By automating damper controls on ovens, as much as a 50 percent reduction in fuel costs can be realized. As the cost of fuel increases, so do the concerns about energy optimization and safety. Learn how to safely use flammability monitoring to effectively automate damper controls and reduce the costs of owning and operating ovens and dryers.
First, a few safety points must be noted. Automating damper controls to modulate the flow of fresh air or exhaust through an oven can reduce the amount of fresh ventilated air moving through the system. In turn, this reduction greatly reduces the consumption of natural gas used for heating any unnecessary added air (figure 1). However, when doing this, care must be taken. If the primary controller was to malfunction and the dampers were not to open, an added risk of fire or explosion presents itself.
The National Fire Protection Association’s Standard for the Safe Operation of Ovens and Furnaces (NFPA 86) states that if a continuous vapor concentration controller is used to control the oven exhaust, then a secondary protection system must be used to prevent an analyzer failure from causing a hazardous condition. This secondary protection system shall have a separate continuous vapor concentration high limit controller for each zone. Otherwise, a fixed damper must be set so that solvent vapor concentration input cannot exceed 50 percent lower flammability limit (LFL) for the highest design solvent.
Achieving Cost Savings via Damper Adjustments
The use of redundant analyzers for secondary safety is preferred over setting damper stops to a worst-case setting of 50 percent LFL for cost savings due to the increased design flexibility of the total system.
If fixed damper stops are used as secondary safety, the damper will not close enough when the oven or dryer is lightly loaded, and the optimization that comes from damper control then is greatly reduced.
When dampers can be sufficiently closed beyond the worst-case position — especially when the ovens/dryers are lightly loaded — no additional air is heated and oven users can achieve a substantial cost savings.
The solvent load in many ovens and dryers contain a mixture of volatile organic compounds (VOCs) in differing proportions and concentrations. In various applications, different product is running through the ovens and dryers, so even formulations are changing from product to product.
Because it depends on the actual process conditions — in particular, the variation in loading from zone to zone from time to time — the flexibility of using a secondary solvent vapor-monitoring analyzer will precisely measure and safely allow proper modulation of fresh air or exhaust from an oven or zone.
Most dryers are used for more than one solvent. Accuracy depends on how the analyzer’s sensor responds to each solvent. This means that precise measurement of solvent vapor concentrations is needed to optimize the control of damper position and achieve maximum energy savings. The measuring principle of the sensor is important to achieve these goals.
The amount of error that results from attempting to measure two different vapors that have different response factors can be understood by taking the ratio of the two factors. Thus, an attempt to measure two vapors — one with a response factor of 0.5 and another with a response factor of 1.5 — could yield a reading of one-third (0.5/1.5) or three times (1.5/0.5) the actual concentration.
A flame-temperature-type sensor is said to have a universal calibration for many common solvent vapors because the response factors are in the range of 0.9 to 1.1. By contrast (figure 2), factors for catalytic sensors can range from 0.8 to 1.3 (a 1.5 to 1 ratio) and infrared sensors can easily reach 0.25 to 2.0 (an 8 to 1 ratio). The more solvents that are contained in the process, the greater the error and the wider the damper will stay open unless precise measurement is continuously taken.
Flame temperature analyzers will react accurately to most flammable substances and usually will 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. Therefore, direct flammability measurement can be used to precisely measure and modulate damper control to ensure no unnecessary air is heated. This optimization can result in as much as 50 percent reduction in fuel costs, so it is not something that should be ignored. The cost of adding solvent vapor analyzers would be quickly recouped (figure 3).
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.
1. NFPA-86-2007 Sec 10.1.8.4: Where a continuous vapor concentration controller is used to modulate the flow of fresh air or exhaust from an oven or zone, a secondary protection shall be required to prevent an analyzer failure from causing a hazardous condition.