Support - Welcome to the solutions page

Support - Welcome to the solutions page

By Grant Laidlaw

Many people ask for assistance in understanding the theoretical and practical aspects of the industry. 

Jaco asks: Grant, can you give us an idea as to where we stand with natural refrigerants? I attended the ATMOS conference and noted comments made that we as a country will need to look at the possibility of using natural refrigerants. Which refrigerants? CO2 was mentioned, but what other alternatives exist? 

Hi Jaco. Yes, this is an interesting topic. I recently went over to Europe, and particularly to Germany, to see what developments are being made in this area. I will publish a detailed account of my findings, but to answer your question: we are looking at ammonia, carbon dioxide, propane, and isobutane. I also visited a company using water as refrigerant for process cooling.

So, let us start with an overview.

For many years, we have been made aware of the problems caused by ozone depleting substances, as well as of the negative effects of the greenhouse gases. Two important international agreements constructed a basis for the law that is being introduced gradually worldwide. The Montreal Protocol (1987) aims at fighting the negative influence of ozone depleting substances on the environment. The second, the Kyoto Protocol adopted in 1997, establishes legally binding commitment of its signatories for the reduction of greenhouse gases.

For the refrigeration/air-conditioning industry, this means changing previous habits and the gradual elimination of refrigerants containing chlorine (CFCs and HCFCs) as well as those contributing to global warming.

The first step is the elimination of CFCs and HCFCs under the Montreal Protocol agreement, whereby developed countries are obliged to reduce production and consumption of these substances.

As an Article 5 developing country signatory to the Montreal Protocol, South Africa will follow in the footsteps of Europe. After 31 December 2009, virgin R22 began to be phased out, with recycled fluid still being allowed for maintenance, and starting 1 January 2015, the use of recycled R22 was completely banned in Europe.

Natural refrigerants

R717 – ammonia (NH3)

Due to its toxicity and flammability, the use of ammonia has been limited to large industrial applications. Because it does not do any harm to the environment, its use is being investigated for smaller plants.

Among ‘natural’ refrigerants, R717 holds one of the top places as an alternative. Production of ammonia all over the world amounts to 120 million tons, and only a small portion of it (up to 5%) is used in the refrigeration equipment.

Ammonia does not deplete the ozone layer (ODP=0) and does not directly contribute to an increase in the greenhouse effect (GWP=0). Gas with a sharp rank smell is harmful for the human body. Tolerance concentration in the air is 0.02mg/dm, which corresponds to its inclusion volume fraction 0.0028%. In combination with the air at volume fraction 16...26.8% and exposed to open fire, ammonia is explosive. The ignition temperature with air is 651°C.

Ammonia vapours are lighter than air and readily soluble in water (one unit of water can dissolve 700 units of ammonia, which excludes moisture freezing in the system). Ammonia is virtually insoluble in mineral oils and does not affect ferrous metals, aluminium, and phosphorous bronze.

According to thermodynamic qualities, ammonia is one of the best refrigerants: as to bulk cold-productivity, it considerably exceeds R22 and has a higher heat transfer co-efficient, which allows the use of smaller diameter pipes under assigned cold-productivity in the heat transfer apparatus. Due to ammonia’s sharp smell, leakage in the refrigeration system can be easily detected. For these very reasons, R717 has found  wide use in large refrigeration facilities. Also, R717 refrigerant has a low cost.

One of the disadvantages of ammonia is the higher value of adiabatic line (1.31) compared to R22 (1.18), which causes a considerable increase in the discharge temperature. Additional difficulties with refrigeration equipment production are caused by ammonia’s high activity towards copper and copper alloys; that is why pipelines, heat exchangers, and accessories are made of steel. Due to ammonia’s high toxicity and combustibility, welded connections are thoroughly controlled.

Having said this, ammonia is a tried and tested refrigerant and the systems are relatively simple.

R744 refrigerant (CO2 carbon dioxide)

A cheap, non-toxic, incombustible, and ecologically clean (ODP=0, GWP=1) substance. The cost of carbon dioxide is considerably lower than that of R134a. Carbon dioxide has a low critical temperature (31̊°C), comparatively high triple point temperature (-56̊°C), high pressures in triple point (more than 0.5MPa), and critical pressure (7.39MPa). It can serve as an alternative refrigerant. It is contained in the atmosphere and the biosphere of the Earth and as well as low price it has the following advantages: simple servicing, compatibility with mineral oils, and insulating and structural materials.

