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.

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.

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.