The evaporator is the coil inside the cold box where the chilled liquid boils at low pressure and absorbs heat from the food and air around it. It works best with as much tube surface as the box allows, kept clear of frost, and it must send only dry vapour back toward the compressor.
Part of Refrigeration
Step four of the cycle: the coil where refrigerant boils inside the cold space, the heat it can carry, and the frost, flooding and liquid-return faults that spoil it.
Boiling Inside The Coil Is What Takes The Heat Out Of The Box
The refrigerant arrives at the evaporator colder than the box. Heat flows from the food and air into the tube, and the liquid inside boils. The heat needed to boil it is the latent heat of the fluid, which is far larger than the heat needed just to warm it. The corpus explains the principle for water in Sensible vs Latent Heat.
The carrier’s rule is simple: coil as much tube as you can in the box. More surface moves more heat for the same temperature difference. The evaporator tube is then connected to the compressor suction (or, in an absorption machine, to the absorber), carrying the vapour away so the pressure stays low enough for the liquid to keep boiling.
What You Need
- Tube in a coil or a grid, steel for ammonia, with enough length that the liquid boils off before it leaves the box.
- A thermometer in the box, away from the coil and the door.
- A drip tray and a drain for defrost water.
- A way to see the outlet: frost on the tube leaving the box is a clue to how wet the vapour is.
Method: Reading An Evaporator
Step 1: Look at the frost pattern. When it is right, the coil frosts evenly along most of its length and the tube beyond it is near room temperature or only slightly cool.
Step 2: If frost runs all the way back past the box, liquid is leaving the evaporator unboiled. The restriction is passing too much, and liquid that reaches a compressor will damage it.
Step 3: If only the first turns frost and the rest is warm, the coil is starved; open the restriction a little or check the charge.
Step 4: Defrost before the frost insulates the coil. A thick coat of frost acts as an insulating layer between coil and air, so the box warms while the machine works harder. Stop the cycle and let it melt, and drain the water out of the box.
Worked Example: Heat Carried Per Kilogram Of Boiling Liquid
For comparison with the corpus water figure, the evaporation heat of water is about 2,260 joules per gram. The evaporation heat of ammonia at its atmospheric boiling point is about 1,370 joules per gram (a recalled property value, flagged for verification).
For an assumed 100 W taken from the box:
- Ammonia boiled per second, ideal: 100 ÷ 1,370 = 0.073 g/s.
- Per hour: 0.073 × 3,600 = about 263 g.
That is a best-case figure: it ignores the fraction that flashed at the expansion device and did no cooling, so the real flow is higher. Even so, a modest flow of liquid carries a modest load; the machine’s size depends on how fast heat leaks into the box, not on how much refrigerant it holds.
Common Mistakes
- Letting the coil frost over. Ice insulates and the box warms.
- Running the restriction too open. The evaporator floods and unboiled liquid returns toward the compressor or absorber.
- Putting the coil against a wall. Air must be able to move around it.
- Packing food against the coil. It freezes, and it blocks the air.
- Leaving defrost water to refreeze in the drain.
Safety
Liquid at the evaporator outlet can wreck a compressor and burn skin
Liquid reaching a compressor can break it, because a liquid is nearly incompressible (Pressure Testing makes the point for water), and the cold tubing can stick to skin. A leak of ammonia into a closed cold room fills it with irritant vapour in a space people enter to fetch food, so a cold room cooled directly by ammonia needs ventilation and a way out from inside. See Ammonia Toxicity.
What’s Next
- Learn next: Working Fluids — which fluids boil usefully at which pressures
- Combine with: Cold Room Design — the box the evaporator sits in