Application of Water Mist to Extinguish Large Oil Pool Fires for Industrial Oil Cooker Protection

Transcription

1 Appliation of Water Mist to Extinguish Large Oil Pool Fires for Industrial Oil Cooker Protetion ZHIGANG LIU, DON CARPENTER, and ANDREW K. KIM Fire Researh Program, Institute for Researh in Constrution National Researh Counil of Canada Ottaa, Canada, K1A 0R6 ABSTRACT Large oil fires ourring in industrial oil ookers are very hallenging to extinguish due to their size and the large amount of hot oil involved. This paper reports a study to use ater mist for large industrial oil ooker protetion. The extinguishing mehanisms of ater mist and orresponding riteria required for extinguishing large pool ooking oil fires ere investigated both theoretially and experimentally. Based on the extinguishing mehanisms and required riteria, to ater mist systems ere developed in the present ork. A series of full-sale fire tests ere onduted in a large industrial oil ooker mok-up. The study shoed that the to ater mist systems presently developed orked effetively to extinguish large ooking oil fires and prevented them from re-igniting. Their extinguishing performane as determined by the type of ater mist system, disharge pressure and hood position in the oil ooker. KEYWORDS: ater mist, fire suppression, ooking oil fire, industrial oil ooker NOMENCLATURE A spray overage (m 2 ) Q & L heat loss from the fuel (kw) C p speifi heat (J/mol.K) T temperature (K) d Diameter of ater drop (m) u veloity (m/s) H heat of fuel ombustion (kj/mol) x fration of ater mist that is suspended in flame f fration of heat that is transferred from flame to fuel Greek L latent heat of evaporation (kj/mol) ρ density (kg/m 3 ) v m& f oil burning rate (kg/ m 2 s) φ stoihiometri air/oil ratio m& ater mist disharge rate (kg/s) Subsripts Q & E external heating flux (kw) f fuel ater INTRODUCTION Industrial oil ookers are major equipment used in food proessing plants for hiken, fish, potato produts, doughnuts and other food produts. They are typially onveyorized fryers, or oasional bath kettles. Industrial oil ookers ontain ooking oil, varying from a fe hundred liters to tens of thousands of liters, and their ooking surfaes vary from 4.6 m 2 (50 ft 2 ) to several hundred square feet [1,2]. A very severe fire ould our in industrial oil ookers by overheated ooking oil reahing its auto-ignition temperature due to a system malfuntion or simple human FIRE SAFETY SCIENCE PROCEEDINGS OF THE EIGHTH INTERNATIONAL SYMPOSIUM, pp COPYRIGHT 2005 INTERNATIONAL ASSOCIATION FOR FIRE SAFETY SCIENCE 741

2 error. The fires are very hallenging to extinguish as they ignite at a very hot oil temperature, spreads rapidly over the oil surfae and forms a large fire involving a very large oil surfae and tons of hot oil. It requires flame extintion over the entire surfae at one, and at the same time, rapid ooling of the oil to prevent re-igniting. Fire suppressants that ontain hemial omponents are not alloed for use in the food proessing industry due to food safety onsiderations. Carbon dioxide is the most ommonly-used agent for the protetion of industrial oil ookers. It is apable of extinguishing flames over the oil surfae, but it annot effetively prevent hot oil from reignition, beause arbon dioxide does not have suffiient ooling apaity to ool the oil belo its auto-ignition temperature. Carbon dioxide is being onsidered for phasing out for use in industrial oil ooker protetion. The feasibility of using sprinkler ater sprays for industrial oil ooker protetion has been previously studied [2]. A series of full-sale fire tests ere onduted in an outdoor faility involving three large industrial oil ooker mok-ups. Test results shoed that ater sprays extinguished oil fires in 10 to 145 s, depending on the ooker size and the sprinkler type. Hoever, fire flare-up generated in fire suppression as pronouned and the interation beteen the fire and the ater spray as very intense. An extensive amount of oil as spilled over the oil ooker and formed large fires on the ground, as oarse ater droplets sank and boiled up in the hot oil. Water mist that onsists of small ater droplets (D v99 < 1000 µm) has been used to suessfully extinguish various types of fires [3-5] as ell as ooking oil fires in a ommerial deep fat fryer ithout ausing signifiant fire flare-up and re-ignition [6]. Hoever, no researh on usage of fine ater mist against large ooking oil fires assoiated ith the industrial oil ookers has been reported. In this paper, a study of the use of fine ater mist for industrial oil ooker protetion is reported. The extinguishing mehanisms of ater mist and orresponding riteria required for extinguishing large ooking oil pool fires are analyzed. A series of full-sale fire tests ere onduted in a large industrial oil ooker mok-up. The performane of to ater mist systems in extinguishing large ooking oil fires as evaluated in the fullsale tests. The impats of the type of ater mist system and their onfigurations, disharge pressure, oil quantity in the ooker, and hood position on the effetiveness of the ater mist systems in suppressing large ooking oil fires ere investigated. EXTINGUISHING MECHANISMS AND CRITERIA REQUIRED FOR EXTINGUISHING LARGE COOKING OIL POOL FIRES An oil fire in an industrial oil ooker is a pool fire situated in an open environment. When ater mist is disharged onto a fire, as shon in Fig. 1, some fine ater drops, x m&, are suspended in the flame and absorb the heat from the flame. Other ater drops, ( 1 - x) m&, penetrate through the flame and reah the oil surfae. They ool the oil as ater drops evaporate and absorb the heat from the oil, and produe a large amount of steam. The flame that is diretly hit by ater mist an be extinguished, hen the flame is ooled don to its lo adiabati temperature limit, resulting in the termination of the ombustion reation of the fuel-air mixture [4]. The energy balane of the flame at the lo adiabati flame temperature, T, during fire suppression an be given as: fl 742

3 Fig. 1. Shemati of a pool fire suppressed by ater mist. Fig. 2. Shemati of an industrial oil ooker mok-up. m & H = xm& L + C (373 - T ) + C ( T - 373)) + m& ( C ( T - T ) + φc ( T - T ) + L ) (1) f ( v pl pv fl f pf fl f 0 pa fl a0 vf The left side of Eq. 1 is the heat released from the ombustion of the fuel and the right side of Eq. 1 is the heat required for heating ater and gas mixture to the flame temperature, T. It is generally aepted that the lo adiabati temperature limit, fl T, in fl hih ombustion annot be sustained is around 1500 K [7]. An oil fire an also be extinguished hen ater droplets reah the oil surfae and ool the oil belo its fire point, as oil vapour generated from the oil is not suffiient enough in supporting the flames. The energy balane of the oil surfae at the fire point during fire suppression by ater mist is: f H m& + Q& = L m& + Q& (2) f E vf f L The left side of the Eq. 2 inludes the energy transferred from the flame bak to the oil surfae and the external heating flux transferred to the oil that an be ignored for the present ase. The right side of the Eq. 2 inludes the energy required for gasifiation of the oil and the energy lost from the oil surfae. During fire suppression, the heat loss from the oil surfae is mainly aused by heating and evaporating ater. Eq. 2 an be reritten as: S = f H - L ) m& - (1- x) m& ( L + C (373 -T )) (3) ( vf f v pl When S 0, suffiient heat is available to maintain the flame on the oil surfae and the ombustion ontinues, hoever, hen S < 0, the heat ill not be suffiient to produe fuel vapour to support the flame, resulting in the extintion of the flame. Normally, the fire point of a ombustible material is higher than its flash point but loer than its 743

