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Extinguishment of large cooking oil pool fires by the use of water mist systems
Extinguishment of large cooking oil pool fires by the use of water mist systems
Liu, Z.; Kim, A.K.; Carpenter, D.
NRCC-46970
A version of this document is published in / Une version de ce document se trouve dans : Combustion Institute/Canadian Section, Spring Technical Meeting,
Kingston, Ontario, May 9-12, 2004, pp. 1-6
EXTINGUISHMENT OF LARGE COOKING OIL POOL FIRES BY THE USE OF WATER MIST SYSTEMS
Z. Liu, A. K. Kim and D. Carpenter
National Fire Laboratory, Institute for Research in Construction National Research Council of Canada, Ottawa, Canada, K1A 0R6,
INTRODUCTION
Industrial oil cookers used in food processing plants have potential involving very severe fire incidents caused by overheated cooking oil reaching its auto- ignition
temperature. It is difficult to extinguish such a large pool fire involving an oil surface of several hundred square feet with a few thousand liters of hot cooking oil. It requires flame extinction over the entire surface at once, and at the same time, the oil must be cooled down from its burning temperature to below its flash point to prevent re- ignition [1]. An appropriate fire suppression system is therefore needed for the protection of industrial oil cookers.
The National Research Council of Canada, with CAFS Unit Inc., initiated a project to study fire hazards associated with industrial oil cookers and water mist fire suppression technology for the protection of large industrial oil cookers. Two water mist systems with different nozzle types were developed and evaluated in full-scale fire experiments. This paper describes these experiments and results. The impacts of the type of water mist system and their configurations, discharge pressure, oil quantity in the cooker, and hood position on the effectiveness of the water mist systems for suppressing large cooking oil fires are discussed.
THEORY
Cooking oil has a high flash point, and its temperature during burning is high (603-733K). Its gasification heat is also high and additional thermal energy is required for fuel evaporation. The cooking oil fire can be extinguished by water mist mainly through cooling the fuel surface, as the rate of supply of fuel vapour or burning rate is reduced sufficiently enough not to support the flame. The energy balance of the fuel surface at the fire point is:
L E f fv c c H L m Q Q f
S =( ∆ − )& + & − & (1)
where fcis fraction of the heat of combustion of the fuel (∆Hc) that is transferred from the flame to the fuel, m&f is fuel burning rate, Q&E is the external heating flux transferred to the fuel, Q&L is the heat loss from the fuel and includes heat loss to open space through the radiation, to the fuel, and to the water:
) ) ( ( ) ( 4 vw w fs pw w fo fs f fs L m C T T L T T K T Q& = + − + & − + δ εσ (2)
where ε is the emissivity of the oil, σ is the Stefan-Boltzmann constant, Tfs and Tfo are the fuel surface and fuel temperatures inside the fuel, respectively, δ is the thickness of the heating layer belo w fuel surface, and Kf is the heat conductivity of the fuel, m& is the w water mist rate that reaches the oil, Tw is the water temperature, Lvw is the latent heat of
evaporation of water.
The energy balance at the oil surface can be rewritten as:
))) ( ( ) ( ( ) ( 4 w fs pw wv w fo fs f fs E f fv c c m L C T T T T K T Q m L H f
S= ∆ − & + & − + − + & + −
δ
εσ (3)
When S ≥0, sufficient heat is available to maintain the flame on the oil surface and the combustion continues, however, when S <0, the heat will not be sufficient to produce fuel vapour to support the flame, resulting in the extinction of the flame. The critical water flow, m& , required to cool the fuel for flame extinction is: cw
) ( ) ) ( ( ) ( 4 w fs pw wv fo fs f fs E f fc c c cw T T C L T T K T Q m L H f m − + − + − + − ∆ = εσ δ & & & (4)
These equations indicate that in order to extinguish an oil fire, the heat transfer from the flame to the oil must be minimised by reducing the fire size and blocking the convective and radiation heat transfer, and at the same time, the oil must be cooled down to reduce fuel vapour available for combustion. They also explain one of the reasons why the oil fire caused by auto-ignition is difficult to extinguish. During fire suppression, the oil surface is cooled down quickly but the oil below its surface is still hot and its temperature is higher than that at oil surface. The oil provides the heat to maintain the flame at the oil surface, instead of acting as an energy sink as observed in other liquid fuel pool fires. Additional water is required to cool the whole oil below its flash point.
FACILITY AND PROCEDURE FOR FIRE EXPERIMENTS A mock-up industrial oil cooker that
consists of a pan and a hood was constructed (see Figure 1). The pan was 3.0 m long, 2.4 m wide and 0.343 m deep. The dimension of the hood was 3.0 m long, 2.6 m wide, and 0.76 m deep. There was a 0.51 m diameter hole at the center of the hood. During experiments, the hood was placed in two different positions: namely a hood-up and a hood-down position. The distance between the ceiling of the hood and the bottom of the pan was 1.56 m at the hood-up position and 1.16 m at the
Two water mist fire suppression systems were developed and evaluated in the experiments. For water mist system I, four FJ nozzles were installed inside the mock-up cooker and placed 0.8 m above the oil surface. The water flow rate of the nozzle ranges from 30 L/min at 414 kPa to 40.9 L/min at 862 kPa discharge pressure. Water drops generated are relatively fine (Dv90 =380 micron at 552 kPa). Its large spray angle (150 degrees) is not changed with changes in discharge pressure.
