This short guide examines the comparative advantages and disadvantages of the different types of fire extinguishers in general use and their respective extinguishing method.
Water
AFF Foam
Dry Powder
Carbon Dioxide (CO2)
Wet Chemical
Class of Fire & Suitability of Fire Extinguisher Type
Water Fire Extinguishers
Advantages
Although little used in Nigeria, water fire extinguishers are good for tackling class A fires involving burning wood, paper, textiles and general combustible materials. They don’t contain any harmful chemicals or residual agents and they are kind to the environment.
Disadvantages
The downside is that they are strictly for class A use only and can make the fire worse or cause harm to the user if used on other classes of fire. Water conducts electricity and thereby presents a risk of electrocution to the user if used on live electrical equipment.
Extinguishing Method
Water fire extinguishers work by cooling and quenching a fire. They absorb heat from the fire and convert it into steam to reduce its temperature below the critical level needed to sustain the fire.
Class of Fire
AFF Foam Fire Extinguishers
Advantages
AFF Foam extinguishers are especially suitable for class B flammable liquid fires such as paints, thinners, petrol, and oil spills. Foam is also very effective on class A fires such as man-made furnishings. They have a higher class A fire rating per litre than their water counterparts and can therefore be more lightweight and portable, making them a good choice for an office fire extinguisher.
Disadvantages
As they are wet they will damage dry goods such as office documents or stock that they come into contact with.
Extinguishing Method
Foam fire extinguishers work by covering a fire with a film of foam that starves it of oxygen and prevents re-ignition, in addition to penetrating absorbent materials and having cooling and quenching properties due to the water content.
Classes of Fire
ABC Dry Powder Fire Extinguishers
Advantages
ABC Dry Powder fire extinguishers are a multi-purpose and highly effective extinguishing agent, making them an excellent choice for use in many circumstances. They are ideal for use in warehouse and factory environments and other areas where the considerable advantages offered by their effectiveness and multi-purpose application outweigh the disadvantage of their residual properties. As their extinguishing medium is dry they will not cause damage to items such as documentation in the way that wet types such as water or foam will.
Disadvantages
The downside is that they leave a very fine powder residue that will disburse on discharge over a wide area and settle into every nook and cranny, making the clean-up process of small fires more problematical.
The powder residue can cause damage to sensitive electronic equipment such as circuit boards, computers, and production machinery, etc. The ammonium phosphate within the extinguishing agent can undergo hydrolysis to form phosphoric acid which is extremely corrosive to ferrous metal and alloys and may also attack some plastics, rubber and coatings. Every particle of this powder will potentially cause damage to such equipment and machinery, and therefore every particle must be removed during the clean-up process. As this is impossible to achieve in practice, its use should be avoided in such environments.
Dry powder introduces exposure hazards in three ways: (a) it causes irritation on contact with skin; (b) it causes irritation and discomfort on contact with eyes, and (c) it causes breathing discomfort on inhalation. For these reasons, the use of dry powder extinguishers should be avoided in environments where its discharged powder residue will come into contact with people.
They should not be used in offices or in contaminant-sensitive environments such as food preparation areas or laboratories, etc.
Dry powder extinguishers must not be located along emergency fire escape routes.
Extinguishing Method
ABC Dry Powder fire extinguishers work by fusing particles of blended ammonium phosphate and ammonium sulphate powder over a fire to form a barrier that starves it of oxygen and prevents re-ignition, and acts to knockdown a fire.
Classes of Fire
Carbon Dioxide (CO2) Fire Extinguishers
Advantages
Carbon Dioxide (CO2) fire extinguishers are safe for use on electrical appliances and are non-residual, making them ideal for use in office environments where sensitive computer equipment would be damaged by using other types of extinguishing agents.
Disadvantages
As CO2 does not cool the fire very well, there is a risk of the fire re-igniting. Fumes from CO2 extinguishers can be harmful if used in confined spaces. The non-insulated horns freeze during discharge, so care must be taken not to touch it.
Extinguishing Method
CO2 fire extinguishers work by displacing oxygen at the source of the fire with an inert gas, thereby starving the fire of the oxygen that is essential to the combustion process. They also have some cooling properties.
Classes of Fire
Wet Chemical Fire Extinguishers
Advantages
Wet Chemical fire extinguishers are specially designed for class F fires in kitchens involving burning cooking oils and fats like deep fat fryers.
Disadvantages
No disadvantage for their intended application. However, it is essential that the intended user should be trained in the proper application of this specialised extinguisher.
Extinguishing Method
Wet Chemical fire extinguishers work by covering burning oil in a cooling layer of foam by means of a long lance applicator. Most class F extinguishers contain a solution of potassium acetate, sometimes with some potassium citrate or potassium bicarbonate added. The extinguisher sprays the solution as a fine mist which cools the fire, while the potassium salts saponify the surface of the burning oil, producing a layer of thick, soap foam over the surface that smoothers it and prevents re-ignition. This saponification only works on animal fats and vegetable oils.
Classes of Fire
Class of Fire & Suitability of Fire Extinguisher Type