4 ignition temperature [7]. The fration of heat of ombustion of the oil, f, that is transferred from the flame to the fuel is in the range of 0.1 to 0.4 [8]. Water flux, spray overage and spray momentum are onsidered to be the three most important harateristis of ater mist required for extinguishing a ooking oil fire [6]. As indiated in Eqs. 1 and 3, the fire annot be extinguished unless the ater quantity disharged from a ater mist system is suffiient enough to extinguish the flame by removing suffiient heat from the flame, or to ool the oil belo its fire point. The ritial ater mist flux required for extinguishing the flame, ( x m& ), and ooling the oil surfae, ( 1 - x) m& ), are given, respetively: ( m& f ( H C pf ( T f 0 ) φc pa ( Ta 0 ) L ) vf xm& = (4) L + C (373 - T ) + C ( ) v pl pv ( f H - Lvf ) m& f (1 - x) m& = (5) L + C (373 - T ) v pl They suggest that ritial ater flux required is mainly determined by oil property, suh as their heat of ombustion, burning rate and adiabati flame temperature. Sine a ertain amount of ater is required to extinguish a fire, ater mist overage must be large enough to over the entire oil surfae, hih enables ater mist to attak the flames and ool the oil over the hole oil surfae. Those flames that are not diretly attaked by ater mist ill not be extinguished and the heat released by the nonextinguished flames ill ounterat the ooling effet of the ater mist on the oil, maintaining the flame on the oil surfae. The ater mist overage, A, is determined by the spray angle, α, and disharge distane, L, to the fuel surfae: α 2 2 A = π ( L tan ) (6) Water mist momentum is the third riterion required in extinguishing an oil fire. It must be suffiient enough to allo ater droplets to penetrate the fire plume and reah the fuel surfae. Water mist ith lo momentum ill be arried aay by the fire plume. To overome the fire plume, the ater mist momentum must be at least equal in magnitude, and opposite in diretion, to the fire plume momentum. The fire plume momentum is mainly determined by the heat release rate of the fire. The maximum upard veloity in a fire plume, u f max, is ahieved in top of the flame [9]: 0.2 u =. Q& (7) f max 1 9 here Q & is the onvetive heat release rate of the flame. Normally its relationship ith the total heat release rate of the flame, Q &, an be expressed as [9]: & = 0. 7Q& (8) Q 744

5 The ater droplet momentum is mainly determined by the exit veloity of a ater droplet at the nozzle, droplet size and disharge distane. Under non-evaporation onditions, the veloity of a ater droplet, u, at the end of the disharge distane an be given [10]: u o u = (9) 0.33ρ g L exp( ) dρ here u o is the exit veloity of the ater droplet at the nozzle. TEST FACILITY AND PROCEDURE An industrial oil ooker mok-up that onsisted of a pan and a hood as built (Fig. 2). The pan as 3.0 m long, 2.4 m ide and m deep. The hood as 3.05 m long, 2.6 m ide, and 0.76 m deep. Both ends of the hood ere open. There as a 0.51 m diameter hole on top of the hood to simulate the onnetion ith the exhaust dut. A burner as entered beneath the pan in hih the heat as distributed relatively uniformly throughout the pan surfae. The propane gas as used in the burner to provide heating. During experiments, the hood as plaed in to different positions: namely a hood-up and a hood-don position. The learane beteen the hood and the pan as 0.46 m at the hood-up position and 0.05 m at the hood-don position. Three thermoouple trees ere plaed in the pan to measure oil and air/flame temperatures. Tree #1 as plaed in the enter of the pan and Trees #2 and #3 ere loated 0.7 m apart from eah other along the diretion from the enter of the pan to the southeast orner of the pan. Eight thermoouples (Type K, 18 gauge) ere attahed to eah tree. Their elevations ere 51 mm, 100 mm, 124 mm, 165 mm, 254 mm, 381 mm, 681 mm and 981 mm, respetively, above the bottom of the pan, hen the oil depth as 125 mm. To pressure gauges ere used to monitor the disharge pressure of the mist system. The first one as loated at the inlet of the mist system and another as loated near one of the nozzles. A flo meter loated at the inlet of the ater mist system as used to measure the ater flo rate. To heat flux meters (air ooled) ere plaed, respetively, at the north and south sides of the ooker to measure the heat release rate of the fire, and possible fire flare-up. Three video ameras ere used in the experiments to reord the testing proess. One as loated at the southeast side of the ooker, the seond one at the est side of the ooker, and the third one as an aerial-vie amera and elevated 7 m from the ground of the east side of the ooker. Fresh ooking oil as introdued into the pan and then heated ontinuously at 3-5 o C/min until it auto-ignited. After the flame had spread over the hole oil surfae, the fire as alloed to burn freely for 30 s, hih ensures that there is suffiient time for people to evauate from the oil ooker before the ativation of a suppression system, aording to requirement of the test protool [1]. Heating as provided during the pre-burning until the start of the ater mist disharge. At the end of the pre-burning period, the ater mist disharge as ativated manually. After the fire as extinguished, the ater mist disharge as maintained for a ertain time period to ool the oil and prevent them from re-ignition. 745

