A continuous ammonia-water refrigerator circulates a strong solution to the generator and a weak one back to the absorber, and the flow needed depends on how far apart the two concentrations are: the narrower the gap, the more solution must be pumped and heated for each kilogram of ammonia delivered. With a 30% strong solution and a 20% weak one, 8 kg of solution must circulate for every kilogram of ammonia driven off.
Part of Refrigeration
The components of a continuous ammonia-water absorption cycle, how the solution circulates between the high and low pressure sides, and the mass balance that sizes the flow.
Two Pressure Levels And Two Solutions Run Through Seven Parts
The corpus carrier’s loop has a generator, condenser, expansion capillary, evaporator and absorber. A continuous machine adds three things the carrier does not draw: a rectifier, a solution heat exchanger and a pump.
| Part | Side | Job |
|---|---|---|
| Generator | High | Heated, drives ammonia vapour out of the strong solution |
| Rectifier | High | Condenses the water carried with the vapour and returns it to the generator |
| Condenser | High | Liquefies the ammonia vapour; see Condensation |
| Restriction | Between | Drops pressure; the carrier gives a capillary of 1 to 2 mm bore and 1 to 2 m length, or a partly closed needle valve |
| Evaporator | Low | Ammonia boils and takes heat from the cold box |
| Absorber | Low | Weak solution dissolves the vapour; heat of absorption must be removed |
| Pump | Between | Lifts strong solution from the low side to the high side |
| Solution heat exchanger | Both | Warms strong solution on its way up with heat from the weak solution on its way down |
The weak solution leaving the generator is at high pressure and the absorber is at low pressure, so it needs a throttle (a second restriction) on its way down, not a bare pipe. The heat exchanger and the second restriction are the recalled standard arrangement, flagged; the carrier’s four-part sketch omits them. Without the exchanger the weak solution carries its heat straight to the absorber, which then has more to reject and takes more fuel to keep running.
The corpus carrier says to use steel for vessels and pipes, because ammonia attacks copper alloys; Ammonia covers the materials.
Circulation Rate Follows From The Two Concentrations
Let x_s be the ammonia mass fraction of the strong solution entering the generator, x_w that of the weak solution leaving it, and treat the vapour as pure ammonia (a simplification; a rectifier works to make it so). Per kilogram of vapour driven off, with F kilograms of strong solution fed in, the ammonia balance is
F × x_s = 1 + (F − 1) × x_w, so F = (1 − x_w) ÷ (x_s − x_w).
Worked Example: A Wider Gap Means Less Circulation
Take the carrier’s 30% as the strong solution and assume a weak solution of 20%.
- F = (1 − 0.20) ÷ (0.30 − 0.20) = 0.80 ÷ 0.10 = 8 kg of strong solution per kg of ammonia.
- Check: ammonia in = 8 × 0.30 = 2.4 kg. Ammonia out = 1 (vapour) + 7 × 0.20 = 2.4 kg. ✓.
Now narrow the gap by raising the weak solution to 25%.
- F = (1 − 0.25) ÷ (0.30 − 0.25) = 0.75 ÷ 0.05 = 15 kg of strong solution per kg of ammonia.
Halving the gap from 10 to 5 percentage points nearly doubles the circulation, from 8 to 15, and so nearly doubles the solution that must be pumped and warmed. This is why a generator that does not drive off enough ammonia, because it is not hot enough or the fuel is poor, hurts so much: the weak solution comes back richer, the gap shrinks and the machine must move more liquid for each unit of cooling. The concentrations are assumptions for the arithmetic, not operating values for a particular machine.
Why Cooling The Absorber Matters As Much As Heating The Generator
Dissolving ammonia in water gives off heat, and a warmer solution holds less dissolved gas (recalled behaviour, flagged). If the absorber warms up, the weak solution takes up less ammonia, the strong solution comes out weaker, and the gap in the example shrinks. The carrier says to cool the absorber with air or water for this reason. A water jacket, a finned coil or a drip over the vessel are the practical ways.
Failure Modes Worth Knowing Before You Build
- Water in the evaporator. The rectifier is not removing enough water. The evaporator stops getting cold at one end and the cold box warms.
- Weak-solution blockage. The second restriction fouls, the generator fills, and pressure on the high side climbs.
- Air in the circuit. It does not dissolve or condense and takes up space, so the pressures read higher than the temperatures suggest. The carrier lists air as a cause of weak cooling.
- Pump failure on a pumped machine. The generator runs dry and overheats while the absorber floods.
Common Mistakes
- Using a pump rated for water on strong ammonia solution. The seals and the pump body must tolerate it.
- Leaving out the rectifier because the carrier sketch has none. It will work at first and fail slowly.
- Starting the fire before the absorber cooling is running.
Safety
The generator is a heated pressure vessel with a toxic and flammable gas inside
A dead pump, a blocked restriction or a failed cooling supply can raise the pressure while the fire keeps burning. The relief device must be on the high side, unisolable and discharge to a safe place; a loss of cooling or of flow should be a trigger to cut the heat at once. The pressure rating of the generator is not something this page can give: it comes from the vessel’s own rating or code, see Pressure Vessels.
What’s Next
- Learn next: Building an Absorber — turning this layout into steel and testing it
- Combine with: Troubleshooting — reading the faults above from a running machine