Cone Calorimeter and Exhaust Duct System - Literature review Example

Cone Calorimeter and Exhaust Duct System Cone calorimeter is a modern gadget or device; it is used to study the behavior of the fire of small samples of a variety of materials when they are in condensed phase. It is usually used in fire safety engineering field. Con calorimeter gathers data concerning combustion products, ignition time, and rate of release of heat, loss of mass and other parameters that are associated with the burning properties. According to Andrew (1994), samples of fuel are allowed by this device to be exposed different fluxes of heat on its surface. The principle behind the heat release rate measurement is usually based on Huggett’s principle. On this, gross heat of combustion of organic material is directly related to oxygen amount that is required for combustion. The name of con calorimeter is derived from the conical shape of radiant heater, which produces almost a uniform flux of heat over the sample surface which is under study. In 1970’s and 1980’s, importance of a bench scale tool that is reliable in measuring the heat release rate was in the process of being released. There were a number of such devices that had already been built before in various institutions. However, none of those devices was found to be appropriate for normal engineering use in the laboratory. This was mainly due to operation difficulty and measurement errors of those devices. It has been argued (Arthur 1995) that other instruments that were built such as substitution burner had the capability of giving good accuracy although they had limitations which included complexity, and installation and maintenance difficulties. This therefore, was an indication that a new device was required that had no such difficulties like the other devices. Later a device that used oxygen consumption principle was invented, it had a successful design, and it was termed as cone calorimeter. This design was first described in 1982 by NBS report. Cone calorimeter basic principle has until today been unchanged although there has several improvements and additions that have been made. The devices that are being used today have few parts that are identical to the one that was made in 1982. The significant changes that have been made include the introduction of systems which measure smoke optically and yield soot gravimetrically. Other major changes are not the functional changes but redesign so as to ease the use and operation reliability. In 1985, first cone calorimeter was built outside NIST. It was built in BRI Japan followed by another one in 1986 at Gent University and afterward in the same year other 3 commercial units were built and sold in the U.S. the number of cone calorimeters placed into service has been increasing significantly from 1986 up to date. Barbrauskas and Parker (1987) maintain that Cone calorimeter is most important bench scale instrument in fire testing field. The release of heat is the key measurement that is required in assessing the development of fire of the products and materials. It was a little bit difficult to measure the items although a technique known as oxygen depletion calorimetry was being used. Cone calorimeter was produced to overcome those deficiencies that were in existing devices. The new device relied on outflow enthalpy measurement of enclosed system. The best measurement method that was found to be fit was oxygen depletion calorimetry. This was based on empirical observation in that the heat that burning materials released was directly proportional to the amount of oxygen used in the process of combustion. The cone calorimeter was the instrument that was used. Its name was gotten from the shape of the heater that had truncated conical shape that was used in irradiating test specimen with fluxes up to 100kwlm squared in that test. The cone calorimeter being used meets all the standards required and can be bought in modular form to be used by those laboratories with interest in production of heat, release of heat, mass loss etc. It has been implied (Barbrauskas 1998) that a full conical calorimeter has various parts, which include conical heater which is wound in truncated cone form and is rated 5000 watts at 230 volts. It has a heat output of up to 100 Kw/m squared. It is for use in vertical and horizontal orientation. The control of temperature is usually achieved by use of 3 thermocouples of type K and a 3-term (PID) controller of temperature. Another system of cone calorimeter is a split shutter mechanism which usually protects the area with sample before the start of the test. This usually enables the stabilization of initial mass measurement before the start of the actual test. This enables the operator to have additional time for checking the system before the start of the test. This time that is added is important for samples that are easily ignitable, and those that ignite irreproducibly when the shutter mechanism is not in the position. Specimen holder is another system used for holding specimens that are 100mm in length 100mm in height and 50mm in thickness in both vertical and horizontal orientation. Load cell is another component in the cone calorimeter where masses are measured and is usually conducted through the strain gauge load cell that has an accuracy of 0.1g. It has a total weight range of 2 kg and a quick electronic facility. This load cell is usually