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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. - PowerPoint PPT Presentation
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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. 3.3.25.3
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 materials—such as quench oil, waterborne
finishes, cooling oil, or cooking oils—that 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. 4.2.1
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 ft2 for 15ft3 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 9.2.2.3
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 ….cont’d
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 770 F (250 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 8.328 lbs at 700 F
Dry air at 70 deg F & 29.9 in Hg weighs 0.075 lb/ft3
Specific gravity of Toluene = 0.87 (Water =1)
Vapor density of Toluene = 3.1 (Air =1)
LEL of Toluene at 700 F = 1.1
Oven exhaust temperature = 3000 F
ELC International Inc. (Loss Control through Engineering)
LELt = LEL77 F {1-0.000436 (t0f –770F)}
= 1.1 { 1-0.000436 (300-77)}
= 0.99
Volume of air at 700F required to render 1 gallon of Toluene
barely explosive (100% LEL)
8.328/0.075 x 0.87/3.1 { (100-0.99)/0.99}
= 3116 ft3 at 700 F (SCFM) per gallon of Toluene
ELC International Inc. (Loss Control through Engineering)
Adjust this factor to 25% LEL by multiplying by 4
3116 x 4 = 12464 SCFM per gallon solvent
Multiply this value by solvent loading in gallons/minute
Assume solvent evaporated per minute is 0.7 gallons
12464 x 0.7 = 8724.8 SCFM
8724.8 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 2 + 8724.8 = 9090.8 SCFM
Correcting this value for oven operating temperature of 3000
F and assuming sea level altitude:
9090.8 x { (460+300)/(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 9.2.14
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 242.5
deg F and the entrained air is 70 deg F then
(X + Y) x 242.5 = 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.
ELC International Inc. (Loss Control through Engineering)
Any Questions?
ELC International Inc. (Loss Control through Engineering)
Thank You