6 WATER MIST SYSTEMS AND THEIR SPRAY PERFORMANCE To ater mist fire suppression systems ith single fluid nozzles ere developed and evaluated in the experiments. The spray performane of both single and group nozzles of to systems as studied. To measure the spray performane in the oil ooker, 23 sampling ups ere plaed on the bottom of the pan to measure ater density distribution in the pan. They ere distributed from the enter of the pan along the longitudinal, transverse and diagonal axis of the pan. The distane beteen the ups as 20 m. The amount of ater olleted in the ups as eighted after the ater mist disharge. The total amount of ater olleted in the pan as determined by measuring the ater depth in the pan. The ater olletion ratio in the pan as defined as the ratio of total ater olleted in the pan to the total ater disharged by the mist system. Water mist system I onsisted of four MistShield nozzles. The nozzles ere installed inside the mok-up and plaed 0.93 m above the bottom of the pan. The spaing of the nozzles as 1.22 m x 1.52 m. The distane from the nozzles to the side alls of the pan as 0.6 m and to the ends of the pan as 0.75 m. The ater flo rate of a MistShield nozzle ranged from 28.2 L/min at 414 kpa to 40.9 L/min at 862 kpa disharge pressure. Water drops generated ere relatively fine. Under a pressure of 552 kpa (80 psi), 50 and 90 perentages of the spray volume ere in drops smaller than 250 and 380 µm, respetively. The spray angle of the nozzle as 150 degrees and not hanged ith an inrease in disharge pressure. Water mist generated by system I overed the hole oil pan surfae. Fig. 3 shos the ater density distribution over the pan under 552 kpa disharge pressure. Higher ater density as distributed underneath the nozzles and the areas here ater sprays overlapped. The ater density at the orner of the pan as the loest, hile ater density at the enter of the pan as to times higher than that at the orner of the pan. Under 414 kpa disharge pressure, the total ater flo rate of ater mist system I as L/min and approximately 84.2% of the ater disharged from the system as olleted by the pan. The average ater density in the pan as 13.2 L/m 2.min. With an inrease in disharge pressure, the ater densities in the pan inreased but its spray overage pattern did not hange due to no hange in the spray angle, as shon in Fig. 3. Water mist system II onsisted of six sirl type nozzles. The nozzles ere installed inside the ooker mok-up and plaed 1.03 m above the bottom of the pan. The spaing of the nozzles as 1.22 m x 1.00 m. The distane from the nozzles to the side alls of the pan as 0.6 m and to the ends of the pan as 0.5 m. The ater droplets generated by ater mist system II ere relatively oarser, ompared to ater mist system I. Under a pressure of 552 kpa, 50 and 90 perentages of the spray volume ere in drops smaller than 300 and 540 µm, respetively. Its ater flo rate varies from 19.1 L/min at 414 kpa to 24.3 L/min at 862 kpa disharge pressure. Its spray angle as substantially dereased ith an inrease in disharge pressure. Its spray angle as 120 degrees at 207 kpa of the disharge pressure, but dereased to 80 degrees at the disharge pressure of 896 kpa. 746

