Heat pipe

Heat pipe is an item that holds and transfers heat energy along connected segments and imposes a limit on how much energy can pass through each segment over time. Each heat pipe segment reduces the temperature of heat flowing through it by an amount that depends on the power being transmitted; this creates an effective maximum useful length for a straight run of heat pipes between a heat source and a heat consumer.

For a heat pipe entity with one input connection on one side and one output connection on the opposite side, the temperature drop per segment is 1 + (P / 15) °C, where P is the power transmitted in megawatts (MW). Because heat exchangers must reach 500°C to generate steam and heat generators (for example, nuclear reactors) have a maximum of 1000°C, the maximum possible temperature difference between a heat source and a heat exchanger is 500°C. The maximum straight-line length L of heat pipe segments that can carry power P and still allow a heat exchanger to reach 500°C is therefore L = 500 / (1 + P/15). For example, a single nuclear reactor outputting 40 MW of thermal power to a single straight line of heat pipes can reach roughly 500 / (1 + 40/15) ≈ 136 heat pipe segments before the temperature has dropped too far to reach 500°C.

A nuclear reactor can itself act as a thermal conductor similar to a heat pipe whether or not it is fueled. In that role the reactor reduces temperature by 1 + (P / 387) °C per reactor entity, with P in MW; measured values indicate the denominator is approximately 386.847. Because a reactor occupies more tiles than a single heat pipe segment, one must compare it to multiple heat pipe segments in aggregate (for a typical reactor footprint that comparison is to about five lines of five heat pipes). Compared to equivalent length of heat pipe, a reactor reduces temperature less per tile: with near-zero transmitted power it reduces temperature about five times less than equivalent heat pipe tiles, and at very high transmitted power it reduces temperature nearly twenty-six times less. As an illustrative example, a straight line of 100 reactors (equivalent to 500 tiles) carrying 1 GW of thermal power will lower the temperature by roughly 360°C.

In the Space Age scenario on Aquilo, heat pipes are also used to prevent entities from freezing. There heat pipes automatically transfer heat into neighboring entities; different entities consume different amounts of heat to avoid freezing, and they do not require the 500°C threshold of a heat exchanger. Heat pipes used for anti-freeze purposes only need to be at least 30°C to keep an entity warm. Heat pipes do not lose heat to the environment in that mode; they only lose heat to the entities they are warming.

• Practical notes:
  • Temperature drop per heat pipe increases with transmitted power; keep runs short or reduce power per line to preserve temperature.
  • Use the formula L = 500 / (1 + P/15) to estimate maximum straight-run length from a heat source to a heat exchanger for given power.
  • Consider using multiple parallel runs to split power and reduce per-segment temperature loss.
  • When using reactors as conductors, account for their larger footprint and more favorable per-tile temperature loss at low power, but worse behavior at extremely high power.