Whether you have a direct gas-fired or an electrically heated oven,
understanding and following the NFPA 86 standard can minimize the risk
of a catastrophic event at your plant.
By J.W. GUANCI III,
PRECISION QUINCY CORP.
What are the most important
components of oven selection? Airflow and its subsequent interaction
with product in a given loading pattern? Sufficient thermal input as it
relates to getting a given load to temperature? Acquiring and recording
data for one’s quality control program? Certainly, this list could go on
for pages, but the answer will always be no.
Are they important?
Certainly. In fact, there are many questions critical to a successful
application and a positive outcome. But, those questions do not address
the structure or framing in which ovens must be manufactured and
applied. Adhering to those rules — outlined in the National Fire
Protection Association’s Bulletin 86 (commonly known as NFPA 86) — is
the single most important thing an oven builder, buyer and operator can
NFPA 86’s most recent
revision was published in 1995. With new editions released every four or
five years, it is a living document that has evolved with the times.
Oven users may be confused when reading NFPA 86 because the listing for
“oven” says simply, “see furnace.” Processors should not be misled.
Although the text says “furnace,” oven users should not conclude that
the section does not apply to them. Because NFPA sometimes interchanges
“oven” and “furnace,” processors should always read further — get into
the details — to be sure they are not overlooking important information.
Currently, there are two
classifications pertaining to ovens: Class A and Class B. I’ll start
with some working definitions. It is important to discuss Class B ovens
first — they are more basic and simpler. Class A ovens include
additional features not found on their Class B counterparts.
Class B ovens are those
units meant solely for clean processes “...wherein there are no
flammable volatiles or combustible materials being heated.”
Conversely, Class A ovens
are those units that can be utilized in processes with present solvents,
volatile materials or other flammable or combustible contents. NFPA 86
cites several materials requiring the Class A rating, specifically
• Paints, powders, inks and
adhesives from finishing processes such as dipped, coated, sprayed and
• The substrate material
• Wood, paper and plastic pallets, spacers or packaging materials
• Polymerization or other molecular arrangements. Potentially flammable
materials such as quench oils, waterborne finishes, cooling oil, or
cooking oils that present a hazard are ventilated according to Class A
It is important to note
that this list is not all-inclusive; rather, it serves as an overview of
the types of materials or processes that are considered Class A in
What’s in it for You?
The answer to that question
is really another question: What’s not in it for you?
Reality: Everyone is
impacted and everything is at stake, every single day — reputation,
bottom line, personnel confidence and morale, market position, profit,
marketing, cost accounting, insurance rates and corporate survival.
Disregarding oven safety
features or removing safeguards is inviting disaster through your
company’s front door. Life becomes an even more dangerous and random
game of chance — like Russian roulette with only one empty chamber.
For example, if just one
safety is jumpered — say, an airflow switch — the likelihood of an
accident occurring has increased exponentially. If flammable fumes build
up in the oven’s work area, an explosion can take place. This would
damage the oven, surely, but what of the other costs?
Other equipment may become
damaged or inoperable. The building itself may be damaged due to flame
or the sheer force of the event. Most important, though, is the human
element — an employee, just someone trying to make an honest living —
may be hurt, incapacitated or killed. The economic, legal and moral
repercussions of such an accident can be widespread, and the shock waves
often are far reaching — not only within the company, but in the
individual lives with which your company is intertwined.
Safety is a big word — a
broad-brush concept that has been drummed into the collective industrial
conscience for quite some time. However, the long-term battle to be safe
is so often won and lost by small, seemingly insignificant practices
(like bypassing a simple switch) that occur day-to-day.
Oven safety — what’s in it
for you? A lot more than one might think.
With working definitions
established, let’s look at how they impact the way ovens are designed,
equipped and applied.
Class B - Electrically heated Class B ovens must be
equipped with the following safety equipment:
Airflow Safety Switch - An airflow safety switch should be integrated
into the recirculation fan assembly, where it will shut down the unit
should the fan cease operation. This prevents the heat source from
continuing to operate without recirculation air moving passed it.
Manual Reset Excess
Temperature Control - Commonly known in the
industry as a hi-limit, this control should be set approximately 50°F
(10°C) higher than the oven setpoint or maximum rated temperature.
Should the unit produce too much heat; the hi-limit will trip and shut
down the heaters once the excess temperature setting is reached.
Backup Contactors - Backup contactors are required as a fail-safe.
