Safety Design Requirements for Class A Ovens ELC International Inc. (Loss Control through Engineering)

Topics of Discussion: Definition General Requirements Safety Ventilation Requirement Example of Safety Ventilation Calculation Exhaust Fan Capacity Test ELC International Inc. (Loss Control through Engineering)

What is a Class A oven? ELC International Inc. (Loss Control through Engineering) An oven or furnace that has heat utilization equipment operating at approximately atmospheric pressure wherein there is a potential explosion or fire hazard that could be occasioned by the presence of flammable volatiles or combustible materials processed or heated in the furnace. NFPA 86 Sec

ELC International Inc. (Loss Control through Engineering) Flammable volatiles or combustible materials can include, but are not limited to, any of the following: (1) Paints, powders, inks, and adhesives from finishing processes, such as dipped, coated, sprayed, and impregnated materials (2) Substrate material (3) Wood, paper, and plastic pallets, spacers, or packaging materials (4) Polymerization In addition, potentially flammable materialssuch as quench oil, waterborne finishes, cooling oil, or cooking oilsthat present a hazard are ventilated according to Class A standards

B149.3 :00 - Valve Train, Combustion Control etc. NFPA 86: 2003 : Safety Ventilation ELC International Inc. (Loss Control through Engineering)

Operator & Maintenance Personnel Training ELC International Inc. (Loss Control through Engineering) All operating, maintenance, and supervisory personnel shall be thoroughly instructed and trained under the direction of a qualified person(s) and shall be required to demonstrate understanding of the equipment and its operation to ensure knowledge of and practice of safe operating procedures. NFPA 86 Sec

ELC International Inc. (Loss Control through Engineering) Operator training shall include: (1) Combustion of fuel–air mixtures (2) Explosion hazards, including improper purge timing and purge flow and safety ventilation (3) Sources of ignition, including auto ignition (e.g., by incandescent surface) (4) Functions of controls, safety devices, and maintenance of proper set points (6) Operating instructions

ELC International Inc. (Loss Control through Engineering) Explosion Relief Shall be designed as a ratio of relief area to furnace volume; 1 ft 2 for 15ft 3 of furnace volume. Hinged panels, openings or access doors with approved explosion-relief hardware can be included in the calculation Explosion relief vents shall activate at a surge pressure not exceeding the design pressure of the oven enclosure

Interlocks ELC International Inc. (Loss Control through Engineering) Conveyors or sources of flammable material shall be interlocked to shutdown on excess temperature or if either the exhaust or recirculation system fails. NFPA 86 Section

ELC International Inc. (Loss Control through Engineering) Building Make up air A quantity of makeup air shall be admitted to oven rooms and building to provide the air volume required for oven safety ventilation and combustion air.

ELC International Inc. (Loss Control through Engineering) Name Plate for Solvent Atmosphere Ovens. Manufacturer's nameplates for solvent atmosphere ovens shall include all the following design data: (1) Solvent used (2) Number of gallons (liters) per batch or per hour of solvent and volatiles entering the oven (3) Required purge time (4) Oven operating temperature (5) Exhaust blower rating for gallons of solvent / hour at the max operating temperature

Safety Ventilation Requirements ELC International Inc. (Loss Control through Engineering) 1. Air circulation shall be used within the oven 2. Safety ventilation shall be based on: i. Volume of products of combustion ii. Weight or volume of flammable material released during the process, based on maximum loading iii. Solvent that requires maximum ventilation per gallon when a combination of solvents is used

ELC International Inc. (Loss Control through Engineering) Safety Ventilation Requirements ….contd The safety ventilating rate shall be designed, maintained and operated to prevent the vapor concentration in the exhaust from exceeding 25% LEL A considerable portion of the fresh air can pass through the oven without traversing the zone where majority of vapors are given off. 25 % LEL introduces a 4:1 factor of safety.

Calculation of Safety Ventilation Rate ELC International Inc. (Loss Control through Engineering) Data Required: Exhaust temperature of oven Solvent data for worst case scenario – Sp. Gravity, Vapor Density and % LEL Solvent loading of the oven based on maximum capacity

ELC International Inc. (Loss Control through Engineering) Temperature Correction: LEL values are normally reported at 77 0 F (25 0 C). This value decreases with rise in temperature. Calculated ventilation air flow shall be corrected for oven exhaust temperature

ELC International Inc. (Loss Control through Engineering) The volume of dilution air that would render vapor from a known volume of toluene barely flammable is determined as follows: One gallon water weighs lbs at 70 0 F Dry air at 70 deg F & 29.9 in Hg weighs lb/ft 3 Specific gravity of Toluene = 0.87 (Water =1) Vapor density of Toluene = 3.1 (Air =1) LEL of Toluene at 70 0 F = 1.1 Oven exhaust temperature = F

ELC International Inc. (Loss Control through Engineering) LEL t = LEL 77 F { (t 0 f –77 0 F)} = 1.1 { (300-77)} = 0.99 Volume of air at 70 0 F required to render 1 gallon of Toluene barely explosive (100% LEL) 8.328/0.075 x 0.87/3.1 { ( )/0.99} = 3116 ft 3 at 70 0 F (SCFM) per gallon of Toluene

ELC International Inc. (Loss Control through Engineering) Adjust this factor to 25% LEL by multiplying by x 4 = SCFM per gallon solvent Multiply this value by solvent loading in gallons/minute Assume solvent evaporated per minute is 0.7 gallons x 0.7 = SCFM SCFM of ventilation air required only for the solvent to keep its concentration at 25 % LEL

ELC International Inc. (Loss Control through Engineering) Add products of combustion (183 SCFM per million BTU) Assume the heat input is 2.0 million Btuh Total Exhaust Fan capacity required is 183 x = SCFM Correcting this value for oven operating temperature of F and assuming sea level altitude: x { ( )/(460+70) } = 13,036 acfm

Exhaust Fan Capacity Test ELC International Inc. (Loss Control through Engineering) On completion of an oven installation, airflow tests shall be conducted on the ventilation systems under the oven operating conditions, with flow control devices at their minimum setting. NFPA 86 Section

ELC International Inc. (Loss Control through Engineering) Exhaust Fan Capacity Test …contd How to Calculate Air Flow 1. Measure velocity of air in the discharge duct by using a velometer, pitot tube or anemometer 2. Multiply velocity by cross sectional area of duct 3. Measure exhaust gas temperature and correct calculated volume to 70 degree F

ELC International Inc. (Loss Control through Engineering) Exhaust Fan Capacity Test …contd The calculated volume is of air flow is an indication of the volume exhausted from the oven, provided the exhaust air does not mix with air external to the oven. In many ovens the exhaust duct is incorrectly placed in location where outside air enters the exhaust system and is exhausted together with the air exhausted from oven

ELC International Inc. (Loss Control through Engineering) Exhaust Fan Capacity Test …contd The amount of outside air entering into the exhaust system is calculated by measuring oven temperature, exhaust air temperature and outside air temperature. If the temperature of oven is 300 deg F, the exhaust is deg F and the entrained air is 70 deg F then (X + Y) x = 300 x X + 70 x Y or 3Y=X The exhaust fan is exhausting 75% of its flow from the oven!

ELC International Inc. (Loss Control through Engineering) Powder Coating Ovens: The safety ventilation required for powder curing ovens shall be calculated by assuming that 9 percent of the mass of the powder is xylene and the remaining mass is inert.

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