Ventilation of Restaurant-Type Cooking Equipment (CFPS)

 

Exhaust systems for restaurant equipment require careful design, because grease condenses in the interior of the ducts. Grease accumulations may be ignited by sparks from the cooking appliance or by a small fire on the cooking appliance caused by overheated cooking oil or fat in a deep-fat fryer or on a grill.
If the duct did not have a grease accumulation, cooking appliance fires could often be extinguished before causing appreciable damage.

Fire risk is especially high in frying because cooking oils and fats are heated nearly to their flashpoints

Ducts. The following should be considered when designing a duct system for commercial cooking equipment:

1. The system should be designed to minimize grease accumulations, with a minimum air velocity of 1500 ft/min (458 m/min) through any duct.
2. Ducts should be arranged with ample clearance from combustible materials to minimize the danger of ignition, incase of fire in the duct.
   
3. Ducts of substantial construction (not lighter than No. 16
   Manufacturers Standard Gauge steel or No. 18 Manufacturers Standard Gauge stainless steel) should be used with all seams and joints having a liquid-tight, continuous weld.
   
4. Systems should be separated to ensure there is no connection with any other ventilating or exhaust system.
   
5. Ducts should be led directly outside the building, without dips or traps, unless automatic grease removers are employed at the dips and traps.
   
6. Openings should be provided for inspection and cleaning. Dampers should not be installed in any duct system, unless required as part of a grease extractor or extinguishing system.
   

DRY CHEMICAL EXTINGUISHING

 

OVERVIEW OF DRY CHEMICAL EXTINGUISHING AGENTS

 Dry chemical extinguishing agents are commonly listed for use on Class B and Class C fires. Dry chemical extinguishing agents are not effective on Class D fires. Multipurpose dry chemical because they are listed for use on Class A, B, and C fires.

The principal base chemicals used in the production of currently dry chemical extinguishing agents are sodium bicarbonate, potassium chloride, urea potassium bicarbonate, and mono-ammonium phosphate.

Dry extinguishing chemical agents are stable at both low and normal temperatures and are considered to be non toxic. However, other physiological conditions such as minor skin and respiratory irritations may occur on exposure to the agents.

Toxicity.

Dry extinguishing chemical agents are stable at both low and normal temperatures and are considered to be nontoxic and noncarcinogenic. However, other physiological conditions such as minor skin and respiratory irritations.

Extinguishing Properties

Smothering, cooling and radiation shielding contribute to the extinguishing efficiency of dry chemicals .but studies suggest that a chain-breaking reaction in the flame is the principal cause of extinguishment.

Cooling

Cooling action of the dry chemical cannot be substantiated as an important reason fot its ability to promptly extinguish fires. To be effective, any dry chemical must be heat sensitive and, as such, absorb heat in order to become chemically active.

Radiation Shielding.

Discharge of dry chemical produces a cloud of powder between the flame and the fuel; this cloud shields the fuel from some of the heat radiated by the flame.

Smothering Action

For special applications, such as kitchen range, hood, duct and fryer fire protection, the extinguishing mechanism for dry chemical is based on the process of saponification. Saponification is the process of chemically converting the fatty acid contained in the cooking medium to soap, or foam, and it accomplishes extinguishment by forming a surface coating that smoothers the fire.

Chain-Breaking Reaction.

The discharge of dry chemical into the flames prevents reactive particles from coming together and continuing the combustion chain reaction. The explanation is referred to as the chain-breaking mechanism of extinguishment.