If the primary contactors become fouled or fused, the backup contactors
will ensure that the heaters shut down if an over-temperature condition
Class A - Electrically heated Class A ovens must have all
of the safety controls specified for Class B. In addition, the following
items are required:
Powered Exhaust Fan - The oven must have an exhaust fan of sufficient
size (measured in cubic feet per minute) to evacuate a sufficient
quantity of fumes, smoke, etc. so that a buildup of hazardous materials
in the oven’s work area does not occur during processing. The amount
(cfm) of required exhaust depends on many variables, including
solvent/volatile material level, temperature, cycle time and cycle
dynamics (ramp rate and soak duration).
Extra Airflow Safety Switch - A second airflow safety switch also is required.
Should the exhaust fan fail for any reason, this second airflow switch
acts as a fail-safe for the primary airflow switch and will shut down
Purge Timer - A timed purge is required prior to heating.
During this period, the recirculation and exhaust fans remove any
flammable vapors and/or gases that may have entered the work area while
the oven was idle.
Extra Kilowatts (if
required) - Electrically heated ovens equipped with Class A
features may require additional kilowatts. Because the oven will be
forcibly removing heat from the work area (via the required exhaust
fan), more heaters may be needed to maintain process temperatures,
depending on the heatup time and the maximum operating temperature. If
this issue is not considered in the design stage, the oven may have a
poor response time or be unable to reach the operating temperature
necessary to run the load.
Extra Backup Contactors (if
extra kilowatts are added)
- If the oven is designed with extra kilowatts, additional backup
contactors are required to provide adequate safeguards. Backup
contactors will ensure that the heaters shut off should the primary
contactors become fouled or fused.
Internal Pressure Relief - The final extra that must be included in an
electrically heated Class A oven is internal pressure relief. If a
buildup of fumes leads to an explosion, this required feature will
ensure that the event has an avenue of “controlled” release. Pressure
relief typically is accomplished by incorporating pressure-relief doors
or a caged pressure-relief panel.
All direct gas-fired ovens
carry the Class A rating automatically because they introduce products
of combustion into the oven work area. These volatile materials must be
exhausted in a forcible manner. Direct gas-fired Class A ovens must
include the following safety features:
Powered Exhaust Fan - A powered exhaust fan serves to forcibly remove
any products of combustion, or process byproducts, to reduce the chance
that a hazardous situation will develop.
Airflow Safety Switches - On gas-fired ovens, as many as three airflow
safety switches are required. All Class A ovens must incorporate airflow
safety switches connected to the exhaust fan assembly and the
recirculation fan assembly. Also, if the burner is equipped with a
combustion blower, an airflow safety switch must be installed on it.
Should any one of the fans fail, the appropriate airflow safety switch
will trip and shut down the burner.
Manual Reset Excess
Temperature Controller - This control should be
set approximately 50°F higher than the oven setpoint or maximum rated
temperature. If the oven reaches the excess temperature setting, the
hi-limit will shut down the burner.
Purge Timer - As with electrically heated units, the purge
timer is intended to ensure that the exhaust fans remove any buildup of
combustible fumes in the work area prior to igniting the burner.
Internal Pressure Relief - The internal pressure relief panel serves as a
“controlled” avenue through which explosive forces may be released —
just as with an electrically heated Class A oven.
High-Low Gas Pressure
Switch - Should the sensor detect that gas flow is too
high or too low, the high-low gas pressure switch will shut down the
oven. Fairly even gas pressure must be supplied to maintain the correct
air-to-fuel ratio for proper flame characteristics and to prevent a
buildup of combustible vapors.
Flame Safety/Spark Ignition – Finally, on direct gas-fired ovens, a flame
supervision device is required. Upon startup, this control sends a high
voltage signal to the spark plug for ignition, and the pilot gas valve
opens to ignite the pilot flame. Once the flame safety system detects a
pilot flame, the high voltage signal stops and the main gas valves open,
allowing the unit to go to high fire and begin processing. If no pilot
flame is detected, the flame safety stops the attempt at ignition.
The final oven category is
an explosion-proof rating. Covered in part in NFPA 70, this type of
construction can be very expensive to purchase when compared to its
Class B and Class A counterparts. Do not confuse an explosion-proof
rating with Class A requirements — explosion-proof ovens are complex
units with requirements too extensive to cover in any detail in this
In review, according to
NFPA 86, there are Class A and Class B ovens. Class A ovens are rated
for flammable materials, while Class B ovens are not. All direct-fired
ovens must carry the Class A rating and be equipped with the appropriate
safety features. Explosion-proof ovens require a host of special and
costly considerations that are outlined in NFPA 70.
Precision Quincy Ovens
602 East Blackhawk
Byron, IL 61010
Oven safety - what's in it for you? A lot
more than one might think.