Pressurizing with dust collectors: Making the right decision
Deciding how to pressurize
As noted earlier, pressurization with dust collectors is best suited to heavy dust loading applications where HVAC filters will not have an acceptable life. In extremely dusty conditions, high-efficiency HVAC filters can quickly become overloaded and may require changeout every few months or every few weeks in some cases, even with pre-filters in place to protect the more expensive primary filters.
Cartridge dust collector filters, by contrast, are designed specifically to handle high dust loads in industrial environments. A dust collector has the capability to automatically pulse-clean its filters using 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 before needing replacement.
To determine the best choice for your application, begin with a cost analysis that evaluates the space to be pressurized and compare the two filtration technologies (i.e., HVAC vs. dust collection filters). What will be the initial cost of the equipment/hardware and the filters themselves? What will be the cost of electrical energy required to operate the system? What is the life expectancy of the filters under the anticipated dust loading conditions? Based on expected changeout frequency, you can arrive at 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, there may also be restrictions as to what types of filters or what level of filtration will comply with industry standards. This may be true for food-grade or pharamceutical-grade applications.
Dust collection equipment selection
If your analysis determines that dust collection is the most cost-effective approach, here are some general guidelines on the types of equipment best suited to pressurizing.
Type of dust collector: Cartridge style dust collectors are the system of choice because they typically offer much higher filtering efficiencies than traditional baghouse collectors, a necessity when protecting workers or sensitive equipment from high levels of fine dust. Cartridge collectors also operate at lower pressure drop, for more efficient performance. The dust collector will be located outdoors, so make sure it is equipped with weather-resistant components and controls.
Wet scrubbers: When pressurizing to protect a space from gaseous contaminants, a wet scrubber system should be used. If dust is present, however, you will still need the cartridge dust collector for particulate filtration installed upstream of the wet scrubber.
Filter media: 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 especially well suited to this use. Typically, an outer layer of extra filtration fibers will ensure the maximum efficiency of the media from the very first day. This technology increases the efficiency and allows maximum filtration with low pressure drop, thus improving the energy performance of the system.
Don’t rely solely on MERV values or filter efficiency percentages to predict performance, however. 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 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 or safety monitoring filters: HEPA or after-filters, also known as safety monitoring filters, may be added to the ductwork downstream of the collector to provide an extra measure of protection for 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 regulations of a specific industry.
Carbon after-filters: This option is available when odor control is desired. For example, for occupied spaces it may be a good practice to use carbon after-filters to prevent outside odors from entering the pressurized space.
Variable frequency drive (VFD) and pressure sensor: A VFD drive provides precise electrical control of dust collector fan speed and is highly efficient in maintaining the desired airflow through the collector. It is a must for pressurized applications, and should always be used in conjunction with a pressure sensor in the room. The two devices will work in tandem to monitor and control pressure.
Air conditioned vs. non-air conditioned spaces
Finally, the approach to pressurizing will vary depending on whether or not the space to be pressurized is air conditioned.
If the space does not have to be 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, see 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% and 20% 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.
It is important to use heavy-duty air handling components to withstand the dirty conditions, a sometimes overlooked step.
General ventilation guidelines for industrial applications recommend a difference of 5% between the supply and exhaust airflow. For most industrial applications, a good standard is to set a pressure differential of 0.04 +/- 2 in. wg. (For more details, see section 7.5 of the ACGIH industrial ventilation manual, above.)
Uncontrolled pressure could have negative effects, creating high-velocity conditions that result in slamming doors and back drafts. Most designers recommend a pressure sensor inside the room to adjust the supply air using a VFD on the fan of the pressurizing unit.
For example: To treat an area with office workers using an extraction system mounted on top of a building, you will need a certain number of air changes per hour, which will determine the airflow. You should then add 5% to 10% more on top of that airflow to create pressurization. So it you need to extract 1000 cfm from the room, you will want a dust collector with 1,100 cfm capacity to make sure you are injecting more air than you are extracting.
If there are some openings or potential leak paths, it isn’t a bad idea to oversize the dust collector slightly or calculate your infiltration and add it to the formula. And as noted, a VFD and pressure sensor should again be used as controls.
Pablo Rocasermeno is Camfil APC’s regional manager for Latin America. He is a mechanical engineer with expertise in ventilation and dust collection systems. The author can be reached at filterman(at)farrapc.com; www.camfilapc.com.