enclosed in a case so as to reduce temperature changes effect. Spark ignition is another component or system in conical calorimeter. It has a 10 KV spark generator that is fitted with safety cut out device. The spark ignition is usually inserted by a lever that is combined with the shutter mechanism in to its position. Another component is the exhaust system, which is manufactured from stainless steel because it is long lasting. It consists of a hood, a ring probe for gas sampling, exhaust fan that is able to adjust controls of flow from 0-50g/s and at a resolution of 0.1g/s. it has measurement of orifice plate flow of differential pressure and thermocouple transducer. A normal operation of this system is at 24l/s. Cone calorimeter has gas sampling unit which comprises a ring sampler, pump, drying columns, refrigerated cold trap and flow control. Oxygen analysis is also done in this machine by paramagnetic oxygen analyzer that ranges from 0-25 percentage with accuracy of 50ppm oxygen. Another major component is smoke obstruction part, where smoke is measured with a laser system by use of photodiodes and 0.5 mW helium neon lasers, with reference and main photo detectors that are complete with alignment cradle and neutral density filters that are 0.3 and 0.8 used for calibration. Heat flux meter is used for setting irradiance on the specimen’s surface form the conical heat. Another component is calibration burner that is used for calibrating heat release rate measured using apparatus that use 99.5 percent methane. Factory Mutual Engineering Corporation (1997) has compiled the information that data collection is done on 16 channel A/D interface. Software in windows is also used in IBM computers or any other compatible computers. Other options available include carbon dioxide and carbon monoxide gas analyzers. Low ambient oxygen attachment is another option used for analyzing samples with low oxygen. Cone calorimeter also has protection doors for screening the operator from hazardous samples smoke. These doors usually allow an influx of normal air from the bottom for natural test area ventilation and also input of test air. These protection doors usually enclose three sides of testing area with steel panel covers on the rear and exhaust covering at the top with the base being covered with the floor of the test area. According to Hirschler (1992) the cone calorimeter usually measures released heat, rate of heat release and effective combustion heat by the principle of oxygen consumption. The calorimeter also measures time to ignition, rate of mass loss, specific extinction area and carbon dioxide and carbon monoxide production during the material burning although it is optional. An automatic calibration system also called ConeCalc is used standard calibration although it can be overridden manually. ConeCalc sets the initial oxygen values at 20.95 percent, obscuration of smoke at 100 percent transmission and other gases at 0. The area for burning is designed with an opening potential so as to allow the unit be used with any other type of small calorimeter for non standard experiment. The platform supporting load cell, cone heater and the sample support system can be replaced by removing it. It can be replaced by another fire model or combustor or free burning item that can be accommodated in the hood. This usually allows the device to be used to asses the heat release from areas or systems like small pool fires or even wood cribs that are used as ignition supply in other test types. The actual cone calorimeter and the sampling ring are usually connected by a gas sampling train which includes three removable glass columns which are accessible from the instrument front side. The whole unit is designed to operate using electricity supply of 230 volts AC +/- 10 percent, 50/60 HZ, 25 amperes. The device should also be supplied with water to cool the meter for heat flux during the calibration process. Connectors used are 6mm in size and are used to supply and return water. Cone calorimeter is a clear indication of advancement in the laboratory scale fire testing of various materials like composites and materials. It has been realized that the data released by the cone calorimetry usually correlate very well with behavior of full scale fire. This method has proved to be very important in development as well as optimization of flame retard ant, fire resistant and smoke suppressant materials. Cone calorimeter provide abundant experimental data that enable the manufacturers of flame retardant and heat resistant products, it also helps the manufacturers with a lot of data for fire protection materials so as to formulate the optimal systems. Therefore, cone calorimeter is used eradicate the specimen with a range of heat fluxes that are not of important to any use in the fire field. Although cone calorimeter is designed for measuring the rate of heat release, which is the key parameter in predicting fire intensity and growth, it has also other uses like testing the smoke and toxic gases which are not main hazards to life in a real fire condition. Cone calorimeter is conducted in accordance to international standards which include BS 476, ASTM E1354 and ISO 5660-1. On the global scene, international organization for standardization (ISO) has been in the process of seeking to develop HRR apparatus since 1970. ISO assessment was that cone calorimeter was best device on which to base