AMMONIA AS A REFRIGERANT. KEY CHARACTERISTICS:
- Ammonia is a colourless gas at atmospheric pressure. It is part of many natural processes, so it occurs in abundance worldwide. 5% out of more than 2 billion metric tons of ammonia in the world is man-made, and less than 2% is used for refrigeration.
- The refrigerant itself consists of one atom of nitrogen and three hydrogen atoms; hence the chemical formula NH3.
- Ammonia’s refrigeration code is R717. Besides water (R718) and air (R729), ammonia is the only refrigerant with zero ozone depleting (ODP) and global warming potential (GWP).
- Being lighter than air, and becoming a vapour upon release, any ammonia leakage first tends to form a cloud that stays near the ground for a short time and then gets dispersed into the sky.
- Ammonia is classified as a B2L refrigerant due to its low flammability threshold (2L) and higher toxicity (B) with a pungent odour.
Flammability is not a major concern when dealing with ammonia as a refrigerant. NH3 ignites at temperatures hardly encountered in conventional HVAC&R applications (above 650°C) and it needs a support flame to burn. In any case, due to its slight flammability, ammonia has to be kept away from ignition sources such as hot surfaces or sparks, for example from electric switches. In facilities that use ammonia as a refrigerant it is recommended to install ventilation and special detectors in all areas where NH3 could leak.
The use of ammonia has rather been limited by its toxicity to use in large industrial or food preservation systems (in places not directly accessible by the general public) or low-charge systems. In these contexts, ammonia is usually the primary refrigerant in secondary systems, while increasingly being used in packaged systems.
Even though small amounts of ammonia are also found in cigarette smoke and even in the air we breathe, inhalation of high quantities of NH3 can lead to serious health complications.A number of technological solutions are implemented in the design of an ammonia system to prevent any leakage and minimize risks. These include for example the use of welded joints, hermetic or semi-hermetic compressors, use of shell and plate heat exchangers as condensers and chillers, installations of the system on a rooftop, amongst others. In addition, to ensure safe maintenance of ammonia systems technical personnel need to have adequate training and follow basic safety procedures. Other safety measures include frequent checks of the system, protecting the system from external damage, as well as a well-adjusted alarm system. If all the right measures are taken, the refrigerant is safe to use. Investigations of incidents involving ammonia-based systems have indeed consistently shown a pattern of human errors, with poor maintenance and ignorance of basic safety protocols as the main causes of incidents. To address the higher toxicity of ammonia considerable efforts have been made to develop ammonia systems with reduced refrigerant charge as the most effective way to improve safety while increasing system efficiency.