7 Water Density in Oil Pan (kg/m 2.min) diag (60 psi) long (60 psi) trans (60 psi) diag (100 psi) long (100 psi) trans (100 psi) Water Density in Oil Pan (kg/m 2.min) diag (60 psi) long (60 psi) trans (60 psi) diag (100 psi) long (100 psi) trans (100 psi) Longitudinal Distane (m) Transverse Distane (m) Longitudinal Distane (m) Transverse Distane (m) Fig. 3. Water density distribution of ater mist system I in the oil pan. Fig. 4. Water density distribution of ater mist system II in the oil pan. The ater olletion ratio of ater mist system II in the pan as loer than that ith ater mist system I. Under 414 kpa disharge pressure, the total ater flo rate of system II as 114 L/min and its ater olletion rate as 80.4%. Its average ater density in the pan as 12.7 L/min.m 2. With inreasing disharge pressure, the ater olletion ratio and the average ater density in the pan ere inreased. Figure 4 shos ater density distribution of ater mist system II over the oil pan under 414 kpa disharge pressure. Its ater spray also overed the hole oil pan, hoever, its spray pattern as different from that observed in ater mist system I. High ater density appeared underneath the nozzle, and the ater density near the edge of the pan as higher than that ith ater mist system I, but the ater density in the enter area of the pan along the longitudinal diretion as lo. The ater distribution pattern as also hanged ith disharge pressure. The ater density along the longitudinal diretion as redued and tended to be uniform, hile the peak ater delivery density along the diagonal diretion as shifted toards the edge of the pan, hen the disharge pressure inreased. These hanges ere aused by the redution in the spray angle ith inreasing disharge pressure. FIRE EXPERIMENTS Canola oil as used as the ooking oil in the experiments. The properties of the anola oil at the room temperature are listed in Table 1. Table 1. Physial property of anola oil [2,11]. Oil Flash Point (K) Auto-ignition temperature (K) Density (kg/l) Speifi heat (kj/kg.k) Heat release rate (MW/m 2 ) Canola Seven full-sale fire experiments ere onduted. The effet of hood position, disharge pressure, the type of the ater mist system on ater mist performane ere examined. During the fire experiments, disharge pressures employed ere 414 and 689 kpa, 747

8 respetively, for ater mist system I, and 689 and 827 kpa, respetively, for ater mist system II. Testing onditions and results are listed in Table 2. Table 2. Fire experimental onditions and results. Test No. Mist system Disharge pre. (kpa) Hood position Pre-burn period (s) Ignition temp. (K) Ex. Time (s) Disharge duration (s) T-1 #1 689 up T-2 #1 414 up T-3 #2 689 up T-4 #2 827 up T-5 #1 414 don T-6 #1 414 don T-7 #1 414 up Fresh anola oil as introdued in the pan in eah experiment and heated ontinuously until it auto-ignited. After the oil as heated over 523 K, oil smoke appeared over the oil surfae, and beame very dense near the auto-ignition temperature. During seven experiments, the oil auto-ignited at temperatures ranging from 624 to 634 K. The flame onsumed all the fuel vapour over the oil surfae that as generated in the heating period and quikly spread to the hole oil surfae one the oil auto-ignited. During free burning, the fire as fully developed, filling inside the ooker and reahing outside the oil ooker. A large amount of dark smoke as produed. The fire size generated in the experiments as approximately MW, based on anola oil s property and the size of the oil pan used in the experiments. Experiments also shoed that at the "hood-don" position, the fire gre more quikly than those at the hood-up position, beause more heat as onfined inside the ooker. Also, under the same heating soure, the fire ith shallo oil depth gre more quikly than that ith deep oil depth and its oil temperature in the pan as relatively high due to higher heating rate. Both ater mist systems extinguished all the large ooking oil fires very effetively ithout substantial fire flare-up and burning oil being splashed outside the ooker. As observed in the experiments, ith the disharge of ater mist, the flame belo the nozzle tip as extinguished quikly by flame quenhing and oxygen dilution as the flame as hit diretly by ater mist and a large amount of steam as produed. Hoever, the flames near the eiling of the hood that ere not diretly hit by ater mist ere not extinguished immediately and a part of them as pushed outside the ooker from to ends of the hood. After a fe more seonds of disharge, the entire flames ere ompletely extinguished