the much needed ISO heat release rate standard. Many fire devices that were designed before were not based on real fire conditions and the measurements that were taken had no significance in engineering. There were just used to fail or pass a specimen according to regulatory necessities. Cone calorimeter can be applied in various areas, which include providing the data that is required or needed for the state of the art models of the fire. It is also used to provide the data which is used for predicting the fire behavior in real scale by simple means correlations or formulas. It is also used to rank the product orders according to their performances. It is also used to pass a product in accordance to the criteria levels. The earliest cone calorimeter applications were in the polymers industries in Hitherto US. It relied on limiting oxygen index (LOI) (14) tests as well as UL94 (15). The LOI (14) gives results that are quantitative and the most suitable for engineering variable. However, UL94 (15) only gives fail or pass test without quantitative information. Mulholland (1997) argues that cone calorimeter has already given several successes when used in more detailed predictions; this is because it has been used in testing the combustible wall and linings of ceiling in rooms. This was achieved in the European EUREFIC research program. Cone calorimeter data were required so as to make predictions that are real. Same kind of problem was approached at Lund University. Another problem that was addressed at NIST was on upholstered furniture, this was addressed in two research projects that were separate. However, work in this area is still continuing at NIST as well as in European Community project in the Europe. Electric cable and wire is very large test for the fire tests in many countries. Vertical cable tray test is one of those products that are tested in many countries. Cone test has proved to be successful in predicting the results of electric wires and cables. Another successful test by cone calorimeter is non-combustibility and combustibility degrees of building products. Canadian building code committee has done a lot of work in the establishment of cone calorimeter data use in areas where code had specified to be either material specific requirements or non-combustibility tests requirements. Earlier fire test methods had simple outputs of data which was only one or even two numbers and one or two curves that were reported. However, cone calorimeter provides a lot of data which includes curves of smoke, heat release and loss in masses. In addition, it provides data for in carbon monoxide, carbon dioxide and other gases. The calorimeters software provides all data automatically and readily for use. There were some challenges that came on the way which included data interchange, data inputting to fire models and also methods of calculation. There are many anticipated applications in coming years. The cone calorimeter has proved its utility in many industries and testing laboratories and research workers. Some of the anticipated trends are polymer development increased utilization hence taking over existing thermal analysis equipment roles like (TGA, DSC, etc). Second anticipation in use of cone calorimeter is approval into building codes and also increases in amounts of product types, this is where bench scale correlations become available. It is also being anticipated that there will be increased interest in data from cone calorimeter by engineers in fire protection department; this is due to increased fire models that are comprehensive, more successful as well as being used. According to Richardson (1992), the cone calorimeter has a stream of oxidant that is usually introduced from the bottom of the chamber. In many experiments, largest flow rates of gas inflowing comes from air supply that is compressed. This is usually provided by an air compressor that is 45 kw usually located in mechanical equipment room. The compressor supplies pressure air at 860 kpa that is reduced to nearly atmospheric pressure by regulators that are two single stages. This two stage breakdown of pressure assists in maintaining a constant flow. The second downstream regulator is usually used in setting desired flow of air. Air then moves into two rotameters that give operator a visual check of inlet flow of air hence approximating its amount. There is a pressure safety valve which helps in preventing rotameters damage when the pressure builds up in this system. This air line then moves to T that brings in second gas, which may be nitrogen, carbon dioxide among others. The air mixture then move to a pipe that is long and straight and equipped with orifice plate-flow meter located in the middle of the length and is used to measure the mass rate of flow of the gas inflowing. Eventually, this gas moves to manifold system where the sample is located ready for the testing process. After the testing the sample, the exhaust gases moves out through the exhaust hood and clean out port to the exhaust duct. Exhaust duct has several components along it which all have it various functions. The first component is soot sampling system to experiment the properties of produced soot. The other component is gas sampling ring which is fitted with heated line to TUHC, water and hydrogen chloride analyzers. Unheated line to oxygen, carbon dioxide and carbon monoxide analyzers are also attached to it. Other