Build an absorption refrigerator from steel vessels and steel tube only, prove every pressure part with water before any ammonia goes in, check the joints for leaks with soapy water, and heat it first with a water bath so the vessel cannot be driven above the bath’s temperature. Start with the batch machine, which needs no pump and no valves, and fit a relief device the operator cannot defeat before the first fire.
Part of Refrigeration
The materials, assembly order, pressure and leak testing, charging and first run for a village-scale ammonia-water absorption refrigerator.
Steel Pressure Parts, Proven With Water, Come Before Any Ammonia
The corpus carrier lists the parts: steel pipe of 12 to 25 mm for the refrigerant lines, a thick-walled steel vessel for the generator, a similar vessel for the absorber, steel or aluminium coil tubing, a heat source, an insulated box, threaded fittings with high-temperature gaskets, and a 25 to 30% ammonia solution. Two of those need a second look.
- Copper is out, and aluminium is doubtful. The carrier says steel, iron or aluminium. A water-bearing ammonia solution is strongly alkaline and attacks aluminium, so use steel (recalled behaviour, flagged); see Ammonia.
- “An old pressure cooker or steam boiler” is not a certified ammonia vessel. The corpus’s own rule for pressure parts, in Vessel Construction, is to use a vessel of known history and proven strength, not one of unknown service. A vessel that held fuel, chemicals or compressed gas is not a candidate.
What You Need
- Vessels of known pressure rating and history for the generator and the absorber; if you must build one, from a code drawing and by tested welders, as that article describes.
- Steel tube for the lines, condenser and evaporator coils. The carrier’s condenser is 5 to 10 m of coiled steel tube.
- A relief device on the high side that cannot be adjusted, shut off or weighted by the operator; see Pressure Relief Valve, which covers the same requirements for steam.
- A pressure gauge rated above the highest pressure the circuit can reach and made for ammonia, not brass-cased with copper internals; see Pressure Gauge.
- A hand pump and water for the proof test.
- A heat source you can remove at once. A water bath, a covered fire pit or a shuttered solar collector all qualify.
Method: Build, Prove, Charge, Run
Step 1: Lay out the circuit on paper. For the batch machine: one vessel for the solution (the generator-absorber) and one for the condenser-evaporator, joined by a short, large-bore steel pipe. For the continuous machine, follow Ammonia-Water System.
Step 2: Fabricate and clean. Remove oil, scale and moisture from every part before assembly. Dirt blocks restrictions and moisture dilutes the charge.
Step 3: Prove every pressure part with water. Fill the vessel completely, bleed the air from the top, raise the pressure slowly with a hand pump to the proof pressure the governing code gives for that vessel, hold it, and look at every weld and joint. The method is in Pressure Testing. This page does not give a number because the proof pressure belongs to the vessel, not to the machine.
Step 4: Assemble, then leak-test with soapy water at low pressure. The carrier suggests testing the empty circuit with air at 1.5 times the expected operating pressure. This page does not follow that: compressed air stores far more energy than water at the same pressure, as Receiver Tank Construction shows by comparing its water test and its air energy. Prove strength with water, then find leaks with a modest air pressure and soap. Bubbles mark a leak. Fix every one before introducing ammonia.
Step 5: Fit the relief device and a gauge, and mark the circuit. A tag stating “ammonia; steel only; no copper alloys” prevents a later brass fitting.
Step 6: Charge outdoors with the strong solution. Wear eye protection and gloves, work upwind, and keep water close. Never charge a warm system.
Step 7: Make the first run with a water-bath heater. Heating through a water bath puts a ceiling on the generator temperature, because the bath cannot be hotter than its boiling point. That is a natural limit on the pressure the vessel can reach, whatever the operator does with the fire (recalled engineering reasoning, flagged). The ceiling also limits how much ammonia is driven off, so check that the cooling is adequate before relying on it.
Step 8: Watch the first cycle from a distance. The relief device must discharge where nobody stands. Record the gauge reading, the time, the heat source and the result in a log kept with the machine.
Worked Example: How Much Tube Surface The Carrier’s Coil Gives
Surface area of a tube is π × diameter × length.
- Smallest case: 12 mm tube, 5 m long: π × 0.012 × 5 = 0.19 m².
- Largest case: 25 mm tube, 10 m long: π × 0.025 × 10 = 0.79 m².
So the carrier’s range for a condenser coil is about 0.19 to 0.79 m² of outside surface. That is a surface to work with, not a guarantee of capacity: how much heat it rejects depends on the air or water around it, which Condensation treats.
Common Mistakes
- Testing with air first. If a flawed joint fails under air, the stored energy is far greater than under water.
- A brass gauge or a bronze valve “just for the test”. It stays in the circuit.
- Charging before the cooling is ready. The first heat has nowhere to go.
- Relying on the fire to control the pressure. Only the relief device does that.
Safety
Never charge, heat or open an untested ammonia circuit
A leak of ammonia solution or vapour burns eyes and lungs, and a vessel that has not been proved with water can fail with the energy of its contents. Work outdoors, upwind, with a second person present and water within reach; keep a way out clear. See Ammonia Toxicity and Pressure Vessels.
What’s Next
- Learn next: Troubleshooting — reading a running machine
- Combine with: System Construction — piping and seals for the circuit as a whole