safety design requirements for class a ovens

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


What is a Class A oven?


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


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


Operator & Maintenance Personnel Training


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


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


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


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


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.


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


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


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


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


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


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


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


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


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


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


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



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



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!


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.


Any Questions?


Thank You

safety design requirements for class a ovens

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