AMMONIA NH3: natural and efficient but toxic

Ammonia with chemical symbol NH3 and refrigerant designation R717, is the only natural refrigerant being continuously used in industrial refrigeration since its first application.

Traditionally, ammonia-based industrial refrigeration has been the standard in the food processing and preservation industries, because of its relatively low cost combined with excellent operating performance.

In developed countries over 80% of the industrial refrigeration and cold storage industry uses ammonia refrigeration plants together with CO2 and hydrocarbon refrigerants. In still developing countries this market is lower, around 40%.

In the food processing factories, ammonia can be used in meat, chicken and fish production facilities, fresh milk and dairy production facilities, wineries and breweries facilities, etc. Some kind of ammonia installations includes ammonia blast freezers, spiral freezers, plate freezers, and freezing tunnels. NH3 is also used in ice cream production machines.

Furthmore, nowadays there is also a growing development of ammonia equipment for air conditioning of public buildings; in fact, large ammonia chillers are now safely used in heating and cooling applications in populated areas, providing high efficiency refrigeration and low energy consumption.

For example, among many worldwide applications, Terminal 5 at London’s Heathrow Airoport should be mentioned. It uses a central ammonia chilling plant to provide comfort environment for passengers and workers. The energy center for the London Olympics (2012) features ammonia chillers as part of the low carbon heating and cooling system that have catered for the needs across the site. Morevore, the Acquatic Center makes use of a separate NH3 chiller. On the other side of the world, New Zealand’s Christchurch airport uses ammonia chillers to cool a freight hub, allowing temporary storage of shipments.

Talking about the physical properties of ammonia, it is a colourless gas at atmospheric pressure with zero ozone-depletion and global warming potential, as well as a short atmospheric lifetime where it does not form any by-product or decomposition products with negative environmental impact. The next table summarizes the most important properties of ammonia and of other natural refrigerants.

According to the safety group designation, ammonia is toxic and flammable (B2), whereas carbon dioxide as well the most part of synthetic refrigerants are non-toxic and non-flammable (A1). Of course, the hydrocarbons are much more flammable than ammonia (A3).

Despite its undisputed energy efficiency benefits, the use of ammonia is restricted in certain applications and geographic regions, due to its higher toxicity and lower flammability characteristics. As a result, R717 is effectively prohibited inside occupied spaces, but can be used in unoccupied areas or outside. However, many advances have been made in recent years to minimize risks for human health, particularly for ammonia installations in populated areas. They include using ammonia in conjunction with other refrigerants in order to reduce and isolate the ammonia charge, such as in secondary systems; using advanced safety equipment; deploying containment casings; ammonia absorption systems.

It is important to note that ammonia has a strong odour making leaks easy to detect. The additional safety equipment required will obviously increase costs, however, manufacturers claim that operational energy and maintenance savings will potentially outweigh the increased initial outlay in the long run.

Considering the compatibility of ammonia with the components in the refrigerating equipment, it is fundamental to highlight that the use of copper must be completely avoided in the part wetted by ammonia because of its high corrosive effects. Therefore, hermetic compressors or semi-hermetic compressors cannot be used with ammonia; only open type of compressors are implemented because it is not possible to directly cool the copper windings of the electric motor using gas. Copper must be avoided in the pipe lines and also in the heat exchangers. Several different solutions have been developed to face this problem: nowadays, stainless steel plate heat exchangers are available for use with ammonia as well as aluminum microchannel heat exchangers. Considering the lubricants, ammonia is not suited for use with polyol ester (POE) and polyvinyl ether (PVE) lubrificants, and it has only limited applications with poly alkylene glycol (PAG) lubrificants. The next figure reports the pressure-enthalphy diagram for ammonia.

p-h diagram for ammonia

The thermodynamic benefits and drawbacks of ammonia can be summarized in the next table, which lists the most important characteristics of an ideal refrigerating cycle +30°C/-30°C with isobaric condensation and evaporation processes and isentropic compression. The reference fluid for the comparisons is R22 because it is the traditional synthetic refrigerant, which presents similar values of saturation temperature-pressure curve. Furthmore, R22 can be considered as a benchmark for efficiency aspects. From the analysis of p-h diagram and of the values reported in the table, we can state that this fluid presents an extraordinary large latent heat of vaporization, which implies that the total mass flow rate a given cooling capacity is lower than that required by traditional refrigerants (around 15% lower than that for R22).

The efficiency of ammonia is very high, thus its Total Equivalent Warming Impact (TEWI), which measures direct and indirect environmental impacts of a refrigerant, is very low as compared to the traditional synthetic fluids.

What is peculiar for the ammonia behavior is the high value of the final compression temperature, which in the case of ideal refrigerating cycle is more than 70°C higher than that of R22. During the real operations, this temperature is even higher because the compressor process is not isentropic as in the ideal case. This implies some critical issues for the compressor resistance associated to the thermal stress involved; most cases of the cases the compressor is cooled by a water jacket and the compression process is subdivided in two separate stages. At the end of the first stage, the compressed gas at an intermediate pressure is cooled in an intercooler or with direct injection of cold condensed liquid.

Among the natural refrigerant, ammonia today is the most-established refrigerant market with large sales in terms of volumes and revenues but, on the other hand, its critical aspects related to toxicity, flammability and material (copper) usage and lubrificants compatibility issues are limiting its application to small refrigerants and air conditioning equipment confining it only to large scale plants.   

References

SHECCO, Guide 2012: Natural Refrigerantis Market Growth for Europe, 2012, Shecco Publications

Correlated topics:

Carbon Dioxide as refrigerant (Newsletter No.1 – October 2012)

Hydrocarbons – From domestic to commercial refrigeration: a developing market

Microchannels: applications and future perspectives