At a time when industry is re-evaluating natural refrigerants, Sandra Van Dijk undertakes a timely risk assessment of ammonia use in refrigeration systems and heat pumps.
Many of the dangers associated with ammonia have been greatly exaggerated, according to Anders Lindborg of Ammonia Partnership, a Swedish consulting firm specialising in the design and operation of refrigeration plants and heat pumps.
Lindborg, widely known in refrigerant circles as “Mr Ammonia” for his lifelong advocacy of natural refrigerants, died in September last year, not long after delivering a paper at an International Institute of Refrigeration (IIR) workshop in which he outlined what he said were misunderstandings associated with ammonia use.
Lindborg believed that ammonia had a reputation it doesn’t deserve and myths that depict it as dangerous continue to influence regulators and society at large.
“Ammonia is unsurpassed as a refrigerant, having excellent thermodynamic qualities that involve environmental advantages,” Lindborg wrote.
“All life is dependent on the recirculation of nitrogen, in which the breaking down of natural substances to ammonia is an essential part. Use of ammonia as refrigerant will continue in the future since society cannot afford not to use it.”
Lindborg wrote that the belief that ammonia is both poisonous and explosive is entirely untrue. Although flammable, ammonia does not explode, he said.
“It flash burns the same as confined smoke in a burning building; burning ammonia has low flame propagation. When describing ammonia, there is often a negative reaction with people claiming it is dangerous, toxic, explosive and has a terrible smell.
“It is true that ammonia is the only refrigerant that has a strong, characteristic smell. But this strong smell is an advantage since the smallest leaks are immediately discovered and corrected. Many other gases have no smell which represents a great potential hazard.”
According to Lindborg, the vaporisation heat transfer capabilities of ammonia are high and the liquid fluid flow rate is low because of ammonia’s high latent heat, and it is this low liquid flow that has limited the use of ammonia for smaller refrigeration capacities.
“However, with new technology it can be an alternative in the future for extremely small systems with charges of some hundred grams; so small charges are no risk but may smell,” he said.
“There is no such thing as the ideal refrigerant. For example, HFC refrigerants are not recommended for large charge systems because leaks are more difficult to prevent and the price of replacing the charge is too high.
“This is a double penalty on top of the environmental challenges created by
their release.”
Although literature on ammonia refrigeration systems dates back more than 100 years, Lindborg believed there was still a lot of ground that hadn't been covered.
Despite this, he believed there was a clear need for extensive documentation to improve accessibility and confidence in the operation of ammonia refrigeration systems.
Compared to the large number of systems in existence, incidents involving ammonia leaks are few and far between. Data and statistics relating to fatal accidents compiled by Lindborg show an annual death rate (ADR) of less than two persons per billion inhabitants per year.
Ammonia is easily identified and can be noticed in concentrations of less than 4-20ppm. It starts to become life threatening at concentrations exceeding approximately 700-1000ppm with exposure of between 20 to 30 minutes.
“There is no universal standard here so opinions differ from one country to another,” Lindborg wrote. “But it is difficult to conceal ammonia, which is the only refrigerant that gives a warning long before the concentration can be considered dangerous.”
The word 'explosive' is used in relation to rapid fire behaviour, with flame propagation of many metres per second (m/s) and a detonation in kilometres per second (km/s).
Since ammonia burns with low energy – about half that of hydrocarbons – the flame propagation is low, about eight centimetres per second (cm/s), according to international standard ISO 817.
Ammonia can self-ignite if the temperature is above 651°C and as a refrigerant is classified in group B2 (low flammability) in accordance with ISO 817 and ASHRAE standard 34.
Ammonia’s flammability range is from 15 to 28 per cent and can only burn in confined space, not outdoors in the open without a supporting flame. Therefore, it is not classified as flammable in connection with outdoor use.
“In order to ignite ammonia, an ignition source with minimum energy is needed and this energy compared to other flammable substances is considerable,” Lindborg said.
Ammonia requires minimum ignition energy of 680 mJ, while methane, ethane and propane require 0.21-0.26 mJ and hydrogen gas requires 0.02 mJ.
“This is why electrical equipment for ammonia systems is placed outside the machinery room to eliminate ignition sources,” he said. “Open flames or boilers are not allowed in ammonia machinery rooms under any safety standards.
“Similarly, naked electric bulbs are a possible ignition source so lighting must have a spray-proof cover as a plastic hood. Fluorescent lighting must also be covered although such light fittings do not heat up during use.
“The fire process is short-lived; after just a few seconds of fire a certain amount of oxygen in the room has been used up and the ammonia/atmospheric oxygen balance is no longer flammable. The fire dies if other material is not ignited.”