For water mist system II, six WL nozzles were installed inside the mock-up cooker and placed 0.9 m above the oil surface. The water droplets generated by the nozzle are relatively coarser (Dv90 =540 micron at 552 kPa). Its water flow rate varies from 17.4 L/min at 414 kPa to 24.3 L/min at 862 kPa discharge pressure. Its spray angle (120 degrees at 207 kPa) is decreased with increase in discharge pressure.
Canola oil was used as the cooking oil in the experiments. The properties of the Canola oil are as follow [2]: (1) density at room temperature: 0.914 kg/l; (2) specific heat: 1.91 kJ/kg K; (3) flash point: 563 K; and (4) thermal conductivity: 0.188 W/m.K.
Three thermocouple trees were placed in the pan to measure the oil and air/flame temperatures. Thermocouple tree #1 was placed in the center of the pan. Thermocouple trees #2 and #3 were located 0.7 m away from thermocouple tree #1 on either side, along the diagonal side of the pan. Eight thermocouples were attached to each tree. Two pressure gauges were used to monitor the discharge pressure of the water mist system. The first one was located at the inlet of the water mist piping system and the second one was located near a nozzle. Two heat flux meters (air-cooled) were placed at the north and south sides of the cooker to measure the heat release rate of the fire and possible fire flare- up generated during suppression. The distance from the heat flux meter to the cooker was 1.0 m and the height of the meter was 1.1 m above the floor.
During the experiments, cooking oil was heated continuously at 5-8oC/min until it auto-ignited. After the flame had spread over the whole oil surface, the fire was allowed to burn freely fo r 30 sec. Heating was provided continuously during the pre-burning period until the start of the water mist discharge.
EXPERIMENTAL RESULTS AND DISCUSSION
In order to determine the spray performance of two water mist systems in the oil cooker, 23 sampling cups were placed on the floor of the pan. They were distributed from the center of the pan along the longitudinal, transverse and diagonal axis of the pan. The amount of water collected in the cups was weighed after the water mist discharge. The total amount of water collected in the pan was determined by measuring the water depth in the pan. The water collection ratio in the pan was defined as the ratio of total water collected in the pan to the total water discharged by the water mist system.
Under 414 kPa discharge pressure, approximately 84.2% of the water discharged by water mist system I was collected by the pan and the average water density in the pan
Figure 2 Oil and fire temperatures at Thermocouple Tree #1 0 200 400 600 800 1000 1200 1615 1625 1635 1645 1655 1665 1675 1685 1695 1705 1715 Time (s) Temperature ( oC) 1.5 cm 2.5 cm 4.5 cm 6.0 cm 25.4 cm 38.1 cm 68.1 cm 98.1 cm
was 13 L/min.m2. Higher water density was observed near the nozzle and the areas where water sprays generated by the nozzles overlapped. With increasing discharge pressure, the average water density was increased, however, the water collection ratio did not change because there was no change in its spray angle.
Water spray produced by water mist system II also covered the whole pan, however, its water collection ratio was lower than that with water mist system I. Under 414 kPa discharge pressure, its water collection rate was 80.4% and its average water density in the pan was 11.5 L/min.m2. With increasing discharge pressure, the water collection ratio and the average water density in the pan were increased due to the reduction in spray angle.
Seven full-scale fire experiments were conducted. After the oil was heated over 250oC, oil smoke appeared over the oil surface, and became very dense near the auto-ignition temperature. During seven experiments, the oil auto- ignited at temperatures ranging from 351oC to 361oC. The flame consumed all the fuel vapour over the oil surface that was generated in the heating period and quickly spread to the whole oil surface once the oil auto- ignited.
During free burning, the fire was fully developed, reaching the overhead hood and filling inside the cooker. A large amount of dark smoke was produced. The fire size was determined by the hood position and oil depth in the pan. At the "hood-down" position, the fire grew more quickly and released more heat than those at the “hood- up” position, because more heat was confined inside the cooker. Under the same heating source, the oil temperature with less oil quantity in the pan was heated to a higher level than that with a large oil quantity due to higher heating rate, resulting in a larger fire.
Figures 2 to 4 show the changes in oil and flame temperatures measured at three locations in experiment 1-1 with water mist system I and a 5 cm depth of oil in the pan. The temperatures measured at the center of
the oil pan were higher than those
measured at the other two locations of the cooker. The flame temperatures near the hood were higher than the others measured at the lower portion of the cooker due to the accumulation of hot gases.
