Everything liquid, solid or gaseous here is made of particles that carry heat. Warm water is redder, cold water bluer. Frozen particles turn into pale squares and lock together into rigid lumps that float or sink like real ice. Vapour is small translucent dots that drift up if lighter than air and pool on the floor if heavier.
Building
Walls are solid slabs. Steel conducts heat, insulation blocks it, glass is in between. Heaters and chillers hold a fixed temperature. A door is an insulated wall you can open with the Stir tool.
Pipes are hollow: fluid inside stays inside. Pick a bore; narrow bores let less through. Ends are open, so drag the end of one pipe onto another to join them (the joint seals itself). Copper pipes trade heat with whatever touches them, insulated ones don't.
Fluid brush pours the chosen fluid at the chosen temperature. Below its freezing point it arrives as solid.
Heat gun / freeze ray add or remove heat where you hold them. Watch the temperature pause at the freezing and boiling points: that pause is latent heat.
Moving things
Liquid pump: click IN, click OUT. Liquid near IN reappears at OUT. Inside pipes it also drags fluid along towards IN.
Vapour compressor: the same two clicks, but it only takes vapour. Vapour that goes through comes out on the high-pressure side, warmer and with a much higher boiling point, so it condenses easily. That warm condensing is where a heat pump gives off heat.
Throttle valve: placed in a pipe, it lets high-pressure fluid back to normal pressure, and only in that direction. A liquid crossing it partly flashes to vapour and turns very cold. That is where a heat pump takes heat in.
Thermostat: place it, then click a compressor to wire them. Above the setpoint it runs the compressor, below it stops it. Thermometers just read.
Fans move air and vapour; faucets pour any fluid, drains swallow anything.
Select & move (G): click anything you built to select it, drag it elsewhere, press Delete to remove it. The eraser does the same with one click.
A heat pump (or fridge) in six moves
Lay a closed loop of copper pipe. Make one part pass through the space you want cold and another through the space that may get warm.
Place a compressor with IN at the end of the cold part and OUT at the start of the warm part.
Place a throttle valve where the warm part flows into the cold part.
Charge the loop with a fluid brush: anything that is vapour at room temperature — refrigerant, nitrogen — is poured as pressurised liquid. Water works too, as a very slow heat pump.
Optionally wire a thermostat to the compressor and box the cold side in insulation with a door.
Turn on the air temperature overlay in the panel to watch the cold pool and the heat leak.
Keys
Tools: G (select/move) V W T F H C P X S D M A E · Space pauses · R resets the demo · 1–7 switch demos · Shift while drawing makes straight lines · Delete clears fluids · ? opens this.
Mixtures and distilling
Water and alcohol dissolve in each other, so a wash behaves as one liquid: it boils at a temperature between the two (a 20% wash boils at 88°), and the vapour coming off is much richer in alcohol than the liquid. Condense that vapour and you have spirit. The enrichment follows the real vapour–liquid curve, including its ceiling: at 96% ABV the vapour has the same strength as the liquid, so no amount of distilling gets you past it. Oil, mercury and the refrigerant do not dissolve in water and stack up by density instead.
The full write-up of the model, its equations and its assumptions is in MODEL.md.
Heat moves about sixty times faster than in real life so that an hour of fridge fits in a minute. Thermal expansion is exaggerated by the World slider. Gravity, latent heats, boiling points, densities and viscosities are real.