the first place - but that's just because it's so easy to make the mistake. As I am going to show you below, the actual impact in gameplay is not trivial at all. And while I'm at it, I am going to show you graphs that visualize precisely how every single possible boiler performs in terms of energy production, both over a certain time and with a certain amount of fuel. I'm also going to provide a spreadsheet that has all these graphs for you to play with, as well as a fuel value list, a MJ calculator, and some general performance metrics. And finally I'll throw in the LUA script I wrote to calculate all these things, so you can even experiment with it ingame. Let's get started. So You Want To Build A Steam Boiler The first question is: how much energy do you need? That will decide the size and type, and therefore the power output, of your boiler. The second question, then, is: what kind of fuel options do you have? Based on that, you decide whether you want to build a solid or a liquid fueled boiler. Here is a table showing you all the different fuel options you have. (side note: blaze rods got un-nerfed slightly to 800 HU in the latest Railraft build, but no FTB pack includes it yet at the time of this posting.) Now, if you were concerned with fuel efficiency, what fuel you have available - specifically, the amount of it - should also prompt you to think about what size and type of boiler you really need. Everyone understands that bigger boilers give you more MJ for your fuel... but by far not everyone understands that that's actually not always true. If your fuel supply is not infinite, but in fact limited, then you may very well run into a situation where the opposite is true. The reason for this lies in the most commonly ignored trait of these devices: Steam boilers are catastrophically inefficient during heatup. Visualizing Energy Output Let's look at a couple of sample fuel profiles. 1 Million Heat Units - Just shy of 10 stacks of charcoal, or just over 10 buckets of fuel. 'Wow, I have 10 stacks of charcoal', the newbie player thinks. 'Maybe I should finally stop messing around with stirling engines and build something really powerful! I bet I could run a really big steam boiler with this!' ...Or can he? The graph I am showing you here is a fuel-energy graph. The horizontal X-axis is labeled with millions of HU, and the vertical Y-axis is labeled with millions of MJ. The hallmarks of a good and efficient steam boiler, then, would be to be listed as high up as possible, as early as possible. The less heat units are required to reach a given amount of MJ, or the more MJ produced with a given amount of HU, the better. And what boilers gives us the most bang for our buck here? The 1 LP variant, the tiniest and most humble of them all. It managed to pull ten times more MJ out of the available fuel than a 36 LP boiler. The 27 HP and 36 HP variants don't even show up in this graph; they produce not a single MJ. 10 Million Heat Units - Three a half wooden chests full of charcoal, or six and a half tank blocks full of fuel Okay, the previous example was a little extreme... let's give more than just the smallest four boilers an actual chance to finish heating up. The only ones that do not finish in this example are the 27 and 36 HP variants. Do note, though, that the amount of fuel used here already borders on ridiculous. Nobody builds up that kind of charcoal stash; you'd need a diamond chest just to store it. Only liquid fuels really make sense to keep in storage at this point, everything else better be supplied by a constant source. It should give you pause to realize that with only the second graph I'm showing you, we're already in "infinite fuel loop" territory. There's two more after this. Here, we see the low pressure boilers, one after the other, execute an upwards turn as they come out of heatup and from then on burn fuel at maximum efficiency. And due to that, one after the other eventually passes the 1 LP boiler in total MJ generated (though it continues to lead up until about 2.75 million HU). At the end of this chart, all the low pressure boilers have ordered themselves into their expected ranking, with 36 LP leading and 1 LP at the bottom. You know what didn't pass the 1 LP boiler, though? Any high pressure boilers. Those are still zigzagging about below, even though the 1, 8, 12 and 18 variants have all finished heatup in this graph. The 8 HP one does it as early as 1.8 million HU, but it still struggles to catch up. Even after well over a golden chest worth of charcoal, the 1 LP boiler still manages to produce more total MJ than any and all high pressure boilers available. So much for the supposed fuel efficiency of HPs, eh? 32 Million Heat Units - Almost three diamond chests full of charcoal, or about the entire contents of a small oil spout Finally, just north of 22.3 million HU, the last of the boilers finishes heatup. The 1 LP boiler is soundly overtaken by everything except the hobbyist engine and its own HP counterpart (which is invisible here due to the sheer scale of the graph). The larger high pressure boilers attempt to hurry upwards and sort themselves into the fan spread of the low pressure ones, but despite pouring an entire oil well worth of fuel down their figurative gullets, they still can't quite manage it yet. From an efficiency standpoint, a 12 LP boiler would still give you better fuel efficiency here than a 36 HP boiler. And obviously, the larger low pressure boilers are even better. There's a gap of more than 75 million MJ between the 36 LP and HP variants. 96 Million Heat Units - Ungodly amounts of just about anything And there we are! After enough fuel input to power a death star for a field trip to Alderaan, everything finished ordering itself into its right and proper place. And here is what people mean when they talk haphazardly about 'efficiency' being the same. Notice how all the high pressure boiler lines are exactly parallel to their low pressure counterparts; they produce MJ at exactly the same rate. However, since the lines are exactly parallel, that also means that the gap will remain forever constant. The 36 HP boiler is still about 75 million MJ below the 36 LP variant, and it will forever remain about 75 million MJ below. That is why using the correct words is important, because while energy output per fuel is indeed identical, overall efficiency is clearly not. You can, of course, keep increasing the size of the graph. As you do, you will keep zooming out to larger and larger scales, and the gaps between the lines will eventually appear to vanish, as you can see happening here with the 8 LP and 8 HP boilers. The gap is still there, and it is forever constant, but if you zoom out far enough it will become negligible in the overall scale of things. Thus we approach infinity, and thus the careless simplification of "high pressure has the same efficiency as low pressure" begins to work. But it works only out there, at infinity. For anything below infinity, it is wrong. How wrong - that depends entirely on your scale.