When handling large quantities of chemicals, especially at elevated pressures, an important part of the risk analysis for any refrigeration system should be “What is the combustion risk of this system?”
Industrial Refrigeration makes use of a variety of chemicals in order to achieve the desired cooling effect of the system. No system can be 100% risk free, but if we are well informed about the physical properties of the chemicals we are using, we can substantially mitigate the risk of a flammable environment occurring; as well as what we need to do in the event one does occur. Documents, such as the Australian Standard for Refrigerating Systems and Heat Pumps (AS/NZ 5149), can assist designers in implementation of well-engineered safety solutions based on good installation practices which are specifically relevant to the modern Australian market.
Combustion in any form requires three components, in sufficient quantity, to occur. These are fuel, oxygen and heat. In reference to industrial refrigeration systems, the refrigerant is the fuel. The heat (ignition source) could take many forms but most commonly in terms of design may be any system element capable of producing a spark.
Figure 1 – For combustion to occur, all elements must be present in sufficient quantities.
When investigating chemical combustibility, the most frequently encountered phrase is the Lower Explosive Limit, LEL for short. This figure, represented as either a percentage or parts-per-million value (ppm), describes at what concentration, in air, the refrigerant will be flammable. Less commonly used, but no less relevant, is the Upper Explosive Limit. This value which describes the concentration at which there is too much refrigerant in the air which restricts the supply of oxygen, thus inhibiting combustion from taking place.
In knowing what these limits are we allow ourselves to design our system such that a dangerous (flammable) environment can never occur, or in the event that it does all potential sources of ignition are removed.
A list of commonly used refrigerants is included below along with applicable flammability limits. Because of the well-known limits of flammability, the associated risks can be greatly reduced if not eliminated. By employing simple, yet highly effective, preventative actions in refrigeration plant design we are free to focus on other factors in selection of refrigerant such as the generated refrigeration effect or the environmental impact.
|ASHRAE Number||Chemical Name||Flammability|
|R290||Propane||Vapour is flammable when concentrations in air are between 20,000ppm and 95,000ppm.|
|R717||Ammonia||Vapour is flammable when concentrations in air are between 150,00ppm and 250,000ppm.|
It is important to note that for the safe operation of an industrial plant, Australian states and territories each have OH&S laws in place with prescribed risk management strategies relating to levels of combustible refrigerant in the atmosphere. Tigger points at lower concentrations (5% and 10% of the LEL) are detailed in such legislation and as such represent more stringent requirements compared with the recommendations within AS5419.