Fire Science: Flame Characteristic and Heat Detectors - Assignment Example

Question 1 In the table the two poly identified are Polymer Ignition time Heat flux PMMA 1000s 10kw/m2 PUR 800s 11kw/s In the first incident we take the area of a frying pan. In the second we take the area of a stool. In order to find the maximum diameter of pan a thread of about 1m in length is used. A knot is tied on one end of the thread. The knot is held firmly at one point on the edge of the frying pan. The thread is then stretched on top of the surface for so that it cuts approximately the diameter of the frying pan. The thread is moved horizontally along the surface while observing keenly to establish the point where the distance from edge where the thread has a knot to the opposite side of the frying pan the thread has a maximum distance. The maximum distance gives the diameter. The same procedure is used in establishing the diameter of the stool surface. Thus diameter of pan=25cm = 0.25m Diameter of stool = 32cm =0.32m For materials with area equivalent to a frying pan Here we are using PMMA Area=  Burning rate is approximately =1.29g/s Heat of combustion = 24.8Kj/g Heat release rate= HCxBurning rate = 24.8x1.29= 31.99Kj Heat flux= 31.99/0.049= 652.86Kj/m2 For materials with area equivalent to a stool Here we are using PUR Area=  Burning rate is approximately =2.11g/s Heat of combustion = 23.7Kj/g Heat release rate= HCxBurning rate = 24.8x2.11= 52.33Kj Heat flux= 52.33/0.08= 654.13Kj/m2 From the calculation it can be seen that the heat flux values are much higher that heat release rate (HRR). In the two cases the heat flux is not significantly different indicating that the two fuels are close in terms of combustion properties. Question 2 = 0.68m2 Here Af=  Flame characteristic The flame will be characterized having different colours along its height. Near the surface of the fuel the flame will be seen to be greenish. Away from the fuel surface the flame will be yellowish. At the tip the flame will have some smoke. The yellow flame is the hottest. Question 3 The fire density of the room Item Quantity Load Table 6.5kg 6.5x13.2=85.8Mj Chairs 1.4x38=53.2kg 53.2x9.3=494.76 Mj Windows (2x3)x2=12m2 12x21.3=255.6 Mj Carpet 12mx15mx2.8kg/m2=504kg 504x33=16632 Mj Cable (1.2)x2= 2.4mx0.5=1.2kg 1.2x11.3=13.56 Mj Total load 17481.72 Mj In the combustion process there is always generation of CO and CO2 due to oxygen deficiency. In the cases where the burning material has some nitrogen in it there will be nitrogen oxides (NOx) being generated including NH3, and the cyanides.HCI will be generated during the combustion of chlorinated plastics even though there might be generation of carbonyl chloride (COCl,), or phosgene. This is likely to be in low concentration that does not reach a hazardous concentration. The behaviour exhibited by fluorinated plastics are expected to be similar to those of chlorinated plastics though with higher level of stability with fluorocarbons being emitted in addition to HF. There will be a variation of proportions of the gases depending on the fire conditions. Some combustion gases such as NO2 will cause irritation in cases where the concentration is not at dangerous levels and this act as a warning of their presence. Direct expose of NO2 to the skin may result to irritation and burns even though it is only at a very high concentration of the gas that there wills immediate distress. For a concentration range of 10–20 ppm there will be mild irritation in nose and through, at 25-50ppm concentration level it may lead to bronchitis or pneumonia while at a concentration level beyond 100ppm the gas can cause death as a result of asphyxiation from lung fluids. Question 4 Heat detectors In a thermal detection system design is such that it depends on the thermal output of the fire. In a fire environment the heat that is generated by the fire will the disbursed in the entire area though both laminar and turbulent convective heat flow originating from heated gases. Turbulent flow comes as a result of the inducement and regulation of the fire plume thermal column effect of the heated air and gases occupying the upper area of the fire surface. The characteristics exhibited from the fire plume and the ceiling jet flow pattern of convective heated gases are determined through the heat release rate for the ceiling height and the diffusion flame combustion. Also proximity to walls, how the room is configured and the ceiling being obstructed are likely to have considerable effect on the transportation of the heat flow that is required in the operation of the thermal detector. The sensitivity of a heat detector in a given fire situation is dependent on the temperature of the gas that is affected by the height of the ceiling, the positioning of the fire detector and also HRR of the fire. In accordance to the operating principles the heat detectors may belong to any of the following three categories: 1. Fixed Temperature Detectors 2. Rate-of-Rise Detectors 3. Rate compensated Detectors Fixed temperature heat detectors This is seen to be the simplest heat detector type whose design is such that an alarm is triggered when a predetermined temperature is reached by the sensing element. This is the same principle upon which a sprinkler head operates. Generally, the temperature of the surrounding air should be higher than the heat rating of the detector before the heat detector element is raised the operating temperature; a condition described as thermal lag. This detectors construction may be a bimetal type, continuous line type or fusible element type. The operation of the fusible element type has similarity with the operation principle of a sprinkler head with eutectic metal melting at a predetermined temperature triggering the release of a tensioned spring thus initiating an alarm to go off. This is a spot type detector. For the case of a continuous line type heat detector, it will generally be compost of parallel wires that are separated from each other by a heat resistive insulation. Upon the melting away of the insulation at a temperature that is predetermined due to exposure to fire, the parallel wires are short circuited thus initiating an alarm. This are usually employed as fire detection devices in tunnels and in cable trays among other places. For the case of a bimetal heat detectors there is utilization of two metals with different coefficient of expansion being joined together. Upon the expansion of the metal strips , each at different rates, there will be deflection of the bimetals towards the metal with lower expansion which then result to closing a circuit that leads to alarm initiation. Rate-of rise heat detectors In these detectors the design is such that they come to function at the point the increase in ambient temperature goes beyond a value that is predetermined, usually 12 - 15 per minute. With these detectors, the normal change in the ambient air temperature is accommodated, this being the air temperature anticipated in the normal situation without fire. One of example of this type of detector is that employ pneumatic tubing, that is filled with air and having a relief vent. When the normal conditions are prevailing, after the air being heated it will expand with the excess volume being expelled through the vent before there is any build up of pressure. But where the air is expanding at a rate exceeding the relief capacity of the vent, we will have a pressure build up which will initiate an alarm. We have both spot and line types of these detectors. Rate compensated detectors Unlike in the rate-of rise heat detectors, for the rate compensated heat detectors the design is such that an alarm will be initiated upon the air of surrounding air reaching a predetermined level this being irrespective of the temperature rise. This design is such that we have temperature sensitive contacts inside a steel shell. The shell’s coefficient of expansion is differs from the internal contacts. There will expansion of the shell before the internal contacts in case there is rapid increase in air temperature leading to a signal production, this being similar to a rate-of-rise detector. In a case where there is a slow HRR from fire, the shell and the interim contacts will heat up in a more even fashion leading to a signal production at a predetermined temperature rating of the detector, which is similar to a fixed temperature heat detector. References Rodney C. (2004). Scientific American Inventions and Discoveries, p.351. John Wiley & Songs, Inc., New Jersey. ISBN 0-471-24410-4 Babrauskas, V. (1995). “Burning Rates,” SFPE Handbook of Fire Protection Engineering, 2nd ed., National Fire Protection Association, Quincy, MA,. Read More