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How can we tell if a fire, once started, will develop and spread? The most important parameter to consider is the rate of heat release. This provides an indication of the likely size of the fire, the rate of fire growth and other attributes that define the fire hazard. Although smoke and toxic gases are the major causes of death in fire, especially domestic fires, these are secondary effects, since if the fire doesn’t develop or burns slowly then little or no smoke will be released.
How can we measure rate of heat release? Traditional methods are based on measurement of the heat lost by the system and the development of an energy balance, and are difficult to perform. The most significant, powerful, simple and most versatile method uses the oxygen consumption principle (OCP). First enumerated in 1917, it was only in the 1980s that the principle was first applied and heat release data became generally available. The OCP relies on the observation that burning most organic material releases 13.1kJ per gram of oxygen consumed with an accuracy of 5%. A few typical values are given in table 1 and compared to the heat of combustion of 1g of material consumed. The equipment therefore needs to measure the change in oxygen concentration.
Table 1. Heats of combustion of various materials.
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Methane
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-12.54
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-50.01
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Polycarbonate
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-13.12
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-29.72
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PET
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-13.21
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-22.00
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PMMA
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-12.98
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-24.98
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Polystyrene
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-12.97
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-39.85
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PVC
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-12.84
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-16.43
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Cotton
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-13.61
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-15.55
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Measuring Heat Release
Fire testers have developed a number of different standards and tests to measure rate of heat release, which are described below.
Cone Calorimeter
The first cone was built at what was then the National Bureau of Standards in 1982. Subsequently, a number of manufacturers produced cones and there are now some 120 cones in operation worldwide. The 100mm square sample, which can be up to 50mm deep, is mounted in a specimen holder on a load cell and exposed to radiant heat from a truncated cone heater which enables irradiance levels from 10 to 100kW.m-2 to be achieved. The volatiles from the specimen are ignited by a continuous spark igniter positioned just above the specimen until it is burning. All combustion products are removed by the exhaust hood and pass along the ducting, pulled through by a fan normally at the rate of 24 l.s-1. A small proportion of the product gas flow is drawn off, cooled, dried and then passed through a sensitive oxygen analyser. Instruments for other measurements on carbon dioxide, carbon monoxide and smoke can be added. All data is collected and processed so that information such as time to ignition, total heat release, peak heat release, heat release at specified intervals, mass loss rate and effective heat of combustion can all be evaluated and reported.
Furniture Calorimeter
In this equipment, items of complete furniture are mounted on a weighing platform under a large hood to capture all the combustion products. Measurements on the rate of heat release may be made readily in the same way as the Cone.
Room Corner Test
This test (ISO 9705) mirrors the size of room (3.6 x 2.5 x 2.4m) in a domestic house and enables various fire scenarios to be studied. It is commonly used for wall coverings ignited by a gas burner situated in the corner. The combustion products exit from an open doorway situated on the small side of the room away from the fire into a large hood and ducting system. Again, this is instrumented similarly so that measurements of heat release can be made.
Issues with Testing Procedures
Since the 1980s a large volume of data has accumulated which is proving invaluable in evaluating fire risk. The cost of testing is lowest for the Cone when compared to the large scale tests using either the furniture calorimeter or the room comer test. Considerable effort has been made over recent years to validate Cone data for larger scale fire scenarios. Nevertheless, attention needs to be paid to criticisms levelled at the use of the Cone. Firstly, it is clear that the specimen size is small. Secondly, there has been criticism that it is difficult to test non-standard specimens, such as composite and layered samples, non-planar materials, and intumescent materials which swell up on exposure to heat to the extent that they make contact with the spark igniter or indeed with the heater itself. Thirdly, the combustion takes place in an open atmosphere containing 21 % of oxygen.
Testing Programmes
One extensive programme, ‘The combustion behaviour of upholstered furniture’, initiated in the European Community and carried out by 11 different fire research laboratories has addressed the first objection about specimen size as far as furniture is concerned, as well as looking at the relation between components and composites. Part of the programme has established clearly that providing precise testing protocols are followed, it is possible to model from Cone tests on individual components through to complete items of furniture. In the event this gives three options for testing furniture for safety in fire:
• Testing of the individual components used in the furniture
• Testing of the composite materials
• Testing the complete item of furniture itself.
The first two options use the Cone calorimeter, while the third uses the furniture calorimeter. Obviously the latter case provides the most accurate data but with the models developed within the programme it is feasible to use the original components themselves.
Dealing with Issues Surrounding the Cone Test
Testing Non-Planar Products
Non-planar products, including corrugated materials or composites such as small electric cables, can be tested in the specimen holder. However, it must be remembered that the different parts of the surface may experience different irradiance levels due to the angle of presentation by the surface orientation and the distance variation of the surface from the cone heater. Also, the surface area exposed to the heater is greater than for a flat surface specimen. The technique may best be used as a comparison for specimens of similar geometry. The results will not be absolute but comparative. Specimens such as cables and layered composites need to be sealed at the ends if they are to yield results that can be applied to larger assemblies.
Testing of Intumescent Materials
Intumescent materials are best tested without any restraint since it is the formation of a continuous protective crust on the surface that provides the necessary protection. The use of a wire grid, initially thought of as appropriate, is not now recommended. However, increasing the distance between the base of the cone heater to the initial specimen surface ensures that the swelling specimen avoids physical contact with the spark igniter or with the heater coils. The material does not become over-exposed to the radiant heat after swelling. If the cone heater is set to 45kW.m-2 at an initial distance of 40mm and then if the specimen swells so that the distance is reduced to 25 mm, the irradiance only increases to 50kW.m-2 and to 51.5kW.m-2 at 14mm. Protocols for testing liquids and materials that retreat from the heat have also been developed.
Atmospheric Variations
The final criticism was that all burns take place in 21 % oxygen. However, Cones have now been produced which enable work to be carried out in atmospheres with less oxygen content.
Reducing the Costs of Cone Calorimeters
A criticism of a different type levelled at the Cone calorimeter is initial cost. Manufacturers have been able to reduce the cost of production to some extent but this is also being addressed by a development in which a thermopile is used to measure the temperature rise in the product gases. This rise is calibrated using a methane burn. The calibration value can be used to give a value of the heat release of materials whose heat release is unknown. The system dispenses with the need for the very accurate oxygen analyser, which is the most costly component of the Cone calorimeter.
Analysing the Information Using Software Packages
The collection of data and its subsequent processing calls for a well-developed software package. The data, all against time, include rate of heat release, effective heat of combustion, as well as the average rate of heat release timed from ignition. Most Cones also give information in respect of smoke and gases such as carbon monoxide and dioxide. Some data is presented as area-based and some on a mass loss rate basis which becomes misleading whenever the mass loss rate falls to low levels or tends to zero. Software now available makes it possible to make all calculations from zero time or from any other time selected, important where smouldering occurs before ignition. Additionally, access to the raw data is available for Cone users to calculate whatever derived parameters they may require.
Conclusions
Already the rate of heat release of materials and products is increasingly being specified in different situations. In addition it appears that the Cone will become even more important in the next few years as models are developed to extrapolate to differing fire scenarios.
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