as the oil vapour generated from the oil as redued, and more steams ere produed to dilute oxygen and fuel available for the flame. Figure 5 shos variation of temperatures measured above the oil surfae at three different loations of thermoouple trees ith time in Test 7. One the ater mist disharge as ativated, the temperatures belo the nozzle tip ere quikly dropped as fine ater drops ooled the flame, hile the temperatures above the nozzle tip suddenly inreased as some flames ere pushed up. After the fire as extinguished, the temperatures far from the oil surfae ere ooled don to K and tended to be uniform. Hoever, the temperatures near the oil surfae ere still high, ranging from 473 to 613 K, as signifiant hot steam as produed. It also shoed tha the temperature in the enter of 748

9 the pan as loer than that at the orner of the pan due to a differene in the ater density distribution over the pan. Temperature ( o C) Thermoouple loation m, # m, #1 disharge 98.1 m, # m, # m, # m, # m, # m, #3 extinguishing 98.1 m, # end of disharge Time (s) Fig. 5. Variation of temperatures above oil surfae ith time. Temperature ( o C) disharge extinguished Thermoouple loation 5.1 m, # m, #1 5.1 m, # m, #2 5.1 m, # m, # Time (s) end of disharge Fig. 6. Variation of oil temperatures ith time. During the experiments, the extinguishing times ranged from 4 to 18 s, depending on the type of ater mist system, disharge pressure, and the hood position. Mist system I as more effetive in extinguishing an oil fire than the system II. Under a disharge pressure of 689 kpa, ater mist system I extinguished the fire in 4 s, hile it took 15 s for ater mist system II to extinguish the same size of oil fire. The exellent performane of ater mist system I as attributed to its high ater density, high ater drop s veloity to reah the oil due to short penetration distane and large spray angle, ompared to system II. Aording to Eqs. 7 and 8, the maximum oil fire plume veloity as: u = 1.9Q& = 1.9( ) 11.76m s (10) f max = / The maximum fire plume veloity is ahieved at the tip of the flame height as the 0.5 veloity is inreased in the proportion to Z of the vertial distane above the oil surfae [7]. The fully developed oil flame height is muh higher than the nozzle loation of to ater mist systems, based on its fire size. It suggests that the fire plume veloity enountered by ater drops should be loer than its maximum veloity. The veloity of ater droplets, on the other hand, dereases ith an inrease in penetration distane as they are shrinking and enountered ith the upard fire plume. Approximate ater droplet veloities reahing to the oil surfae under non-evaporation onditions, disharged from to ater mist systems, are listed in Table 3. The ater droplet veloity is lo for small droplets (<100 µm) but it substantially inreased ith an inrease in droplet size and disharge pressure. The droplet veloity disharged from the system I is higher than that from the system II, hen they reah the oil surfae, due to their short penetration distane. Considering heavy mass density of ater drops, their high disharge veloity and nozzle loation in respet to the oil surfae, the momentum of most ater droplets disharged from to ater mist systems under given orking pressure ere strong enough to overome the fire plume momentum, and drops ould penetrate through the fire plume and reah the oil surfae. As shon in Fig. 5, the gas temperatures that ere measured around 2 mm above the oil surfae almost simultaneously dropped ith ativation of the ater mist disharge, indiating that fine drops did reah the oil surfae. 749

