NAFA Guide To Air Filtration Fifth Edition Chapter 6 Air Cleaners presented by the National Air Filtration Association

Air Cleaners Different types of air cleaners - Media Electronic Air Cleaners (aka-- Electrostatic Precipitators Agglomorators Service requirements for air cleaners Standards for operation To be exact, every type of air filter could be called an “air cleaner.” In the industry, we usually refer to air cleaners as those units that are meant for special applications such as source removal in one area or room, are self-powered and are separate from the main HVAC system of a building. We will discuss the different types of media and electronic air cleaners and the bigger cousins called agglomorators along with the advantages of disadvantages for each and the service requirements.

Media Air Cleaners Most media-type air cleaners use HEPA filters and molecular (activated carbon) afterfilters to clean the air. Media units are portable and most use a HEPA filter for air cleaning. Many also add a molecular filter for odor control. They utilize standard 110 volt electrical plug-in connection for power. Advantages of this type of unit is portability, ease of filter change and disposal and cost. Disadvantages include proprietary filters (higher cost) and possible noise factors.

Media Air Cleaners Air cleaners also come in whole-house residential styles that add-on to the existing HVAC unit While many NAFA members do not focus on residential units, know that HVAC dealers are getting into the filter business through the residential route with whole-house units that work well. Advantages are they clean and entire area, not just one room, and they are typically inexpensive to install and replace. Disadvantages include a higher pressure drop which can exceed the unit specifications and inability to wash and replace.

Electronic Air Cleaners Two-stage electronic air cleaners utilize: an ionizer section a collector section While electronic air cleaning (also called electrostatic precipitation or ESP) has been around for many years, it’s popularity comes and goes with applications. It works by causing incoming particles to receive a high-voltage positive charge in an ionizer section. These charged particles then pass into a collector section where oppositely charged plates collect the particles. The major advantage of electronic air cleaners is the ability to remove and wash the collector section and replace it and ESP’s are good for removal of smaller particles (used extensively in cigarette smoke removal). Another advantage is most units have very little pressure drop when added to an existing system. Disadvantages are they need maintenance and are sometimes less efficient due to increased airflow through the unit or increases in humidity that tend to negate the charge.

Efficiency The ASHRAE 52.2 filter test method has no specific procedure for testing the loading of electronic air cleaners – carbon fouls units The efficiency is highest when the plates are clean. Velocity and humidity sensitive. Slide is self-explanatory – the carbon in the ASHRAE Test Dust fouls the electrical connects in an ESP. Obviously it is difficult to determine when an ESP has reached it’s dust collection capacity.

Prefilters Low-efficiency air filters upstream of an electronic air cleaner: Filters will act as air distribution panels to equalize the airflow. Will capture large particles that could bridge the plates and cause a short or arcing excessive power consumption snapping noise Elimination of arcing helps control ozone generation. Slide is self-explanatory

Factors Affecting ESP Design & Efficiency Their successful use depends on the following: Particle Size Velocity through the filter Voltage on Ionizer and Collector Collector Plate Spacing Collector Plate Loading Ionizer Spacing Ozone

Service Requirements Electronic air cleaners should be designed for easy cleaning. Important design considerations: Insulators: high quality arc-resistant material and mounted out of the airstream. Easy access to the ionizer and collector cells. Some provision for isolating a section in large multi-component units if one becomes defective. Self-explanatory

Standards for Electronic Air Cleaners UL 867 “Standard for Electronic Air Cleaners: Unites States Covers electrical and fire safety requirements. It makes no comment about the air-cleaning ability of electronic air cleaners CSA C22.2 No. 187-Mi986 Canadian Standard: Specifically states (Par.i.5) that the Standard does not specify the effectiveness of air cleaners with respect to the removal of airborne particles. Association of Home Appliance Manufacturers Association (AHAM) AC-1-2006 UL 867 covers fire and ozone limitations for ESP’s – now set at 0.050 ppm. In America, the Association of Home Appliance Manufacturers (AHAM) tests air cleaners for efficiency and rates them using a Clean Air Delivery Rate (CADR) scale. This will be discussed more fully in Chapter 7.

Applications Not Suitable for Electronic Air Cleaners Conditions of very high humidity Systems where there can be a build up of explosive gases. Unattended equipment where periodic maintenance procedures are not assured.

Commercial ESP                                                              This is a picture of a large precipitator that you might find in a manufacturing plant. These units have self-washing cycles to keep the collector cells clean. These units still have application (when specially configured) to remove very specific contaminants created by commercial or industrial processes (smoke from solder flow making printed circuits and cooking smoke removal).

Dust Disposal The removal of dust that an electronic air cleaner has captured can be accomplished in two ways: It can be washed off the collector plates. If it has agglomerated on the plates, it can be allowed to blow off the plates. The blow-off is then captured by a mechanical filter downstream of the agglomerator cells

Washable Electronic Air Cleaners When the collector plates became loaded with dust so that the performance of the electronic air cleaner is reduced, the power is shut off and the plated are washed. Both small and large units have collector sections that can be washed. I many larger units, the washing device is installed inside the unit and runs at selected intervals.

Washing Removable component assemblies. Manual wash in place. Washing tanks Manual wash in place. Power spray with detergent Automatic wash in place. Built in spray nozzles Mobile washers. Large installations Here are examples of those washing systems.

Agglomerator Unit This picture is courtesy of Cincinnati Air Filter Company – this shows the very large agglomerator cells on the left and the after filter (roll filter) on the right. This unit is vintage 1957 and was removed and replaced with a mechanical filtration system.

Agglomerator Units Dry collector plates with a strong bond between the small dust particles and the collector plates. The particles on the plates form agglomerates. As the agglomerates grow larger, their attraction to the plate is reduced and finally these agglomerates may be introduced back into the airstream. While the theory of aggomeration is sound, the application was difficult. Many units were sold as “no maintenance” and the collector plates carbonized the dirt that remained on them and became useless. Many of these units are replaced with media filters.

Afterfilters Automatic Roll Filters Extended Surface Filters Capture agglomerates – larger particles Because the face velocity is not uniform, it can cause variations in efficiency. Extended Surface Filters MERV 14 More uniform air flow creates conditions for the agglomerator to operate at optimum efficiency. All agglomerators had after filters and most were automatic roll filters. The filter material did not need to be highly efficient since the particles were of a larger size. Some did have higher efficiency filters downstream of the unit.

Sample Questions Electronic Air Cleaners cannot be tested for loading using the ASHRAE 52.2 test because? The carbon in the Test Dust shorts out the electrical connections of the unit. One advantage of a self-contained ESP is? Washable cell/capture of smaller particles/good for source capture/portable

Thank you. Other Questions ?