A refrigerant is chosen by four tests: it must boil well below the temperature of the cold space at a manageable pressure, it must be producible with what a settlement has, it must not destroy the materials it touches, and its leak must be survivable. Ammonia, which boils at −33 °C at atmospheric pressure, passes the first two best and fails the last two unless the system is built in steel and kept ventilated.
Part of Refrigeration
How to compare candidate refrigerants by boiling point, working pressure, local availability, material compatibility and leak hazard.
The Boiling Point At Atmospheric Pressure Tells You Which Side Of The Atmosphere The Machine Works On
The refrigerant has to boil in the evaporator at a temperature below the cold space. Its normal boiling point, the temperature at which it boils at atmospheric pressure, therefore sets the pressure the cold side must run at.
- A fluid that boils below the cold-box temperature at atmospheric pressure (ammonia at −33 °C, propane at −42 °C, sulfur dioxide at −10 °C) lets the low side run at or above atmospheric pressure. A leak then pushes refrigerant out.
- A fluid that boils above the cold-box temperature at atmospheric pressure (diethyl ether at 34.6 °C) needs the low side held below atmospheric pressure to boil at all. A leak then pulls air in, and with a flammable fluid that makes an explosive mixture inside the machine.
- A fluid that never becomes a liquid at atmospheric pressure (carbon dioxide, which sublimes at −78.5 °C) needs the whole circuit at high pressure.
The boiling points and the carrier’s remarks on each fluid are in the table. The relative weights are my own arithmetic from molar masses, with air taken as 29 g/mol.
| Fluid | Normal boiling point | Weight against air | Main drawback |
|---|---|---|---|
| Ammonia | −33 °C | Lighter (17 against 29) | Toxic, flammable in a band, attacks copper |
| Propane | −42 °C | Heavier (44 against 29) | Highly flammable, no smell unless added |
| Sulfur dioxide | −10 °C | Heavier (64 against 29) | Highly toxic, corrosive |
| Diethyl ether | 34.6 °C | Heavier (74 against 29) | Extremely flammable, explosive; low side below atmospheric |
| Carbon dioxide | −78.5 °C (sublimes) | Heavier (44 against 29) | Very high pressures; suffocates |
The weight column decides where a leak collects. A lighter-than-air vapour tends to rise out of a room; a heavier one sinks to the floor, into pits and into cold-room corners, where people crouch to lift crates.
What You Need To Decide
- The cold temperature you need. A box at a few degrees above freezing and a freezer at −18 °C need different pressures from the same fluid.
- The climate for rejecting heat. The warmer the air, the higher the condensing pressure. For carbon dioxide, which has a low critical temperature of about 31 °C (a recalled figure, flagged), warm air can leave no liquid to form at all.
- What the settlement can make. The carrier lists ammonia as producible from decayed urine, by the Haber process, or by heating ammonium chloride with lime; Industrial Chemistry covers those routes.
- What the pipe and fittings are made of. Copper and brass cannot go with ammonia.
Method: Pick And Check A Fluid
Step 1: Fix the coldest temperature the machine must reach. That sets the lowest evaporating pressure you need.
Step 2: Find the fluid’s vapour pressure at the highest summer air temperature plus a margin for the condenser. This is the highest pressure the circuit will see in operation, and it sets the strength the pipe and vessel must have. Use that fluid’s own property table. If you cannot find one, do not build.
Step 3: Check what the fluid does to the materials you will use. Pipe, seals, gaskets, solder, fittings.
Step 4: Plan the leak. Where does the vapour go, who is nearby, how will you know? Ammonia announces itself by smell; propane and carbon dioxide do not unless something is added.
Worked Example: Where A Leak Collects
Using molar masses (N 14, H 1; C 12; S 32, O 16) and air at 29:
- Ammonia, NH₃: 14 + 3 = 17, and 17 ÷ 29 = 0.59 times air.
- Propane, C₃H₈: 36 + 8 = 44, and 44 ÷ 29 = 1.52 times air.
- Sulfur dioxide, SO₂: 32 + 32 = 64, and 64 ÷ 29 = 2.2 times air.
These are vapours at room temperature. A release from a pressurised liquid is cold and carries droplets, and a cold ammonia cloud can hang low at first before it warms and rises (a recalled behaviour, flagged). A leak detector or a ventilation opening placed on the wrong level protects nobody.
Common Mistakes
- Choosing by availability alone. The fluid that is easiest to make may be the one that kills fastest in a closed room.
- Using a fluid from the table without its pressure data. Boiling point at atmospheric pressure is not a working pressure.
- Assuming “non-toxic” means safe. Carbon dioxide is not poisonous but displaces air in a closed space.
- Mixing fluids to improve performance. A mixture has no single boiling point and complicates every calculation.
Safety
Every fluid in this table can hurt or kill if it escapes
Ammonia irritates and burns, sulfur dioxide is highly toxic, ether and propane burn or explode, and carbon dioxide displaces breathable air. Whichever you choose, keep the plant outdoors or in a separate ventilated building, and never work alone near a charged circuit. See Safety.
What’s Next
- Learn next: Ammonia — the practical choice and its two big drawbacks
- Combine with: Other Refrigerants — why the alternatives are harder