Vapor quality
Low charge ammonia systems are not a compromise
Written by: Michael Elstrøm
Most large ammonia systems become expensive the moment the charge exceeds local limits. Not because of performance — but because of regulation, safety handling and complexity.
But if your application does not require temperatures below -35°C, ammonia will still deliver lower energy consumption.
Reducing the charge without sacrificing performance
Traditional pump circulated systems are built around overfeed evaporators filled with large amounts of liquid. That design is simple — but inefficient. A DX (direct expansion) reduces refrigerant charge, removes pumps and reduces piping. With 1000 kg of ammonia, you can build a 250,000 m³ cold store.
What DX changes
- No circulation pumps
- Lower refrigerant charge
- Less piping
- Same total system cost
- Higher efficiency potential
Where traditional systems lose energy
Pump circulated systems waste energy by transporting liquid continuously, creating pressure losses and higher compressor energy use. At part load, evaporators fill with liquid and performance drops further.
Evaporator design decides your efficiency. An efficient DX evaporators use many small pipes and minimise pressure drop and lowers the energy usage.
Control strategy is where performance is won
Vapor quality control improves evaporation and can reduce energy consumption by 20–50%, especially at part load. Water in your ammonia system reduces heat transfer and increases energy consumptions and creates control instability. It must be removed.
System comparison
- Pump systems: high charge, inefficient at part load, high maintenance.
- DX systems: Low charge, high efficiency, simpler maintenance.
Defrost is an overlooked opportunity
Using vapor quality control reduces piping and system charge. Ambient air defrost can reduce cost further.
Defrost options
- Hot gas: fast but higher charge
- Vapor quality controlled: lower charge
- Ambient air: low cost but slower
Lower charge means lower risk
DX systems reduce ammonia volume and allow fast isolation, minimising risk. Low charge DX systems is not new technology, it has been used since 2014 across multiple applications.
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