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Pressurizing with Dust Collection Keeps Dirty Air Out

July 19, 2013
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In many powder/bulk handling environments, areas such as offices, labs, and electrical and server rooms need to be protected from dirty outdoor air conditions, from dust or fumes generated by an adjacent production process, or even from excess humidity that might seep in through walls or other openings. Dust infiltration in such spaces can create unpleasant working conditions while causing problems with critical equipment, especially electronics.
    Pressurization is a well-known ventilation technique in which a positive or negative atmospheric pressure is applied to prevent dust infiltration. Positive pressure (known as “inflating the building”) keeps particulate or gaseous contaminants out of a room, creating an air barrier between the outside and the inside. When you enter a positively pressurized environment from outside, you will feel the “whoosh” of air escaping due to the higher pressure of the air inside the room. Positive pressure can keep contaminated outside air from entering an office, server room, or other space.
    Conversely, in a pharmaceutical facility where potent compounds are used, negative pressure may be applied – sometimes in conjunction with containment systems – to prevent the dust generated in a manufacturing process from cross-contaminating other areas of the plant.
    To maintain air quality during either positive or negative pressurization, proper air filtration is a must. High efficiency ASHRAE-grade HVAC filters or HEPA filters are the most common solution, but cartridge-type industrial dust collectors can offer an effective and sometimes overlooked alternative – especially in situations where dust loads are very heavy. Following are the key questions to ask when considering this strategy:

1. What applications are appropriate for pressurizing with dust collectors?
The most common industries include cement and lime production, metal and coal mining, pharmaceutical processing, and grain processing. But the technique can really be used wherever high volumes of dust are generated. Areas that are most commonly protected through pressurization include control rooms, server rooms, manufacturing clean rooms, compressor rooms, offices, quality control labs, substations, and electrical equipment and motor control center (MCC) rooms.

2. How do I decide if pressurizing is right for my facility?
Begin by considering the type of dust, conditions inside and outside the space, and what (or whom) you are trying to protect. If workers in an office area are subjected to unacceptable levels of dust or fumes, you will need to clean up the air to protect occupants against health hazards, and ensure compliance with OSHA regulations for exposure.
    If you are protecting electronic equipment costing $100,000 from damage, it will be worth investing in a $20,000 pressurizing system to protect that equipment. However, if a space is not occupied by workers and equipment is not of high value, pressurization may not be worth the expense.

3. What is the impact of climate?
The local climate might also impact your decision on whether to pressurize. In the tropics, if you inject large amounts of warm, moisture-laden air from outside to pressurize a building, the additional air conditioning load could be cost-prohibitive. However, in colder climates or during winter season, you can take advantage of the “free cooling” and use your pressurizing system for conditioning, saving substantially on air conditioning costs required to keep servers and electrical equipment from overheating.
    Return on investment should typically be less than two years for pressurization to be cost-effective, and field experience shows that the payback is often much faster. As noted before, sometimes pressurization is necessary to guarantee a certain air quality in a space. The technology is best applied to new construction, expansion or renovation projects. Retrofitting of existing facilities can be more costly and complex depending on the set-up and location of the room to be pressurized.

4. Is dust collection or HVAC filtration the best choice?
Pressurization with dust collectors is intended for very heavy dust loading applications where HVAC filters will not have an acceptable life. In extremely dusty conditions, high-efficiency HVAC filters can become rapidly overloaded and may require change-out every few months or even every few weeks in some instances.
    By comparison, cartridge dust collector filters are designed specifically to handle high dust loads in industrial environments. Dust collector filters are automatically pulse-cleaned with very brief bursts of compressed air that blow dirt off the filter surfaces and down into a collection device. When used in a pressurization system, high-efficiency cartridge filters can last for years between change-outs.
    A cost analysis will allow you to compare HVAC vs. dust collection filters. What is the projected first cost of the equipment/hardware and the filter elements? What is the anticipated cost of electrical energy required to operate the system? What is the life expectancy of the filters under the dust loading conditions? Based on change-out frequency, you can work with your filtration supplier to develop an annual cost estimate that takes into account not only the price of the filters but such factors as labor cost to change the filters, equipment downtime, inventory, disposal, etc.
    If you are in a highly regulated industry such as pharmaceutical or food manufacturing, there may also be restrictions as to what types of filters or what level of filtration will comply with industry standards.