Fixed systems are of two types:

• Total Flooding
• Hand Hose line/ Local Application

Uses and Limitations of Dry Chemical Systems

• Due to the rapidity with which dry chemical extinguishes flame, dry chemical is used on surface fires involving ordinary combustible materials (Class A Fires).
• There are several areas in the textile industry, notably opener-picker rooms and carding rooms in cotton mills, where regular dry chemical has been used effectively.
• Multipurpose dry chemicals  becomes sticky when heated, it is not recommended for textile card rooms or other locations where removal of the residue from fine machine parts may be difficult.
• Dry chemicals should not be used in installations where relays and delicate electrical contacts are located, as the insulating properties of dry chemical might render such equipment inoperative.
• Dry chemical extinguishing systems can be used in those situations where quick extinguishment is desired and where re-ignition sources are not present.
• Regular dry chemical will not extinguish fires that penetrate beneath the surface or fires in materials that supply their own oxygen for combustion.

In total flooding applications, a predetermined amount of dry chemical is discharged through fixed piping and nozzles into an enclosed space or enclosure around the hazard.

Hand hose line systems containing regular or ordinary dry chemical have been used to a limited extent for quick spreading surface fires on ordinary combustible material.

Required Delivered Density

 

Required delivered density (RDD) is the minimum rate of water application that, if delivered to the top of the fuel package, is capable of providing early suppression.

Suppression can be considered to occur when the heat release rate of the fire is quickly knocked down and is prevented from resurging.

Actual delivered density (ADD) is the actual rate of water application that a particular configuration of flowing sprinklers is capable of delivering to the top of a fuel package, depending on the strength of the upward fire plume.

Density/Area Curves Concept

Density/Area Curves Concept 

The density/area curves have traditionally provided some flexibility in system pipe sizing. Under provisions of NFPA 13, meeting any point on the appropriate curve is acceptable. This permits the use of higher densities over smaller areas or lower densities over larger areas.  Higher densities with smaller areas will generally result in larger branch-line piping but lower main sizes and lower overall water supply requirements. For this reason, a high-density, small-area point is often selected as the most economical.

minimum orifice sizes were specified for spray sprinklers protecting general storage, rack storage, rubber tire storage, roll paper storage, and baled cotton storage. For design densities of 0.34 gpm/ft2 (13.9 mm/min) or less, standard response sprinklers with a nominal orifice co efficient of K = 8.0 (Km = 115) or larger must be used. For design densities exceeding 0.34 gpm/ft2 (13.9 mm/min), standard- response sprinklers with a nominal orifice coefficient of K = 11.2 (Km = 160) or larger and that are specifically listed for storage applications must be used.

WATER DISTRIBUTION AND SPRAY COOLING CHARACTERISTICS OF SPRINKLERS

 

At present, there is no method of predicting the actual amount of water that will be delivered to a specific unit of floor area under actual fire conditions, especially since spray patterns vary with water discharge pressures

Figure 16.1.6 demonstrates the variability of the spray pattern of a typical pendent spray sprinkler with a nominal orifice coefficient of K = 5.6 (Km = 80) when discharged under non fire conditions at selected pressures. A 7 psi (0.5 bar) operating pressure is considered minimum by NFPA 13 , At that minimum pressure, the extent of the spray pattern roughly approximates that of the maximum light hazard spacing permitted by NFPA 13, when the sprinkler is located 8 ft (2.4 m) above the floor. The spray pattern enlarges as the operating pressure is increased to about 70 psi (4.8 bar), then begins to contract at higher pressures, becoming more elliptical in shape at the upper end of the allowable pressure range.

Convective Heat Flow in Fires (Sprinkler operation )

Convective Heat Flow in Fires

Heat is released from a fire in several forms: radiation, conduction, and convection. It has been determined that convective heat transfer is most important in activating sprinklers.

Convective heat transfer involves heat transfer through a circulating medium, which, in the case of fire sprinklers, is the room air .The air heated by the fire rises in a plume entraining other room air as it rises When the plume hits the ceiling, it generally splits to produce a ceiling gas jet The thickness  of this ceiling jet flow is approximately 5 to 12 percent of the height of the ceiling above the fire source, with the maximum temperature and velocity occurring 1 percent of the distance from the ceiling to the fire source. The heat-sensing elements of sprinklers within this ceiling jet are then heated by conduction of the heat from the air.