Ammonia is the refrigerant a settlement can actually make: it boils at −33 °C at atmospheric pressure, it can be prepared from decayed urine or by heating ammonium chloride with lime, and it works at pressures a steel pipe can hold. Its price is that it is toxic, flammable in air between 15 and 28%, and corrosive to copper and copper alloys, so every pipe, valve and fitting must be steel.
Part of Refrigeration
The properties of ammonia as a refrigerant, where it comes from, what it attacks, and how a system should be built around those facts.
Ammonia Combines A Useful Boiling Point With Ingredients A Settlement Has
The refrigeration carrier calls ammonia the best option for a rebuilding civilization, and lists its advantages: very effective heat transfer, producible from natural materials, a strong smell that makes a leak immediately noticeable, no ozone damage, and working pressures that are not extreme. The carrier also lists its disadvantages: toxic above 300 ppm, flammable at 15 to 28% in air, and corrosive to copper alloys.
Its boiling point of −33 °C at atmospheric pressure means that a low side held near atmospheric pressure can already give freezing temperatures, so a leak on the low side pushes ammonia out and not air in. At ordinary ambient temperatures it is a gas unless held under pressure. For orientation, its saturated vapour pressure is roughly 8 to 9 bar absolute at 20 °C and 13 to 14 bar at 35 °C (recalled property values, flagged for verification). The point is the steep rise with temperature: a vessel that is comfortable on a cool day is working much harder on a hot one, and a vessel in a fire cannot be expected to hold. This is a property of the fluid, not a design pressure, and the rated pressure of any vessel or pipe must come from the vessel’s own rating; see Pressure Vessels.
Where The Ammonia Comes From
Industrial Chemistry lists the routes in order of difficulty: the Haber process from nitrogen and hydrogen (high pressure, a catalyst, a plant of its own), decomposed urine (small quantities, concentrated by distillation), and heating sal ammoniac with slaked lime, which releases ammonia gas that can be captured in water. For an absorption machine the product used is not pure gas but a 25 to 30% solution in water, the carrier’s charge strength, as Ammonia-Water System explains.
Ammonia dissolves very readily in water, which is what makes the absorption cycle work. It also means that moisture getting into a compressor-type system is hard to remove from the ammonia, and that water in the refrigerant raises its boiling temperature (recalled behaviour, flagged).
What You Need
- Steel or iron for everything the ammonia touches: pipe, vessels, valves, gauge elements, fittings. Do not accept brass, bronze or any other copper alloy anywhere in contact with it, even in a gauge or a valve seat.
- Materials with caution. The carrier also lists aluminium. Aluminium is used with dry ammonia in some industrial plants, but a water-bearing ammonia solution is strongly alkaline and attacks aluminium (a recalled behaviour, flagged), so in an ammonia-water machine treat steel as the default and aluminium as unproven. Treat zinc coatings the same way (flagged).
- A site that is outdoors or ventilated, separate from living quarters.
- A supply of clean water nearby for emergencies.
Method: Preparing To Build With Ammonia
Step 1: List every material that will touch the fluid. Walk the whole circuit pipe by pipe and valve by valve. One brass fitting is enough to start a leak at that joint.
Step 2: Choose seals and gaskets that ammonia does not destroy. Check each one against the fluid before use; Gaskets and Seals and Gasket Types cover the corpus’s options for other fluids, and none of them is a statement about ammonia.
Step 3: Pressure-test with air or water, not ammonia. The empty circuit is tested before any ammonia goes in; the carrier says to brush soapy water on the joints.
Step 4: Mark the circuit. Label it as ammonia, with a note that copper alloys must never be fitted, so a later repair does not substitute a brass part.
Worked Example: A Leak Is Detectable Far Below The Level That Hurts
Using the carrier’s figures: smell at 5 to 50 ppm and toxic at 300 ppm. Even at the top of the smell range the toxic figure is 300 ÷ 50 = 6 times higher, and at the bottom it is 300 ÷ 5 = 60 times higher. A person in a room that smells faintly of ammonia has a considerable margin.
That margin is real but not unlimited. Smell fades with exposure, a person who is asleep or drunk will not notice, and a burst or fracture can release far more than a slow leak. The smell is an alarm, not a measurement of safety. The carrier’s own figure for danger is 2,500 ppm, which it says can be fatal within 30 minutes.
Common Mistakes
- A brass pressure gauge on an ammonia line. The gauge’s internal parts are copper alloy. Use one made for ammonia.
- Assuming that smelling nothing means no leak. The nose adapts.
- Mixing ammonia with household bleach or other chlorine products. They react to give toxic gases (recalled chemistry, flagged); keep them apart.
- Draining ammonia solution to a pond or stream. Even low concentrations harm fish; see pH and Ammonia.
Safety
Ammonia burns eyes, lungs and skin, and burns in air
The carrier gives 300 ppm as the toxic level, 2,500 ppm as potentially fatal in 30 minutes, and 15 to 28% in air as flammable. Liquid ammonia on skin freezes and burns; get it off with plenty of water for at least 15 minutes, as the carrier says. Never enter a space after a large release without a way out and a second person. See Ammonia Toxicity.
What’s Next
- Learn next: Ammonia Toxicity — ventilation, detection and response
- Combine with: Absorption Refrigeration — the cycle that makes the best use of ammonia