10 Table 3. Droplet veloity at the oil surfae generated by to ater mist systems. Drop size 50 (µm) 100 (µm) 200 (µm) 300 (µm) 400 (µm) 500 (µm) Veloity from 414 (kpa) System I (m/s) 689 (kpa) Veloity from 689 (kpa) System II (m/s) 827 (kpa) The ritial ater quantity required to extinguish the oil fire by ooling the flame or by ooling the oil an be approximately estimated from Eqs. 5 and 6. Based on the knon properties of other ooking oils and hydroarbon fuels [2,12,13], the heat of ombustion and the burning rate of anola oil are given as 39,200 kj/kg and kg/m 2 s. The heat of gasifiation of anola oil is assumed to be 970 kj/kg, the same value as ethyl alohol in onsideration of their similar burning rate (0.04 kg/ m 2 s) [13]. The fration of heat of ombustion of the oil that is transferred from the flame to the fuel is assumed to be 0.1 in onsideration of fine ater drops and their vapors in the flame. The stoihiometri air/oil ratio of anola oil, like most of hydroarbon fuels, is assumed to be 14.7 [7,13]. Heat apaity of the anola oil, steam, air and ater is given, respetively to be 1.3, 2.1, 1.15, 4.2 kj/kgk [13]. The oil surfae temperature is assumed to be its ignition point, sine its pre-burning period is very short. Thus, the ritial ater quantities required for extinguishing an oil fire are given, respetively, as: 0.046( ( ) ( ) 970) 2 xm& = = 8.6kg / m min (11) ( ) ( ) 0.046( ) 2 (1- x) m& = = 2.8kg / m min (12) ( ) They are suggested that to extinguish an oil fire, more ater is needed to ool the flame than to ool the oil due to the existene of fine ater drops and vapor in the flame as ell as the oil property. In the urrent ork, the average ater densities of to systems over the pan ere 13.2 and 12.7 kg/m 2.min, respetively, under the loest disharge pressure of 414 kpa employed in the experiment. They both ere higher than the ritial ater quantity required for extinguishing a anola oil fire. In addition, the ritial ater flux required is redued during fire suppression as the fire and its burning rate are redued. Inrease in the disharge pressure resulted in an inrease in the ater flo rate and spray momentum. Hoever, it did not mean that the performane of the ater mist system ould be improved ith an inrease in disharge pressure. For ater mist system I, an inrease in disharge pressure from 414 to 690 kpa redued extinguishing time from 7 s to 4 s. Hoever, for ater mist system II, an inrease in disharge pressure from 689 to 827 kpa resulted in inrease in extinguishing time from 15 to 18 s. This is beause an inrease in disharge pressure did not hange the spray angle of system I and its spray distribution pattern, but redued the spray angle of system II, resulting in the redution in the spray overage area and hanges in ater density distribution. Some loal ater densities ere redued ith an inrease in disharge pressure. As observed in the experiments, during the fire suppression ith ater mist system II, the flame as pushed out only from to ends of the hood under lo disharge pressure, hile under higher 750

11 disharge pressure, the flame as pushed out, not only from to ends of the hood, but also from to sides of the ooker due to the redution in spray overage. Hood position in the oil ooker also had an impat on the extinguishing performane of the ater mist system. For ater mist system I under a disharge pressure of 414 kpa, the extinguishing time as redued from 7 s to 5 s, hen the hood as plaed from the up to the don position. This is beause the learane beteen the nozzle tip and the hood eiling as redued in the don position, resulting in redution in the amount of hot gases and flames near the eiling that ere not hit by ater mist. Water mist also demonstrated a strong ooling apability in fire suppression. Large amount of hot oil as quikly ooled belo its auto-ignition temperature. Fig. 6 shos the variation of oil temperatures measured in Test 7 ith 1000 L of oil. The average oil temperature as ooled don to 603 K from the burning temperature of 628 K during 23 s of ater mist disharge. The oil ooling rate by ater mist as approximately 65 K/min. The oil temperatures measured at the enter of the pan ooled don more quikly than those at the orner of the pan due to the differene distribution of ater density. In other experiments, the average oil temperature ith 300 L of ooking oil in the pan as ooled don to K from its burning temperature during s of ater mist disharges. The average oil temperature ith 1000 L of ooking oil in the pan as dropped to K during 23 to 24 s of ater mist disharges. As the hot oil as quikly ooled don, no oil re-ignition as observed in the experiments. Experiments also shoed that near the end of the ater mist disharge, some ater droplets sank into the oil and started