components that are attached to exhaust duct are laser extinction beam, thermocouples for measuring the temperature difference, exhaust orifice plate and pressure difference transducer and a high temperature fan at the end. The laser photometer used in standard cone calorimeter could not be used as it depends on the duct pressure or the pressure that is inside the duct. The adjustment are made such that the inside pressure is positive due to adjusted inflow supply and the speed of exhaust fan. Therefore, an air forcing purge system was designed to cater for a positive pressure in the duct. A flow of 13 cm cubed per second in each side of smoke meter was found enough in preventing the optics from getting soot up with no artificial reduction of the smoke path. Smoke meter is usually situated downstream of the analyzer sampling ring so that the introduction of room air doesn’t affect the analysis of the gas. Underwriters Laboratories (2002) has complied the information on several measures that have been taken towards preventing any accidents that may occur in the machine. If the specimen can auto-ignite in apparatus, a deflagration is likely to occur in the combustion chamber hence the need to take several measures or precautions to minimize such dangers explosion. The measures that have been put in place include blow out door panel. The combustion chamber is glazed in all sides by use of Pyrex. The left side has two doors that are used for normal operations, and there are also fitted with latches that are positive sealing. The back panel is hinged also and can as well work as a door although its primary role is to function as blow out panel. This panel is fixed on the frame by a friction fit on the support flame which is released in case the pressure increases in the chamber. The pressure forces it out hence reduce accident or explosion; furthermore, this door is fitted with pairs of connecting rods that limit panel from moving far. This further reduces the accident that may occur when this panel blows out and hit another object or even a person. To protect cone calorimeter against methane explosion, calorimeter is calibrated using calibration burner that is fed with methane which is of high quality. If burner was to be used exhaust system turned off with the absence of pilot ignition spark and enclosure door closed, then an explosion may result. In order to avoid this, then an interlock system was installed which blocks methane flow to calibration burner if exhaust burner is not turned. To protect the explosion from the steam occurring inside the cold plate shutter which may be caused by turning on the heater without water may be prevented. This is usually through the installation of an interlock which prevents operator of the machine from turning heater when there is no flow of cooling water. Rotarmeters and flow meters that are used in metering inflow of various gases can be damaged or even ruptured in case over-pressure conditions developed in the feed line. To reduce these dangers, blow out valves are installed in front of each meter. These valves are rated at 138 kpa release pressure and below 240 kpa for maximum operating pressure. Some of the limitations of using this apparatus include inaccuracy of the parameters being used in the practice. Pressure may not be up to the required levels as they may be required hence making results inaccurate. Another limitation on this apparatus is occurring of the explosions in the combustion chamber due to pressures that may build within the chamber. This may end up causing an explosion that may destroy the apparatus even if the safety measures are put in place. This apparatus is very expensive to buy and maintain in many laboratories. It also requires a specialist who has been trained on how to use it and obtain the required data correctly and accurately. Cone calorimeter is a device that enables direct comparison of materials combustion rates and ranks them according to their fire performance. This is a very important aspect towards improving polymers properties. Use of cone calorimeter has enabled the improvement of fire properties on various materials. It also helps to pass or fail material in accordance to fire criteria which include heat release rate value and smoke obstruction level. It also helps in assessing response to a material if exposed to large fire. References Andrew, J 1994, Fire and flammability of furnishings and contents of buildings, ASTM International, California. Arthur, F 1995, Fire standards in the international marketplace, ASTM International, California. Barbrauskas, V & Parker W, 1987. Ignitability measurements with the cone calorimeter. Elservier. London. Barbrauskas, V 1998, Specimen Heat Fluxes for Bench scale Heat Release Rate Testing, Oxford press, London. Factory Mutual Engineering Corporation, 1997, Hand book of Industrial Loss prevention, McGraw-Hill, New York. Hirschler, M 1992, Fire hazard and fire risk assessment, ASTM International, California. Mulholland, G. 1997. Smoke and Soot Dterminations in the Cone calorimeter, American society for testing materials, Philadelphia. Richardson, L 1992, Determining Degrees of Combustibility of building materials-national building code of Canada, Elsevier Applied Science Publishers, London. Underwriters Laboratories, 2002, Tests for Flammability of plastic Materials for Parts in Devices and Appliances, Underwriters press, Boston. Read More