It took me 4 days to figure out the ratios related to nuclear power, so I figured I'd share.
The main thing to remember is that Factorio generally follows the laws of thermodynamics; if your nuclear reactor produces 40 megawatts of heat, you can get a maximum of 40 megawatts of electricity out the other side.
(Except that you can potentially turn 8 gigajoules of uranium fuel cell into 40 gigajoules of heat. Don't ask me how that works.)
Each uranium fuel cell will power a nuclear reactor for 200 seconds.
A powered nuclear reactor outputs 40 megawatts of heat, plus an additional 40 megawatts for each powered nuclear reactor directly adjacent to it. (100% neighbor bonus per adjacent reactor).
A perfect square of reactors has the highest theoretical efficiency, but at 3x3 and above you'll have reactors surrounded on all 4 sides and have no way to load the uranium fuel cells.
The true maximally efficient layout is a 2 by X rectangle; this gives you access to every reactor. This can be extended as far as you want; every additional 2 reactors will provide an additional 320 megawatts of heat output.
If you run an odd number of reactors, you should have a 2 by X rectangle with one reactor dangling off the end.
|Reactors||Heat output (MW)||MW per reactor|
|Reactors||Heat output (MW)||MW per reactor|
|n even||160n - 160||160 - 160/n|
|n odd, >1||160n - 200||160 - 200/n|
Nuclear reactors have a maximum temperature of 1000 ° C.
Unlike boilers, nuclear reactors will not slow or stop their fuel consumption if their output isn't being used; they'll constantly use up fuel cells at the normal rate of 1 fuel cell / 200 seconds. If you overbuild reactors, you can end up wasting a lot of fuel cells without realizing it. Heat Pipes
Heat pipes are used to transfer heat from your nuclear reactors to your heat exchangers.
If your heat pipe is too long, your reactors will max out at 1000 ° C before your heat exchangers can reach a steady state of 500 ° C, and and you'll start to waste heat.
Each heat exchanger takes a maximum input 10 megawatts of heat and uses it to heat water into steam.
They only work when they're above 500 ° C, and have a maximum temperature of 1000 ° C.
Temperatures above 500 don't increase efficiency; the exchanger will just store the heat, which it can then use later.
|n even||16n - 16|
|n odd, >1||16n - 20|
The maximum length of heat pipe you can use depends on the combined distance of your heat exchangers from your reactors. The more heat exchangers you want to put on a single length of heat pipe, the shorter that heat pipe has to be to ensure minimal heat loss; e.g. you can put 4 heat exchangers at the end of ~135 heat pipes, but you can put 16 heat exchangers only at the end of ~50 heat pipes.
The most heat exchangers I've been able to fit on a single length of heat pipe is 30 heat exchangers on 44 heat pipes; any more than that incurs significant heat loss.
Each steam turbine take a maximum input of 60 units of 500 ° C steam per second and outputs 5.82 megawatts of electricity; the 5.8 megawatts listed on the tooltip is rounded.
The true value can be comes from the following facts:
It takes 1 kilojoule of thermal energy to raise 5 units of water/steam 1 ° C You've heated water from 15 ° C to 500 ° C; 500 - 15 = 485 Consuming steam in turbines gives you the same amount of energy back as electricity Steam is consumed by steam turbines at a rate of 60 units/second 485 / 5 * 60 = 5820 KW, or 5.82 MW
As each heat exchanger produces 10 MW, the optimal ratio is 500 steam turbines for every 291 heat exchangers.
Each offshore pump outputs 1200 units of water per second.
Optimal ratio is 1 offshore pump for every 20 steam turbines; or, 25 offshore pumps for every 291 heat exchangers.
Remember that water in pipes still obeys Factorio physics; if you pipe your water a long distance, you may not get the full 1200/s.
Final ratio: 25 offshore pumps : 291 heat exchangers : 500 steam turbines.
Here are the total requirements for certain amounts of reactors, with everything rounded up to guarantee maximum energy production
|Reactors||Heat exchangers||Offshore pumps||Steam turbines||Total electricity (MW)|
/u/asdjfsjhfkdjs calculated the convergents of 500:291, and found that 7 : 4 and 55 : 32 are both fairly accurate approximations. If you want a ratio that's a little easier to remember, those are probably your best bet.
As far as I can tell, the absolute smallest perfect-ratio setup possible is:
292 nuclear reactors
400 offshore pumps
4656 heat exchangers
8000 steam turbines
…which would require an input of 2.92 uranium fuel cells per second and output a cool 46.56 gigawatts of electricity.
EDIT NOTE: The original version of the post used the wrong output for steam turbines (5.8 MW instead of 5.82). I've confirmed that the true value is indeed 5.82, and updated everything accordingly.