Moving and heating large amounts of air can be extremely expensive. Many manufacturing processes must use chemical solvents in the production of their products. As a result, hot air dryers are frequently used as a means of evaporating those solvents.
Most think the first approach to cost control is reducing ventilation air to the point which just assures complete drying at the desired production speed. However, this could lead to fire or explosion due to a dangerous build-up of flammable vapors, caused by insufficient ventilation. The following methods will produce better, safer results:
- Control of Flammable Vapors - To prevent such incidents, the National Fire Protection Association (NFPA) developed NFPA 86—the national standard for the safe operation of ovens and dryers. Section 184.108.40.206 of the standard states "...the safety ventilation rate shall be designed, maintained and operated to prevent the vapor concentration in the oven exhaust from exceeding 25 percent of the LFL". This standard provides a method for either estimating or calculating the minimum amount of ventilation air required to achieve this. In most cases, the estimation method requires the use of 12,000 cu/ft of air per gallon of solvent evaporated.
- Using Solvent Vapor Monitors - The cost of heating large volumes of ventilation air is high. However, NFPA 86 allows a substantial reduction in air in cases "where a continuous solvent vapor concentration indicator and controller is provided...” When such instruments are installed to continuously sample the exhaust of an oven zone, the vapor concentration in that zone is allowed to rise as high as 50% LFL.