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BULLETIN
TC-940
Class A Oven Equipment for Solvent Processing
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.



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 and subject to changes in this standard. All flammable solvent processing applications should be reviewed by the factory.
SIZING SAFETY VENTILATION AND HEAT LOSS

Safety ventilation exhaust provides a supply of fresh air to ensure that the flammable vapor concentration in the oven remains below the Lower Flammable Limit (LFL) 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 solvent with LFL 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 Flammable Limit (LFL) after the LFL 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 Flammable 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 Flammable 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 Flammable 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.

Batch Processing 0.1 Gallon Per Batch
Continuous Processing 0.1 Gallon Per Hour
Oven
Temperature
Batch
Exhaust
CFM
Heat Loss
KW
BTU/HR
100
46
0.4
1435
150
51
1.1
3827
200
55
1.8
6219
250
83
3.5
12,056
300
88
4.5
15,405
350
94
5.5
18,754
400
100
6.5
22,103
450
106
7.5
25,452
500
112
8.4
28,800
Oven
Temperature
Continuous
Exhaust
CFM
Heat Loss
KW
BTU/HR
100
21
0.2
638
150
23
0.5
1742
200
26
0.8
2897
250
28
1.2
4107
300
31
1.6
5375
350
34
2.0
6707
400
37
2.4
8107
450
40
2.8
9580
500
43
3.3
11,132
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