How a cooling system actually works
Every AC unit, fridge, cold room, and heat pump is the same four-part loop moving a refrigerant around in a circle. Heating and cooling are the same machine — a heat pump is just an AC run for the heat it rejects instead of the cold it makes.
The vapor-compression loop
Refrigerant flows clockwise. The top edge is hot and high-pressure; the bottom edge is cold and low-pressure. The two heat exchangers (condenser & evaporator) are where heat actually moves; the compressor and valve set the pressures that make it possible.
Compressor
Raises pressureCold low-pressure vapor leaves the evaporator. The compressor squeezes it into hot high-pressure gas. This is where almost all the electrical energy goes — and what we're trying to minimize.
⚙ Work in ≈ enthalpy rise (h₂ − h₁). Lower the pressure lift and this work drops.
Condenser
Rejects heatThe hot gas flows through a coil exposed to a cooler medium (outdoor air, water, or the ground). It dumps its heat and condenses into a high-pressure liquid.
⚙ Heat out = h₂ − h₃. A colder sink means a lower condensing temperature — the single biggest efficiency lever.
Expansion valve
Drops pressureThe liquid is forced through a tiny orifice. Pressure crashes, a little flash-boils, and the temperature plummets — turning warm liquid into a cold low-pressure mixture.
⚙ Enthalpy is conserved (h₄ = h₃). The drop in pressure is what creates the cold.
Evaporator
Absorbs heatThe cold mixture passes through the space or product you want to cool. It boils as it absorbs heat, leaving as vapor — and the loop starts over.
⚙ Useful cooling = h₁ − h₄. This, divided by compressor work, is your COP.
The one number that matters: COP
Coefficient of Performance = useful heat moved ÷ electrical work in. A COP of 4 means every 1 kW of electricity moves 4 kW of heat. Because you're moving heat rather than making it, COP is almost always greater than 1 — that's why heat pumps beat resistive heaters.
COP = (h₁ − h₄) / (h₂ − h₁)
COP = (h₂ − h₃) / (h₂ − h₁)
The smaller the gap between evaporator and condenser temperature, the less the compressor works and the higher the COP. Everything in the simulator comes back to widening or narrowing that gap.