Heat exchangers and heat transport: pipe to pipe, gas to liquid

Often you want to move heat from one place to another without mixing the substances: cool hot exhaust without leading it into the cooling loop, or couple a water loop to a gas loop. That is what heat exchangers are for. At the end of every chain sits a radiator that hands the heat to the surroundings.

A network already transports heat#

The good news first: within one pipe network you do not need to transport heat. The game treats a network like a single container; temperature and pressure are the same everywhere. If a Wall Cooler and a Pipe Convection Radiator hang on the same network, the cooler's heat reaches the radiator without any pump.

You only need heat exchangers when two separate networks should exchange heat.

The exchangers at a glance#

Type	Ports	Key figure per game data
Small Direct Heat Exchanger - Gas + Gas	two inputs, one network each	exchange area 1
Large Direct Heat Exchanger - Gas + Gas	two inputs, one network each	exchange area 8
Super-Large Direct Heat Exchanger (Gases)	two ports	exchange area 28
CounterFlow Heat Exchanger - Gas + Gas	two pairs of input and output	counterflow
CounterFlow Heat Exchanger - Gas + Liquid	gas and liquid	counterflow
CounterFlow Heat Exchanger - Liquid + Liquid	two liquid pairs	counterflow

The direct heat exchangers sit between two networks and let heat pass from the warmer to the colder network. The larger the exchange area, the more heat per tick. The small one comes from one kit, the large one from one kit plus a Kit (Pipe) and a steel sheet, the super-large one from three of each. All exist for gas and gas, gas and liquid, and liquid and liquid.

The counterflow exchangers have one input and one output per side. Two streams pass each other in opposite directions. According to the game code they are passive: no power, no pump of their own. Each tick they let through as much on each side as makes input and output equal in pressure – without a pressure drop nothing flows. So plan a pump or a pressure regulator that drives the stream through the device, and check with an analyzer before and after it whether it flows. All four pipes must be connected. On a gas side below 101.325 kPa the transfer drops in proportion to the pressure; a liquid side counts in full as soon as 0.1 % of its volume is liquid. Setting appears in the logic list but has no effect.

How close the temperatures get#

The cold side cannot take up more heat than the hot side gives off. If two networks exchange until both are equally warm, their shared temperature follows from the heat capacities:

• T = (C₁ · T₁ + C₂ · T₂) / (C₁ + C₂), with C = amount × heat capacity

An example with two gas networks: 500 litres of nitrogen at 5,000 kPa and 400 K are 752 mol, C₁ = 752 · 20.6 ≈ 15,500 J/K. The cold network also has 500 litres of nitrogen, but only 100 kPa and 290 K: 20.7 mol, C₂ ≈ 427 J/K. The shared temperature is about 397 K – the hot network cools by just 3 K. A thin gas network can hardly take heat from a dense one.

Gas to liquid#

That is why liquid pays off as a coolant. According to the game data water stores 72 J per mole and kelvin and takes up 0.018 litres per mole:

Content per litre	Amount	Heat capacity
nitrogen, 100 kPa, 20 °C	0.041 mol	0.85 J/K
nitrogen, 10,000 kPa, 20 °C	4.1 mol	85 J/K
water	55.6 mol	4,000 J/K

The same hot gas network as above (C ≈ 15,500 J/K, 400 K) on a water loop of 200 litres at 290 K (C = 800,000 J/K) ends up at about 292 K. The water itself only warms by a little over 2 K.

With water mind two limits: below 273.15 K it freezes in the pipe into chunks of ice, and at 100 kPa it boils at about 384 K in the game. Liquid lines hold at most 6,079.5 kPa.

Getting rid of the heat#

In the end the heat has to go to the surroundings. Radiators do that, depending on the world through the air (convection) or radiation:

Radiator	Convection	Radiation	meant for
Pipe Convection Radiator	1.05	0.79	atmosphere
Pipe Radiator	0.23	3.46	vacuum
Medium Convection Radiator	9.38	3.00	atmosphere
Medium Radiator	1.50	30.0	vacuum
Large Extendable Radiator	0.74	73.6	vacuum, liquid; only when extended

For comparison: a straight pipe piece has convection 0.005. The large radiator reports through EnergyConvected and EnergyRadiated how much it moved. The medium ones list these values in their logic too, but have no data port – there you measure with a pipe analyzer on the loop.

Two rules always apply:

• A radiator on a loop that is colder than its surroundings pulls heat in. On Vulcan, where nights do not drop below about 127 °C, and on Venus at 464 °C it only cools very hot loops.
• Radiation radiators also absorb heat in the sun. The medium radiator has a high value of 15 for that. The large extendable radiator additionally loses radiation in full sun – about 46 instead of 73.6, as long as the surroundings are colder than its contents. Turn it edge-on to the sun with Horizontal, and retract it during storms: it only takes weather damage while extended.

A typical cooling chain#

1. Wall coolers pull heat from the rooms into a gas loop.
2. A CounterFlow Heat Exchanger - Gas + Liquid hands it on to a water loop.
3. The water loop runs outside through insulated liquid lines.
4. Radiators outdoors shed the heat.

Insulate every stretch on which no heat should be exchanged – otherwise a warm return line heats your rooms up again.

Where next#

• Heating and cooling: wall heater, wall cooler, air conditioner, radiators – devices, outputs and thermostat
• Evaporating, condensing, freezing: using phases on purpose – moving heat with evaporation and condensation
• Pipes and lines: gas, liquid, insulated – networks and insulated lines
• Substance values: Gases