With the discharge of water mist, the flame below the nozzle tip was quickly extinguished and dark smoke disappeared from the cooker as fine water drops reached the hot oil and produced a large amount of steam. The flames near the ceiling of the hood were not directly hit by water mist, and they were not immediately
0 100 200 300 400 500 600 700 1615 1625 1635 1645 1655 1665 1675 1685 1695 1705 1715 Time (s) Temperature ( o C) 1.5 cm 2.5 cm 4.5 cm 6.0 cm 25.4 cm 38.1 cm 68.1 cm 98.1 cm
Figure 4 Oil and fire temperatures at Thermocouple Tree #3 Figure 3 Oil and fire temperatures at Thermocouple Tree #2
0 100 200 300 400 500 600 700 1615 1625 1635 1645 1655 1665 1675 1685 1695 1705 1715 Time (s) Temperature ( oC) 1.5 cm 2.5 cm 4.5 cm 6.0 cm 25.4 cm 38.1 cm 68.1 cm 98.1 cm
from two ends of the hood. However, with the production of a large amount of steam and the restriction of fuel supply from the oil, the flame near the ceiling of the hood was extinguished.
Both water mist systems were effective in extinguishing the cooking oil fires. The extinguishing performance was mainly determined by the type of water mist system, discharge pressure, and the hood position. The use of water mist system I extinguished the fires in 4 to 7 sec, depending on the discharge pressure and the hood position. At the “hood-up” position, an increase in discharge pressure from 414 to 690 kPa resulted in the decrease in extinguishing time from 7 to 4 sec, because of increase in water flow rate and spray momentum. When the hood was in the “down” position, the clearance between the nozzle tip and the ceiling of the hood was reduced, resulting in a reduction in the
thickness of the hot layer near the ceiling. Under 414 kPa of discharge pressure, the extinguishing time was reduced from 7 to 5 sec, compared to the hood in “up” position.
The use of water mist system II extinguished the cooking oil fires in 15 to 18 sec, depending on discharge pressure. The extinguishing time was longer than that using water mist system I. This may be attributed to its large water droplets, low collected water rate and non-uniform water density distribution, compared to water mist system I. With an increase in discharge pressure, the extinguishing time was further increased, because the spray angle of water mist system II was reduced, resulting in the reduction in the spray coverage area and the change in water density distribution.
The oil quantity in the pan had no impact on the extinguishing time, but it had a direct impact on the cooling time of the oil. With 300 L of cooking oil in the pan, the average oil temperature was cooled down to 215 – 302oC from its burning temperature during 20 – 29 sec of water mist discharges. With 1000 L of cooking oil in the pan, the average oil temperature was dropped to 332 - 335oC during 23 to 24 sec of water mist
Figure 5 Heat flux and discharge pressure at experiment 1-1 0 10 20 30 40 50 60 70 80 90 100 110 1615 1625 1635 1645 1655 1665 1675 1685 1695 1705 1715 Time (s) Heat Flux (kW/m
2) & Pressure (psi)
heat flux (south) heat flux (east) discharge pressure
discharges. As shown in Figures 2 to 4, the oil temperature near the oil surface was very sensitive to the water mist discharge. It quickly dropped to a low temperature during discharge, however, it suddenly increased as the water mist discharge stopped, because of mixing with other portions of the hot oil. The oil temperature in the lower portion of the cooker was cooled down slowly. After 20 to 30 sec of discharge, no re- ignition occurred in the experiments.
During the experiments with two water mist systems, no
substantial fire flare-up was observed and no burning oil was splashed outside the cooker. Figure 5 shows the heat flux and the discharge
pressure measured in experiment 1-1. The heat flux increased to nearly 25 kW/m2 as the fire became big during the free burning period. With the discharge of water mist, the heat flux quickly decreased, indicating that there was no substantial fire flare-up and no re-ignition during the
experiment.
SUMMARY
A large fire quickly formed in the industrial oil cooker once the oil auto- ignited. The fire size was determined by the oil quantity in the cooker and the hood position. Both water mist systems developed in the current study were effective in extinguishing the cooking oil fires mainly by cooling the fuel surface. The extinguishing performance depended on the type of the water mist system, discharge pressure, and the hood position. Water mist system I with fine drops and large spray angle had a better performance than water mist system II with coarse drops and narrow spray angle.
ACKLOWDGEMENTS
The study was conducted under a joint research project between CAFS Unit, Inc. and the National Research Council of Canada. The authors would like to acknowledge the assistance of Mr. Ping- Li Yen of CAFS Unit, Inc. in this work. The authors would also like to thank all the other NRC staff involved in this project for their contributions.
REFERENCES
1 Soonil Nam “Application of Water Sprays to Industrial Oil Cooker Fire,” 7th
International Symposium on Fire Safety Science, Massachusetts, USA, June 2002.
2 Przybylski, R., “Canola Oil: Physical and Chemical Properties,” Canola Council of