At the same time, while using carbon dioxide, water cooling of the refrigerating machine condenser increases efficiency of the refrigerating facility. High critical pressure also has a positive aspect connected with low levels of compression and as a consequence, the compressor’s effectiveness becomes considerable.

Possible perspectives exist in using carbon dioxide in the air-conditioning systems of automotive applications as well as in trains. Suggested usage includes domestic refrigerators and heat pumps. The systems tend to be complex, which in itself represents a challenge. At the moment, components are at times difficult to source.

Hydrocarbon refrigerants (HC): R290 (propane) and R600A (isobutane)

Hydrocarbon refrigerants are commonly used in domestic refrigerators, and now we are seeing large commercial and even industrial systems operating with propane as the refrigerant.

I was very interested to come across the pictured propane chiller operating on R290. I will publish more details on this project in the near future.

RACA Oct Grant 4Propane chiller.
Image credit: Grant Laidlaw

RACA Oct Grant 1Refrigeration rack operating on R290.

RACA Oct Grant 2Clearly marked R290.

RACA Oct Grant 3MMR290 sensor/alarm in plant room.

The use of large quantities of propane represents an obvious safety risk that has to be managed; I will also expand on this aspect in future publications. The opportunity exists to retrofit HCFC systems to R290; again, I will go into more detail later.

In the case of R600A, the refrigerating capacity is very low (about 60% of that of R134A). Compressors used with R600A must have a much higher displacement but have the same size motor as for R-134A appliances. The compressor is therefore a new model with a different displacement/motor combination.

All refrigerants must be very pure. They must have extremely low levels of moisture and other contaminants. The use of impure HCs will cause the following problems: A high level of moisture will saturate the filter drier, freeze in the capillary tube, and can lead to compressor damage and failure.

A few HC refrigerant manufacturers add stenching agents for the critical concentrations of HC refrigerant so that a leakage may be smelled. In addition, HCs used as a fuel do not have the correct composition for a refrigerant. It will not give the correct refrigeration capacity, and its use may result in higher running costs and poor reliability.

Only use refrigerant grade R600A and R290.

The first step is the elimination of CFCs and HCFCs under the Montreal Protocol agreement whereby developed countries are obliged to reduce production and consumption of these substances.

Hydrocarbons are flammable when mixed with air and ignited. The concentration of the hydrocarbon in air must be between the lower and upper flammable levels as shown below for R600A.

  • If the concentration is below the lower flammability level (LFL) of approximately 1.8%, there is not enough HC for combustion.
  • If the concentration is above the upper flammability level (UFL) of approximately 8.9%, there is insufficient oxygen for combustion.

The LFL is approximately equal to 35g/m3 of HC refrigerant in air. For safety reasons, a practical limit of 8g/m3 of HC in air should not be exceeded in a closed space. For example, in a room 6m × 3m × 2.5m high, the practical limit for an HC charge is 360g. If the total charge of 360g is released into the room, it will not be enough to produce a flammable mixture. The resulting concentration will be about 20% of the lower flammability level if the refrigerant was evenly dispersed in the entire room. But this may not be the case, and therefore the practical limit should never be exceeded.

Consider the fire triangle, all three components have to be present for fire to be maintained — remove one and combustion is not possible.

Looking at European trends, natural refrigerants seem to be the future. Ammonia and carbon dioxide mainly in industrial applications, with carbon dioxide moving into other applications. Isobutane dominates the domestic and light commercial refrigeration areas. Propane is used in commercial/industrial refrigeration and even in air-conditioning sectors. Water — yes, specialised, but watch this space.

valve brazed into the side of a disposable and purchase 1 kg of refrigerant.

Jaco, thanks for the question. This is an interesting topic and I feel that we are in for substantial changes in our industry.


Thank you for all your questions. Send your problems (and sometimes your creative solutions) to acra@netactive.co.za with “Solutions Page” in the subject line. You may include pictures.



 


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