OSHA
requires ovens processing flammable solvents or vapors to conform to the
National Fire Protection Association Standard 86 (1999). NFPA 86 defines
ovens for these applications as Class A ovens. In order to keep flammable
vapor concentration at safe levels, Class A ovens include specific items
of additional equipment to provide safety ventilation. This additional equipment
is sized based on the volume of solvent being evaporated and the maximum
operating temperature.
Safety
ventilation also depends on the processing method. In batch processes, where
all parts heat up at the same time, the flammable vapor concentration in
not constant. As the work temperature rises the vapor concentration also
rises to a peak value and then tapers off. This requires greater safety
ventilation than in continuous processing where parts are heated sequentially
and the vapor concentration remains constant.
Direct - fired gas ovens include
Class A equipment to remove products of combustion. Combustion venting
is
sized at 183 standard cubic feet per minute of exhaust per 1,000,000 BTU/HR
of burner rating. Depending on the amount of flammable solvent processed,
a larger powered forced exhauster may be required to handle both combustion
venting and safety ventilation.
Electrically-heated ovens require
that the following be added:
Powered Forced Exhauster to provide
safety ventilation .
Capacity
Carbon
Steel
to 850°F*
Stainless
Steel
Over 850°F
HP
Outlet
Diameter
Height
208V
230V
460V
208V
230V
460V
80
CFM†
X808
X802
X804
N/A
N/A
N/A
1/8
4"
16"
130
CFM†
X1308
X1302
X1308
SX1308
SX1302
SX1304
1/3
4"
20"
325
CFM
X3258
X3252
X3254
SX3258
SX3252
SX3254
1/3
6"
23"
650
CFM
X6508
X6502
X6504
SX6508
SX6502
SX6504
1/2
6"
23"
975
CFM
X9758
X9752
X9754
SX9758
SX9752
SX9754
1
8"
23"
*80 CFM available up to
550°F only.
†80 CFM and 130 CFM exhausters not available on Walk-In Ovens.
Factory Mutual will not approve an exhauster smaller than 325 CFM capacity
as they require a minimum exhauster tip speed.
Larger capacity exhausters
are available.
All exhausters have a damper which can be adjusted to reduce exhaust rate
to the required volume.
Electric ovens equipped with an exhauster may require additional heat input
to offset exhaust heat loss—please consult factory.
Powered Forced Exhauster Air
Flow Safety Switch
to prove exhauster operation and shut heat down on exhauster failure.....................EAFS
Purge Timer to provide
time period of sufficient duration to exhaust four oven volumes of fresh air
prior to turning the heat on ...................................PT
SIZING SAFETY VENTILATION
AND HEAT LOSS This information is
based on NFPA Standard 86 (1999 edition) and subject to changes in this standard.
All flammable solvent processing applications should be reviewed by the factory.
Safety ventilation exhaust provides
a supply of fresh air to ensure that the flammable vapor concentration in the
oven remains below the Lower Exposure Limit (LEL) at all times. When more than
one flammable solvent is present, safety ventilation is based on the solvent
requiring the greatest amount of ventilation.
Direct-fired gas heated ovens require
combustion venting which must be added to the safety ventilation exhaust rate.
Combustible solids or substrates
do not require safety ventilation unless flammable constituents are released
when these materials are heated. Powder coating curing ovens require safety
ventilation to be calculated as though 9 % of the powder weight is being evaporated
as the flammable solvent Xylene.
Exhaust rates must be corrected for
maximum operating temperature. As oven operating temperature increases, greater
exhaust is required to compensate for the decrease in air density. Similarly,
at altitudes over 1000 feet, the exhaust rate must be increased to compensate
for lower air density.
Examples of actual calculations can
be found in NFPA 86. Estimated safety ventilation exhaust rate and heat loss
can be scaled from charts shown below.
Batch Processing
For batch oven applications
below 250° F, the safety ventilation exhaust rate must be sized
at a rate of 440 standard cubic feet per minute for each gallon of
solvent introduced into the oven in a batch. Between 250° F and
500° F, this exhaust rate must be increased by a multiplier of
1.4. Above 500° F, the 1.4 multiplier is not appropriate and a
correction factor must be determined by tests run by the solvent manufacturer.
In addition, for solvents where the volume of air necessary to render
1 gallon of solvent barely explosive exceeds 2640 standard cubic
feet,
the exhaust rate must be increased. The increase is made by multiplying
by a factor created by taking the volume of air necessary to render
1 gallon of solvent barley explosive and dividing it by 2640 standard
cubic feet.
Safety ventilation and heat loss for a batch process can be estimated
using the chart below. This chart shows the exhaust rate and heat
loss for 0.1 gallon of solvent. The data is based on a a solvent with
LEL of 2640 standard cubic feet and on an installation located below
1000 feet in altitude.
Continuous Processing
For continuous
processing ovens, the safety ventilation exhaust rate must be sized
to keep the concentration of solvent in the oven atmosphere below
25% of the Lower Explosive Limit (LEL) after the LEL has been corrected
for operating temperature.
The specific gravity and vapor density of the solvent are used to
determine the gallons of vapor created by 1 gallon of evaporated solvent.
Combining this information with the Lower Explosive Limit determines
the air volume rendered barely explosive when 1 gallon of solvent
is evaporated. This is multiplied by a factor of 4 to limit the vapor
concentration to 25% of the Lower Explosive Limit.
The safety ventilation exhaust rate is obtained by multiplying this
volume by the number of gallons of solvent evaporated per minute.
Safety ventilation and heat loss for a continuous process can be estimated
using the chart below. This chart shows the exhaust rate and heat
loss for 0.1 gallon of Xylene. Xylene has specific gravity of 0.88
(Water = 1) and vapor density of 3.7 (Air = 1). The Lower Explosive
Limit by volume is 0.9%. The amount of air rendered barely explosive
per gallon evaporated is 2899 standard cubic feet. The calculated
exhaust required per gallon of Xylene evaporated in continuous processing
is 193 standard cubic feet per minute.
