Air, water, and non-condensable gases enter almost all ammonia refrigeration systems —from installation through daily operation— and work silently until capacity losses, higher energy consumption, or, in the worst case, an unplanned shutdown appear. Here is where these contaminants come from, what they do to the system, and how to remove them.
Every ammonia system should be hermetically sealed. In practice, none truly is: maintenance, no matter how careful, exposes it to contamination. The good news is that the entry pathways are known, the effects are predictable, and removal follows established procedures. The bad news is that it is almost never detected in time, because the system keeps "running" while losing efficiency.
At Thermomac we are IIAR members and work with these technical criteria. This post is based on the technical paper «Contaminants in an Ammonia Refrigeration System: Causes, Effects, and Their Elimination» (Mauricio Quiroga y Oscar Gómez, GEA — IIAR International Technical Paper #3, 2016).
Where Contaminants Come From
Pure anhydrous ammonia has an enormous affinity for water, so as soon as there is an entry pathway, it takes advantage. The most common:
- Air entry during vacuum operation. In the low-temperature stage, when the system operates below atmospheric pressure (at 14.7 psia, saturation is around −28 °F / −33.3 °C), any leak draws air inward, and moisture enters with the air.
- Leaks at mechanical points: packing glands and valve stems, compressor mechanical seals, gaskets, and flanges.
- Air trapped in piping, equipment, and fittings that did not undergo a proper vacuum and purge process before connection, both during construction and subsequent service.
- During oil charging into the system.
- Low-quality ammonia charge (non-refrigeration grade), reused, or from containers or cylinders with air.
- Poor evacuation (vacuum) and system preparation before charging the ammonia.
Ammonia Quality Is Not a Detail
When it is produced, ammonia is pure and dry, with only 6 to 20 ppm of water. Water is added afterwards, for transport. That is why the grade matters, and it is worth being clear about it when charging:
- Premium grade (metallurgical): up to 33 ppm of water (typically less than 10 ppm), maximum 2 ppm of oil.
- Refrigeration grade: up to 0.02% (200 ppm) of water, typically 50 ppm.
- Commercial or agricultural grade: up to 0.5% (5,000 ppm) of water.
The difference between grades is two orders of magnitude in water content. Charging commercial grade into a refrigeration system is bringing the problem in through the front door.
What Damage They Cause
Each contaminant strikes in a different way, but they all end up in the same place: loss of performance and money.
Water concentrates on the low-pressure side. Due to the vapor pressure difference between water and ammonia (at 35 °F, ammonia is at 66.3 psia absolute versus only 0.10 psia for water), water barely evaporates and stays behind. In pump-recirculated systems it accumulates in the low-pressure vessels and suction accumulator; in flooded systems, in evaporators and heat exchangers; in direct expansion, in the suction accumulators. There it raises the saturation temperature, forces operation at lower suction pressures, and brings with it internal freezing, ice crystals, reduced oil lubrication properties, oil migration to evaporators, heat transfer deficiencies, and capacity loss.
Air brings unwanted chemistry. Atmospheric air always contains some CO₂, and it reacts with ammonia:
ammonia + carbon dioxide ↔ ammonium carbonate ↔ urea + water
Ammonium carbonate is corrosive to steel and urea appears as sludge. In addition, as a non-condensable gas, air directly raises the condensation pressure, pushing energy consumption upward.
How to Detect It Before It Is Too Late
The problem with these contaminants is that they give no obvious symptoms until the percentages are already high —and by then you have already been paying for lower suction pressures and compromised efficiencies. That is why it pays not to wait for symptoms:
- Include in the maintenance plan a quality analysis of the ammonia, to determine the purity percentage, especially in systems with significant charges.
- Monitor the condensation pressure against the saturation pressure: if it rises without explanation, there are probably non-condensables present. It is considered significant when air exceeds 0.5% of the receiver space.
How to Eliminate Them
Water. First, determine the magnitude and origin, and resolve the root cause. In systems with a small ammonia charge (for example, a chiller of 50 to 350 kg), the quickest approach is often to replace the charge with refrigeration-grade ammonia. In large systems, an ammonia dryer (still) is used, which heats the solution in stages to evaporate the ammonia and retain the water; a practical criterion from the equipment: if the temperature of the remaining mixture is below 62 °F (16.7 °C), it still contains ammonia.
Non-condensable gases. They are eliminated by purging —manual, semi-automatic, or automatic. The key is to purge where the greatest volume of air accumulates: the coldest and lowest-velocity points, such as the top of the liquid receiver and the discharge header of the condensers. There are three purging concepts: direct venting of the air-refrigerant mixture; compressing the mixture, condensing most of the refrigerant, and venting the non-condensable-rich vapor; or condensing the refrigerant in a small evaporator and then venting.
Practical rule: contaminants enter wherever maintenance touches the system. A good vacuum before charging, refrigeration-grade ammonia, purging at the correct points, and periodic quality analysis cover 90% of the problem.
In Summary
Every ammonia system is exposed to contamination, from installation and throughout its entire life. Each contaminant has a different effect, but all converge in operational deficiencies: less capacity, higher energy consumption, higher operating and maintenance costs. In critical cases, the outcome is unplanned shutdowns, downtime, major repairs, and product loss —always far more expensive than regular and effective maintenance. The underlying recommendation is simple: know the state of your system regarding these agents, invest in the equipment to reduce them, and sustain a maintenance program that keeps them under control.
From Thermomac
As IIAR members, at Thermomac we evaluate and treat contamination in ammonia systems: ammonia quality analysis, non-condensable diagnosis, purge sizing, and water removal schemes, as well as vacuum and charging setup for new installations. If you notice high condensation pressures without apparent cause, or suction pressures that do not close, those are usually the first symptoms where we start.
Want to know how contaminated your system is and what it is costing you? Contact us and we will assess it.
Technical source: Mauricio Quiroga y Oscar Gómez (GEA Refrigeration), "Agentes contaminantes en un sistema de refrigeración con amoníaco: causas, efectos y su eliminación", IIAR International Technical Paper #3, 2016 IIAR Industrial Refrigeration Conference & Exhibition, Orlando, Florida.
