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Natural gas kilns and curing ovens typically operate at high temperatures of up to 1500°C. The solvents used to bind ceramic material together are vaporized in the kilns; the high temperatures are needed to cure the organic polymers to the substrate material. Moving and heating large amounts of air can be extremely expensive. As a result, the cost of heating large volumes of ventilation air is high.
Many think that the first approach to controlling costs is to reduce ventilation air to the point where it ensures complete drying at the desired production rate. However, this could lead to a buildup of flammable vapors caused by insufficient ventilation.
To prevent such incidents, the National Fire Protection Association (NFPA) developed NFPA 86-the National Standard for the Safe Operation of Ovens and Furnaces. In accordance with this guideline, when continuous solvent vapor monitoring analyzers are installed to keep track of the flammability levels of the oven or kiln, the vapor concentration is allowed to rise from 25% lower flammable limit (LFL) to as high as 50% LFL. This allows ventilation to be controlled and even cut in half, so excess air does not need to be heated to such high temperatures. The amount of natural gas that is needed to operate these processes safely is therefore reduced.
SafetyAutomating damper controls to modulate this flow of fresh air and/or exhaust through an oven can further reduce the concentration of ventilated air moving through the system. In turn, the consumption of natural gas used for heating any unnecessary added air is reduced.
When adding automated dampers to control the ventilation rates, additional safety and cost savings can be achieved without taking the risk of human error-but care must be taken. If the primary controller was to malfunction and the dampers did not open, the system has an added risk of fire or explosion.
NFPA-86 therefore states that if a continuous vapor concentration controller is used to control the oven exhaust, a secondary protection system must be used to prevent an analyzer failure from causing a hazardous condition.1 This secondary protection system should have a separate continuous vapor concentration high-limit controller for each zone. Otherwise, a fixed damper must be set so that the solvent vapor concentration input cannot exceed 50% LFL "worst case" (for the highest design solvent load).
Cost SavingsDue to the increased design flexibility of the total system, the use of redundant analyzers for secondary safety and cost savings is preferred over setting damper stops to a "worst case" fixed setting of 50% LFL. If fixed damper stops are used as secondary safety, the damper will not close enough when the oven/kiln is lightly loaded, and the optimization that comes from damper control is greatly reduced. When dampers can be sufficiently closed beyond the "worst case" position, especially when the ovens/kilns are lightly loaded, no additional air is heated and companies can achieve substantial cost savings.
The solvent load in many ovens and dryers contains a mixture of volatile organic compounds (VOCs) in differing proportions and concentrations. In various applications, different product and formulations are running through the ovens and dryers. The secondary solvent vapor monitoring analyzer is adaptable to actual process conditions (e.g., the variation in loading from zone to zone, batch to batch, and time to time). This flexibility enables the precise measurement and safe modulation of fresh air or exhaust from an oven or zone (see Figure 1).
Technical ConsiderationsMost dryers are used for more than one solvent, and accuracy completely depends on how the sensor responds to each solvent. This means that the precise concentration of solvent vapors should be measured with the highest possible level of accuracy to optimize the control of damper position and achieve maximum energy savings (see Figure 2).
The measuring principle of the sensor is important to achieve these goals. The flame temperature sensor is said to have a "universal calibration" for many common solvent vapors because the response factors are in the range of 0.9-1.1. By contrast, factors for catalytic sensors can range from 0.8-1.3 (a 1.6:1 ratio), and infrared sensors can easily reach 0.25-2.0 (an 8:1 ratio). The more solvents that are contained in a process, the greater the error and the wider the damper will stay open-unless precise measurement is continuously taken.
Flame temperature analyzers react accurately to most flammable substances and usually measure both single solvents and mixtures with the same high degree of accuracy. Unlike some sensors, flame temperature analyzers were developed for one specific purpose: to directly measure flammability.
Optimized ProcessDirect flammability measurements can be used to precisely modulate damper control and ensure that no unnecessary air is heated. This optimization can result in a reduction in fuel costs by as much as 50%; the cost of adding solvent vapor analyzers can be quickly recouped.
In addition, a reduction in the oven/kiln exhaust rates will decrease demand on the VOC destruction oxidizers. This would be the case when a thermal oxidizer has reached its maximum rated airflow capacity, preventing the addition of new processes without a sizable investment.
Instead of adding another oxidizer for the new equipment, it may be desirable to reduce the exhaust from the existing process ovens by recirculating a portion of the exhaust air. Such a solution is possible when the ovens are equipped with solvent vapor analyzers. (The cost of these flammability analyzers can also be quickly recouped.)
When selecting a flammability analyzer, do not assume that one size fits all. The appropriate analyzer for a previous application is not necessarily the right choice for another. It is beneficial to select an analyzer that can handle future changes in sample stream composition. However, the specific details of each application need to be examined closely in order to prevent disaster. The instrument of choice may vary from one application to the next, but the correct process flammability analyzer will always be fast, accurate and fail safe.
For more information, contact Control Instruments Corp. at 25 Law Dr., Fairfield, NJ 07004; call (973) 575-9114; fax (973) 575-0013; email firstname.lastname@example.org; or visit www.controlinstruments.com.