Clean Air for Less

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by Jim Reid

Optimizing your air quality system

Are you paying more for clean air than you need to? When it comes to dust and fume extraction, “more and bigger” isn’t always the answer.

There are two main drivers for your air quality costs: the volume of air you need to move and the square footage of filter media you need to buy. An understanding of the fundamentals of air movement and filtration can help you make informed choices for your next air quality investment.

Understanding Airflow
There are two main methods of removing dirty air from a facility.

Exhaust and make-up air systems use large fans to blow contaminated air out of the facility into the environment and pull cleaner outdoor air back into the facility. These systems are cheap to install, but heating or cooling outdoor air to factory temperature can result in significant energy costs over time. They also may put facilities out of environmental compliance.

Filtration systems pull contaminated air through a filter media and return clean air back to the facility. These systems can be ambient (cleaning air for the entire facility) or source capture (capturing fumes at the point where they are generated).

Regardless of which type of system you choose, one of the first metrics you need to understand is the airflow rate needed for effective capture, measured in cubic feet of air per minute (cfm).

CFM is one of the primary drivers of your overall air quality costs: the more air you need to move, the more energy it will cost to move it. Greater airflow requires a larger (and more expensive) machine with a more powerful blower. It also directly impacts the second big cost driver, filter media. Filters are sized according to “air-to-cloth ratio.” This is the ratio of filter media to the volume of air being pushed through. The more air you are moving through your filters, the larger the square footage of filter media required.

The cfm required to reduce particulate levels to acceptable exposure limits is directly tied to the capture method you select. The closer to the source you are able to capture fumes, the more concentrated they will be and the less air you will need to move to capture them. For example, consider a single welder operating within a medium-sized facility.

• Capturing directly at the source using a fume gun may require as little as 80 cfm.

• A backdraft table for small parts (18-24 inches) may use 1,000 cfm.

• A whole-facility ambient or exhaust system may require as much as 10,000 cfm.

The same basic principal applies to robotic welding and other processes. Containing fume- and dust-generating applications within a hood reduces the amount of air that needs to be moved and cleaned. Sizing the hood correctly for your application and making sure it is fully enclosed will improve capture efficiency and minimize opportunities for fumes and particulates to escape into the ambient air throughout the facility.

Reducing filter costs
Minimizing cfm is your first step in reducing filter costs. However, there are other things you can do to improve filter life and further cut costs.

First, you want to make sure you don’t use too little filter media. While fewer or smaller filters may sound like a less expensive option, the loss in filter life will more than eliminate savings gained by reducing filter media. Again, this goes back to air-to-cloth ratio. The higher the air-to-cloth ratio, the deeper particulate is driven into the media. Instead of settling on the surface where it can be easily shaken off, particulate rapidly clogs the filters, reducing their life and increasing the energy required to move air through them. Sizing your filter media for your air volume and the volume of particulates produced is one of the most important considerations in air quality system design.

Other factors that impact filter life include:

• Filter media: Filter media will depend on the size, type and volume of your particulate. It’s important to select the right filter media for your application.

• Filter cleaning: An automated mechanism that periodically pulses or shakes the filters will significantly lengthen filter life by shaking
dust off the outside of the filters
into the collection bin before it becomes entrapped in the filter media. This can increase filter life by 30-50 per cent.

• Orientation: Vertical filters last longer than filters that are installed horizontally, because dust can be more easily pulsed off the filter and does not settle onto filters below.

Designing for maximum cost efficiency
Designing for minimum cfm and maximum filter life will go a long way towards optimizing your air quality system. But there are many other factors to consider as well, from facility layout to the specific human health and safety dangers of your dust and fumes. An air quality systems engineer can help you evaluate your facility, processes and air quality goals to help you find the right balance.

System design takes a number of factors into consideration:

Computer modeling can help engineers evaluate different options and select the most cost effective approach to meet air quality goals.What is the type and volume of fumes and particulates produced, and how dangerous are they? Highly toxic fumes, such as those containing hexavalent chromium, manganese or silica, may require more aggressive measures, including ambient capture systems to remove fumes that escape from your primary source capture system. Combustible dusts also require special considerations.

What are the sources of fumes and particulates? How large are the pieces being worked on, and are they moving or stationary? For some applications, source capture is simply not practical or feasible due to the size of the pieces produced or the mobility of the welder.

What are your overall air quality goals? Are you just trying to stay within compliance, or do you have more ambitious goals in mind? Many companies are pursuing more stringent air quality standards to meet recruiting, retention and productivity goals.

What does your facility layout look like, and what are your space constraints? Ideally, your air quality system design will go hand-in-hand with the design of your production lines. Integrating fume capture into production equipment design, instead of adding them as an afterthought, can reduce cfm requirements by as much as 50-75 per cent–a significant cost savings.

What are the current airflow patterns in the building? How do dust and fumes propagate throughout the facility? Working with your airflow patterns, instead of against them, will minimize cfm requirements.

Evaluating all of these factors can be challenging. RoboVent uses computer modeling to evaluate factors such as particulate size and volume, airflow patterns in the facility and other variables. With modeling, engineers can optimize placement of source and ambient capture equipment to make sure you are getting the most utility out of each piece. This approach avoids over or under engineering to find the solution that produces the desired air quality outcomes in the most efficient and cost effective way possible.

Understanding some fundamentals of air quality system design can help you avoid expensive mistakes. Time spent up front evaluating your goals and requirements will pay dividends in the future through reduced energy, maintenance and equipment costs. SMT

Jim Reid is the general manager for RoboVent.

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