to boil in the hot oil. Hoever, no oil as spilled over the oil ooker in the experiments, sine the bubbles generated in the boiling of fine ater droplets ere very small. The steam quikly disappeared from the oil after the end of ater mist disharge, indiating that there as no more ater in the oil. The results obtained from the experiments ere onsistent ith the theoretial analysis, shoing that ooling of the flame and ooling of the oil ere the primary extinguishing mehanisms of ooking oil fires by ater mist. To extinguish a ooking oil fire, it must have suffiient ater flo rate to ool the flame and oil, suffiient overage area to over the hole oil surfae and suffiient spray momentum to penetrate through the flame. SUMMARY Flame and oil ooling are to dominant extinguishing mehanisms of ater mist for large ooking oil pool fires. It requires that the employed ater mist systems shall have suffiient spray overage, ater flo rate and spray momentum. Both ater mist systems developed in the urrent ork ere effetive in extinguishing the ooking oil fires. Their extinguishing performane as determined by the type of ater mist system employed, disharge pressure, and the hood position. Water mist system I had a better performane than ater mist system II due to their fine drops and large spray angle. Inrease in disharge pressure improved the performane of system I but resulted in longer extinguishing time for system II due to the hanges in the spray overage and ater density distribution. Water mist extinguished the ooking oil fire more quikly at the hood don position than at the up position due to a redution in the amount of hot gases and flames near the eiling of the hood. Water mist also effetively ooled hot oil and prevented it from re-ignition. No oil splashing as observed in the experiments. The urrent ork also studies the flame quenhing by ater mist, the ritial ater quantities 751

12 and spray momentum required for extinguishing a fire and more future orks on these analyses are needed. ACKNOWLEDGEMENTS The study as onduted under a joint researh projet ith CAFS Unit, In. The authors ould like to aknoledge the assistane of Mr. Ping-Li Yen of CAFS Unit, In. and all the other NRC staff involved in this projet, as ell as Drs. S. Nam and H. Z. Yu of FM Global for their tehnial assistane in this researh ork. REFERENCES [1] Fatual Mutual Researh Corporation, Draft Performane Requirements for Water Mist Systems for the Protetion of Industrial Oil Cookers, May [2] Nam, S., Appliation of Water Sprays to Industrial Oil Cooker Fire, 7 th International Symposium on Fire Safety Siene, Massahusetts, USA, June [3] Bak, G.G., An Overvie of Water Mist Fire Suppression System Tehnology, Proeedings: Halon Alternatives Tehnial Working Conferene, Albuquerque, Ne Mexio, USA, [4] Mahinney, J.R., Bak, G.G., Water Mist Fire Suppression Systems, Chapter 14, Setion 4, The SFPE Handbook of Fire Protetion Engineering, 3rd Edition, [5] Liu, Z. and Kim, A.K., A Revie of Water Mist Fire Suppression Systems Appliation Studies, J. of Fire Protetion Engineering, 11 (1), [6] Liu, Z., Kim, A.K., Carpenter, D.W., Kanabus-Kaminska, J.M., Yen, P-L., Extinguishment of ooking oil fires by ater mist fire suppression systems, Fire Tehnology, 40 (4), Otober [7] Drysdale, D., An Introdution to Fire Dynamis, John Wiley and Sons, Chihester, [8] Rasbash, D.J., Relevane of Firepoint Theory to the Assessment of Fire Behaviour of Combustion Materials, International Symposium on Fire Safety of Combustible Materials, Edinburgh University, pp , [9] Heskestad, G., Fire Plumes, Flame Height, and Air Entrainment, Chapter 1, Setion 2, The SFPE Handbook of Fire Protetion Engineering, 3rd Edition, [10] Williams, A. Combustion of Sprays of Liquid Fuels, Unin Brothers Limited, UK, [11] Przybylski, R., Canola Oil: Physial and Chemial Properties, Canola Counil of Canada Publiation, [12] Koseki, H., Natsume, Y., and Iata, Y., Combustion of High Flash Point Materials, Proeedings of the Fire and Materials 2001 Conferene, San Franiso, CA, USA, January [13] Appendies B and C of The SFPE Handbook of Fire Protetion Engineering, 3rd Edition, National Fire Protetion Assoiation, Quiny, MA,