5. What type of dust collector should I use?
If your analysis points toward dust collection as the most cost-effective approach, a cartridge-style dust collector will be the system of choice. Cartridge collectors offer much higher filtering efficiencies than conventional baghouses, a necessity when protecting workers or sensitive equipment from fine dust contamination. Cartridge collectors also operate at lower pressure drop, for more energy-efficient performance. The dust collector will be located outdoors, so make sure it is equipped with weather-resistant components and controls.
    If you are pressurizing to protect a space from gaseous contaminants, a wet scrubber system may be needed. However, if dust is mixed with the gases, you will still need the cartridge dust collector for particulate filtration installed upstream of the wet scrubber.

6. What are the guidelines for filter selection?
A very high-efficiency filter (MERV 15 or 16) is recommended for pressurizing applications. Cartridge filters using nano fiber or other high-efficiency filtration media are well suited to this use. Don’t rely solely on MERV values or filter efficiency percentages to predict performance. Although these measures are useful for comparing different filters, it is more important to ascertain that emissions will be at or below required thresholds. Ask the filter manufacturer for a written guarantee of emissions performance stated as grains per cubic foot.
    As noted, cartridge filters will only need infrequent change-out in pressurizing applications, but it is good practice to replace filters every two to three years. An older filter may develop a hole or leak after time and will no longer deliver the guaranteed efficiency.
    HEPA after-filters, sometimes known as secondary or safety monitoring filters, may be added to the ductwork downstream of the collector as a safeguard in critical applications. In the unlikely event of an air leak through the dust collector filters, the after-filters will provide backup protection. In certain cases, HEPA filters will be required to comply with industry-specific regulations. If control of outside odors is desired, carbon after-filters are another option to prevent outside odors from entering the pressurized space.

7. What control devices are required?
A variable frequency drive (VFD) and pressure sensor are essential for pressurized applications. The VFD drive provides precise electrical control of dust collector fan speed and is highly efficient in maintaining the desired airflow through the collector. It should always be used in tandem with a pressure sensor in the space to monitor and control air pressure.

8. How do I calculate dust collector airflow?
If the space to be pressurized is not heated or cooled, dust collector airflow should be calculated based on ventilation needs for indoor air quality + air leakage through cracks and openings (i.e., using standard formulas for infiltration). False ceilings, raised floors, and other construction details may also impact the calculations. For suggested velocities across openings and their corresponding pressures, refer to table 7-1 of the ACGIH “Industrial Ventilation: A Manual of Recommended Practice”.
    If the space to be pressurized requires heating or cooling, dust collector airflow should average between 10–20 percent of the HVAC unit airflow at a given capacity. This approach assumes the HVAC system has been properly sized to account for infiltration and will ensure you do not overwork the HVAC system by injecting too much humidity. Heavy-duty air handling components are recommended to withstand the dirty conditions.
    General ventilation guidelines for industrial applications recommend a difference of 5 percent between the supply and exhaust airflow. In most cases, a good rule of thumb is to set a pressure differential of 0.04 +/- 2 in. wg. For more details refer to section 7.5 of the ACGIH industrial ventilation manual. Uncontrolled pressure could have negative effects, creating high velocity conditions that result in slamming doors and back drafts. As noted, this can be prevented by using a pressure sensor inside the room and a VFD on the fan of the pressurizing unit to control and monitor pressure.
    Pablo Rocasermeno is Camfil APC’s regional manager for Latin America. He is a mechanical engineer with expertise in ventilation and dust collection systems. Camfil APC is a leading manufacturer of dust collection equipment and part of Camfil, the largest air filter manufacturer in the world. Rocasermeno can be reached at 800-479-6801, 870-933-8048, or filterman@farrapc.com. For more information on Camfil APC, visit www.camfilapc.com.

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