The cost of natural gas used for heating process ovens and dryers can be substantial, but using solvent-vapor analyzers can lower oven-heating costs.

Many manufacturing processes must use chemical solvents in the production of their products. As a result, hot air dryers frequently are used as a means of evaporating those solvents, according to Control Instruments Corp., a maker of gas-detection sensors in Fairfield, N.J.

In one of its online application notes, the company acknowledges that moving and heating large amounts of air can be expensive. Often, the first approach to cost control is reducing ventilation air just to the point at which complete drying is ensured at the desired production speed. However, cautions Control Instruments, this could lead to fire or explosion due to a dangerous buildup of flammable vapor caused by insufficient ventilation.

To prevent such incidents, the National Fire Protection Association developed NFPA 86, the national standard for the safe operation of ovens, dryers and furnaces. Section 10.1.6.1 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 LEL.

LEL is the lower explosive limit, which is the minimum concentration of solvent vapors in air that, given a source of ignition, will sustain combustion. NFPA 86 provides a method for either estimating or calculating the minimum amount of ventilation air required to achieve the LEL. In most cases, the estimation method requires the use of 12,000 ft 3of air per gallon of solvent evaporated.

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 are provided...”.

When such instruments are installed to continuously sample an oven zone’s exhaust, the vapor concentration in that zone is allowed to rise as high as 50 percent LEL, according to NFPA 86, Sections 10.1.6.1 and 10.1.8. This translates to the following money- and fuel-saving results, according to Control Instruments.
  • Reduction of ventilation air without affecting existing production rates or violating NFPA safety directives.
  • Production-speed increase without increasing existing air or fuel costs.
  • Additional savings by reusing rather than expelling some of the hot exhaust stream back into an oven zone.
  • Reduction of oven exhaust rates to lower the demand on VOC-destruction oxidizers.
Other advantages to adding analyzers often are overlooked because they do not seem to be associated with the problem in another part of the plant, says Control Instruments. For example, when a thermal oxidizer reaches its maximum rated airflow capacity, it may prevent the addition of new process lines without a sizeable investment. Instead of adding another oxidizer for the new equipment, the manufacturer could reduce the outflow from existing process lines by recirculating a portion of the exhaust air. The cost of the solvent vapor analyzers would be recouped quickly, Control Instruments notes.

For more information on gas analyzers, visit www.controlinstruments.com.

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